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Since the Bronze Age, mankind has relied on metal work to grease the wheels of industry. Step forward several thousand years, and the industrial revolution ensured our dependence on machines, and in particular, on their not breaking down or exploding and killing us all.

Enter non-destructive testing, which allows us to test the pieces of metal in structures and machinery in a manner akin to a GP's visit rather than an autopsy.

Alternating Current Potential Difference is such an NDT technique, used to determine the size (in particular depth) of a crack in a piece of metal. Finding and sizing cracks is important, as cracks indicate a structural weakness that is likely to end in failure. Accurately measuring the size of a crack allows us to make predictions as to the likelihood and timing of such a failure. By monitoring the depth of a crack over a period of time, we can observe the rate at which the crack is growing, and more accurately predict the likely timing of a failure.

ACPD works by making use of the skin effect of current flow on the surface of metallic objects. We start by injecting a high frequency AC voltage (~5kHz) across the piece of metal being tested. Ideally the two electrical contacts are placed equidistant from the crack being tested, with the current flowing perpendicular to the axis of the crack. Then we take voltage (potential difference) readings across the metal, using a probe that ensures the potential difference is measured over the same distance each time. We need two voltage readings, one across the crack, and a second reference measurement as close to the crack as possible.

The skin effect basically means that the current will flow along the surface of the metal, so if there's a crack, the current will travel further and the potential difference will be greater -- the deeper the crack, the higher the voltage. So, we can determine the crack size by comparing the crack voltage to the reference voltage as follows: Crack Depth = Δ/2 (Vc/Vr - 1) Where:   Δ is the probe separation.   Vc is the crack voltage.   Vr is the reference voltage.

A related technique for detecting and sizing cracks without the need for electrical contact is ACFM (Alternating Current Frequency Measurement).

Limitations of ACPD
• The surface being tested must be very clean.
• Accurate orientation of the probe is very important.
• Hard to deploy in hard to access locations.
• Cannot be used sub-sea. Alternative Methods of Crack Sizing
• Alternating Current Frequency Measurement
• Ultrasonic measurement
• Mechanical measurment (using feeler gauges)
• Post-mortem examination
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