Our experience is mainly obtained during participating in national and international projects on manufacturing and testing of composites, and it will be illustrated by examples. In European projects (partly sponsored by the European Union) industries making cars, in aerospace and in shipping have participated. Therefore the demands on the materials show a wide variation in properties, quality and price.

Composite Sandwich Inspection

One European project, which will be finished in 1996, has 10 partners, from 6 countries. There are 6 industries covering aerospace, cars and ships, and 4 research institutes. the aim of this project is to find the damage tolerance of sandwich structures and to improve it. Non-destructive evaluation is used to characterise the damage and to follow the damage growth during mechanical testing.

Impact defects are an important type of damage for sandwich materials. Here the internal damage recorded by ultrasonic scanning may be 10 times greater than the visible damage on the surface. In general, ultrasonic inspection has proven most useful for evaluation of sandwich structures, but other techniques were also used in the project. The surface shape of impact damage can be mapped non destructively using ultrasonic scanning.

An outcome of the project will be a design guideline including One chapter on non destructive evaluation of sandwich structures, comparing different non destructive techniques with respect to sensitivity, speed, price, mobility and so on.

Laminate Inspection

In another European project, fibre composite laminates were characterized by ultrasonic scanning. The internal structure (number of layers, fibre orientation) and defects (delaminations, impacts) may be found, using A-, B- and C-scan, as well as time- of-flight scan (D-scan). This will also be illustrated by examples.

The skin in sandwich materials may be characterized using high ultrasonic frequency as for laminate evaluation. However, the whole cross section of a sandwich, especially with a foam core calls for a lower frequency due to the high damping.

Acousto ultrasonic or in-plane transmission technique was used for measuring the velocity in composite laminates, hereby enabling non destructive determination of mechanical properties such as modules of elasticity.

Conclusion

Ultrasonic techniques are very useful for characterising fibre composite materials. Some of the techniques used for sandwich structures demand access to both sides of the material. To obtain the optimum results from ultrasonic scanning laboratory conditions are demanded. Therefore the scanning techniques are very useful for characterising materials during development work on test specimens. The precise characterisation by ultrasonic scanning may also be useful for calibrating other non destructive inspection methods more suitable for work in the field.