Papers by Author: P. Juijerm

Abstract: Different classes of metallic materials (aluminum alloys, steels, titanium alloys) were mechanically surface treated by deep rolling and laser shock peening and isothermally fatigued at elevated temperature under stress control. The fatigue tests were interrupted after different numbers of cycles for several stress amplitudes and residual stresses and FWHM-values were measured by X-ray diffraction methods at the surface and as a function of depth. The results summarize the response of the surface treatment induced residual stress profiles to thermomechanical loading conditions in the High Cycle Fatigue (HCF)- as well as in the Low Cycle Fatigue (LCF) regime. The effects of stress amplitude, plastic strain amplitude, temperature and frequency are addressed in detail and discussed. The results indicate that residual stress relaxation during high temperature fatigue can be predicted for sufficiently simplified loading conditions and that thermal and mechanical effects can be separated from each other. A plastic strain based approach appears to be most suitable to describe residual stress relaxation. Frequency effects were found to be not very pronounced in the frequency range investigated.

Abstract: The precipitation-hardened aluminium wrought alloy AA6110-T6 (Al-Mg-Si-Cu) was mechanically surface treated (deep rolled) at room temperature. The cyclic deformation behavior and s/n-curves of deep rolled AA6110-T6 have been investigated by stress-controlled fatigue tests at room and elevated temperatures up to 250°C and compared to the polished condition as a reference. The effect of deep rolling on fatigue lifetime under high-loading and/or elevatedtemperature conditions will be discussed. The stability of near-surface residual stresses as well as work-hardening states (FWHM-values) was investigated by X-ray diffraction methods. Residual stress- and FWHM-depth-profiles before and after fatigue tests at elevated temperature are presented. It was found that the investigated AA6110-T6 aluminium alloy shows cyclic softening during stress controlled fatigue tests at room and elevated temperatures. Below a certain stress amplitude at a given temperature, deep rolling can enhance the fatigue lifetime of AA6110-T6 as compared to the untreated state through cyclically stable near-surface work hardening as indicated by stable FWHM values. From the s/n data of deep rolled and polished AA6110-T6, an effective boundary line for the deep rolling treatment in a stress amplitude-temperature diagram can be established.

Abstract: The cyclic deformation behavior of deep rolled and polished aluminium wrought alloy AlMg4,5Mn in the temperature range 20-300°C has been investigated. Results of quasistatic tension and compression tests of untreated specimens in the temperature range 20-300°C are presented. To characterize the fatigue behavior for stress-controlled tests as a function of test temperature, s-n curves, cyclic deformations curves and mean strains as a function of number of cycles are given. The residual stress- and work hardening states near the surface of deep rolled aluminium alloy AlMg4.5Mn before and after fatigue tests were investigated by X-ray diffraction methods. The investigated AlMn4.5Mn aluminium alloy shows cyclic hardening until fracture at all stress amplitudes in stress-controlled fatigue tests at 25-150°C. With increasing temperature the deformation behavior shifts from cyclic hardening to cyclic softening. Below a certain stress amplitude at a given temperature deep rolling led to a reduction of the plastic strain amplitude as compared to the untreated state through cyclically stable near-surface work hardening as indicated by stable FWHM-values. This reduction in plastic strain amplitude is associated with enhanced fatigue lives. The effectiveness of deep rolling is governed by the cyclic and thermal stability of nearsurface work hardening rather than macroscopic compressive residual stresses. Since nearsurface work hardening is known to retard crack initiation, deep rolling is also effective in temperature- and stress ranges where macroscopic compressive residual stresses have relaxed almost completely, but where near-surface work hardening prevails. Above certain stress amplitudes and temperatures, deep rolling has no beneficial effect on the fatigue behavior of AlMg4.5Mn. This is a consequence of instable near-surface microstructures, especially instable near-surface work hardening.