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Ultrasound enters non-destructive testing practice

From the Technical University of Munich … Engineers who want to investigate the stability of a bridge without cutting into it are in the same position as a doctor seeking a full picture of a patient’s health without resorting to a scalpel. They now have a non-destructive testing (NDT) technique that gives a precise picture of the inner world of objects combining a computerized model and experiment.

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Technical University of Munich researchers model the ultrasound-enabled testing method on a concrete cube.

Ultrasound measurements are among the most frequently applied technologies in NDT. The principle is similar to the echolocation bats use to gather information on their surroundings, possible obstacles and prey using ultrasonic sounds. The bat localizes the direction to an object by sensing which ear registers the echo of a sound first. The smaller the time difference between the arrival of the signals at the respective ear, the smaller the angle between the direction of the sound emitted and the object. If the echo arrives at both ears at the same time and volume, the object is located exactly in the direction the sound was emitted.

Conventional NDT methods also use this principle to localize structural defects by means of ultrasound. Measurements detect the speed, wavelength and time difference with which the ultrasound waves or their reflections arrive at sensors mounted on the surface of the test object. These measurements are then compared with data from a non-defective reference object to expose variances.

“This standard procedure can basically be used to identify and localize defects,” says Professor Ernst Rank, who heads the TUM Chair for Computation in Engineering. “But it only returns a limited amount of information on their exact position, extent and orientation.”

WHOLE WAVE COUNTS

In order to solve the problem, the scientists first mathematically modeled the material properties of the aluminum plates used in the experimental part of their project. They then used this model to calculate the propagation of the ultrasound waves and their reflection. The virtually determined measured values were compared with the results from real experiments in which the ultrasound waves were sent through real aluminum plates.

They discovered that the measured values of the virtual and real sensors differed from one another, since special factors within the structure can lead to a specific scattering or refraction of the ultrasound waves. The researchers used a trick to get around this error source: The difference between measured real and virtual waves at the sensors is reintroduced as a signal and used to calculate an improved model. This method, referred to as waveform inversion, thus utilizes the entire information content of the wave field measured.

A method developed in the 1980s by the Spanish geophysicist Albert Tarantola was long considered practically unfeasible due to the very large amount of data required. It was not until the development of innovative algorithms and advances in computer technology in recent years that the highly computation-intensive application of wave form inversion could be realized. “We can surely ask ourselves whether the additional computational effort is worth it,” says information scientist and mathematician Robert Seidl. “But when we look at the benefits of our method, the answer has to be yes, the effort is more than worth it.”

One of the most important tasks of the near future will be applying and validating waveform inversion with real structures in construction and mechanical engineering. This is planned for a research project in which Professor Rank’s team will partner with Professor Christian Grosse’s team as NDT chair.