Chengkai Jiang

A new form of equivalent stress for combined axial–torsional loading considering the tension–compression asymmetry of polymeric materials

Although the von Mises criterion has been generally adopted to obtain the equivalent stress for materials under multiaxial loading, it does not consider the influence of the tension–compression asymmetry of pressure-sensitive materials. For polymeric materials, to include the non-negligible effect of the tension–compression asymmetry, this work proposes a new form of equivalent stress in which the tensile and compressive yield strengths are presented. This form is compared with the work of von Mises, Bai and Christensen with the support of experimental data. It was found that the proposed form is more suitable for polymeric materials under combined axial–torsional loading conditions. The implications of the present findings for designing stress-controlled combined axial–torsional loading experiments are also discussed.

Accelerated aging test of hydrogenated nitrile butadiene rubber using the time–temperature–strain superposition principle

The effect of physical aging on the long-term performance of hydrogenated nitrile butadiene rubber (HNBR) is very important to its engineering applications. In this paper, aging tests of HNBR have been conducted systematically at different temperatures and levels of uniaxial tensile strain. Fracture strain was chosen as the index of the severity of aging. Applied strain shows an equivalent effect on aging acceleration to that of temperature; the higher the temperature and strain level are, the faster the aging is. A WLF-type time–temperature–strain superposition principle (TTSSP) is proposed to study the long-term aging performance of HNBR with short-term test data. With the constructed master curves, time and cost can be significantly reduced for the evaluation of the long-term aging behavior of the rubber materials.