Experimental investigation of crack paths.

The knowledge of crack path mechanism could improve the safety issues, design and finally reduce the cost of the maintenance or production of structures in aerospace and energy industries. However, the physical mechanism behind the crack path development is not still completely understood although many criteria have been developed to predict the crack path. It is even more challenging to predict the crack trajectory in areas such as multi site damage zones where there are interactions between cracks. A study has been undertaken on sets of cracks with different interaction properties, both numerically, using a finite element (FE) method, and experimentally, using Thermoelastic Stress Analysis (TSA) where the effectiveness of three of the most common criteria was assessed. It was shown that the crack paths are not always repeatable as expected by FE models. It was found that the crack path criteria are capable of an acceptable prediction only in the early stages of the crack growth. Furthermore, the Stress Intensity Factors (SIF) only partially control the crack path and it has been recognised that the T -stress is one the influential parameters of the crack trajectory. Despite the vital role of T-stress, not only in directional stability problems but also in crack growth rate and the shape and size of the plastic zone ahead of the crack tip, little attention has been paid to experimentally determine the T-stress. Therefore, based on both Muskhelishvili's and Williams' approaches, methodologies were developed to determine the SIF and the T -stress from both stress field and displacement data generated artificially and using a finite element method. These methodologies were successfully employed to experimentally determine the SIF and the T -stress for different types of notched and fatigue cracked specimens manufactured from Al 7010 T765 I using TSA and Digital Image Correlation (DIC) technique. It was shown that the Muskhelishvili's approach is equivalent to the Williams' 2 terms stress solution for SIF determination. However, the 2 terms solution is not sufficient to determine the T -stress and, three or more terms are needed both from the stress and displacement fields. Results obtained from the stress field are numerically unstable if more than four terms are used. However, results obtained from the displacement field show more robustness with an increased number of terms.