Method and apparatus for quantitative nondestructive evaluation of metal airfoils using high resolution transient thermography

An apparatus for determining a thickness of an object having a surface which can be visualized as an array of pixels, comprising:

• a heat source for rapidly heating the surface of an object;
• a means for recording pixel intensity in a sequence of IR images;
• a means for determining pixel contrast data from recorded pixel intensity;
• a means for determining a derivative of the contrast data based on a frame number value assigned to an IR image;
• and
• a means for determining thickness of the object relative to other portions of the object based upon pixel contrast and image frame number.

A high speed infrared (IR) transient thermography method for determining the actual thickness of an object, comprising the steps of:

• a) acquiring pixel intensity data from a predetermined sequence of IR image frames of the object using an IR sensitive focal-plane array camera, each image frame comprising a plurality of pixels;
• b) obtaining a value representing an average pixel intensity from pixel intensity data corresponding to a thermally thick reference portion of the object;
• c) determining pixel contrast data by subtracting said value representing average pixel intensity from pixel intensity data for a pixel;
• d) repeating steps (b) and (c) for each frame of said sequence of IR image frames to develop a temperature-time (T-t) contrast curve for said pixel;
• and
• e) determining actual thickness at a resolution element of the object corresponding to the pixel using said contrast curve, said determination based on a heat-flow invariant relationship between an inflection point in the contrast curve and a heat-flow characteristic time dependent upon a thermal diffusivity of the object.

A high speed infrared (IR) transient thermography method for determining the actual thickness of an object, comprising the steps of:

• a) acquiring pixel intensity data from a predetermined sequence of IR image frames of the object and an adjacent separate reference article imaged along with the object using an IR sensitive focal-plane array camera, each image frame comprising a plurality of pixels and said reference article having a thermally thick reference portion of substantially similar heat conduction characteristics as the object;
• b) obtaining a value representing an average pixel intensity from pixel intensity data corresponding to a thermally thick reference portion of the reference article;
• c) determining pixel contrast data by subtracting said value representing average pixel intensity from pixel intensity data for a pixel;
• d) repeating steps (b) and (c) for each frame of said sequence of IR image frames to develop a temperature-time (T-t) contrast curve for said pixel;
• and
• e) determining actual thickness at a resolution element of the object corresponding to the pixel using said contrast curve, said determination based on a heat-flow invariant relationship between an inflection point in the contrast curve and a heat-flow characteristic time dependent upon a thermal diffusivity of the object.

A high speed infrared (IR) transient thermography method for determining the actual thickness of an object, comprising the steps of:

• a) acquiring pixel intensity data from a predetermined sequence of IR image frames of the object and an adjacent separate reference article imaged along with the object using an IR sensitive focal-plane array camera, each image frame comprising a plurality of pixels, and said reference article having a one or more discrete portions of known thickness and substantially similar heat conduction characteristics as the object;
• b) determining inflection points in pixel T-t intensity curves corresponding to a resolution element of the object and one or more resolution elements corresponding to different portions of known thickness on the reference article;
• and
• c) determining an actual thickness of the object at the resolution element of the object by comparing relative times of occurrence of the inflection points determined in step (b) and extrapolating or interpolating a thickness value for the object from the portions of known thickness on the reference article.