Resonant Inspection a "new" NDT technique by Godfrey Hands *

Resonant Inspection a "new" NDT technique

by Godfrey Hands *

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ABSTRACT

This new Inspection Technology is not only restricted to conventional NDT applications, but is also able to perform many inspection functions in one test. The technology can detect cracks, voids, hardness variations, dimensional variations, bonding problems, parts with missing manufacturing processes, misshaped parts and changes in material properties. It is primarily suitable for inspecting mass-produced components, although some high value individual components can be condition monitored to detect changes in their structural integrity. The Author's company offers a rapid response testing service using RI to manufacturing industries.

Table of contents

• Introduction
• Operational Theory
• Test Set-Up
• Vibration Modes and Spectra
• What can we detect and with what detection limits ?
  Cracks, Dimensional variations, Hardness variations, Changes in Material Properties, Parts with different shapes, Parts with a missing production process, Lack of Bonding
• What materials can we inspect ?
• Testing Speed
• Applications in mass-production
• Resonant Inspection Services
• Suppliers of Resonant Inspection equipment
• References

Introduction

Resonant Inspection is a "new" NDT technique that was originated by scientists at Los Alamos National Laboratory in the USA, and has been developed for industrial applications during the last four years of commercialisation by an American company Quatrosonics Inc. It is a whole-body resonance inspection that is particularly suited to inspecting smaller mass-produced hard components, and one test will inspect the complete component without radiation, the need for scanning, immersion in liquids, chemicals, abrasives or other consumables.

Operational Theory

Hard components have their own Resonant Frequencies, for example a bell will ring with one specific note. This note is actually a combination of several pure tones, each representing a different resonance mode of the bell or harmonics of them. Wine glasses also have resonant frequencies. The tone of the "ringing" depends upon the size of the glass, a small glass ringing at a higher note than a large glass. This tells us that Resonant Inspection can differentiate between components of different sizes. A bell and a glass of the same size will ring at different frequencies. This tells us that the resonant frequency is dependant upon the material of the tested component. (in practice, it depends upon the material properties or "stiffness" of the object). In addition, a good bell or wine glass will ring true, whilst a cracked bell or wine glass will ring with a "cracked" note or will "clunk" instead of ringing. This tells us that we can detect cracks with Resonant Inspection. So what's new ? People have been "inspecting" things by hitting them with a hammer and listening to them ringing for centuries. Computers and modern electronics technology have enabled us to take the human element out of the inspection process, thus measuring more frequencies and recognising more subtle changes than are detectable with the human ear. This also allows us to automate the process (thereby eliminating "operator error"), and also allows us to move into the ultrasound region to detect smaller differences.
Resonant Inspection operates by exciting a component with a sine wave excitation at one specific frequency (thereby putting all of the energy into that one frequency) then quickly sweeping all of the individual frequencies through the required test range. A hammer striking the component will put all the energy into a broad spectrum (from DC up to hundreds of kilohertz), with only a small amount at the resonant frequencies. This swept sine-wave approach allows a much improved signal to noise compared to the hammer blow technique. A narrow band filtered receiver, typically only several Hertz wide, will follow the swept sine-wave. This .vastly improves the signal to noise ratio and raises the detectability of the inspection by orders of magnitude compared to the old hammer method.

Test Set-Up

For Resonant Inspection, we normally locate the component to be tested on three or four piezo transducers. It is not necessary to scan the component with the transducers, nor to rotate a component past the transducers, as one test will evaluate the whole-body or complete component. One of the transducers normally acts as a transmitter, exciting the component, whilst one or two more of the transducers act as receivers, measuring the amplitude of vibration at the specific frequency of the transmitter or at one of its harmonics. Further transducers can be used to support the component in the test. These transducers have ceramic tips (to prevent wear of the transducers and to provide a good transfer of energy between the component and transducer), which whilst normally being hemispherical, can also be ground to a user specific shape if required.

Vibration Modes and Spectra

Figure of vibration modes Components vibrate typically with Torsional (twisting), Flexural (bending) and Extensional (stretching) modes. The figure here demonstrates some of these modes.
Figure of a typical spectrum . Also shown is a typical spectrum between 100 and 400 kHz from a small cylindrical component. This shows many resonances of different amplitudes.	Figure of a small piece of above spectrum If we concentrate on a small section of this spectrum (between 225 and 250 kHz), we can see three specific resonances.
If a defect is introduced into the component, then two of the resonances will change. A crack will reduce the stiffness of a component, and therfore the resonant frequency will have a lower frequency. If a component is rotationally symmetric (e.g. a cylinder), there will normally be two resonances at the same frequency from the X and Y axes (diameters of the cylinder), plus another resonance from the length (Z axis) of the cylinder. A defect on the outside diameter (only extending for a small amount of the circumference, but precisely in the direction of vibration of one of the axes) will only affect the X or the Y resonance, not both. In this case, the X or the Y component of the two superimposed resonances will shift low, and we have an apparent "splitting" of the resonance into two distinct peaks. This becomes apparent from the spectrum of the component with a defect introduced into it, where one of the resonances is not affected by the defect, one of the resonances splits, so only one axis is affected, and one splits and shifts, showing that the two axes are affected, but to different degrees.
Figure of same piece of spectrum of component with defect	This next figure identifies some of the resonant peaks of another component, describing torsional, bending and "breathing" or extensional modes of resonance. Figure with identified modes or resonance from Quatrosonics

What can we detect and with what detection limits ?

Cracks on steel components (typically 3mm x 0.3mm on components with dimensions 15-20mm).

Cracks on small steel components (typically 1mm x 0.1mm on components with dimensions 10-15mm).

Cracks on ceramic components (typically 2mm x 0.05mm on components with dimensions 15-25mm).

Dimensional variations of about 0.1%, or 0.025mm on components with dimensions typically 25mm (provided the production variation is smaller than this).

Hardness variations typically of 4 or 5 Rockwell C points, also larger variations that do not affect 100% of the component.

Changes in Material Properties

Parts with different shapes such as radii etc.

Parts with a missing production process such as coating, thread rolling, hole drilling, grooves etc.

Lack of Bonding

What materials can we inspect ?

Almost any "hard" materials such as metals, powder metal parts, ceramics and in certain circumstances also composites. What can't we inspect ? "Soft" materials, assemblies and very large components

Testing Speed

Resonant Inspection is a fast inspection technology. Typical testing times are between 1.5 and 4 seconds per piece, but in some circumstances, significant deviations from this can occurr. For some simple operations, testing times of less than 100 milliseconds per component are possible, and some condition monitoring applications may require testing times as high as 10 seconds per piece.

Applications in mass-production

Resonant Inspection is very suited to inspecting mass produced parts, and is easily able to detect "outliers" or components that differ from the normal production.

Resonant Inspection Services

The author's company offers a "fire-brigade" type of service to manufacturing industries who operate with a "Just-In-Time" manufacturing and delivery schedule, for when their production processes get out of control and their customer (such as the automobile industry) urgently needs acceptable components to fulfill their own production requirements. This service with portable testing equipment means that manufacturers can have Resonant Inspection services (subject to availability of manpower and equipment) only as and when they need it, eliminating the need for large investments and manpower training for this new technology.

Suppliers of Resonant Inspection equipment

Within the USA, Resonant Inspection equipment can be purchased from Magnaflux at 3624 West Lake Avenue, Glenview, Illinois, 60025 USA, Phone +1 847 657 5329, Fax +1 847 657 5388. In the rest of the world, Krautkramer & Co. GmbH, Robert-Bosch-Strasse 3, D50330 Huerth, West Germany, Phone +49 2233 601-0 are the suppliers of the technology. See a press release from Krautkramer: Innovation in nondestructive testing: Automated resonance testing of mass-produced parts made out of metal, powder metal, ceramic, stone, composites.
The manufacturer is Quatrosonics Inc. 4209 Balloon park N.E. Albuquerque, NM 87109
Phone +1 505 343 8549, Fax +1 505 343 8436

Author

Godfrey Hands
Godfrey Hands has been involved in NDT since 1968, and has specialised in Ultrasonic inspection since 1973. His carreer developed towards the (semi) automated inspection of mass-produced components in 1978, and he has been involved in Resonant Inspection since it's introduction to Europe in 1993. In 1996, he decided that Resonant Inspection was a technology that was too beneficial to restrict to his employer at that time, and he started his own company. This is now located in the British Midlands, at the heart of the British automobile component manufacturing industry, to support them with his Rapid Response Inspection Service, using RI and conventional NDT. He is married with three children and one grandchild, and his wife is a partner with him in the business.
Godfrey Hands Ltd.
Nuneaton, England
Phone +44 1203 320812, Fax +44 1203 320813
E:Mail Godfrey@hands-ndt.co.uk
Homepage: www.hands-ndt.co.uk

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