Cold Working Holes in Multi-Layer Members

Increasing the life cycle of critical components is a common goal in many vehicle industries. One of the most common ways to increase the fatigue resistance of fasteners holes is the process of cold expansion. This method introduces a compressive stress field in the region around the hole that slows the propagation of cracks. Determining the life cycle benefits gained from cold expansion is difficult due to the complex nature of the residual stress field. Many groups have attempted to accurately predict how this field is generated and what factors can cause major variations in the resulting stress field. There are still many factors related to the cold expansion process that have not been quantified. By creating a script in the computer language Python it was possible to generate a number of different models quickly and efficiently in the finite element program Abaqus. While not all the models that could be created were initially found to be convergent, the script proved useful in creating a varied number of models to assist in determining which factors were leading to the convergence problems. Confidence in the script's ability to produce accurate models was established by generating models that mirrored the conditions found in other literature, so that a direct comparison of results could be made. For this work two factors were considered for analysis, the effect of starting hole size and multi-layer expansion. The results showed that even within the range of recommended starting hole sizes, a difference in the residual stress field was evident. If the hole was expanded beyond the recommended size a threshold was reached and a severe weakening of the residual stress field was noticed. In the case of two plate expansion, changes in the residual stress field were only observed at the interface region of the plates. For the entrance face of the second plate in the expansion, this change was highly beneficial. The results from the two plate expansion suggest that artificially creating a multi-layer stack-up may be a useful tool to improve the residual stress field at the entrance surface of a plate.