A good combination: Model and experiment for a deeper look

November 21, 2016
Computer simulation of ultrasound measurement of a concrete-cube. Credit: Chair for Computation in Engineering / TUM

Doctors performing medical check-ups want a full picture of the patient's health without using the scalpel. Engineers who want to investigate the stability of a bridge without cutting into it are in the same position. Non-destructive testing methods play a major role in guaranteeing quality and safety, driving substantial interest in refined methodologies. Researchers at the Technical University of Munich (TUM) have developed a method that gives a precise picture of the inner world of objects combining a computerized model and experiment.

Ultrasound measurements are among the most frequently applied technologies in non-destructive testing. The principle is similar to the echolocation that 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 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 with the same volume, the object is located exactly in the direction the sound was emitted.

Conventional non-destructive testing 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 Prof. Ernst Rank, head of the TUM Chair for Computation in Engineering. "But it only returns a limited amount of information on their exact position, extent and orientation."

When the whole wave counts

In order to solve this 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 . 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.

The effort is worth it

This 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 wave form inversion with real structures in construction and mechanical engineering. This is planned for a research project in which Prof. Rank's team will partner with Prof. Christian Grosse's team at the Chair of Non-destructive Testing (NDT).

Explore further: Lego-like wall produces acoustic holograms

More information: Robert Seidl et al, Iterative time reversal based flaw identification, Computers & Mathematics with Applications (2016). DOI: 10.1016/j.camwa.2016.05.036

Related Stories

Lego-like wall produces acoustic holograms

October 14, 2016

Research Triangle engineers have developed a simple, energy-efficient way to create three-dimensional acoustic holograms. The technique could revolutionize applications ranging from home stereo systems to medical ultrasound ...

Lightning-fast materials testing using ultrasound

April 1, 2011

For years, ultrasound has proven to be a valuable tool in non-destructive materials testing. However, the demands of modern production conditions are increasing all the time. Researchers at Fraunhofer have now developed a ...

Researchers check quality of chocolate with ultrasound

June 1, 2016

Belgian chocolate is a world-famous delight. But producing a divinely delicious bar of chocolate that has a beautiful gloss, makes that wonderful sound when you break it, melts in your mouth, and maintains all these qualities ...

Acoustic imaging with outline detection

September 21, 2015

Reverberated sound can make objects visible. The sonar is used in the shipping industry to acquire information about the seabed or shoals of fish, while gynaecologists use ultrasound images to study foetuses in the womb. ...

UA engineers twist physics laws to boost sonic science

August 17, 2016

For decades, advances in electronics and optics have driven progress in information technology, energy and biomedicine. Now researchers at the University of Arizona are pioneering a new field—phononics, the science of sound—with ...

Recommended for you

Ancient barley took high road to China

November 21, 2017

First domesticated 10,000 years ago in the Fertile Crescent of the Middle East, wheat and barley took vastly different routes to China, with barley switching from a winter to both a winter and summer crop during a thousand-year ...

New paper answers causation conundrum

November 17, 2017

In a new paper published in a special issue of the Philosophical Transactions of the Royal Society A, SFI Professor Jessica Flack offers a practical answer to one of the most significant, and most confused questions in evolutionary ...

0 comments

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.