Issue 35

J. Albinmousa, Frattura ed Integrità Strutturale, 35 (2016) 182-186; DOI: 10.3221/IGF-ESIS.35.21 182 Focussed on Crack Paths Multiaxial fatigue crack path prediction using critical plane concept Jafar Albinmousa ( King Fahd University of Petroleum & Minerals, Dhahran 31261, Saudi Arabia A BSTRACT . Prediction of fatigue crack orientation can be an essential step for estimating fatigue crack path. Critical plane concept is widely used due to its physical basis that fatigue failure is associated with certain plane(s). However, recent investigations suggest that critical plane concept might need revision. In this paper, fatigue experiments that involve careful measurement of fatigue crack were reviewed. Predictions of fatigue crack orientation using critical plane concept were examined. Projected length and angle were used to characterize fatigue crack. Considering the entire fatigue life, this average representation suggests that it is more reasonable to assume the plane of maximum normal strain as the critical plane even though fundamentally the plane of maximum shear strain is more likely to be the critical one at early initiation stage. K EYWORDS . Multiaxial fatigue; Crack growth; Critical plane; Crack path. I NTRODUCTION here is still no unified definition of fatigue crack initiation. Yet, there is an agreement that there are three classes of fatigue crack growth: microscopic, small and macroscopic [1]. Still these classes are not exactly defined and there is an overlap between them. In general, cracks with lengths less than 10 2 μm are considered microscopic and their growth is governed by the microstructure texture. Based on the literature, such crack size marks the initiation size [2]. Cracks with lengths between 10 2 to 10 3 μm are considered small and such range represents what is so-called early growth stage [3, 4]. It is widely accepted that fatigue correlations are valid during initiation and early growth stages. After that, fracture mechanics is used to predict propagation life that is dominated by growth of macroscopic cracks. The concept of critical plane was developed based on the experimental observations that fatigue cracks initiate at specific planes [3, 5]. The current practice for evaluating critical plane models consists of three steps. Considering a maximum shear strain damage parameter, the first step is to search for critical plane by transforming hysteresis loops at different planes using plane stress-strain transformation relations. The critical plane is the plane at which shear strain is maximum. This plane may only correspond to the orientation of the inception or early initiation of crack, with length scale in the order of microns. In the second step the corresponding axial and shear stresses are obtained from the hysteresis loops on this critical plane. Finally, fatigue life is predicted by coupling the damage with a life equation. This procedure implies that crack will initiate at the plane of maximum shear strain and it will grow, within early growth stage, in the same orientation. Of course, this is not always the case as fatigue cracks may grow in a zigzag paths. Recently, Albinmousa and Jahed [6] examined the predictions of fatigue crack orientation on smooth specimens subjected to multiaxial loading using critical plane concept. They showed that reversed analysis for predicting fatigue life by assuming the critical plane as the experimentally observed plane showed that the predictions of strain-based models were mostly non-conservative. T