_ Tech Alert Ultrasonic NDE of fibre- reinforced composites

A new technique for measuring the wavespeed and attenuation of ultrasound in the thickness direction, in extremely thin laminates, and the excitation and detection of Lamb waves in the lengthwise direction has been developed at Texas A 8z M University. Thus both in-plane and out-of-plane measurement can be made. Damage in the form of transverse cracking in cross-ply graphite/epoxy laminates was studied using these two techniques.

The results obtained showed that for the through-the-thickness measurements the change in attenuation was a good measure of matrix cracking, while wave-speed is not. But in the in-plane measurements both wave speed and attenuation are sensitive measures of matrix cracks.

A strong correlation between laser- induced fluorescence intensity and tensile strength of several ester- based and other polyurethane systems has been demonstrated. The strong correlation exists only for the right combination of excitation and detection wavelengths. Various parameters of the process, including excitation and detection wavelengths, have been experimentally investigated. Successful laboratory experiments employing fibre-optic bundles indicate that practical implementation of the method is possible.

useful mass density variations in ceramics, even those that cannot be discerned by a visual inspection of film images.

The availability of very sensitive and stable electronic systems for image analysis opens up opportunities for using neutron radiography under conditions not available before.

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A portable instrument to determine the physical condition of fabric- reinforced polyurethane fuel tanks used by the US Army is one immediate application, but the method may also be applicable to other elastomeric materials.

Towards fully automatic welding

The work followed a review of ultrasonic nondestructive evaluation techniques applicable to fibre-reinforced composites. The techniques are briefly described and key references are cited.

Further information: a 92 page report, reference 41 lZNDT, microfiche price f 14.50 + f2.18 VAT, or paper copy price f30.75 + f2.15 postage and packing, is available from TechAlert.

Further information: an 81 page report, reference 3959-NDT( M), microfiche price f 14.50 + f2.18 VAT, or paper copy price f 30.75 + f 2.15 postage and packing, is available from TechAlert.

Inspecting ceramics by using neutron radiography

Laser-induced fluorescence of polyurethanes

Laser-induced fluorescence has been used as the basis for a new nondestructive inspection technique for polyurethane fabrics at the Southwest Research Institute in work for the US Army. The work is being pursued to produce a portable nondestructive test instrument to determine the physical condition of fabric- reinforced polyurethane fuel tanks. These portable tanks are folded up after use and can deteriorate both in use and in storage. Current practice is to throw them away after five years, regardless of their apparent condition.

The use of neutron radiography for the nondestructive evaluation of green and fired ceramics to assess the flaw population, particularly in the early stages of processing, has been investigated by the US Army Materials Technology Laboratory. It was shown that useful studies of ceramics could be conducted using small neutron sources (califomium- 252 and a D, T accelerator) and an ordinary film densitometer.

A report describes the use of neutron radiography for detecting spherical voids, cracks and binder dispersion in ceramic bodies. It illustrates how simple densitometric equipment is able to map out very

Work at the Idaho National Engineering Laboratory has shown that a non-contacting ultrasonic sensing system using laser sound generation and an electromagnetic acoustic transducer can be used to sense the welding process.

The work is part of a welding programme directed at modelling the gas-metal arc welding process and sensing physical properties of the weld using optical and ultrasonic sensors. The sensors monitor the properties of the weld pool and the geometry and defect generating conditions, for input to an intelligent controller. The ultrasonic system described is being developed as part of the sensing control system for a completely automatic welder. The results of feasibility work show that the laser and electromagnetic acoustic sensor system is able to detect weld pool depth, fill level and fill slope.

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NOT International June 1990 191