Determination of strain components in low-cycle multiaxial fatigue tests on tubes

Many thermal cycling tests are characterized by an equibiaxial stress distribution. To simulate this kind of loading under isothermal conditions at room temperature, a test facility for multiaxial isothermal tests on tubes has been developed. The tubes are subjected to a longitudinal tension compression load within a superimposed circumferential load using internal and external pressures. Owing to the gradient of stresses and strains over the wall thickness, the stress-strain distribution at the inner tube wall differs markedly from the values outside. Therefore they have to be calculated depending on the measured forces, pressures and the dimensions of the specimen. In the present paper, measurement and evaluation of the stress-strain distribution over the tube wall are described.