Issue 41

D.-Q. Wang et alii, Frattura ed Integrità Strutturale, 41 (2017) 143-148; DOI: 10.3221/IGF-ESIS.41.20 143 Focused on Crack Tip Fields Crack tip strain evolution and crack closure during overload of a growing fatigue crack De-Qiang Wang, Ming-Liang Zhu, Fu-Zhen Xuan Key Laboratory of Pressure Systems and Safety, MOE; School of Mechanical and Power Engineering, East China University of Science & Technology, Shanghai 200237, China mlzhu@ecust.edu.cn Jie Tong Mechanical Behavior of Materials Laboratory, School of Engineering, University of Portsmouth, Portsmouth PO1 3DJ, UK A BSTRACT . It is generally accepted that fatigue crack growth is retarded after an overload, which has been explained either by plasticity-induced crack closure or near-tip residual stress. However, any interpretation of overload effect is insufficient if strain evolution in front of crack tip is not properly considered. The current understanding of overload-induced retardation lacks the clarification of the relationship between crack closure at crack wake and strain evolution at crack tip. In this work, a material with low work hardening coefficient was used to study the effect of overload on crack tip strain evolution and crack closure by in-situ SEM observation and digital image correlation technique. Crack opening displacement (COD) and crack tip strain were measured before and after the overload. It was observed that the evolution of crack tip strain follows the crack opening behaviour behind the crack tip, indicating a smaller influence of overload on micro-mechanical behaviour of fatigue crack growth. After the overload, plastic strain accumulation was responsible for crack growth. The strain at a certain distance to crack tip was mapped, and it was found that the crack tip plastic zone size correlated well with crack growth rate during post-overload fatigue crack propagation. K EYWORDS . In-situ SEM; Overload; Fatigue crack growth; Digital image correlation. Citation: Wang, D.-Q., Zhu, M.-L., Xuan, F.- Z., Tong, J., Crack Tip Strain Evolution and Crack Closure during Overload of a Growing Fatigue Crack, Frattura ed Integrità Strutturale, 41 (2017) 143-148. Received: 28.02.2017 Accepted: 15.04.2017 Published: 01.07.2017 Copyright: © 2017 This is an open access article under the terms of the CC-BY 4.0, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. I NTRODUCTION verloading behavior of a material has been widely investigated as variable amplitude loading is the load form that engineering structures and components often undergo. It is generally accepted that overloading of a fatigue crack often induces crack growth retardation and thus can influence fatigue life. To date, a variety of experimental and simulation work have focused on the influence of overload parameters, e.g. overload ratio, base stress intensity factor range, O