===_New equipment measures to be taken before total failure.

To test this, real joints from a disused mast, fitted with the pressure system, were fatigue tested to destruction. The tests showed that, even with an invisible surface-breaking crack less than 2 mm long underneath the galvanizing and paint, the pressure dropped to below half its original value in less than a minute. The measured life to total failure gave ample margin for action to be taken to strengthen the joints by mechanical clamping.

TWI, Abington Hall, Abington, Cambridge CB 1 6AL, UK

Infrared thermography improves PCB inspection Separate units of the Thermovision 800 Series from AGEMA Infrared Systems have been brought together to form a stand-alone system for real-time temperature measurement of electronic components. Using burst recording techniques, the system can analyse dynamic and static thermal patterns of PCBs, hybrid circuits and racking systems down to the thermal analysis of discrete components.

Called Magnus, the system can be used in the research and development of initial PCB designs or during final production testing on the shop floor. Used in development, it enables designers to study the optimum component selection and positioning for maintaining even temperature distribution over the board and to study new designs for bonded heat sinks and component cooling. As a quality assurance tool, the system enables users to study the behaviour of PCBs under power and devise suitable techniques for product testing and fault location on the production line.

A microscope attachment for the Agema Thermovision 800 series allows images and measurement of devices as small as 1.6mm × 1.6mm

As a non-contact measurement system, it offers a choice of three scanners, each supported by a Brut system controller linked to a super VGA monitor offering 256 colour levels or a continuous grey scale. The system works on the principle that all objects emit thermal radiation. Each scanner converts infrared radiation emitted by the board or component under test into an electronic video signal. The surface of the object is then scanned 25 times per second to produce a TV-like real-time thermal image of the object for examining transient events. If necessary, the spatial resolution of the system can be enhanced by reducing the scanning rate.

To achieve maximum accuracy, there is a microprocessor- controlled measurement system. Two miniature reference sources are built into the scanner and by scanning these references every horizontal scan, the gain and level of the system can be precisely controlled, achieving a temperature accuracy of better than +2% or

_2°C.

For situations requiring more flexibility, the thermoelectrically cooled Thermovision 870 SWB scanner operating in the short-wave band 2-5.6 #m is suitable as it does not suffer from the handling restrictions of cryogenically cooled scanners. However, the cryogenically cooled long-wave (8-1 2 #m) 880 LWB scanner and the short-wave (2-5.6/~m)' 880 SWB provide better spatial resolution.

A wide range of bayonet-action lenses enables Magnus to be adapted for measuring small objects at a distance or larger objects closer up. An extensive range of spectral filters enables objects with particular spectral characteristics such as laser beams to be analysed more easily. Using a filter it is possible to extend the temperature measurement range of the 880 LWB scanner to between -30°C and +2000°C.

The analogue signal produced by the scanner is converted into a 12 bit digital signal which is then stored in 12 bit format (with 4096 levels) on a standard 105 Mbyte Winchester disc. For applications where real-time sequences are captured using burst recording techniques an optional 1.3 Gbyte hard disc is available. Single images or sequences can subsequently be archived on standard 3.5" DOS diskettes, tape streamer or DAT tapes (depending on the chosen system configuration) or on an optional optical disc. The images can then be retrieved and analysed with full dynamics.

The system controller software supports advanced image handling and analysis. Using a keyboard and trackball, operators can make precise settings of measurement and presentation parameters within a windows environment which allows display layouts to be tailored to specific needs.

The system offers users the ability

334 N DT& E International December 1991

=New equipment to obtain true temperature measurements of objects irrespective of their individual emissivities. The problem with many thermal imaging systems is that they only provide a general picture of the thermal behaviour of objects since they do not take into account their differing emissivities. The emissivity of an object depends on the material from which it is made. Using the Equal software, operators simply take two reference images of the PCB under test at different temperatures above ambient. By capturing another raw image of the board powered up under normal operating conditions, the software calculates the true temperature of each of the image points from the three images obtained and produces an equalized image on screen.

Agema Infrared Systems Ltd, Arden House, Leighton Buzzard, Bedfordshire L U7 7DD, UK

Art i f ic ial defect saw

An artificial defect saw that will allow customers to make NDT calibration standards from their own material or in samples of finished or semi-finished products has been announced by Wells Krautkramer. An artificial defect cutting service is also available from the company.

The saw will produce Class A artificial defects with a high level of dimensional accuracy, straightness and depth consistency in most ferrous and non-ferrous materials.

It comprises two electrically driven precision saw blades for cutting external and internal defects respectively. The external blade will cut artificial defects in the external surface of rod, tube, bar, flat plate and any engineering components that will fit in its integral vice. The internal blade is designed to enter holes greater than 20 mm diameter to cut artificial defects on the inside of tubes or machined products

VC-lO undergoing fuselage testing with an array of 288 acoustic emission sensors from Physical Acoustics

such as gun barrels or engine cylinders.

Wells Krautkramer, Castle Vale Industrial Estate, Sutton Coldfield, West Midlands B76 8AY, UK

Acoustic emission system monitors VC-10 f leet

In 1990 a decision was made by the Royal Air Force upon the recommendation of British Aerospace to re-qualify the fuselages on the VC-10 fleet by proof pressure test. This led to trials being carried out to evaluate the effectiveness of acoustic emission to detect structural problems.

Acoustic emission has been used by Physical Acoustics on a number of commercial aircraft fuselage tests and is routinely used for monitoring re-qualification tests on the F-111 air frame, the Ariane IV Spelda production test, and pressure vessels in the process industry. During the trials it was established that 280 sensors were required to monitor the complete pressure shell.

Late in 1991 Physical Acoustics were awarded the contract to supply 288 channels of 'Spartan'-based acoustic emission equipment with technology transfer and training so the Royal Air Force could undertake the acoustic emission monitoring of the proof tests themselves.

This equipment was delivered and commissioned in November 1991 and the first test completed successfully in December in conjunction with British Aerospace, who were responsible for the test. The aircraft pressurization was carried out using large mobile compressors. The rest of the fleet is being put through the proof-test programme with the Royal Air Force running the tests, and operating the acoustic emission systems.

It is hoped to correlate acoustic emission with subsequent maintenance findings and build up a data base to aid future VC-10 maintenance. Meanwhile the acoustic emission monitoring provides additional assurance during proof testing that a major structural failure will not occur.

Physical Acoustics Ltd, Norman Way, Over, Cambridge CB4 5QE, UK

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