Active thermography and IRNDT

Active thermography is one of the important research areas of the Thermomechanics/Laser technologies department. It includes the Infrared Nondestructive Testing (IRNDT) which is modern method for inspecting surface layers and thin parts made of various materials.

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

The state-of-the-art measurement system available in our laboratory extends the range of possible applications of the technique. Thanks to its modularity it can be utilized not only for research and development, but also for a modern industrial production. Our research center can be your partner for analyses, verification studies, determination of parameters, and implementation of the measurement system in your laboratories or production lines. We offer inspection of prototype parts and a practical verification of a usability of the system specific setup for a desired application. Afterwards, we can be your partner in designing, technical realization and fine-tuning of a measurement procedure for your application.

Thermographic testing highlights

Some basic information about infrared non-destructive testing. It is an informative description only. Please, take into account that the parameters or usability ranges can differ based on inspected materials, excitation and evaluation methods, measurement configurations, application requirements etc.

  • Active infrared thermography is a technique based on a thermal excitation and an observing of a material response by an infrared camera.
  • It is used for detection of defects or inhomogeneities in materials, for example cracks, voids or foreign bodies.
  • Defects or foreign particles in an inspected part can be detected if their thermal properties are different from the inspected material.
  • The method can be generally used for both off-line and on-line inspection.
  • Sample heating is mostly in the range of tenths of degree or degrees of Celsia (it depends on sample properties, size and excitation method).
  • The inspection time is generaly from a few second to minutes (based on the used method). However, measurements taking tens of minutes or a few hours can be also performed for some special applications.
  • It is possible to inspect materials in a high range of thermal properties.
  • A standard maximum inspection depth is a few millimetres. Surface and sub-surface inspection is also possible.
  • A maximum spatial resolution generally starts from about one millimetre. However, it depends on an application and equipment used (can be better with a special equipment or worse for a large area inspection).
  • The evaluation is mostly visual (by an operator), however, an algorithms and methods for automated evaluation could be developed for some specific cases.

Active thermography

Scheme of the active thermography principle.

Active thermography uses an external source for measured object excitation, that means introducing an energy into the object. Various excitation sources can be used for the active thermography and nondestructive testing, for example laser heating, flash lamps, halogen lamps, electrical heating, ultrasonic excitation, 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 then a distortion of temperature field. This distortion is detected as temperature differences on the material surface. Another possibility is to use thermo-physical processes in the material, when 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. It is the case of ultrasound excitation for example.

IRNDT

Scheme of IRNDT principle.

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 – available also at the NTC research centre – include Lock-In, Pulse or Transient evaluation techniques. Continuous excitation can also be used in some cases.

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

The IR nondestructive testing system available at the NTC research centre is modular. It means that various excitation sources can be combined with various infrared cameras and various evaluation methods depending on application, tested material, measuring time demands, size of a tested area, etc. The modularity allows universal usage of the system for various industrial, scientific and research applications.

Lock-In thermography (periodic excitation method)

A modulated periodic source is used for the excitation. The phase and amplitude shift of the measured signal are evaluated and the analysis can be done by various techniques. Halogen lamps, LED lamps, ultrasound excitation or an electric current are suitable excitation sources. It has the advantage that it can be used on large surfaces and it puts a low thermal energy on the part being inspected. The disadvantage is a longer measurement time and dependence of detection capabilities on a geometrical orientation of defects (except of an indirect excitation such as ultrasound). The Lock-In method is suitable for testing components with a low thermal diffusivity and it has many modifications for various specific applications (such as Lock-In Ref, Lock-In Online, etc.).

Pulse thermography (pulse method)

A very short pulse – usually in the units of milliseconds – is used to excite the object. The cooling process is then analyzed. A flash lamp is typically used as an excitation source. The advantage of this method is the speed of the analysis and a possibility to estimate the defects depth. The disadvantage is a limited depth of the analysis, a limited area that can be inspected (with regard to a usable power of excitation sources) and a dependence of detection capabilities on geometrical orientation of defects.

Transient thermography (thermal wave method)

In principle, the excitation and evaluation are similar to the pulse thermography, however, the pulse length is much bigger. Less powerful excitation sources are required compared to the pulse thermography. It is therefore possible to analyze larger areas and the measurement time is shorter than in the case of Lock-In thermography. As in the pulse thermography, the sensitivity of the method is limited by the geometrical orientation of defects. Halogen lamps are the suitable excitation source for this type of evaluation.

Continual excitation

The simplest method usable only in special applications.