Modeling and characterizing impact damage in carbon fiber composites by thermal/infrared non-destructive testing

Thermal/infrared non-destructive testing (T/I NDT) is a particular application of IR thermography. T/I NDT is typically classified for passive and active, as well as for steady-state (stationary) and transient (non-stationary, or dynamic). Active T/I NDT can be classified by: (1) the type of thermal stimulation, (2) the arrangement of a sample and a thermal stimulation source, and (3) the size and shape of stimulated area. T/I NDT has proven to be a convenient technique for the detection of impact damage in composite materials due to the following: (1) graphite-based composites are similar to a blackbody by absorption/radiation properties in the infrared (IR) wavelength band, (2) their thermal conductivity is lower than that of metals but higher than of many non-metals thus ensuring reasonable temperature signals at convenient observation times, (3) impact damage leads to thin but laterally-extended air-filled defects which produce considerable thermal resistance to the in-depth heat flux, and (4) T/I NDT is a fast, remote and illustrative technique which, unlike ultrasonic inspection, does not require immersing a sample into water. This paper describes some approaches to thermal detection and characterization of impact damage in carbon fiber reinforced plastic (CFRP) of whose inspection is an important issue in several industrial areas, first of all, in aero space where subsurface defects might lead to catastrophic consequences. Realistic solutions of T/I NDT theoretical problems can be obtained by using 3D numerical models of heat conduction. Direct solutions allow better understanding of heat propagation in defect areas while inverse solutions ensure the evaluation of defect parameters, such as defect depth, size and thickness. Several characterization algorithms are available, with a one-sided T/I NDT procedure being better suited for the characterization of defect depth, while defect thickness is best evaluated in a two-sided procedure. In the case of CFRP composites, the defect characterization approaches are well developed, including the technique of dynamic thermal tomography, which enables a considerable reduction of surface clutter and allows the imaging of separate layers of a composite test sample. (C) 2014 Elsevier Ltd. All rights reserved.