Numerical simulations of cracked round bar test: Effect of residual stresses and crack asymmetry

The Cracked Round Bar (abbreviated as CRB in the following) test is used as standardized accelerated testing method for simple ranking of polyethylene pipes based on measuring number of cycles to failure of the cyclically loaded CRB specimen. It can also be used for the slow crack growth rate evaluation of the tested material. This paper deals with the issue of asymmetrically growing cracks in CRB specimens and possible influence of the asymmetry on the results of CRB tests, both the number of cycles to failure and the crack growth rates. It is assumed that the asymmetry is caused by presence of residual stress in the CRB specimens. In the first part of the paper, residual stress in two polyethylene pipes (produced using different extrusion speed) is investigated. Then CRB specimens made from these pipes are also examined regarding the residual stress and the results are compared. The found residual stress distributions are used as input for the following finite element method (FEM) simulation of crack growth in the CRB specimens. The FEM model results are then evaluated and fatigue crack growth rates for asymmetrically growing cracks are calculated. They are compared to the results for symmetrically growing cracks and influence on the results and its significance is discussed. In the end, CRB specimen lifetime prediction is carried out and the influence of the asymmetry on the obtained number of cycles to the failure is also addressed.