Materials Science Forum Vol. 681

Abstract: The influence of different parameters of laser shock processing applied to a precipitation-hardened aluminium alloy 6082-T651, on residual stress, surface tophraphy and microhardness was investigated. Processing was performed with an innovative Nd:YLF laser with the power densities of 2 and 4 GW/cm², with a uniform pulse duration of 18 ns. Laser shock processing experiments were performed with the closed ablation method to ensure a higher shock-wave pressure. In the first phase, the study was focused on an evaluation of surface topography, with the record of the surface roughness profile and with the surface evaluation at a scanning electron microscope JEOL JXA-8600M. Then followed measurement of microhardness HV_0.2 in the cross section region. In the second phase comparison of residual stresses which were measured using the X-ray diffraction, was performed. Laser shock processing turned out to be a very efficient technique to improve surface properties. On the basis of the micro plastic deformation induced by shock waves, an increased dislocation density in the specimen surface was obtained. The gradient of dislocation piling through the specimen depth improved the variation of microhardness and residual stresses, which, in turn, improves fatigue strength of the material under dynamic loading.

Abstract: Local rolling and other mechanical tensioning techniques can be highly effective at reducing residual stress and distortion in thin plate welds prone to buckling. However, the issues of high capital cost and low scalability currently prevent wider adoption of such processes. Pre-weld rolling aims to address these issues and can be applied easily to each component prior to fabrication. The results of an initial trial are presented, and indicate that post-weld distortion can be reduced by an average of 38% when correct rolling parameters are used. Finally, the mechanism by which pre-rolling acts to modify the state of residual stress around a weld line is discussed.

Abstract: In common with all mechanical strain relief residual stress measurement methods, extra care must be taken when making measurements on components containing highly triaxial residual stress fields which are close to yield. The introduction of a free surface, created as part of the measurement procedure, can lead to plastic redistribution of the residual stress field. Usually, this is not accounted for in the elastic inversion algorithms of the experimental procedure. This paper demon­strates the usefulness and accuracy of deep-hole drilling (DHD) method [1] in a component predicted to contain a triaxial residual stress field. Previous measurements [2] are compared with the results of a DHD simulation on a type 316H stainless steel pipe containing a repair weld offset from an original girth weld. The influence of different material models was also studied.

Abstract: Residual stress measurement techniques using mechanical strain relaxation depend on a number of physical quantities and are therefore sensitive to errors associated with the measured data. The resulting stress uncertainties can easily become significant and compromise the usefulness of the results or lead to misinterpretation of the behaviour of the residual stress distributions. It is therefore essential to develop an error analysis procedure for the measurements undertaken. Error analysis procedures for the deep hole drilling (DHD) method are developed to consider triaxial residual stresses. A modified deep hole drilling method, called the incremental deep-hole drilling (iDHD), was applied to measure the near yield residual stress distributions in a cold water quenched aluminium 7010 alloy forged block. The experimental results are used to illustrate the errors.

Abstract: Laser shock peening offers potential advantages over conventional peen technologies in terms of the depth of the residual stresses that can be induced, and improvements in surface roughness. In this study the application of laser peening to thin aluminium plates such as are used in aerospace applications is investigated. Peening of thin plates presents challenges in balancing the peen intensity to prevent overpeening that will actually lower the stress field. Strain profiles for different laser peening parameters were obtained using synchrotron X-ray diffraction at the ESRF, France. Results are presented and discussed of the residual strain profiles in terms of the laser power density and the number of peen passes. When the power density and number of passes are increased the compressive strain magnitudes are also increased, as has been observed in previous studies. However, the strain components longitudinal and transverse to the peen line are not identical to each other, with the transverse component being much less compressive.

Abstract: In this work, a methodology to quantify the effect of the drilling operation, during the application of the incremental hole-drilling technique (IHD) for measuring residual stresses in laminate composites, in particular, the polymer matrix composites (PMC), is presented. This technique will allow the optimization of the drilling procedures and its parameters, enabling the quantification of the drilling effect. This quantification is obtained by using an experimental calibration procedure followed by a numerical simulation of the whole process. The direct comparison of the experimental and numerical results will allow quantifying the effect of the drilling operation. As example, the methodology was applied to the case of carbon/epoxy cross-ply laminate [0°/90°]_5s. The holes have been made by using two different drilling procedures, but the same tool geometry. High speed milling powered by air compression, a process usually employed in the case of the application of hole-drilling technique to metal alloys and a conventional computer numerically controlled (CNC) milling machine, were used. The results seem to show that incremental hole-drilling could be a reliable technique to determine residual stresses in fibre-reinforced polymers.

Abstract: The deformation behaviour of four super duplex stainless steels of the grade SAF 2507 (UNS S32750) were studied by X-ray diffraction experiment with in-situ uniaxial tensile load. The steels had different nitrogen contents, between 0.2 and 0.33%, and/or different volume fractions of the ferrite, between 37% and 49%, in balance with austenite. The development of phase-specific stresses under external loading up to over 10% tensile strain was followed. The X-ray diffraction measurements revealed that load partitioning between the phases changed with increasing applied load, as the ferrite and austenite exhibited different deformation hardening behaviours. At the onset of macroscopic yielding and low plastic strains, a load transfer from γ to α occurred due to higher yield strength and strain hardening rate of the ferrite but vice versa at larger plastic strains when the austenite hardened more rapidly than the ferrite. It was also concluded that both the yield and tensile strengthen of the steels increased with increasing nitrogen content due to increased strengthen of the austenite by additional solid solution hardening, whereas a higher volume fraction of austenite contributed to higher tensile strength.

Abstract: Damage accumulation due to fatigue significantly reduces the safety of railway vehicles. Shattered wheel rim failures are the result of large fatigue cracks that propagate roughly parallel to the wheel tread surface. The large stress, most likely due to wheel/rail impact or material discontinuity, is responsible for the initiation of shattered rims. The voids and inclusions of sufficient size in a stress field will also lead to failure of wheels. Significant improvements have been made in recent years to prevent the shattered rim failure. The ‘new’ wheels have a better resistance to the shattered rim failure, due to the fact that the circumferential residual stress on tread of a new wheel must be compressive to comply with requirements of international standard EN 13262. However, this may not necessarily apply for millions of ‘old’ wheels that are still currently in use. At the moment the residual stress measurements are carried out using destructive methods (such as slitting or hole drilling), or using quantitatively ultrasound method obtaining the average stress across the whole section. The main objective of this research was to apply non-destructive neutron diffraction method to quantitatively measure residual stress distribution of the wheel rim in as manufactured condition.