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1.1 This practice describes a procedure for detecting subsurface flaws in composite panels and repair patches using Flash Thermography (FT), in which an infrared (IR) camera is used to detect anomalous cooling behavior of a sample surface after it has been heated with a spatially uniform light pulse from a flash lamp array.

1.2 This practice describes established FT test methods that are currently used by industry, and have demonstrated utility in quality assurance of composite structures during post-manufacturing and in-service examinations.

1.3 This practice has utility for testing of polymer composite panels and repair patches containing, but not limited to, bismaleimide, epoxy, phenolic, poly(amide imide), polybenzimidazole, polyester (thermosetting and thermoplastic), poly(ether ether ketone), poly(ether imide), polyimide (thermosetting and thermoplastic), poly(phenylene sulfide), or polysulfone matrices; and alumina, aramid, boron, carbon, glass, quartz, or silicon carbide fibers. Typical as-fabricated geometries include uniaxial, cross ply, and angle ply laminates; as well as honeycomb core sandwich core materials.

1.4 This practice has utility for testing of ceramic matrix composite panels containing, but not limited to, silicon carbide, silicon nitride, and carbon matrix and fibers.

1.5 This practice applies to polymer or ceramic matrix composite structures with inspection surfaces that are sufficiently optically opaque to absorb incident light, and that have sufficient emissivity to allow monitoring of the surface temperature with an IR camera. Excessively thick samples, or samples with low thermal diffusivities, require long acquisition periods and yield weak signals approaching background and noise levels, and may be impractical for this technique.

1.6 This practice applies to detection of flaws in a composite panel or repair patch, or at the bonded interface between the panel and a supporting sandwich core or solid substrate. It does not apply to discontinuities in the sandwich core, or at the interface between the sandwich core and a second panel on the far side of the core (with respect to the inspection apparatus).

1.7 This practice does not specify accept-reject criteria and is not intended to be used as a basis for approving composite structures for service.

1.8 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.

1.9 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.

5.1 Typically, FT is used to identify flaws that occur in the manufacture of composite structures, or to identify and track flaws that develop during the service lifetime of the structure. Flaws detected with FT include delamination, disbonds, voids, inclusions, foreign object debris, porosity, or the presence of fluid that is in contact with the backside of the inspection surface. For example, the effect of variable ply number (or thickness), bridging, and an insert simulating delamination on heat flow into a composite is shown in Fig. 1 (left). Bridging (Fig. 1, right) or delaminated areas show up as hot spots due to discontinuous heat flow, causing heating to be localized close to the inspection surface. With dedicated signal processing and the use of representative test samples, characterization of flaw depth and size, or measurement of component thickness and thermal diffusivity, may be performed.

FIG. 1 Variation of Heat Flow Into a Composite With Variable Ply Thickness (Scenarios 1, 3, and 4), Bridging (Scenario 2) And an Insert (Scenario 5) (Left), And a Post Layup Line Scan Showing Bright Spots Attributed to Bridging (Right) (Courtesy of NASA Langley Research Center)

5.2 Since FT is based on the diffusion of thermal energy from the inspection surface of the specimen to the opposing surface (or the depth plane of interest), the practice requires that data acquisition allows sufficient time for this process to occur, and that at the completion of the acquisition process, the radiated surface temperature signal collected by the IR camera is strong enough to be distinguished from spurious IR contributions from background sources or system noise.

5.3 This method is based on accurate detection of changes in the emitted IR energy emanating from the inspection surface during the cooling process. As the emissivity of the inspection surface falls below that of an ideal blackbody (blackbody emissivity = 1), the signal detected by the IR camera may include components that are reflected from the inspection surface. Most composite materials can be examined without special surface preparation. However, it may be necessary to coat low-emissivity, optically translucent inspection surfaces with an optically opaque, high-emissivity water-washable paint.

5.4 This practice applies to the detection of flaws with aspect ratio greater than one.

5.5 This practice is based on the thermal response of a specimen to a light pulse that is uniformly distributed over the plane of the inspection surface. To ensure that 1-dimensional heat flow from the surface into the sample is the primary cooling mechanism during the data acquisition period, the height and width dimensions of the heated area should be significantly greater than the thickness of the specimen, or the depth plane of interest. To minimize edge effects, the height and width dimensions of the heated area should be at least 5 % greater than the height and width dimensions of the inspection area.

5.6 This practice applies to flat panels, or to curved panels where the angle between the line normal to the inspection surface and the IR camera optical axis is less than 30°. Analysis of regions with higher curvature can result in streaking artifacts due to nonuniform heating (Fig. 2).

FIG. 2 Thermal Scan of a Complex Composite Shape (Left) Showing Less Effective Heating of a High Curvature Saddle-Region, Resulting in a Darker Diagonal Streak in the Thermographic Image (Right) (Courtesy of NASA Langley Research Center)