Me5656 - Mechanics of Contact & Lubrication Term Project Shot Peening Process

Shot peening is a process where compressive residual stresses are generated at the surface of the processed part by impacting small hard particles to the surface in order to increase the fatigue resistance of the part. In this work, a general literature survey of shot peening process is presented. Survey is mainly focused on mechanics and numerical simulation of the process. Effects of various process parameters on resulting residual stresses and surface characteristics are summarized. In the second part, effect of surface roughness is investigated through numerical simulations. For this purpose, a 2D finite element model is developed in which substrate surface is modeled as a random rough surface. Results show that surface roughness has a substantial effect on the impact characteristics when the particle size is close to the average surface roughness. For large enough particles the effect of roughness is limited to the region near to the surface, but higher stresses are seen to be generated at the surface as a result of localization of deformation. Baran YILDIRIM 11/24/2009 2 1. Description of shot peening process: Shot peening is a cold working process where the surface is impacted with small spherical hard metal or ceramic particles (also called “shot”) at velocities between 20 to 150 m/s. The typical diameter of particles used in shot peening process ranges between 0.5 to 2.5 mm. In shot peening, plastic deformation of the surface occurs rather than abrasion. The process consists of multiple repeated impacts to the substrate. Each particle, when impacted to surface, acts as a tiny hammer creating a small indent (dimple) and local plastic deformation. Deformation due to impact forms a zone with compressive residual stresses just beneath the surface. Since cracks generally propagate from the surface, with the compressively stressed layer, the chance of crack propagation is reduced. Thus, resistance to fatigue and stress corrosion failure of a material can be greatly increased. With increased resistance to failure, applying shot peening to a part can considerable extend the part’s life [20]. Surface hardness also increases due to cold working aspect of the process. The effectiveness of the process is often measured by the amount of compressive residual stresses induced in the part and the integrity of the surface. Many factors can affect the quality of shot peening process. These can be categorized as:  Particle diameter  Particle impact velocity and impact angle  Hardness and structure (thickness, shape, etc.) of shot-peened part  Roughness of shot-peened surface  Peening time  Peening coverage (percentage of a surface that has been impacted by peening particles) Particles used in shot peening process are generally made of hard steel, ceramic or glass. The parts to be processed are ductile metals such as aluminum or steel. The hardness of the impacting particles should be much higher relative to the material that is being processed. Since the process involves high level of deformation and straining at the surface of the processed part, brittle materials cannot be shot peened. Baran YILDIRIM 11/24/2009 3 2. Mechanics of shot peening process During the impact of particles in shot peening process, contact can be considered as fully plastic for all cases. This can be validated by a simple analysis on loading conditions at the surface [21]. The following equation gives the equation of motion for contact: