Test methods for the determination of the stress–strain behaviour of oriented polypropylene (OPP) tubes in uniaxial compression

In order to assess the performance of thin-walled oriented thermoplastic tubes under large strain biaxial forming conditions at various temperatures and strain rates, an understanding of the tensile and compressive behaviour of the tube along its principal directions is required. Both the tensile and compressive behaviour of these materials is required since thermoplastics are known to exhibit differences in their behaviour in tension compared with compression. Suitable test methods for obtaining the compressive stress–strain curve up to large strains, and at various temperatures and strain rates, for thin-walled tubular materials have not been developed to date. This study presents methods for obtaining the axial compressive properties of thin-walled tubes from compression tests on ring samples, whose height is slightly less than the wall thickness of the tube. The choice of specimen height was critical in avoiding buckling of the sample during compression, and enabling the generation of stress–strain curves up to large strains. Compressive properties in the through-thickness direction are obtained from tests on miniature square cross-sectional specimens machined from the wall of the tube.