Indirect impedance-based nde through instrumented fixtures effects of fixture material on defect-detection capabilities

Electromechanical impedance allows for a quick assessment of the mechanical integrity of the part under test by providing insights into its dynamic response. In previous studies, it has been shown that electromechanical impedance signatures obtained via directly bonded piezoelectric transducers can successfully be used for non-destructive evaluation of manufactured parts. Indirect electromechanical impedance measurements, through an instrumented fixture or testbed, has also been introduced as a promising solution for quick evaluation of manufactured parts. While such indirect impedance measurements alleviate the need for individual parts to be instrumented, they increase the complexity of the measurement system. Factors such as fixture design, part-fixture interface, and clamping force are found to impact measurement sensitivity to manufacturing defects and anomalies. In this paper, the effects of instrumented fixture material on indirect electromechanical impedance measurements are investigated. Three geometrically-identical fixtures are considered in this study. Two are commercially available clamps made of aluminum and steel, while the third is an additively manufactured replica made of PLA through a fused filament fabrication process. The fixtures are instrumented with monolithic piezoelectric transducers for electromechanical impedance measurement. Strain gauges are used to measure the clamping force exerted by the fixture on the part under test. Test specimens are made of machined steel blocks including defectfree (control) parts along with others featuring manufacturing defects. The impact of fixture's material properties, as compared to the part under test, on the defect-detection capabilities of indirect impedance-based NDE is evaluated.