S. Kunaporn

Waterjet Machining and Peening of Metals

An experimental study was conducted to determine the influence of high-pressure waterjet (WJ) peening and abrasive waterjet (AWJ) machining on the surface integrity and texture of metals. A combination of microstructure analysis, microhardness measurements, and profilometry were used in determining the depth of plastic deformation and surface texture that result from the material removal process. The measurement and evaluation of residual stress was conducted with X-ray diffraction. The residual stress fields resulting from treatment were analyzed to further distinguish the influence of material properties on the surface integrity. It was found that waterjet peening induces plastic deformation at the surface layer of metals as good as shot peening. The degree of plastic deformation and the state of material surface were found to be strongly dependent on the peening conditions applied.

Mathematical Modeling of Ultra-High-Pressure Waterjet Peening

Waterjet peening is a recent promising method in surface treatment. It has the potential to induce compressive residual stresses that benefit the fatigue life of materials similar to the conventional shot peening process. However, there are no analytical models that incorporate process parameters (i.e., supply pressure, jet exposure time, and nozzle traverse rate, etc) to allow predicting the optimized peering process. Mathematical modeling of high-pressure waterjet peening was developed in this study to describe the relation between the waterjet peening parameters and the resulting material modifications. Results showed the possibility of using the proposed mathematical model to predict an initial range for effective waterjet peening under the variation of waterjet peening conditions. The high cycle fatigue tests were performed to validate the proposed model and fatigue test results showed good agreement with the predictions.

Fatigue Performance of High-Pressure Waterjet-Peened Aluminum Alloy

An experimental study of high-pressure waterjet peening on 7075-T6 aluminum alloy was conducted to investigate the effects of waterjet on high-cycle fatigue life and fatigue crack growth. Unnotched hourglass-shaped circular cross section test specimens were fatigue tested in completely reversed rotating bending (R =S min /S max = -1) to determine fatigue life behavior (S-N curves). Single-edge-notched flat tensile test specimens were tested in the tension-tension fatigue crack growth tests (R=S min /S max =0.1) to determine fatigue crack propagation behavior (da/dN versus AK). Surface characteristics and fracture surfaces were evaluated by scanning electron microscopy (SEM). Results show that waterjet peening can increase high-cycle fatigue life, delay fatigue crack initiation, and decrease the rate of fatigue crack propagation.

Residual Stress Induced by Waterjet Peening: A Finite Element Analysis

The concept of multiple droplet impacts resulting from ultra high-pressure waterjet (UHPWJ) was used to develop a mathematical model to describe the effect of interfacial pressure on the underlying workpiece material. A non-linear elastic-plastic finite element analysis (FEA) was carried out in this study using the interfacial pressure model to predict residual compressive stresses. This three-dimensional FEA model was based on quasi-static considerations to provide prediction of both magnitude and depth of residual stress fields in a 7075-T6 aluminum alloy (A17075-T6). Results of the FEA modeling were in good agreement with experimental measurements. Effects of applied pressures on the residual stress fields are also presented and discussed as a method of estimating high-pressure waterjet induced compressive stresses under varying process conditions for peening.