Typically, non-destructive evaluation of composites — by ultrasound or other means — is conducted either before or after the cure process but many defects vanish and form during cure. NASA Langley Research Center, in collaboration with Analytical Mechanics Associates, developed an automated ultrasonic scanning system for in-situ cure monitoring and defect detection of composites in an autoclave or an oven.
The non-destructive system consists of an ultrasonic, portable, automated C-scan system with an attached ultrasonic contact probe. The scanner is placed inside an insulated vessel that protects the temperature-sensitive components of the scanner. A liquid nitrogen cooling system keeps the interior of the vessel below 38 °C. A motorized X-Y raster scanner is mounted inside an unsealed cooling container made of porous insulation boards with a cantilever scanning arm protruding out of the cooling container through a slot.
The cooling container that houses the X-Y raster scanner is periodically cooled using a liquid nitrogen (LN2) delivery system. Flexible bellows in the slot opening of the box minimize heat transfer between the box and the external autoclave environment. The box and scanning arm are located on a precision cast tool plate. A thin layer of ultrasonic couplant is placed between the transducer and the tool plate. The composite parts are vacuum bagged on the other side of the tool plate and inspected. The scanning system inside the vessel is connected to the controller outside the autoclave. The system can provide A-scan, B-scan, and C-scan images of the composite panel at multiple times during the cure process.
The system provides real-time monitoring of defect formation and movement during cure. This not only offers a better understanding of defect sources and sinks but also the ability to more accurately validate process models for the prediction of cure process defects. The system also shows the through-thickness location of any composite manufacturing defects during cure with real-time localization and tracking. This has been demonstrated for both intentionally introduced porosity (i.e., trapped during layup) as well as processing-induced porosity (e.g., resulting from uneven pressure distribution on a part).
The technology can be used as a nondestructive evaluation system when making composite parts in an oven or an autoclave including thermosets, thermoplastics, composite laminates, high-temperature resins, and ceramics.