A Lightweight Optical Approach to Detect Incipient Heat Damage

Composite materials, widely used in aircraft, reduce manufacturing cost, improve structural performance, and boost fuel efficiency. However, composites are susceptible to hidden heat damage, which may occur from fire, exhaust impingement, overheating, or during repairs. The earliest heat damage stage, referred to as"incipient"heat damage, may reduce upper use temperature, cause matrix mass loss, and reduce mechanical flexural strength. We propose an innovative approach that detects this very early matrix damage via changes in the mechanical flexural response of a structure. We use a small lightweight optical imaging detector that can rapidly monitor large structures and identify regions exhibiting incipient heat damage. We combine small mechanical excitation of the structure under test with a lightweight camera that images the surface resonant frequency, directly probing the local flexural strength of the composite material. This technique, called the Fast Imaging Non-Destructive Inspection Technique (FINDIT), can directly and nondestructively test the mechanical flexural properties of composite material. Flexural damage has been shown to provide a first warning of incipient heat damage before visual manifestation. FINDIT quantitatively measures the associated tilt-tip surface changes, and may be automated, removing subjective judgment factors, rendering the approach fully functional under adverse circumstances.