Creep of structures subjected to cyclic loading.

The development over the last decade of the reference stress method for estimating the deformation of structures composed of time dependent Maxwell material is reviewed, together with the implications of recently derived energy theorems based on idealized material models. Experiments are described which confirm predictions implicit in two energy theorems which extend the concepts of a plastic limit load and a plastic shakedown state to situations involving time hardening creep. The influence of constitutive relationships on stress redistribution effects which in turn affect the deformation of structures subjected to both constant and cyclic histories of loading are considered, and it is argued that the two energy theorems derived for time hardening materials provide conservative bounds which permit the designer to estimate deformation of structures composed of a wider class of materials with related constitutive relationships. An empirical method is proposed for estimating structural creep deformation due to cyclic loading. The method applies to structures composed of materials whose creep law for constant uniaxial stress is known, but knowledge of the form of the creep law for time varying stress is not required, as use is made of data obtained from a single cyclic creep test and results are obtained from a weighted time hardening calculation. In order to check the proposed procedure calculations were performed for a two-bar structure in which stress redistribution effects were particularly severe. At worst the errors in the predicted deformation rate corresponded to a 2% error in the applied load. The results also suggest that in most practical situations the actual solution is likely to correspond to an optimal upper bound provided by one of the energy theorems. The method also permits this optimal bound to be applied to structures composed of a wider class of materials with related constitutive relationships.