Practical guided wave inspection and applications to structural health monitoring

The application of ultrasonic guided waves for the long range inspection of large structures is reviewed. The technique is in routine commercial use on pipes and this technology is described. However, the method is much more difficult to apply on complex structures where the reflections from multiple features are not resolved in time. Here it is necessary to look for changes relative to a baseline condition as part of a structural health monitoring strategy; this requires a method to compensate for benign changes such as temperature variations, and possible methods for achieving this are discussed

RCNDE Engineering Doctorate in NDE

The UK Research Centre in NDE (RCNDE) runs an engineering doctorate programme. The qualification is equivalent to a PhD but with much more industry involvement, the idea being that students spend a large fraction of their time in the sponsoring company. The motivation behind the original government funding was to increase the level of NDE engineers in industry and, by not simply calling the degree a PhD, to appeal to a different intake who actively want to pursue an industrial, rather than academic, career. The programme has been running for over 10 years and over 30 students have graduated; his paper discusses experience with the degree and the benefits delivered to industry

Comparison fo Reflection Coefficient Minima with Disperion Curves for Ultrasonic Waves in Embedded Layers

Waves which propagate along an embedded layer in a multilayered plate offer potential for the nondestructive characterisation of the layer and of the boundary conditions between the layer and the adjoining layers. Because the energy of these waves is concentrated in the layer, they may be expected to be considerably more sensitive to the local properties than Lamb waves which occupy the whole plate, and relatively insensitive to variations in the properties of other layers in the system

Current Deflection NDE for Defect Screening

The measurement of perturbations in the magnetic field resulting from the deflection of an injected electric current around a defect has been shown to have many potential applications in NDE. Characteristics of such a technique include through-wall sensitivity to defects at tens of millimetres of lift-off between sensor and sample, which makes the technique suitable to a number of traditionally difficult-to-inspect scenarios where extensive insulation and/or coating removal is undesirable. This paper presents a study into the feasibility of using Magnetoresistive (MR) sensors to measure current deflection from corrosion-like defects via an internal or external scan of the pipe. Prediction of the signal due to different defect geometries have been facilitated by a validated FE model, and current deflection measurements using Anisotropic MR sensors have allowed prediction of the sensitivity of the technique. The various practical challenges are explored which include the misalignment of the sensors with the target components of the magnetic field, the influence of nearby ferromagnetic objects, and the effect of pipe features such as bends and crossings. The results suggest that low-powered, inexpensive MR sensors in a gradiometric configuration can be used to measure current deflection from defects occurring on the inner or outer wall of pipes carrying a few amps of current while the coating, insulation and (non-ferromagnetic) cladding remain intact. This has many potential applications, particularly in the oil and gas and power industries

Feasibility and Reliability of Grain Noise Suppression in Monitoring Highly Scattering Materials Using Baseline Subtraction

This paper studies the feasibility and reliability of using the baseline subtraction method [1] to suppress grain noise in monitoring highly scattering materials. Monitoring is usually done with permanently installed sensors but this is not always possible and here we investigate the feasibility of subtracting A-scans extracted from repeat C-scans. It is important that the transducer standoff and angle relative to the testpiece are set consistently in the repeat scans and the influence of errors in these settings has been investigated. The successive C-scans can be registered by cross correlation and the effect of errors in the registration is illustrated. The experimental results demonstrate that the residual grain noise after baseline subtraction is around 15dB lower than the original grain noise; this will give a significant improvement in defect sensitivity. Successive tests may be carried out at different temperatures and with different transducers of similar specification. Compensation methods for temperature variations [2] and transducer frequency response changes are then proposed, and their effectiveness is tested experimentally. The addition of these two effects reduces the typical improvement in the signal to noise ratio obtained via baseline subtraction to about 10 dB which is still potentially valuable in some applications

Structural Health Monitoring of Pipelines Using Permanently Installed Guided Wave Sensors

Guided wave measurements are an established inspection method for pipelines in a variety of industries. Permanently installed guided wave sensors (gPIMS) have been used in the past to reduce the access cost for pipes in hard to reach locations which require repeated inspections. Making use of the existing gPIMS technology a method was devised to move from the current single inspection approach to a structural health monitoring system. With changes due to the environmental conditions of the pipe, such as temperature variations or content changes, the usefulness of simple baseline subtraction methods is very limited as temperature compensation methods do not allow for the full removal of these effects. Using a dataset of historic measurements and statistical signal processing it was possible to identify and track defects in pipe sections. The procedure is based on Independent Component Analysis (ICA) and allows the collected measurements to be separated into components of the constant baseline and those components associated with signal changes. Further processing of these components allows the removal of those caused by environmental variations or noise, leaving only components related to real changes of the pipe structure. Datasets were obtained from two different field trials, one of which was open, with known defect locations and positions, while the other was a blind trial. It was found that with the presented method it was possible to detect and track the growth of defects in the pipe structure with reflection coefficients of down to 0.5%. Defects at welds and on pipe bends were also detected and tracked successfully. Structural changes of this order cannot be detected reliably using standard guided wave processing methods so this new approach promises a significant advance in monitoring performance

The Correlation of Ultrasonic Measurements with Toughness Changes During the Environmental Degradation of Adhesive Joints

Several factors have held back the more widespread use of adhesives. These principally are the detrimental effect of moisture on bond strength and also the lack of a suitable non-destructive testing technique for detecting strength loss due to environmental attack. It is the latter problem that this work attempts to answer. The focus of this work has been to look at the bonding of aluminium to aluminium using epoxy based adhesives, as would be used in the aerospace industry. Bonding of aluminium has been performed in the aerospace industry for many years, and there has been much work done to improve the durability of this type of joint. It has been seen that the improvement in corrosion resistance that can be achieved by treating aluminium prior to bonding has a significant effect on the durability of the bond produced. This is not surprising when it is often seen that a joint which has been exposed to a hot-wet environment will fail along the interface between the aluminium and epoxy, as opposed to through the adhesive when the joint has remained dry [1]. Therefore it is this interface region that is to be examined when searching for environmental attack. The most common form of pretreatment that is used when environmental attack is a concern is anodisation of the surface to be bonded. Anodising produces a thin oxide layer on the aluminium surface, typically 1 –3 μm thick. Joints that have been anodised are considerably more durable than joints that are not anodised, but they will still exhibit interfacial failure after exposure to hot-wet environments [1]. The problem for NDT techniques is that the oxide layer which we need to inspect is orders of magnitude smaller than the bounding layers; the aluminium being 1–5mm, and the adhesive being 0.1–0.5mm thick, as shown in Figure 1. Ultrasonics has appeared to be the most promising technique for inspecting for degradation of adhesive joints, and it is this technique on which we have concentrated our efforts [2–4]

Guided Wave Propagation in Pipes Embedded in Concrete

Long-range guided wave testing (GWT) is routinely used for detection of corrosion defects in industrial pipelines. The application of the method to pipes embedded in concrete results in unpredictable test ranges due to different contact conditions between the pipe and the embedding concrete. When the coupling is intact GWT test ranges tend to be considerably reduced compared with pipes in air due to very high attenuation rates resulting from energy leakage into the embedding concrete; while in cases where the contact between the concrete and pipe is partial, e.g. due to the concrete cracking, the attenuation would be smaller, resulting in increased practical inspection ranges. In this study, we investigate the influence of the acoustic coupling between the pipe and concrete on the guided wave attenuation. Full-scale guided wave tests have been conducted on two 8”-dia., 6-meters long steel pipes fully- and partially-embedded in concrete over an axial length of 0.4-meters. Measurements of attenuation of the T(0,1) guided wave mode are performed over a frequency range of 10-35 kHz. Tests are compared with model predictions, explicit 3D finite-element (FE) and semi-analytical finite-element (SAFE) simulations. The attenuation is found to be very large in a fully-embedded pipe while much smaller in partially-embedded pipes. It is shown that the attenuation is not linearly proportional to the extent of the circumferential fraction embedded in concrete. This is due to mode conversion in which the ultrasonic energy is concentrated in modified guided wave modes in the free fraction of the pipe in the partly-embedded cases; model studies are being used to investigate the physics of this behavior. The extent of circumferential contact between the pipe and the concrete governs the propagation of the guided waves. For a pipe fully embedded in concrete practically no signal would be transmitted past a relatively short propagation distance due to leakage of energy. However, for a pipe partially embedded in concrete, measurable signal propagates

Study of Metal Magnetic Memory (MMM) Technique Using Permanently Installed Magnetic Sensor Arrays

The Metal magnetic memory (MMM) technique is marketed as a non-destructive testing method to evaluate stress concentration for ferromagnetic materials by measuring variations of the self-magnetic leakage field (SMLF) distribution on the specimen surface. This method has been implemented as a periodic screening inspection tool by several companies, and has been reported to be capable of detecting stress concentration in some instances. However, recent literature has suggested that the performance can be unreliable, with many false-calls; however, the reason for the false calls has not been satisfactorily investigated. In this paper, the normal and tangential components of the stress induced SMLF on the surface of specimens made from L80 steel, which is commonly used for pipes in the petrochemical industry, were measured by two permanently installed magnetic sensor arrays under the earth’s magnetic field. The results show that the SMLF changes are very small (about 10 ~ 30 A/m) in all cases for the material tested and occur mainly in first elastic load cycle. A Finite Element (FE) model has been used to predict the spatial distribution of the magnetic field due to localized MMM. The FE studies show that the defect size and the applied background magnetic field will influence the SMLF signal; however, the signal due to MMM could be swamped by any ferromagnetic material near to the magnetic sensors. The simulation and experimental results demonstrate the high probability of false calls with the MMM technique even if there were a significant MMM effect in the material used