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Updated: March 21, 2012 22:28 IST

IGCAR develops sensors to inspect defects in materials

R. PRASAD
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Test specimens with two circular defects – one big and one small (right) and a rectangular and a circular defect (left) imaged using nanofluid optical sensor developed by Dr. John Philip, IGCAR, Kalapakkam
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Test specimens with two circular defects – one big and one small (right) and a rectangular and a circular defect (left) imaged using nanofluid optical sensor developed by Dr. John Philip, IGCAR, Kalapakkam

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.

Advantages

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.”

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This has reference to Panicker's question. I agree with you on the importance of inspection tools for non- metallic composites materials of aircraft and small marine vessels. However, flux leakage technique can be adopted only for ferromagnetic materials. I am sure, one day, scientist would come up with a sensor for such applications.

from:  Mahendren
Posted on: Mar 27, 2012 at 09:10 IST

With regard to Pavan's question, it is true that MFL is a technique suitable only for magnetic materials. MFL is being routinely used for inspection of many ferromagnetic materials and the present one seems to have many advantages over the existing ones as elaborated by Prasad . The demagnetization is not a major problem after testing.

from:  Chamy
Posted on: Mar 26, 2012 at 11:16 IST

I am very happy to answer your questions: (1) What is the smallest size of defect that can be detected by this sensor? Ans: ~ 500 micron or more (2) Can it detect sub-surface defects? If so what is the maximum depth below the surface at which a defect can be detected by this method? Ans: Yes, we have measured defects at a depth of 6 mm below surface (3) What are the advantages of this "optical method" over conventional magnetic particle inspection (Wet/Dry, AC/DC, Normal/Fluorescent) methods? Ans: Ours is truly a non contact technique. The techniques mentioned by Udhishtir are really messy as the particles come into contact with the test specimen.

from:  John Philip
Posted on: Mar 26, 2012 at 11:06 IST

Excellent work by IGCAR. Can this method detect defects within non- metallic composites like those used in aircraft and small marine vessels? The innovation could save countless lives if the system is quickly incorporated into handheld detectors for use by Aircraft maintenence engineers and marine and loco safety engineers.

from:  H.Panicker
Posted on: Mar 26, 2012 at 09:23 IST

The main limitation of this method is that it is limited to all
materials which can be easily magnetizable. Also the issues with this
method is demagnetization should be followed, else if this tested
material is used in sensitive applications like dials, compass etc, erroneous results will be detectable. Are these issues taken care?

from:  Pavan Kumar G
Posted on: Mar 24, 2012 at 10:16 IST

Wow! Never knew that there so would be so much interest in this section
of the news. Way too technical for me.

from:  Sadhan
Posted on: Mar 24, 2012 at 08:12 IST

The color pattern can be seen with naked eye.The fringes are due to
Diffraction of white light.

from:  V.Mahendran IGCAR
Posted on: Mar 22, 2012 at 22:23 IST

I am the first author of APL paper,crack of 500 micron at 1 mm depth we
could image,it can detect sub surface defects in our paper we have reported imaging crack at 6 mm depth below the surface.Main advantages
are,our sensor is fully reversible, by naked eye one can see the buried
defects as color pattern.It can confer shape, orientation, length and
width qualitatively.

from:  V.Mahendran IGCAR
Posted on: Mar 22, 2012 at 22:20 IST

This methodology seems to be a cheap way of bringing out the qualitative information on defects. The scientists at IGCAR deserve appreciation for their achievement. Hope this is further pursued to produce a patented commercial product.

from:  Naru
Posted on: Mar 22, 2012 at 18:35 IST

I think this would be the good way of detecting magnetic defects. This may avoid using magentic crack detector and Eddy current tester,i believe.If this can be used for detecting the sub surfce defects (i.e depth oriented)then this is the most cost effective method. Nice to hear that Digging out the innovation fied is on high at IGCAR.

from:  Sudha
Posted on: Mar 22, 2012 at 12:08 IST

This is not the first time we hear about positive contributions from
Indira Gandhi Centre for Atomic Research (IGCAR), Kalpakkam whose motto is to "provide the means to realize your dreams towards seeking an excellent scientific pursuit At IGCAR, we believe in EXCELLENCE WITH RELEVANCE Our strengths are the highly stimulating and scholarly environment, knowledgeable and motivating faculty and state-of-the-art facilities." In a country often remembered for negatives, dismal atmosphere of endemic corruption and unethical manipulations, we feel justly proud of excellent contributions from institutions like IGCAR.

from:  N.G. Krishnan
Posted on: Mar 22, 2012 at 08:58 IST

Interesting article on an interesting technology development. Will the Author of the article or the Scientists from IGCAR clarify the following three points?
(1) What is the smallest size of defect that can be detected by this sensor?
(2) Can it detect sub-surface defects? If so what is the maximum depth below the surface at which a defect can be detected by this method?
(3) What are the advantages of this "optical method" over conventional magnetic particle inspection (Wet/Dry, AC/DC, Normal/Fluorescent) methods?

from:  Udhishtir
Posted on: Mar 22, 2012 at 06:52 IST

Is the colour fringes seen, is it diffraction or polarisation? I would like to see a monocromatic image preferably in yellow light(sodium vapour). I'm sure they would have faced problems on residual magnetic field and how they resolved it, would be interesting .
I feel this can be related to reflective Photoelasticity too. As Glass can behave as Polaroids. If this is photoelastic, what you see is stress caused by magnetic field. By quantifying the fringe pattern you can identify the amount/shape of magnetic flux there by the shape may be location of defect inside the material.
There was a sensor made by one of the Professors in IIT Madras which was based on thin film.
the scope of such sensor can be made very versatile if we can combine the optical effect too.
Such Non-destructive testing (NDT) or Non-destructive Analysis (NDA) can be brought to not only identifying defects but also for stress analysis.
this material is "smart". Great work. But more can be done. Good Luck.

from:  senthil
Posted on: Mar 22, 2012 at 03:29 IST
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