Volume 22, Issue 2 (March 1994)
Development of an Improved Compression Specimen Geometry for Unreinforced Polymers
One of the recommended compression specimen geometries for plastics in ASTM D 695, Test Method for Compressive Properties of Rigid Plastics, is a straight-sided cylinder or prism. This end-loaded specimen tends to barrel at high strains. Additionally, if modulus is desired, the standard suggests that the specimen have extra length to avoid end constraints. Therefore, compression specimens for modulus can fail at a different stress level than compression specimens for strength. To overcome these problems, a compression specimen with a gage section of reduced area which is end-loaded but side supported was developed. Additional experimental observations of the thermosetting polymer of interest included a nonlinear compressive stress-compressive strain response which required a parabolic constitutive relationship to model, an observation of a significant anelastic material response, and a material response termed anelastic buckling. These observations of material behavior had not previously been observed.