Specimen size effect on stress-strain response of foams under direct-impact

Stress-strain response of rigid closed-cell polymeric foam under direct impact loading conditions is investigated, focusing on the specimen size effects. Cylindrical specimens with two different length-to-diameter ratios are impacted at different projectile velocities. Stereovision high speed photography in conjunction with 3D digital image correlation is utilized to study the full-field deformation of specimens under impact loading. A non-parametric analysis is then conducted to extract the local stress-strain curves within specimens. The proposed analytical method takes into account the concurrent influences of inertia stresses and material compressibility. The inertia stress within the area of interest is evaluated using the full-field distributions of acceleration and density. The calculated inertia stress is then superimposed with the stress measured at the boundary to enable the determination of full-field stress distribution over the entire gauge area. The results obtained in this work confirm that the effects of inertia stresses become more pronounced as specimen length-to-diameter ratio increases; whereas the degree of strain and strain rate variability is also elevated.