Defective regions produce magnetic resistance, which in turn leads to leakage of magnetic flux
Detecting and imaging structural defects like cracks, holes etc, present in components made of ferromagnetic materials like pipelines, railway tracks and tubes has now become easy with optical sensors. These sensors were developed by Dr. John Philip and his team at the Indira Gandhi Centre for Atomic Research (IGCAR), Kalpakkam near Chennai. The results of their work were published recently in the Applied Physics Letters journal.
The work provides a “methodology for extracting defect feature information from optical images,” notes the paper.
The optical sensor has oil droplets (about 200 nanometres in diameter) containing a few nanoparticles of magnetic materials, about 6.5 nanometres in size. The oil droplets are present as an emulsion with water.
Since the sensor contains magnetic particles, it responds to magnetic fields. “The sensor is magnetically polarizable,” said Dr. Philip, Head of SMART Section at IGCAR. He is the senior author of the paper.
The sensor works on the principle that defective regions in a material produce magnetic resistance, and this in turn leads to leakage of magnetic flux (field lines). “The leakage of magnetic flux will be right outside the point where the defect in the material is present,” Dr. Philip explained. “The nanofluid-based optical sensor can detect such leakages.”
The material whose structural integrity is to be evaluated has to be first magnetised. This can be done by using two strong magnets kept on either ends of the material to be tested. The sensor, which is sandwiched between two glass plates, is kept on top of the magnetised material. The sensor is then illuminated with white light.
Magnetic flux passes through the material the moment it is magnetised. The magnetic flux leaks if the material has any structural defects, and the nanofluid inside the optical sensor immediately forms an one dimensional array or chain along the direction of the magnetic field. “When it forms an one dimensional array, the spacing between the droplets satisfies the criterion to diffract one particular colour in white light,” he said.
The colour that is diffracted or reflected depends on the inter-droplet spacing. “When the defect is large, the magnetic flux leakage is more, and the spacing between the droplets is smaller. The reflected colour is violet,” he explained. Alternatively, if the defect is small, the spacing between the droplets is more and the reflected light is red or orange.
While the optical sensor can provide the result immediately, the dimension of the defect can be found using certain modelling. “We can map different shapes (geometry) of the defects,” Dr. Philip said.
“Future perspectives include the fabrication of large flexible films for the inspection of large components and development of suitable pattern recognition software for rapid inspection of components,” the paper notes.
The sensor has several advantages over existing techniques. For instance, the optical sensor can be repeatedly used as the one-dimensional array formed in the nanofluid is “perfectly reversible.”
“It takes less time to detect flaws in the material, allows direct visual inspection of the defects, and does not destroy the material being tested,” he explained. “The sensors are very cheap — a one inch by one inch sensor would cost just a few hundred rupees.”