Numerical and Experimental Analysis of Cyclic Strains in Coiled Tubing Surface Defects for Fatigue Life Assessment

Abstract

Abstract A defect in the surface of coiled tubing (CT) can have expensive ramifications. The defect may lead to failure in a few bending cycles and research is underway to study the influence of defects on CT fatigue strength. Existing semiempirical models are based on measured flaw dimensions and fatigue lives measured experimentally. Although the dimensions and overall flaw geometry are "known," an important feature of the flaw that has not been used is the radius of curvature at the notch root. The notch root radius is a measure of the "sharpness" of a flaw. Although empirical models assess the shape of a notch indirectly, they do not incorporate a direct measurement of the notch root radius. Since technology is emerging that is capable of measuring notch root radii, a method is needed to assess its influence on fatigue. To assess the effect of flaw geometry, Finite Element Analysis (FEA) can be used, with sophisticated kinematic hardening rules to model the cyclic strain behavior of the material at the notch root, and reveal its dynamic behavior. However, obtaining the experimental data necessary to validate the predictions is difficult. The measurement of cyclic strains at the roots of physically small defects in CT has never been attempted due to the small size of the flaws, and limited access to them in CT fatigue testing machines. Therefore, specialized defect geometry was developed for this study specifically to accommodate the placement of small strain gages in the notch root. Computationally intensive FEA modeling and strain gage measurements were conducted on defect-free tubing, to establish baseline tubing behavior, then repeated with the notch geometry. Strain gages had to be replaced every cycle of loading due to the large strain ranges. The tedious gage installation procedure resulted in repeatable measurements of notch root cyclic strains, and good agreement with FEA predictions was achieved. Some additional FEA results are presented with increasingly sharper notches. Estimates from conventional notch strain analysis techniques based on the elastic stress concentration factor and elastoplastic material properties were compared to the FEA results and a modified approach was identified that agreed well over the range of geometries examined in this study.