DEGREE PROJECT IN VEHICLE ENGINEERING, SECOND CYCLE, 30 CREDITS STOCKHOLM, SWEDEN 2020
Modelling and simulations of window attachments for a surface warfare ship
MOHAMMAD MEKDADI
KTH ROYAL INSTITUTE OF TECHNOLOGY SCHOOL OF ENGINEERING SCIENCES Modelling and simulations of window attachments for a surface warfare ship
Mohammad Mekdadi
Examiner - Associate Professor Per Wennhage, KTH
Supervisor - Dan Eld, FMV
Master of Science Thesis
Stockholm, Sweden, 2020 Abstract
In recent years it has been discovered that the window installations on the Visby class corvette are prone to break. The aim of this report is to perform a structural analysis of three different window installations. Thus, to find which window installation minimizes the risk of leaks and cracks in the attachment connecting the window glass and the hull. The three window installations consist of the existing window installation and one of SAAB Kockums alternative solutions installed in two different ways, with and without an additional damping mass. The model that will be used in the simulations will consist of a rectangular shaped carbon fibre composite sandwich plate that represents the side structure of the maneuver deck. Furthermore, it will include three windows where only the middle positioned window will contain the given installation details in order to reduce the complexity of both the modelling and the simulations. Three load cases were simulated in ANSYS Workbench. The first load case called "hogging" consisted of a bending moment along the vertical sides of the model. The second load case called "slamming" consisted of a vertical force pointing upwards along the bottom of the model. Lastly, a torsion load case was simulated. In all load cases the existing window installation were subjected to the largest strains along the inner edges of the attachment. In the slamming load case, the alternative solution without the double damping mass was exposed to least strains around the inner edges of attachment compared to the other window installations. For the hogging and the torsion load case the alternative solution with double damping mass produced least strains around the inner edges of the attachment. But the alternative solution without the double damping mass was also able to reduce the strains around the inner edges compared to the existing window installation. In conclusion, the alternative solution without an additional damping mass is in overall minimizing the strains along the inner edges of the attachment in which the most leaks and cracks have been observed. It has especially been efficient in reducing strains in the slamming load case. Even though the window installation with an additional damping mass best withstands hogging and torsion, the slamming load case is the most common scenario. Therefore, the window installation that best withstands the slamming load case should be prioritized. Thus, the alternative window installation without the additional damping mass is the best alternative because it best withstands slamming but also reduces the strains along the inner edges compared to the existing window installation in the other load cases. Sammanfattning
Under de senaste åren har det upptäckts att fönsterinstallationerna på Visby-klasskorvetten har en benägenhet att brista. Syftet med denna studie är att utföra en hållfasthetsanalys på tre oli- ka fönsterinstallationer. Därav är målet att hitta den fönsterinstallation som minimerar risken för läckor samt sprickor i smygskarven som kopplar fönsterglaset med skrovet. De tre fönsterinstalla- tionerna består av den befintliga fönsterinstallationen samt en av SAAB Kockums lösningsförslag installerat på två olika sätt, med och utan dubbel dämpningsmassa. Modellen som ska användas i simuleringarna kommer att bestå av en rektangulär skiva bestående av en sandwichkonstruktion med lamineringar av kolfiberkomposit. Skivan ska motsvara sidan av manöverbryggan. Modellen in- kluderar tre fönster där endast mittenfönstret kommer att modelleras med de givna detaljerna på samtliga fönsterinstallationerna. Detta för att förenkla både modelleringen och simuleringarna. Tre olika lastfall kommer att simuleras i ANSYS Workbench. Det första lastfallet kallas "hoggingsom kommer utgöras av ett böjmoment längs de vertikala sidorna om modellen. Det andra lastfallet kallas slammingöch består av en vertikal kraft riktad uppåt längs modellens undersida. Slutligen, består det sista lastfallet av en kraft på respektive vertikala sida längs modellen som ska ge upphov till en vridning på modellen. I samtliga lastfall gav den befintliga fönsterinstallationen upphov till störst töjningar längs innerkanterna på smygskarven (kanterna närmast fönsterglaset). I slamming lastfallet gav lösningsförslaget utan dubbel dämpningsmassa upphov till minst töjningar längs inner- kanterna på smygskarven jämfört med övriga fönsterinstallationer. I hogging och vridning lastfallen gav lösningsförslaget med dubbel dämpningsmassa upphov till minst töjningar längs innerkanterna på smygskarven. Även lösningsförslaget utan dämpningsmassa lyckades dämpa töjningarna längs in- nerkanterna på smygskarven jämfört med den befintliga fönsterinstallationen. Sammanfattningsvis har lösningsförslaget utan dubbel dämpningsmassa i helhet minskat töjningarna längs innerkanterna på smygskarven där de flesta läckor och sprickor har observerats. Framförallt har lösningsförslaget varit effektiv i slamming lastfallet. Även fast lösningsförslaget med dubbel dämpningsmassa bäst mot- står hogging och vridning, är slamming lastfallet det mest förekommande lastfallet. Därför bör den fönsterinstallation som bäst dämpar töjningarna vid slamming prioriteras. Därav är lösningsförslaget utan dubbel dämpningsmassa det bästa alternativet då den är effektivast mot slamming samt att den reducerar töjningarna kring innerkanterna på smygskarven jämfört med befintliga fönsterinstal- lationen i resterande lastfall. Acknowledgement
I would like to thank a number of persons who helped and encouraged me during the time of this project.
First, I would like to thank Dan Eld, my supervisor at FMV for giving me the opportunity to write my thesis at FMV that has been very interesting and helped me gaining new experiences and develop my engineering skills. I am very grateful for his support and guidance throughout the project that has helped me moving forward.
I would also like to thank my supervisor and examiner at KTH Royal Institute of Tecknology Professor Per Wennhage for providing me with help and advice regarding the theory and the software used in this project. He never hesitated to have meetings and discuss questions that arose during the project which was greatly appreciated. Furthermore, I would like to thank PhD Gustav Hultgren for his advice in ANSYS.
Lastly, I want to thank my loving family and my friends for their support and for always standing by my side throughout my studies at KTH. Without them none of this would have been possible. Contents
1 Introduction 1 1.1 Background ...... 1 1.2 Problem formulation ...... 1 1.3 Initial Assumptions and Simplifications ...... 2
2 Theory 3 2.1 Structural Loads ...... 3 2.1.1 Longitudinal strength loads ...... 3 2.1.2 Hogging and Sagging ...... 4 2.1.3 Slamming ...... 6 2.2 Sandwich Structures ...... 7 2.3 FEM...... 8 2.3.1 ANSYS Workbench ...... 8 2.3.2 Large deflections ...... 9
3 The Structure of Corvette Visby 10 3.1 The Hull Structure of HMS Visby ...... 10 3.2 Reinforcements ...... 11 3.3 The existing window installation ...... 13 3.3.1 The window attachment ...... 13 3.3.2 Sikaflex ...... 14 3.3.3 Silicone Seal ...... 15 3.4 Alternative window installations ...... 15
4 Methodology 17 4.1 The model of the maneuver deck ...... 17 4.1.1 3D-model of the hull ...... 17 4.1.2 3D-Model of the Reinforcement ...... 18 4.1.3 Fibre Orientation ...... 19 4.2 Modelling of the Existing Window Installation ...... 20 4.2.1 Meshing model ...... 23 4.3 Modelling of Alternative Window Installations ...... 24 4.3.1 Meshing model ...... 24 4.4 Load Cases and Boundary Conditions ...... 25 4.4.1 Hogging load case ...... 25 4.4.2 Slamming load case ...... 26 4.4.3 Torsion load case ...... 27
5 Results and analysis 28 5.1 Hogging load case ...... 28 5.1.1 Existing window installation ...... 29 5.1.2 Alternative window installation (without double damping mass) ...... 30 5.1.3 Alternative window installation (with double damping mass) ...... 31 5.2 Slamming load case ...... 32 5.2.1 Existing window installation ...... 32 5.2.2 Alternative window installation (without double damping mass) ...... 33 5.2.3 Alternative window installation (with double damping mass) ...... 34 5.3 Torsion load case ...... 35 5.3.1 Existing window installation ...... 35 5.3.2 Alternative window installation (without double damping mass) ...... 36 5.3.3 Alternative window installation (with double damping mass) ...... 37 5.4 Summary of Results ...... 38
6 Discussion 39
7 Conclusion 41
8 Future Work 42
References 43
Appendix A A-1 A.1 Dimensions of the 3D-model of the hull ...... A-1
Appendix B B-1 B.1 Sikaflex 265 glue strings in the attachment of the alternative window installations subjected to different load cases ...... B-1 1
1 Introduction
1.1 Background
The modern corvettes were used back in World War II. The advantages were their smaller size combined with improved maneuverability. A corvette can be described as a small warfare ship which typically weights between 500 tons up to 3000 tons[1]. Sweden is one of the countries that still are operating corvettes today. The latest class of corvette in Sweden is the Visby class corvette, see figure 1. The Visby class corvette was built for the Royal Swedish Navy and constructed by SAAB Kockums under the lead of FMV (the Swedish Defense Materiel Administration). The first Visby class corvette, HMS Visby was delivered in December 2009. HMS Visby weights 640 tonnes (fully equipped) and its hull is entirely made of carbon fibre composite sandwich.[2]
In recent years it has been discovered that the window installations on the corvette are prone to break. The attachment that connects the window with the hull is detached from the window glass over time when the corvette is operated under normal conditions. Consequently, the risk of water entering the gap between the window and the hull increases. Thus, during cold temperatures when water expands, the risk of the attachment being detached is rapidly increased. This has been a recurrent problem causing costly and time consuming repairs. Repeated observations of cracks and leaks along the window attachments led to the conclusion that this problem is mainly caused the by overall window installation design.
The existing window installation has been used for decades and has worked sufficiently well, therefore it has not been prioritized until now. SAAB Kockums have finished a study on the existing installation and proposed four alternative solutions for the window installation. The remaining work in SAAB Kockums study is to verify which alternative solution is best suited based on how well it minimizes the risk of the attachment being detached.
Figure 1: Visby class corvette [3].
1.2 Problem formulation
On behalf of FMV, a conceptual study of three different window installations will be done. The three window installations consist of the existing window installation and one of SAAB Kockums alternative solutions installed in two different ways. The objective of this study will be to find which of the three window installations that best withstands the common load cases and minimizes the risk of leaks and 2
cracks in the attachment connecting the window glass and the hull. More precisely, the goal will be to find the window installation that minimizes the tensions i.e the strains along the contact area between the attachment and the window glass were most leaks and cracks have been observed. The modellings and simulations will be implemented using the software tools Solid Edge and ANSYS Workbench. The existing installation and the alternative installations will be 3D modelled in Solid Edge and simulated in ANSYS Workbench under different load cases, using its finite element tool.
1.3 Initial Assumptions and Simplifications
In order to delimit the work, the analysis will be based on a simplified representation of the side structure of the maneuver deck. The simplified model will consist of a rectangular shaped carbon fibre composite sandwich plate that will include a window positioned in the middle with the given installation details. In addition, the model will contain the surrounding windows, but they will be designed as two rigid rectangular bodies without detailed installation. This, in order to simplify the modelling but at the same time considering the surrounding windows’ contribution to the window positioned in the middle. So, by considering this simplified model, larger parts of the ship can be neglected in the modelling which reduces the complexity of the analysis but at the same time includes the areas of interest. A more detailed description of simplified model will be discussed later in this study in the methodology section.
The first step will be to model the existing window installation and analyze its weaknesses. Furthermore, with some modifications on the obtained model the alternative solutions can be modelled and analyzed to find whether they generate better results or not. The mentioned problems have been observed on different windows around the maneuver deck including the window denoted as number 11, see figure 2. Window number 11 will be the simplest window to reproduce for this study and for future testing due to its size and geometries. Therefore, all models and simulations will be based on window number 11 throughout the whole study.
Figure 2: Schematic view from the side of the full size Visby class corvette and its maneuver deck. 3
2 Theory
2.1 Structural Loads
Before doing any simulations, it is important to have an idea of what type of loads that will be simulated and analyzed. For a corvette there are many different loads acting on the structure. The loads are subjected to various patterns and magnitudes which causes deformations and stresses on the structure. There are three main categories of loads acting on the hull structure: longitudinal strength loads, trans- verse strength loads and local strength loads. In this study, focus will be on longitudinal strength loads as they define the overall strength of the ship’s hull which is most interesting in the simulations. Loads that are characterized as longitudinal strength loads are hogging, sagging and slamming that will be explained more in detail in sections 2.1.2 and 2.1.3.[4]
2.1.1 Longitudinal strength loads
The longitudinal loads play an important role when analyzing the overall strength of a hull structure. Due to the ship’s slender profile it can be regarded as a beam if it is observed globally. The loads that belongs to the longitudinal strength loads are following
• Bending moment
• Shear force
• Torsional moment
When analyzing longitudinal strength loads there are two main categories; static longitudinal loads and dynamic longitudinal loads.[4]
The static longitudinal loads are caused by the difference between weight and buoyancy when the water is still. According to Archimedes principle, when a body (in this case the draught of the ship) is completely or partially immersed in a fluid, the fluid exerts an upward force on the body equal to the weight of the fluid displaced by the body [5]. The resulting force that causes the longitudinal loads is as follows
F F F ρgV mg (1) net = buoyanc y − wei ght = − where ρ - Density of the fluid, in this case 1000kg/m3 (water). g - gravitational acceleration, 9.81m/s2. V - Immersed volume [m3]. m - Mass of the ship [kg].
Dynamic longitudinal loads are induced by waves. As the loads from waves varies over time, they become dynamic loads. Depending on the shape and position of the waves in relation to the hull, the ship will be subjected to different loads. The behavior of a wave can be seen in figure 3. 4
Figure 3: The fundamentals of waves induced loading.[6]
2.1.2 Hogging and Sagging
As seen in figure 3, the largest buoyancy loads are induced from the top of a wave, also called the crest while the loads are decreased in the trough. This behavior results in two phenomena called hogging and sagging. When analyzing these phenomena, the ship can be considered as a bending beam for simplicity. The locations of crests and trough is what defines the two phenomena, hogging and sagging see figure 4.
Figure 4: Dynamic longitudinal load phenomena known as hogging and sagging.[4]
The hogging phenomenon is experienced when the ship is on top of a wave crest. If comparing figure 3 and 4 it can be concluded that the crest is inducing an increased buoyancy load upwards in the midship region while the troughs are doing the opposite in the outer regions. By looking at the ship as a beam, it can be seen that the waves are causing a "hogging" global bending moment on the body. Additionally, a shear force is obtained between the middle region of the ship that is pushed upwards and the two outer regions that are pushed downwards when observing the phenomenon globally. When a ship is in a wave trough, a "sagging" bending moment and a shear force are acting on the hull but in opposite directions compared to the hogging phenomenon.[4]
However, the two phenomena presented above, hogging and sagging, are extreme cases. In figure 4, the waves are almost equal to the length of the ship which causes very high longitudinal bending stresses on the hull [4]. But, as the Visby corvette encountered problems with the window installation under normal conditions, these extreme cases are most likely not behind the observed leaks and cracks. However, the extreme case will give an idea of a ship’s behavior when exposed to longitudinal loads. Furthermore, according to Association of Classification Societies (IACS), the magnitude of the dynamic longitudinal 5
bending moments can be estimated as follows [4]
2 Mw( ) 0.19C C L BC 0 (2) + = + 1 2 1 b
2 Mw( ) 0.11C C L B(C 0 0.7) (3) − = − 1 2 1 b + where Mw( ) - the wave bending moment of hogging [kN m] + − Mw( ) - the wave bending moment of sagging [kN m] − − C1 - constant depending on ship length
³ ´1.5 300 L1 10.75 − L1 300m − 100 ≤ C1 10.75 300 L1 350m (4) = ³ ´1.5 ≤ ≤ L1 350 10.75 − 350m L − 100 ≤ 1
C2 - distribution factor along ship length
L1 - ship length [m] B - ship breadth [m]
Cb0 - block coefficient
The distribution factor C2 is a constant varying between 0 and 1 depending on what region along the ship length is considered. The distribution factor C2 1 for the region 0.4L 0.65L, starting from the end = − of the ship. Otherwise, C2 0 for regions beyond 0.4L 0.65L. The block coefficient takes into account = − the ship’s volume under water. Therefore, the length, breadth and the draft are used to calculate this volume, see figure 5.
Figure 5: Volumes that are used to compute the block coefficient.[7]
’ 6
The block coefficient is a factor between the displacements of the ship’s volume under the water and the rectangular box and the total volume of the rectangular box. For a ship that is 64 m long, 10 m broad and 1.5 m draft the block coefficient is 0.6 [7]. It can be assumed that the Visby corvette has approximately the same block coefficient because of its similar properties with a length of 73 m and a breadth of 10.4 m [2].
The above equations, 2 and 3, are supposed to return the approximate value of the maximum magnitude of the wave bending load. By inserting the known variables in equation 2 and 3, the obtained bending moments are following
Mw( ) 4.6311 107 Nm + = · Mw( ) 5.8092 107 Nm − = − ·
The above computed value will give an upper limit of the bending moment magnitudes that will be helpful when constructing the load cases in the simulations.
2.1.3 Slamming
Another more common phenomenon is slamming. Slamming is a kind of wave impact that occur when a ship slams hard into a water surface which particularly happens in heaving and pitching motions. This can be observed when the ship is moving with high relative velocity in rough seas. The highest probability of slamming is on the fore part of the ship where the relative vertical velocity between the ship and the waves is largest [8]. This is demonstrated in figure 6.
Figure 6: Slamming phenomenon on the fore part of the ship.
Slamming loads do reach high loads over small time periods. Usually, slamming loads are larger than other wave loads. Consequently, slamming may cause local damage on the ships or buckling on the deck for improperly constructed ships. The Visby class corvette has a maximum speed of 35 knots [2], which is considered as a high speed ship [9]. In general, for high speed ships, frequent slamming will accelerate fatigue failures of the hull. Furthermore, slamming is strictly nonlinear and a three-dimensional phenomenon. However, when performing the simulations later in this study the bottom slamming loads will be treated as linear and two-dimensional for simplicity.[10] 7
2.2 Sandwich Structures
The hull of the Visby class corvette is entirely made of carbon fibre composite sandwich structure. Thus, it will be important to understand the fundamentals of sandwich structures as it will be included in the model and the simulations. The main principle of sandwich structures is to maintain a lightweight component that has the same strength as a heavier solid component. Thus, the goal is to achieve a high stiffness and strength to weight ratio. A sandwich structure consists of two thin and stiff faces that are separated by a less stiff core, see figure 7.
Figure 7: Main parts of a sandwich structure [11]
The faces are adhesively bonded to the core which must be strong enough in order to obtain a high performing resistance against shear and buckling. The behavior of a sandwich is almost identical to an I-beam. The basic principle of an I-beam is to have more material in the flanges far from the centre of bending axis. Meanwhile, just enough material is used in the web connecting the two flanges in order let the flanges work together and resist shear and buckling. This idea can be compared to sandwich structures where the function of the face materials are the same as the flanges and the core takes the place of the web. The difference is the use of different materials and the continuous supports in each section of the sandwich. However, both sandwich structures and I-beams are beneficial for counteracting external bending moment.[11]
The flexural rigidity of an object subjected to a bending moment is normally the product of the Young’s modulus E and the moment of inertia I. But for a sandwich design, the flexural rigidity must be reformulated as the Young’s modulus varies over the z-axis, see definition of the axes in figure 8
Figure 8: Sandwich panel subjected to a bending moment 8
If assuming a sandwich with thin faces and a weak core the flexural rigidity can be approximated as
2 bt f (t f tc ) D E + (5) = f 2 where E f is the Young’s modulus of the face material and t f and tc are the thickness of the face material and the core respectively in meters. The width of the beam corresponds to the variable b.[11]
2.3 FEM
The finite element method (FEM) is a mathematical method that divides a global structure into several structural elements which can be separately analyzed. Each element has a number of nodes that are connected to surrounding elements. At each node, the value of the displacements is interpolated. The idea is to reduce the complexity by approximating the solution within an element using simple functions instead of using functions across the whole structure. Together, the elements form a solution for the global structure. Depending on the problem, the design can be modelled with different element types. If considering a three-dimensional problem which is the case in this study, the relevant elements consists of 3D-solids and 3D-shells [12]. FEM is an efficient method when analyzing more complex geometries that includes multiple material properties and boundary conditions. However, in order to reduce the computational cost, the geometries should be kept as simple as possible, as too detailed models do not always generate more accurate results. For instance, there are components that do not have any structural contribution. Therefore, a good strategy is to exclude such parts and reduce the complexity of the FEM design.
2.3.1 ANSYS Workbench
The software that will be used for the FEM analysis is ANSYS Workbench. After creating a 3D CAD model in Solid Edge it is imported into ANSYS Workbench. Then, the 3D model is meshed into several elements that can be automatically generated by choosing option "program controlled". The most commonly used element types in ANSYS Workbench are presented in figure 9.
Figure 9: Element types in ANSYS Workbench [13]
According to above figure there are two types of 3D elements: 3D-solid and 3D shell. In Workbench a 3D-model is originally meshed with SOLID186 elements. The 20-node second order element SOLID186 is 9
initially hexahedral. But, by combining some nodes, the SOLID186 element can be modified to 10-nodes second order tetrahedral elements (SOLID187), as well as other shapes.[14]
Another element that is relevant in this study is 3D-shell. It is originally a 2D-element that is defined as a 3D because it is not restricted to the XY-plane like a 2D element. But it can deform out of plane. A shell element does not have a physical thickness but in ANSYS a real constant is used to assign a thickness to a shell element. It can be used to model sandwich panel skins [12]. The fibre orientation properties of the shell are defined in another software called ANSYS ACP.
2.3.2 Large deflections
In ANSYS Workbench solver there is an option called "large deflection". By turning on large deflection the solution will be based on an updated stiffness matrix and a changing load vector [15]. That means ANSYS considers the changes in stiffness due to changes in shape of the simulating part. A hyperelastic material such as silicone rubber has a nonlinear elasticity [16]. In such a case, large deflection setting needs to be enabled so that the stiffness matrix is updated. 10
3 The Structure of Corvette Visby
3.1 The Hull Structure of HMS Visby
As mentioned before the hull of the Visby corvette is entirely made of carbon fibre composite sandwich. In the blueprints provided by FMV the hull is marked as H100/60 which stands for a 60mm thick core that is made of Divinycell H100, a foam material with density 100kg/m3. The properties of the Divinycell H100 are listed in table 1.
Property H100 Density [kg/m3] 100 Tensile Strength [MPa] 3.5 Young’s Modulus [MPa] 130 Shear Strength [MPa] 1.6 Shear Modulus [MPa] 35 Poisson’s ratio 0.4
Table 1: Divinycell H100 material properties [17].
Divinycell is a PVC (polyvinyl chloride) cellular foam and strong compared to other foams. Cellular foam surfaces are easy to bond to thanks to its solidity [11].
The face materials bonded on each side of the core is a carbon fibre composite laminate. A ship has to withstand loads in multiple directions, therefore it has been assumed that the laminates consists of multiple layers (also called plies) in a quasi-isotropic lay-up. A quasi-isotropic lay-up is a stack-up of plies with different fibre orientation. The idea is to maintain the same strength and stiffness through any direction within the plane of the laminate [18]. In figure 10, the difference between a unidirectional and a quasi-isotropic stack-up are presented.
Figure 10: A comparison between a unidirectional and a quasi-isotropic stack-up [19]
In order to achieve a high performing carbon fibre laminate apart from fibre orientations, the layup must be symmetrical, and each lamina should have the same layer thickness. In addition, each layer should contain the same fibre type and geometry. Each fibre orientation provides a specific strength property to the overall stack [18];
• 0o layer - Provides axial strength
• 45o layer - Provides shear and torsional strength ± 11
• 90o layer - Provides transverse strength
In ANSYS there is a predefined material, epoxy carbon fibre woven 230 GPa wet. As there are no details provided about the carbon fibre used in the faces, the epoxy carbon fibre woven 230 GPa wet will be used in the simulations in this study. This carbon fibre is a ply with fibre directions 0/90 degrees resulting in a higher strength in those directions. The material properties of this material are listed in the table below. The laminates are approximately 3.6 mm thick over the whole hull structure.
Figure 11: Epoxy carbon fibre woven 230 GPa material properties [20]
Summing up, the hull construction consists of two 3.6 mm thick face material made of a quasi-isotropic carbon fibre composite and a 60 mm thick core made of Divinycell H100. In addition, the 3.6 mm thick face material consists of multiple sets of 0o/90o/ 45o plies stack-up in the real structure like the stack-up ± in figure 11. However, when performing the modelling in ANSYS this will be simplified to one set of 0o/90o/ 45o stack-up but will generate similar material properties, see section 4.1.3. ±
3.2 Reinforcements
Another sandwich structure in the Visby corvette is the reinforcements. The side structure of the hull consists of one vertical and one horizontal reinforcement hand laminated on the back, see figure 12a. 12
(a) Reinforcements on the side structure of the maneuver (b) Detailed blueprint of the reinforcements on the side deck. structure of the maneuver deck.
Figure 12
The reinforcements are made of three beams connected to each other where the vertical reinforcement consists of one beam and the horizontal reinforcement consists of two beams denoted as 208, 210 and 214 in figure 12b. All beams have the same cross section but different lengths, the cross section is shown in figure 13. The reinforcements are made of a 60 mm thick Divinycell H100 core with 3.6 mm thick carbon fibre face material, like the hull structure. Yet again, epoxy carbon fibre woven 230 GPa wet will be used as face material. It will be assumed that these faces are quasi-isotropic because the reinforcements have to withstand loads in multiple directions.
Figure 13: Cross section of the reinforcements.
The purpose of adding reinforcements is to provide an additional strength to the structure. The strength of the reinforcement is strongly dependent of the direction of the loads and therefore, both a vertical and a horizontal reinforcement are included. 13
3.3 The existing window installation
The existing window installation consists of multiple parts, see figure 14. The hull in this context is the area above the window frame. The dotted area represents the core material Divinycell H100 while the dashed area surrounding the core is the epoxy carbon fibre. The thickness of the carbon fibre laminate remains constant along the structure, but the core is tapered around the window glass.
Figure 14: Schematics of the existing window installation.
The window is kept in place by the pink colored components, see figure 14. The components are attached by a screw in the window frame. However, these components are not intended to carry any loads from the window, only to keep the window in the right place horizontally.
Another component in the window installation is the EMC (electromagnetic compatibility) gasket. The purpose of the EMC gasket is to establish a connection to the layers inside the window glass, enabling window heating and protection from EMI/EMP (Electromagnetic interference/Electromagnetic pulses). The EMC gasket does not have any damping effects and therefore will not contribute to the structural analysis.
3.3.1 The window attachment
The window attachment consists of a carbon fibre sheet that connects the window glass and the sur- rounding hull, see part named attachment in figure 14 and see front view of the attachment in figure 16. The carbon fibre sheet is 2 mm thick and consists of a stack-up of layers with fibre directions 0o/90o. Again, the epoxy carbon fibre 230 GPa wet will be used as material. The attachment is bonded directly to the hull and the window, as demonstrated in figure 15. The attachment is not designed to carry any loads and therefore it is not quasi-isotropic. Its main purpose is to cover the Sikaflex joint between the window glass and the hull in order to protect the structure from EMI/EMP. This because Sikaflex 265 provides a weak isolation from EMI/EMP. 14
Figure 15: The attachment connecting the window and the hull, cross-section view.
Figure 16: The attachment connecting the window and the hull, front view.
The problems observed on the windows are mainly related to the window attachment. More precisely, it is the region encircled in figure 14. The area of attachment that is connected to the window glass has been detached over time under normal conditions consequently leading to penetration of water. Cracks on the window glass outer layer has also been observed at certain occasions. However, these cracks are thought to be the result of mishaps during transportation and/or installation of the windows.
3.3.2 Sikaflex
Sikaflex 265 is a glue specifically used for glass. It is used to attach the window glass with the hull corresponding to the sealant in figure 14. Furthermore, Sikaflex 265 has been used to glue the attachment to the window glass (note that the connection between the attachment and the hull is hand laminated). In ANSYS Sikaflex 265 is not an existing material and therefore, the material will be created and added to the material library in ANSYS. The glue will be modelled as an elastic isotropic material. Such material has the same elasticity independently of what direction though the material is considered. This assumption is justified in [21]. The material properties of Sikaflex 265 are listed in table 2. 15
Property Sikaflex 265 Density [kg/m3] 1200 Tensile Strength [MPa] 6 Young’s Modulus [MPa] 2.7 Shear Strength [MPa] 4.5 Shear modulus [MPa] 0.91 (see equation 6) Poisson’s ratio 0.48
Table 2: Sikaflex 265 material properties [22][23].
The shear modulus can be easily calculated if assuming isotropic material properties as follows
E G (6) = 2(1 v) + where the Young’s modulus E and the Poisson’s ratio v are given. E, v and the computed shear modulus will be added to the material properties of Sikaflex 265 in ANSYS and further used in the simulations.
3.3.3 Silicone Seal
The silicone seal is positioned between the window and the hull, see figure 14. Silicone rubber is a soft material with relatively low modulus of elasticity characterized as a nonlinear material. The advantage is that it can reduce stress concentration due to its ability to absorb a high amount of energy. But silicone seals are not good at transferring high shear forces. The stiffness of a seal is strictly dependent on the geometry, and in this case the seal is cylindrical along the window’s four sides.[24]
The contact between the silicone rubber and the window glass and the hull is a cylinder to flat surface contact. This contact area could be described as a line. Such a contact requires a numerical method and models such as Hertzian elastic contact cannot be applied. This because of the nonlinear elastic properties of silicone rubber. The contact area is small due to the geometries of the bodies. The loads are distributed over this area and therefore, there will be rapid changes in stress surrounding the contact. When performing a FEM simulation, this contact will require a very fine mesh in order to not miss the rapid changes of stresses. Even a simple Hertz contact demands a very fine mesh to obtain accurate results. Another problem associated with nonlinear material contacts is that the solution is loading-path dependent. Thus, the applied loads must be divided into several small steps and solve for each iteration. Consequently, a fine mesh together with small steps loads will be time consuming and require much memory [25]. Details on how this has been resolved is presented in section 4.2.
3.4 Alternative window installations
In the first alternative solution, see figure 17a, the silicone seal and the outer sealant are removed. Instead these gaps are filled with Sikaflex 265. The laminate that covered the outer sealant has been removed in order to have a thicker seal of Sikaflex 265. Furthermore, a carbon frame has been added to the installation. It is bonded right above the EMC gasket. The aim of the carbon fibre frame is 16
to maintain a more controlled mounting of the window glass. In the second alternative solution, an additional damping mass made of Sikaflex 265 has been added right above the carbon frame, see figure 17. Otherwise, the rest of the components are identical in both alternatives.
(a) Alternative window installation without double damp- (b) Alternative window installation with double damping ing mass. mass.
Figure 17
Another component that differs the two alternative window installations from the existing window installation is the attachment connecting the window glass and the hull. Instead of bonding the carbon fibre sheet directly to the window and the hull, Sikaflex 265 will be inserted in between them, see figure 18.
Figure 18: A new concept of the attachment connecting the window and the hull, cross-section view.
The Sikaflex 265 will glue the carbon fibre sheet along two lines: on the hull and on the window glass. The reason why an area in the middle is kept without glue is to let the carbon fibre sheet flex vertically. Recommendations from SAAB Kockums is to have a width of at least 10 mm for the Sikaflex 265 attaching the carbon fibre sheet. In order to preserve a flush installation of the window attachment, a thickness of 1 mm will be used for the Sikaflex 265 glue. 17
4 Methodology
4.1 The model of the maneuver deck
The model that will represent the maneuver deck will consist of the area enclosed by the green square, see figure 19. As mentioned earlier in the introduction the simplified model will exclude the two outer windows because it is assumed that they do not have any direct effect on the middle-positioned window.
Figure 19: Rectangular model of the maneuver deck side.
The reason why the two windows besides the middle positioned window are kept in the model is because their stiffness may have an effect on the middle positioned window. In ANSYS glass has a Young’s modulus of 69.9GPa in all directions. This can be compared with the epoxy carbon fibre woven which has a Young’s modulus of approximately 59.2GPa in X and Y directions, see figure 11. In a sandwich structure the face material contributes mostly to the overall stiffness. Therefore, by comparing the Young’s modulus i.e. the stiffness of the window glass and the face material of the sandwich structure in which the hull is entirely made of, it appears that both materials have a stiffness of the same order of magnitude. This means that the window glass will contribute to the overall stiffness of the model and may even act as local supports as glass is slightly stiffer than epoxy carbon fibre woven. Therefore, two additional windows are included in the model apart from the middle positioned one, as they can have an effect on the middle positioned window.
4.1.1 3D-model of the hull
The simplified side structure of the hull has been 3D-modelled in Solid Edge. In total there are three windows where the middle window has a detailed installation. The two outer windows consist of two rectangular glass blocks that are directly attached to the hull. The different views of the 3D-model are shown in figure 20. 18
Figure 20: 3D-model of the side structure hull
The dimensions of the 3D-model of the side structure hull are based on the real dimensions of the enclosed area in figure 19. The dimensions have been taken from blue prints of the Visby class corvette provided by SAAB Kockums. The dimensions of the 3D-model are presented in appendix A.1.
4.1.2 3D-Model of the Reinforcement
The reinforcements on the side hull structure consists of three beams connected to each other. The attachment connecting the three beams has been simplified by modelling the whole structure as one body, see figure 21a. The reinforcements are then attached to the back of the side hull structure model, see figure 21b. 19
(b) Reinforcements and side structure hull assem- (a) 3D-model of the reinforcements. bly.
Figure 21
The model in figure 21b forms the foundation of the remaining modellings in this study. The next step is to model the existing window installation and the two alternative installations. The only part that will be modified on the 3D-model of the hull from now on is the middle positioned window, the other parts remains the same for all models.
4.1.3 Fibre Orientation
Both the hull and the reinforcements are sandwich structures. In order to consider this kind of structure the face materials covering the surfaces of the hull and the reinforcements will be designed in ANSYS. ANSYS has a tool where shells with specific thickness, made of 3D shell elements, can be created on desired surfaces. The fibre orientations and layup of plies of the shells can then be edited in ANSYS ACP. The thickness of the shells has been set to 3.6 mm and the composite layup are listed in table 3.
Ply Thickness orientation [mm] 0o 0.9 45o 0.9 45o 0.9 − 90o 0.9
Table 3: Composite layup of the face materials on the hull/ reinforcements.
The 3.6 mm composite consists of four equally thick layers which together have a fibre orientation code 0o/45o/ 45o/90o. This type of layup has quasi-isotropic properties. The Young’s modulus and shear − modulus in each direction through plane of the face material are generated in ANSYS in the diagram below. 20
Figure 22: Young’s modulus and shear modulus in each direction through the plane of the face materials on the hull/ reinforcements.
As seen in figure 22, a constant stiffness is maintained through all directions in the plane which represents the characteristics of a quasi-isotropic material.
4.2 Modelling of the Existing Window Installation
The existing window installation was 3D modelled in Solid Edge. As seen in figure 23, all components are included if comparing the model with the existing schematic model in figure 14. However, by including all components the model will require a larger number of elements and nodes in the mesh. Firstly, such a mesh model will have a high computational cost. Secondly, the ANSYS student license does have a limit on number of nodes and elements which consequently will result in failures. Thus, important areas such as the attachment connecting the windows and the hull cannot be finely meshed. So, instead of having many elements with insufficient mesh quality, a better idea is to reduce the model and have a higher mesh quality in the areas of interests. 21
Figure 23: Initial 3D-model of the existing window installation, cross-section view.
Thus, some modifications must be done to the current model in order to focus the simulations on the problem area. Firstly, the components attaching the window horizontally are removed because they are mainly assigned to keep the window in correct position but are not designed to carry any loads. Therefore, the window is bonded to the hull in ANSYS in order to keep it in the right position. Secondly, the EMC- gasket has been taken away because it is not relevant for the structural analysis as its main task is to transfer heat to the window glass. Moreover, the rounded edge on the hull which surrounds the window glass has been replaced with a sharp edge in order to achieve a more efficient mesh. Lastly, the cylindrical silicone seal has been replaced by an octagon shaped silicone seal, see figure 24b. This is done because of the difficulties with the contact between the silicone seal and the hull/window, which was discussed in section 3.3.3. The complete simplified 3D-model of the existing model can be seen in figure 24a
(a) Simplified 3D-model of the existing window instal- (b) A close-up view of the octagon shaped silicone seal lation, cross-section view. on the simplified 3D-model.
Figure 24 22
Moreover, in the simplified model the size of the middle window glass is kept the same and instead the hull has been compensating the space of the EMC-gasket. The carbon fibre sheet in the attachment connecting the window and the hull is unlike the rest not quasi-isotropic. Instead, the fibre orientation code is 0o/90o which is the exact property of the epoxy carbon fibre (230 GPa) wet. The carbon fibre sheet is 2 mm thick and will consist of two 1 mm layers stackup of 0o/90o epoxy carbon fibre,
Ply Thickness orientation [mm] 0o 1 90o 1
Table 4: Composite layup of the carbon fibre sheet.
The Young’s modulus and the shear modulus in each direction through the plane of the carbon fibre sheet are displayed in the diagram below.
Figure 25: Young’s modulus and shear modulus in each direction through the plane of the carbon fibre sheet.
It is clear that the carbon fibre sheet is weaker in the 45o degrees regions and therefore do not have ± a constant stiffness in all angles in the plane of the material as for the quasi-isotropic case. 23
4.2.1 Meshing model
The meshing of the existing window installation consists of multiple element types that are automatically generated in ANSYS for the geometry. The automatically generated mesh has been modified with a finer mesh on the middle window including the attachment and the other components around the middle window. The final mesh model used for the simulations is presented in figure 26. The total number of nodes and elements are listed in the table below.
Number of Number of nodes elements 62907 29775
Table 5: Number of nodes and elements of the mesh of the existing window installation.
(a) Mesh of the existing window installation, front view. (b) Mesh of the existing window installation, back view.
(c) Mesh of the existing window installation, cross- sectional view. (d) Mesh of the carbon fibre sheet attachment.
Figure 26 24
4.3 Modelling of Alternative Window Installations
In these models, the silicone seal and the outer sealant are removed and replaced with Sikaflex 265. A carbon frame has been added around the window. The difference between the alternative solutions is the double damping mass that also is made of Sikaflex 265. The 3D-models of the two alternative window installations are shown in figure 27
(a) 3D-model of alternative window installation WITHOUT (b) 3D-model of alternative window installation WITH additional damping mass, cross-section view. additional damping mass, cross-section view.
Figure 27
4.3.1 Meshing model
The mesh designs of the front and the back of the overall side structure hull follows the same idea as for the existing window installation, see figures 26a and 26b. The mesh of the carbon fibre sheet in the attachment is also the same as for the existing window installation, see figure 26d. However, there are different components used both in the attachment connecting the window glass and the hull as well as new components around the middle window. The same idea as in the existing window installation has been maintained where a finer mesh has been applied for the smaller components around the middle window. The Sikaflex 265 glue strings are finely meshed as well. The mesh designs of the alternative window installations are presented in figure 28. The total number of nodes and elements are listed in the table below.
Alternative window Number of Number of installation nodes elements Without 59911 30711 double damping mass With 68638 33549 double damping mass
Table 6: Number of nodes and elements of the mesh of the alternative window installations. 25
(a) Mesh of alternative window installation WITHOUT dou- (b) Mesh of alternative window installation WITH double ble damping mass, cross-sectional view. damping mass, cross-sectional view.
Figure 28
4.4 Load Cases and Boundary Conditions
In this study, three different load cases will be examined: hogging, slamming and torsion. The idea is to find how well each window installation withstands the mentioned load cases. In other words, which window installations that causes least strain on the attachment connecting the window glass and the hull. Thereby, the conclusion will be based on a comparison between the results of the different window installations subjected to the same loads. Thus, the exact magnitudes of the applied loads are not important. Therefore, the loads have been chosen arbitrarily for each load case.
The applied boundary condition has been set the same for all load cases in order to achieve the same prerequisites for all load cases. It is assumed that the connection between the side structure of the maneuver deck and the ceiling is a fixed support which means that it does not move in any direction. The reason why the ceiling has been used as the fixed support and not the floor is because of the slamming load case. If the floor is set as a fixed support, it will not be possible to apply a force vertically along the floor. Therefore, the ceiling has been chosen to be the fixed support in order to keep the same boundary condition for all load cases. However, to set the ceiling as a fixed support is a rough approximation of the reality because the ceiling can still deform. But this is still just a model and therefore not a full representation of the reality.
4.4.1 Hogging load case
In equation 2 the maximum hogging moment were estimated to be around 4.6311 107 Nm. However, · this hogging moment is considered as an extreme case that most likely will not occur. However, this estimation will help giving an idea of what the upper limit is and an arbitrarily value less than this limit will be chosen in the simulations. The applied loads and the boundary condition for the simulations are demonstrated in figure 29. 26
Figure 29: Hogging load case.
The boundary condition at position C represents the ceiling of the corvette that is supposed to be placed right above the side structure model. The applied hogging moment has been arbitrarily set to 104 Nm which is evenly distributed along the two sides of the model, see positions A and B in figure ± 29.
4.4.2 Slamming load case
The slamming load is assumed to be an evenly distributed force along the bottom side of the side structure, in position A. Slamming is usually the largest load subjected to ships, and therefore it will be larger than the other loads in the simulations. The total magnitude of the force is arbitrarily set to 105 N. A fixed support is located along the top of the model. The applied load and the boundary condition are demonstrated in figure 30.
Figure 30: Slamming load case. 27
4.4.3 Torsion load case
The third and last load case that will be analyzed is torsion. Two evenly distributed forces with opposite directions will be applied on each side of the model, in positions A and B. The total magnitudes of the forces has been arbitrarily set to 103 N. This load case could appear when waves slams into the side of ± the hull structure.
Figure 31: Torsion load case. 28
5 Results and analysis
Three different load cases will be simulated in ANSYS. In all simulations the equivalent von Mises strain will be evaluated in order to find how much the attachment is strained and thereby, which window installation performs best in each load case. The equivalent von Mises strain is not an optimal option when analyzing composites in which the attachment is made of, as it does not fully describe the tensions in each ply of the composite. However, in this study the carbon fibre composites in the 3D-models have been modelled as homogeneous. Thus, the equivalent von Mises strain is sufficient to find the overall strains of the composite but will ignore the strains in specific plies. Moreover, strain is a unitless quantity describing the ratio of the change in dimension (deformation) to the original dimension on the body of interest.
The complete 3D model of the maneuver deck will be simulated in ANSYS Workbench. However, In the results focus will lay on the middle window and its surrounding components. Therefore, after simulating the load case on the whole model, the middle window will be cut from the solution and presented for each load case, see enclosed area in figure 32. In addition, the cross section of the middle window will be presented for each load case in the results as well.
Figure 32: The enclosed area represents the cut off area for the results.
Furthermore, large deflection has been enabled in ANSYS Workbench solver settings for the simula- tions on the existing window installation but disabled for the other two alternative solutions because they do not contain any hyperelastic material (silicone rubber). Lastly, in the results, the output values of the strains have been re-scaled for every window installation in each load case. This, in order to simplify the comparison of the results between the different window installations
5.1 Hogging load case
The hogging load case presented in section 4.4.1 is applied on each window installation presented below. 29
5.1.1 Existing window installation
In figure 33a it can be seen that the existing window installation do have higher strains on the lower 5 areas of the attachment. Especially the bottom corners that reaches strains up to 1 10− m/m. This · means that there are higher stress concentrations towards the corners if assuming that strain is linearly proportional to stress which is justified by Hooke’s law σ E². Along the inner edges of the vertical = 6 sides of the attachment, strains up to 4 10− m/m are observed. The top area of the attachment do have · lower strains close to the window glass but the strains become larger towards the corners. Lastly, when looking at the cross section, the silicone seal, outer sealant and the damping mass are exposed to strains.
(a) Equivalent (Von-Mises) strain of the attachment on the existing window installation.
(b) Equivalent (Von-Mises) strain of the existing window installation, cross- sectional view.
Figure 33 30
5.1.2 Alternative window installation (without double damping mass)
Compared to the existing window installation there are larger strains along the outer edges of the 6 attachment. The top side of the attachment reaches strains up to 4 10− m/m in regions that reached · 6 strains up to 2 10− m/m on the existing window installation. But, along the inner edges close to the · window glass the strains have been reduced. Especially around the middle regions on the vertical sides but also along the lower horizontal inner edge. The Sikaflex glue connecting the glass and the carbon fibre sheet is subjected to small strains compared to the rest of the model, see figure 43b in appendix B.1 for a close up view.
(a) Equivalent (Von-Mises) strain of the attachment on the alternative window installation without double damping mass.
(b) Equivalent (Von-Mises) strain of the alternative window installation without double damping mass, cross-sectional view.
Figure 34 31
5.1.3 Alternative window installation (with double damping mass)
6 When adding an additional damping mass the low strain areas (strains up to 2 10− m/m corresponding · to light blue) along the horizontal sides of the attachment are smaller compared to the installation that excludes the extra damping mass. However, the strains are still low along the inner edges close to the 6 window glass. Additionally, the vertical sides have larger areas of strains less than 2 10− m/m compared · to previous window installation. The strains along the corners have been significantly reduced compared to the other two window installations. Lastly, in the cross sectional view, it can be seen that the Sikaflex 6 glue is exposed to larger areas of strains up to 4 10− compared to the previous window installation but · is still considered low compared to the rest of the model, see figure 46b in appendix B.1 for a close up view.
(a) Equivalent (Von-Mises) strain of the attachment on the alternative window installation with double damping mass.
(b) Equivalent (Von-Mises) strain of the alternative window installation with double damping mass, cross-sectional view.
Figure 35 32
5.2 Slamming load case
The results in this section are based on the slamming load case in section 4.4.2.
5.2.1 Existing window installation
On the existing window installation there are higher strains up to 0.0003 m/m located on regions along the vertical sides including the inner edges close to the window glass. Along the inner edge of the lower 5 horizontal side, lower strains less than 5 10− m/m are observed. In the cross sectional view it can be · observed that the silicone seal, inner sealant and the damping mass are exposed to larger strains up to 0,0011944 m/m meaning that they do have a damping effect to the surrounding structure.
(a) Equivalent (Von-Mises) strain of the attachment on the existing window installation.
(b) Equivalent (Von-Mises) strain of the existing window installation, cross- sectional view.
Figure 36 33
5.2.2 Alternative window installation (without double damping mass)
Compared to the existing window installation, the vertical sides have significantly lower strains, especially 5 towards the inner edges that reaches strains less than 5 10− m/m around the middle areas. In addition, · the strains on the inner edge of the top horizontal side is reduced. However, the strain on the inner edge of the bottom horizontal side has increased to strains up to 0.0001 m/m compared to previous window 5 installation that reached strains up to 5 10− m/mon the same area. ·
(a) Equivalent (Von-Mises) strain of the attachment on the alternative window installation without double damping mass.
(b) Equivalent (Von-Mises) strain of the alternative window installation without double damping mass, cross-sectional view.
Figure 37 34
5.2.3 Alternative window installation (with double damping mass)
In overall, by adding an additional damping mass the strains larger than 0.0003 m/m, corresponding to the light green colored areas, have been reduced compared to previous window installation. The bottom 5 vertical inner edge reaches strains less than 5 10− m/m compared to the previous window installation · that reached strains up to 0.0001 m/m on the same area. However, along the other inner edges of the attachment there are larger strains compared to previous window installation, especially the right inner edge that reaches strains up to 0.0003 m/m
(a) Equivalent (Von-Mises) strain of the attachment on the alternative window installation with double damping mass.
(b) Equivalent (Von-Mises) strain of the alternative window installation with double damping mass, cross-sectional view.
Figure 38 35
5.3 Torsion load case
The results in this section are based on the slamming load case in section 4.4.3.
5.3.1 Existing window installation
On the attachment of the existing window installation, strains with magnitudes up to 0.00067064 m/m corresponding to the green colored areas are observed around the inner edges along the two horizontal sides. The inner edge of the left vertical side of the attachment is also subjected to large strains up to 0.0004 m/m compared to the rest of the model. On the left vertical inner edge, lower strains between 5 1 10− 0.0001 m/m can be observed. · −
(a) Equivalent (Von-Mises) strain of the attachment on the existing window installation.
(b) Equivalent (Von-Mises) strain of the existing window installation, cross- sectional view.
Figure 39 36
5.3.2 Alternative window installation (without double damping mass)
In the alternative window installation without double damping mass, the strains on the attachment are significantly decreased compared to the existing window installation, but there are still larger strains towards the inner edges of the attachment which reaches magnitudes up to 0.0004 m/m. Additionally, the strains on the right vertical side of the attachment have decreased compared to the existing window 5 installation where most of strains reaches magnitudes up to 6 10− m/m. ·
(a) Equivalent (Von-Mises) strain of the attachment on the alternative window installation without double damping mass.
(b) Equivalent (Von-Mises) strain of the alternative window installation without double damping mass, cross-sectional view.
Figure 40 37
5.3.3 Alternative window installation (with double damping mass)
When adding an additional damping mass, the strains along the inner edges of the attachment have been further reduced compared to the alternative window installation without the additional damping mass. Especially the corners of the attachment are subjected to lower magnitudes of strain. The order of magnitudes of the strains are the same as for the window installation without the additional damping 5 mass, but the regions with strains up to 0.0001 m/m have been reduced to strains up to 6 10− m/m. ·
(a) Equivalent (Von-Mises) strain of the attachment on the alternative window installation with double damping mass.
(b) Equivalent (Von-Mises) strain of the alternative window installation with double damping mass, cross-sectional view.
Figure 41 38
5.4 Summary of Results
The above results have been summarized in table 7. The window installation that best reduces the strains towards the inner edges of the attachment has been marked with an X.
Window Installation Hogging Slamming Torsion Existing (X) Without additional X damping mass With additional X X damping mass
Table 7: The most optimal window installation for each load case based on how well it reduced the strains towards the inner edges of the attachment
In the hogging load case the existing window installation was in overall subjected to smaller strains on the attachment. But towards the inner edges of the attachment there were slightly larger strains compared to the other window installations. In overall the exciting window installation performed and therefore it has been marked with an X in parenthesis. However, if just considering the strains around the inner edges of the attachment, the window installation with an additional damping mass was the performed best in the hogging and torsion load case while the window installation without an additional damping mass performed best in the slamming load case. 39
6 Discussion
The results proved that each window installation had both strengths and weaknesses. In the hogging load case, the attachment on the existing window installation were subjected to least amount of strain in overall. However, if taking into account the area were most leaks have been observed which is the inner edges of the attachment, the alternative solutions have reduced the strains within these areas. Furthermore, by adding an additional damping mass, the strains around the bottom corners of the attachment have been reduced and the low strains around the inner edges are still kept low.
In the slamming load case the existing window installation was subjected to largest strains both overall and around the inner edges of the attachment. The alternative solution without the additional damping mass was able to significantly reduce the strains on the vertical inner edges. The horizontal edges have larger strains than the vertical edges. In the alternative solution with the additional damping mass, the low strains are moved towards the outer vertical edges of the attachment. Compared to the first alternative solution, the strains on the overall attachment are lower compared to the first alternative window installation. But, the strains are larger along the inner edges of the attachment, especially towards the bottom right inner corner.
In the torsion load case the existing window installation is yet again subjected to the largest strains. The alternative solutions proves to be efficient in reducing the strains on the attachment where the alternative solution with additional damping mass shows least overall strains on the whole attachment as well as towards the inner edges.
Another important factor to consider in the alternative solutions is the new concept of the attachment. Instead of hand laminating the carbon fibre sheet directly to hull, two strings of glue made of Sikaflex 265 have been placed on the window glass and the hull respectively. But, even though the Sikaflex 265 is a much softer material than the carbon fibre sheet, the strains of the glue and surfaces the carbon fibre sheet that the glue is bonded to, did not differ much along the inner edges of the attachment in all load cases, see appendix B.1. Therefore, when comparing the existing window installation with the alternative installations along the inner edges, it is fair enough to just compare the strains of the carbon fibre sheets of each installation. This because the Sikaflex 265 glue is not causing any higher risk of separation of the attachment from the window glass and the hull as the strains of the Sikaflex 265 glue string is not significantly larger than the strains in the surfaces it is bonded to.
However, if considering the outer edges of the attachment the Sikaflex 265 glue is subjected to larger strains than the surfaces it connects to, see appendix B.1. So, when taking into account these edges, the strains of the Sikaflex 265 glue cannot be disregarded when comparing it with the existing window installation. For all load cases, the strains of the Sikfalex 265 glue in the outer edges of the attachment in the alternative solutions were larger compared to the same area on the existing window installation. Thereby, this proves that the glue is not as efficient when connecting two carbon fibre components (the hull skin and the carbon fibre sheet of the attachment) as connecting the window glass and the carbon fibre sheet. This is reasonable as Sikaflex 265 is originally a glue made for glass. According to the results, the existing window installation has a lower risk of separation in the outer edges of the attachment if taking into account the Sikaflex 265 glue. But, most of the leaks and cracks have been observed 40
around the inner edges of the attachment and therefore should be prioritized when choosing the most suitable window installation. Thus, based on that aspect, the alternative window installations are better alternatives.
However, if the glue string that connects the attachment to the hull is detached, there is a risk of water penetrating under the attachment. The new attachment concept has a gap between the two glue strings. Thus, if water leaks through the glue in the outer edges of the attachment there is a possibility that water could reach all the way to the inner edge of the attachment. This could result in larger strains around the inner edges of the attachment, especially during cold temperatures when water expands. But the Sikaflex 265 glue is a softer material than carbon fibre which the attachment is made of. Therefore, the glue could withstand larger strains, but as the strains in the glue string attaching the attachment to the hull is subjected to much larger strains compared to the adjacent contact areas there could be a risk of separation. 41
7 Conclusion
The objective of this study was to find the window installation that minimizes the risk of leaks and cracks in the attachment i.e the strains of the attachment. Summing up the results, the alternative solution without the double damping mass had least strains around the inner edges of attachment for the slamming load case. For the hogging and the torsion load case the alternative solution with double damping mass had least strains around the inner edges of the attachment. But even the alternative solution without the double damping mass was able to reduce the strains around the inner edges compared to the existing window installation.
In conclusion, the alternative solution without an additional damping mass is in overall minimizing the strains along the inner edges of the attachment in which most of the leaks and cracks have been observed. It has especially been efficient in reducing strains in the slamming load case. Even though the window installation with an additional damping mass best withstands hogging and torsion, the slamming load case is the most common phenomenon. Therefore, the window installation that best withstands the slamming load case should be prioritized. Thus, the alternative window installation without the additional damping mass is the best alternative because it best withstands slamming but it also reduces the strains along the inner edges compared to the existing window installation in the other load cases. Furthermore, as mentioned in the discussion, the Sikaflex 265 glue string along the outer edges of the attachment in the alternative window installations were subjected to large strains. Therefore, a recommendation when performing measures around the window, is to include the outer edges in the measurements. This in order to find if there are any higher risk of leaks compared to the existing window installation that could be important to be aware of for future applications. 42
8 Future Work
The results of this study are based on approximated load cases with arbitrarily chosen magnitudes of loads. A future work could be to reconstruct a more detailed load case based on real loads measured around the middle window. By measuring the loads with strain gauges both the magnitude and the direction of the loads can be obtained. This would hopefully generate more accurate results that better represents the reality.
Another aspect that could be interesting is to look at the performance of the alternative window instal- lations with the initial attachment concept. Thereby, a better understanding on how the new internal components consisting of the additional carbon frame, Sikaflex 265 seal and the double damping mass affects the results. Also, it could be interesting to see how the results changes when replacing the attach- ment with the new attachment on the existing window installation. These actions would acquire help to acquire a better knowledge of what contribution every component has to the overall structure. Thus, it is possible to draw a conclusion on what components that should be prioritized. 43
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Appendix A
A.1 Dimensions of the 3D-model of the hull
Figure 42: Dimensions of the 3D-model of the side structure hull based on the real dimensions (according to blue prints provided by SAAB Kockums) of the enclosed area in figure 19. B-1
Appendix B
B.1 Sikaflex 265 glue strings in the attachment of the alternative win- dow installations subjected to different load cases
(a) Sikaflex 265 glue string connecting the carbon fibre sheet and the hull on the alternative window installation without double damping mass under hogging load case.
(b) Sikaflex 265 glue string connecting the carbon fibre sheet and the window glass on the alternative window installation without double damping mass under hogging load case.
Figure 43 B-2
(a) Sikaflex 265 glue string connecting the carbon fibre sheet and the hull on the alternative window installation without double damping mass under slamming load case.
(b) Sikaflex 265 glue string connecting the carbon fibre sheet and the window glass on the alternative window installation without double damping mass under slamming load case.
Figure 44 B-3
(a) Sikaflex 265 glue string connecting the carbon fibre sheet and the hull on the alternative window installation without double damping mass under torsion load case.
(b) Sikaflex 265 glue string connecting the carbon fibre sheet and the window glass on the alternative window installation without double damping mass under torsion load case.
Figure 45 B-4
(a) Sikaflex 265 glue string connecting the carbon fibre sheet and the hull on the alternative window installation with double damping mass under hogging load case.
(b) Sikaflex 265 glue string connecting the carbon fibre sheet and the window glass on the alternative window installation with double damping mass under hogging load case.
Figure 46 B-5
(a) Sikaflex 265 glue string connecting the carbon fibre sheet and the hull on the alternative window installation with double damping mass under slamming load case.
(b) Sikaflex 265 glue string connecting the carbon fibre sheet and the window glass on the alternative window installation with double damping mass under slamming load case.
Figure 47 B-6
(a) Sikaflex 265 glue string connecting the carbon fibre sheet and the hull on the alternative window installation with double damping mass under torsion load case.
(b) Sikaflex 265 glue string connecting the carbon fibre sheet and the window glass on the alternative window installation without double damping mass under torsion load case.
Figure 48 TRITA -SCI-GRU 2020:230
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