Non-destructive testing of materials

Magnetic field imaging

The visualisation of metallic objects located beneath or within an optically opaque boundary, such as steel reinforcing bars and cables embedded within pre-stressed and reinforced concrete, is a very important problem and a central research theme within SISP. We have been active in this area since 1990, and have published over 50 journal and conference papers on this theme. These systems generate images of the targets either by exploiting eddy current generation and detection when a time-varying magnetic field is impressed on the material by resonant transmitter coils, or by direct visualisation of steady-state magnetic fields using solid state sensors. Some of our systems have been or are being commercialised.

Corrosion of steel in prestressed and reinforced concrete structures is a global, multibillion dollar problem. Most techniques used to assess its severity are either semi-destructive or hazardous. In my group, we have been developing radically new methods. In 1999, we commenced work on a multi-sensor array inductive scanner for rapid imaging of reinforced and pre-stressed concrete. Earlier research suggested that before the method could be adopted by industry, a number of outstanding issues needed to be addressed, including: miniaturisation, increased detection range, image de-blurring, quantitative information extraction, corrosion detection and scan time reduction from one hour to approximately ten minutes. In our final report, we confirmed that all of these critical research targets had been realised and in several cases exceeded. The system could now generate, within seven minutes, spatially accurate, virtual 3D images of steel embedded in concrete. In addition, the system could also identify regions of corrosion and qualitatively assess its severity. The system (ISG1) is now being commercialised.

Read a copy of one of our final EPSRC reports in PDF format.

Early ISG1 system

Following the development of ISG1, it was recognised that in theory, the electromechanical scanning assembly could be replaced by a 2D array of solid state magneto-inductive sensors, to produce live, streaming-video of steel embedded in concrete (or any other electrically inert material).

The Solid state 1D scanner

Our latest system is a solid state real-time magnetic field camera (mFIC), initially for deployment in the nondestructive evaluation (NDE) arena, but later for exploitation in many other fields, including medicine, security, manufacturing, quality assurance and design involving magnetic fields. As its name suggests, it has no moving parts and will generate video rate images of metal targets located beneath an opaque layer.

Initially, a 1D line array of 33 sensors was fabricated, which allowed the generation of an image using a single pass of the electromechanical scanner. The system can now generate in seconds, images that at one time required over two hours to produce. The full 2D system is now under construction, which will comprise 1089 individual sensors. With the deployment of the 2D array, it will be possible to stream video-rate images (at ten frames per second) to the dedicated control unit and display these in real-time on the graphics screen.

The system under construction comprises a coil, 400 × 400 mm in area, which may be excited by either a steady-state or time varying current source. Below the plane of the coil are located 33 rows of solid-state, tri-axial magneto-inductive sensors, with each row comprising 33 sensors. The outputs of the sensors are connected to application specific integrated circuits (ASICs), which in turn generate pulse-code modulated (PCM) bit streams respecting the x, y and z field components detected. The outputs of the ASICs are multiplexed and fed to three sub-master controllers, with each sub-master controlling up to twelve rows. The sub-masters route data to a single DSP master controller, which transfers the data to the host computer. The design allows each sensor to be individually activated, avoiding cross-talk between adjacent sensors when scanning over an area. (It is reiterated that at the time of writing, a single row is employed in conjunction with a scanner to generate the images.)

Other non-destructive testing research

  • Research and development of ultrasonic systems for detection and imaging of fault conditions in pre-stressed and reinforced concrete, including visualization of steel embedments and voids. These systems have  exploited the analysis of the frequency content of the returned echoes  for fault identification, or the Synthetic Aperture Focusing Technique (SAFT) applied to signals received from multiple transducers to  generate 3D images of the internal concrete matrix.
  • Research and development of inductive scan imaging systems for the visualisation of steel reinforcing bars and cables embedded within pre-stressed and reinforced concrete. These systems generate images of the steel by exploiting eddy current generation and detection when a time-varying magnetic field is impressed on the material through the use of a resonant transmitter coil. Extensive image processing software development is required to improve the spatial and axial resolution of the raw images. Download Inductive Scan Imaging research report.
  • Development of inductive imaging systems for detection of faults in prestressing wire of large bore water pipes.
  • Research and development of instrumentation for fault detection in pre-stressing cables exploiting the principle of electrical time domain reflectometry.
  • Development  of intelligent, autonomous sensors for monitoring fatigue damage of welded steel components. This work is on-going and is funded by the DTI.
  • Development of remote, autonomous ultrasonic sensors for monitoring flood conditions in the reinforcing cross-beams of oil rigs.

 

For more information, visit the Signal Wizard website at: www.signalwizardsystems.com

Project main investigator: Professor Gaydecki

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