At the Vicenza Crack Paths conference in 2009, Professor M Neil James (University of Plymouth and Co-Editor of the International Journal of Fatigue, published by Elsevier) and Professor John R Yates (at the time, at the University of Sheffield and one of the Co-Editors of Fatigue & Fracture of Engineering Materials & Structures, published by Wiley-Blackwell) discussed their observations that as journal editors an increasing number of authors and research groups, particularly in Europe, were working on the topic of characterization of crack tip stresses using more than one fracture mechanics parameter. They decided to try to organize an international conference on this topic, with the assistance of the Italian Group on Fracture and, in particular, the sterling efforts of Professor Luca Susmel who, at that time, was at the University of Ferrara in Italy. The 1st International Conference on Characterisation of Crack Tip Stress Fields was duly organized and held in the skiing resort of Forni di Sopra in the Italian Dolomites in January 2011 and attracted some 50 senior researchers working in this research area from across Europe and the Americas.
The conference was intended to promote discussion and the free exchange of ideas in the characterization of crack tip stress fields using fracture mechanics parameters and, in particular, to consider the issue of multiparameter characterization. It was focussed around invited keynote lectures and seminar-type discussions with rapporteurs capturing key points for plenary sessions. The conference was deemed by delegates successful enough to initiate the organization of a second conference due to take place in Malaga, Spain in early-March 2013. It was also felt that the topic was sufficiently significant to arrange a conference Special Issue jointly shared between the International Journal of Fatigue and Fatigue & Fracture of Engineering Materials & Structures. Essentially, the Italian Group on Fracture will host a website giving access to the joint-Special Issue and accessing papers submitted to both the journals. This will provide a unique repository for papers from the conference and ensure that aspects of fatigue and fracture are covered to the greatest extent. The organizers and Guest Editors extend their thanks to Elsevier and to Wiley-Blackwell, as well as to the Italian Group on Fracture for making this possible. A short history of the Italian Group on Fracture is included in the Special Issue.
Single parameter characterization of the crack/notch tip field using fracture mechanics parameters like K, J or crack tip opening displacement (CTOD) has been extremely powerful in advancing predictive technologies for critical or sub-critical crack growth. It has also become clear over the last 40 years that single parameter approaches have limitations particularly in dealing with crack growth phenomena arising from crack tip shielding, often resulting from the plastic enclave surrounding a crack. Influences of this enclave on the crack tip stress field ahead of the crack are maximized during cyclic loading. In the case of a parameter like the stress intensity factor, K, which characterizes the crack tip field via an elastic approximation, it is not surprising that any set of plasticity-induced circumstances which perturb the size of the plastic enclave and its associated strain field lead to predictive difficulties. Over the last 30 years, notable areas of activity related to such difficulties include short cracks, plasticity-induced closure, variable amplitude, multiaxial loading and notch effects.
Thus attention has been directed towards incorporating the T-stress into life prediction methods. The T-stress is the second term in a Williams-type expansion of the crack tip stresses and it affects the extent and shape of crack tip plasticity. It would therefore be expected to be influential in plasticity-related crack growth phenomena and a number of publications have demonstrated this to be true. The situation is further complicated where a crack experiences multiaxial loading and Modes II and III fracture mechanics parameters are also necessary. Alongside this, other groups have focused attention on incorporating additional elastic fracture mechanics parameters into crack/notch tip characterisation, which describe the effects of an Eshelby-type ‘plastic inclusion’ on an elastic stress field.
The Guest Editors hope that this fully peer reviewed overview of current thinking around crack tip characterization in relation to fatigue and fracture is of use to the research community.