Berthold Scholtes

Cyclic deformation and near surface microstructures of shot peened or deep rolled austenitic stainless steel AISI 304

Cylindrical specimens of the austenitic stainless steel AISI 304 were shot peened or deep rolled with different peening intensities, and rolling pressures, respectively. The resulting near surface properties were characterized by cross sectioning transmission electron microscopy (TEM), residual stress and phase analysis as well as interference line half-width and microhardness measurements. Cyclic deformation curves were obtained by hysteresis measurements under stress control with zero mean stress. The microstructural alterations in the fatigued surface regions were again characterized by the above mentioned methods. The investigations revealed that both shot peening and deep rolling lead to a complex near surface microstructure, consisting of nanocrystalline regions, deformation bands and strain induced martensitic twin lamellae with high dislocation densities in the austenitic matrix. These microstructural changes severely influence the cyclic deformation behaviour: Plastic strain amplitudes and cyclic creep were drastically decreased by shot peening and especially by deep rolling. Both surface finishing methods were found to decrease crack initiation and propagation rate. Remarkably, the initial residual stress profile and surface strain hardening were not completely eliminated even by applying high cyclic stress amplitudes. This is due to the fact that during cyclic loading dislocation cell structures were only formed in greater depths whereas the nanocrystalline layer remained stable. In the case of deep rolled surfaces, the martensitic layer was even increased by fatigue-induced martensite formation.

Cyclic deformation and near surface microstructures of normalized shot peened steel SAE 1045

Cylindrical specimens of the normalized plain carbon steel SAE 1045 were shot peened and cyclically deformed under stress control. A special cross-sectioning technique was applied to prepare near surface regions for transmission electron microscopy. The resulting properties of these regions were characterized by X-ray diffraction and microhardness measurements. The cyclic deformation curves are affected characteristically by dislocation arrangements induced by shot peening. The stability of these dislocation arrangements and the depth profile obtained by residual stress and interference line half-width value measurements depend strongly on the stress amplitude. High stress amplitudes lead to the formation of dislocation cell structures in the shot peened layers, accompanied by the total disappearance of compressive stresses and the decrease of interference line half-width values down to the initial level. Thus, a clear correlation between stress relaxation, cyclic deformation behaviour and microstructural alterations in shot peened regions is revealed. Experiments carried out on shot peened hollow specimens indicate that the cyclic deformation behaviour is not merely a function of dislocation arrangement and density but also of the thickness of strain hardened near surface material in relation to the diameter of the specimen.

Determination of residual stresses in ceramics and ceramic-metal composites by x-ray diffraction methods

Abstract A survey is given of the research done in the field of X-ray residual stress determinations on ceramics and ceramic-metal composites. The principles of X-ray residual stress determinations are described and helpful hints are given for X-ray stress determinations in ceramic materials. X-ray elastic constants of common ceramics are included. A number of interesting examples of X-ray residual stress determinations on ceramics and ceramic-metal composites are presented.

Deformation Asymmetry of AZ31 Wrought Magnesium Alloy

In this work, the mechanical behaviour of an AZ31 wrought magnesium alloy due to tensile, compressive and four-point bending tests was analysed. Several specimens of this material were taken in different directions from a rolled plate. Tensile and compression tests allowed to characterise the mechanical properties of the material in the rolling and cross rolling directions. To take the effect of the anisotropy induced by the rolling procedure itself into account, the bending tests were carried out using specimens with four different orientations. Six strain gages per specimen allowed to observe the evolution of the strain with the bending moment during the tests. All bending specimens were bent until a total compressive deformation of 2.5%. It was observed that the corresponding tensile strain was significant lower and slightly different for each orientation of the specimens. In addition, a characteristic non-uniform distribution of deformation twinning was observed. The induced residual stresses after bending were characterised by X-ray diffraction (XRD) and incremental hole-drilling (IHD). Due to the different mechanical behaviour in tension and compression, an asymmetric residual stress distribution after bending could be observed. The neutral axis was, in all cases, shifted towards the tensile side. This observation agrees with the strain measurements during bending tests. In addition, an example of the influence of the asymmetric deformation behaviour during fatigue was given.

Isothermal strain-controlled quasi-static and cyclic deformation behavior of magnesium wrought alloy AZ31

Abstract The low-cycle and high-cycle fatigue behavior of the Mg-base wrought alloy AZ31 is investigated in the temperature range 20–300°C using total-strain-controlled push–pull tests. It is shown that the mechanical properties of the material are substantially influenced by the direction dependent formation of deformation twins and resulting deformation asymmetry. As a consequence tensile mean stresses develop during total-strain-controlled fatigue tests without mean strain.

Use of the hole-drilling method for measuring residual stresses in highly stressed shot-peened surfaces

The same shot-peening treatment was applied to five steels with different mechanical properties. The induced residual stress profiles were analyzed using X-ray diffraction and incremental hole drilling (IHD). The results of both techniques showed that IHD can still be successfully used for measuring shot-peening residual stresses, even if these exceed the yield strength of the bulk material. Expected errors due to the plasticity effect are reduced by the strain hardening of the surface. For an assessment of the reliability of IHD data, strain-hardening variation was quantified by microhardness measurements to estimate the yield strength of the plastified layer. All the main calculation methods for IHD were applied. The results were compared and discussed with respect to the characteristics of each method.

Experimental and theoretical investigation of the residual stress distribution in brazed ceramic-steel components

Abstract The residual stress state of brazed ceramic-steel components was investigated by neutron diffraction experiments. In addition, X-ray residual stress measurements were performed at the surface of the specimens. The neutron data show clear evidence of plastic deformation in the steel during cooling from the brazing temperature. Finite element calculations modelling the temperature dependent elastic-plastic behavior of the materials correspond very well to the observed residual stress distributions.

Determination of residual stresses in autofrettaged steel tubes by neutron and X-ray diffraction

Abstract The residual stress state of an autofrettaged 30 CrNiMo 8 steel tube has been investigated by neutron and X-ray diffraction. The results of X-ray measurements on the face of a ring cut out of the tube agree well with the neutron results obtained on the central part of an unmachined specimen. The experimentally determined residual stress distributions resemble only to a first approximation that expected from theoretical estimations. As expected, the tangential stress component at the inner diameter was found to be largest and is compressive. However, the distributions of the tangential and radial residual stress components show large deviations from the expected values. The differences between experiment and theory are probably due to neglecting the Bauschinger effect, the decarburization of the surface layers and the residual stresses produced by an alignment procedure before the autofrettage process. Annealing the tube for 1 h at 450 °C has little effect on the residual stress distribution, except for a reduction in the steep gradient of the tangential stress component close to the inner surface.

RESIDUAL STRESSES INTRODUCED BY MACHINING

A short survey is given about the elementary processes influencing the generation of residual stresses during chip-forming machining operations. Typical residual stress distributions, introduced by widely used machining processes are presented and their consequences on the fatigue behaviour of metallic materials are briefly discussed.

Residual Stress Relaxation and Cyclic Deformation Behavior of Deep Rolled AlMg4.5Mn (AA5083) at Elevated Temperatures

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.