This study demonstrates the capability of inductively coupled piezoelectric sensors to monitor the state of health throughout the lifetime of composite structures. A single sensor which generated guided elastic waves was embedded into the stacking sequence of a large glass fiber reinforced plastic plate. The progress of cure was monitored by measuring variations in the amplitude and velocity of the waveforms reflected from the plate’s edges. Baseline subtraction techniques were then implemented to detect barely visible impact damage (BVID) created by a 10 Joule impact, at a distance of 350 mm from the sensor embedded in the cured plate. To investigate the influence of mechanical loading on sensor performance, a single sensor was embedded within a glass fiber panel and subjected to tensile load. The panel was loaded up to a maximum strain of 1%, in increments of 0.1% strain. Guided wave measurements were recorded by the embedded sensor before testing, when the panel was under load, and after testing. The ultrasonic measurements showed a strong dependence on the applied load. Upon removal of the mechanical load the guided wave measurements returned to their original values recorded before testing. The results in this work show that embedded piezoelectric sensors can be used to monitor the state of health throughout the life-cycle of composite parts, even when subjected to relatively large strains. However the influence of load on guided wave measurements has implications for online monitoring using embedded piezoelectric transducers.
The layer wise construction of laminated composites offers the potential to embed sensors within composite structures.
One possible solution is the embedding of sensors that are inductively coupled to an external probe; which allows for the
efficient contactless transfer of electrical signals to the sensor. Embedding sensors within structures is an attractive
option, due to the physical protection offered to the sensor by the host structure. However, for embedding sensors to be
viable, sensor integration must result in minimal degradation of the laminates mechanical performance. This work
focuses on designing embedded inductively coupled sensors for structural performance. A suitable sensor coating for the
sensor unit was identified using interlaminar shear strength testing. Sensors were then embedded into quasi-isotropic
four-point bend flexural strength specimens, and different embedding strategies demonstrated. In addition to providing
the sensor with physical protection, embedding sensors within a composite host offers the additional benefit of
monitoring the curing process of the surrounding composite. A single inductively coupled sensor was embedded into a
large glass fiber epoxy plate, and the measured guided wave pulse echo response used to monitor the curing process.
This novel cure monitoring technique was then benchmarked against direct scanning calorimetry.