Navy to sell know how

Navy to sell know how

combinations of these. The geometry of the structure is reduced to these terms and included in the program through teletype, tape-reader and punch-tap...

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combinations of these. The geometry of the structure is reduced to these terms and included in the program through teletype, tape-reader and punch-tape interfaces. This rules out a simple treatment of threedimensional framework structures.

lndt APPLICATIONS Navy to sell know how In May this year, the Royal Navy took a couple of hours off from the cod war to bring its high technology to the marketplace. The Admiralty Materials Laboratory (AML), Poole, Dorset, showed its acousticemission system: the latest fruit of a long programme aimed at rationalizing inspection. The occasion was arranged by the National Research Development Corporation (NRDC), which deals with the commercial use of systems developed by the AML. The object was to explain the working and application of the AML system to the technical press to inform potential buyers and users. The NRDC was optimistic; AMLEC, a portable electromagnetic flaw detector was the last product of the AML programme and was a notable commercial success on its introduction in 1968. D. Birchon, the leader of the research group, explained the system, which he has already described to a more

specialized audience. 1,2 His description was a very clear explanation of acoustic emission and the general aims of the programme. The system is basically a defect locator. The /EVIL hope to follow this up with defect characterization. The overall aim is to reduce the cost and improve the trustworthiness and efficiency of testing materials and structures for the Royal Navy. The research at AML is very much tied to achievement of objectives. The stress wave emission work began in 1966 and has now reached the construction of a working package. The equipment is carried in a five-ton standard commercial lorry and has 20 transducers wired into a computer. This system allows the location of defects on large, fairly simple structure within minutes once everything is set up (Fig.l). The computer is a Hewlett Packard 2100A digital computer with a 16 kbit capacity. There is a basic program for cylinders, spheres, plates and

The transducers on the structure are the basic source of data which is processed by the program. These transducers are pzt crystals in brass mountings designed to shield them from unwanted effects. The signals from each of up to 20 transducers are conveyed by 100 m of cable to the computer. The cable was designed at AEE Winfrith to give minimum noise. They are multiscreen coaxial cables (Fig.2). The limit to the number of transducers and the length of cable is intrinsically much higher than these limits chosen for convenience. The processing of the signal begins at a pre-amp designed at AERE Winfrith. It can handle signals as low as 3 ~tV with little noise and amplifies 20 dB. The main amplifier is a 10 stage log with steps of 10 dB so that the gain can be selected. The computer program identifies transducers by number. Four signals are received and compared with timing and transducer number and spurious signals are rejected by a list of allowed combinations. The locations are found and stored and arranged in order of most frequent

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Recorders ! 1 / I I " " Fig.2 A five-ton lorry holds all the instrumentation needed for defect location. An operator sat in the back o f lorry can locate flaws a cable-length away f r o m the structure.

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Fig.1 The signals from up to 20 transducers are processed by a computer programmed for the particular geometry of the test structure. The results appear on an easily interpreted defect location plan

N O N - D E S T R U C T I V E T E S T I N G . OCTOBER 1973

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signal. In this way 150 flaws could be found. These are plotted on a Hewlet Packard x-y plotter. A typical cycle time is 0 . 5 - 4 s.

cylinder. However, emission can travel through skin and cause a good deal of difficulty. Also a defect cannot be placed in the thickness.

The setting up of the system is where experience is most useful. The location of transducers is learnt in this way. The threshold is chosen by the level of the noise down to 3 ~V. The frequency of transducer is chosen by finding the quietest frequency between 0.1-0.9 MHz and selecting the pzt crystals accordingly.

Their system, of course, needs a crew. The crew would consist of a computer operator, structural engineer and an electronics engineer. The designers feel that given good background knowledge such a crew should be very capable in six months.

The system can thus handle a wide range of structures. It is however limited where frameworks are used. It should be possible to tell the computer to ignore a cylindrical strut on the structure and then test it' separately as an independent

References 1 Birchon, D. 'The philosophy of using non-destructive testing', Annual conference of the Non-Destructive Testing Society of Great Britain, Loughborough, 12-15 September 1972 2 Birehon, D., Warren, R. H., Wingfidd, P. 'Structural validation: a positive approach to ndt', Journal of Mechanical Engineering Science 14 (June 1972)54

Ultrasound tests ball bearings A test that gives trouble to ball bearing manufacturers and aircraft makers is assessing the condition and appearance of bails used on the main shaft of aeroengine bearings. Possible faults include overheating cracks, unequal distribution of carbon, marks from oxidization, corrosion pitting and hard impact marks.

drive mechanism. While being continously rotated, bails are subjected to ultrasound which both refracts at the surface of the ball and propagates into the ball. If the wave meets a flaw it is reflected back along its path, detected, displayed on a cathode ray tube and recorded on an ultra-violet chart.

The superficial examination of ballbearings, either with the naked eye or with the aid of a magnifying lens is a lengthy operation. It presents certain risks, as much from the fatigue of the operator as from the low magnification used.

The transducer is submerged in a vertical position and emits pulses

NON-DESTRUCTIVE

The balls pass through solvent before the test because traces of grease or other superficial impurities give spurious recordings. Oil is used in the immersion tank to reduce the risk of oxidation and to hinder the formation of air bubbles which also give spurious records. The ball rests on three driving and guiding rollers, two of which are driven by an electric motor providing rotation about a horizontal axis. One of the rollers can be stopped periodically enabling the ball to rotate about a n e w axis and the ultrasonic beam to scan the complete ball. Axis of transducer

Axisof bearing ball

B a '~__-~. l / ~ to be tested Path of ultrasound/4 whenthereisa / L I ~ I .. "~x ~

Ransome Hoffman Pollard Aerospace Bearings Division have recently installed a Sofranel ultrasonic nondestructive testing machine (Fig.3) designed specially for testing aeroengine mainshaft ball bearings. The machine can detect surface and subsurface defects, granular burning, overheating, oxidation and corrosion stains, and indentations on balls from 16-25 mm in diameter. The equipment, although supplied by Sofranel, was the result of a joint project by Sofranel and Krautkramer and will be used on both Concorde engines and the new RB2-11 engines for the Lockheed Tristar. An automatic feed mechanism passes balls through a cleaning tank into an oil immersion tank where they are picked up by an electric

in the direction of the ball at an angle calculated so that a beam of transverse waves of short wavelength is reflected with maximum efficiency by the wedge effect produced by a fault. This is arranged by having the vertical axis of the transducer and the ball slightly displaced relative to each other (Fig.4). Because the size of traceable faults decreases as the wavelength decreases, a single t r a n s m i t t e r receiver working at a frequency of 12 MHz and with a diameter of 5 mm is used.

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Fig.4 Arrangement o f the transducer and ball bearing: the vertical axes of each are displaced relative to one another

Scanning time is adjustable from 15 s to 6.5 min depending on ball size. After removal from the oil bath, balls are either accepted or rejected then placed on a storage area for draining. The process is completely automatic and does not require any manual intervention. Fig.3 The Sofranel ultrasonic nondestructive testing equipment designed f o r the inspection o f aeroengine mainshaft ball bearings

T E S T I N G . O C T O B E R 1973

Ransome Hoffman Pollard Ltd, RHP Aerospace Bearings Division, Stonehouse, Gloucestershire, UK

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