William D. Dover

The Use of A-C Field Measurements to Determine the Shape and Size of a Crack in a Metal

An instrument designed to measure the a-c field accurately has been built and fully tested on steel, aluminium, and titanium. These field measurements can be interpreted in terms of crack size, which provides a new technique for nondestructive testing (NDT) that requires no prior calibration. This paper describes the basic electronic measuring system, theoretical derivations of the electrical-field distribution, and application to industrial problems such as crack measurement in threads, shafting, welded connections, etc.

The use of AC-field measurements for crack detection and sizing in air and underwater

The use of AC-field measurements for crack sizing and detection is now common in many industries. It has been shown to be a simple and rapid method of sizing surface cracks, both in air and under water. Associated with the development of the Crack Microgauge has been the formulation of mathematical models of the electromagnetic fields, which has removed the need for calibration in many practical applications. This work is reviewed briefly. New versions of the instrument have recently been produced and novel applications are described.

Progress in the Development of a Wave Action Standard History (WASH) for Fatigue Testing Relevant to Tubular Structures in the North Sea

Fixed tubular steel platforms were first installed in the North Sea in 1966. They are subjected to significant fatigue loads due to wave action, and consequently a large number of relevant fatigue tests have been carried out on representative tubular joints. In 1979, the Wave Action Standard History (WASH) Working Group was formed to develop standard load spectra for fatigue testing relevant to offshore structures. This paper reviews some of the work that has been carried out in support of the Working Group, but it is not an official Working Group document. It is concluded that construction of realistic load spectra representative of wave loading on tubular structures in the North Sea is feasible at the current state of the art. The basic steps needed for implementation of a tentative standard load spectrum are described.

Electromagnetic induction in anisotropic half-space and electromagnetic stress model

An analytical solution to the electromagnetic induction problem involving both electrical anisotropy and magnetic anisotropy has been derived. The method of solution can be extended to electromagnetic induction problems in infinite plates and layered media. A theoretical model for measuring magnetic field change under a uniaxial stress based on anisotropic electromagnetic induction has also been developed. An experiment designed to measure the effect of mechanical stress on magnetic permeability has been carried out. Theoretical predictions of relative magnetic field change are in good agreement with the experimental data.

Detection and Measurement of Cracks in Threaded Bolts with an a.c. Potential Difference Method

This paper describes theory related to a programme of tests made on titanium and Inconel bolts to detect and measure fatigue cracks in the threads. The Crack Microgauge instrument, which uses the a. c. potential difference method of detection, was used in the experimental investigation. A continuous sequence of probe signals, taken between successive crowns of the thread as the bolt is turned, yields a characteristic pattern of response in which there is a main signal, obtained when the probe spans the crack, accompanied by a sequence of satellite signals, which arise when the probe spans the threads adjacent to the crack. The theory developed in the paper provides the relation between crack depths and probe signals measured and it explains and quantifies the system of satellite signals found on the threads upstream and downstream of the cracked thread. Since the electrical skin depths and the radii of the bolts involved were much larger than the length scales of the crack and the thread, a two-dimensional thick-skin theory for the electromagnetic field is appropriate. Satisfactory agreement between theory and experiment was obtained. The tests were made on threads with artificial notches cut at the thread roots, which have a symmetrical satellite system, and on naturally occurring fatigue cracks, which give rise to asymmetric satellite systems. The theory shows that there is a relation between the asymmetry and the location of the crack on the thread. Only a few complete measurements of satellite asymmetry were recorded in the tests, which were completed before the mathematical theory was developed. The work suggests that in future tests of this type it would be beneficial to have more detailed investigation of the satellite signal asymmetry so that the location of the cracks on the threads can be more closely specified.

Fracture Mechanics Analysis of Fatigue Crack Repaired Joints

Fatigue life predictions based on fracture mechanics calculations are required to satisfy an increasing level of safety demanded by industry. These predictions are mainly used to schedule NDT inspections and with inspection data make structural integrity assessments. The periodic inspection-assessment process can lead to the implementation of a fatigue crack repair by crack removal. Fracture mechanics analysis is used again to determine whether or not a repair will be effective. For the case of tubular joints, in offshore structures, once repairs have been shown to be ineffective it is usually required to install a clamp to maintain the continuity of joint members if the structure is still required for production. In this paper a fracture mechanics analysis of crack repaired joints based on Y factors is presented. The analysis is used to predict fatigue life after crack removal and is validated against T-butts experimental data. The analysis is also extrapolated for the prediction of fatigue life of crack repaired tubular joints

Thin-skin AC field in anisotropic rectangular bar and ACPD stress measurement

Abstract This paper presents some theoretical and experimental studies on the magnetostrictive anisotropy (stress-induced anisotropy) in a long rectangular steel bar that were recently conducted at the University College London. In order to obtain the thin-skin field in the anisotropic bar, the validity of the general boundary conditions at the interface between the conductor and the exterior (air), which were established in earlier studies for isotropic media, were firstly examined. The thin-skin field solution of Zhou and Lewis [Zhou J, Lewis AM. Thin-skin electromagnetic fields around a rectangular conductor bar, Journal of Physics D: Applied Physics, 1994;27:419–425] for the isotropic bar was then extended to include magnetic anisotropy. Using the extended thin-skin field solution, an electromagnetic stress model has been presented by relating the stress level to a quantity that can be used as a measure of the amount of magnetic anisotropy in the material. For the application of this model, experimental stress measurements on mild steel rectangular bars with different dimensions were conducted based on the Alternating Current Potential Difference (ACPD) technique. The ACPD stress measurement results showed that the theoretical stress model could predict the stress values with the calibration of a magnetic anisotropic parameter.

Uniform ac field in anisotropic bar and alternating current potential difference stress measurement

Some theoretical and experimental studies are presented on magnetostrictive anisotropy (stress induced anisotropy) in a long circular steel bar. An explicit analytical solution to the problem of uniform axial alternating current (ac) distribution in a cylindrical bar which retains magnetic and electrical anisotropy was established. By relating the stress level to the amount of magnetic anisotropy in the material, an electromagnetic stress model was then developed for cylindrical bars under uniaxial stress using the analytical solution. For the application of this model, experimental stress measurements on mild steel bars with different radii were conducted based on the alternating current potential difference (ACPD) technique. During the test, a form of alternating current field measurement (ACFM) instrumentation, known as the ACFM crack microgauge, was used to inject a uniform ac current into the steel bars and to measure the electrical field on the bar surface. The ACPD stress measurement results showed that the theoretical stress model could predict the stress values with the calibration of a magnetic anisotropic parameter.

Design of Crack Removal Profiles Based on Shape Development of Surface Defects

A crack removal has the purpose to stop crack propagation and prevent future crack reinitiation, thus fatigue life of the component is extended. A profile design for the removal of fatigue cracks that grow under bending loading is presented. The design is based on the preferred shape development of surface defects under bending loading. Preferred shape development of surface defects was experimentally identified and validated with observations previously reported by other authors. The design of a crack removal profile consists on determining a notch geometry and its dimensions to cut out a crack. The crack removal profile presented here induces crack reinitiation, if it would occur, on the surface plate at the notch ends rather than in the bottom of the notch, so original plate thickness is reinstalled for propagation and inspection is easier. This repair procedure is recommended for the removal of fatigue cracks in tubular joints of offshore jackets steel structures.© 2003 ASME

The influence of diffusion coatings on the fatigue strength of copper

It has been found possible to increase the fatigue strength of copper by selectively alloying a thin layer at the surface. The three alloy systems copper-nickel, copper-silver and copper-zinc were investigated, and the fatigue strength was shown to be independent of the degree of solute penetration and of its mode of distribution, when tested at room temperature in the normal laboratory environment. The greatest increase in fatigue strength was obtained with the copper-zinc surface alloy, for which an increase of 15% was recorded. The increased fatigue strength is thought to be dependent both on the new value of τIII, the threshold stress for stage III hardening in single crystals, and on the relatively greater thermal diffusivity of the foreign atoms introduced; the improvement in fatigue resistance was limited to alloy systems in which the solutes possessed a higher diffusion rate than the copper.