This disclosure generally relates to non-destructive inspection of structures or parts, and more particularly to systems and methods for characterizing or evaluating anomalies in material, such as wrinkles in fiber-reinforced plastic material and designed periodic material variations such as low-observable coatings with periodic features for reducing susceptibility to radar detection.
New, lightweight composite materials and designs are being used more extensively in the aerospace industry for commercial aircraft and other aerospace vehicles, as well as in other industries. The structures using these composite materials may be formed using multiple plies or layers of fiber-reinforced plastic material that may be laminated together to form a lightweight, high-strength structure. Fabrication of composite laminate structure for aerospace applications can result in unwanted out-of-plane and/or in-plane wrinkling of plies that can impact performance of the structure based on the size of the wrinkle. Quality assurance and certification for production parts in industries such as the aircraft industry requires that the part be built to meet certain design standards and specifications. For some parts there may be a standard acceptance criteria based on wrinkle size. Accordingly being able to accurately detect and measure the size of any wrinkles in a structure or part is desirable.
Some wrinkles can be identified visually from the surface. However, they cannot be quantified visually, so in an abundance of caution the worst case may be assumed unless means for measuring the size of the wrinkle (often in terms of wavelength L divided by maximum amplitude D) can be provided. Also, wrinkles deeper in the structure cannot be seen visually from the surface at all. Ultrasonic methods have been developed to identify and quantify wrinkles. However, the main drawback of ultrasonic methods is that they cannot “see” beneath a wrinkle (and no access is available to the back side of the structure), so the amount of “good” material is unknown. Therefore, the maximum thickness of a wrinkle from its highest peak to the back surface may have to be assumed. This may result in overly conservative knockdowns of strength predictions, and needless and costly repairs. Another drawback of the ultrasonic method in general is that it takes significant time to collect the data.
Therefore further improvements in methods for non-destructive determination of the size of wrinkles in composite material would be advantageous.