| Resumo : |
Several sources of residual stresses characterize the entire gear manufacturing chain. In-between processes, their redistribution has to be the condition to satisfy the equilibrium principle. Nevertheless, the state-of-the-art reveals an excessive concern on optimizing the processes individually. As direct consequences, the accuracy of the prediction models is lowered, the control on the production dispersion is jeopardized and the product design is not optimized. The hypothesis herein stated is that the residual stress intensity and heterogeneity induced by previous processes are significant to the surface integrity of a subsequent process. Consequently, the objective is the identification of the residual stress factors induced along the gear manufacturing chain with relevant influence on the final surface integrity. An experimental investigation was conducted on the interaction between soft machining and heat treatment, and between shot peening and grinding. This study originally proposed to structure the investigation into a convergent approach. Whereas every parameter modification is to be done at the first process, the second process is kept the same. The ground gears were submitted to contact fatigue tests, to identify the relevance of the interaction effects to the part lifetime. The results proved that the previous process can exert a noteworthy influence on the final integrity state. The interaction is understood by a model of springs, into a self-equilibrated potential energy concept. The primary factor from the previous process is the hereafter named unstable area of residual stresses (UARS). It proportionally drives the modifications induced to the surface after the next process. They happen either as distortion or as change in the stress state itself, depending on the equilibrium disturbance mode that the manufacturing process represents. The effect follows the domain size under analysis. Residual microstresses carry and reflect microstructural inhomogeneity and roughness patterns between neighboring processes. The correlation was enabled due to a novel assessment method of the residual stress heterogeneity state, through the Gauss integral breadth parameter. The interaction showed also to be present to the gear fatigue behavior. Both the failure mode and the lifetime correspond to the region where the interaction effects were mostly observed. The comprehension of the residual stress interaction effects derives the newly introduced concept of Design for Residual Stress (DRS). The manufacturing chain can be designed with the purpose of providing an optimal residual stress state, with a consequent potential for enhancing the gear load carrying capacity. |