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.