FE analysis of strain constraint around the crack tip under cyclic loading

Abstract

PurposeThe purpose of this paper is to deal with the FE analysis of strain constraint around the crack tip under cyclic loading and its utilization using crack growth prediction strip yield model (SYM). During cycling, the constraint develops based on the load history. The monotonic loading is analyzed mostly, but during cyclic loading the conditions are different. The constraint is analyzed after several loading cycles applied in upwards part of the cycle and the formula for its development is proposed.Design/methodology/approachThe study is based on the 3D FE analysis of middle-cracked tension specimen M(T). The strain constraint is described by Newman’s factor α. The variability of constraint factor α was analyzed for several load levels and specimen thicknesses. The crack is considered as non-propagating with straight crack front. The material is modelled as elastic-perfectly plastic. The SYM is modified by implementing variable constraint and the experimental results are compared with the simulation.FindingsIn major part of the loading cycle, it was found by FE analysis, that the constraint factor αg is lower after overloads than when creating monotonic plastic deformation on the same load level. The value of αg is governed by the ratio of thickness B over the plastic zone size rp. By implementing the variable constraint factor into the SYM, the improvement of the predicted specimens lives under variable amplitude loading was shown.Originality/valueThe new phenomenon on the variability of strain constraint during cyclic loading is presented. The development of constraint factor αg during cyclic loading is different from the monotonic loading and should be accordingly implemented into prediction models.