Non-destructive evaluation of additively manufactured materials: developing a coupled acoustic emission and thermoelastic stress analysis approach

Abstract

A method combining two non-destructive evaluations techniques, acoustic emission and thermoelastic stress analysis, is proposed to monitor cyclically loaded monolithic and additive manufactured metallic samples for crack growth. Current literature was used to investigate additive manufacturing processes and their current quality control methods. A review of acoustic emission and thermoelastic stress analysis theory and recent applications is also provided. It was found that there was currently a gap in monitoring methods for additive manufactured parts under fatigue loading for crack growth due to internal defects. Acoustic emission and thermoelastic stress analysis have never been combined to monitor crack growth in monolithic or additive manufactured metals in such a way and would be ideal methods for crack growth monitoring under cyclic loading due to their inherent benefits. Thermoelastic stress analysis is a full-field non-contact non-destructive evaluation technique which can monitor surface stress fields in materials subjected to cyclic loading. Acoustic emission is ideal for monitoring fatigue damage due to its continuous monitoring capabilities of energy released by crack growth. The proposed method consisted of acoustic emission capturing continuously and saving any waveforms exceeding a set threshold while the thermoelastic stress analysis captured data at regular intervals. Acoustic emission energy was extracted from captured waveforms to provide crack initiation detection as well as a measure of crack severity. Thermoelastic stress analysis provided a crack tip location and enabled crack tip tracking as well as crack growth rate monitoring to complement the acoustic emission energy findings. Acoustic emission could not locate the crack because it only used a single piezoelectric sensor but could detect crack growth anywhere on the sample while thermoelastic stress analysis could give the crack location only if the crack was near the surface or within the field of observation. The method successfully detected cracks and tracked crack growth in monolithic aluminium samples, additive manufactured aluminium and additive manufactured titanium alloy samples under fatigue loading conditions. This method involves little modification to the part being observed, the surface must be painted with a thin coat of matt black paint for thermoelastic stress analysis, and a sensor must be attached to the part for acoustic emission. Acoustic emission energy detected crack growth before thermoelastic stress analysis in most tests however this was due to the experimental method, acoustic emission captures continuously while thermoelastic stress analysis is at regular time intervals. Issues with experimental method came to light in early tests but were addressed in the final tests using additive manufactured titanium samples. Future work includes testing additive manufactured samples with built-in subsurface defects and developing a chip-based system in order to make a smaller and cheaper monitoring system.