Date post: | 12-Jan-2017 |
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Design Optimization of Aerospace Sandwich
Composites for Strength and Stiffness
K. PadmanabhanAR and DB Project No: 1650SMBS, VIT University-Vellore.
May 2013
Objectives• To design optimize aerospace
sandwich composites for maximum flexural strength and stiffness ( current)
• To design optimize aerospace sandwich composites for minimum weight and cost ( future)
Theoretical Background• GR Froud’s publication : Maximum
strength occurs when skin weight equals core weight and maximum stiffness occurs when skin weight equals half of core weight. (General )
• LJ Gibson’s publication: Maximum stiffness occurs when skin weight is one fourth of core weight ( Metal skin rigid polymeric foam core)
• Available literature is scanty in general on design optimization.
Ref: 1. G. R. Froud, “Your Sandwich Order, Sir?”, Composites, July (1980) p 133. 2. L. J. Gibson, Materials Science and Engineering, 67 (1984) 125-135.
Current Approach• Semi-empirical approach. • To experimentally determine the strength and
the stiffness optimization rules for sandwich composites fabricated out of glass and/or carbon fabric skins and cores made of rigid cellular solids with closed pores.
• Different core densities and thicknesses to be experimented.
• To refine the FEA models in accordance with the design optimization rules. For example employ layer effect in flexure to sandwich composites.
• Correlation of the shape factors with the observed stiffness and strength.
Finite Element Approaches
Experimental Approach • Hand Wet Lay Up and Hand Wet Lay Up
followed by Vacuum Bagging fabrication of sandwich composites.
• Conventional machining of test specimens
• Flexural testing in Instron 8801 and Structural UTM
• Failure analyses and feedback
Fabrication..
Testing and Failure Analyses
An Instron 8801flexure set up.ASTM D 790 M &ASTM D 7250 M
Compressive face skin failure & core crushing
FEA Results and Discussion
Shear Strain
Shear Stress
Results and Discussion PUF Sandwich Composites
125 kg/cu. m density and 10 mmthick rigid foam. G/E skin with100 GSM and 280 GSM weave.
Results and Discussion PUF sandwich Composites
125 kg/ cu. m density and 50 mm thick rigid PUF foam . G/E Skin with 260GSM weave.
Results and Discussion PIR Sandwich Composites
125 kg/cu. m density and 10 mmthick rigid foam. G/E skin with100 GSM and 280 GSM weave.
` Polyisocyanurate foamsare more fire resistant thanpolyurethane foams. Otherwisethe mechanical properties of the respective sandwich composites are in the samerange for similar densities andthicknesses.’ Ref: Lloyd insulations brochure on rigid foams, 2012.
Future Plan• To continue with different rigid foam densities and
thicknesses for simulation and experiments. • To conduct more simulation and experiments to find
thumb rules on strength and stiffness optimization in rigid foam core and fibre/matrix skin sandwich composites.
• To include resin bond tests between core and skin ( Shear and peel tests).
• To endeavour to achieve accomodative behaviour between core and skin for design optimization through choice of resin.
• To include layer effect in skin in design optimization• To include flexural parameters in design optimization
( like bending modulus and span to depth ratio).
Acknowledgement• AR and DB for the financial support and
advice.• VIT management for the equipment
purchase and support.• CAMPT DST-FIST facility and Structures
lab, VIT, for mechanical testing. • My project associate and students.