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Abstract
The strain rate, temperature, and microstructure-dependent, tensile-yielding behavior of three semi-crystalline polymers, namely high-density polyethylene (HDPE), polyamide 6 (PA6) and low-density polyethylene (LDPE), was investigated. It is found that, depending on the strain rate and temperature, the three polymers exhibit markedly different tensile deformation behavior, especially the shape of the stress-strain curves. LDPE exhibits a uniform extension and shows no obvious geometrically unstable effect, such as necking, during the overall tensile process. HDPE and PA6, on the other hand, show clear necking and cold-drawing phenomena during the uniaxial tensile process. When considering the effect of strain temperature on necking, significant differences between HDPE and PA6 emerge. For both, the heterogeneous necking disappears and homogeneous deformation occurs with increasing temperature. For HDPE, the homogeneous deformation takes place in the vicinity of the melting temperature, while for PA6, it takes place close to the glass transition temperature instead. The conventional yield point, corresponding to the force maximum in stress-strain curves, becomes less defined as the testing temperature is increased. It is applicable, to some extent, to combine the Brereton analysis and Considère construction to predict such a point quantitatively. However, this combination can only be suitable for homogeneously deformed material. In addition, it is found that the special, double yielding behavior will take place under certain deformation conditions for all three semi-crystalline polymers. With respect to judging the appearance of the double yielding of polymers, it seems that it can be estimated qualitatively by plotting the compression residual strain-applied strain curves of the samples.
Acknowledgements
The authors gratefully acknowledge the financial support of the National Natural Science Foundation of China (Grant Nos. 20734005, 50503014, and 50533050), the Doctoral Research Foundation granted by the National Ministry of Education, China (Grant No. 20060610029), and the Special Funds for Major Basic Research (Grant No. 2005CB623808). G. F. Shan also acknowledges the Chinese Government for awarding a scholarship under the State Scholarship Fund and the University of Sydney to pursue the study in Centre for Advanced Materials Technology (CAMT).