Composite inspection

A method of detecting inconsistencies in a composite structure, the method comprising:

• directing a pulsed laser beam towards the composite structure comprised of a number of composite materials, wherein wide-band ultrasonic signals are formed in the composite structure when radiation of the pulsed laser beam is absorbed by a surface of the composite structure;
• detecting the wide-band ultrasonic signals over a duration of time to form data, wherein the data comprises an ultrasonic A-scan spectrum;
• processing the data to identify a structure signal in a frequency domain of the ultrasonic A-scan spectrum, wherein processing the data comprises:
• applying a moving time-window in a time domain to an ultrasonic A-scan to form time-windowed signals, wherein the moving time-window has a Gaussian shape sized to contain between two and five plies of the number of composite materials;
• and
• determining at least one of a mean frequency or a maximum frequency of a Fourier spectrum taken for each of the time-windowed signals, including determining a maximum frequency of a windowed signal of an ultrasonic A-scan using the equation Sn=Σk=1pαk*Sn-k, where Sn is an A-scan signal at sample point n, p is a quantity of coefficients, αk is a kth Fourier coefficient, and Sn-k is the A-scan signal at prior sample point n-k;
• and
• determining whether the inconsistencies are present in the number of composite materials by comparing the structure signal of the ultrasonic A-scan spectrum to a structure signal of a composite structure standard.