Strain-controlled fatigue life and modeling of conduit polymers

Strain-controlled fatigue lives of conduit polymers, viz., nylon 6, polypropylene (PP) and calcium carbonate filled black colored polypropylene (PP-blk) were studied. Thermal and mechanical analyses were conducted before fatigue tests. Thermal characteristics, such as the degree of molecular degradation, glass transition temperatures, and melting points were determined. Tensile strength, elastic modulus, and Poisson's ratio were obtained from tests under quasi-static loading. Fatigue lives were measured under different displacement ranges and temperature conditions. Four different temperatures were selected to represent low (-40 degrees C), room (25 degrees C), and high (65 and 125 degrees C) temperature conditions. Hysteretic heating was found to be significantly operative in PP specimens. By optimizing the previously developed unified strain model [1], strain fatigue lives were predicted based on the studied materials.