ADVANCED MATERIALS & PROCESSES | APRIL 2025 20 transparent to the laser light, and so water is used. The shockwave generated results in plastic deformation of the near-surface, and it is this plastic deformation that creates the residual stress field. Modeling of the LSP process is complex and difficult, and there have been few, if any, attempts to couple the physics of the laser/surface interaction with the formation of the plasma and the generation of the shock wave into the material. Most modeling studies begin with the shockwave and its propagation sustain higher shock pressures than aluminum. An illustration of a typical LSP process is shown in Fig. 1. Prior to peening, the surface of the component is normally first coated with a sacrificial ablative layer. Typically, black vinyl tape is used but paint or metallic foils have been reported. LSP can also be applied without an ablative coating; however, the ablative layer protects the surface of the component from thermal effects during peening. LSP without an ablative layer tends to use less energy per pulse to minimize thermal effects on the target surface. After surface preparation, a pulsed laser light is fired at the component surface. The ablative tape (if used) is partially ablated by the extreme heat generated. A high-pressure plasma is created through ionization, and a shockwave is generated. To increase the efficiency of the process, an inertial tamping layer is used. This confines the plasma and directs more of the generated pressure into the component. The tamping layer needs to be TABLE 1 — EXPERIMENTAL RESIDUAL STRESS ANALYSIS METHODS Method Advantages Disadvantages Costs Laboratory (surface) x-ray diffraction Fast Easy to obtain results Depth profiles can be obtained using layer removal Near-surface only (<10 µm) Layer removal can be timeconsuming and may require corrections to be applied to the data A measurement campaign could be conducted for $1-10K depending on number of data points required. Incremental hole drilling Fast Easy to obtain results At least semi-destructive, with typical hole sizes of 2 mm diameter and depth On the order of $500/hole or less. Contour method Full cross-sectional map of residual stress is obtained Destructive (sample must be cut in half). Care needs to be taken if stresses within the first millimeter from the surface are required with accuracy. On the order of $5K per cut, depending on complexity of component and number of trial cuts required to optimize the process. Neutron diffraction Nondestructive and can penetrate deep into material Long lead time to access. Few global facilities. Near-surface stresses (within 1-2 mm) either cannot be obtained or require corrections to be applied. Beamtime may be available freely for engineering science studies. But personnel costs for preparation, execution, and analysis mean that a measurement campaign will likely cost at least $15-50K. Synchrotron x-ray diffraction May be nondestructive, depending on sample geometry Long lead time to access. Few global facilities. Typically cannot determine sufficient strain components to allow for a calculation of stress. Not all SXRD instruments are optimized for near-surface strain measurement without the need for corrections to be applied. Beamtime may be available freely for engineering science studies. But personnel costs for preparation, execution, and analysis mean that a measurement campaign will likely cost at least $15-50K.
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