An Integrated Global-Local System for the Detection and Monitoring of Damage Progression in Heterogeneous Materials

Abstract

In this study, global measurements from low-impact velocity experiments and local strain measurements from embedded and surface mounted optical fiber Bragg grating (FBG) sensors were used to obtain detailed information pertaining to damage progression in two materials, multi-dimensional laminate woven composites and Polymethyl-methacrylate (PMMA) acrylic. The woven composites and PMMA specimens were subjected to multiple strikes at 2m/s until perforation occurred. The impactor position and acceleration were monitored to obtain dissipated energies and contact forces. FBG sensors, which were embedded and mounted on the surface, at different critical locations near regions of penetration-induced damage, were used to obtain local strains. Measurements of initial residual strains and both axial and transverse strains corresponding to matrix cracking, delamination and fiber breakage were obtained for the composites, and impact induced surface deformations were obtained for the PMMA. From the FBG sensor response spectra, optical fiber sensor and host material damage were separated by an analysis based on the signal intensity, the presence of cladding modes, and the behavior of individual Bragg peaks as a function of evolving and repeated impact loads. A comparison by number of strikes and dissipated energies corresponding to composite material perforation indicates that embedding these sensors did not affect the integrity of the woven systems, and that FBG sensor measurements can provide accurate failure strains even after the initial loss of sensor integrity, which generally occurred prior to complete host material failure. The measurements from surface mounted and embedded FBG sensors were used with the global measurements to develop maps of failure paths for the host woven composite materials, and to determine the accuracy of FBG measurements.

These characteristic maps were obtained by identifying relations between the impact contact force and the local strain fields that corresponded to five distinct regimes of composite and sensor response spanning behavior from initial impact to complete material penetration. The FBG sensors were also used to obtain residual impact relaxation strains, and to uniquely determine the mode of relaxation, a measurement that is not possible with conventional strain gauges. The PMMA specimens were impacted and quasi-statically deformed. The global measurements, for both loading regimes, were used with local measurements from surface mounted FBGs to obtain critical information related to strain-transfer, how the strain evolves from the point of impact, how the sensor debonds, and how catastrophic overall failure can be related to sensor failure. The current experimental approach indicates that local measurements can be used with global measurements to obtain a framework that can be effectively used to monitor damage progression in different host materials, and it can be potentially used to mitigate damage, if it is detected at an early stage of damage progression.