Sisäjännitteitä

Sisäjännitteitä, known in English as internal stresses or residual stresses, are mechanical stresses that exist within a material even in the absence of external forces or loads. These stresses arise during the material's manufacturing process or through subsequent treatments. They are essentially locked-in forces within the material's structure.

Common sources of internal stresses include plastic deformation, rapid cooling (quenching), welding, casting, and shot peening. For instance, during quenching, different parts of a material cool at different rates, leading to differential expansion and contraction, which in turn creates internal stresses. In welding, the localized heating and cooling cycle around the weld joint introduces significant residual stresses.

The presence and distribution of internal stresses can have a profound impact on a material's mechanical properties. Tensile residual stresses can decrease a material's fatigue life and increase its susceptibility to stress corrosion cracking. Conversely, compressive residual stresses can enhance fatigue resistance and reduce the likelihood of cracking.

Understanding and controlling internal stresses is crucial in many engineering applications, particularly in components subjected to cyclic loading or corrosive environments. Techniques like stress relief annealing, where a material is heated to a specific temperature and then cooled slowly, can be employed to reduce or eliminate undesirable residual stresses. Non-destructive testing methods, such as X-ray diffraction or ultrasonic testing, are used to measure and map these internal stress distributions.