Alternating Current Potential Difference
ACPD is an electromagnetic crack depth sizing technique and relies on the fact that an alternating current flowing in a thin skin on the surface of a component will be disturbed by the presence of a crack. A current is injected (or sometimes induced) into the component in such a way as to flow perpendicularly across the crack.
A series of voltage measurements are then taken both across and adjacent to the crack, using either a single moveable probe (for on-site inspection) or a series of fixed probes (for monitoring). The ratio between the voltages measured across the crack and adjacent to it can be related to the extra path length taken by the current around the crack faces, from which the crack depth can be calculated.
Using an alternating, rather than a direct current, means that the currents are concentrated in a thin layer at the specimen surface. One advantage of this is that much lower currents are required so there is no complication arising from the specimen heating up. Another advantage is that the 2D nature of the electric fields allows theoretical modelling of the current flows, even over complex structures, which results in improved depth sizing accuracy.
The ACPD potential drop technique requires two connections between the instrument and the inspection site - namely the current output, and the voltage input from a single probe for inspection and sizing, or from one of a number of probes during crack growth monitoring.
When an alternating current is passed through a conductor, the so-called “skin effect” forces the current to flow in a thin layer on the outer surface. This means the effective cross-section carrying the current is small, so that currents of less than an Amp can generate relatively high surface voltages. This feature offers benefits compared with the direct current method (DCPD), particularly on large specimens.
Materials of high permeability or conductivity thus have relatively small skin depths. At a frequency of about 5kHz, for example, ferromagnetic mild steel has a skin depth of order 0.1mm, high conductivity materials such as aluminium, tungsten and zinc have skin depths of 1-2mm, and low conductivity metals such as titanium, stainless steel and Inconel have skin depths of 5-8mm.
The input current produces a surface voltage which is measured by a two point contacting probe. The value of the measured voltage is dependent on the strength of input current, the separation of the two measuring points, the skin depth, the material conductivity and the specimen geometry.
The technique is mainly used for crack sizing because the need for good electrical contact makes it unsuitable for crack detection. Crack detection (as well as crack sizing in cases where surface contact is not possible) is best carried out with a non-contacting technique, such as a.c. field measurement (ACFM).
For a given material, keeping the current and the probe gap constant, and ensuring a uniform current distribution, makes the measured voltage dependent only on the conductive metal path length between the probe tips. When the probe straddles a surface-breaking crack, this path length is obviously longer than with no crack. A combination of two voltage readings, one across a crack and the other on an adjacent un-cracked area, allows separation of the effect of the crack from the other effects.
In this way, crack depth can be calculated without the need for any calibration.
The main technical benefits of ACPD are:
In TSC’s ACPD equipment, the current input and voltage measurement are generally kept apart. The advantage is that it allows the current input points to be widely separated to produce a more uniform electric field, giving more accurate sizing. In addition, this approach also allows many voltage probes to be associated with a single input current for monitoring purposes, making the instrument more versatile.
General advantages of ACPD include:
- ACPD works on any metal type, including welds.
- Probes are inexpensive. This makes the ACPD Technique ideal for monitoring, on-site or in a laboratory environment, using spot-welded pins.
- Very small increments in crack depth can be detected by comparing readings taken periodically with the same fixed probe.
- Provides a detailed, accurate crack profile, including information on crack bridging.
- In a metal with large skin-depth, compared to the voltage probe size, inclination of the crack to the metal surface can be estimated.
- The a.c. skin effect concentrates currents at the surface so the currents required are much smaller than for d.c. potential drop.
- Defect depth can be measured with no upper limit.
- TSC staff have between them over 50 years experience in the application of ACPD in a wide range of applications.
- Products for crack measurement or continuous monitoring generally available ex-stock.
- Experienced engineers and product development teams can offer bespoke ACPD systems.
- First class customer support.
- TSC’s ACPD Systems are used by influential universities and major certification bodies worldwide.
As long as care is taken over field input placement and probe deployment as described above, ACPD is a very easy technique to use to obtain accurate crack sizing in a wide range of applications. Since the mid-1980's, staff at TSC and University College London built up a wealth of experience in the successful use of ACPD. Please contact us to discuss further
ACPD Mk IV Instrument
TSC's ACPD probes, when used with MK IV instrumentation, provide the capability to measure crack depth at different points along a defect and to provide 'continuous' monitoring of crack profile and growth. The probes and instruments have been developed to minimise spurious signals from induction effects leading to more reliable and repeatable measurements.
The MKIV system provides: acpd mk iv
- On site crack depth profile measurement using a hand-held probe.
- Long-term crack growth monitoring using multiple fixed or movable array probes.
- Rugged unit, IP54 rated.
- Windows software for ease of operation and compatibility with other Windows applications.
- Full data storage for back-up, off-line view and audit purposes.
ACPD Probes are available to meet general or specific inspection needs - a full range of probe types are available including:
- Probes for awkward geometries.
- Probes suited to non-magnetic materials.
- Probes for underwater use.
- Probes for high temperature use.
Probes can be made with tip spacing between 2.5mm and 20mm with quickly detachable probe tips which can be sharply pointed for rough surface inspection or rounded for high quality surfaces.
Cable accessories are available such as twisted pair cables for connection between the MK IV ACPD instrument 16 way multiplex card and spot welded pins, and an ACPD current output cable for connecting current input to a sample. See MK IV Accessories.