Coin-tap test

Today the coin-tap test is a widely used technique on wind turbine rotor blades for inspection of thin GFRP laminates for disbonded and delaminated areas. However, since the sensitivity of this technique depends not only on the operator but also on the thickness of the inspected component, the coin-tap testing technique is most sensitive to defects positioned near the surface of the laminate. Therefore, there has been an increasing demand for alternative non-destmctive testing techniques which is less operator dependent and also more sensitive to delaminations and disbonded areas situated beyond thicker GFRP-laminates. 

Of historical interest may be the use of a half dollar coin (the lighter weight 250 not as efficient). During the early 1940s the coin tap test was used very successfully in evaluating the performances of plastics, particularly RP primary aircraft structures. With a good ear (human hearing ear) there was (and is) a definite different sound between a satisfactory and unsatisfactory RP product. The unsatisfactory product would contain voids, delaminations, defects in fiber distribution, etc. In... 

Adams, R. D., Allen, A M. and Cawley, P., The coin-tap test for laminated structures, Proc, 11th World Conf. on NDT, Las Vegas, USA 1985. 

Tap testing coin tapping on a laminate structure is not recommended, but it will detect near-surface internal damage if such damage is of sufficient size. 

Tap test. In this method, a coin is used as a special tapping hammer. Tone differences indicate inconsistencies in the bonded joint. Sharp, clear tones... 

Tap test In this method, a coin is used as a special tapping hammer. Tone differences indicate inconsistencies in the bonded joint. Sharp, clear tones indicate that the adhesive is present and adhering to the substrate to some degree. Dull, hollow tones indicate a void or unattached area. Some improvement in the tap test can be achieved using a solenoid-operated hammer with a microphone pickup. The resulting electrical signal can be analyzed on the basis of amplirnde and frequency [5,9]. 

Sonic Inspection. Sonic and ultrasonic methods are, at present, the most popular NDT techniques for use on adhesive joints. Simple tapping of a bonded joint with a coin or light hammer can indicate an unbonded area. Sharp, clear tones indicate that adhesive is present and adhering to the substrate in some degree duU, hollow tones indicate a void or unattached area. Ultrasonic testing basically measures the response of the bonded joint to loading by low-power ultrasonic energy. 

The bonded assemblies are then subjected to nondestructive tests designed to determine the presence of voids. Some adhesives are formulated to be x-ray opaque in order to simplify void detection. Tapping the surface with a coin will detect very large voids. However, this... 

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