Non-Destructive Testing Method & Application For The Screw Thread Of Drilling Tools Based On Its Self-Magnetization

The screw thread of oil drilling tools is the easiest part to get failure, because of the harsh working condition which is full of diverse loads, and the influence of its own structural factor which results in the stress concentration. Micro-magnetic field will occur in the crack parts result from the self-magnetization which dues to the interaction of both geomagnetism and diverse loads. It's really a new non-destructive method to judge whether there is a gap or not, which is quite different from the normal testing ways such as magnetic particle inspection, ultrasonic inspection and magnetic flux leakage testing, because there is no need to bring any additional magnetization fields.The calculation on the transverse crack in the screw thread part according to both magnetic dipole model and the finite element model show that, the wave form testing result of the micro-magnetic field around the crack is cap-like, if Hall sensor array is used to scan the screw thread transversely. Furthermore, the experiment also proves the conclusion of this thesis calculation.On the other hand, a series of contradistinctive experiments between this methods and the normal non-destructive testing referred above indicate its advantage in some areas such as convenience, intelligence and so on. Above all, it is widely used to detect both the exterior and the interior transverse crack. Meanwhile, the longitudinal crack, holes and inner wall corrosion can also be found at the same time basing on its self-magnetization. Even more, it covers all kinds of screw threads of drilling tools, in spite of its shape or spec.As is known, the screw threads of drilling tools are complicated in structure and multiplex in specification. Several different testing devices have been designed, including vehicle type, cantilever type, chuck type and clip type etc, and a new testing instrument has been explored used in some corporations according to this theory. The testing result shows that, the instrument has some advantages, for example it has a high inspecting sensitivity which can find the smallest transverse crack that is10 mm×0.2 mm×0.5mm, and the smallest hole that isφ1 .6mm. Besides, simple structure, convenient operation and high reliability are also the outstanding strength of this new instrument, which will lead to a wide application prospect of this kind of instrument.