Determining the Depth of Occurrence of Defects in Multilayer PCM Structures by Acoustic Methods Based on the Mechanical Impedance Value

Abstract

Dedicated low-frequency acoustic-testing methods are widely used to detect flaws in three-, five-, and seven-layer structures with a honeycomb core made of polymer composite materials (PCM), with the impedance technique being the main one. However, this technique allows one to establish only the mere fact of the presence of a flaw but not the depth of its occurrence. To obtain information on the depth of occurrence, it is necessary to numerically measure the mechanical impedance on the surface. The complete impedance-transducer frame system has been simulated by the electromechanical analogy method in order to establish the relationship (and derive respective dependences) between the external load applied to the sensor, expressed as the total mechanical impedance on the article surface, and the change in the values of measured electrical parameters on sensor’s piezoelectric elements. Relevant dependences have been derived for the transmission factor, which is the modulus of the ratio of voltages across receiving and emitting piezoelectric elements, and for the phase shift between these voltages. By combining these dependences, hodographs have been produced that represent the graphs of the dependences in the amplitude-phase plane, similar to how information is displayed in state-of-the-art impedance flaw detectors. The dynamic contact compliance (significantly affecting the efficiency of impedance testing technique) of a dry point contact between materials of the outer PCM layers and widely distributed and commercially available impedance sensors (for example, PADI-8) has been determined. Model hodographs constructed with allowance for contact compliance were used to run an experiment on revealing delamination and starved-joint flaws at various depths in a seven-layer honeycomb structure of an aircraft engine nacelle. It has been confirmed that the depths of defects are effectively discriminated both by the magnitude of mechanical impedances and by the characteristic indication from cell walls on the C-scan over the entire article surface.