Magnetic nanoemulsion aided optical defect detection in carbon steel components: Effect of defect width variation on optical contrast

Abstract

Optical detection of defects of varying widths in ferromagnetic steel components is carried out using polyacrylic acid stabilized magnetic nanoemulsions (MNEs). Magnetic field lines are leaked from the defect regions, due to permeability variation, when a defective ferromagnetic steel component is magnetized. Under the influence of such magnetic flux leakage (MFL), the MNE droplets exhibit orientational ordering, where the inter-droplet separation within the chains varies with the strength of the magnetic field. Exploiting this tunable optical grating phenomenon, wide area naked eye visualization of the surface opening and buried defects is carried out. The defect morphology is accurately estimated using a double thresholding-based image processing technique. The optical contrast varies non-monotonically with defect width for the surface opening defects, which is due to the formation of the surface dipoles on the defect edges. The surface dipoles are strongly coupled for lower defect width due to smaller spatial separation, which causes the MFL to be lower. With increasing defect width, the surface magnetic field lines are diffused leading to a lowering of the MFL. In between, for ∼2 mm wide defect, MFL is the highest and the generated optical contrast is found to be the strongest. The MFL profiles are numerically estimated using finite element modeling and experimentally verified using a Hall probe. The cross correlation between the generated optical contrast and MFL magnitudes clearly demonstrates the non-monotonic effect of defect width on the optical contrast of the MNE-based sensors for large area naked eye non-destructive defect detection in ferromagnetic materials.