The Deformation Behaviors And The Strength Of Welded Joints With Mechanical Properties Mismatch

Mechanical properties of weld metal and HAZ of welded joints usually differ from those of base metal.The mismatches in properties strongly influence the strength and the fracture toughness of welded joints. Because the limit yield load and crack drive force are important input parameters of defects assessment procedures, plenty of researches were focused on the cracked specimens with strength mismatch. Approaches for assessing the strength of welded joint with mechanical property mismatch are prerequisites for desighning weldment properly in engineering. Quantitive relations between joint strength and strength mismatch factors including geometric size of HAZ and weld metal are important to practical design of joint strength. In the present paper, the finite element analyses and tensile test are carried out for welded joints with strength mismatch, to completely investigate the above problems in four aspects as follows.Firstly, the finite element analyses of tensile specimens are carried out for different welded joints with mechanical property mismatch. The results show that yield stress and relative fraction of softened HAZ has more effects on strength loss of weld joints, and beyond same critical values of which the tensile strength of joint will decrease strongly. Overmatch weld metal can compensate the yield strength and tensile strength in HAZ-softened weld joints. The length of specimen has certain effect on the determination of yield strength. The strengths of mismatch joints increase with increasing the specimen width, and reach constant as specimen widths come to certain values. In addition, the accuracy of joint strength estimation with finite element method was verified by tensile test on the joints with weld metal undermatch and HAZ softening. A practical method of strength assessment for mismatch joints was presented.Secondly, based on results of the finite element analyses, the stress distribution in tensile plate specimen with HAZ softened butt-welded joint show the characteristics as follows : the Mises equivalent stress is higher than the applied stress in weld metal and base metal adjacent to softened HAZ. At the interfaces between HAZ and weld metal and base metal , Mises equivalent stresses change in step manner, their amplitudes are determined by applied stress and the differences of yield stresses in the both sides. The Mises equivalent stress increase with the power law in the welds and base adjacent to softened HAZ The integration of the decrease in the Misesequivalent stress in the softest zone equals to sum of that of the increase in the weld metal and base metal zones. Based on the results of finite element computations, analytical expressions were derived to relate the distribution of Mises equivalent stress to the strength mismatches factor, fractions of HAZ and weld metal and the geometric parameters of specimen. In addition, the mechanism of effects of joint mismatch on their strength has been discussed on basis of the stress distribution in joint.Thirdly, Based on the stress distribution in tensile plate specimen with softened HAZ butt-welded joint, an approach to predict the tensile strength of such joints is presented as follows: (1) calculate equivalent stress distributions according to the strength mismatch factors and specimen sizes of a given welded joints, (2) determine overall deformation by integrating local elongation along the specimen on basis of the relation of equivalent stress and strain, (3) obtain the nominal stress strain curve to determine the joint strength. This analytical approach is successfully used to accurately predict the tensile strengths of strength mismatch joints proposed in present work and other literatures. Further, the accuracy of this analytical approach was verified by the tensile test on the evenmatch and overmatch joints of pipeline steel.Finally, The functional relations between strengths of mismatch joints and specimen width have been presented by fitting equations to the data, which are calculated by the analytical method. The accuracy of these functional relations were verified by tensile tests on specimens, which have 5 different specimen widths and were cut from 12 kinds of mismatch joints. By determining the strength value of the specimen with a certain width, and measuring the strength mismatch factors of HAZ and weld metal instead of measuring the their lengths, the strengths for infinite and arbitrary specimen width can be calculated by these functional relations, hi this way, the practical approach was provided to assess the strength of mismatch welded joint.