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The mechanics of the coin-tap method of non-destructive testing

1988; Elsevier BV; Volume: 122; Issue: 2 Linguagem: Inglês

10.1016/s0022-460x(88)80356-0

1095-8568

P. Cawley, Robert Adams,

Non-Destructive Testing Techniques

The physical basis of the coin-tap method of non-destructive inspection has been investigated. It has been shown that the characteristics of the force input to a structure during a tap are changed by the presence of a defect such as a disbond or delamination beneath the surface of the structure, the impact duration increasing and the peak force decreasing as the defect becomes larger. The spectra of the force-time histories demonstrate that the rate of reduction of force amplitude with frequency increases as the defect becomes larger. It has been shown that defects such as disbonds and delaminations may be modelled as springs, the stiffness of which is that of the layer(s) above the defect. The change in the impact characteristics as this spring stiffness is varied has been investigated theoretically on both a rigid structure and on thick and thin beams. On a rigid aluminium structure, a spring stiffness of 100 MN/m would readily be detectable, whereas on the beams the sensitivity of the test is reduced due to an increased variation in the impact characteristics with position over good areas of the structure. Initial experimental results indicate that a spring stiffness of around 30 MN/m is at the limit of detectability. In an aluminium structure, this corresponds to a defect 1 mm deep and 6 mm in diameter. The method has the advantage over higher frequency ultrasonic testing that no coupling fluid is required between the transducer and the structure. Initial results also suggest that the method is more reliable on thin structures than the mechanical impedance technique.