Experimental study and computer simulation on degradation of z-pin reinforcement under cyclic fatigue

This paper presents an experimental study on the fatigue degradation of the bridging forces of z-pins embedded in polymer laminates. Carbon/epoxy laminate specimens were reinforced in the through-thickness direction with two volume contents and two sizes of z-pins. The specimens were fatigue tested in the through-thickness (mode I) direction to measure the deterioration in the bridging forces of the pins. Degradation of the debonding and friction pull-out forces of the pins with the number of load cycles was determined over a range of fatigue displacements. The degradation of the bridging forces is dependent on the diameter of the pins. Reducing the diameter of the pins reduced the degradation rate of the debonding and friction forces. However, the friction force always degraded more rapidly under fatigue loading than the debonding force. Using the fatigue data and experimental observations, a micromechanics model for calculating the degradation of bridging tractions was developed and the lifetime of z-pinned laminates subjected to mode I fatigue was simulated. In the modeling, z-pins are treated as a series of external forces applied to the DCB laminates which provides bridging forces to the opening crack during delamination. Considering the degradation of bridging force under cyclic loading, the maximum bridging force in the bridging law is assumed to be a power law function decreasing with loading cycles N and maximum z-pin displacement δ_max. The classic beam theory is utilized to calculate the energy release rate consumed by the newly created crack surface. The fracture energy method is used as the delamination criterion, in which the unpinned DCB fracture toughness versus loading cycles is assumed as an exponential function determined by previous test results. Computer simulation results show that z-pins can effectively reduce the delamination growth rate under both load-control and displacement-control conditions.