Influence of temperature on ultrasound measurements

Timur Turkovic, winter term 22/23

Ultrasonic measurements are a powerful tool in nondestructive testing. When using ultrasonic measurements, the influence of temperature on the sensors and the measurement parameters must be taken into account. Depending on temperature and application, suitable sensors must be chosen.

Actuators/Sensors

A major influence on ultrasonic measurements in extreme temperature conditions is the reaction of the sensors to thesetemperatures.

Piezo actuators/sensors

One of the most used sensor types for ultrasonic measuring is the piezoelectric sensor, which utilizes the piezoelectric effect to create/measure ultrasonic waves. This effect however is based on a material propertywhich is lost after surpassing the so-called Curie temperature T_c of the material. The Curie temperature usually is not much higher than 150°C.

One solution for this limitation is using materials like bismuth titanate or LiNbO3 with a higher Curie temperature. These however tend to have a lower piezoelectric coefficient, making measurements with them more difficult. Examples for such materials can be found in Tabel 1.

Another solution is the use of a waveguide/delay line/wedge that sits in between the material and the piezoelectric element, ensuring that the elements temperature stays below the Curie temperature. An example for such a setup can be seen in Figure 2 The drawback of this method is that dissipation can occur in the material in between. Additionally, the method chosen for bonding the filler material to the piezoelectric element must withstand the temperatures and has to additionally have a suitable thermal coefficient to match the transducer as well as the test piece. A protective layer like this can additionally be used for impedance matching, as the impedance of the test object often differs from the impedance of the piezoelectric material. ^[1]

EMAT (electromagnetic acoustic transducer)

EMATs use electromagnetic mechanisms to generate acoustic waves in conducting materials. Therefore, they do not need to be in direct contact with the test object. 

Below the Curie temperature Tc, the Lorentz mechanism remains dominant. Above Tc the   magnetostrictive mechanism becomes more efficient as the magnetic moments rearrange from ordered domains (ferromagnetic) to a disordered (paramagnetic) state. This magnetic phase transition lowers the magnetostrictive constant and is accompanied by large changes in the efficiency of electromagnetic ultrasound generation leading to the use of these detectors as a method of studying phase transitions in magnetic alloys. In the paramagnetic state the generation of ultrasonic waves has a low efficiency, although the detection is still possible. ^[2]

Temperature dependence of ultrasonic velocity in materials

Another effect that must be accounted for is the change of the wave velocity inside the object caused by a change in temperature. These changes are usually negligible with small variations but as the variations get bigger the changes gain significant influence on the measurement. Therefore, the relationship between the temperate of the object and the travelling speed of the ultrasonic wave must be known in order to get correct results. Additionally, it must be considered that several materials retain a changed wave velocity even after cooling down. In order to account for these variations careful studies of the wave velocity must be conducted to ensure that,  even after several heating and cooling cycles, the correct velocity is used. ^[3]

Literature

1. High Temperature Ultrasonic Transducers: A Review. Kazys, Rymantas & Vaškelienė, Vaida. 2021.
2. S.E. Burrows, Y. Fan, S. Dixon. High temperature thickness measurements of stainless steel and low carbon steel using electromagnetic acoustic transducers. NDT & E International. 2014, Vol. 68.
3. Influence of small temperature variations on the ultrasonic velocity in concrete. Niederleithinger, Ernst & Wunderlich, Carolin. 2012.