Material fatigue testing

Machine parts exposed to cyclical variable stresses can be damaged by tensions significantly lower than their tensile or yield strength determined by a tensile (static) test. In such a case, we speak of material fatigue, and the resulting damage is termed fatigue fracture. Studies show that 80 % of all failures to machines parts in operation are caused by material fatigue. Therefore, studying the workings of material fatigue and determining respective material characteristics are of major importance.

Nowadays, material fatigue characteristics are very well explored, and the information about fatigue behaviour in materials can be found in a number of survey publications and textbooks, collections from specialized conferences, as well as in a number of expert publications published by technical journals, some of which are focused exclusively on material fatigue.

The onset of research on material fatigue dates back to 1820’s. The tests were initially performed on specific mechanical parts, not on specimen, and were rather unsystematic.  The first person to perform a systematic tests to determine bend stress on train carriage axles was the German railway engineer August Wöhler in the second half of the 19th century (1852-1870). His trailblazing work boasts three achievements: he was the first person to use specimen for testing, he built a rotating bending test device, the principle of which is still used today, and he experimentally determined the dependence of the number of cycles to failure on the stress applied. This dependence, i.e. the so called fatigue curve, is called the Wöhler curve in Europe, or the S-N curve in the English-speaking world. This dependence, that is, the dependence of the number of cycles to failure (N) on stress amplitude (σ), has been the most commonly used fatigue characteristic.

Formerly, the Wöhler curve used to be determined for a high number of cycles in a region, which is now called the high-cycle region. This region represents a time-restricted fatigue limit, and a permanent strength, with the lower limit being approx. 100 000 cycles. This limit is an agreed one, and has no physical meaning.

Since the mid 20th century, research work also began to concentrate on the low-cycle region, which can be characterised by a region of 100 to 10 000 cycles, and by a quasi-static region, which includes a life cycle of up to about 100 cycles and where the tensile force throughout the load cycle is approaching the ultimate tensile strength.

Individual regions differ from each other according to the size of the plastic deformation amplitude. In the high-cycle region, the stress is almost elastic, and the plastic deformation amplitude at fatigue limit is in the order of 0,00001. By contrast, in the low-cycle region, the plastic deformation amplitude can reach up to the order of percent.

Fatigue is a process, in which structural and associated physical and mechanical properties of a material are changed as a result of oscillating or cyclical mechanical loading. The loading is always lower than the tensile strength, and, typically, also lower than the material’s yield strength. A material can withstand static load without any major damage, while under cyclic loading, the material accumulates damage, i.e. accumulates plastic deformation which at the final stage of the process manifests itself by the initiation and propagation of fatigue cracks. The process ends in fatigue fracture.

Material fatigue and its attributes constitute a complex scientific field which combines a number of research disciplines, exploring materials, mechanical properties, strength or simulations.