Residual Stresses VII, ICRS7

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Authors: Ru Lin Peng, Sten Å.H. Johansson, Torsten Ericsson, Lena Thorsson
Abstract: In this paper we report our attempts to use the neutron and X-ray diffraction techniques to characterize residual stresses in specimens manufactured by laser FreeForm technique. The aim of our work has been to understand how residua stresses develop during forming and their possible correlation with the microstructure, in particular the pore density which varies with the laser scan speed. The specimens under investigation were built by layer-sintering a ferritic-steel powder with a Cu-based-alloy powder as the binder material. Limited results have been obtained for the ferrite steel phase and are presented in the paper.
Authors: Andrew King, A.D. Evans, Philip J. Withers, C. Woodward
Abstract: The Rolls-Royce Wide Chord Fan Blade (WCFB) is an innovative aerospace component, manufactured from Ti-6Al-4V alloy. The blade roots experience fatigue loading during flight, and are subject to fretting fatigue where they contact the fan disc. Shot peening (SP) and laser shock peening (LSP) are mechanical surface treatments which have been applied to the blade root to improve fatigue strength and damage tolerance. Both techniques cause local plastic deformation of the material, resulting in a layer of compressive residual stress. The compressive stress from LSP may reach a depth of 2mm in titanium, compared with only 0.25mm for conventional SP. These compressive stresses are balanced by subsurface tensile stress. LSP is reported to cause less work hardening than shot peening. It is important to understand the stability of the residual stresses during service conditions so that accurate fatigue life assessments can be made. Fatigue testing has been carried out on blade root section test pieces and on notched three point bend samples. Residual strain and stress measurements have been made on both types of sample. Residual strain measurements have been made using monochromatic and pulsed time-of-flight neutron diffraction techniques. The large penetration lengths of these types of radiation allow accurate residual strain measurements to be made non-destructively, deep inside a component. Because no material removal is required, no stress relaxation occurs, allowing the measurement of balancing tensile stresses. LSP and SP samples have been studied, both in the as-received condition and after fatigue.
Authors: Janez Grum
Abstract: Induction surface hardening creates very desirable residual stresses in the hardened surface layer. Residual stresses are always of a compressive nature and are usually present to the depth of the induction-hardened layer. By the appropriate selection of grinding wheel and grinding conditions and taking into account the physical and mechanical properties of the workpiece material very favourable compressive residual stresses in the hardened surface layer can be retained. How is it possible to assure a desirable surface and surface layer quality after induction hardening and fine grinding? Finding an answer to this question requires a very good knowledge of the process of grinding on the micro-level as well as knowledge of mechanical and heat effects acting on the layer of the workpiece including the type and condition of the grinding wheel. An allinclusive consideration of the numerous influences of the kind and condition of the tool on the changes on the surface and in the surface layer of the workpiece in the given machining conditions is described by the term “surface integrity”.
Authors: Denis Thibault, Raynald Simoneau, Jacques Lanteigne, Jean-Luc Fihey
Abstract: The strains induced by hammer-peening were measured by strain gauges on a mild steel plate during the hammer-peening operation. This process has recently been robotized by the research institute of Hydro-Québec (IREQ), so the hammer-peening performed for this study was done with the help of a Scompi robot. The resulting stresses calculated from the strain measurements were compared with residual stress measurements made with the hole drilling technique. The comparison shows a very good correlation of the two sets of measurements. Residual stress measurements were also made in the hammer-peened zone: as expected we found a highly biaxial compressive state of stress in this zone. An unexpected region of transverse tension was found at the end of the hammerpeened zone. This region can be very critical if hammer-peening is made with the objective of improving fatigue behaviour.
Authors: Masatoshi Sudo, Tomohiko Iwase, Yoshiyuki Hattori, Madoka Nakajima
Abstract: In this study, we examined how the press formability of A1 alloys sheets and the generation of residual stresses was influenced by pre-coating resins, lubricants, die shoulder radius, the punch shoulder radius, and so on. The use of a pre-coated layer offers advantages by preventing direct contact between sheet metal and the face of the die, and also by enabling the use of volatile oil lubricants that keep the factory environment cleaner. The circumferential residual stress (σC ) of a cup becomes lower when the lubricant is less effective, and when the die shoulder radius is smaller.
Authors: H. Tsuda, Dong Ying Ju, T. Uchiyama, Y. Sunayama, R. Oba
Abstract: Water cavitation technique can be applied to modify the surface strength of materials as an attractive new peening route. By inducing cavitation of ultrahigh speed water due to water-jet, the numerous impacts induced by the cavitation bubbles impact can produce compressive residual stress on material surface in the similar way as that by shot peening. In the present paper, an automobile part of steel gear shaft with complex shape is processed by water peening process. Compressive residual stress induced by water peening was measured to investigate the surface strengthening effect as a quantitative factor. The residual stresses on the tooth surface of the gear are measured by X-ray diffraction method. The effect of process conditions such as water outlet pressure, standoff distance of the nozzle, and peening time are discussed. To investigate the effect of heat treatment, carburizing-quenching is implemented to the gear shaft. By comparing the residual stresses of the tooth before and after water peening, the effectiveness of water peening process on surface strengthening is verified for a quenched gear shaft.
Authors: Koichi Akita, Hirotomo Tanaka, Yuji Sano, Shin Ichi Ohya
Abstract: Microscopic residual stress distribution on laser-irradiated materials was measured using a synchrotron radiation source. Intense laser pulses were irradiated in water to high tensile strength steel and austenitic stainless steel without any surface coating. Residual stress was measured in a laser-irradiated spot (0D), line (1D) and area (2D) on the materials in order to clarify the evolution process of residual stress. Tensile residual stresses were observed in the laser single pulse irradiated spot (0D). In the line (1D) and area (2D) irradiation, however, the tensile residual stress gradually changed into compressive side as the density of irradiated laser pulses increased. In case of laser irradiation in water, resulting residual stress is considered to be the sum of a tensile stress component by thermal effect and a compressive one by plastic deformation due to shock wave. The tensile stress component remains constant even if the laser pulse density increases, because the thermal effect may be reset every pulse, whereas the compressive stress component increases with laser pulse density, until the saturation of plastic deformation. As a result, the surface residual stress changes into compression with increasing the laser pulse density.
Authors: F. Galzy, H. Michaud, Jean Michel Sprauel
Abstract: In the new types of diesel motors, the crankshaft loads increase dramatically. For that reason, to satisfy the requirements and needs of its motorist’s customers, ASCOMETAL has developed a special bending fatigue test applied to “Baldwin” specimens reinforced by deep rolling. Such test is representative of the main in-service loads existing in crankshaft crankpin fillets (one of the weakest point of the component). Thus, using this test, the mechanisms leading to the increase of the fatigue resistance brought by the deep rolling treatment, could be identified. This reinforcement is mainly linked to crack arrest due to both a decrease of the in-depth stress concentration factor and to remaining compressive residual stresses induced by the deep rolling. In this paper, the experimental methods (in particular X-rays diffraction) used to characterize the stress fields generated by the deep rolling treatment are described first. The equations of equilibrium and compatibility of the particular toric symmetry of the crankshaft crankpin fillets have been used then to analyze the surface and in-depth stress profiles of the tested sample. This allowed characterizing the shake down and stress relaxation induced by fatigue. The crack propagation behaviour and the associated fracture limits were also defined.
Authors: J.B. Li, X.Y. Gai, D.L. Wang, S.Y. Ma, Vincent Ji
Abstract: The work hardening effect of the shot peening affected layer of hardened and low temperature tempered spring steel was investigated using the method for determining the yield strength of a metallic surface with biaxial residual stress. The results show that for the surface layer of the specimens, the microhardness and half-width values of X-ray diffraction lines is decreased, whereas the yield strength is increased during shot peening. Thus, shot peening leads the surface layer of steel in hard state to work hardening instead of work softening.

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