Abstracts:A FE seam weld model has been built for advanced fatigue structural stress analysis. The shell element model allows both weld toe and weld root fatigue assessment. In order to validate the model, this project deals with the complete analysis of a last decade manufactured vehicle for which experience feedback is possible. The vehicle structure is mainly built with steel sheets and the assembly is performed by seam welding. Shell element model is appropriate for FEA. The entire FEM preparation is presented hereafter and each step is detailed: 3D CAD weld building, mid-surface idealization, weld leg imprint creation, weld mesh connection and load application. Fatigue analysis is performed based on a fatigue limit approach and IIW FAT data. For every structure location a Haigh diagram is built. Different diagrams are presented for plain metal, cut edge, weld toe and weld root. The FEA results are post processed and each appropriate tensor component is taken into account. For plain metal, stress variation along the worst principal stress orientation is calculated. In welds, for both toe and root locations, the structural stress computation based on grid point forces extraction is described. Longitudinal shear stress in weld and at weld toe as well as weld longitudinal normal stress calculations are detailed. Fatigue results are displayed for each location over the whole chassis model. Finally, vehicles built during the last decade are inspected back from the customers experience field. Observed crack types and locations are compared to fatigue analysis predictions.

Abstracts:The 3rd Edition of API 579-1/ASME FFS-1 2016 Fitness-For-Service includes a new Part 14 dedicated to fatigue assessment. An important section in this part covers the fatigue assessment of welded joints. In this paper, an overview of the fatigue methods for welded joints is provided and extensions are recommended. First, an overview is given of the classical fatigue method used in the ASME B&PV Code based on smooth bar fatigue curves in conjunction with a fatigue strength reduction factor. In addition, the mesh insensitive structural stress method is outlined using an equivalent stress parameter based on fracture mechanics considerations in conjunction with a master S-N curve based on the analysis of over 2000 high and low cycle S-N test data. The resulting master S-N curve approach is applicable to high cycle fatigue and low cycle fatigue if a Neuber correction is introduced. In this paper, a new structural strain method is presented to extend the early structural stress based master S-N curve method to the low cycle fatigue regime in which plastic deformations can be significant while an elastic core is present. With this new method, some of the inconsistencies of the pseudo-elastic structural stress procedure can be eliminated, such as its use of Neuber’s rule in approximating structural strain beyond yield. The earlier mesh-insensitive structural stress based master S-N curve method can now be viewed as an application of the structural strain method in the high cycle regime, in which structural strains are linearly related to traction-based structural stresses according to Hooke’s law. Thus, both low cycle and high cycle fatigue behavior can now be treated in a unified manner. In the low-cycle regime, the structural strain method characterizes fatigue damage directly in terms of structural strains that satisfy a linear through-thickness deformation gradient assumption, material nonlinear behavior, and equilibrium conditions. A PVRC Joint Industry Project is currently sponsoring work on the structural strain method that will lead to its incorporation in the next edition of API 579-1/ASME FFS-1.

Abstracts:During a still running European Project the feasibility of vehicle light weighting technologies for the manufacturing of light urban electric vehicles with new standards of mechanical performance will be demonstrated. One of the innovative technologies of the project, the Electromagnetic Pulse Technology (EMPT), which is a high speed crimping method, will be applied for joining of particular structural parts of the body, especially to join different lightweight materials such as Al- or Mg-tubes and Al- and Mg-cast or forged nodes.

Abstracts:Load capacity of keyway couplings is usually calculated according to standards (e.g. DIN6892, DIN743) based on nominal stress and simplifying assumptions dating back to several decades. Detailed modeling of keyway couplings is still a research topic, because of the complex mechanical behaviour involved. Moreover, the current standards apply only to usual geometries the designer sometimes needs to depart from, especially for the sake of compacity.

Abstracts:In this study, several aspects regarding effect of quality on the fatigue strength in welded cut HSS have been investigated and are discussed. A novel numerical algorithm has been developed which assesses the welded surface and calculates and quantifies weld quality parameters and the presence of defects which are critical in fatigue applications. The algorithm is designed for implementation in serial production. It will provide robust and reliable feedback on the quality being produced, which is essential if high strength steels are utilized and high quality welds are necessary for the structural integrity of the welded component. Two welding procedures which can increase the weld quality in as welded conditions have been assessed. It was found that by using these methods, the fatigue strength can be increased with 20% compared to normal weld quality. Furthermore, two fatigue assessment methods ability to account for increased weld quality in low cycle and high cycle fatigue applications has been studied. One of these methods showed sufficient accuracy in predicting the fatigue strength with small scatter and also account for increased weld quality. The influence of surface quality on cut edges was studied and the fatigue strength was estimated using international standards and a fatigue strength model for cut edges. It was found that the fatigue strength in testing was 15-70% higher compared to the estimation, thus proving a weak link between the international standard and fatigue strength.

Abstracts:In this work, three types of structural modified epoxy adhesives were used to investigate the effect of stress concentrations on the fatigue behavior of notched bulk specimens. SN curves of un-notched and notched specimens were determined at constant amplitude and R = 0.1 in the range between Nf = 10³ (LCF) and Nf = 10⁶ (HCF). The following key conclusions were made: (i) fatigue strength was reduced due to the presence of notches, especially at the HCF; (ii) adhesives showed different values of notch sensitivity with values for the adhesives lower than typical values of metals; (iii) for un-notched samples fatigue strength was between 62 and 78% of tensile strength for Nf = 10³ and around 50% for Nf = 10⁶; (iv) for notched samples fatigue strength was between 67 and 78% of the tensile strength for Nf = 10³ and around 40% for Nf = 10⁶; (v) fractography evidenced the presence of voids and shear yielding around the notches, (vi) unnotched samples showed the same fracture behavior for both LCF and HCF with crack formation at the external surface. For notched samples there was a significant distinction between LCF and HCF with cracks forming at the notch root.

Abstracts:In the present work, the Fatemi-Socie approach is adopted in order to analyze the fatigue endurance of welded joints under multiaxial loads. This critical plane criterion has already been successfully applied to plain or notched components, however, it is not spread in the assessment of welded joints, yet. This work is focused on the practical implementation issues related to this particular application, which has not been discussed in the literature. The described procedure is adopted for the assessment of one hundred experimental tests and some preliminary results are shown. The specimen under investigation is a pipe-to-plate fillet joint made out of structural steel (S355JR). The tests were performed under both uniaxial and multiaxial, i.e. combined in-phase and out-of-phase bending and torsion, load conditions with a constant amplitude at the laboratories of the University of Pisa, Italy.

Abstracts:The design of parts and structures in the vehicle development process is highly influenced by strength and stiffness requirements. Under cyclic loading a change in the local stiffness of joints can be observed which motivates analyses in order to tackle stiffness changes in structures. The fatigue and stiffness behaviour has been obtained for several joining techniques and material combinations: spot and seam welds, remote laser welds and high velocity bolting for steel and/or aluminium partner sheets. Specimens under shear and peel loading as well as hat profiles under torsion have been tested under constant amplitude loading. Quasi-static tests and micrographic evaluations have been conducted in order to determine material parameters and the setup of FE models. Online stiffness measurements during fatigue life have been performed. In this paper an approach to describe degradation in specimens representing a joint detail is presented. The stiffness degradation curves for the various joining techniques are compared and a numerical description of the stiffness degradation behaviour for further simulations is derived. The concept of simulating stiffness changes in conventional joining techniques is introduced. First numerical analyses of stiffness changes in spot welded specimens are presented. Several sensitivity analyses, e.g. with regard to R-ratio or loading amplitude, are performed in order to check the numerical stability of the implemented method.

Abstracts:The roads represent an important heritage owned by the French Ministry of Transports. Even more than corrosion, the fatigue is the principal aging process that affects the durability of steel bridges. Several examples illustrate in the article the importance of affecting a right consideration to the fatigue design of bridges.

Abstracts:The electromagnetic pulse technology (EMPT) is a joining process similar to the explosion welding. In the process, a flyer-sheet is accelerated towards a target-sheet using electromagnetic fields. During impact high normal and lateral contact forces are generated which join the sheets on an atomic level. This process can be automated for joining thin sheets and is therefore especially suited for mass production, e.g. in the automotive industry. The resulting joints are not comparable to ones made by common MIG/MAG or laser welding processes since no heat is induced. This leads to the question if the standard approaches to assess the weld in terms of their fatigue behavior can be used also and if a reliable fatigue assessment can be achieved. In the paper, the fatigue strength of EMPT-welded connections under constant amplitude loading is presented. Common assessment approaches like the notch stress approach are applied to assess the fatigue life. A comparison between numerically derived fatigue life and experimental tests shows a good agreement and proves the applicability of established approaches.