Active thermography

Active thermography is an advanced nondestructive testing procedure, which uses a thermographic measurement of a tested material thermal response after its external excitation. This principle can be used also for non-contact infrared non-destructive testing (IRNDT) of materials.

The IRNDT method is based on an excitation of a tested material by an external source, which brings some energy to the material. Halogen lamps, flash-lamps, ultrasonic horn or other sources can be used as the excitation source for the IRNDT. The excitation causes a tested material thermal response, which is measured by an infrared camera. It is possible to obtain information about the tested material surface and sub-surface defects or material inhomogeneities by using a suitable combination of excitation source, excitation procedure, infrared camera and evaluation method.

Modern thermographic systems with high-speed and high-sensitivity IR cameras extend the possibilities of the inspection method. Modularity of the systems allows their usage for research and development applications as well as in modern industrial production lines.

Thermovision nondestructive testing of components can be carried out on a wide range of various materials. Thermographic inspection of material can be regarded as a method of infrared defectoscopy, that is capable of revealing material imperfections such as cracks, defects, voids, cavities and other inhomogeneities. The thermographic testing can be provided on individual components in a laboratory or directly on technology facilities that are in duty.

Active thermography uses an external source for measured object excitation, that means introducing an energy into the object. The excitation sources can be classified by the principles:

Various excitation sources can be used for the active thermography and nondestructive testing, for example laser heating, flash lamps, halogen lamps, electrical heating, ultrasonic horn, eddy currents, microwaves, and others. The measured object can be heated by an external source directly, e.g. by halogen lamps or hot air. The material inhomogeneities or defects cause a distortion of the temperature field. This distortion is detected as temperature differences on the material surface. Another possibility is to use thermophysical processes in the material, where mechanical or electrical energy is transformed into thermal energy due to defects and inhomogeneities. It creates local temperature sources, which cause temperature differences detected on the object surface by infrared techniques, such as in the case of ultrasound excitation.

A lot of methods were developed for active thermography for the nondestructive testing measurement evaluation. The evaluation methods selection depends on application, used excitation source and excitation type (pulse, periodic, continuous). In the simplest case, the response is evident from a thermogram directly. However, it is necessary to use advanced analysis techniques in most cases. The most common methods include Lock-In, Pulse or Transient (Step thermography) evaluation techniques, with continuous excitation used in some cases:

A high-speed cooled infrared camera with a high sensitivity is commonly used for IRNDT applications. However, an uncooled bolometric infrared camera can be used for specific applications. It can significantly reduce acquisition costs of the measurement system.