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Integrating self-healing materials with structural health monitoring (SHM) represents a significant advancement in materials science and engineering. In applications such as aerospace, self-healing composites with SHM can detect and repair damage autonomously, enhancing safety and reducing maintenance time, which significantly improves structural durability by maintaining integrity and extending material service life. This research employs an experimental approach to validate the self-healing process of self-healing metal matrix composites reinforced with shape memory alloy fibers, by utilizing lead zirconate titanate (PZT) piezoelectric transducers mounted on the surface of the composite. A three-point bending test is used to induce damage in the metal matrix composites by applying a load at its midpoint while supporting it at both ends; then, self-healing is used to return the specimen to its original state. The ultrasonic signals from the PZT sensor were compared at three stages: pristine state, during bending (i.e., damage), and after the healing process, to evaluate the effectiveness of the healing process and health monitoring technique adopted for this study. Real-time monitoring using digital image correlation, laser profilometry, and mechanical testing was used to validate the damaged/healed signal state—demonstrated by improved recovered signal amplitude, reduced scatter energy, a notable decrease in damage indices root-mean-square deviation, normalized scatter energy, and stabilized wave propagation. The successful self-healing composite design and health monitoring results confirm the restoration of the specimen, which regained 84% of its initial shape deformation at 135 °C for 30 min (i.e., during the first step of healing). Complete shape restoration was achieved by raising the temperature to 150 °C for 90 min (during the second step of healing), recovering approximately 96% of the original flexural strength after healing. 

This paper presents a recent investigation into the electromechanical behavior of thermally reduced graphene oxide (rGO) as a strain sensor undergoing repeated large mechanical strains up to 20.72%, with electrical signal output measurement in multiple directions relative to the applied strain. Strain is one the most basic and most common stimuli sensed. rGO can be synthesized from abundant materials, can survive exposure to large strains (up to 20.72%), can be synthesized directly on structures with relative ease, and provides high sensitivity, with gauge factors up to 200 regularly reported. In this investigation, a suspension of graphene oxide flakes was deposited onto Polydimethylsiloxane (PDMS) substrates and thermally reduced to create macroscopic rGO-strain sensors. Electrical resistance parallel to the direction of applied tension (x^) demonstrated linear behavior (similar to the piezoresistive behavior of solid materials under strain) up to strains around 7.5%, beyond which nonlinear resistive behavior (similar to percolative electrical behavior) was observed. Cyclic tensile testing results suggested that some residual micro-cracks remained in place after relaxation from the first cycle of tensile loading. A linear fit across the range of strains investigated produced a gauge factor of 91.50(Ω/Ω)/(m/m), though it was observed that the behavior at high strains was clearly nonlinear. Hysteresis testing showed high consistency in the electromechanical response of the sensor between loading and unloading within cycles as well as increased consistency in the pattern of the response between different cycles starting from cycle 2. 

Abstract Strain sensors are the primary, direct sensing element in many sensors with applications in robotics, wearable sensors, structural health monitoring, and beyond. Cutting edge applications are increasing demand for sensors that can survive and measure large strains (> 5%). Presently, the most common strain sensors are composed of a serpentine metal foil which can survive strains up to about 5% with a gauge factor (GF) of about 2 (measured as change in resistance divided by initial resistance all over strain). Research into nanoparticle-based strain sensors commonly reports surviving strains up to 50% and gauge factors around 200. Unfortunately, most nanoparticle-based strain sensors are composed of expensive, toxic materials and require high precision synthesis methods. The reduced Graphene Oxide (rGO) based sensors can be synthesized easily with common materials and methods. Study of strain sensing capabilities have revealed that rGO strain sensors can survive strains beyond 15% with gauge factors (sensitivity) on the order of 200. Suspensions of graphene oxide (GO)’s flakes were deposited on flexible Polydimethylsiloxane (PDMS) substrates to create specimens with different area densities of 0.69, 0.80 and 091 mg/cm2 of GO. Specimens were thermally reduced to create rGO-based strain sensors. Resulting sensors were tested under tension applied at a rate of 0.1 mm/sec starting from 0% strain until failure. Resistance of the sensors in the direction aligned with the direction of the applied tension were measured at each 1 mm-increment of tension. Sensitivity and the strain to failure of the sensor were calculated and compared in specimens with different GO area densities. Our study suggests that with increasing the area density of graphene oxide (GO) during the synthesis of rGO, the survivability of the rGO subjected to large strains can be improved while still demonstrating a high sensitivity. This study can help tailor rGO-based strain sensors especially to the applications where high strain survival (> 30%) is required while benefiting from a reasonably good GF (> 30).