As mentioned previously this series of tests included periodic thread inspection. To the knowledge of the author, this was the first time that any drill collar experimental fatigue investigation had the facility to detect, monitor and size cracks during testing. The inspection facility was considered from the outset of the design process. Access to the threaded regions was made possible by the torque / un- torquing system of the tongs and overhead crane, with the linear roller track providing the clearance for one of the members. The non-destructive testing (NDT) method of inspection used was the ACFM (Alternating Current Field Measurement) technique, with MPI (Magnetic Particle Inspection) applied as a secondary method.
4.6.1 The ACFM Technique
ACFM is an electromagnetic technique capable of detecting and sizing (length and depth) defects in metal components. It was developed within the NDE Centre, Department of Mechanical Engineering at UCL [4.14], and is an extension of the ACPD (Alternating Current Potential Drop) crack detection and measurement technique. The basis for both techniques is that an alternating current flows in a thin “skin” near the surface of any conductor. If the surface under examination is defect free, the current flow will be undisturbed. However, if a crack is encountered, the current will flow around the ends of the crack and down the faces of the crack, as shown in Fig. 4.18. Associated with the surface current flow is a three-dimensional
magnetic field in the free space above the surface, which, like the current flow, is disturbed by the presence of a defect. Fig. 4.19.
The difference between the two techniques is that ACPD relies on measurements in the electric field on the surface under inspection, whereas ACFM measures and interprets perturbations in the magnetic field above the surface under examination. The sizing capability for both methods is based on mathematical modelling of the electric and magnetic fields that surround a crack. Theoretically predicted disturbances have shown good correlation with those actually measured [4.15]. This has provided the ability to make quantitative measurements of the magnetic field disturbances and to relate them directly to the size of the defect that would have caused such a disturbance. This has eliminated the need for calibration of the system prior to inspection. These field perturbations are interpreted through a Crack Microgauge [4.16] and displayed through software on a PC.
An ACFM inspection considers two component parts of the magnetic field above the surface, X and Z in Fig.4.19. The X component, Bx, is parallel to the crack, and the Z component, Bz, is perpendicular to the metal surface. With uniform current flow in the Y-direction, and no defect present, the magnetic field in the X-direction, Bx, is uniform. The presence of a surface discontinuity diverts the current away from the deepest part and concentrates it near the ends of the defect. This produces a strong peak in the Bz signal above the ends of the crack, while the Bx signal drops in strength. Figure 4.20 illustrates the nature of the Bx and Bz signals above a surface defect. Interpretation of crack depth is based upon the ratio of background to minimum Bx levels, whilst crack length is determined from the peaks and troughs in the Bz signal. The magnitudes of these two parameters are recorded as a function of time, as the ACFM probe traverses the surface under inspection. Removal of the time base for the measurement of Bx and Bz and plotting one against the other, results in what is known as a “butterfly plot”. The “butterfly plot” assists in defect detection, as a when a defect is encountered the “butterfly plot” will form a loop starting and finishing in the same region of the output display. This enables the operator to
distinguish between spurious indications and true crack signals. Figure 4.21 illustrates a typical ACFM defect indication.
ACFM thread inspection equipment has been available for several years [4.17], and can be either a single or an array of miniature probes designed for detecting cracks in thread roots. The probe incorporates an induction coil for the generation of surface current, and a pick-up coil for the measurement Bx and Bz in the vicinity of a defect. As ACFM is a non-contacting technique the thread probe can be operated with a “shoe”. This is a piece of moulded plastic in the shape of the thread root profile, which is fitted over the probe coils to protect them from wear. For this series of tests, a single thread root probe. Fig. 4.22, was used for all inspections. This was connected to a U9 Crack Microgauge and laptop computer, as shown in Fig. 4.23. Results of the inspections are presented in chapter 5.
4.6.2 Magnetic Particle Inspection
The most common method for detecting defects in rotary shouldered connections is the wet fluorescent magnetic particle inspection (MPI) technique [4.17]. Indeed, this method is often the first choice of NDT on ferromagnetic materials [4.18].
The component to be inspected is magnetised either by bringing it into contact with the poles of a strong magnet, often a hand held yoke, or by passing a heavy alternating current through a coil wrapped around the surface under inspection. Where a discontinuity is encountered in the path of the magnetic field minute poles are set up at the discontinuity. The surface under inspection is covered in fine magnetic particles, which are attracted to the poles of the discontinuity. The magnetic particles can be applied to the surface either dry or wet. For a dry inspection, the particles are in the form of powder which is dusted over the surface. The particles for wet inspection are smaller than those used in the dry method and are suspended in a light petroleum distillate, which is sprayed onto the surface. Because of the small particle size the wet method is more sensitive to fine surface defects. The magnetic particles are coloured or fluorescent in order to make them visible, and are detected
either through contrast with the surface or under ultraviolet light (for the fluorescent particles). Implicit to this is that the surface must be clean and free from dirt and grease, and as close to “bright metal” as possible.
In this series of tests, MPI was only applied as a secondary NDT method to the pin connector, after a positive defect indication from the ACFM system. A line of sight to the defect is required, therefore it was impossible to inspect the NC-26 box thread roots due to the tight geometry of the connection. The technique is also restricted to the measurement of the defect length. No estimate of crack depth can be made, which is important when considering crack aspect ratio at connection failure.