Cracks In Metallic Structures Research Articles

A New Approach to Improve the Probability of Detection of Eddy Current Inspection

Eddy current testing is now a widely used Non Destructive Test (NDT) method for determining the size of cracks in metallic structures. The probability of detection (POD) is a key metric in assessing the effectiveness of nondestructive evaluation techniques. The POD analysis using common method is studied and modified. The eddy current sensor effective is discussed to support the proposed method. The new method is shown better result in improve the POD than the common one. The estimates of POD as a function of crack size will be valuable for future model-assisted POD study.

Statistical Assessment of Probability of Detection for Automated Eddy Current Nondestructive Evaluation Inspection

Eddy current inspection is widely used in nondestructive evaluation to detect cracks in metal structures. The impedance plane measurement response, collected using our motion controlled eddy current inspection system, is used in the analysis. A scalar reduction from the impedance plane response is used to minimize human-factor variation. We apply a noise interference model to the data from a large-scale experiment taking measurements on fastener rivet holes in multilayer structures with fatigue cracks, and estimate the probability of detection (POD). The estimates of POD as a function of crack size will be valuable for future model-assisted POD studies.

Wireless interrogation of passive antenna sensors

Recently, we discovered that the resonant frequency of a microstrip patch antenna is sensitive to mechanical strains or crack presence in the ground plane. Based on this principle, antenna sensors have been demonstrated to measure strain and detect crack in metallic structures. This paper presents a wireless method to remotely interrogate a dual-frequency antenna sensor. An interrogation horn antenna was used to irradiate the antenna sensor with a linear chirp microwave signal. By implementing a light-activated switch at the sensor node and performing signal processing of the backscattered signals, the resonant frequencies of the antenna sensor along both polarizations can be measured remotely. Since the antenna sensor does not need a local power source and can be interrogated wirelessly, electric wiring can be eliminated. The sensor implementation, the signal processing and the experimental setup that validate the remote interrogation of the antenna sensor are presented. A power budget model has also been established to estimate the maximum interrogation range.

Evolution of residual stresses with fatigue crack growth in integral structures with crack retarders

Bonded straps are investigated for their ability to retard a growing fatigue crack in metallic structures. The evolution of the residual stresses in the vicinity of the strap with fatigue crack growth has been studied. Cracks were grown in single edge-notched tension (SEN(T)) specimens reinforced with either a titanium or a carbon fibre reinforced plastics (CFRP) strap. The residual stress evolution has been measured in situ during crack growth using neutron diffraction, and modelled with a finite element approach. The peak residual stresses induced by the mismatch of the coefficient of thermal expansion between the strap and plate materials were seen to be fairly constant with crack growth. Good correlation between the experimental and the modelling results was found, except at very long crack lengths for a specimen that exhibited considerable fracture surface roughness at long crack lengths. The difference was attributed to wedging of the fracture surface changing the expected stress state, rather than any effect of the strap.

Modelling composite repairs to cracked metal structures

Composite patches (usually made from carbon-fibre reinforced plastic (CFRP)) are frequently used for localised structural strengthening. They can also be used to repair fatigue cracks in metallic structures in situations where welded repairs are unsuitable: for example, cracks in stress concentrations may re-appear soon after a welded repair is attempted. This application is, however, hampered by the difficulty of predicting the fatigue life extension produced by the repair. This paper presents work which shows that the growth of patched and unpatched fatigue cracks can be predicted using finite-element analysis, allowing the benefits of a potential patch repair to be quantitatively assessed. Simulations of the crack propagation are performed and show a good correlation with test results. The tested repair was successful in significantly reducing the fatigue crack growth rate.

Fatigue crack detection in metallic structures with Lamb waves and 3D laser vibrometry

The paper presents the application of ultrasonic guided waves for fatigue crack detection in metallic structures. The study involves a simple fatigue test performed to introduce a crack into an aluminium plate. Lamb waves generated by a low-profile, surface-bonded piezoceramic transducer are sensed using a tri-axis, multi-position scanning laser vibrometer. The results demonstrate the potential of laser vibrometry for simple, rapid and robust detection of fatigue cracks in metallic structures. The method could be used in quality inspection and in-service maintenance of metallic structures in aerospace, civil and mechanical engineering industries.

Structural health monitoring using scanning laser vibrometry: III. Lamb waves for fatiguecrack detection

Guided ultrasonic waves have shown great potential for structural health monitoring.Various types of transducer can be used for actuating and sensing of these waves. Thisincludes non-contact approaches such as optical/laser techniques. Classical laser methodsusually involve high energy interferometers. The paper demonstrates that a commerciallaser vibrometer, designed for vibration/modal analysis, can be used for crack detection inmetallic structures. The study involves a simple fatigue test in order to initiate and grow acrack. Lamb waves generated by one bonded piezoceramic transducer were sensed using amulti-point scanning laser vibrometer. The results demonstrate the potential of laservibrometry for simple, rapid and robust detection of fatigue cracks in metallic structures.

Analytical modelling of fatigue crack propagation

An analytical approach to the prediction of fatigue crack growth is discussed from the viewpoint of analytical mechanics and mechanics of solids. Conditions of fatigue crack non‐propagation and stability are formulated in terms of the principle of virtual work for systems with unilateral constraints. Additional variables are introduced for the description of damage accumulated ahead of cracks and conditions at the damaged crack tips. Applications are given to high‐cycle and low‐cycle fatigue cracks in metallic structures. A brief survey is included of recent studies of fatigue and related phenomena based on the presented theory.

Inspecting Printed Wiring by Using the Eddy-Current Testing Approach

Eddy-current testing (ECT) is used for inspecting cracks in metal structures. We propose a planar-type ECT probe consisting of a meander and a mesh coil, and discuss the possibility of using ways of this probe to inspect disconnections in printed wirings. This paper describes (1) finding disconnections in printed wiring and (2) discriminating between various patterns of printed wiring and disconnections. Experimental results confirm that it is possible in principle to use ECT for inspecting printed wiring.

Techniques for depth-selective, low-frequency eddy current analysis for SQUID-based nondestructive testing

Conventional eddy current techniques are widely used for detection of surface-breaking cracks in metal structures. These techniques have limited success in the detection of deep, nonsurface-breaking flaws that require low frequency eddy currents, for which inductive pick-up probes have drastically reduced sensitivity. High resolution, Superconducting QUantum Interference Device (SQUID) magnetometers, which are very sensitive to do or low frequency magnetic fields, have been developed for detection of subsurface flaws. We have now extended SQUID NDE by utilizing a sheet inducer to produce an extended eddy current parallel to the surface in a conducting plate. The magnitude of the induced current density inside the plate reduces with the depth; however, the current component at a certain phase angle may increase with the depth. At a particular phase angle, the current density on the surface becomes zero, while the current inside the plate is large, so that the magnetic signal at that phase angle due to the surface structures can be minimized. With this method, we have detected simulated cracks in the sides of plugged holes in a thick plate, a hidden corrosion area in a specimen which consisted of two painted aluminum plates joined with sealant, as well as crack defects adjacent to fasteners in the second layer of lap joined aluminum plates. We present a theoretical model for simulation of the phase-related magnetic signal due to a flaw, which shows the relation between the phase angle and the depth of the flaw. The theoretical phase analysis is compared with the experimental results.

Eddy current testing probe composed of planar coils

This paper presents a new eddy-current probe composed of a micro-planar mesh coil and meander coil. The probe can be used to detect the existence and the size of cracks in metallic structures. Experimental results for the sensed output voltage of this device are presented. The output signal is shown to have a discrete nature. Also, the results show that the signal strength is weak and that an offset voltage exists. Improved probe characteristics are obtained by connecting two mesh coils in series, stacking the coils on top of each other and orienting the two coils 180/spl deg/ apart.

Determination of crack size and shape using relaxation methods

The relaxation method is used together with surface-potential values in order to determine the size and shape of surface cracks in metallic structures. The method is based on the ac potential difference technique for sizing surface-breaking cracks. The original problem is transformed to a form where the X and Y coordinates are expressed as functions of the potential and stream functions. The crack shape is then given by the set of points formed by the computed values of X and Y along the boundary where the potential vanishes. The procedure is tested using synthetic input data, and including the effect of noisy data. An experimental technique is proposed in order to obtain the required input data.

Thin-skin electromagnetic fields in the neighbourhood of surface-breaking cracks in metals

Electromagnetic non-destructive evaluation techniques are widely used to detect and size surface-breaking cracks in metal structures and components. The precise distribution of the electromagnetic field around such a crack depends on the frequency of the applied field, the material properties of the metal and the crack geometry. In many situations, the skin depth of the electromagnetic field in the metal is small compared with the crack dimensions. If this is the case, the crucial parameter that determines the way the electromagnetic field in air couples to the field in the metal is m = μ 0 l / μδ , where μ and μ 0 are the metal and free space permeabilities respectively and l / δ is the ratio of the crack length scale l to the skin depth δ . If the metal is ferromagnetic, m can take a wide range of values and the distribution of the electromagnetic field around the crack is very different in the two limiting cases m = 0 and m ≫ 1. In the first case, the magnetic flux emerging from the crack is directed into the metal surface whereas in the second case, the flux is directed into free space. In this work, the distribution of the electromagnetic field around a surface-breaking crack is determined for arbitrary values of m . The theory is developed for cracks of general shape and numerical calculations of the free-space components of the magnetic field are made for rectangular and semi-elliptical shaped cracks. The numerical predictions are found to be in good agreement with experimental measurements of the magnetic field above a rectangular slot, cut in a flat plate of mild steel.

Inverse problem methodology and finite elements in the identification of cracks, sources, materials, and their geometry in inaccessible locations

Inverse problem methodology is extended, through the more difficult geometric differentiation of finite-element matrices, to identify the location, material, and value of unknown sources within an inaccessible region using exterior measurements. This is done through the definition of an object function that vanishes at its minimum when the externally measured electric field matches the electric field given by an assumed configuration that is optimized to match measurements. The method is demonstrated by identifying the shape, permittivity, charge, and location of an electrostatic source through exterior measurement. The procedure is then extended to eddy current problems for the identification of the location and shape of cracks in metallic structures. An example demonstrates that when dealing with eddy current problems the least squares object function used by others has multiple local minima so that gradient methods have to be combined with search methods to identify the one absolute minimum. Procedures are also given for handling situations with no cracks and overdescribed cracks. >

Cold Working Method Retards Metal Fatigue Cracks

‘STOPCRACK,’ a new repair procedure for retarding growth of fatigue cracks in metal structures of aircraft and other applications, has been introduced by Fatigue Technology Inc.