Indirect Tensile versus Two Point Bending Fatigue Testing

This paper describes a comparison between an indirect tensile fatigue method and the currently preferred European standard two-point bending, trapezoidal fatigue test. A study was undertaken using both tests to determine the fatigue properties of a standard UK asphalt mixture at two temperatures of 10 and 30 8C. The fatigue life, in addition to using the traditional 90% reduction in initial stiffness, was also computed using a fatigue failure point based on the transition between the quasi-stationary phase (associated with a uniform stiffness decrease and the development of micro-cracks) and the failure phase (associated with localised crack propagation). This fatigue failure point, determined by taking the peak of the product of loading cycles and stiffness versus loading cycles, was also converted into an alternative form used to determine the fatigue failure point for the indirect tensile fatigue test. Separate fatigue relationships were produced for the four combinations of test method and temperature based on the phenomenological initial tensile strain versus fatigue life relationship. Although the indirect tensile fatigue test produced shorter fatigue lives compared to the two-point bending fatigue test, it was also possible to combine all four fatigue relationships to produce one unique fatigue function for the 20mm dense bitumen macadam asphalt mixture.