Methodology Used for Characterizing the Fracture and Fatigue Behavior of Thermoplastic Elastomers

Abstract

The fracture and fatigue behavior of thermoplastic elastomers (TPE) is of prime practical importance for demanding engineering applications. The fatigue behavior of several TPE grades used for superior industrial application was characterized under displacement-controlled cyclic loading conditions by local strain based Wöhler curves (LSWC) and by fatigue crack growth (FCG) curves. The LSWC method is based on the determination of the local strain in diabolo-shaped cylindrical or flat specimens and the identification of cycle number-to-failure, Nf, values in the Fmax/Fmin-Nf diagrams. While these LSWC curves were successfully created for injection molded TPEs investigated using cylindrical specimens, the extrusion grade TPU did not reveal fatigue failure using flat specimens with 0.2 mm thickness. The LSWC curves were used for both supporting material development efforts and dimensioning cyclically loaded components. Notched Plane Strain (PS) tensile specimens were tested under displacement-controlled loading conditions and the crack length was measured optically. Tearing energy values have been calculated for the PS specimens and subsequently FCG curves were generated in terms of crack growth rate, da/dN, and tearing energy, T. The majority of the TPE grades investigated revealed extensive crack tip blunting. While the slope of the stable FCG revealed rather minor differences, the apparent fatigue threshold values were observed in the range of appr. 1.5 decades. The endurance strain limit has been estimated (1) by direct measurements in LSWC tests (2) by the determination of threshold values in the FCG curves and calculation of critical strain values and (3) by using Essential Work of Fracture (EWF) approach.