High‐energy X‐rays offer the large penetration depths that are often required for determination of bulk properties in engineering materials research. Photon energies of 150 keV and more are available at synchrotron sources, depending on storage ring and insertion device. In addition, synchrotron sources can offer very high intensities on the sample even at these energies. They can be used not only to obtain high spatial resolution using very small beams, but also high time resolution in combination with a fast detector. This opens up possibilities for a wide range of in situ experiments. Typical examples that are already widely used are heating or tensile testing in the beam. However, there are also more challenging in situ experiments in the field of engineering materials research like e.g. dilatometry, differential scanning calorimetry, or cutting. Nevertheless, there are a number of applications where neutron techniques are still favorable and both probes, photons and neutrons, should be regarded as complementary. A number of in situ experiments were realized at the GKSS synchrotron and neutron beamlines and selected examples are presented in the following.
Investigations on fatigue crack growth retardation due to single tensile and periodic multiple over load in strength undermatched laser beam welded 3.2 mm thick aerospace grade aluminium alloy 2139T8 sheets are conducted. The effect of overload on the fatigue crack propagation behaviours of the homogenous base metal and welded panels (200 mm wide, centre cracked) was compared using experimental and FE analysis methods. The effective crack tip plasticity has been determined in homogeneous M(T) specimens using Irwin's method and in both homogeneous and laser welded specimen by calculating crack tip plastic strain using FE analysis for single tensile overload. The crack retardation due to the overload in welded specimens are described by the Wheeler Model. The crack tip plastic zone size in the welded specimen was determined by FE analysis using maximum plastic zone extension at the mid sheet thickness. The results show that the Wheeler Model can be implemented to the highly heterogeneous undermatched weld to describe the crack retardation in fatigue following single tensile overload. Fatigue crack growth retardation due to single overload is found to be larger than the base metal. However, after periodic multiple overload, shorter crack retardation has occurred for undermatched welds than the base metal.
In order to determine retardation mechanisms due to overload and to predict the subsequent evolution of crack growth rate, investigations are conducted on crack retardation due to single tensile overload in laser weld and base material of AA6056-T6 Al alloys sheets. The effect of such overloads with different load ratios on the fatigue crack propagation behavior of the homogenous base metal and welded C(T) 100 specimens was compared in terms of crack growth rate and fracture surface features using experimental and FE analysis methods. The retardation due to overload is described in term of affected regions ahead of the crack tip. The size and shape of the crack tip plastic zone and the damage profile induced during the application of the overload in base material are predicted by FE analysis in conjunction with a porous metal plasticity model. The results show that the mechanisms of retardation in undermatched welds are substantially different from that of homogenous base material. The more significant crack retardation due to overload has been observed in the laser weld of AA6056-T6. Based on SEM observations of fracture surfaces and damage profiles predicted by the proposed FE model, the shape of the crack front formed during the application of overload can be correlated. During the overload, the crack-front extends to a new shape which can be predicted by the ductile damage model; the higher the load the more curved the resulting crack-front. These outcomes are used to determine the dominated retardation mechanisms and the significance of retardation observed in each region ahead of crack tip and finally to define the minimum crack growth rate occurred after overload.
Abstract. A conical slit cell for depth-resolved diffraction of high-energy X-rays was tested at the high-energy materials science beamline HEMS at PETRA III and used for the analysis of residual stresses in a laser beam welded steel overlap joint. With a conical slit width of 20 µm and beam cross-sections below 100 µm, depth resolutions well below 1 mm were achieved. The residual stress distributions obtained from the steel joint were in very good agreement with previous results from neutron diffraction measurements, although they were still noisier because of inferior grain statistics.
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