Modeling of Stress Development During Thermal Damage Healing in Fiber-reinforced Composite Materials Containing Embedded Shape Memory Alloy Wires

Fiber-reinforced composite materials are susceptible to damage development through matrix cracking and delamination. This article concerns the use of shape memory alloy (SMA) wires embedded in a composite material to support healing of damage through a local heat treatment. The composite material contains a thermoplastic matrix that allows healing at elevated temperatures. The woven in SMA wires, oriented in the out-of-plane direction of the composite material, are used to close the delamination upon heating. Several case studies were performed. The influence of the SMA wire fraction, the degree of prestraining of the SMA wires, the glass transition temperature of the amorphous thermoplastic matrix, and the healing temperature have been considered. The delamination is compacted while heating up to the healing temperature. The thermal expansion coefficient of the thermoplastic matrix is much larger than that of the SMA wires causing a compressive thermal stress in the composite material upon heating. Prestraining of the SMA wires is not a priori required to obtain a compressive stress at the delamination interfaces at the healing temperature. After cooling to room temperature residual stresses occur in the composite material and SMA wires if the SMA wire composition at the start of the heat treatment differs from that at the end. The conditions under which no residual stresses develop have been determined. SMA wire fractions have been calculated to achieve a preset stress level in the composite material at the healing temperature and a stress-free state after healing. Finally, the use of high strength wires to replace SMA wires has been considered and shown to be a worthwhile alternative.