Fracture behavior under monotonic and low cycle loadings in carbon steel pipes and welded pipe joints with a defect

To evaluate the structural integrity of power plant piping during earthquakes, fracture behavior under monotonic and low cycle loadings was examined, using 4-inch carbon steel pipes made of base metal and welded pipe joints with a circumferential defect at room and elevated temperatures in air. In the monotonic bending tests, the maximum loads increased slightly with the loading rates for base metal pipes and welded pipe joints. The maximum loads in welded pipe joints were about 10% higher than those in base metal pipes for various displacement rate. In the cyclic bending tests, the number of cycles to failure, Nf was closely correlated with the ratio of the load amplitude, Pa to the maximum load, Pmax obtained in the monotonic bending tests, in spite of the difference in the maximum loads for base metal pipes and welded pipe joints. The number of loading cycles during an earthquake was assumed as about 100 cycles in design. The relationship between Pa/Pmax and the number of cycles to failure showed that the allowable load amplitude ratio was estimated as about 0.7