Multiaxial Failure Stress Locus of a Polyamide Syntactic Foam at Low and High Strain Rates

The mechanical response of a polyamide syntactic foam under combined tension-torsion loading is measured experimentally at quasi-static (10−3 s−1) and high rates of strain (103 s−1). The dynamic experiments were conducted on a newly developed tension-torsion Hopkinson bar (TTHB) equipped with high-speed photography equipment. Quasi-static experiments were carried out using a universal screw-driven machine. The multiaxial high-rate experiments demonstrate the ability to achieve synchronization of longitudinal and shear stress waves upon loading of the specimen. The capacity to achieve force and torque equilibrium under combined loading on materials characterized by relatively low-stress wave propagation velocities is also demonstrated. Approximately constant strain rate conditions were attained. The failure envelope of the polyamide foam studied was analyzed over a wide range of stress states including pure torsion, shear-dominated combined tension-shear, tension-dominated combined tension-shear, and plain tension. Additional low rate and dynamic experiments in tension, compression, and torsion were conducted at higher than ambient and sub-ambient temperature conditions using bespoke temperature conditioning equipment, to assess the temperature dependence of the material. The multiaxial failure stress locus was constructed in the normal versus shear stress space as well as in the principal stress space from experiments conducted at low and high rates of strain. The failure stress locus of a polymer syntactic foam and its rate and temperature dependence are presented for the first time. The newly developed TTHB apparatus allows for the direct measurement of the failure stress locus of lightweight materials and therefore for the evaluation of existing failure criteria.