Infrared cameras produce images of heat radiation emitted in the infrared spectrum from objects. This is why infrared cameras allow to see in the total darkness. In Non Destructive Testing and Evaluation (NTD & E) by thermography, infrared cameras are used to observe how the heat propagates in materials as the material is heating up or cooling down. Invisible defects within the inspected material strongly affect the diffusion of heat. Thus, defective areas may look cooler or hotter in respect to non defective areas of the sample. This difference of temperature caused by non defects or non uniform material is visible through infrared cameras. This is how Infrared thermography works when used for non destructive testing applications. It is usually called IR NDT. It has the advantage of being non intrusive, unlike X-Ray where both sides of the material under inspection must be accessed. Because it is relatively easy to use and simple to understand, IR NDT is largely used in the aerospace industry to test the integrity of the fuselage of planes, in the automotive industry and everywhere where is it needed to test materials, tubes, welds, uniformity, quality etc ...

Advanced tutorial about Infrared Thermography for Non Destructive Testing (NDT)

Partial reproduction of the publications "Active infrared thermography with applications", IEEE Canadian Review — Spring 2008 by C. Ibarra-Castanedo (Visiooimage inc.) et al. and "Active Infrared Thermography Techniques for the nondestructive testing of materials" from the book "Ultrasonic and Advanced Methods for Nondestructive Testing and Material Characterization", ed. C H Chen, by C. Ibarra-Castanedo (Visiooimage inc.) et al.

1. Introduction

Active infrared thermography [1] is a nondestructive testing and evaluation (NDT&E) technique requiring an external source of energy to induce a temperature difference between defective and non-defective areas in the specimen under examination. A wide variety of energy sources are available, the most common types can be divided into optical, mechanical or inductive, although many other sources can be employed. Figure 1 shows typical examples of heat sources of these three excitation types.

1.1 Optical excitation : With optical excitation, defects are stimulated externally, that is, the energy is delivered to the surface of the specimen, where light is transformed into heat. Thermal waves propagate by conduction from the surface through the specimen until they reach an internal discontinuity that either slows down or speeds up their propagation (depending on the thermal properties of both the specimen and the discontinuity). This can be seen as hot or cold spots on the specimen’s surface with an infrared camera. Optical devices include flashes (for pulsed heat stimulation, see Figure 1), infrared lamps (for step heating) or halogen lamps (for periodic heating), among others. Optical excitation is the most widely used form of excitation in thermography for NDT&E [2] that was originally used to develop the classical thermographic techniques such as pulsed and lock-in thermography, described in section 2.

1.2 Mechanical excitation : In the case of mechanical excitation also known as vibro-thermography (VT) , the energy is applied into the specimen by means of mechanical oscillations using, for example, a sonic or ultrasonic transducer that is in contact with the specimen (usually a coupling media is employed); see Figure 2 . In this case, the defects are stimulated internally; the mechanical oscillations transmitted into the specimen spread in all directions inside it. The mechanical energy is dissipated at the discontinuities in the form of heat waves that travel to the surface by conduction.
Ultrasonic excitation has received considerable attention in recent years. The technique known as vibrothermography (also ultrasound thermography [3] or thermosonics [4]) is typically used in the inspection of cracks and micro-cracks [5] in metallic structures, which are very difficult to inspect with optical excitation. As in optical excitation, pulsed thermography (better known as burst thermography in the case of ultrasonic excitation) and lock-in thermography are used.

In either lock-in or burst configuration, mechanical excitation such as vibro thermography is extremely fast, although it is necessary to relocate the transducer (and to immobilize the specimen again) to cover a large area for inspection. Hence, VT is more suitable for relatively small objects. It is the most appropriate technique to inspect some types of defects, e.g. micro cracks. On the contrary, it does not perform very well in some other cases in which application of optical techniques are straightforward, e.g. water detection. But probably the most inconvenient aspect of VT is the need of a coupling media between the sample and the transducer, and the need of holding the specimen. On the other hand, there is only minimal heating of the inspected specimen since energy is usually dissipated mostly at the defective areas, although there is some localized heating at the coupling and clamping points.

1.3 Inductive excitation : Inductive excitation can be applied internally to electro-conductive-materials, generating eddy currents at a specific depth (determined by the frequency of the excitation), heating up the specimen and the eventual internal defects. Surface or subsurface defects produce variations on the eddy current patterns, changing the temperature distribution. As with the previous excitation forms, these temperature variations can be detected on the surface with an infrared camera.
Eddy current thermography [6] or Induction heating thermography [7] is the latest development in the field of active thermography. It is receiving considerable attention at the moment (year 2008) from researchers around the world [8]-[9]. As in the case of optical and mechanical excitation, inductive stimulation can be deployed in the form of pulses (pulsed thermography) or amplitude modulations (lock-in thermography), which are discussed in section 2 below.

1.4 Data acquisition and configurations : A typical data acquisition setup for active thermography is depicted in Figure 3. For simplicity, only optical excitation is portrayed in this illustration. Similar configurations are used for mechanical and inductive excitations. The experiments can be performed in reflection, that is, both the heating and the recording are performed from the same side, or in transmission, that is, the surface is heated from one side and data is acquired from the opposite side [2].

When using an external stimulation (as with optical excitation), the reflection approach is best suited to detect defects located close to the heated surface; the transmission approach allows detecting defect close to the rear surface because of the spreading effect of the thermal front. It should be noted, however, that it is not always possible to use the transmission approach since the rear surface is not always accessible. Similarly, in the case of complex structures made of various thicknesses of different materials, the defect depth cannot be estimated since the thermal waves travel the same distance regardless of the defect depth. In the case of internal stimulation (mechanical or inductive), the choice of one configuration over the other is a matter of finding the best location to position the heat source, which is often limited by accessibility.

2. Pulsed vs. Lock-in vs. Step techniques: advantages and trade-off

Regardless of the excitation mode being used, there are basically three thermographic techniques: pulsed, step and lock-in. The experimental and theoretical aspects are different for each of these techniques and so are the typical applications.

2.1 Pulsed thermography: Pulsed thermography (PT) is one of the most popular thermal stimulation methods in active thermography [1]-[2]. One reason for this is the quickness of the inspection relying on a short thermal stimulation pulse, with duration going from a few milliseconds for high conductivity material inspection (such as metal) to a few seconds for low conductivity specimens (such as plastics). In addition, the brief heating prevents damage to the component.
Depending on the excitation source, it might be interesting to observe both the heating phase (while the pulse is applied) and the cooling phase, or only the surface cooling phase. For instance, in optical PT there is no interest in observing the thermal changes during the excitation since these images are often saturated. More importantly, this early data does not contain any information about the internal defects yet. However, images prior to the excitation (cold images) are very useful at pre-processing stages and for some advanced processing techniques. Conversely, thermal changes in vibrothermography are very fast – a few seconds – and important information can be found at any instant, during heating or cooling. In this case, the whole profile needs to be analyzed.
There are many processing techniques such as: thermal contrast, differential absolute contrast (DAC), principal component thermography (PCT), thermographic signal reconstruction (TSR), and pulsed phase thermography (PPT). References [10] to [13] describe all these techniques in detail.

Advantages, disadvantages and applications of pulsed thermography: Pulsed thermography is fast (from a few seconds for high conductivity materials to a few minutes for low conductivity materials) and easy to deploy. There are numerous processing techniques available although many of them are more complex when compared to lock-in thermography. Thermal based techniques are affected by non-uniform heating, emissivity variations, environmental reflections and surface geometry. These problems however, are dramatically reduced using advanced processing algorithms, e.g. PPT, DAC and TSR.

2.2 Step heating thermography : Step heating uses a larger pulse (from several seconds to a few minutes). The temperature decay is of interest; in this case, the increase of surface temperature is monitored during the application of a step heating pulse. Variations of surface temperature with time are related to specimen features as in PT. This technique is sometimes referred to as time-resolved infrared radiometry (TRIR). TRIR finds many applications such as evaluation of coating thickness – including multilayered coatings, determination of coating-substrate bond integrity or evaluation of composite structures [14]. Although at the moment only optical excitation has been used in step heating, there is no limitation to the use of other excitation forms.

2.3 Lock-in thermography : In lock-in thermography (LT) [15], also known as modulated thermography [16], the specimen is stimulated with a periodic energy source. Typically, sinusoidal waves are used, although it is possible to use other periodic waveforms. Internal defects, acting as barriers for heat propagation, produce changes in amplitude and phase delay of the response signal at the surface. Different techniques have been developed to extract the amplitude and phase information. Fourier analysis is the preferred processing technique since it provides single images, ampligrams or phasegrams (the weighted average of all the images in a sequence). The resulting Signal-to-Noise Ratio (SNR) is therefore very high. Phase data in particular is very interesting in NDT&E [17] as it is less affected than raw thermographic data by non-uniform heating, emissivity variations at the surface, reflections from the environment and surface geometry [18].

Advantages, disadvantages and applications of lock-in thermography: Given that LT requires to perform an experiment for each and every inspected depth and as there is a stabilization time before reaching a permanent regime, inspection by lock-in thermography is in general slower than other approaches such as pulsed thermography. A complete LT experiment is carried out by inspecting the specimen at several frequencies, covering a wide range from low to high frequencies, then a fitting function can be used to complete the amplitude or phase profiles for each point (i.e. each pixel). Nevertheless, there is a direct relationship between depth and the inspection frequency that allows depth estimations to be performed from amplitude or phase data without further processing. Furthermore, the energy required to perform an LT experiment is generally smaller than in other active techniques, which might be interesting if a low power source is to be used or if special care has to be given to the inspected part, e.g. cultural heritage pieces, works of art, frescoes, etc. [20]. Some typical applications include determination of coatings thickness, detection of delaminations, determination of local fiber orientation, corrosion detection and inspection of cultural properties. [21], [20]

3. References click to expand...