Research On Fatigue Design And Assessment Of Ultrasonic Peening Treated Welded Joints On Hot Spot Stress Method

Hot spot stress method plays an important role in the fatigue assessment of welded structures. This paper mainly includes three parts: the analysis of hot spot stress concentration coefficient and its influence factors of several commonly used welded joints, the fatigue assessment of welded joints of Q345 B based on hot spot stress method and the fatigue design by hot spot stress of ultrasonic peening welded joints of medium and low strength steel, high strength steel, aluminium alloy and titanium alloy.First the hot spot stress concentration coefficient of butt joint(BJ), transverse cruciform joints(TJ), longitudinal fillet welded plate joints(LJ) and side gusset plate joint(GJ) was researched. The results show that: the geometric parameters near the weld toe has very little effect on the hot spot stress concentration coefficient of welded joint, but the plate thickness of TJ and LJ has a certain impact on the hot spot stress concentration factor while the gusset length LJ and GJ has an obvious influence on the hot spot stress concentration factor.Then the fatigue strength of Q345 B is evaluated by hot spot stress. Research shows that: the correlation coefficient is 0.849 when the nominal stress approach is used while the correlation coefficient improves about 8.48 % to 0.921 when used hot spot stress, which means the data dispersion decreases and we can use the same S-N curves to analyze the fatigue design, which greatly reduced the dependence of fatigue level on the form of welded joints.Finally, the hot spot stress design of ultrasonic peening welded joints of medium and low strength steel, high strength steel and titanium alloy was researched. Studies show that:(1) The hot spot stress design curve of ultrasonic impact treated butt welds, non-load-carrying transverse cruciform joints and non-load-carrying longitudinal fillet welds of medium and low strength steel can be expressed with one design curve: Nlog108.27logD-=s and the fatigue strength can uniformly applied to FAT135 at 2×10~6 times.(2) The hot spot stress design curve of ultrasonic peening welded joints of high strength steel can be expressed with one design curve: Nlog101.29logD-=s and the fatigue strength can uniformly applied to FAT190 at 2×10~6 times.(3) The hot spot stress design curve of different high strength steel with yield strength of more than 355 MPa should be further divided according to the strength level, not all use FAT140.(4) It is recommended that the hot spot stress design curve of ultrasonic peening welded joints of aluminum alloy should select different FAT value according to the strength of material, not just using FAT50 as IIW recommended.(5) The hot spot stress design curve of ultrasonic peening welded joints of titanium alloy can be expressed with one design curve: Nlog105.30logD-=s and the fatigue strength can uniformly applied to FAT150 at 2×10~6 times.