In order to provide a defect size (length and depth) the ACFM software requires input of four parameters:
The background magnetic field strength (Bx measured away from the defect).
The field strength at the deepest part of the defect (Bx minimum value).
The distance between the Bz signals (peak & trough) associated with the crack
ends. The sensor lift-off (normally 0 unless the surface is coated).
Figure 8 – Software screen. Signal form calibration standard scanning.
The length (c), (measured on the sample, or from an encoder) is converted to actual defect length.
The software the uses the ratios of (a) and (b) - a dimensionless number, which is independent of instrument gain, material permeability or conductivity (provided the skin depth is small) and compares this with values in the look-up tables for the given lift-off to give defect depth.
The use of a unidirectional input current provides a number of advantages:
On metals with a small skin-depth (such as ferritic steel), the interaction between the current and a defect has been extensively modeled so that defect sizes can be ob-tained without resorting to calibration on slots.
Currents will be forced to flow further down a crack face (compared to circular cur-rents). This means that deeper cracks can be sized.
Input current strength and magnetic field perturbations decay relatively slowly with height above the surface. This means that the technique is less sensitive to changes in lift-off and can be used to inspect through coatings 6mm (1/4") or more thick.
The ability of measure open to the surface fissures without the necessity to extremely clean (white metal) the weld region makes ACFM a natural alternative to other traditional NDT methods (MPI, ACPD, ET, surface wave UT).
Based on its advantages ACFM method was chosen to evaluate surface crack detected by MPI on FPSOs turrets welds.
3.2 UT TOFD
TOFD (Time of Flight Diffraction) method is based on ultrasound crack tips (top and bottom) diffractions. This diffraction happens when an ultrasound beam is introduced by an emitter transducer in the welded zone. Reflected and diffracted waves are collected by another angled receiver transducer. On bottom part of figure 9, an A-scan presentation represents the ultrasound signal converted in an electrical one. Blue signal belongs to a