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.

A study was carried out on laser and resistance spot welds in overlapped sheets of dual phase advanced high strength steel (DP780) and deep drawing steel (DC04) of 2?0 mm in thickness. The aim of the study was to investigate the fatigue performance of these joints under tensile shear loading as well as the monotonic performance for applications in the automotive industry. The mechanical properties, failure behaviour and fatigue life analyses of spot welds in similar and dissimilar joints were investigated by experimental and numerical methods. The structural stress concept was used to describe the fatigue lives of spot welded specimens. The results revealed different failure types with different fatigue behaviours for laser and resistance spot welds under the application of cyclic loads at 'high load' and 'low load' levels.

a b s t r a c tExperimental and numerical studies were conducted to characterize laser and resistance spot welds to gain an understanding of load carrying capacity, temperature distributions and residual stress states of different joint geometries used in the automotive industry. Different laser spot weld path geometries are compared with conventional resistance spot welds to find the residual stress distributions in each. It was found out that the weld region in laser spot welding is surrounded by a compressive region which has higher compressive stress values and larger size than that of resistance spot welds. Simulations showed good agreement with experimental temperature distributions, and were able to qualitatively predict the residual stress distributions in each of the weld geometries. The thermal history at known failure locations within the welds and the influence of the weld geometries on cooling rate are also discussed.

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.

This study is carried out to investigate the laser spot welds (LSWs) of advanced high strength steel sheets of 1?0 mm thickness for application to the automotive industry. Mechanical properties and failure behaviour of LSWs for transformation induced plasticity steel sheets (TRIP800) subjected to monotonic coach-peel loading are investigated by experimental and finite element (FE) simulation methods. Microstucture of LSW and fracture surfaces of the welded specimens were examined by scanning electron microscopy (SEM) to describe the microstructural features and to clarify the crack initiation mechanism respectively. Based on simulation solutions equivalent plastic strain was obtained to describe local deformation of LSW joints. Experimental results revealed a 'plug type' of failure mode of the circular LSW joints under 'peel-coach' loading condition. This type of ductile failure is the most common failure mode for spot welds used in the automotive industry. Numerical simulation of damage process was compared with experimental results and this revealed that fracture path was successfully predicted.

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.

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The present study is carried out to investigate fatigue crack initiation and kinking behaviours of spot welded coach peel (CP) specimens of low carbon steel sheets subjected to cyclic loading by experimental and finite element analysis methods. Evaluations of fatigue crack growth stages were performed by crack tip plastic strains and J integral analyses and also by microhardness measurements on process zone. According to the experimental and analytical results, fatigue crack initiation and growing stages in the spot welded CP specimens can be divided to three stages. Stage I corresponds to 'gap sharpening stage' observed at the beginning steps of cyclic loading with crack growing on the interface plane between the overlapped sheets. Stage II corresponds to kinked crack initiation and propagation through the sheet thickness observed after applying a certain number of loading cycles. Stage III corresponds to crack propagation through the width of the specimens observed at the final step of the fatigue crack propagation. The FE results of the crack paths and crack locations are in good agreement with those of experimental observations.