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FLIR Technical SeriesApplication Note for Research & Science

NDT in Composite Materials with Flash, Transient, and Lock-in ThermographyMarkus Tarin and Ralph RotolanteMoviTHERM, Inc.

AbstractComposites are now used in everything from aircraft to prosthetics. This paper will discuss three thermographic NDT techniques currently in use for the detection and measurement of defects in composites such as impact damage, delamination, voids, inclusions, and stresses. The particular technique that should be employed for optimum results, whether pulsed flash, pulsed transient, or lock-in, will depend upon the sample material and thickness, and the test environment. Choice of camera type also varies widely, from cooled to uncooled, with large format 640 × 480 pixels now available.

IntroductionInfrared cameras are used in a wide variety of military and civil applications. These cameras typically operate with a temperature sensitivity of about 15 mK for cooled and 80 mK for uncooled, certainly sensitive enough for most applications.

Occasionally, and especially in many NDT applications, a more sensitive measurement is needed. Infrared Pulse and Lock-In Thermography give a much better sensitivity—100 times to 1000 times better.

Pulse and Lock-In Thermography are in use for NDT applications worldwide: for defect imaging, stress imaging, solar-cell shunt imaging, solar-cell carrier-density/lifetime imaging, and semiconductor photo- and electro-luminescence imaging.

A variety of active-sources, various infrared camera types, and a selection of analytical tools are available for monitoring the responses of the materials to the active-sources.

This document presents an overview of the rich choice of NDT tools for NDT inspection of composites and illustrates the advantages of Pulse (Flash and Transient) and Lock-In for different structure types.

In this paper, the names of the techniques reference the heat waveform: pulsed, which includes flash heat, transient heat; and Lock-In, which involves repetitive heat as shown in Table 1.

Techniques are further categorized by the method of excitation, as shown in Table 2.

What is common to all these techniques is the use of infrared cameras. The most common types for composite inspection are shown in Figure 1. The uncooled, microbolometer type offers the most affordable solution. Wide angle and microscopic lenses are available for special applications.

Table 1. Classification of IR Thermography Techniques

Table 2. Pulsed and Lock-In Excitation Sources

Figure 1. Typical infrared cameras used for composite inspections.

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Pulsed Thermography – Principles of OperationIn pulsed thermography, the stimulus is applied with a xenon flash lamp for a flash pulse or with a halogen lamp in the transient case. In both cases, the equipment set-up is as indicated in Figure 2.

Solving the Heat Conduction Equation tells us that the thermal propagation time to the depth of 2 mm to a subsurface defect is about 40 ms in aluminum and for 2 mm of graphite epoxy is about 30s. This means Flash is better for materials of high thermal diffusivity, e.g., metals. Materials with a low thermal diffusivity, e.g., composites, have a long thermal propagation time, which limits flash thermography to the detection of shallow defects.

The Heat Conduction Equation gives us the curve fit in Figure 3 for the surface temperature of a sample with a defect after a heat pulse.

A good treatment of the underlying mathematics is given by Maldague1.

The reconstructed profiles in black are from an analytic approximation function. Common analytical functions that are used include a root-approximation-model, an exponential-approximation-model, and a pulse-phase calculation which performs a Fourier Transform of the recorded signal and gives the phase spectrum as a function of the frequency. The first and second derivatives of these curves often yield the best images.

The main advantages of the flash thermography are short measuring times and the ability to perform depth-resolved inspections, providing excellent performance for inspection of thin layers and for the detection of shallow defects.

The main disadvantages of flash thermography are a limited inspection area due to the energy of the flash lamps, a depth range limited to shallow defects, and a high-thermal loading on the sample.

The main advantages of transient thermography are that it is applicable for large-area measurements, halogen lamps are affordable heat-sources, and the low thermal load on the inspected component. The main disadvantage of transient thermography is that it is not suited for thick samples.

A rough rule-of-thumb for composite defect inspection is up to 3 mm for flash thermography (see Shepard2), up to 6 mm for transient thermography (and up to 12 mm for Lock-In thermography, discussed below). Of course, many material and configuration variables are present, so each application must be evaluated individually.

Pulsed Thermography – ExamplesFlash thermography with a high-power xenon lamp is a powerful tool for thin samples with shallow defects, such as corrosion under paint. Typical lamps are 6 kJ with 2 ms pulses. A dramatic example is shown in Figure 5, a helicopter panel with corrosion under the paint. This is a situation where transient or lock-in thermography would not work.

Transient thermography becomes important for large structures, as shown in the auto factory image in Figure 6. Depending upon the minimum defect size to be investigated, sections of an airplane fuselage or an AWACS dome as large as 4 ft by 6 ft, can be imaged in a single shot, as shown in Figure 7. Typical set-ups will have two to four halogen lamps (1 kW each), used with a 10 sec pulse and a 20 sec cool-down.

Figure 2. Equipment set-up for Pulse Thermography—Flash or Transient

Figure 3. Typical temperature signal of one pixel at the surface of a part after applying a heat pulse.

Figure 5. Flash thermography is well-suited for very thin samples.

Figure 7. AWACS dome showing (a) Photograph of the dome and workstation technician (b) Phase image of damaged region (not visually seen) (c) Normal region of the dome.

Figure 6. Inspection of dashboards for voids.

Figure 4. Transient is best for composite samples that are large in area and/or thick.

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Lock-In Thermography – Principles of OperationIn lock-in thermography, the dynamic stimulus can be applied from a wide variety of sources. For composite inspection, this includes halogen lamps, ultrasound, and mechanical stimulation. In all cases, the equipment setup is as shown in Figure 8.

Mathematically, in the lock-in case, the recorded temperature information is transformed into the frequency domain. In each pixel, the measured temporal evolution of the temperature is Fourier-transformed for all images of the recorded sequence. Phase and amplitude information are derived and presented as an image.

Advantages of the lock-in method include:• The summation causes a noise-filtering which enhances the contrast in

the resulting images. • Depth range for the phase information is twice that of the pulse mode,

~ 2µ with µ =.

• Lock-in allows detection of thermal waves with a sensitivity of 100 to 1,000 greater than the best thermal camera – down to µ-Kelvin range.

• The phase image is insensitive to external effects, such as sunlight, reflections, dirt, and emissivity differences – problems common to conventional thermography.

• The phase information is insensitive to uneven distribution of the applied heat.

• Large areas can be examined within a few minutes from a distance via non-contact measurement.

• An un-cooled IR-camera is normally sufficient.• Affordable heat sources are widely available (e.g. halogen lamps).• Visualization of deeper defects is possible than with pulse thermography.

Figure 9 shows the measuring principle, using CFRP material with a sinusoidal excitation. The curves are the typical temperature signals at the surface. Curves are shown for pixels with and without delamination. Note that the signal is evident in both amplitude and phase.

Lock-In Thermography – ExamplesThe most common excitation source for lock-in is the affordable halogen lamp. Figure10 shows an example of voids in a new composite honeycomb. Other defects like delaminations or inclusions also can be detected reliably with lock-in.

This challenging measurement of Figure 10 was done at a frequency of 0.05 Hz, and for two periods - a forty second measurement acceptable for many applications. The panel is 1-inch thick with defects on the front and back sides.

An important source of heat is ultrasound. Typical settings are 100 W at 20 KHz with a 200 ms burst frequency for synchronization. A disadvantage of the ultrasound technique is that it can be destructive—care is required in the temperature rise.

Figure 11 shows a test of Carbon-fiber Reinforced Ceramic (CFRC) with cracks originating from drilled holes.

A powerful tool for laboratory and factory is the use of mechanical excitation for heat generation through the thermoelastic effect, shown in Figure 12.

Applications of this technique include measurement of fatigue limits, imaging of stress patterns, crack propagation studies, and imaging of

Figure 8. Schematic of equipment setup for lock-in measurements.

Figure 9. Example of the lock-in measurement principle measurements.

Figure 10. Voids in composite Al-honeycomb structure.

ωρ pCK2

Figure 11. Top: Visible image. Bottom: Phase image. Fine cracks originating from the drilled holes are clearly evident.

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vibration patterns, as shown with turbine blades in Figure 13. The stress images of Figure 13 are equivalent to having 320 x 240 = 77,000 strain gages on the sample. Engineers often compare results such as these to FEA calculations.

Polymers present a challenge for thermography because they are often orientation-dependent, as shown in Figure 14. The different thermal characteristics of the fibers are evident in the figure. Features of 1 mm size are evident. A standard microscope lens could be used with this FLIR MW camera to examine detail as small as 5 µm.

SummaryIt is evident from the discussion above that pulsed flash, pulsed transient and lock-in thermography are important tools for research and for quality assurance in most areas of manufacturing. The particular technique used must be determined on a case-by-case basis. A starting point for the choice is shown in Figure 15 - do you need a blink…a glance…or a stare.

References1. X. Maldague, Theory and Practice of Infrared Technology for

Nondestructive Testing, John Wiley & Sons, 2001.2. S. Shepard, “Understanding Flash Thermography,” Materials Evaluation,

454 (2006).

About the AuthorsMarkus Tarin is President and CEO of MoviTHERM.Ralph Rotolante heads the East Coast Operations of MoviTHERM

Figure 12. Compression and expansion give temperature changes proportional to the stress.

Figure 13. Vibration mode on a turbine blade tested on a shake table.

717 Hz – 1st Mode

2136 Hz – 2nd Mode

3418 Hz – 3rd Mode

5911 Hz – 4th Mode

Tests performed on shake table.

Figure 14. Stress patterns on a polymer sample.

Figure 15. Time, power, and temperature regimes for pulsed flash, pulsed transient and lock-in.

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