False failure in flexural fatigue tests

Abstract

Flexural fatigue tests are typically run under displacement or in a straincontrolled mode. In these tests, either the oscillatory displacement amplitude or strain amplitude applied to the bottom of the specimen is kept constant. The evolution of loading required to cause fatigue is then measured. Load amplitude decreases with the number of cycles, and the specimen is considered to have failed when the load is half its initial value. This failure criterion may be erroneous when non-fragile fracture mixtures prepared with high bitumen contents or modified binders are tested. In these cases, mixtures exhibit a visco-plastic behaviour and increasingly less stress is necessary to cause strain without cracking. Mixtures are hardly deteriorated when the fatigue failure is determined, and may be subjected to a larger number of load repetitions. It is then recommended to control the evolution of loading more effectively and regard as valid only tests where load decreases sharply to very low levels, making sure that the three stages in the fatigue process have occurred.