ADVANCED MATERIALS & PROCESSES | NOVEMBER/DECEMBER 2024 16 PIEZO-SPECTROSCOPIC/ RAMAN METHODS Raman spectroscopy can be applied as a technique to measure residual stress in materials by analyzing the vibrational modes of bonded atoms. Like diffraction-based methods, a shift in the Raman peak is experienced in the presence of residual or applied stresses. This method uses a laser as the energy interacting with the material and has a footprint of micro- meters enabling mapping of stress/ strain at micro-scale. The penetration depth is on the order of micrometers and is material dependent. The Raman method is specifically used in ceramic materials where retained strain is small and therefore challenging to measure with diffraction-based methods. Typically, the Raman method determines residual stresses assuming a hydrostatic type of stress. Polarized Raman measurements can give biaxial residual stress data[14]. MECHANICAL METHODS Residual stress measurements via mechanical methods involve the measurement of strain relaxation resulting from cutting (drilling, sectioning) of the component or sample. The strain relaxation is measured via strain gages. Examples of the different mechanical relaxation methods and typical set-up are shown in Fig. 4. The strains are converted to stresses using Hooke’s Law. There are three well-known mechanical methods. Figure 4a shows hole drilling, which involves drilling small holes in the material and measuring the release of strain with strain gages. The contour method creates 2D maps of residual stress by cutting an object into two pieces and measuring surface height maps along the free plane created by the cut (Fig. 4b). And the slitting method measures through-thickness residual stress normal to a plane cut through a component or sample (Fig. 4c). For more details regarding different mechanical methods see references 15-18. Table 2 gives a summary of mechanical residual stress measurement methods. ULTRASOUND METHODS Ultrasound methods measure changes in ultrasonic speed as result of residual stresses in the sample or part. The changes provide a measure of the stress averaged along the wave path. The acousto-elastic coefficients necessary for the analysis are usually calculated using calibration tests. Different types of waves can be employed, but the most common techniques use critically refracted longitudinal waves. The best sensitivity is obtained when the wave propagates in the same direction as the stress. This method can be applied non- destructively. The challenges with ultra- sonic measurement methods are that the data are sensitive to the state of stress in the material, and also to a number of variable material properties, such as phase composition, grain size, and crystallographic texture[19]. Fig. 4 — Schematic illustration of mechanical methods and associated experimental settings: (a) hole drilling, (b) contour, and (c) slitting. (a) (b) (c)
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