Showing papers in "Ndt & E International in 1993"

Nondestructive evaluation of materials by infrared thermography

Abstract: Overview of nondestructive evaluation (NDE) using infrared thermography theoretical aspects experimental apparatus external thermal stimulation - methods and image processing internal thermal stimulation - methods and image processing quantitative analysis of delaminations inspection of materials with low emissivity by thermal transfer imaging thermal diffusity measurements of materials thermal tomography thermal NDE of nonplanar surfaces applications of infrared thermography to high temperatures.

Non-contact ultrasonic measurements on steel at elevated temperatures

Abstract: Non-contact ultrasonic measurements have been made on ferritic and austenitic steel specimens as a function of temperature from ambient to 1200°C, using a pulsed laser to generate and a reference beam laser interferometer to receive the ultrasound. The generation efficiency is found to remain surprisingly constant in both thermoelastic and ablation regimes over a wide temperature range. The sensitivity of the laser interferometer is also found to be temperature independent to a first approximation. However, it is typically reduced by 3–6 dB by convection currents above ∼ 900°C. Both the compression and shear velocities decrease with rising temperature. The former is measured with a precision of 1 in 103, the latter rather less accurately with the present configuration. Compression wave attenuation increases steadily below 600°C in both materials. There is a peak in attenuation in ferritic steel between 600 and 750°C, which is absent in austenitic steel. It coincides with a steeper decrease in ultrasonic velocity and is believed to be due to the martensitic structural phase transformation. The attenuation rose more rapidly in both materials as 1000°C was approached. The material attenuation varied with heat treatment, a value in the range 1–1.5 dB cm−1 being recorded at 1000°C. Complicated effects were observed during heat treatments at 1000°C and above. Both attenuation and forward scattering data were consistent with some annealing out of sub-structure, in addition to austenitic grain growth. Finally, there was evidence of lattice softening at the highest temperatures investigated. The data suggest that thicknesses of steel in the range 100–250 mm should be inspectable with a scaled-up system, depending upon various factors such as the presence of oxide scale, provided high power lasers are employed for generation and reception and an optimum bandwidth is chosen.

Thermal nondestructive testing of carbon epoxy composites: detailed analysis and data processing

Abstract: A detailed description of results for thermal testing of carbon fibre reinforced plastic composites is presented. Theoretical analyses of 1D and 2D heat transfer problems illustrate the connections between all the parameters involved. Experimental results are obtained for standard carbon epoxy composites simulating the various types of defects (delaminations, Teflon inserts, cut-out and added plies). Particular image processing techniques as well as thermal tomography are also discussed.

Lamb wave propagation in composite laminates and its relationship with acousto-ultrasonics

Abstract: The acousto-ultrasonic technique has many potential applications in the NDE of composite materials. However, problems of poor reproducibility and of the sensitivity of the results to precise instrument settings have restricted its application in industry The waves employed in acousto-ultrasonics are chiefly Lamb waves which propagate in the plane of the laminate. In this paper, Lamb wave propagation in composite laminates both with and without defects is investigated both numerically and experimentally. Acousto-ultrasonic parameters based on both predicted and measured responses are calculated, and tests are carried out using a commercially available instrument. The factors which lead to the poor reproducibility of acousto-ultrasonic results are discussed and possible improvements to the technique are proposed.

Error analysis of tomographic filters. II: Results

Abstract: The convolution backprojection algorithm (CBP) has been studied from the point-of-view of the already published error formula which relates the error in reconstructions, under certain ideal conditions, to the Fourier-space derivatives of the filter functions employed in CBP. The results, for simulated images representing a damaged nuclear reactor fuel assembly and a cross-section of human brain, indicate that the error pattern obtained is in concurrence with the theory. Experimental results for a pixel size of 20 μm are also included.

Model for predicting ultrasonic pulse-echo probability of detection

Abstract: This paper describes a mathematical model for predicting the theoretical probability of detection (POD) of planar buried defects, for conventional ultrasonic pulse-echo inspection. A model for the scattering of ultrasound by well-oriented planar defects is combined with noise theory to produce a calculated capability of detection, based on the likelihood that the defect signal exceeds the specified threshold. The problem of false indications, the recording of a defect when none is present, is also addressed by the model, showing how any improvement in POD predictions must be considered in parallel with the associated change in the probability of false indication. We use examples to illustrate how the model may be used to check on proposed inspections, showing how factors such as reporting threshold, probe scan pattern and the criterion for the number of probe positions at which an indication must be seen before a defect is recorded, all affect the probability of detection and of false indication. The effects of defect roughness on detection probabilities are studied. We also use the model to quantify the uncertainties that result in POD predictions, when defect properties such as orientation, roughness, aspect ratio and depth within the specimen are themselves uncertain. The problems of equipment and human error are not addressed, although we discuss how, if these can be quantified, they may be incorporated into the model.

Acoustic emission source location on a cylindrical surface

Abstract: Acoustic emission (AE) is a stress wave, generated in a structural material when it is stimulated by perturbations. One of the important features of this technique is to detect the location of the AE source in the structure from the differences in the times of arrival of stress waves at several sensors placed on it. A generalized mathematical formalism for the evaluation of the path travelled by an acoustic wave on the surface of a structure, on the concept of geodesics, is presented. Based on this concept, a mathematical method is developed for the calculation of the coordinates of an AE source on a cylindrical surface by three AE sensors. This concept has not been attempted analytically so far, to our knowledge.

Multi-frequency fibre optic sensors for in-process laser welding quality monitoring

Abstract: Laser welding is a high temperature machining process in which there are two principal methods of beam absorption: absorption by the plasma and Fresnel absorption by the surface. The plasma absorption and reradiation generates shorter wavelength light waves whereas the heated surface reradiates in the longer wavelengths. This paper describes the experimental relationships between weld quality and the multi-frequency fibre optic sensor signals during laser welding. An analysis of the likely meaning of the signals is presented together with an appraisal of the technique as an on-line weld monitor.

Application of microfocus X-ray radiography in materials and medical research

Abstract: The advantages of real-time microfocus radiography are evaluated for several applications. These advantages are mainly the small focal spot size (5 μm) leading to a very low geometrical unsharpness (0.1 mm) for a magnification of 21 ×; the positioning of the specimen very close to the source (15 mm) leading to high magnifications (100 ×) and the minimal scattered radiation recorded on film. This radiographic technique was used to study the electrolytic codeposition of a Cu-layer on a metal substrate. Enlarged radiographic images of archaeological findings revealed the smallest detail. This helps in the restoration of pieces contaminated outside and remaining intact inside. For medical research microfocus X-ray radiography provided useful information on the reaction of bone tissue on specific osteoporosis medication, on the vascular changes in experimentally induced liver tumours and on the sequential deterioration of kidney stones.

Estimation of residual stresses in railway wheels by means of stress-induced magnetic anisotropy

Abstract: A nondestructive inspection technique, based on stress-induced magnetic anisotropy, provides a means of quantifying the residual stress distribution in railway wheels. An automatic prototype device has been constructed; calibration measurements were carried out to examine the sensitivity of the technique to metallurgical effects in the wheel grades of interest. The device was used to obtain residual stress information on new and thermally damaged wheels representing a range of service conditions. The validity of the technique is examined using additional strain gauge measurements. A relationship is established between the output of the device and net circumferential stress within the wheel rim.

Characterization of microstructures in 17-4-PH stainless steel by magnetic Barkhausen noise analysis

Abstract: Magnetic Barkhausen noise (MBN) signals have been used to characterize the microstructures in quench-aged precipitation hardening 17-4-PH stainless steel. The MBN data have been compared with hardness and coercivity. Microstructures were evaluated by optical, scanning electron and transmission electron microscopy. It was found that MBN signals correlate well with the microstructures manifested by the presence of intra-lath spherical and inter-lath copper-rich non-magnetic precipitates. The presence of coherency strains at the peak hardness regions of the aged specimens could be inferred from the MBN data. MBN signals were found to provide a better criterion for the detection of the maximum in coherency strains than the usual hardness data.

Effects of bending stresses on magnetic flux leakage patterns

Abstract: The effects of stress on magnetic flux leakage patterns induced by pits in pipeline samples are examined using separate arrangements to produce surface or through wall bending stress. The test samples are long strips cut axially from a sample of pipeline. The first sample is a single strip bent as a beam to induce surface stresses. The second is a composite beam with two steel strips separated by a bonded glass-fibre and wood layer bent to give nearly uniform but opposite stress induced in each steel strip. Maps of the radial magnetic flux leakage amplitudes from identical defects in the two beams were compared for response to bending stress. The results show that surface stress alone is insufficient for considering the effects of stress on magnetic flux leakage amplitudes. The uniformly stressed composite beam showed considerably higher response to stress than the single strip sample with bending producing predominantly surface stress.

Three-dimensional flaw reconstruction and dimensional analysis using a real-time X-ray imaging system

Abstract: We have implemented a three-dimensional point reconstruction method based on stereo radiography techniques utilizing a real-time radiography (RTR) laboratory. The continuous radiographic image coupled with computerized position control and feature point determination eliminates problems associated with film detectors (development time, occluded/crossed disparities due to missing data between views). Furthermore, the RTR laboratory geometry affords precise control over sample movements using a stepper motor-controlled positioner. We used a simple image-based calibration method to accurately determine the X-ray source and sample position relative to the detector, and ‘'pincushion’' distortion inherent in the electrostatic inverting image intensifier tube was reduced to less than ±0.5 pixels using a two-dimensional third-order polynomial-based image warp. These calibrations resulted in an overall depth accuracy of ±0.2 mm, making this system competitive with computed tomography-based measurements. Moreover, this system achieves good results using two views of a sample rotated 10–30° between views, so it works equally well with symmetric and plate-like geometries. The stereo radiography equations were generalized to support arbitary sample manipulation (translation, rotation) and an arbitrary number of views using a fixed source/detector geometry. The overall system performance was determined by measuring lengths and orientations of simulated cracks (0.3 mm diameter copper wires) embedded at different depths within an aluminum plate, and by measuring the locations of coolant tube axes within a jet engine turbine blade sample.

Ultrasonic characterization of defects in steel using Multi-SAFT imaging and neural networks

Abstract: Due to the recent improvements in nondestructive inspection (NDI) techniques, even small or weak inhomogeneities can be detected and discrimination between critical and non-critical defects becomes more crucial. In practice, welded regions of steel components are of special interest. Characterizing a detected inhomogeneity may be done by using cross-sectional images, which are reconstructed from ultrasonic B-scan data. However, in this application high-resolution images are very hard to obtain, because the geometry of most objects severely limits the amount of ultrasonic information which can be collected. In this paper it is demonstrated that optimized data acquisition may yield ultrasonic B-scan data containing responses of multiple wave paths, which can be used to provide multiple focused images. Furthermore it is shown that the application of neural networks may give additional information on the shape of the defect, provided that correct pre-processing of the ultrasonic B-scan data is applied.

Estimation of the stress state inside metals using stress perturbing waves and probe waves

Abstract: A new method to detect the stress conditions inside metals such as the limiting stress value of elastic deformation is proposed. It is based on the nonlinear dependence of ultrasound velocity on stress. First, the nonlinear dependence of sound velocity on stress is observed in an Al alloy. Then equipment which consists of a wave generator for electromagnetic monopole stress perturbing pumping and a 50 MHz ultrasonic sound velocity change detecting unit is presented. In this system the stress perturbation is applied at the desired measuring region in the metal along with the probe wave and changes in the wave are observed. Finally, it is applied for the estimation of the elastic deformation limit of Al alloy and the usefulness of the method is confirmed.

Evaluation of defects in axisymmetric composite structures by thermography

Abstract: Thermography has been developed to evaluate delaminations, voids, missing layers and cloth joints in axisymmetric composite structures made by filament winding and hand lay-up techniques. The composite structures were made of carbon/epoxy, carbon/phenolic, carbon/polyimide and high silica glass/phenolic. Thermography has been found to be simple, fast, very effective and reliable to locate these defects.

Influences on signal/noise ratio during ultrasonic inspection of composites

Abstract: This paper investigates the correlation between transducer properties, defect size and depths, and the signal/noise ratio during ultrasonic inspection of polymeric composites. Samples of different fibre/matrix systems were produced, including artificial defects of various size and shape. These specimens were inspected ultrasonically with transducers in the frequency range from 1 to 150 MHz. It could be seen that the 5 MHz transducer possessed the highest signal/noise ratio in most cases. A transducer selection catalogue incorporating special usage terms for carbon fibre/epoxy resin composites is introduced.

AE testing of a low-alloy steel pressure vessel

Abstract: Testing pressure vessels in the field is generally conducted within a given range of test pressures, determined beforehand according to specified criteria. Thus monitoring of flaw growth using AE is carried under restricted conditions. To study the AE activity of flaws in welded seams of a pressure vessel (made of low-alloy steel) at different stages of growth and at different strains, tests were conducted on both spherical and cylindrical pressure vessels containing different artificial flaws. For given test conditions, the relationships between AE activity of artificial flaws and measured strains were studied. These tests form an important basis for the testing and assessment of engineering pressure vessels in the field.

Digital colour radiography

Abstract: The production of digital colour radiographs is discussed and the interpretation and possible advantages of such radiographs are examined. Two specific processes are identified. One is direct colour radiography in which radiographs obtained at different beam energies are colour coded and combined to form a true colour image. The other is colour subtraction radiography in which the primary radiographs are subtracted and the resulting difference radiographs are colour coded and combined. Both formats generate colour radiographs in which intensity and colour are related to the material composition of the object. They are found to have some advantages over single energy, monochrome pictures. They provide greater latitude and contrast, in terms of object structure, improved image interpretation, by facilitating long range visual correlation and feature interpolation, quantitative information for analysis of single and multiple radiographs, and the possibility of elemental analysis. The direct colour radiographs are additive with respect to the primary colours and, like monochrome radiographs, provide good intensity contrast. The subtraction radiographs retain the primary colours and offer better colour contrast but poorer intensity contrast. Because of the subtraction process, they are noisier images.

Internal ultrasonic inspection of flexible pipe

Abstract: Methods for internal ultrasonic inspection of flexible pipe have been investigated through experiments with a short sample of Coflexip pipe. Ultrasonic backscatter methods using normal and non-normal incidence have been used for qualitative high contrast ultrasonic imaging of the inner surface of the pipe. Analysis of the internal cross-section has been performed based on the use of a non-contact ultrasonic caliper, and processing procedures which enable calculation of, and compensation for, eccentricity of the tool in the pipe. The methods developed can be used to quantitatively estimate the thickness of the internal carcass, and perform high resolution topographic mapping of the inner surface.

NDT reliability and product quality

Abstract: To assure the reliability of products and their fitness for use, an approach is described that starts from the required inspection effectiveness targets. Use is made of parameters which characterize the performance and reliability of nondestructive procedures. The effectiveness of the nondestructive inspection techniques that is required to reach the targets is defined by means of these parameters. This approach allows selection of nondestructive inspection systems based on general objectives regarding detection performance and reliability.

AE monitoring and data analysis for large spherical tanks

Abstract: The successful assessment of structural integrity by means of AE depends on the quality of the recorded AE data and on correct post-test data analysis. AE monitoring and signal analysis of tests on two large, spherical tanks were carried out using a 32-channel AE system (Spartan, from PAC, USA). Water pressurization was used on one, whilst the other was tested on-line. The effects of both the residual and loading stresses on the surface-opening cracks produced by multiple welding passes were assessed by means of AE. AE measurements generally take a rather long time and background noise cannot always be restricted during the test period (a major problem with on-line testing). It is possible to improve the detection of AE in the field by varying the pressure of the operating medium by 10% P ( P = operating pressure of tank). The relationship between the source (a crack growing with load changes) and its associated AE parameters is also discussed.

A single procedure to measure ultrasound characteristic wave impedance, attenuation and propagation velocity in materials

Abstract: A simple experimental procedure which can be used to measure the ultrasound characteristic wave impedance, attenuation and velocity propagation in materials is described. The method is based only on the measurement of reflection coefficient vs frequency for a slab-form sample of the material under test. Interesting features are that there are no particular requirements for the sample sizes and that only commercially available instrumentation is used. A first experimental implementation of the technique using a vector network analyser is reported. The results obtained confirm the performance estimated theoretically.

Reconstruction of convex flaws using backscattered ultrasound

Abstract: The paper deals with high frequency scattering. The backscattered amplitude for a convex flaw is proportional to a function depending on the Gaussian curvature at the point where the wavefront makes contact with the flaw surface. A numerical algorithm is employed for reconstruction of the flaw shape and orientation from the known amplitude of the backscattered wave. Experimental results obtained for two-dimensional defects (oval, triangle) produced in a thick aluminium plate are in good agreement with the numerical reconstruction. The proposed method enables identification of an image of arbitrary convex flaws in real time using a 386-based PC.

Polycrystalline diamond cutting element control using high frequency C-scan image processing

Abstract: The quality control of drilling tools used in the oil industry essentially consists of testing the cutting elements distributed around the petroleum tool. This study presents a nondestructive testing (NDT) method for the detection of different kinds of defects inside polycrystalline diamond cuttine elements (PDC). This ultrasonic method, based on high frequency (100 MHz) C-scan image processing, allows the detection of three major types of defect such as diamond layer debonding from the substrate, cracks and thermal defects of the diamond layers. These defects induce a perfectly quantified behaviour to mechanical abrasion resistance and fatigue tests. Experimental results show good correlation between our ultrasonic measurements and classical abrasion resistance tests. In fact, a PDC sample exhibiting defects detected by the C-scan technique has been submitted to a mechanical fatigue process and optical microscopic analysis. These tests have shown that each kind of defect has an influence on one or more mechanical characteristics. Using C-mode scanning acoustic microscopy (with a 50 μm pixel size), allows an extremely precise and quantified level of erosion resistance of the cutting elements without systematic resort to expensive destructive tests carried out during the reception of the batch. The rejection of samples showing harmful defects will eliminate the erratic and sometimes unexplained destruction of some tools which were considered to be defect-free.

Ultrasonic inspection of line pin post insulators and solid core insulators in switch gear stations

Abstract: Cracks in high-tension porcelain insulators are one of the most common sources for disturbances in transformer and sub-stations. The cracks can be detected before they cause a failure by using ultrasonic testing. A crack mechanism is presented as well as the manual ultrasonic technique and newly developed ultrasonic equipment which allows testing on energized insulators.

A neural network-based technique for quantitative wire rope inspection

Abstract: The possibility of solving the problems encountered in the quantitative inspection of broken wires with a neural network is discussed and a new algorithm is presented. In comparison with conventional methods, the performance of inspection is much improved with the neural network-based technique.

Study of the nondestructive inspection frequency based on structural reliability considerations

Abstract: To ensure the safety and reliability of mechanical components and structures, in-service nondestructive inspection (NDI) is frequently employed in the aircraft, petrochemical, nuclear and non-nuclear power and other industries. In the present paper, the effect of NDI on structural reliability is studied within the context of probabilistic fracture mechanics. Special attention is given to the investigation of optimal in-service nondestructive inspection frequency based on structural reliability considerations. Using the proposed analytical method, an appropriate in-service inspection (ISI) schedule can be planned. This ISI plan gives the least maintenance cost while retaining minimally required reliability assurance for the structure.

Linear mottling on X-ray radiographs of automatic TIG welded aluminium alloy studied by X-ray topography

Abstract: Linear mottling on X-ray radiographs of TIG welds in an aluminium alloy has been investigated by X-ray topography and X-ray spectroscopy. The experimental results showed that the radiographic mottling was due to X-ray diffraction. Detailed X-ray topographic images of large diffracting grains of the weld zone were obtained. The spectra of diffracted beams were measured using energy-dispersive spectrometry. The diffraction was identified as 111- and higher orders of Bragg reflection from the diffracting FCC grains.