In addition, it is often not possible to make deformation measurements directly at the stress-free surface, which provides the zero-stress datum. In this case, the zero-datum challenge is to cut the material in a geometrically accurate way that does not induce additional local residual stresses. The deformation responses from the resulting local redistribution of the residual stresses are measured, from which the originally existing residual stresses are inferred. In the relaxation type of residual stress measurements, some stressed material within the specimen is cut away so as to expose a new stress-free surface that acts as the zero-stress datum of the measurements. Perhaps only when the method completely maintains the integrity of the component can the method truly be called “non-destructive.” Nonetheless, we follow convention and refer to such methods as “non-destructive.” If the need for the creation of a stress-free reference specimen requires the destruction of a second specimen, then the effect is just to transfer the damage elsewhere. Conversely, non-destructive methods often require cutting the sample to gain sufficient access to the location of interest, or to collect reference samples to establish the zero-stress datum. In many cases destructive methods do little damage, for example, some methods are described as “semi-destructive” because the damage is slight or can be repaired. The distinction between the two measurement classes is not as clear as it may seem. Non-destructive methods, which rely on measuring some phenomenon that relates directly to the stress, such as the spacing between the atoms, or a secondary effect, such as a change in magnetic or vibration spectra that can be related back to the underlying stress state. Relaxation (also called “destructive”) methods, in which material is removed and the resulting deformations used to infer the residual stress prior to cutting, and Residual stress measurement methods can be classified broadly into two general classes: Consequently, all residual stress measurement methods must explicitly include some method for establishing a zero-stress datum. Residual stresses are “absolute” quantities, in contrast to applied stresses, which are “relative” quantities, (i.e., relative to the unloaded datum). Unfortunately, this convenient circumstance does not exist for residual stress measurements. This greatly simplifies measurement procedures and data interpretation. It is intended as a “commentary” rather than as a review and is aimed towards incoming practitioners and researchers to the field to help guide them on the issues that need to be addressed so as to make effective residual stress measurements.įor applied stress measurements, the unloaded condition is taken as the zero datum. This paper describes the character and causes of various experimental and theoretical challenges that exist when making residual stress measurements. These and other characteristics make residual stress measurement very challenging, so that significant knowledge, judgment and skill are essential to make effective measurements. Residual stresses are more complex to measure because they cannot easily be added and/or removed in a quantified manner, as can be done when considering applied stresses. They are very difficult to predict, and so reliable measurement is essential. Residual stresses are introduced throughout all stages of manufacturing and can vary during component life. They add to the more apparent stresses arising from the applied loads and can lead to serious failures if not adequately accounted for. In addition, they also often have a complex 3-dimensional character. Residual stresses have a “hidden” character because they exist in a material in the absence of any external loads. The most significant feature for success is a highly skilled and knowledge practitioner. Conclusionsĭespite the various challenges that need to be overcome, residual stress measurements can be successfully undertaken in practice. Resultsįive major challenges for residual stress measurements, and the approaches used for their resolution, are identified. Various of the most common residual stress measurements methods are considered and the challenges associated with them are identified and classified. The objective here is to identify and describe the various features that make residual stress measurement methods challenging and to consider the ways that these challenges can be addressed in practice. They cannot easily be added or subtracted in a quantified manner, as is done when measuring applied stresses, and so are much more challenging to measure. Residual stresses have a “hidden” character because they exist in a material without the presence of any external loads.
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