Characterization of wrinkles and periodic variations in material using infrared thermography

A method for measuring an out-of-plane wrinkle in a structure made of a composite material, the method comprising:

• (a) developing a calibration curve for an infrared imaging system based on correlation of infrared image data and optical cross-section measurement data acquired from reference standards made of composite material, at least some of the reference standards having at least one wrinkle;
• (b) acquiring infrared image data from an inspected area on a surface of the structure using an infrared camera of the infrared imaging system, the infrared image data being indicative of the presence of an out-of-plane wrinkle under the surface of the inspected area;
• (c) generating thermographic line profile data based on the infrared image data;
• (d) processing the thermographic line profile data to estimate a value of a first wrinkle dimensional parameter of the out-of-plane wrinkle;
• (e) processing the thermographic line profile data to estimate a value of a second wrinkle dimensional parameter of the out-of-plane wrinkle based in part on the calibration curve;
• (f) calculating a value of a wrinkle parameter which is a function of the first and second wrinkle dimensional parameters;
• and
• (g) determining a status of the composite structure in dependence on whether the value of the wrinkle parameter calculated in step (f) is inside or outside an allowable range of values.

A method for measuring an in-plane wrinkle in a composite structure, comprising:

• (a) moving an infrared camera to a location whereat a field of view of the infrared camera encompasses an inspection area on a surface of the composite structure;
• (b) activating at least one flash lamp to output light that illuminates at least portions of the inspection area;
• (c) activating the infrared camera to acquire infrared image data while the field of view of the infrared camera encompasses at least the inspection area;
• (d) processing the infrared image data to identify selected points along an in-plane wrinkle;
• and
• (e) determining a wavelength and a maximum amplitude of the in-plane wrinkle based on the selected points using image processing software.

A method for calibrating an infrared imaging system, comprising:

• (a) forming a first reference standard made of a type of composite material and having a wrinkle;
• (b) forming a second reference standard made of the same type of composite material and not having a wrinkle;
• (c) collecting first infrared image data from the first reference standard;
• (d) collecting second infrared image data from the second reference standard;
• (e) generating first thermographic line profile data based on the first infrared image data;
• (f) generating second thermographic line profile data based on the first infrared image data;
• (g) calculating a change in the first thermographic line profile data along a line in the first reference standard;
• (h) calculating a change in the second thermographic line profile data along a line in the second reference standard;
• (i) calculating a difference between the respective changes;
• (j) cutting the first and second reference standards to expose cross sections, wherein the cross section of the first reference standard intersects the wrinkle;
• (k) imaging the exposed cross sections of the first and second reference standards to produce respective optical cross sections;
• (l) measuring an amplitude of the wrinkle which appears in the optical cross section of the first reference standard;
• (m) calculating a conversion factor based on the difference calculated in step (i) and the amplitude measured in step (l);
• and
• (n) constructing a calibration curve using the conversion factor.