Lightweight Structures and Novel Steel Alloys Resistant to Impulsive LoadingLapo Mori, Ph.D., Northwestern University, 2008.
Major Professor: Dr. Horacio D. Espinosa.
Blast protection can be achieved with several different approaches. One approach consists in using sandwich panels with thin faces and low density hard cores. These structures offer significant impulse mitigation possibilities provided that panel fracture is avoided. Here steel square honeycomb and pyramidal truss core sandwich panels made of stainless steel were tested under impulsive loads simulating underwater blasts. Fluid-structure interaction experiments were performed to demonstrate the benefits of sandwich structures with respect to solid plates of equal weight per unit area, identify failure modes, and assess the accuracy of finite element models for simulating the dynamic structural response. These two hard core sandwich structures showed a 30% reduction in the maximum panel deflection compared with a monolithic plate of identical mass per unit area. The failure modes consisted of core crushing, node imprinting, tearing and stretching of the front face sheet for the pyramidal truss core panels.
Theoretical assessment of sandwich panel structures with soft cores are predicted to exhibit significantly lower impulse transmission and reduced deflections compared to monolithic panels and hard core sandwich structures of equivalent specific mass. We have experimentally investigated the impulse response of stainless steel sandwich panels with a soft I-core topology. The study affirms that usage of soft cores is beneficial and that performance enhancements, measured by the reduction in panel deflection of the monolithic plate, can be as high as 68%. The experiments also indicate that the performance improvement is impulse dependent with the best performance achieved at lower impulses. This appears to be a consequence of core densification and node failure under high impulse loading conditions. The performance of sandwich panels is highly dependent on the manufacturing accuracy. Initial imperfections can lead to premature failure and loss of structural integrity.
A relevant alternative consists in using solid plates made of higher strength materials optimized for equibiaxial stretching and shear. This study presents a novel TRIP steel that shows significant improvement in both uniform ductility and shear localization resistance. Achieving performances similar to the sandwich panels with a solid plate made of a TRIP steel has also advantages in terms of cost and manufacturing.