Infrared thermographic methods for wrinkle characterization in composite structures

A method for inspecting a composite structure comprising:

• (a) acquiring infrared imaging data from an inspected area on a surface of the composite structure using an infrared camera, said infrared imaging data being indicative of the presence of wrinkles under the surface of the inspected area;
• (b) processing the infrared imaging data to estimate a value of a first wrinkle dimensional parameter of the wrinkles under the surface of the composite structure in the inspected area;
• (c) subsequent to step (a), acquiring ultrasonic imaging data from the inspected area on the surface of the composite structure using an ultrasonic transducer array;
• (d) processing the ultrasonic imaging data to estimate a value of a second wrinkle dimensional parameter of wrinkles under the surface of the composite structure in the inspected area;
• (e) calculating a value of a wrinkle parameter which is a function of the first and second wrinkle dimensional parameters;
• and
• (f) determining a status of the composite structure in dependence on whether the value of the wrinkle parameter calculated in step (e) is inside or outside an allowable range of values.

A method for measuring features in a composite structure made of a composite material and having a thickness, the method comprising:

• (a) establishing a reference database containing transfer functions that relate true wrinkle wavelength to measured wrinkle wavelength in a multiplicity of reference standards made of the same composite material and having the same thickness as the composite structure to be inspected, the reference database further containing, for each transfer function, associated wrinkle dimensional data representing at least known depths, amplitudes and wavelengths of wrinkles in the multiplicity of reference standards;
• (b) activating at least one flash lamp to output light that illuminates an area on a surface of the composite structure;
• (c) activating an infrared camera to acquire infrared imaging data representing a temperature of the surface of the composite structure in at least a portion of the area illuminated in step (b);
• (d) processing the infrared imaging data to identify the presence of wrinkles in a portion of the composite structure underneath at least a portion of the illuminated area;
• (e) measuring a time required to get a maximum contrast between the wrinkled portion and a surrounding non-wrinkled portion of the composite structure;
• (f) measuring a wavelength of wrinkles imaged by the infrared imaging data;
• (g) estimating a depth of the wrinkled portion based on the time measured in step (e);
• (h) using the estimated depth and the measured wavelength to retrieve a transfer function from the reference database;
• and
• (i) applying the retrieved transfer function to the measured wavelength to estimate an actual wavelength of the wrinkles in the wrinkled portion of the composite structure.