Potential difference methods for measuring crack growth: A review

The present authors proposed the electric potential CT (Computed Tomography) method for measuring two- and three-dimensional cracks in a body from the potential distributions observed on surfaces of the body. By using the boundary element equations and applying the concept of inverse approaches, two crack identification schemes were formulated the inverse boundary-integral-equation method and the least residual method. Numerical simulations and experiments showed that the proposed method was useful for the quantitative determination of location, size and shape of two- and three-dimensional cracks. In the present paper optimization techniques are incorporated in the least residual method to achieve an efficient determination of the location, size and shape of cracks. Vectorization of DO loops in the program of inverse analysis is also made to speed up the computation on a supercomputer. A numerical simulation on the identification of a three-dimensional corner crack demonstrates that an efficient and high-speed identification of cracks can be made by the inverse analysis incorporating optimization procedures on a supercomputer.

Crack identification by the electric potential CT inverse analyses incorporating optimization techniques

DC potential drop has long been used for automatic crack length monitoring, yet details about its set‐up, causes of errors, precautions to achieve optimum performance and actual long term stability and accuracy under practical conditions have seldom been documented in detail A DCPD system was established and details about its long term performance under varying loading and temperature parameters were evaluated Necessary precautions to reduce error and improve accuracy are discussed The system resolution is 0003mm when applied to a compact tension specimen The long term system accuracy achieved was ± 0012mm Two approaches for improvement were also attempted It was found that the switch‐off approach at least doubled the accuracy and the reversing polarity approach raised the accuracy by nearly ten times The improved system has been applied to a large pipe fracture specimen The resulting empirical calibration data agrees closely with independently obtained finite element results

An improved DC potential drop system for crack length measurement

A hierarchical crack identification scheme is proposed based on electric potential measurements. It consists of three stages for progressive estimation of the size and the location of a crack embedded in an electric current carrying body. The scheme includes two different identification procedures, applied on the different stages, which in the proposed method are internally connected as one whole operation, since each stage uses the results from the previous one. The identification procedure is organized in such a way that it can be stopped after each stage, for each stage provides information for the presence of the crack and its dimensions. The completion of the first stage gives a comparatively rough estimate of the size of the crack, classifying it as a negligible, small, medium or large one. The second stage obtains intervals to which the parameters characterizing the crack belong. In the last stage a precise enough estimate of the crack parameters is obtained by applying optimization technique. The ...

Hierarchical crack identification from electric potential measurements using pattern recognition approach and optimization technique

The electric potential drop technique is an effective crack monitoring method, especially in harsh environment conditions With this method, the calibration curve, the relationship between potential drop across the crack and the crack sizes, is needed This work applied a cell-centered finite volume discretisation with unstructured quadrilateral mesh to simulate the potential drop within cracked specimens The numerical model is implemented with C++ language The program was used to simulate electric potential drop in 2D specimens with the two most common through-thickness crack problems, the single edge crack and central crack types with the crack length to specimen width ratios from 01 to 08 The resulting calculated calibration curves conform well with the analytical solutions

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A finite volume simulation of electrical potential drop in 2D cracked plates

An evaluation of the electropotential method for deriving the crack length in a sheet metal specimen is carried out. Comparisons are made between an existing analytical and numerical method for calculating the relationship between electropotential and crack length. The numerical method allows an exact representation of the specimen geometry which enables the influence of asymmetrical crack growth on the electropotential to be examined. A case study to analyse this effect on the electropotential is discussed which indicates that underestimation of the crack length will occur which becomes increasingly significant as the voltage probes across which the electropotential is measured are placed closer to the crack. The paper shows that a compromise must be made with regard to voltage probe positioning between the desired sensitivity of the potential reading and tolerated vulnerability to asymmetric crack growth. Comparisons between experimental and theoretical derived potential crack length relationshi...

D W J Pulle

Papers

Crack length measurement: Analysis of the electropotential method using a finite element method