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 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.