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ACPD in Detail Deployment in Special Situations CT - Specimens Access on a standard compact tension (CT) specimen obviously prohibits the field wire and probes contact arrangement used on a flat plate. Experience has shown that the best positions for the current inputs are on the edge faces either side of notch as shown in Figure 5. This gives a good, linear dependence of voltage on crack length over a large length range. The current input points may conveniently be combined with the fittings for a clip gauge, if used. The relatively small size of CT specimen means that there is an inevitable redistribution of current when the crack approaches through-wall. This process then gives a non-linear variation in voltage with crack length.
Figure 5 - ACPD connections for a CT specimen. The voltage contacts are best placed in a three-point arrangement along the centre-line of the notch as shown in Figure 5. The middle contact is then common to both probes. The cross-crack probe points should be symmetrically placed on the top and bottom notch faces close to the start of the 'V'. The third contact making up the reference probe should be placed 10mm or so further away from the 'V' on one notch face. Note that the measurement of the cross-crack probe spacing must be made around the 'V'. If the CT specimen material is thin-skin and the crack front is fairly straight (as is usual in CT specimens cracking) then no account need be taken of the fact that the voltage readings are not taken at the crack edge. If, on the other hand, the material is thick-skin then a modifier must be used to take account of the stand-off distance between the centre contact and the crack. Modifiers are discussed further in the section on sizing. Large Specimens When the inspecting or monitoring on large specimens such as tubular joints, there is a limit to the region over which the current density is uniform and strong enough to measure. The usable width of the field is roughly equal to half the separation between the current input points, centred on the line between these points as shown in Figure 6. If it is required to inspect an area outside the range, the field input points should be moved (either by physical disconnection and reconnection, or by switching between an array of inputs).
Figure 6 - Extent of useable injected field. Threads Most cracks in threads occur along the thread roots but the position around the root radius is not generally known. For small to medium threads (up to say 10mm pitch) it is generally best to make the voltage probes contact across adjacent crowns as shown in Figure 7, using a purpose-built probe. If the thread is on a solid component, such as a bolt, the field is best injected along the axis. This gives a uniform distribution around the thread circumference. If there arrangement is not possible it may be better to induce the field using a coil that moves with the voltage probe as shown in Figure 7.
Figure 7 - ACPD connections for bolt inspection. The reference voltage should be taken further around the same thread as the crack voltage at a point where the voltage is seen to return to a steady value. Voltage readings taken across the thread immediately adjacent to a crack often show a satellite signal (especially on thick-skin material). This is a drop in voltage below the normal background level caused by a diversion of current away from the nearby crack. Examples of the voltage traces expected are shown in Figure 8. When sizing, modifiers should be used if the crack is short or the material is thick-skin to take account of the stand-off distance between the thread crown and the crack.
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