Zhiyi Su

Novel Transceiver Rotating Field Nondestructive Inspection Probe

Metallic tube inspection techniques using eddy current probes have evolved over the years from those employing a single bobbin coil to rotating coils and arrays, in an attempt to improve the speed and reliability of inspection. This paper presents a novel eddy current transceiver probe design that uses a rotating electromagnetic field. The transceiver coils consist of three identical windings located 120° apart on the same physical axis. A three-phase sinusoidal current source is used for exciting the coils. The phase voltages are identical in amplitude, but 120° apart in phase. The rotating magnetic field generated by the three-phase current is sinusoidal in space and time and so are the induced eddy currents in the tube wall. The sensor achieves mechanical rotating probe functionality by electronic means and eliminates the need for mechanical rotation. The terminal voltages of the three-phase windings can be measured during the scan. The defects axial and angular position can be estimated by analyzing the amplitude and phase of the sum of the three terminal voltage signals. The probe is sensitive to defects of all orientations and is as effective as conventional rotating pancake coil probes while offering the advantages of high inspection speed and greater reliability, since the probe does not rotate mechanically. A 3-D finite-element model based on reduced magnetic vector potential Ar, V-Ar formulation was developed to simulate and predict the response of the probe to a variety of defects. A prototype unit consisting of a probe connected to a three-phase constant current source and data acquisition system was developed and tested. Experimental results validating the simulation model and demonstrating the feasibility concept are presented.

Monotonicity principle in pulsed eddy current testing and its application to defect sizing

This article applies monotonicity principle to time constants in pulsed eddy current testing. This principle allows for a non-iterative imaging method for defects in conducting materials. The source free response in pulsed eddy current testing consists of linear combination of exponential decaying waveforms. A major challenge of this method is to accurately estimate the time constants from measurement of such responses. Preliminary results of estimating dominant time constants are presented. Potentially, this information can be used to predict the size of a volumetric defect regardless of its location, i.e. this method is robust in the case of both surface and buried defects.

Analysis of EC-GMR data for detection of cracks under fasteners (CUFs)

EC-GMR measurements have been applied for detection of sub-surface corrosion and cracks under fastener (CUF) head. Generally, fastener signal amplitude is larger than the amplitude of the crack signal indication, rendering crack detection a challenging task. Also, another challenge in the analysis of field signals is the stitching problem, commonly present in a raster scan of a row of fasteners. We propose a method based on robust sparse coding (RSC) representation to alleviate the stitching problem and enhance defect detection capability. For the dictionary, we simulate multi-layer geometries using finite element (FE) modeling. Results on simulated and field data demonstrate the feasibility and robustness of the proposed algorithm.

Model-based study for evaluating the sensitivity of eddy current GMR probe inspection of multilayer structures

In eddy current nondestructive testing (EC-NDT) of a multilayer riveted structure, rotating current excitation, generated by orthogonal coils, is advantageous in providing sensitivity to defects of all orientations. The signal of a defect is not only a function of its size but also of the geometrical features in vicinity to the rivet such as edges, adjacent rivets and properties of the layered structure. Numerical models can be used to provide fast and accurate estimates of defect signals. In this paper the sensitivity of the eddy current system with rotating current excitation and GMR sensors is evaluated by considering the effect of rivet permeability, lift-off, thickness of first layer as well as defect orientation related to adjacent rivets and edges on the signal measured. A numerical model capable of simulating these combinations of defect and test geometry parameters at an acceptable computation time is used. A meta-model is developed based on these simulation results and utilized for sensitivity e...

Optimization of coil design for near uniform interrogating field generation

The detection of a crack under fastener heads (CUF) in a multi-layered aircraft structure remains a challenge in non-destructive evaluation (NDE). An EC-GMR system using a linear eddy current (EC) coil with giant magnetoresistive (GMR) sensors located on the axis of symmetry is proposed for detecting discontinuities in conducting materials. The signal received from sensors is greatly influenced by the interrogating field. This paper describes a detailed parametric study, using a finite element model predicted signals in conjunction with a multi-parameter optimization problem for coil design. The sensor performance is assessed using quantitative measures based on Probability of detection (POD) with respect to different crack geometries.