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.