R. F. Mostafavi

Analysis technique for interaction of high-frequency rhombic inducer field with cracks in metals

In the nondestructive evaluation (NDE) involving the ac field measurement (ACFM), a current carrying structure is required to induce the eddy current in the work-piece and a probe to sample the field. Due to its flat profile, slender shape, and other advantages, the rhombic wire loop is a suitable inducer for developing linear flexible arrays for the ACFM inspection of large surfaces of ferrous and nonferrous metals. This paper introduces an analysis technique for the evaluation of the interaction of the field of the rhombic inducer carrying a high-frequency current, with long surface cracks of uniform depth in flat metal plates. The technique is accurate and very efficient computationally. It uses the two-dimensional Fourier transform together with a special boundary condition at the metal surface. The boundary condition takes into account the thin-skin nature of the eddy current in the metal as well as the flux leakage at the crack mouth. The analysis technique benefits from the use of scalar potential functions and can be extended to simply or multiply connected wire loops. Also, it is applicable to high-frequency (thin-skin) eddy current problems. Using the analysis technique, the tangential field below a rhombic inducer along its long diagonal when the inducer is located above the surface of aluminum and steel is given in the presence and absence of a crack. This field was found to have a nonuniform phase distribution. Near a crack, the phase change is significant, even for shallow cracks. The role of the nonuniform phase in the detection sensitivity is addressed. Also, simulated ACFM crack responses using an inducer with a linear probe attached along the long diagonal are presented and discussed. To support the validity of the analysis technique, experimental results obtained for some of the simulations are also reported. In addition to its application in predicting crack responses, the technique can be used for model-based inversion of crack signals.

AC fields around short cracks in metals induced by rectangular coils

In the alternating current field measurement method, information about the magnetic field distribution above the metal surface is used to detect and size surface-breaking cracks in metals. In this measurement, the incident field is produced by a set of wires carrying a high-frequency current. This paper presents a mathematical technique for the theoretical prediction of the distribution of the magnetic field around a short crack due to a current-carrying rectangular coil located horizontally above the metal surface. The crack is assumed to have a profile that can be approximated by a rectangular boundary. The mathematical technique is developed and its experimental verification is discussed. The technique, an extension of one recently reported by the authors, is accurate and computationally efficient. The effects of crack length and depth on the crack signal, obtained theoretically, are illustrated and discussed.

1-D probe array for ACFM inspection of large metal plates

The theory and experiments behind the development. of a novel one-dimensional ACFM probe array system for nondestructive evaluation of ferromagnetic and nonferromagnetic metal plates are explained. In this linear array, each element consists of a linear pick-up coil and a rhombic inducer arranged specifically for high detection sensitivity. The special sequential switching , arrangement of the array allows the fabrication of the array, theoretically, to any required length on flexible or ordinary printed circuit. boards. The switching circuit, the signal acquisition system, and the signal processing method, are presented, and the limitations that the switch imposes on the array length are addressed. Example signals from the array as applied to crack and saw-cut notch samples in different rectangular metal blocks (cut from metal plates) are shown, and methods for the inversion of the crack signal to the crack depth are discussed.

High Sensitivity AC Field Measurement Using Rhombic Inducer

The principle behind the ac field measurement (ACFM) technique has been explained in [1]. In this technique, a thin-skin eddy current is induced in the metal under test. The current is perturbed by defects in the metal surface and the result is reflected in the magnetic field above the surface. A probe is used to detect perturbations in this field.

Small filters based on slotted cylindrical ring resonators

The realization of a small four-pole Chebyshev filter using four matched slotted cylindrical ring (SCR) resonators, is explained. This filter, which does not need tuning screws, is low-cost to manufacture and suitable for mobile communications. The filter has a bandwidth of 75 MHz centered at 1.73 GHz. To obtain the coupling distances between the resonators, a new technique based on the quasi-magneto-static finite-difference method, is also reported.

Resonant Frequency of Slotted Ring Resonator

The slotted ring resonator has some promising features for the realisation of compact microwave filters. For the first time, using the finite difference technique and conformal mapping method, the effects of geometrical parameters (i.e.: the resonator length, inner radius, gap width and wall thickness) on the resonant frequency of this resonator, are investigated. The fine tuning of the resonant frequency in practice, is also addressed and in this connection, experimental results are presented.

Crack Signal Saturation in High Sensitivity ACFM Technique

Recently, we proposed the high sensitivity ac field measurement (ACFM) technique for detecting and sizing surface cracks in metals [1–2]. This non-destructive evaluation (NDE) technique requires an inducer which possesses a field region with odd symmetry for positioning the probe. When properly orientated in this field region, a single linear probe acts as a differential probe with the added advantage of a phase contribution due to the crack. Both a rectangular coil and a rhombic wire loop can be used as inducers to provide the necessary field. In Fig.1, the positions of the probe with respect to these inducers are shown. As can be inferred from this figure, the probe is coupled to the magnetic field tangent to the metal surface.

Simple electromagnetic detection of variations in properties of metal surfaces

Recently, a significant improvement in the sensitivity of the nonuniform AC field measurement technique for the detection of cracks was reported. In this paper, the results of the application of the improved technique in the detection of heat-effected zones and ground welds are presented and briefly discussed. In such applications, the use of a long probe is essential.

A Novel Linear Array System for Inspection of Large Metal Surfaces

Recently, much effort has been focused on the development of arrays for the non destructive testing (NDT) of metal structures in offshore, nuclear and aerospace industry, eg: [1–2]. The main feature of these arrays is electronic scanning which reduces (or eliminates) mechanical noise and makes rapid scan of large areas possible. Some arrays can also provide opportunity for direct signal processing of data. This paper is concerned with a linear array using the non-uniform field ac field measurement (ACFM) technique which is also known as the surface magnetic field measurement (SMFM) method [3].

Unloaded Q-Factor of Slotted Cylindrical Ring Resonator

The slotted cylindrical ring (SCR) resonator has been recognised as a promising resonator for developing compact microwave filters. Using a new quasi-magneto-static finite-difference (QMSFD) method, the effects of geometrical parameters (i.e.: the resonator length, inner radius, gap width and wall thickness) on the unloaded Q-factor of this resonator, are investigated. Experimental and theoretical results of the Q-factor are compared. In addition, comparison is carried out against the published data.