This ppt describes the parameters that are important in the DO of sandwich structures for superior strength and stiffness

1. Design Optimization of
Aerospace Sandwich
Composites for Strength
and Stiffness
K. Padmanabhan
AR and DB Project No: 1650
SMBS, VIT University-Vellore.
May 2013

2. 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)

3. 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.
G. R. Froud, “Your Sandwich Order, Sir?”, Composites, July (1980) p 133
. L. J. Gibson, Materials Science and Engineering, 67 (1984) 125-135.

4. 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.

5. Finite Element Approaches

6. 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

7. Fabrication..

8. Testing and Failure Analyses
An Instron 8801
flexure set up.
ASTM D 790 M &
ASTM D 7250 M
Compressive face skin
failure & core crushing

9. FEA Results and Discussion
Shear Strain
Shear Stress

10. Results and Discussion
PUF Sandwich Composites
125 kg/cu. m density and 10 mm
thick rigid foam. G/E skin with
100 GSM and 280 GSM weave.

11. Results and Discussion
PUF sandwich Composites
125 kg/ cu. m density and
50 mm thick rigid PUF
foam . G/E Skin with 260
GSM weave.

12. Results and Discussion
PIR Sandwich Composites
125 kg/cu. m density and 10 mm
thick rigid foam. G/E skin with
100 GSM and 280 GSM weave.

13. ` Polyisocyanurate foams
are more fire resistant than
polyurethane foams. Otherwise
the mechanical properties of
the respective sandwich
composites are in the same
range for similar densities and
thicknesses.’
Ref: Lloyd insulations brochure on rigid foams, 2012.

14. 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).

15. 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.