Innovative Contact-Free Material Testing Method

Abstract

In non-destructive ultrasonic testing, a large part of the sound is already reflected at the interface between air and material. Therefore, a coupling medium such as water or gel must be applied and the test head has to be moved along the workpiece. This requires a complicated assembly and an increase in test duration. In order to ensure quality assurance by means of a non-contact method and at the same time to provide a new option for inline process monitoring, the Austrian company Xarion has developed a novel optical microphone: The laser-based Eta250 Ultra ultrasonic microphone (Figure 1) is based on the principle that sound waves influence the wavelength of light. This change is measured with a miniaturized interferometer and converted into a proportional output voltage. This ultrasonic measuring process can be performed without a coupling medium and without contact, which significantly reduces the work in testing, and enables automated testing with the help of robotics. According to the company, the frequency range from 10 Hz to 2 MHz in air shown in Figure 2 is unique worldwide. Thanks to its small and handy size, the test head (Figure 3) can easily be mounted on robot arms or process monitoring systems. When performing ultrasonic measurements for non-destructive testing, a coupling agent, such as a gel, usually has to be used, explains Dr. Balthasar Fischer, founder and managing director of Xarion. A basic problem here is that non-destructive testing of materials ideally should be contactless, to avoid, for example, contamination of the material, or damage to the specimen with water, or to facilitate robotic testing. However, air is only partially suitable as a coupling medium in comparison to water or gel. A lot of ultrasound signal is lost at the material-toair interface.