Fatigue is the process of gradual, restricted and permanent structural change in a material subjected to cyclic loading. Cyclic loading may lead to crack formation, multiplication of cracks and eventually fracture. Fatigue tests are most commonly conducted on smooth bar products or products with a pre-existing crack that grows as the test progresses.
High-cycle fatigue (HCF) tests are typically conducted on smooth bar products in force control. These tests determine the number of cycles to fracture for each product, and the data is compiled. Nominal stress levels in HCF tests are generally low significantly below the material's yield strength. As a result, product lives may last hundreds to millions of cycles, creating a need to conduct HCF tests at high frequencies in order to minimize testing time.