Some comparisons in the behaviour of materials at elevated temperatures under uniaxial and under multiaxial stress

Abstract

Most data on the mechanical behaviour of materials at elevated temperature concerns the influence of a uniaxial stress. For many purposes such information may suffice but there is increasing awareness of its inadequacy when used in the design or service life estimation of engineering components operating under more complex stress systems. Such stress systems do not only arise from the external stresses that are applied but may also result from thermal effects, particularly at interfaces and joints, or from the special geometrical features of a component.

Some experimental techniques to provide information on creep behaviour under multiaxial stresses are described together with a discussion and evaluation of the results obtained. It is noted that data from uniaxial stress tests can be used to predict such behaviour when the material is isotropic and is not subject to volume changes, microstructural instability or creep damage. Frequently, materials do not fulfill these conditions and information is presented on the influence of some of these complicating features both on creep rate and on fracture.