Measurement and characterisation of the electric stress field in geological and man-made materials (E-Stress)

The aim of this project is to investigate a new technique, using a unique sensor technology - the Electric Potential (EP) Sensor, invented at Sussex, for the measurement of electric field activity related to the build-up of stress in geological and man-made materials. This will result in a sensor design and the fabrication of a batch of EP sensors, capable of coupling to the electric field produced in a representative selection of rock and concrete samples. Choice of material and testing regimes will be advised from project partners and collaborators. The EP sensors will be used concurrently with strain gauges and acoustic emission sensors in bespoke acquisition and analysis instrumentation. Project partner, British Geological Survey (BGS), has committed the in-kind use of a state-of-the-art rock mechanics laboratory for all stress testing programmes. The sensor instrumentation system will correlate EP sensor outputs with stress and internal damage processes occurring in samples subjected to the rock testing system. The correlated EP sensor output is referred to as E-Stress. There is conclusive evidence that supports the existence of electrical activity in stressed rock and concrete however, the measurements made to date, by other workers, are of weak current signals at the pico to micro Amp level, a difficult measurement even in a laboratory environment. There is also evidence that various materials exhibit pre-fracture signal activity which can be of great importance to structural health and geoscience. The investigator has already published preliminary results that identify large (Volts) electric pre-fracture signals in a range of rock samples. This combination of a large available signal, an appropriate sensor, and expertise in the application of the technology, presents a unique opportunity with specific expert partners to develop a new measurement tool with predictive capability and insight to internal processes. To achieve these objectives the work packages will be:

WP1 - Initial planning meetings, consultations with BGS and Arup in materials to test. Trial tests at BGS for familiarisation and sensor instrument planning.

WP2 - Develop an EP sensor design capable of detecting electric field in both rock and concrete samples. The investigator has experience in applying EP sensors to couple to the electric signals produced in the diversity of rock and concrete under stress. The sensors will be integrated with strain gauge and an acoustic emissions system into one portable instrument using a high quality data acquisition and processing system. This system will also require work on software analysis of the signals for correlation.

WP3 - The application of the EP sensor and instrumentation within a rigorous material testing regime to destruction at BGS. Three testing programmes will be refined in consultation with partners and all data recorded to storage systems.

WP4 - Analysis and correlation to calibrate EP sensor as an E-Stress instrument for use in research and industry. Results will be disseminated and presented to international conferences and industry networks. Work will conclude by exploring the commercial opportunities with technology licensee, Plessey Semiconductors to fabricate custom sensors.

The work is novel and timely since the investigator's preliminary publication and interest from BGS. The results will be of direct interest to the project partners: Arup, for structural health monitoring (SHM) of civil infrastructure and to BGS, as a tool for monitoring natural hazards caused by rock fracture or slippage. There is a route to commercialisation through the support of Plessey Semiconductors.

This technology cuts across discipline boundaries with the work in this proposal being of interest to structural engineers, seismologists and material scientists who are interested in the properties of rock and concrete. A sensor which can measure stress changes by monitoring the electric field presents a non-invasive and simple tool that can provide fundamental data on the processes occurring within the material, up to the point of failure. This will assist scientists who study structural materials by providing them with additional information on which to base their analysis.

Academia: The unstable nature of the land has always presented challenges that need to be addressed in buildings and infrastructure resilience to slippage or subsidence. This has led to considerable investment in characterising construction materials. Thus, the study of the properties and behaviour of materials used within buildings and civil structures is an active area of research. New sensor technologies which can reveal complementary information to aid analysis techniques of laboratory scale materials will benefit such studies in the civil engineering research community.

Geotechnical scientists looking at the properties of geological rock samples using traditional measurement methods will benefit from an alternative and simple sensor technology, capable of detecting process driven electric stress changes with precursory information. This will give them further insight into the molecular dynamics of the rocks under test conditions and help to develop ideas on why precursory signals occur within them. In particular, the British Geological Survey will use the final version of the sensor in laboratory analysis of rock and would look to deploy it in the future as a network of sensors, at sites around the UK known to be unstable.

The sensors engineering community will also benefit from such a diverse technology by providing a new tool for electric field measurement which is robust and suitable for deployment in unattended sensor networks for long term monitoring applications.

Industry: The interests of industry frequently align with academia and so the electric field sensor technology proposed will be of direct relevance to civil and structural engineers in the construction and consultation sectors. These industries either embed/retrofit sensors to monitor the structural health or make repetitive onsite observations of national infrastructure and buildings. The scientific knowledge gained from the proposed laboratory studies of materials, will assist consultants in specifying construction materials and devising maintenance plans for their clients. In addition, they will be informed of the future applicability of E-Stress sensing as a field deployable technique on a range of materials and its use for structural health monitoring.

Specifically, the E-Stress sensor and the results it produces will be of interest to Arup and its research division in providing complementary information to inform structural engineers.

Economic and Societal: This sensor technology has already made an impact on society with its application in medical instrumentation, currently in commercial production. This proposal aims to evaluate the applicability of the technology to monitoring stress changes in rock and man-made materials. The impact of developing a robust, reliable, easily deployed sensor will be considerable. This could provide an economic solution to retrofitting existing buildings and structures with long term monitoring for structural health as well as larger scale slippage and subsidence. The crucial factor is its significance to worker/human safety, especially with regard to repeated onsite observations being obsolete if these sensors are employed. It will help in shaping policy and legislation on an international level and provide savings to the national economy through better mitigation of damage and need for repair.