Elevated temperature properties of pinned composite lap joints

Reinforcement in the through-thickness direction with pins improves the elevated temperature strength and damage tolerance of polymer composite joints. The failure modes and ultimate strength are experimentally determined for carbon/epoxy single-lap joints at temperatures between 20 and 250°C. Pinning improves the ultimate strength, elongation-to-failure limit, and work of fracture of the joints over the entire temperature range, by factors that depend on the orientation, volume content and diameter of the pins. The dominant strengthening mechanism at elevated temperature is crack bridging by the pins, which increases the ultimate joint strength by as much as 60%. However, rate-dependent mechanisms increasingly negate the benefit of pins as the temperature rises above approximately 125°C. This threshold temperature is 40°C less than the glass transition temperature for the laminate resin. For tests conducted at a constant rate of increase of load, rate dependence appears as a reduction of the failure load with increasing test temperature. A simple model derived by introducing rate-dependent sliding between the pins and the laminate into a previous model of the mechanics of pin bridging fits the data for strength reduction well, if the sliding rate is assumed to increase exponentially with temperature. The model predicts that the ultimate strength will decrease as the loading rate decreases, for loading that increases linearly with time. The fractional reduction of strength for a given temperature is approximately independent of the pin half-length and area fraction; and greater reduction should occur for larger pin diameter if the pin half-length and volume content are fixed.