Developing an Analytical Model for Predicting the Residual Stresses Induced by Shot Peening with Considering the Effect of Initial Surface Treatment

Sherafatnia, Khalil | 2019

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 51530 (08)
  4. University: Sharif University of Technology
  5. Department: Mechanical Engineering
  6. Advisor(s): Farrahi, Gholam Hossein; Mahmoudi, Amir Hossein
  7. Abstract:
  8. Shot peening is a cold-working process commonly used in industry to improve the fatigue performance, stress corrosion resistance and surface nano-crystallization of metallic parts. This process extends fatigue life via two mechanisms: Preventing the crack growth due to compressive residual stresses and, preventing the crack initiation because of increased material hardness. These mechanisms are the results of the bombardment of the component's surface with small spherical particles. In this research, an analytical model is developed for estimating the residual stress distribution induced by shot peening process. The modifications of the developed analytical model are related to elasto-plastic unloading of shot impingements, introducing a new stress-depth distribution model suitable for full coverage, considering the friction coefficient and strain rate effects and the fraction of kinetic energy transmitted to the treated material. The residual stress distribution induced by shot peening is obtained using Hertzian contact theory and elastic-plastic evaluation after yielding occurs during impingement and rebound of shots. In this model, cyclic plasticity behavior of the material is incorporated into the constitutive equation. In order to predict more realistic residual stresses, the elasto-plastic unloading phase of shot impacts is modeled using two nonlinear kinematic hardening models considering the Bauschinger effect. Moreover, the effect of the Coulomb friction between target surface and shots is evaluated. To doing so, the interior stresses caused by tangential tractions of friction force are determined analytically. The distribution of the shot peening residual stresses is calculated implementing the linear elasticity and the mentioned plasticity model. In addition, the Almen intensity parameter of the shot peening process is obtained analytically. The analytical model is so developed that the effects of the previous loading history is considered. The present model is able to predict redistribution of residual stresses in shot peening process by considering the initial conditions of target surface. Initial stress distribution and yield stress variation produced by previous surface treatments are taken into account by measuring residual stresses and hardness profile near surface layers. An analytical parametric study is performed to evaluate the influence of initial conditions induced by surface pretreatments on shot peening residual stress field. The results of analytical model are validated by experimental data obtained in the literature as well as by our own measurements. Initial residual stresses were induced using a four-point bending rig and grinding. Incremental center hole drilling and X-ray diffraction techniques were employed to measure residual stresses on bent, ground, shot peened, bent plus shot peened, ground plus shot peened and reshot peened specimens. The analytical results generally agree with the experimental measurements
  9. Keywords:
  10. Residual Stress ; Shot Peening ; Cyclic Plasticity ; Surface Treatment ; Analytical Modeling

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