Structural Monitoring Systems has developed sensors manufactured from a number of materials, for a range of applications.
Sensors for Metallic Structures
The original sensors were manufactured using silicone with silicone Pressure Sensitive Adhesive (PSA). The pressure sensitive adhesive allows the sensor to be removed from the storage box and installed directly onto the surface of the component in much the same way as a postage stamp or tape. The silicone allows the sensors to conform to complex geometries with minimal internal residual stresses.
Sensors are also manufactured from polymers such as polyimide and FEP for applications with different environmental and geometrical requirements. Polymer sensors can be manufactured to longer lengths, but are only able to conform to simple component geometries.
It is important to note that the sensors are the same whether used in a laboratory or an in-service component. This allows the response of the sensor configuration to be characterized in the laboratory for a specific component, and then for these results to be transferred directly to an in-service application.
Sensors for Composite Structures
SMS has also developed sensors for composite structures. These sensors are able to monitor Barely Visible Impact Damage (BVID) using standard sensors installed on the back face of an impacted structure and disbond using SMS’ Through the Thickness or TTT sensors.
SMS has determined that by placing standard surface sensors, with detection galleries widely spaced, on the back face of an impacted structure BVID can be reliably detected. The impact on the structure causes micro-cracks in the resin matrix which can be detected using the CVM™ system. Impacts as low as 5 J, causing indentations as small as 0.1 mm have been successfully detected on standard aerospace panel lay-ups.
The second form of composite sensor being developed is a Through the Thickness, or TTT sensor. The sensor involves creating a small (diameter < 1 mm) blind hole through the bonded structure and into the adhesive layer, which is then connected to a flow meter and vacuum source. If the adhesive layer fails, the damage forms a leakage path for air that is then detected by the flow meter. The size of the detectable damage is determined by the spacing of the holes in the structure. The sensors have been used to detect poor manufacturing processes and damage due to impact on the whole structure, with results confirmed using standard c-scans and thermography.