Universal Strain‐Life Curve Exponents for Thermoplastics and Elastomers under Tension‐Tension and Torsion

Abstract

The fatigue behavior of 28 amorphous and semi‐crystalline thermoplastic polymers and elastomers is tested under strain controlled sinusoidal tension‐tension (TT) and torsion (T) at room temperature and analyzed via strain‐life (total strain amplitude versus fatigue lifetime) and crack propagation rate versus total strain amplitude curves, analogous to Paris’ law. Investigating fatigue is extremely time‐ and resources consuming, so universal relationships between the exponents of the strain‐life power‐law and material properties are of high importance. For a brittle failure mechanism (I), the strain‐life curves are found to have fixed exponents of BTT,I = −0.27 and BT,I = −0.22, respectively, while the crack propagation versus strain amplitude in TT has an exponent of mda/dN,I = 4. For ductile failure (II), fixed strain‐life curve exponents in TT of BTT,II = −0.35 and in torsion of BT,II = −0.48 with mda/dN,II = 2.8 are obtained. In torsion, most semi‐crystalline polymers show brittle and ductile failure depending on the applied strain amplitude, so the strain‐life curve exponent changes accordingly. The universal exponents for strain‐life and crack growth‐strain amplitude curves offer a significant simplification to rapidly estimate, predict, and simulate the fatigue behavior of polymers.

Mechanical fatigue of recycled and virgin high‐/low‐density polyethylene

“…Strain‐number of cycles to failure ( S–N ) curves of virgin and recycled polymers: (a) HDPE and (b) LDPE. The data for the virgin PE are taken from Hirschberg et al 51 [Color figure can be viewed at wileyonlinelibrary.com]…”