Experimental Evaluation of the Possibility of Through Defects Detection in Thermally Expanded Graphite Workpieces by Acoustic Method

The paper presents the results of evaluating the possibility of detecting defects (e.g., through holes) in workpieces made of thermally expanded graphite used as sealing materials using a non-contact air-coupled acoustic inspection system. The deviation of the acoustic signal amplitude when it passes through a sample of thermally expanded graphite in defective and defect-free regions is used as an informative parameter. The obtained dependences of the change in the amplitude of the passed signal through the sample on the size of the artificial through hole, density and thickness of the object showed a significant influence of these parameters on the detectability of the defect, which should be taken into account in the development of inspection technologies and in the process of its implementation. The change in the amplitude of the signal passed through the object directly depends on the increase in the parameters of the sample (density, thickness) and the size of the through hole, which increases the sensitivity of acoustic control and detectability of defects.On the basis of the obtained dependences the criteria for rejection of workpieces can be developed at the stage of production or immediately before manufacturing of their products. The influence of defect positioning relative to the acoustic axis of the transmitter-receiver system is shown, which leads to a decrease in the informative parameter, and consequently to a decrease in sensitivity. This dependence is characteristic for all thicknesses and densities of objects, as well as for all sizes of artificial through holes when control is carried out immediately after defect formation. When acoustic inspection is carried out some time after the defect formation, the maximum of this dependence may shift due to the shift of the amount of substance to fill the defect cavity, which is confirmed by microscopy. The obtained results can be used in the development of workpieces acoustic control parameters.