Four-Point Fatigue Testing of Pressurized Composite Pipe

This technical paper presents a comparative study of the fatigue strength of high-pressure composite pipes and high-pressure composite pipes containing joints. The test specimens used in this experimental investigation were exposed to cyclic bending stresses and to cyclic bending stresses in combination with constant or cyclic internal pressures generated by water, brine, and crude oil. The extrinsic influence of elevated temperature on fatigue performance was also examined. A new four-point bending fatigue machine was developed at the University of Akron to accomplish the testing. For each test specimen, two types of failure were distinctly observed. After a number of repeated cycles, the fluid under internal pressure began to gradually leak through the fine microscopic cracks in the matrix. The fine microscopic cracks were oriented in the circumferential direction of the pipe. However, despite the occurrence of "weeping failure" of the pipe, internal pressure could be easily maintained. After about 10-100 times the number of cycles required for "weeping," the fiber reinforcements of the pipe gradually fractured and the internal pressure could no longer be maintained. The loss of pressure is referred to as "pipe failure." In these tests, the primary parameter controlling failure was orientation of the fiber reinforcements in the body of the pipe. Those fibers aligned along the pipe axis revealed a substantial improvement in cyclic fatigue resistance. The existence of a joint in the test specimen showed secondary importance while contributing to degradation of the fatigue resistance of the specimen. Both types of failure were found to be dependent on temperature, over the range tested (22-66 degrees C, and also dependent on the type of internal liquid used. The specimens tested using crude oil at 66 degrees C as the internal fluid revealed the lowest fatigue resistance.