Self-sealing of microcrack damage in structural fiber reinforced composites

Fiber reinforced composites are used in a variety of applications such as cryogenic storage tanks and composite face sheets for sandwich structures. The inherent thermal fatigue of cryogenic vessels leads to the formation of microcracks, which allow gas phase leakage across the tank walls. In composite face sheets mechanical fatigue or low velocity impact such as a tool drop can induce microcracking, thus leading to an increase in water absorption into the core material. Self-sealing functionality provides a potential solution to these problems. In this work, we adopt a dual microcapsule healing chemistry comprised of silanol terminated poly(dimethyl siloxane) plus a crosslinking agent and a tin catalyst, which is stable to 150 °C. The microcapsules are first incorporated into a woven glass fiber/epoxy composite and cured at 120 °C to yield a final glass transition temperature of 130 °C. The composites are damaged by cyclically driving an indenter tip to a prescribed load into opposing surfaces of the sample. As a crack propagates through the polymer matrix, microcapsules rupture and release the polymer, crosslinker, and catalyst, which combine to react in the crack plane. Incorporating 13 vol% 41 μm capsules or 17 vol% 25 μm capsules into the composite matrix leads to 100% of the samples sealing as evaluated using a pressurized gas flow cell. More recently, we have successfully integrated microcapsules into a [0/90] _s uniweave carbon fiber/epoxy composite to evaluate sample permeability after thermal cycling. Composite samples with self-sealing functionality were able to prolong the average life expectancy by 63% (649 more thermal cycles) over traditional carbon fiber/epoxy composites. A 134% life extension (959 more cycles) was observed over samples with microcapsules, but no catalyst.