PASSIVE DAMAGE DETECTION

Research and development of metastable material sensors and detection diagnostics for passive structural health monitoring applications are being conducted at San Diego State University. The metastable sensor materials change phase as a function of strain from a nonferromagnetic parent phase to a ferromagnetic product phase. The martensitic phase transition occurs gradually and irreversibly as a function of applied strain. As such, the ferromagnetic signature of sensor elements can be used as a passive means of detecting and monitoring peak strain. Sensor elements can be strategically attached to the surface of structures or embedded as an integral part of the structure, e.g., in composite material applications. The figure below shows the ferromagnetic response for a sensor element as a function of tensile displacement.

Remote Structural Health Monitoring

Remote health monitoring can then be accomplished through a network as illustrated below. Power is required only during sensor interrogation thus opening monitoring possibilities to a wide range of opportunities that otherwise would be too expensive to consider. The network illustrated below shows an array of sensors attached externally to a civil engineering structure such as a bridge, building, dam, etc. Data are collected periodically and remotely sent to the host station where they are displayed on the Internet for remote viewing. Civil infrastructure applications of the technology have been most thoroughly developed and applied through a licensing agreement with Strain Monitor Systems, Inc. of San Diego, CA. Further applications of the general approach are limited only by the imagination, for example the incorporation of embedded sensors in composite materials is now being explored.

Passive Monitoring in Composite Materials

Research has been conducted to explore the embedment of wire arrays in carbon-fiber-reinforced, polymer matrix composite materials. Tensile properties as a function of strain between 0-4% have been determined and further studies are being directed at impact testing. Promising results indicate that the approach has technical merit although much development and process optimization work will be needed before commercialization of the embedded sensor technology can be realized.

Sensor elements can also be integrated into structural composites in conjunction with a simple LRC circuit. The element is positioned inside an induction coil so that the resonant frequency of the embedded circuit reflects the degree of peak strain in the composite. The resonant frequency of the embedded circuit can be interrogated using an externally positioned frequency scanner. There is no probe-to-sensor lift-off effect using this approach as long as the scanner is positioned close enough to energetically couple to the embedded circuit. Further research and development are needed to fully recognize the potential of the technique.