Study of thermal stresses induced surface damage under growing plasma channel in electro-discharge machining

Steep temperature gradient induces thermal stresses, which can cause a network of micro-cracks, damaging the electro-discharge machined surface. In this context, review of different models reveals few limitations concerning too many assumptions and too simplified boundary conditions. Considering the limitations of the earlier models, the close form solution of three-dimensional heat conduction transient is used as forcing function to obtain the mathematical expression of thermal stresses. The temperature distribution function is obtained considering electro-discharged machine surface, which is both axially and radially infinite. Spatial and chronological variation of the heat source has been considered in undertaking a parametric study to characterize the thermal stress distribution. The study reveals that the induced thermal stresses exceed the ultimate stress of the material beyond the crater, which is the most crack prone. From the nature and the magnitude of thermal stresses, it is concluded that crack is initiated from the top edge of the crater and propagates towards the bottom of the crater, which is further corroborated from the photograph of the crater at suitable magnification.