Issue 33

J.M. Vasco-Olmo et alii, Frattura ed Integrità Strutturale, 33 (2015) 191-198; DOI: 10.3221/IGF-ESIS.33.24 191 Focussed on characterization of crack tip fields Experimental evaluation of plasticity-induced crack shielding from crack tip displacements fields J.M. Vasco-Olmo, F.A. Díaz University of Jaén, Departamento de Ingeniería Mecánica y Minera, Campus Las Lagunillas, Edificio A3, 23071, Jaén, Spain jvasco@ujaen.es , fdiaz@ujaen.es A BSTRACT . In this work it is proposed a methodology for the evaluation of plasticity-induced crack shielding from the analysis of the crack tip displacements fields measured by digital image correlation. This methodology is based on the evaluation of the stress intensity factors determined from the displacements fields measured at the vicinity of the tip of a growing fatigue crack. For the characterisation of the crack tip displacements field, CJP model has been implemented. This model considers the shielding effects due to plasticity generated during fatigue crack growth. For the purpose of the current work, several fatigue experiments at different R -ratios have been conducted on Al2024-T3 compact tension specimens. In addition, compliance based methods have been adopted to perform a comparison of the results with those obtained by DIC. Results show a good level of agreement, illustrating the enormous potential of DIC technique for the study of fracture mechanics problems. K EYWORDS . Plasticity-induced crack shielding; Fatigue; Stress intensity factor; Digital image correlation. I NTRODUCTION atigue crack growth generates a plastic zone at the crack tip and along the crack flanks, which may induce plasticity-induced crack closure [1]. This phenomenon is one of the crack tip shielding mechanisms that reduces the stress intensity factor range originated at the crack tip. However, there are still some issues that remain unresolved or misunderstood regarding crack closure, which are mainly associated with problems in its measurement and interpretation [2]. Recently, the use of digital image correlation (DIC) has become very popular for the analysis of structural integrity problems. Thus, different methodologies based on the calculation of stress intensity factors (SIFs) from the displacement fields measured around a tip of growing cracks have been developed [3 – 6]. In particular, the method developed by James et al. (CJP model) [6] which takes into account the effect of plasticity generated around the crack tip during fatigue growth. Thus, it considers an analysis of the shielding effect over the surrounding elastic field. In the current work, it has been performed an experimental evaluation of plasticity-induced crack shielding from the displacements fields measured at the vicinity of the crack tip using DIC. Thus, CJP model [6] has been implemented for the characterisation of the crack tip displacements fields. Moreover, for the purpose of this work, fatigue tests at different R -ratios on Al2014-T3 CT (compact-tension) specimens have been conducted. Results have been compared with those obtained using the strain offset calculation, showing a good level of agreement which highlights the potential of DIC to evaluate the shielding effect during fatigue crack growth. F