What is residual stress? Methods for detecting residual stress.

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Residual Stress refers to the self-equilibrated stress that remains within a material even in the absence of external influences. It is a type of inherent stress or internal stress, which usually arises during the manufacturing process due to uneven plastic deformation or phase changes. The common causes of residual stress include the following: mechanical processing such as cutting, bending, and stretching; these processes lead to uneven plastic deformation of the material. Thermal processing: such as welding, casting, forging, etc.; these processes cause uneven thermal expansion and contraction of the material. Phase transition: During the phase transition of a material, the volume difference between the old and new phases leads to the generation of residual stress. The effect of residual stress is usually harmful; it can cause the workpiece to warp or twist during use, and may even lead to cracking. Of course, if the residual stress is within a normal and reasonable range, it can also be utilized to improve the fatigue strength and wear resistance of the parts. So how can we determine whether the residual stress is within a normal and reasonable range? This is where a residual stress detector comes in; we can use the data obtained from residual stress testing to determine the magnitude of the residual stress. There are various methods for detecting residual stress, and the main methods currently used in the industry include the following: (1) Ultrasonic stress detection method. This is a technique that utilizes ultrasonic waves for stress detection; it is based on the theory of ultrasonic elastodynamics, and makes use of the inherent relationship between the speed of ultrasonic waves in a material and the stress present there. The stress state within a material is analyzed accurately by measuring parameters such as the propagation speed of ultrasonic waves and their reflection within the material. Ultrasonic stress detectors offer advantages such as high precision, non-destructiveness, speed, and simplicity, and are widely used in fields such as aerospace, petrochemicals, machinery manufacturing, and rail transportation. (2) Stress testing by blind holes: Stress testing using blind holes is a common method for detecting residual stresses, and it is suitable for isotropic elastic materials. It works by attaching strain gauges to the area of the workpiece that is to be tested, and then making a small blind hole at the center of the strain gauge in order to release the residual stress. The magnitude and direction of the residual stress are determined by measuring the amount of strain that is released. (3) Stress detection by X-ray diffraction: X-ray diffraction is also a commonly used non-destructive testing method for measuring the residual stress on the surface of materials. Its basic principle is to use X-rays to irradiate the material, and to measure residual stress by analyzing the diffraction information. When residual stress is present in the material, the interplanar spacing changes, resulting in a shift of the diffraction peaks. By measuring the displacement of the diffraction peaks, the magnitude and direction of the residual stress can be calculated.