Many structures and components fail because of fatigue. Fatigue occurs when a material is subjected to repeated loading and unloading. If the loads are above a certain threshold, microscopic cracks will begin to form and grow. Eventually a crack will reach a critical size, the crack will propagate suddenly, and the structure will fracture.

Two types of **experimental** axial fatigue tests are available at OCAS: high cycle fatigue (HCF, fatigue behaviour above 100 000 cycles) and low cycle fatigue (LCF, fatigue behaviour below 100 000 cycles) tests. In addition to this, large test samples can be tested in resonant bending fatigue.

In HCF testing, a cyclic load is applied to the specimen until failure (load control). Different loading conditions exists, these are characterized by the ratio R.

Normal ratios are R = -1, R = 0.1 and R = 1. In high cyclic fatigue testing on sheet material, the most commonly used ratio is R = 0.1 (i.e. only tensile stresses). For each sample (each load), the number of cycles to failure is recorded and a fatigue curve is drawn up.

In LCF testing, a cyclic strain is applied to the specimen (strain control) until failure. The stresses involved are usually higher than for high cycle fatigue and the ratio R is normally -1 (i.e. tension – compression).

At OCAS we also use **finite element analysis** (FEM) to calculate the stresses at critical locations and estimate the fatigue life. FEM results can be validated by in-house exprimental fatigue tests.