where expertise comes together - since 1996 -

The Largest Open Access Portal of Nondestructive Testing (NDT)

Conference Proceedings, Articles, News, Exhibition, Forum, Network and more

where expertise comes together
- since 1996 -

Technical Discussions
Edouard G. Nesvijski
Edouard G. Nesvijski
03:52 Aug-13-1998
Pelly's questions

In reply to Pelly’s questions regarding waves in a thin plate I would like to make some comments that may considerably differ from the traditionally accepted theory.
But first of all
What do you mean by “plate”? Is it a really very thin plate or is it a thin layer on a base? We have several different problems here.

Regarding Lamb waves.
If the plate serves as a structural element its minimal thickness (or thinness) is always limited by stiffness of the structure. That is why if the ultrasonic wavelength corresponds to the plate’s thickness, we have two groups of Lamb waves: symmetric and asymmetric. They differ by phase and group velocity, spread of shifts and stresses along the plate thickness. Each of the Lamb wave groups represents a family of different wave modes depending on their frequency. High dispersion of velocity is observed here. According to my physical notion these groups are complex configurations of longitudinal and shear stresses caused by special boarder conditions of the plate. In this case it is impossible to separate them. Then there are no pure longitudinal or shear Lamb waves. Sometimes for very thin plates it is possible to consider only the first modes of symmetric and asymmetric waves, but it is an artificial approach convenient for theoretical modeling. In reality we use ultrasonic impulses for materials testing which theoretically have unlimited spectrum of frequencies. Practically the spectrum is limited only by transducer frequency bands and equipment measuring possibilities (relation of noise to threshold levels). This uncertainty characteristic of the ultrasonic pulse method is one of the main difficulties of measuring the Lamb wave parameters.
Regarding transducers.
There are different angles of inducing ultrasonic impulses into the material according to Snell law, but this critical angle exists only for one fixed frequency. Impulse signal, as a rule, has a wide frequency spectrum. I do not know a technical solution for getting critical angles for all the spectrum components! Transducer design also affects testing in this case. Traditionally plate (with liquid couplant) contact transducers or air couplant or dry point contact transducers generate stresses in the plate with different directional diagrams, thus different spectrums of pulse ultrasonic waves.
Regarding measurements.
It is necessary to remember that theoretically we consider only models of the waves, where we use notions of only phase or group velocities. As far as I know, the existing ultrasonic equipment do not measure these parameters in a pure form. It is very strange that this problem seems to be ignored. Do we know what we are measuring?
I welcome any discussion and comments for searching the truth.
Good luck, Pelly, but keep in mind that your road is full of stones and lambs.



Product Spotlight

IK4-IDEKO develops an ultrasonic train wheel inspection system for CAF

The Basque technology centre IK4-IDEKO has developed a state-of-the-art ultrasonic inspection system
for the train wheel. This system secures a sound condition of train wheels and is thus a significant contribution to rail transport safety. The device was delivered to CAF recently and its use allows the manufacturer to become an approved supplier of rolling stock in Italy, as it meets the demanding homologation standards of this country. \\\\r\\\\n

Ultrasonic Testing Immersion Tanks with Unmatched Scanning Features

TecScan’s non-destructive testing Ultrasonic Immersion Tanks & scanners are designed for high perf
ormance and demanding NDT testing applications. Our Scan3D™ line of High Precision Immersion Tanks are specifically designed for automated ultrasonic testing of complex composites parts used in aerospace and industrial applications.

Immersion systems

ScanMaster ultrasonic immersion systems are designed for high throughput, multi shift operation in a
n industrial or lab environment. These fully integrated systems provide various scanning configurations and incorporate conventional and phased arrays technologies to support diverse applications, such as inspection of disks, bars, shafts, billets and plates. All of ScanMaster immersion systems are built from high accuracy scanning frames allowing for scanning of complex parts and include a multi-channel ultrasonic instrument with exceptional performance. The systems are approved by all major manufacturers for C-scan inspection of jet engine forged discs. Together with a comprehensive set of software modules these flexible series of systems provide the customer with the best price performance solutions.

Aerospace Systems - Automated Ultrasonic Inspection

USL are specialists in the design and manufacture of turnkey ultrasonic inspection systems for aer
ospace applications. From monolithic composites to complex honeycomb structures. This video shows just a few examples of what is possible, find out more at: www.ultrasonic-sciences.co.uk

We use technical and analytics cookies to ensure that we will give you the best experience of our website - More Info
this is debug window