Issue 30

P. Lopez-Crespo et alii, Frattura ed Integrità Strutturale, 30 (2014) 244-251; DOI: 10.3221/IGF-ESIS.30.31 244 Focussed on: Fracture and Structural Integrity related Issues Study of short cracks under biaxial fatigue P. Lopez-Crespo, B. Moreno, A. Garcia-Gonzalez, J. Zapatero Department of Civil and Materials Engineering, University of Malaga, C/Dr Ortiz Ramos s/n, 29071 Malaga, Spain plopezcrespo@uma.es A. Lopez-Moreno Department of Materials Science and Metallurgy Engineering, University of Jaen, Campus Las Lagunillas 23071 Jaen, Spain A BSTRACT . In this paper a methodology for evaluating crack initiation under biaxial conditions is presented. The methodology consists of evaluating the crack length automatically with digital processing of high- magnification images of the crack. The methodology was applied to study five different strain conditions on a low carbon ferritic-pearlitic steel specimen with tubular shape. A hole of 150 μm diameter was drilled to enforce the crack to initiate at a particular spot. Different combinations of axial and torsional strains were analysed during the initiation stage of the crack. The setup employed allowed detection of the crack to within 6 μm from the edge of the hole on average and monitoring of the crack during early stages. Fatigue crack propagation curves clearly showed oscillations due to microstructure. It was also observed that these oscillations decreased as the torsional component of the strain was increased. K EYWORDS . Biaxial fatigue; Proportional loading; Fatigue crack growth; Crack initiation I NTRODUCTION nderstanding the behaviour of engineering components under biaxial load is crucial for a number of industries, including aerospace, automobile and power generation industries. Different models have been proposed in an attempt to characterise the behaviour under such conditions [1]. Stress-based models can be used to predict the fatigue life if the plastic strains are small [2]. Strain-based models are typically employed in the low-cycle fatigue regime where significant plasticity may occur [3]. Within strain-based models, critical plane approaches aim at predicting not only the fatigue life but also the crack direction of the crack [4, 5]. Energy models are based on evaluating the energy accumulated per cycle on the material, and use it as a damage parameter [6, 7]. Depending on the loading regime, an important portion of the life can be consumed in the initiation stage of the crack. Initiation stage includes crack nucleation and micro-crack growth up to a length of around 1 mm. During the initiation stage, the aforementioned models for predicting crack propagation under biaxial loads no longer can be applied, because they are based on continuum mechanics. The material microstructure and the surface morphology are critical to the crack growth during the initiation stage [8]. During the initiation, fatigue crack growth is different to that predicted by the above mentioned models, and oscillations are commonly observed on the growth curves. These are caused by grain and phase boundaries and other micro-structural features that continuum mechanics models do not take into account [9]. The current work aims at studying the fatigue crack growth behaviour of small cracks under different biaxial conditions. U