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ACPD in Detail Practical Deployment The ACPD 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). This section describes the general rules for deployment on simple specimens such as flat plates or uniform cylinders. Deployment on more complex geometries is described in the next section. Current Input The current is generally injected into the specimen by direct contact. The two contacts should be placed equidistant on either side of the crack site and with the crack lying perpendicular to the line between the contacts (see Figure 1). The contacts should be about 150mm away from the crack, if possible. The further apart the contacts are the more uniform the field is. On ferritic steel, the field contacts are most conveniently attached by magnets. On non-ferrous materials they have to be fixed on mechanically (e.g. screwed onto spot-welded studs, spot welded directly to the surface, or attached to suitable points with crocodile clips).
Figure 1 - Injected current input on a flat plate. In certain situations, such as for axial cracks on a cylinder, an injected field is not very practical. In this case it is better to use the field leads to induce a current into the specimen by deploying them above the specimen surface perpendicular to the crack (see Figure 2). For other situations where it it not possible to inject a current, but where it is also difficult to lay inducing wires, special purpose ACPD voltage probes can be produced which carry a self-contained induction coil.
Figure 2 - Induced field input for axial crack in cylinder (cut-away view). Voltage Probe Connection It is important to maintain good electrical contact with the sample with both pins for several seconds while the voltage display settles. For this reason, probe pins are usually sprung so that contact is maintained even if the probe is moved slightly. Such probes should be held down with the springs fully compressed to minimise the area of loop between the pins. If stable readings cannot be obtained, it is normally because the electrical contact is not good enough. The metal surface must be cleaned to bight metal. Taking Measurements In its simplest deployment, the ACPD technique requires three parameters in order to produce a crack depth; these area voltage reading with probe straddling the crack, a voltage reading with the probe adjacent but not straddling the crack, and the spacing between the voltage probe contacts. The voltage readings are in arbitrary units because only the ration of the two is used. If the position of a crack is not known, the voltage probe should be moved along a line perpendicular to the expected crack edge. The display should be watched fro any sudden changes in reading. The reading will jump up from one steady value to another higher value when the leading probe contact crosses the crack. Not that these values should be stable, repeatable and non-zero; non-repeatable or very low readings indicate loss of electrical contact by either the voltage probe or the field input. Having determined the crack location, the two required voltages should be obtained as shown in Figure 3. The probe should be edged slowly towards the crack until the voltage reading jumps up. This value should be recorded as the cross-crack voltage, Vc. Readings should be observed on the instrument front panel for stability. A reading should be stable within +/- 1 digit for several seconds before being captured. The captured value should be checked to ensure it matches the value observed on the instrument before capture. The probe should then be slowly backed off until the voltage reading drops down again. This value should be recorded as the reference voltage, Vr. The purpose of this procedure is to obtain voltage readings as close together as possible to minimise the effect of any variation i input field strength. Such variations occur, even with a nominally uniform input field, around short cracks or cracks in materials with a large skin depth.
Figure 3 - Accurate measurement of ACPD voltages. When use on flat plate, the spacing between the voltage probe contacts along the metal surface will be the same as the straight-line separation in air. This can then be measured with a ruler. On any other surface, the separation along the metal surface will be longer and should be measured directly, using flexible tape, for example, at the site where the readings are taken. The voltage changes shown in Figure 3 apply for a long uniform depth crack in a thin-skin material. If the crack is short compared to its depth (i.e. length < 5 x depth), or is in a thick-skin material, the voltage is not constant either side of a crack - instead the distribution contains sharp cusps (as shown in Figure 4). In such cases, it is important to measure the voltages at the cusp points (as close t the crack edges as possible) otherwise the crack depth will be underestimated. In cases where it is not possible to measure the cusp voltages (such as when monitoring crack growth with fixed probes) it is possible to correct for this underestimate if the separation between the crack and the nearest voltage contact is known (refer to TSC for details).
Closing Comments 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. Over the last twenty years, staff at TSC and University College London have built up a wealth of experience in the successful use of ACPD, and will be pleased to help with any applications not covered in this document. ACPD Measurement / Introduction / ACPD In Detail / Practical Deployment 1 2 3 Next |
