The effects of strength mis-matching on J-integral and failure assessment line (FAL) of electron beam welded thick TC4-DT titanium alloy joints have been investigated.Elastic and elastic-plastic finite elements (FE) calculations on centre crack tensile (CCT) specimens were made,and the condition of 20% strength mis-matching was considered.The results indicate that the limit load of the joint can be substituted by that of the base metal.The error of substitution is less than 5%.The failure assessment lines for both over-matching and under-matching joints are increasingly close to that of the base metal,with the ratio of weld width to specimen width (H/W) decreasing.Moreover,the failure assessment lines of the joints can be devised using the stress-strain curves of the base metal,which will be convenient for the practical application of engineering structures The effects of strength mis-matching on J-integral and failure assessment line (FAL) of electron beam welded thick TC4-DT titanium alloy joints have been investigated. Elastic and elastic-plastic finite elements (FE) calculations on center crack tensile (CCT ) specimens were made, and the condition of 20% strength mis-matching was considered. The results indicate that the limit load of the joint can be substituted by that of the base metal. error of substitution is less than 5%. failure assessment lines for both over-matching and under-matching joints are increasingly close to that of the base metal, with the ratio of weld width to specimen width (H / W) decreasing. More over, the failure assessment lines of the joints can be devised using the stress-strain curves of the base metal, which will be convenient for the practical application of engineering structures