The sensor will be able to locate defects and discontinuities such as fatigue, cracks, corrosion pits, metallic inclusions and abrasion in ferromagnetic components and structures, says team leader John Philip of the Indira Gandhi Centre for Atomic Research (IGCAR) in Tamilnada. It might also be used to inspect magnetic materials used in the aircraft industry, such as those used to make turbines, for example.

Current techniques, such as "magnetic flux leakage" (MFL), that detect defects in ferromagnetic materials work by measuring stray fields near cracks using a sensor. However, such methods require sophisticated instrumentation and complex signal processing to correlate the MFL signal with the defect signature. Moreover, sophisticated algorithms are then needed to recreate a 2D or 3D image of the defect. Alternative techniques that employ magnetic particles to visualize flux lines around an inclusion, for example, are messy because the magnetic particles must adhere to the component being tested and are difficult to remove afterwards.

"Our new sensor overcomes all of these challenges because it does not come into contact with a sample – it is placed a few millimetres above – and does not involve any complex signal collection or analysis," said Philip. "It is a simple visual test for relatively large defects (a few millimetres in size) buried inside a sample."

Philip's team developed a special nano-emulsion to make its sensor that consists of a colloidal suspension of single-domain superparamagnetic Fe3O4 nanoparticles around 6.5 nm in size that are magnetically polarizable – that is, they respond to a weak magnetic field. The particles are capped with a monolayer of anionic surfactant (sodium dodecyl sulphate) so that they do not agglomerate.

In the absence of a magnetic field, the nanodroplets are randomly oriented but when the nanofluid comes into contact with a magnetic defect, the droplets align in a chain-like fashion along the field created by the defective region. This chain of particles diffracts incident white light to produce bright colours and the colours obtained depend on the defect features. "This approach allows us to directly visualize the defect, with a violet colour being seen at the point where the magnetic flux leakage reaches its maximum – that is at the edge of the defect," explained Philip.

Rectangular and cylindrical defects

The researchers tested their technique on steel plate specimens containing artificially fabricated rectangular and cylindrical defects. The samples were magnetized prior to the experiments. The images obtained clearly show a colour spectrum on both sides of the defects that follows their shape – straight lines are seen for the rectangular defects and curved lines for the cylindrical ones.

Spurred on by these results, the team now plans to make a large-area flexible-film-based sensor using its nanofluid and develop the appropriate pattern recognition software. Such a device could be used by non-specialists to rapidly analyse defective regions in a wide variety of ferromagnetic materials, says Philip.

The current work is detailed in Applied Physics Letters.