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Structural Analysis/Optimization for Mass Properties Engineers

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Structural Analysis/Optimization for Mass Properties Engineers Structural Analysis/Optimization for Mass Properties Engineers Training Primer Course Objective: This class is a one-day session that covers structural design and analysis considerations and their impact on Mass Properties. This course is for working engineers outside of the Stress or Loads organization to provide insight into the decision tools and processes affecting structural integrity. Specific emphasis will be given to identify and explain the typical methods utilized by Structural Analyst and how these can sometimes adversely affect the optimization of the structure. Illustration of several accepted practices and their impact will be covered in the case studies. Course Outline: (8 hours) Morning (8am – 11:30am) Overview of course objectives Basic Airframe Configuration Issues Common Structural Design Features Understanding Design Drivers and the Mass Properties Impact Strength Designed Stiffness Designed Fatigue Designed Damage Tolerant Safe Life Afternoon (1:00pm – 5pm) Common practices Hidden Gotcha’s Case Studies / Examples Aero Commander Aloha Incident 767-300 Freighter Cargo Barrier Shear and Moment Optimization This Primer covers the following Topics Basic Strength and Material Review Metals properties Composites properties Failure Theories and Analysis Static Loading (Brittle & Ductile Materials) Alternating Stresses Cumulative Fatigue Loads Review Ground Air Ground Cycle for Aircraft Cycle Counting methods Copyrighted SAWE 2004 Page 1 of 18 Structural Analysis/Optimization for Mass Properties Engineers Training Primer Foreword This document is intended as a quick refresher for topics that the student is already familiar with either through their education or professional background. This is not intended to be a comprehensive treatise or textbook on structural analysis. The basic knowledge presented here is the foundation upon which the lecture material will expand upon. The intent is to re-familiarize the students with the underlying principals of Structural Analysis, Strength of Materials and Failure criteria development. The actual class will start with the presumption that the student is familiar with these concepts. That does not imply that they will be discussed directly during the class. The intent of providing this information is to give everyone common basis to understand the concepts discussed in the class. These will serve as the basis for explanations on how current structure may not be designed to these criterion and what that means with respect to structural sizing. Much of the in class discussion will address how to identify what criteria is being used to establish the structural requirements. Basic Strength and Material Review The fundamental basis of all structural analysis is an understanding the mechanical properties of the material being used. In fact, degree or level of understanding by the designer and analyst often determines how aggressive or conservative a design approach is taken. The old stress adage, “When in doubt, make it stout… out of things we know about.” still holds true to most structural engineers. This can lead to non-optimized designs or even worse, unsafe designs due to a lack of understanding of the mechanical properties and failure mechanisms of the materials being used. Today the basic materials used in aerospace or transportation are metals and composites. There are even more exotic product forms out there, CMC’s, nano structures, advanced crystalline forms etc. However the distinct differences in these two basic product forms will be sufficient to illustrate the importance of understanding the material properties and their influence on design. The Basics The engineering materials in use today are often characterized by several basic parameters in common usage. Some differences occur using composite laminates, or hybrid structures. The most common of these is the conventional Stress-Strain Diagram. Data collected from tensile or compressive tests can be used to determine the nominal stress in the part from: P σ = A0 Where A0 is the original cross-sectional area of the part and P is the applied load. Copyrighted SAWE 2004 Page 2 of 18 Structural Analysis/Optimization for Mass Properties Engineers Training Primer Nominal strain for a part can also be determined from strain gage reading taken during testing from the following: δ ε = L0 Where L0 is the original length of the part and δ is the change in length. The corresponding values for stress and strain can then be plotted as a graph, with stress on the ordinate and strain on the abscissa. This plot is known as a stress-strain diagram. This provides a quick means of obtaining data about a material’s strength properties independent of the actual part configuration. A typical stress strain curve is shown below. As can be seen, a significant portion of the stress-strain diagram shows a linear relationship between stress and strain. This is often referred to as the linear-elastic portion of the stress-strain curve. This fact was discovered in 1676 and became known as Hooke’s Law, after Robert Hooke. This can be expressed as: Copyrighted SAWE 2004 Page 3 of 18 Structural Analysis/Optimization for Mass Properties Engineers Training Primer σ = Eε Where E represents Young’s modulus for the material. This equation represents a very good approximation of the initial straight lined portion of the stress-strain diagram. In addition, Young’s modulus then represents the slope of this line. The modulus of elasticity or Young’s modulus is one of the most important mechanical properties for material characterization. However, E can only be used if a material has linear-elastic behavior over the entire loading spectrum. Metals properties Metals used in these applications are typically selected for a specific set of properties. Key properties, depending on application are: resistance to flow or creep at high temperature, Stiffness, Tensile and Compressive Strength, durability, toughness and resistance to fracture. Many of these properties can be found in the military handbook MIL-HDBK-5 or the FAA Metallic Material Properties Development and Standardization (MMPDS). Typical product forms include; sheet and plate, extrusions, forgings, and castings. For many of these materials, more than one set of design allowables are common. The type of allowable or its level of statistical certainty is known as its Basis. The general categories for allowables are A, B, or S, depending on how the allowables were established. ∞ A basis – The allowable value above which at least 99% of the population values will fall with a 95% confidence level. ∞ B basis– The allowable value above which at least 90% of the population values will fall with a 95% confidence level. ∞ S basis – The minimum guaranteed value from the governing material specification with no statistically defined confidence level. A basis allowables are required for single load path structures where the failure of that load path would result in the loss of structural integrity for a vehicle. B basis allowables are more commonly applied to redundant load path or “fail safe” structure. S basis allowables are very conservative and are typically applied only in applications of new material and/or product form, where the required testing is not yet complete and the perceived mechanical property benefits outweigh the use of more common materials. Most metals exhibit elastic behavior under relatively low loadings. The level of loading is material dependant. Elastic behavior means that the specimen returns to its original shape or length when the applied load is removed. This elastic behavior occurs when the strains in the specimen are with in the first portion of the previous chart. This portion of the curve is actually a straight line. In other words the stresses are proportional to the strains. The upper stress limit of this linear relationship is defined as the proportional limit, typically designated as σpl. As the stresses exceed the proportional limit, most materials will respond elastically, however, the return curve tends to bend and flatten out such that a greater amount of strain corresponds to an increment of stress. This behavior Copyrighted SAWE 2004 Page 4 of 18 Structural Analysis/Optimization for Mass Properties Engineers Training Primer continues until the stress reaches the elastic limit of the material. After this point on the curve, permanent deformation occurs. This increase in stress above the elastic limit will result in the breakdown of the material causing the permanent deformation. This is known as the yield point or stress for the material, σY. In contrast to elastic loading, any load that causes yielding of the material permanently changes the properties of that material. Some materials have two distinct yield points, an upper yield point that occurs first, followed by a sudden decrease in load carrying capability until the lower yield point is reached. If load equal to this lower load point is applied, the material will continue to deform without any increase in load. At this time a material is often referred to as being perfectly plastic. Strain hardening is the next portion of the diagram. After a material has finished yielding, additional load can be applied to the specimen. This results in the rising portion of the curve that becomes flatter until it reaches its apex, known as the ultimate stress, σu. This rise is referred to as strain hardening. This type of hardening can increase the apparent elastic limit of a material by trading off some of the materials ductility. The last part of the curve is correlated with the “necking” behavior of the part. After reaching the ultimate stress, the cross-sectional area of the part begins to decrease in a local area of the specimen. This is due to the slip planes formed with the material and the actual strains produced are due to the shear stresses. This results in a “neck” or reduction of cross-section in a local area as the specimen elongates. Since the cross-sectional area is decreasing, the load carrying capability of the structure continues to decline until the specimen fractures.
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