CONSTRAINT EFFECT AND FRACTURE OF MISMATCHED WELD JOINTS. E. Ranatowski Technical University, Mechanical Department, St. Prof. S. Kaliskiego 7, 85-796 Bydgoszcz, Poland ABSTRACT The paper deals with some aspects of the fracture resistance of mismatched weld joints with taken into consideration the constraint effect. At first the attention is focused on the relation between microstructure and selected mechanical properties. Physical models for the development of microstructure have the potential of revealing new phenomena and properties. This aspect is illustrated with presenting a brief consideration of the constraint effect in relation: microstructure - mechanical properties. The same problem is account in the macroscopic scale of the heterogeneous weld joints. After formulating a simplified model of mismatched weld joints a concise review of stress was made at interface between zones (W) and (B). Conclusion from above analysis form a constraint parameters K W un/ovwhich were used to an assessment of the fracture parameters such as ratio of driving forces δ δ R un and R ov. INTRODUCTION In recent years the study of fracture from microstructure of materials by means of modern theoretical physical methods is an active field in the theory of fracture. A rational approach towards the design of welding alloys, joints and procedures can benefit from the development of quantitative and reliable models. Considering the completeness of theory and the practical necessity of weld joints designing for strength, fracture toughness, some questions must be answered. One is how the macromechanical quantities can be derived from the microscopic mechanism taking into account local structural heterogeneous fluctuation of materials in weld joints. Another is how deduce a quantitative description of the strength toughness properties of a welded joints with non-matching weld metal or HAZ. Obviously, this is a complicated and difficult problem to be solved. Above question is very important under joint design of engineering structures. Welding is probably the most popular manufacturing process for joining metals used in structural applications. Some groups of weld joints are often highly inhomogeneous. For example this can the place during the welding of toughened steel and strain or age hardened steel, etc. In this situation we will focus our attention on a model in which the weld metal or part of the heat affected zone (HAZ) is imitated by layer (W) - fig. 1a. Strength mismatching occurs as an overmatching - fig. 1b or as an undermatching - fig. 1c. The essential physical phenomena affecting the mechanical properties of this model occur at the interfaces of zones (B) and (W) - fig. 1a. The presence of the interfaces in these models naturally gives rise to mechanical constraint on the weld joints. A fracture safe design also can be influenced by constraint. The analysis of failure in a structural component depends on two inputs, the fracture behaviour and deformation behaviour - both depend on constraint. Current work has concentrated more on looking at constraint effects on the fracture behaviour.
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