Stronger. Stronger,Lighter. Lighter, Smarter Smarter. The Sandwich Concept
Elass Meeting – Pornichet -June 2017
Valerio Corniani C.Eng MRINA – Diab Global Marine Manager From Panel to Beam Stress distribution in a homogenous cross section
Bending stress distribution Shear Stress Distribution
Centre of bending
-200 -150 -100 -50 0 50 100 150 200 0 0,1 0,2 0,3 0,4 0,5 0,6
01/18/16 Presentation name 3 Structural Sandwich Young Modulus of skin material is MUCH HIGHER than modulus of core Ef >> Ec
250
200
150
100 Young Modulus(GPa) 50
30-200 MPa 0 Steel Aluminium GRP Foam core
01/18/16 Presentation name 5 Bending of a sandwich
6/29/2018 6 Bending of a sandwich
Top skin subjected to compression and bottom skin subjected to tension whereas the core is subjected to shear
6/29/2018 7 Sandwich – Flexural Rigidity
3 2 3 2 Ef tf d Ectc D = E z dz = 2 + 2Ef tf + 12 2 12
Ef t f
d Ec t z c t Ef f
Stiffness contribution from the two skins bending around their individual neutral axis
6/29/2018 8 Sandwich – Flexural Rigidity
3 2 3 2 Ef tf d Ectc D = E z dz = 2 + 2Ef tf + 12 2 12
Ef t f
d Ec t z c t Ef f
Stiffness contribution from the two skins being removed from the centre of bending – Steiners theorem
6/29/2018 9 Sandwich – Flexural Rigidity
3 2 3 2 Ef tf d Ectc D = E z dz = 2 + 2Ef tf + 12 2 12
Ef t f
d Ec t z c t Ef f
Stiffness contribution from the core material bending around its own neutral axis
6/29/2018 10 Sandwich – Flexural Rigidity - Example
3 2 3 2 2 Ef tf d Ectc Ef tf d D = E z dz = 2 + 2Ef tf + D = 12 2 12 2 14.5750.4 % ++ 98.53562.3 % + + 1.1 39.0 % = 3615.9 Nmm2/mm (width)
2 1800 g/m 0/90 biaxial glass Ef = 17.8 GPa 20 mm H60 tf = 1.70 mm 2 1800 g/m 0/90 biaxial glass Ec = 58.5 MPa
Assume that the skins are very thin in relation to the core thickness and that the core material is weak/soft in relation the the skins.
6/29/2018 11 Sandwich – 3 ways to increase flexural rigidity
E t d 2 D = f f 2 1. Increase the stiffness of the skins, change from glass to carbon fibres / aluminium or steel 2. Increase the thickness of the skins 3. Increase the core thickness
• Increasing the core thickness is by far the most weight efficient way to increase rigidity of a sandwich! 6/29/2018 12 Bending of a sandwich – strain distribution
Hooke’s Law: i = Ei i
But … Ef >> Ec
Sandwich6/29/2018 Theory 13 Bending of a sandwich – stress distribution
d
No approximations Ec << Ef Ec << Ef and tc >> tf
Mx y Mx f = Ef D tf d
6/29/2018 14 Bending of a sandwich – shear distribution
No approximations Ec << Ef Ec << Ef and tc >> tf
Tx B(z) Tx tc tc d i (z) = , c (z = 0) = Ec + Ef tf D D 2 4 2 Ec << Ef T x c d 6/29/2018 15 The Sandwich Concept
37
• Weight 7 9.2 • Strength 3.5
• Stiffness 1 1.06 1.1 1 1
t 2t 4t Design of Sandwich Structures
Force
Compression
Shear
Tension Reaction Reaction Tensile Failure
Sandwich Concept 18 Compression Failure
Sandwich Concept 19 Contoured Foam Core Shear
Sandwich Concept 20 Skin Buckling
Sandwich Concept 21 Main properties of Sandwich Force
Compression
Shear
Tension Reaction Reaction
Skins: Core: - Tensile and compressive strength - Shear strength - Stiffness of the skin in compression - Compressive stiffness - Shear stiffness - Compressive strength QUESTIONS ?