ASTM E2582-21

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Designation: E2582 − 19 E2582 − 21
Standard Practice for
Infrared Flash Thermography of Composite Panels and
Repair Patches Used in Aerospace Applications
This standard is issued under the fixed designation E2582; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
This standard has been approved for use by agencies of the U.S. Department of Defense.
1. Scope
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.
This practice is under the jurisdiction of ASTM Committee E07 on Nondestructive Testing and is the direct responsibility of Subcommittee E07.10 on Specialized NDT
Methods.
Current edition approved July 1, 2019Feb. 1, 2021. Published August 2019March 2021. Originally approved in 2007. Last previous edition approved in 20142019 as
E2582E2582 – 19.- 07(2014). DOI: 10.1520/E2582-19.10.1520/E2582-21.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E2582 − 21
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.
2. Referenced Documents
2.1 ASTM Standards:
D3878 Terminology for Composite Materials
E1316 Terminology for Nondestructive Examinations
3. Terminology
3.1 Definitions—Terminology in accordance with Terminologies D3878 and E1316 and shall be used where applicable.
3.2 Definitions of Terms Specific to This Standard:
3.2.1 aspect ratio, n—the diameter to depth ratio of a flaw; for irregularly shaped flaws, diameter refers to the minor axis of an
equivalent rectangle that approximates the flaw shape and area.
3.2.2 discrete discontinuity, n—a thermal discontinuity whose projection onto the inspection surface is smaller than the field of
view of the inspection apparatus.
3.2.3 extended discontinuity, n—a thermal discontinuity whose projection onto the inspection surface completely fills the field of
view of the inspection apparatus.
3.2.4 first logarithmic derivative, n—the rate of change of the natural logarithm of temperature (with preflash temperature
subtracted) with respect to the natural logarithm of time.
3.2.5 inspection surface, n—the surface of the specimen that is exposed to the FT apparatus.
3.2.6 logarithmic temperature-time plot, n—a plot of the natural logarithm of the surface temperature with preflash temperature
subtracted on the y-axis versus the natural logarithm of time on the x-axis, where time t=0 is taken to be the midpoint of the flash
event; either temperature or radiance may be used to create the plot.
3.2.7 log plot, n—see logarithmic temperature-time plot.
3.2.8 second logarithmic derivative, n—the rate of change of the first logarithmic derivative with respect to the natural logarithm
of time.
3.2.9 thermal diffusivity, n—the ratio of thermal conductivity to the product of density and specific heat; a measure of the rate at
which heat propagates in a material; units [length /time].
3.2.10 thermal discontinuity, n—a change in the thermophysical properties of a specimen that disrupts the diffusion of heat.
4. Summary of Practice
4.1 In FT, a brief pulse of light energy from a flash lamp array heats the inspection surface of a composite specimen, and an IR
camera monitors the surface temperature (or radiance) as the sample cools.
4.2 The surface temperature falls predictably as heat from the surface diffuses into the sample bulk over a period t* (Eq 1).
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
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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 Re-
search Center)
L
*
t 5 (1)
πα
where:
L = the specimen thickness, and
α = the thermal diffusivity.
4.3 Internal thermal discontinuities (for example, voids, delaminations, or a wall or interface between the host material and a void
or inclusion) modify the local cooling of the surface, and the corresponding radiation flux from the surface that is detected by the
IR camera.
4.4 Fundamental detectability of a flaw will depend on its size, depth, and the degree to which its thermal properties differ from
those of the surrounding host material. For a given flaw-host combination, detectability is a function of the aspect ratio of the flaw.
The minimum detectable flaw size increases with the depth of the flaw. Detectability is highest for larger flaws that are closer to
the sample surface and have thermal properties that are significantly different from the host matrix material.
4.5 Operational parameters affecting detectability include component surface emissivity and optical reflectivity, data acquisition
period, flash lamp energy, and camera wavelength, frame rate, sensitivity, optics, and spatial resolution.
4.6 This practice describes a single-side access examination, in which the flash lamp or heat lamp array (excitation source) and
IR camera (temperature sensor) are both located on the same (inspection) side of the component or material under examination.
4.7 In common practice, signal processing algorithms (for example, Thermographic Signal Reconstruction, Principal Component
Analysis) are used to enhance detectability of flaws that are not detectable in the raw IR camera signal, and to assist in evaluation
and characterization of indications.
5. Significance and Use
5.1 Typically, FT is typically 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 back side 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
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FIG. 2 Thermal Scan of a Complex Composite Shape (Left) Showing Less Effective Heating of a High Curvature Saddle-Region, Result-
ing in a Darker Diagonal Streak in the Thermographic Image (Right) (Courtesy of NASA Langley Research Center)
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.
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 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 local surface normal angle between the line normal to the
inspection surface and the IR camera optical axis is less than 30 degrees from the IR camera optical axis.30°. Analysis of regions
with higher curvature can result in streaking artifacts due to nonuniform heating (Fig. 2).
6. Equipment and Materials
6.1 IR Camera—The camera should be capable of uninterrupted monitoring of the sample surface for the entire duration of the
acquisition. The camera should allow automatic recalibration, non-uniformity correction, or other operations that interrupt the
continuous data stream to be disabled during the data acquisition period. The camera should provide real-time digital output of the
acquired signal. The camera output signal response should be approximately linear over the (post-flash) temperature range of the
sample. The camera wavelength should be in either the 2–5 micron range or the 8–14 micron range, selected such that the test
material is not IR translucent in the spectral range of the camera. The optics and focal plane should be sufficient so that the
projection of 15 contiguous pixels onto the sample plane is less than or equal to the minimum flaw area that is to be detected.
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6.2 Flash Lamp Array—At least one flash lamp should be employed to provide uniform illumination to the sample surface. The
full width at half maximum duration of the flash pulse should be less than five milliseconds. The array should be placed to avoid
a direct path of the flash energy into the IR camera lens opening. The flash lamp array should be enclosed in a protective hood
to prevent workers in the inspection area from direct exposure to the flash, or alternately, the apparatus should be operated in a
partitioned area with appropriate safety warnings to prevent inadvertent exposure.
6.3 Heat Lamp Array—A heat lamp array may be used as an alternative to a flash lamp array for the purposes of illuminating the
sample surface. At least one heat lamp (for example, 500W linear halogen bulb in a parabolic reflector) should be employed to
provide uniform illumination. The lamps should be enclosed in a reflector and covered by an optically transparent window that
suppresses IR radiation in the camera wavelength range (for example, borosilicate glass). The duration of the heating pulse should
be less than 25 % of t* (see Eq 1). The array should be placed to avoid a direct path of the reflected energy into the IR camera
lens opening.
6.4 Acquisition System—The acquisition system includes the IR camera, flash lamps, and a dedicated computer that is interfaced
to both the camera and flash lamps. The acquisition system should be capable of synchronizing the triggering of the flash lamps
and IR camera data acquisition. The system should allow data to be acquired before, during, and after the flash occurs.
6.5 Analysis Software—The computer software should allow acquired sequences to be archived and retrieved for evaluation, and
allow real time display of the IR camera signal, as well as frame-by-frame display of previously acquired flash sequences which
have been archived. The software should allow viewing of the logarithmic temperature-time for specified pixels. Additional
processing operations on each raw image sequence (for example, averaging, pre-flash image subtraction, noise-reduction,
calculation of first or second logarithmic derivatives) may be performed to improve detectability of subsurface features.
7. Reference Standards
7.1 Detectability Standard—A reference standard with known thermal discontinuities
...