ABSTRACT Generally, a polymer only experienced one yielding when deformed. However, some semicrystalline polymers, mainly including polyethylene-like polymers and their blends, exhibit double yielding phenomenon where two stress maxima appear in their stress-strain curves. Recently, this unusual yielding phenomenon was observed in many other polymers, such as random copolymers, and structurally different polymer blends and composites. Some mechanisms based on crystal deformation and evolution during yielding, mainly for polyethylenes, were proposed to explain double yielding, including melting and recrystallization mechanism, shearing and slipping of crystal blocks mechanism, and reorientation and destruction of lamellae mechanism. Additionally, some other mechanisms for the polymers with unique phase morphology were suggested closely in relation to their morphology, such as matrix-dispersed phase separately yielding and yielding of one component in different locations. The present paper thoroughly reviewed the double yielding behaviors in various polymers, and their possible mechanisms. This will give a clear and overall picture to the relationship between yielding behavior and the corresponding microstructure changes in semicrystalline polymers as well as some other polymer maerials mentioned in this paper.
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