# A Novel Low-Cost DIC-Based Residual Stress Measurement Device
## 1 Introduction  
Residual stresses are stresses locked into a material through plastic deformation. On the macroscopic scale, they can be due to heat treating, machining and secondary processing whereas, on the microscopic scale, they are typically associated with discontinuities in the thermal expansion coefficients, yield stresses, rigidities or phase changes [1]. While compressive residual stresses can sometimes be beneficial, for example by increasing apparent tensile strength, tensile residual stresses can reduce fatigue strength [2] or induce cracking [3] which leads to premature failure. For this reason, quantification of residual stresses is crucial. The residual stresses in polycrystalline materials are categorised into three major types [4; 5; 6]. Type I relates to the macroscale residual stresses distributed across the dimensions of a part and contribute to its distortion by long-range gradients of plastic deformation in the part. Type II refers to mesoscale changes on the scale of grain, which are normally caused by the strain incompatibility between grains and the heterogeneous microstructure such as dislocation cells inside grains. Type III, microscale stresses describe the variation of the stress within a grain usually as a result of dislocations and other crystalline defects. These stresses are additive, meaning the sum of these stresses contributes to the overall residual stresses. The nature of external force application means that near-surface residual stresses are often the most influential in terms of structural integrity [7]. As the total stress is equal to the sum of these stresses added to the applied stresses [8], characterising residual stresses, specifically Type I stresses which impact the material performance and lifecycle on a macroscale, is essential for any design and development, manufacturing and quality control applications. Some critical industries include aerospace, where the demand for fuel-efficient and lightweight aircraft has fuelled the need for high-strength and damage-tolerant alloys for thinner web and wall features [9]. Another notable example includes nuclear energy where the residual stresses at a welding zone must be monitored to avoid stress corrosion cracking in nuclear power plants [10]. Residual stresses are of significant interest for many novel scientific methodologies as well. Metal additive manufacturing techniques rely on rapid heating and melt-back cycles which can result in undesired residual stresses in the part [11]. This can limit the functionality and achievable print geometry [12; 13; 14]. Another topic of interest is composites, where stresses can be generated within the laminates during the autoclave heating and cooling cycles [15]. Although numerical predictions can be used to estimate residual stress, typically experimental validation is required. Semi-destructive techniques are dependent on inferring the original stress from the displacement incurred by completely or partially relieving the stress by removing material, leaving its overall structural integrity intact for testing, repair or use [16]. These methods rely on the measurement of deformations due to the release of residual stresses upon the removal of material from the specimen [17] and allow for residual stress measurements at different stages of the material's life [18]. The three most widely used semi-destructive techniques are the deep-hole-drilling [19; 20], ring-core drilling [21; 22] and the hole-drilling methods [23; 24]. The hole-drilling method involves introducing a small hole into the surface of a component at the centre of a strain gauge rosette to measure the relieved strains, and the results are then processed to determine the residual stresses originally present at the hole location [25; 26; 27]. It has the advantages of good accuracy and reliability, standardized test procedures and convenient practical implementation. The damage caused to the specimen is localized to the small, drilled hole, and is often tolerable or repairable.