8.51 in the metal. No direct mechanical coupling is needed, so this method can be used for very high speed inspections or in a vacuum. The surface waves generated are ideal for detecting surface cracks over large surfaces, such as a hull plate. Plate and crack stopper dimensions have to be checked. Ultrasonic thickness measurements are inherently more accurate than flaw detection, especially here where plate and crack stopper geometries are simple and known in advance. Ultrasonic reflection behavior in regular shapes is well understood, so even very small deviations from normal ultrasonic response can be readily interpreted to indicate thickness deviations. The most important present limitation in practical thickness measurements (for simple shapes) is actually the electronic signal processing. Accuracies of 0.01% full thickness (depending on surface finish) are claimed for some commercial units, but 0.1% accuracies are more reasonable for high speed production inspection. Large piezoelectric chips have been fabricated with multiple ultrasonic transducer electrodes on 1 mm center spacings. A single large chip could easily scan the entire length of a plate fastener, so this technique has been chosen for fastener inspection. The associated electronics can collect data at rates of 2000 samples per second per electrode, but at the highest inspection speed lateral resolution is limited to about 4 mm (8.28). An ultrasonic couplant is required, but this presents no problem because the fasteners are relatively small and the inspection need not be performed in a vacuum. In general, accuracies and resolutions an order of magnitude better than those assumed here can be achieved,but usually at inspection speeds an order of magnitude slower than needed for colony construction requirements. Furthermore, the higher accuracies are not supported by extensive field test data. VIII.8.3: Component Quality Assurance VIII.8.3.1: Plate Inspection : Inspection of the steel hull plates for surface cracks is the most critical inspection problem. High speed fully automated inspection of steel plates is currently practiced in some advanced rolling mills. A scaled down version of an
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