Non-Destructive Crack Detection by Ultrasound-Thermography
- Ultrasound-excited thermography (also known as vibrothermography) is a superior method for the detection of cracks in wood boards, panels and surfaces.
- The ultrasonic thermography provides a highly accurate, precise and fast detection without destructing the inspected material.
- As non-destructive detection method, the ultrasonic vibrothermography outperforms the on-line thermography in accuracy.
Crack and Flaw Detection with Ultrasonic Thermography
Advantages of ultrasonic thermography detection:
- High accuracy and precision
- Rapid inspection (in a few seconds or less)
- Deep inspection range
- Non-destructive testing
Thermography methods are based on infrared technology and can provide data about the sub- surface structure of a material by observing differences in thermal emission from the surface using infrared cameras to record the data. The emission depends on heat conduction in the material. Depending on the way the heat transfer is generated, thermography methods are divided into passive and active. In active thermography, heat transfer can be initiated by external energy excitation using electromagnetic radiation or ultrasound (a.k.a. ultrasonic vibrations) and is dependent on physical properties of the material such as thermal conductivity and diffusivity, density, moisture content etc. If a defect below the surface has better insulating properties than the rest of the material, the defect acts as a barrier for the heat transfer, so that the emissivity from the surface above the defect is higher (Meinlschmidt, 2005).
Ultrasound-excited thermography (UET) is a variant of vibro-thermography (Maldague 2001). Unlike most thermography methods, the ultrasound-excited thermography is a contact method. A sonotrode is brought into physical contact with a test piece in order to excite the object with a mechanical wave. Heat is generated locally in the cracks and/or other disbonds by friction where a direct conversion of mechanical into thermal energy occurs (Maldague 2001). The initiated heat transfer results in heat emission from the surface of the object. A local increase in temperature is reached within milliseconds and is imaged by an infrared camera as a bright IR source on a dark background. (Cho et al. 2007).
- Cho J., Seo Y., Jung S., Kim S., Jung H. (2007): Defect detection within a pipe using ultrasound excited thermography. Nuclear Engineering and Technology 37:637-646.
- Lukowsky D., Meinlschmidt P., Grote W. (2008): Ultraschallangeregte Thermographie an Holzverklebungen – Entwicklung einer Prüfmethode. Holztechnologie 49:42-47.
- Meinlschmidt P. (2005): Thermographic detection of defects in wood and wood-based materials. Proc. of the 14th international Symposium of non-destructive testing of wood, Hannover, Germany.
- Popovic D. (2015): Crack Detection and Classification of Oak Lamellas using On-Line and Ultrasound Excited Thermography. Master Thesis – Lula University of Technology Sweden, 2015.
- Popovic D.; Meinlschmidt P.; Plinke B.; Dobic J.; Hagman O. (2015): Crack Detection and Classification of Oak Lamellas Using Online and Ultrasound Excited Thermography. Pro Ligno, 11(4): 464-470.