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9.1 Non destructive testing 9.1.1 Visual inspection Delaminations, misplaced plies, excess resin bleed, dry fabric, porosity, tooling marks and surface defects are amongst the many defects which can easily be observed by visual examination. In addition to geometric and dimensional checks, visual checks of composite quality can often easily identify processing errors or material quality problems. Sometimes defects will not be visible to the naked eye, or will be within the bulk of a component so more complex non destructive examination is required: 9.1.2 Tap testing Tap-testing is a common method for determining voids and defects in cured laminates due to it’s cheap, efficient nature. The method involves tapping the surface of the lami- nate with a hard object, usually a coin, and listening for a change in the pitch/ volume of the noise produced. Advantages ■ Cheap, simple and quick technique Disadvantages ■ Method relies heavily on user ■ Uncured materials won’t have the necessary properties to transmit noise/ with stand the force of impact 9.1.3 Ultrasonics Ultrasonics is similar to the tap-test method described above, except that it deals with the frequency of sound that is out of the range of human hearing (ie., greater than 20,000Hz). The ultrasonic signal is transmitted to the part to be inspected via a trans- ducer, and received either by the same unit (pulse-echo methods) or by a transducer behind the part (through-thickness methods). Recent advances in technology mean that air-coupled pulse-echo ultrasonic units are now commercially available. Advantages ■ Able to detect a wide range of defects ■ Can give information of 3-D location of defect within the laminate ■ Can be slow- resolution dependent on speed Disadvantages ■ Level of skill required to accurately interpret ■ Unsuitable for use on uncured material 64 - GTC-6-0417PDF Image | GUIDE TO COMPOSITES
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