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Significant effects of stress relief on fatigue crack propagation properties of welded joints have been pointed out [i].The stress relief effects on fatigue strength of welded joints with reinforcement have been investigated [2,3].These investigations have revealed that stress relief could sometimes improve the fatigue crack propagation properties and the fatigue strength of welded joints containing tensile residual stresses.However, in the practical case, there are many kinds of weldments.Some of them may have compressive residual stresses around stress concentration sites at which fatigue cracks initiate, and stress relief for those weldments may deteriorate fatigue strength. This report describes the variable effects of stress relief on the fatigue properties of transverse butt welded joints with two kinds of plate thickness.The material used in this investigation was JIS SM50B steel for welded structures.The chemical composition and mechanical properties of the steel are given in Table i. The specimens were fabricated so that the direction of stressing was parallel to the rolling direction of the plate.Butt welded plates were made by shielded metal arc welding in flat position using a 4 mm diameter covered electrode of JIS D5016.The heat input was from 15 to 20 kJ/cm. The number of passes was two for both the first weld side and the second weld side for 9 mm thick joints; it was 29 for the first weld side and 12 for the second weld side for 40 mm thick joints.Details of the pass sequence are given in NRIM Fatigue Data Sheet No.514]. T r a n s v e r s e butt welded specimens of 50 mm wide were machined from the welded plates and fatigued on electrohydrauiic fatigue testing machines in laboratory air at room temperature.The stress ratio was zero.In order to measure the fatigue crack propagation rates, two step programmed tests were conducted.That is, the stress range was periodically varied between A~ H and A~ L keeping the maximum stress constant to obtain beach markings.The fatigue crack propagation rate was calculated by direct division of the beach mark spacing by the number of stress cycles in each step. Some specimens were thermally stress relieved by heating them for 13 hours at 630 deg C. The heating and cooling rates were less than 35 deg C/min above 200 deg C.Residual stresses parallel to the loading direction were measured along the plate thickness by strain gage method [5]. Gages were adhesively bonded at the location of weld toe after removing the weld reinforcement, and stresses were calculated according to (i) at each stratiform cutting of the opposite surface.When the thickness was reduced to half, additional gages were bonded to the machined surface, and (2) was used to determine the stresses: Int Journ of Fracture 24 (1984) R82 Z ~i = E 2~ 3(t-z) dz (i) 2 dz (t-z) 2 Z v