Effects of residual stresses on the fatigue behaviour of welded steel structures

Residual stresses due to the welding process in steel structures can significantly affect the fatigue behaviour. Usually, high tensile residual stresses up to the yield strength are conservatively assumed at the weld toes. This conservative assumption can result in misleading fatigue assessments. Areas with compressive residual stresses may be present in complex structures, where the details are less critical than predicted. This is shown in the paper by the example of fillet‐welded stiffener ends, where beneficial compressive residual stresses cause the initiation of fatigue cracks at other locations in less‐strained areas. Another example for the effects of residual stresses concerns the stress initiation and propagation at a structural detail under fully compressive load cycles. Fatigue cracks are possible here due to high tensile residual stress fields. The conclusion is that the welding‐induced residual stresses should be known in advance for a reliable fatigue assessment, which becomes possible to an increasing extent by numerical welding simulation.     

Residual stress formation after re‐contouring of micro‐plasma welded Ti−6Al−4 V parts by means of ball end milling

During the regeneration of damaged components such as compressor blades, the weld repair is followed by machining processes. Excess weld material is removed in a cutting process in order to reproduce the final contour. Therefore, both processe have to be considered regarding the resulting surface and functional properties of the repaired component. In this study, bead on plate welds are produced on 10 mm Ti‐6Al‐4 V sheets using micro‐plasma welding with different levels of welding current. They are subsequently re‐contoured via ball end milling using tools of different cutting edge radii. The residual stress depth profile is measured using X‐ray diffractometry. It is shown that the final residual stress depth profile is mainly influenced by the milling process when machining with rounded cutting edges.     

Eigenspannung im mit gasdruckunterstütztem Feingussverfahren hergestellten langfaserverstärkten Aluminium‐Matrix‐Verbundwerkstoff

Residual Stress in Continuous Fibre Reinforced Aluminium Matrix Composites Prepared by Modified Investment Casting The residual stresses between matrix und fibres in the continuous γ‐Al₂O₃ fibre reinforced aluminium alloy (AlZn6Mg1Ag1) matrix composites prepared by modified investment casting were measured with x‐ray diffraction as well as simulated with FEM. It was indicated as expected that tensile residual stress exists in the Matrix und compressive residual in the fibre. The average value of the residual stress in both matrix and fibre in the composites is not very significant. However it is distributed very unevenly. Next to the interface between matrix and fibre there is a small zone in the matrix with relative great tensile residual stress. The effect of fibre volume percentage on the residual stress in the composite was also analysed. With increase of the fibre volume percentage the tensile residual stress in the matrix increases while the compressive residual stress in the fibre decreases. If the fibre volume percentage in the composite exceeds 65 %, the maximal tensile residual stress will reach the yield stress of the matrix alloy and local plastic deformation will occur.     

Makroskopische Eigenspannungen in einem selektiv langfaserverstärkten Aluminiumbauteil

Macroscopic residual stresses in selectively long fiber reinforced aluminum parts The macroscopic residual stresses between reinforced and unreinforced zones in with different volumetric contents selectively (locally) long fiber reinforced aluminum samples were measured with the borehole method as well as simulated with FEM. The results indicate that there exist obvious macroscopic residual stresses in the samples, which are distributed very unequally. The residual stresses acquired from both methods agree well with one another. It was found that the macroscopic residual stresses at most locations in both the reinforced and unreinforced zones were not harmfully high. However, in the unreinforced zone very close to the boundary, a small zone of very large tensile residual stress was found parallel to the boundary. In the reinforced zone, a small zone with a quite large tensile residual stress was found in the vertical direction of the fiber. These high tensile residual stresses should be taken into consideration in structure designing and further production of the parts. The influence of fiber volumetric contents in the reinforced zone on the macroscopic residual stresses was analyzed with date comparisons. It was indicated that the fiber volumetric contents did not change the distribution trend very. However, with the increase of fiber volumetric contents the macroscopic residual stresses in the selectively long fiber reinforced samples rise.     

Röntgenographische Spannungsanalyse in einzelnen Kristalliten – Mess‐ und Auswerteroutinen sowie Einflussgrößen

X‐ray Stress Analysis in Single Crystallites – Measuring Method and Analysis Routines The X‐ray stress analysis in single grains of a coarse grained material can be performed by using the single crystal method. In this work theoretical basics and measuring algorithms for the single crystal method are presented. Test measurements on an iron‐silicon alloy were carried out for different wave lengths and for different lattice planes. It is demonstrated that the Mo‐Kα radiation is well suited for the X‐ray stress analysis in single grains of the material investigated.     

Minimizing Stress and Distortion for Shafts and Discs by Controlled Quenching in a Field of Nozzles

A coupled gas‐dynamical and thermo‐mechanical model for simulation of the gas flow, gas and specimen temperature, phase, stress, strain, and displacement transient‐fields during quenching of cutting discs and shafts of steel is introduced. The material properties (e. g. density, conductivity, heat capacity, hardness) are obtained by homogenization procedures. The material behaviour is described as an extension of the classical J₂‐plasticity theory with the extension of temperature and phase fraction dependent yield criteria. The coupling effects such as dissipation, phase transformation enthalpy, and transformation induced plasticity (TRIP) are considered. Simulations were carried out for cutting discs of knives, and for shafts made of steel SAE 52100 with varying diameter. For the validation of the simulations, these work pieces were heated in a roller hearth kiln up to 850 °C, and than quenched in a field of nozzles in which the heat transfer coefficient was known and could be locally adjusted by the volume flow of each nozzle. The phase fractions, surface hardness, distortion, and residual stresses were measured. The simulated and measured results fit quite well. According to optimization‐simulations the shafts were quenched with a certain heat transfer coefficient distribution. The bigger diameter parts of the shaft were more intensively quenched by an increased gas flow so that the hardness profiles were equalized and the residual stresses at the edges were significantly reduced.     

Eigenspannungsmessungen an einem geschweissten Kastenprofil

Residual stress measurement on a welded box beam section The topic of this work is the measurement of the residual stress state of a welded box girder section. The investigation was performed in three main steps consisting of firstly the determination of the global residual stress state, secondly the measurement of local residual stress condition at the positions of interest, and thirdly the superposition of global and local stresses to the complete residual stress state at the respective position. The stress determination was performed using the cut‐compliance method (CC method) and X‐ray measurements. Both measurements methods complement each other very well. The CC method provides information at a great depth below the surface, while the X‐ray method offers high resolution at low depth. The results of both methods have been compared and were in good agreement, validating themselves. The investigations at the corners of the welded box girder section revealed tensile residual stresses. At and just below the surface influences of the thermal cutting process of the steel sheet have been found, resulting in a lowering of the tensile stresses. However, in the middle of the beam section high tensile stresses of up to 430 MPa have been found, nearly reaching the proof stress of the material. The consequences of such high tensile residual stresses on design and use of the beam will be discussed.