A. Valiente

Premature failure of prestressed steel bars

Abstract The details of two failures of prestressed steel bars are reported and a failure analysis is carried out using Linear Elastic Fracture Mechanics: the fracture toughness was measured using two standardized procedures and the pertinent stress intensity factor computed by numerical methods. The analysis of the failures show that the usual design requirements for prestressing bars fail to warn against brittle failures if damage exists. Some procedures, based on the concept of damage tolerance, are suggested to avoid similar accidents.

Stress corrosion failure of large diameter pressure pipelines of prestressed concrete

Abstract The failure of a 1.5 m diameter prestressed concrete line for water supply was examined. The water pressure opened a hole of 0.5 m 2 in the pipe wall by breaking the concrete into fragments and by tensile severing of a number of coils of the wire winding. Flexural and tensile testing of samples of the broken materials showed no damage to the concrete, but showed significant losses of strength and ductility in the prestressing steel wire. The SEM analysis of the external and fracture surfaces of the circumferential wires revealed shallow cracking and corroded areas as expected from a stress corrosion cracking process. The failure analysis presented in this paper shows that such a process was able to exhaust the damage tolerance of the affected tube until the pipeline burst under the work pressures.

Influence of heating source position and dilution rate in achieving overmatched dissimilar welded joints

Abstract The investigation addresses the overall performance of black and white joints (BWJ) of low carbon steel (LCS) and stainless steel thin sheets achieved by laser hybrid welding. Assuming that the structural integrity is directly influenced by the processing temperature, a thermal simulation of BWJ of thin sheets was developed. Afterwards, the base metals apportionment at joint formation, namely their distinct dilution rate, was originally estimated from the top surface temperature variation. Defect-free laser hybrid dissimilar welds were experimentally obtained using the selected filler metal and the energetic input from the process simulation, even for a critical analysed case of heat source displacement from the weld gap centreline towards LCS. Detailed macro and microstructural examination of the BWJ and related microhardness analysis results are presented. The tensile tests results indicate that in the case of transversally loaded BWJ, the positive difference in yield between the weld metal and the base materials protects the weld metal from being plastically deformed; the flat transverse tensile specimens loading up to failure reveals large strains in LCS, far away from the weld.

Specific properties of ferritic/austenitic Dissimilar Metals Welded Joints

This paper addresses several peculiar properties of ferritic/austenitic welded joints (Black and White Joints-BWJ) of thin sheets, which are emerging nowadays due to cost saving and satisfactory service performance. Starting from 3D thermal field modelling approached by considering Goldak’s double ellipsoidal heat source, together with a contribution of the authors, a 2D FEM-based model was used for arc welding of low carbon steel (CS) and stainless steel (SS) of thin sheets (3 mm). Conclusion helped to conduct experiments of BWJ arc welding (MMA and TIG), and laser-GMA hybrid welding, whilst infrared thermographic measurements were compared with the FEM results. Besides microstructural examination and mechanical characterization (hardness and tensile), fracture toughness and stress corrosion tests were performed on the BWJ laser hybrid welded. Whilst the model has offered important qualitative information about the asymmetric thermal field and the heat apportionment on the molten metal pool formation, the experiments have shown inadequate microstructures and constitutions of the welds for MMA and TIG welding, with respect to laser-GMAW hybrid joints. The description of the experimental program focused on B&W laser-GMA hybrid welding of thin plates aiming a characterization procedure using fracture mechanics methods is briefly presented.

Laser shock processing influence on constitutive behaviour of graded structures produced by laser welding of ferritic to austenitic stainless steel

Abstract The main objective of the present investigation was to assess the constitutive behaviour of the graded structures produced by laser shock processing (LSP) on laser welded joints made between thin sheets of ferritic and austenitic stainless steel. First, the evolution of surface strain patterns during uniaxial loading of the as welded and LSP treated joints was experimentally obtained by using a video-image correlation technique. Overall and local constitutive behaviour across the weld, heat affected zone and parent metals resulted by coupling the applied load level with the corresponding strain average of each zone. The differences found in mechanical resistance of the LSP treated zones when comparing with the as welded ones were due to the processing features, namely the generation of compressive stresses and the development of graded surface structures. Special attention was paid on changes occurred in the ferritic steel side of the joint, which acts as a mechanical cut-out fuse in structures working in energy related applications. Then, by extrapolating the fatigue fracture behaviour of macroscopic notches to the surface cavities and irregularities from which surface topography results it was found that LSP improves the ferritic steel side fatigue resistance. Additionally, the growth of eventual surface flaws is delayed by the compressive stress state induced by LSP, as indicated by in-depth X-ray measurements.