Väsyttämislujuus

Väsyttämislujuus, also known as fatigue strength or endurance limit, is a material property that describes the stress level below which a material can endure an infinite number of load cycles without fatigue failure. Fatigue is a phenomenon where a material weakens and eventually fails under repeated or fluctuating stresses, even if those stresses are well below the material's ultimate tensile strength.

The concept of väsyttämislujuus is particularly important in engineering applications where components are subjected to cyclic loading, such as in aircraft, bridges, engines, and machinery. If a stress applied to a material is consistently below its väsyttämislujuus, the material is theoretically expected to last indefinitely. However, if the stress exceeds this limit, the material will eventually develop cracks, which grow with each load cycle until the component fractures.

Determining the väsyttämislujuus of a material typically involves fatigue testing, where specimens are subjected to varying stress levels and the number of cycles to failure is recorded. A fatigue curve, also known as an S-N curve (Stress-Number of cycles), is generated from these test results. For some materials, like many steels, the fatigue curve becomes horizontal at a certain stress level, indicating the endurance limit or väsyttämislujuus. For other materials, such as aluminum alloys, the fatigue curve continues to decrease without reaching a distinct horizontal asymptote, meaning there isn't a true fatigue limit, but rather a very high fatigue strength at a large number of cycles.