Materials Science Forum Vols. 524-525

Abstract: Residual stress induced by laser single pulse irradiation was analyzed using a dynamic finite element code, ABAQUS/Explicit. The effects of the magnitude and length of a surface pressure pulse having a circular top-hat shape on the final residual stress in Ti-6Al-4V were investigated. A high peak pressure and/or a long pulse duration was effective in generating large compressive residual stress deep beneath the surface. However, large tensile residual stress features occurred near the centre and edge of the laser spot on the surface for high pressure and/or long pulse durations due to a radial focusing effect. Use of shorter pulse durations avoided this. The peak pressure (3GPa) required to induce a surface compressive residual stress across the whole area of the spot was slightly higher than the threshold pressure needed to plastically deform the surface.

Abstract: Laser peening has been applied to silicon nitride (Si3N4) ceramics without any pre-coating. X-ray diffraction study revealed that plastic strain was introduced into the surface layer of the ceramics. Compressive residual stress was also imparted, which became larger with increasing peak power density of irradiated laser pulses. Surface roughness significantly increased due to ablative interaction of the surface with laser pulses. A Weibull plot of four-point bending test results clearly showed the increase of the bending strength and Weibull modulus by laser peening in spite of the increase of the surface roughness.

Abstract: By X-ray diffraction (XRD) measurements, material conditions of rolling bearings are red that point to a variety of load possibilities especially at raceway surfaces. Due to unambiguously distinguishable damage symptoms, according to H. Muro, it is differentiated between the surface and sub-surface failure mode in the literature. Surface distress of different intensity can be generated by particle-contaminated lubricants that result in raceway indentations. These micro-Hertzian contacts may lead to changes in residual stress and line broadening and in the microstructure. Another cause of surface distress is boundary lubrication. Relevant position and nature of the failure mechanisms are characterized. In case of initial material stabilization, the time alterations of the XRD parameters correlate with the statistical parameter of the 10 % bearing life. Contrary to the L10 value for sub-surface fatigue, which leads to spalling only a long time after incipient material softening, in the surface damage mode the L10 life roughly coincides with the beginning of the instability phase. Surface pitting or gray staining turns up frequently with low XRD indication of material aging. Here, scanning electron microscopy and electron microprobe analysis point to corrosion fatigue. The interaction between material and lubricant opens research in the field of tribology.

Abstract: The residual stresses around clearance-fit mechanical fasteners have been found to be similar to those around cold expanded holes where compressive hoop stresses close to the fastener hole are balanced by far-field tensile stresses. This compressive zone has been shown to prolong fatigue lifetimes around fastener holes. Constant amplitude fatigue loading was applied to single plate rivet specimens for varying numbers of cycles to investigate the redistribution of these stresses after fatiguing. Synchrotron diffraction was used to map the evolution of the residual stresses around the rivets. Little change in the hoop stress local to the rivets occurred until visible fatigue cracks were observed suggesting that relaxation of these stresses is due to the cracks rather than their cause.

Abstract: Experimental analysis can be very costly and time consuming when searching for the optimal process parameters of a new shot-peening configuration (new material, new geometry of the part…). The prediction of compressive residual stresses in shot-peened parts has been an active field of research for the past fifteen years and several finite elements models have been proposed. These models, although they give interesting qualitative results, over-estimate, most of the time, the level of the maximal compressive stresses. A better comprehension of the phenomena and of the influence of the parameters used in the model can only carry a notable improvement to the prediction of the stresses. The fact that the loading path is cyclic and is not radial led us to think that a model including kinematic hardening would be better adapted for the modelling of shot peening. In this article we present the results of a simulation of a double impact for several constitutive laws. We study the effect of the chosen constitutive law on the level of residual stresses and, in particular, we show that kinematic hardening, even identified on the same tensile curve than isotropic hardening, leads to lower stress levels as compared with isotropic hardening. Furthermore, the overall shape of the stress distribution within the depth is significantly different for the two types of hardening behaviour. Further, in order to check the modelisations, local measurements were carried on with X-ray diffraction on a large size impact and correlated with the topography of the impact.

Abstract: This work presents an experimental and numerical study of the thermo-mechanical problem of a steel plate impacted by a shot. The temperature rise is estimated and its effect on the compressive residual stress is analyzed. The simulations show that the value of the compressive residual stresses at the surface of the plate is modified when thermo-mechanical effects are included in the model as compared with simulation including hardening effects only. To validate this numerical study, an experimental device has been developed to measure the temperature rise after the impact. The experiment consists of the impact of a shot on a metallic plate. The temperature measurement is performed by an infrared camera located on the side of the plate opposite to the impact. Comparison between these experimental measurements and the numerical solution gives good agreement (to within 5%).

Abstract: The further development of innovative forming processes like sheet metal hydroforming is only possible with the help of detailed knowledge about the workpiece properties and their formation depending on the process strategy. Up to now, the knowledge about the formation of macroscopic residual stresses in high-pressure sheet metal forming (HBU), regarding the influence of the sheet material properties, is still insufficient. The characteristics of the specific forming procedure HBU lead to specific stress and strain gradients in the sheet cross-section, and therefore lead to a characteristic distribution of the induced macroscopic residual stresses, particularly in the workpiece bottom zone. This paper decribes the investigations on the influence of the sheet material flow curve on the macroscopic residual stress distribution in the workpiece bottom.

Abstract: The reliability of a mechanical component depends to a large extent on the physical state of its surface layers. This state includes the distribution of residual stresses induced by machining. Residual stresses in the machined surface and subsurface are affected by the cutting tool, work material, contact conditions on the interfaces, cutting regime parameters (cutting speed, feed and depth of cut), but also depends on the cutting procedure. In this paper, the effects of cutting sequence on the residual stress distribution in the machined surface of AISI 316L steel are experimentally and numerically investigated. In the former case, the X-ray diffraction technique is applied, while in the latter an elastic-viscoplastic FEM formulation is implemented. The results show that sequential cut tends to increase superficial residual stresses. A greater variation in residual stresses is observed between the first and the second cut. Moreover, an increase in the thickness of the tensile layer is also observed with the number of cuts, this difference also being greater between the first and the second cut. Based on these results, the residual stress distribution on the affected machined layers can be controlled by optimizing the cutting sequence.

Abstract: The aim of this work is to study the influence of residual stresses on the properties of textured duplex stainless steel (DSS). The properties of both phases in DSS were studied using Xray diffraction whilst external load was applied “in situ” to the sample. The interpretation of experimental data is based on the diffraction elastic constants calculated by the self-consistent model taking into account the anisotropy of the studied material. Carrying out measurements in both compression and tension by using neutron diffraction, important differences in the evolution of lattice strains were noticed. An elastoplastic model is used to predict the evolution of the internal stresses during loading and to identify critical resolved shear stresses and strain hardening parameters of the material. The influence of the initial residual stresses on the yield stresses of the phases is considered. The difference between tensile and compressive behaviour of the steel is explained when the initial stresses (measured in the as received non-loaded sample by diffraction methods) are taken into account in model calculations. The yield stresses in each phase of the studied steel have been experimentally determined and successfully compared with the results of the elastoplastic self-consistent model.

Abstract: In April 2005 the materials science beamline EDDI (Energy Dispersive DIffraction), which the HMI operates at the Berlin synchrotron storage ring BESSY, started user service. The high energy white synchrotron beam up to about 150 keV used for the diffraction experiments is provided by a superconducting 7 Tesla multipole wiggler. Starting with some basic information on the technical parameters of the beamline, its set-up and measuring facilities, the paper focuses on the application of white beam diffraction to the analysis of residual stress fields in the near surface zone of polycrystalline materials. The concept of a program system is introduced, which we offer to our users for preparing and evaluating their measurements performed at the EDDI beamline.