Correlations Between Indentation Hardness and Yield Stress in Thermoplastic Polymers

In mechanical testing empirical correlations are often used for estimating yield stress values from hardness values. Such empirical correlations are only valid within particular classes of materials and in the case of polymers they may be dependent on deformation rate, temperature and kind of deformation (tensile or compressive). For a better understanding of these complex correlations, instrumented indentation tests, tensile tests and compressive tests were performed on different semicrystalline and amorphous thermoplastic polymers. Indentation hardness and Martens hardness values were compared with tensile and compressive yield stress. The ratio of hardness values to tensile yield stress depends very strongly on the basic deformation mechanism (crazing, voiding or shear yielding) of the polymers. Therefore, under tensile load there is no general linear relationship between hardness and yield stress. In contrast, in the case of compressive loading hardness and yield stress are much better linearly correlated.