Tech Alert

Measuring damage in thick composites An ultrasonic technique has been used in combinat ion with a theoretical analysis to characterize the average spacing of transverse matrix cracks in cross-ply laminates, defects that do not show up well with ultrasonic C-scans When combined with fracture mechanics, the technique should be a useful tool for determining the remaining life of cross-ply laminates in which this type of damage has occurred. The work was performed at the Northwestern University, IL, USA.

A report describes expe,riments undertaken using specimens of forty-ply thick, graphite/epoxy, cross-ply laminates. Specimens were tested in their relatively undamaged state following curing and again following thermal stressing by immersion in liquid nitrogen.

Amplitudes of through transmission, oblique incidence, ultrasonic waves were measured. As a check, X-rays were used to measure crack spacing in the specimens. Knowledge of the transducer spot size then permitted the plotting of ultrasonic amplitude ratios as a function of the number of cracks covered by the beam. For a given number of cracks covered, these ratios showed relatively large variations from one location to another, but average values showed consistent decreases in transmission with the increasing number of cracks. X-rays are not part of the proposed ultrasonic technique; they were merely used to verify its accuracy.

The ratios were then plotted onto a theoretically derived curve relating the ratios to the number of cracks covered by the beam. Using this number and the known transducer size, the average crack spacing was then calculated. The current damage state was then compared with a 'characteristic damage ' state (the limiting condition when further tensile stress produces delaminations and fibre fracture, as

opposed to matrix cracking). Finally, fracture mechanics was applied to determine the remaining life.

Further information: a 15-page

report, reference 1772-NDT(M), price £10.00 + £1.50 VAT, is available from TechAlert. If a paper copy is required the price will be £23.90 + £1.65 postage and packing.

Thermal wave imaging of defects

in metals and ceramics

Developments in the method, theory and application of thermal wave imaging are described in a report from the John Hopkins University for the US Army.

Significant results from the work include the development of a new thermal wave imaging technique using an ion beam as the excitation source. It was shown that three different processes are effective in creating thermal images. The technique encompasses a variety of experimental configurations, each supplying different physical information.

Photothermal or thermal wave imaging was first used in the early 1980s. Since then it has been considerably developed to become an important tool for locating and characterizing defects in structural materials and electronic devices and for studying the microstructure of materials. Applications of thermal wave imaging have included grain boundary studies in metals, studies of void location and sizing, and closed crack visualization using both thermal and thermoelastic methods.

Contrast mechanisms have been of dominant interest and several distinct mechanisms have been shown to contribute to the images obtained. These mechanisms encompass the range of physical processes that create the thermal image. An intimate knowledge of these mechanisms is needed for thermal wave imaging to become a

quantitative tool for material characterization.

A theory of multilayer films has been developed which has shown that both thermal and (for optical sources) reflections from optical boundary conditions contribute to the temperature field within the specimen and hence affect the ability of thermal wave imaging methods to determine the depth (or thickness) of buried flaws or layers. In addition, an analysis has been made which shows that thermal barriers affecting heat flow in an heterogeneous material are of two types, one depending exclusively on thermal properties, the other depending on both thermal and optical properties.

Further information: a 235-page report, reference 1731-NDT(M), price £10.00 + £1.50 VAT, is available from TechAlert. If a paper copy is required the price will be £49.90 + £2.30 postage and packing.

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NDT International October 1987 313