This paper examines the evaluation of stress intensity factor (SIF) for surface cracks in a non-uniform residual stress field. A technique that based on utilizing the crack face traction (CFT) integral implemented in the interaction integral method using an open source code for models with a particular crack-block mesh density was presented. 3-D flat plate models with different crack-block mesh densities were employed under different loading conditions to examine the accuracy, conditions and easiness to use of the present technique. The influence of the crack-block mesh density on the calculated SIF solutions due to the application of non-uniform tractions over the crack face was discussed. A considerable improvement of SIF solutions calculated by crack face tractions was observed when the crack-block mesh density becomes fine enough. The effectiveness of the present technique was then validated for different 3-D numerical examples of welded joints under different loading conditions. The present technique was thereafter employed to evaluate SIF for a surface crack in a butt welded joint using real welding residual stress. The SIF solution calculated due to the application of welding residual stress was verified with that given by mechanical loading (without welding residual stress).