2.2 Small dents: bottomside MFL contour plots, before and after annealing: Figure 3 shows MFL signal contour plots (radial component) for small dents from 1.0mm deep up to 3mm deep (0.5mm was also tested but is essentially similar to the 1mm deep signal). MFL signals are shown prior to and after stress-relief annealing, for a flux density of 1.8T. Before annealing, (Figure 3 – Preanneal) MFL signals exhibit the following features:
1) a centre peak region which, for shallow dents, is similar to that seen for a typical corrosion pit (see Figure 2) and
2) four ‘shoulder peaks’ at approximately 45o to the applied field direction.