Low cycle fatigue of welded joints: new experimental approach

摘要

To take into account the reduced fatigue strength of welded joints, a reduction life factor applied on fatigue curves (J_f value) was introduced into the RCC-MR [Design and Construction Rules for Mechanical Components of FBR Nuclear Islands, AFCEN, 1993], for the design and construction of fast breeder reactors. To better assess this factor, previous work showed that mechanical behavior of a welded assembly is influenced by the geometry of the weld and by the interaction of the different cyclic plastic behavior of the two materials: base metal (BM) and weld metal (WM). A new procedure (named FFAST) was performed on welded joint specimens extracted from butt-welded pipe connections (uniaxial tensile-compressive load). An innovative experimental approach is proposed to study the local mechanical behavior of the welded joint specimens and then determine the J_f parameter. The main advantage of the method is to avoid problems due to the relative stiffness of weld part versus the BM part of the specimen. A continuous recording of the stress and strain in the weld allows an estimation of the mechanical behavior and finally the fatigue life of the joint. Observations of the crack surface show two different crack initiation zones near the weld depending on the load level. Calculations of the tests and comparison with experimental results are presented. These studies make it possible to assess in a practical way the J_f design method. It appears that J_f value cannot be considered as a single value for it is influenced by several factors depending on the weldment and on the load level.