![]() The average contour determines the deformations caused by residual stress redistribution and is used to compute residual stresses through an elastic finite element model of the specimen. The contour method determines residual stress by cutting an object into two pieces and measuring surface height maps along the free plane created by the cut. Destructive testing is often far cheaper to implement than non-destructive testing. These techniques involve destruction of the object being measured and are typically used from a research and development perspective. The three primary categories are listed below: Destructive This is particularly true for many non-destructive techniques. ![]() Owing to the complexity of some of the measurement techniques, the measurement must be performed in a specialised facility. The approach to be used is often dependent on the information required. These are broadly categorised into three areas: Destructive, semi-destructive and non-destructive. There are many techniques used to measure residual stresses. It has long been recognised that, for non-welded materials under fatigue loading conditions, only the tensile parts of the applied stress cycle contribute to fatigue crack growth (see the lower part of Figure 1.) Conversely, for joints in the as-welded condition, the effects of welding residual stress need to be added to those of applied cyclic stress, with the result that the whole fatigue cycle (tensile and compressive) gives rise to fatigue damage (see the upper part of Figure 1).įigure 1 Effect of welding residual stress on fatigue damage.įortunately, the effects of welding residual stresses on both fracture and fatigue have been written into codes and standards in such a way that most users are unaware of them, and do not need to consider them explicitly. Nevertheless, there are particular cases where quantification of residual stresses is necessary. The undesired stresses also have an effect on the fatigue performance. For example, thick-walled structures in the as-welded condition are more prone to brittle fracture than a structure that has been stress-relieved. Residual stresses are often invisible to a manufacturer, unless they result in significant distortion, but can negatively affect structural integrity. Typically, however, residual stresses have negative effects. This can be achieved through laser peening, which imparts compressive residual stresses to the surface of an object, allowing for the strengthening of thin sections or the toughening of brittle surfaces. For example, residual stresses are implemented in the designs of certain applications for positive effects. What Effects Do They Have?ĭepending on the application, residual stresses can be positive or negative. Once the load is removed, the material tries to recover the elastic part of the deformation, but is inhibited from full recovery due to the adjacent plastically deformed material. When a material undergoes deformation, one part is elastic and another plastic. Residual stress also occurs when plastic deformation is non-uniform through the cross-section of an object undergoing a manufacturing process, such as bending, drawing, extruding and rolling. The volume difference causes expansion or contraction of the material, resulting in residual stress. When a material undergoes a phase transformation, a volume difference between the newly formed phase and the surrounding material, which has yet to undergo phase transformation, occurs. Consequently, the inner portion will have a residual tensile stress and the outer portion of the component will have a residual compressive stress. As the material in the centre tries to cool, it is constrained by the cooler outer material. During cooling, the surface cools at a quicker rate, compressing the heated material at the centre. The differing thermal contractions develop non-uniform stresses. The difference in cooling rates experienced by the surface and interior of the object results in localised variations in thermal contraction. When an object is cooled from a high temperature (eg after welding), there is often a large difference in the rate of cooling throughout the body. There are three primary reasons for these stresses arising: Thermal Variations Residual stresses are generated when an object (especially a welded component) is stressed beyond its elastic limit, resulting in plastic deformation. Click here to see our latest technical engineering podcasts on YouTube. In others, they affect susceptibility to fracture and fatigue. In some cases, residual stresses result in significant plastic deformation, leading to warping and distortion of an object. Residual stresses are those stresses that remain in an object (in particular, in a welded component) even in the absence of external loading or thermal gradients. National Structural Integrity Research Centre.Structural Integrity Research Foundation.
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