Quasi-static and high strain rates compressive response of iron and Invar matrix syntactic foams

The present work is focused on developing iron and FeNi36 Invar matrix syntactic foams and studying their properties under quasi-static and high strain rate compression. The quasi-static compression is conducted at a strain rate of 10^-3s^-1. High strain rate testing is performed using a split-Hopkinson pressure bar (SHPB) at strain rates up to 2500s^-1. One of the limitations of the SHPB method is that it does not provide useful results at the intermediate strain rates where the specimens do not fail completely during the test. In the present study, a recently developed repeated testing scheme is applied to obtain results at such intermediate strain rates. Syntactic foams containing 5 and 10wt.% hollow glass microballoons (GMBs) are synthesized using the metal powder injection molding (MIM) process for this study. The results show that the yield strength decreases with increasing GMB content. The quasi-static yield strengths of 5 and 10wt.% GMB syntactic iron foams were found to be 14% and 17% lower than that of iron. Similarly, 5 and 10wt.% GMB syntactic Invar foams had 35% and 51% lower yield strength than Invar alloy. However, weight-related strength was found to increase with GMB content and exceed the respective data of other iron and steel foams. High strain rate testing revealed a weak trend towards yield strength increase with strain rate. However, the indication of strain rate sensitivity is not very strong due to scatter in data. Metallographic analyses conducted on failed specimens showed that the extent of matrix plastic deformation decreased with increasing strain rate. Particle crushing was observed at all strain rates.