Vibratory Residual Stress Relieving- A Review

Abstract

All manufacturing and fabricating processes such as casting, welding, machining, molding, heat treatment, plastic deformation during bending, rolling or forging introduce residual stresses into the manufactured object. They are caused by rapid, unequal cooling as opposed to the stresses caused by external loading. These stresses affect fatigue life and dimensional stability of material. Different destructive and non destructive techniques are used to relieve these undesirable residual stressess such as thermal stress relief method. Thermal stress relief methods have limitations like economic and technical infeasibility. To overcome the limitations of these methods in past few decades various methods have developed like shot peening, Vibratory Stress Relief. In this paper different methods of vibratory stress relief are reviewed. Keywords - Residual stress, SubHarmonic VSR, Vibratory stress relief, I. INTRODUCTION Residual stresses reside within the metal work piece. They are caused by rapid, unequal cooling as opposed to the stresses caused by external loading. Residual stress could be caused by localized yielding of the material, because of a sharp notch or from certain surface treatments like shot peening or surface hardening. Among the factors that are known to cause residual stresses are the developments of deformation gradients in various sections of the piece by the development of thermal gradients, volumetric changes arising during solidification or from solid state transformations, and from the differences in the coefficient of thermal expansion in pieces made from different materials. Residual stresses induced in a body after welding may lead to dimensional instability and it reduces strength and fatigue life of component that is why it is important to measure and check these residual stresses. To reduce residual stresses different stress relieving methods are used like vibratory stress relief techniques, shot peening and mechanical tumbling. To check the effectiveness of stress relief methods measurement of these stresses before and after the treatment is necessary. For that purpose different measurement techniques are used like mechanical, X-Ray, ultrasonic, and magneto-elastic methods. In this paper latest vibratory stress relief techniques are discussed and their relative advantages and limitations against other stress relief methods are studied. II. TECHNIQUES OF REDUCING RESIDUAL STRESSES In industries different destructive and non destructive techniques are used to relieve the undesirable residual stressess such as thermal stress relief, shot peening, Vibratory Stress Relief (VSR). 2.1 Heat Treatment(Furnace methods) It is the well-known method which is used to relieve internal stresses. Thermal Stress Relieving is generally applied to metallic materials that have been cold-worked, formed, machined, flame-cut, or weld- fabricated to reduce residual stresses for dimensional stability or reduced risk of premature failure in service. In this traditional method, the part is heated under carefully controlled conditions to several hundred degrees, maintaining this state till majority of the stresses have been relieved. It is then followed by controlled cooling to prevent introduction of new residual stresses. In many cases, this process is reiterated many times during the manufacturing process before the part is ready for use. During this process, the yield point of the material is substantially lowered, which allows for plastic flow in the material. This plastic flow in the material, consequently, causes redistribution and subsequent reduction of internal residual stresses. Although thermal methods have been widely used in industry for the past century, they have their own limitations in certain situations. In some cases, the cost of treatment of the part is very high in terms of energy and capital required. Additionally, oxide scales on the surface are formed that require subsequent finishing processes. 2.2 Shot peening Shot peening is a cold working process used to increase the fatigue properties of metal components. During the peening process, compressive residual stresses are induced in the work piece by bombarding it with small spherical media called shot. It is a surface cold worked process used to minimize stress corrosion cracking and other modes of failure.Excessive peening can result in formation of many small cracks in the component.