Alternating Current Field Measurement
Alternating Current Field Measurement (ACFM) is a technique used to detect and size surface breaking cracks in metallic objects.
Finding cracks is important because once a crack forms it becomes a weakness and a focal point for stress -- much like the weak link in a chain. And once you have a weakness, it's only a matter of time before you have a failure, and that tends to end badly.
ACFM was developed in the 1980s, based on the development of the ACPD technique of crack sizing. The ACFM technique extends ACPD by not requiring electrical contact.
This is a significant advantage because it means we can use the technique underwater, remotely, and we no longer need to clean the surface being tested. No big deal? It is when you realize it takes a full day to clean a subsea member less than 3m2 in surface area -- at a cost of almost $50k US per day, not requiring the cleaning means massive savings during any inspection campaign.
Similarly, in topside inspections you can test objects that are still being used, even if they're hot or irradiated, without having to remove coatings or shutdown production.
How Does it Work?
Take a deep breath -- an Electrical Engineering degree will help here, especially without the diagrams:
Like ACPD, ACFM makes use of the skin effect of metal to find and measure cracks. ACFM starts by inducing a high frequency uni-directional (AC) input current into the metallic surface being tested. A probe is then used to measure the magnetic field above the surface. The field has three components, Bx is perpendicular to the induced field, By is parallel to it, and Bz is perpendicular to the metallic test surface.
If there is no crack (aka defect), the field will be uniform in the Bx direction, and By and Bz will be zero. Once a crack is encountered, current will be diverted away from the deepest part, concentrating it on the ends of the crack. This will be observed by strong peaks and troughs in the By and Bz components, and a broad dip observed on Bx. Generally it's the Bx and Bz components that are measured, which will give us the crack length and depth respectively.
The software used to analyse ACFM results traditionally graphs the Bx and Bz components against time to allow for crack detection. Bx and By are also charted against each other, with the resulting butterfly graph particularly useful for crack sizing.
Does not require surface cleaning and works through paint coatings.
Gives both depth and length sizes for detected cracks.
Works on welds, ferritic, and non-ferritic metals.
Can be deployed remotely and does not require contact. So can be used underwater, in irradiated environments, and on hot surfaces.
Alternatives to ACFM
MPI - Magnetic Particle Inspection
Technique that involves spraying a fluorescent magnetic dye over the suspected crack area (new welds are a good example). The dye will be pulled into the crack, so by wire brushing the area the only visible paint will be in the crack! Good for crack detection and length sizing but useless for estimating crack depth. Requires extensive cleaning of the test surface and the removal of any coating.
UT - Ultrasonic Testing
By applying an ultrasonic pulse to the metal object and watching the return pulse we can look for distortions in the echo that would come from a crack in the metal. Excellent unambiguous results that can give crack depth and length. Very labour intensive and also requires extensive cleaning and coating removal.
ACPD - Alternating Current Potential Difference
Uses a similar technology to ACFM, and gives excellent unambiguous results for crack depth and length. Also requires extensive cleaning and coating removal.