Finite element multi-impact simulations using a crystal plasticity law

For optimal functionality, materials are expected to have the desired properties which include materials hardness, fatigue life, corrosion properties and yield stress. Over the past two decades, surface nanostructuration has been the most used technique for inducing such properties on materials especially the metallic ones. Currently, several methods of nanostructuration of materials components are available for use. However, ultrasonic shot peening (USP) has been widely considered the best method. The method employs the use of shot peening process whose advantage is multiple impacts on the materials for improved surface integrity and microstructural changes. On the other hand, determination of the causes and effects of microstructural changes is very critical in achieving accurate results. It can be performed through finite element simulations.

Doctor Thomas Rousseau, Assistant Professor Cecile Nouguier, and Professor Thierry Hoc from the LTDS – Université de Lyon in France together with Professor Phillipe Gilles from Paris La Defense Cedex in France employed a finite element simulation approach in investigating the changes in the microstructure surfaces under various impacts including normal, multiple or oblique for various ultrasonic shot peening (USP) processes configurations. Crystal plasticity law formed the basis of the model. The authors also paid attention to the possibility of predicting the expected changes in the microstructure through the formulation of the crystal plasticity and the impacts that the process (USP) have on the microstructure and stress of the materials. This research work is published in International Journal of Plasticity.

The research team achieved their primary goal of numerically analyzing the effects induced by the USP process on the parameters such as the residual stress of the materials under various impacts through the crystal plasticity formulation method. The agreement of the results resulting from the comparison of the results obtained by other methods, past studies, and the available theoretic work proved beyond doubt the accuracy and effectiveness of this research.

For the first time, this research has successfully proposed the use of crystal plasticity-based law in finite element multi-impact analysis and simulation. The most initial stage was to demonstrate that indeed the law can be used in the production of both tensile macroscopic curves and their misorientations that results from normal impacts. This is very critical in the analysis of the impact-induced effects. It was also observed that high values of average kernel misorientation occur near the grain boundaries especially in the cases of the deformed grains. A similarity of the same results was reported in the previous studies. According to the researchers, neglecting the effects of the grain boundaries in the early stages of the USP process will not affect the results. The high efficiency of the USP process for working with microstructure changes as compared to other available techniques such as conventional shot peening was also confirmed.

According to the authors, the multiple disorientations observed in the alloy microstructure were induced by the multi-impacts applied during the process. It was the initial step towards the formation of the surface nanostructures. Nanostructured surfaces can be formed by this process at lower impact velocities by considering the multiple oblique impacts over other impacts in cases where the peened surfaces have grain splits.