No abstract

Carbon fiber–epoxy composites have become prevalent in the aerospace industry where mechanical properties and light weight are at a premium. The significant non-destructive evaluation challenges of composites require new solutions, especially in detecting early-stage, or incipient, thermal damage. The initial stages of thermal damage are chemical rather than physical, and can cause significant reduction in mechanical properties well before physical damage becomes detectable in ultrasonic testing. Thermochromic fluorescent probe molecules have the potential to sense incipient thermal damage more accurately than traditional inspection methods. We have designed a molecule which transitions from a colorless, non-fluorescent state to a colorful, highly fluorescent state when exposed to temperature–time combinations that can cause damage in composites. Moreover, this molecule can be dispersed in a polymer film and attached to composite parts as a removable sensor. This work presents an evaluation of the sensor performance of this thermochromic film in comparison to ultrasonic C-scan as a method to detect incipient thermal damage in one of the most widely used carbon fiber–epoxy composite systems. Composite samples exposed to varying thermal exposures were used to evaluate the fluorescent thermal sensor films, and the results are compared to the results of ultrasonic imaging and short-beam shear tests for interlaminar shear strength.

Effect of thermal aging on the lap-shear properties of an acrylic-cored laminated steel was investigated by shear testing of specimens following exposure in air to the paint bake temperature of 180°C. The laminated steel consists of two thin steel layers bonded to a polymer core. It was found that thermal aging caused an increase in shear modulus and a decrease in ductility. Shear strength initially decreased, but then increased with aging time. Fractographic, thermal gravimetric, and differential scanning calorimetric analyses indicated that the decrease in shear strength and ductility is likely caused by void formation resulting from evaporation of the volatiles in the acrylic. These voids increased the local stresses. The subsequent increase in shear strength is attributed to the increase in adhesive cross-linking from thermal curing which hardens the adhesive. The increase in shear modulus with increase in aging time is ascribed primarily to thermal curing.