Лако-красочные материалы — производство

In active thermography, a proper energy source must supply or transfer sufficient energy to the sample to generate an adequate temperature contrast, and this is the first objective of these testing methods. The amount of energy must also be capable of generating a contrast, without damaging the component [6].

Pulse thermography and lockin thermography use different heat sources. Lockin thermography requires a periodic or sinusoidal heat source, with the penetration depth of the thermal waves being altered by frequency modulation, good conductors being excited by higher frequencies, and materials with a low thermal conductivity being excited by low frequencies. In the case of external excitation of thermal waves, a temperature modulation must be generated on the surface ofthe sample that is then conveyed to inner regions of the sample. The following mechanisms provide for a transfer of heat:

• heat conductivity

• radiative heat transfer

• convection

For pulse thermography, a short heat pulse is supplied to the sample, with the duration of the pulse depending on the thermal conductivity of the material (Figure 7.25): the higher the thermal conductivity, the shorter the pulse duration.

The following excitation mechanisms are used:

• Laser: When a sample is irradiated with laser light, the electromagnetic irradiation is absorbed by the sample, and heat is produced. Irradiation energy is propagated through electromagnetic waves (photons), so that it can be conveyed to a compo­nent without any contact. Optical irradiation sources are therefore all suited to the noncontact generation of thermal waves. In conventional photothermal imaging, laser light was used owing to its point excitation capacity. The radiation from a laser source is easy to focus and to modulate, even in the case of very high frequencies of up to several MHz. One advantage of this method is that, for excitation, a wavelength outside the sensitivity range of the detector can be used. However, provisions must be taken to prevent unwanted radiation from falling on the detector, and this can be achieved by using a filter. Lockin thermography, on the other hand, is based on the heating of an area, and the low divergence of a laser beam does not provide any benefit here. Furthermore, the necessary safety precautions required are a fundamental disadvantage of all high-performance laser sources, particularly in industry.

• Light-emitting diodes(LEDs): These are considerably less expensive and serve as an alternative to laser diodes for use as an excitation source. Power LEDs, such as those used in car manufacture, have a luminance of up to 6000 candela. However,

their optical performance is low, which is a disadvantage. Currently, LEDs are undergoing rapid development and, if their performance continues to grow, they will represent an interesting alternative for conventional flash lamps and radiators as they provide flash sequences at any desired rate for lockinthermography.

• Halogen lamps: These are particularly well suited to the illumination of large areas. As their spectrum comprises both the visible and infrared (IR) ranges, filters must be used when halogen lamps are employed as the excitation source. According to the Wien radiation law, halogen lamps have their maximum radiant excitation at a wavelength of 1 pm. Consequently, the long-wave portion of their emission spectrum is within the sensitivity range of thermography cameras. This coherent noise must be filtered out, for example by using water or polycarbonate filters, because medium-wave infrared cameras (MWIR) function in the adjacent range of 3-5 pm. With current-controlled temperature modulation, the maximum frequency is limited to 1.85 Hz, and computational correction is required due to nonsinusoidal optical output. Computer-controlled lamps with wattages of 1 to 6kW are well-suited heat sources for all types of low-frequency application, provided that the test object absorbs their radiation. If the surface is metallic bright, it is generally painted black for this reason.

• Flash lamps: These are only suitable for pulse thermography. With energies of up to 12 kJ, the flash lamps used frequently in the photographic industry make it possible to achieve an energy absorption of 1.5 J cm~2 on a black surface measuring 20 x 20 cm2.

• Hot air. A hot-air blow-dryer provides heat via a modulated stream of air. In the wavelength range of 8-12 pm the atmosphere is virtually transparent, and conse­quently the thermography camera will not record the hot air. The optical properties of the sample are also irrelevant for excitation. Although the thermal waves are easily generated in practice, the upper frequency limit is 0.04 Hz due to the inertia of the hot wires.

• Microwaves. When microwaves are absorbed by the sample, the molecular rotation vibrations induced will result in a heat transfer. Microwaves work in a range from 1 GHz to 3,000 GHz; this method is confined, however, to electrical non-conductors.

• Electrical heating. Direct modulation of the voltage applied is well suited to the testing of electrical conductors, with defects being visualized as ‘hot spots’. An example of application is the detection of leakage current in solar cells.

• Induction. An elegant solution for noncontact thermal wave excitation in electrical conductors is the generation of an oscillating electromagnetic field using an induction coil and a suitable high-frequency generator. In electrical conductors, eddy currents are highly attenuated and only exist within a thin layer near the surface (the ‘skin effect’). The depth of penetration of the eddy current (d) is described by:

where c0 is the speed of light, w the frequency, and Г a material constant which is proportional to the electrical conductivity. The d-value for copper, for example, which determines the heating zone within the material, is 50 pm at 1 MHz. Consequently, with induction heating the thickness of the layer in which heat can be generated is variable, which is in fact an overall advantage. The heating response of flawless regions of a sample differs from that of defective regions, thus making it possible easily to detect flaws in contact joints of composite laminar materials (adhesive bond, soldering, welding), or near-surface fatigue cracks. Intensity modulation is achieved by varying the distance between the inductor and the sample surface.

• Ultrasound: A relatively new method for the detection of cracks, for example, is excitation by power ultrasound. Vibration is excited in a sample by means of an ultrasound generator with an electrical power of up to 5 kW. As a result, crack edges may rub against each other, or plastic deformations may arise in the defective region, and the heat induced as a consequence can be detected on the surface. This method is very rapid, and very sensitive, although there is a risk that the sample will be damaged.

7.6.6