A hybrid optico-acoustic NDE approach for deformation and damage monitoring

To account for both surface and volume effects and to provide cross-validation when using in situ nondestructive evaluation (NDE) methods, a hybrid optico-acoustic approach combining digital image correlation (DIC) and acoustic emission (AE) was developed, validated, and tested. The designed system includes hardware and software linkages between the two sensing modalities with feedback both from the mechanical testing device as well as from other complementary NDE methods including ultrasonic testing (UT) and infrared thermography (IRT). This hybrid monitoring approach targets the early identification of damage precursors which vary depending on the material tested. To corroborate the reliability of the hybrid setup, various tensile, fracture and fatigue testing procedures were implemented for different types of materials. Of interest herein was the evaluation of each individual NDE method's monitoring capabilities of deformation and damage in conjunction with their spatial and temporal resolutions. In carefully designed metal alloy microstructures with well-characterized mechanical properties, AE validated DIC-observed strain band formations on the sample's surface, which further enabled targeted microscopic analyses that proved the origin of the NDE findings. In the case of fracture, DIC and UT validated AE information regarding both crack initiation and growth in Mode I testing of metallic alloy samples. Finally, in fatigue loading of fiber reinforced composites, IR measurements confirmed DIC indications of early damage formation consistent with AE recordings. Post-processing of NDE datasets combined with actual loading information allowed the calculation of mechanical parameters (elastic-plastic transition stress, strain localizations, traction separation laws, energy dissipation, localized Poisson ratios) that have been used to construct physics-based computational models for multi-scale deformation and damage evolution, as well as remaining life-estimations.