Showing papers in "Research in Nondestructive Evaluation in 1995"

Acoustic emission theory for moment tensor analysis

Abstract: Acoustic emission (AE) is extensively applied to nondestructive evaluation of materials and structures. In the conventional AE measurement, several AE parameters are detected and analyzed to elucidate characteristics of microfracturing behaviors in materials. Although theoretical treatment of AE waveforms was proposed more than one decade ago, the quantitative analysis was neither practical nor applicable to general AE waveforms. The crack mechanisms associated with AE generation consist of crack kinetics and crack kinematics. It has already been demonstrated that deconvolution analysis is available for determining crack kinetics. As for crack kinematics, it is known that the moment tensor analysis is promising, but has only been applied to marginal cases. In this respect, a practical procedure for the moment tensor analysis is recently formulated, selecting P wave portion from the full-space Green's function of homogeneous and isotropic material. A multi-channel observation is utilized to locate...

Elastic property measurement using Rayleigh-Lamb waves

Abstract: A nondestructive technique is described for the measurement of elastic constants of isotropic plates using ultrasonic Rayleigh-Lamb waves. The experimental method employs continuous harmonic waves and a pair of variable-angle contact transducers in pitch-catch mode. The phase velocity of the R-L waves at a particular frequency is determined from the phase shift over a measured path length. This simple experimental technique can measure phase velocity over the range 1–10 mm/µs with an error of less than 0.5% over a frequency range of 50 kHz-2 MHz. Individual symmetric and antisymmetric modes can be generated through the selection of transducer angle and frequency. Young's modulus and Poisson's ratio for the material are calculated from measurements of frequency and phase velocity by a nonlinear least squares solution to the dispersion equations. The sensitivity of the nonlinear least squares function to the measurement region of the dispersion curve is investigated. It was found that estimations of material properties are more accurate and less sensitive to small experimental errors when only selected frequencies and R-L modes are used in the least squares calculation. This technique is demonstrated with several isotropic materials and with both thick (6 mm) and thin (0.8 mm) plates. Values for elastic constants determined by the contact transducer Lamb wave technique compare favorably with values measured using the pulse-echo-overlap method. The uncertainty in measurements of Young's modulus and Poisson's ratio was less than 1% and 2%, respectively. The technique has advantages over more traditional methods for measuring elastic properties when it is desirable to use wavelengths greater than the plate thickness, when properties may vary with frequency, or when it is necessary to measure in-plane elastic properties of thin plate structures.

Microwave near-field imaging with open-ended waveguide—Comparison with other techniques of nondestructive testing

Abstract: The results of an experimental study of microwave imaging with an openended waveguide at 30 GHz for NDE of dielectric materials are presented. Using a hybrid measurement method, the near-field point spread function is investigated. Improvement of sensitivity and resolution via deconvolution also is discussed. A comparison of results obtained with more prominent NDE methods as ultrasonics, X-rays, and thermal waves demonstrates the performance of the technique.

Dielectric and magnetic measurements: A survey of nondestructive, quasi-nondestructive, and process-control techniques

Abstract: A review of the most common methods for nondestructive permittivity and permeability measurements is presented. Transmission-line techniques, coaxial apertures, open resonators, surface-waves, and dielectric resonator methods are examined. Measurements on bulk, thin materials, and thin films are addressed. Measurement fixtures that can be used as sensors are highlighted. The frequency range of applicability and typical uncertainties associated with each method are addressed.

Quantitative Mapping of Pore Fraction Variations in Silicon Nitride Using an Ultrasonic Contact Scan Technique

Abstract: An ultrasonic scan procedure using the pulse-echo contact configuration was employed to obtain maps of pore fraction variations in sintered silicon nitride samples in terms of ultrasonic material properties. Ultrasonic velocity, attenuation coefficient, and reflection coefficient images were obtained simultaneously over a broad band of frequencies (e.g., 30 to 110 MHz) by using spectroscopic analysis. Liquid and membrane (dry) coupling techniques and longitudinal and shear-wave energies were used. The major results include the following: Ultrasonic velocity (longitudinal and shear wave) images revealed and correlated with the extent of average through-thickness pore fraction variations in the silicon nitride disks. Attenuation coefficient images revealed pore fraction nonuniformity due to the scattering that occurred at boundaries between regions of high and low pore fraction. Velocity and attenuation coefficient images were each nearly identical for machined and polished disks, making the method readily applicable to machined materials. Velocity images were similar for wet and membrane coupling. Maps of apparent Poisson's ratio constructed from longitudinal and shear-wave velocities quantified Poisson's ratio variations across a silicon nitride disk. Thermal wave images of a disk indicated transient thermal behavior variations that correlated with observed variations in pore fraction and velocity and attenuation coefficients.

Theory and Implementation of NDT Data Fusion

Abstract: Scientific measurements from single or multiple sensors are usually incomplete and uncertain. A process making use of the concept of data fusion has been developed to try to encompass this problem by combining information from multiple sensors. The objective to synergistic use of information from multiple sources is to reduce uncertainty and increase the confidence level of a measurand. The implementation of data fusion to the field of NDT is relatively new. This paper summarizes the achievements of current research on data fusion applied to NDT. A theoretical data fusion strategy is described and experimental results generated from weld inspection are presented.

Microwave and millimeter wave nondestructive testing: A succinct introduction

Abstract: Microwave nondestructive techniques have a long history dating from the early 1950s, with a strong flurry of activities in the 1960s and 1990s. However, these techniques still are not widely known in the nondestructive testing (NDT) community and often are referred to as “emerging techniques” or “others.” It is only during the last two or three years that some NDT conferences have allocated a whole session or two solely to the topic of microwave NDT. For those involved in the research and development of microwave and millimeter-wave NDT techniques who have long appreciated the advantages of such techniques, this increased visibility is a welcome and encouraging change.

A General Formulation for Admittance of an Open-Ended Rectangular Waveguide Radiating into Stratified Dielectrics

Abstract: A general approach is presented for calculating the aperture admittance of a rectangular waveguide radiating into layered dielectric media. The two specific geometries of stratified, lossy dielectric media that are addressed terminate into either an infinite half-space or a perfectly conducting surface. The geometries describe two prevalent categories of layered dielectric composites and coatings that often are encountered in practical microwave nondestructive evaluation applications. Solutions are found initially by constructing a complete set of field components and subsequently enforcing the continuity of power flow across the aperture interface of the waveguide. Final results are presented as a superposition of transverse electric and magnetic components of the aperture admittance. The solutions presented allow the systematic calculation of admittance in the presence of arbitrary multilayer media, which in turn may be related to experimentaly measurable quantities of interest. With the practical assumption of dominant mode incidence on the aperture, the final expressions may be implemented without intense computational power, which often is desirable in practice where inaccuracies due to random errors and instrumentation sensitivity render incorporation of more rigorous solutions inefficient. Numerically simulated data also are presented to verify and interpret the results.

Transient Fields of Pulsed Transducers in Solids

Abstract: The radiation and propagation of elastic waves in solid media are of fundamental importance in ultrasonic NDE. Due to the limitation of analytical approaches, numerical procedures are most appropriate for solving the governing equations. In this paper, the finite element method is used to predict the transient fields of pulsed transducers in a solid medium with axisymmetric geometry. Comparisons with other models are performed. The transducer field interactions with spherical voids are also presented.

Microwave compressive strength estimation of cement paste using monopole probes

Abstract: In-situ evaluation of the compressive strength of existing concrete structures using a direct approach is the objective of this investigation. The principle factor affecting the compressive strength of a concrete structure is its water/cement (w/c) ratio. The reflection properties of four cement paste samples with w/c ratios of 0.35, 0.40, 0.50, and 0.55 were measured using several monopole probes at microwave frequencies of 5, 9, and 12 GHz. The effect of several parameters, such as the operating frequency, the probe length (h), and the properties of the cement paste (w/c ratio) on the measurement sensitivity, were studied theoretically as well. For a given monopole probe (fixed diameter), h and the operating frequency can be optimized such that any slight change in the w/c ratio causes a large change in the reflection coefficient. Lower microwave frequencies are shown to be more sensitive in detecting variation in the reflection coefficient of these samples as a function of the w/c ratio. After...

Microwave inspection of luggage for contraband materials using imaging and inverse-scattering algorithms

Abstract: Microwave inspection techniques can determine frequency-dependent complex permittivity of local regions within a nonmetallic suitcase. These dielectric data can provide the additional information needed to identify contraband material, such as explosives and drugs. Although X-ray techniques provide high-resolution images of luggage contents, they cannot easily discriminate between various organic materials such as foodstuffs, paper products, and plastics. However, because common highexplosive materials and compressed-powder narcotics have dielectric properties similar to those of common plastics, dielectrometric evaluation can discriminate between contraband materials and other organics based on their moisture content or, in the case of cosmetic products, the presence of polar solvents. This paper describes the microwave luggage-inspection problem and presents an approach to discriminating and characterizing contraband materials in this environment.

Determination of All Elastic Constants of Orthotropic Plate Specimens from Group Velocity Data

Abstract: This paper presents a novel method of nondestructively determining all nine elastic constants of fiber-reinforced orthotropic plate specimens. Group velocities of bulk quasi-longitudinal (QL), pure transverse (PT), and surface skimming pseudo-longitudinal modes are measured in glass-fiber (GF) and carbon-fiber (CF) reinforced Poly Ether Ether Ketone (PEEK) specimens by a point-source/point-receiver (PS/PR) technique. First, the pure index longitudinal moduliC 11,C 22, andC 33, are obtained from the longitudinal (L) group velocities measured in three principal directions. Next, the pure index shear moduli,C 44,C 55, andC 66, are determined by measuring the shear horizontally (SH) polarized PT group velocities in the symmetry planes. Finally, the mixed index elastic constantsC ij (i ≠ j) are calculated from QL group velocities measured in the symmetry planes and using analytic formulas.

Ultrasonic high resolution images for defect detection in ceramic materials

Abstract: The final ceramic integrity was investigated by means of high resolution ultrasonic images. To ensure a better evaluation of the defect shape, both spectral and time-domain parameters were used to build the images. A new investigation technique, based on an indirect defect detection, is proposed which exploits the filtering effect that the flaw induces on the forward propagating ultrasonic wave.

A Photothermal Line-Scanning System for NDT of Plasma-Sprayed Coatings of Nuclear Power Plant Components

Abstract: The main pump of the nuclear power plant primary circulation system is one of the most important and critical components of the pressurized water reactor type nuclear power plant. For instance, the failure of plasma-sprayed coatings on the pump's shaft seal rings leads to shutdown of the entire reactor. However, suitable methods for NDT of these coatings have not been found in spite of the obvious need. In this study, a photothermal NDT technique based on line-scanning laser heating and infrared radiation detection was applied. The measurements were modelled numerically and then performed in four series. First, a newly coated seal ring pair was inspected; two adhesion defects were found. The defective pair was subjected to a trial run emulating the real conditions of use and after that was re-inspected. Twenty new adhesion defects were found. Another newly coated seal ring pair was inspected and found to be faultless, and it was installed in the nuclear power plant for a year. During the next maintenance stop the pair was inspected again, and a group of three adhesion defects was found. The detecting of adhesion defects in both the unused and stressed coatings demonstrates the applicability of the photothermal method.

Ultrasonic Nondestructive Evaluation of Inhomogeneous Plane Strain in Elastic Medium

Abstract: The possibility to enhance the efficiency of ultrasonic nondestructive testing making use of non-linear effects of wave propagation is investigated theoretically. The quasi one-dimensional problem of non-linear longitudinal wave propagation in elastic medium undergoing inhomogeneous plane prestrain is solved. The solution establishes the relations between the physical and geometrical properties of the medium, parameters of its predeformed state and the characteristics of the wave with smooth arbitrary initial profile. The non-linear sine-wave propagation in the medium with two traction-free surfaces is considered in more detail. The algorithm for evaluation of the parameters of plane strain on the basis of wave velocity and its profile evolution measurement data is proposed.

Transportable Infrared Line Scanner Based Equipment for Thermal Nondestructive Testing

Abstract: Transportable equipment for on-site thermal nondestructive testing is described. The equipment is constructed from two separate units: a hand-held infrared line scanner and a remote unit containing all the necessary control devices. The sample surface is heated by manually scanning a line source over the surface and the resulting surface temperature rise is simultaneously measured with the line scanner. The heating line is generated either by a line-focused laser beam or by a radio frequency induction coil. The obtained surface temperature distribution of the monitored area is presented as a pseudo-color image. The feasibility of the method is demonstrated with a carbon-glass fiber composite sample.

Anomalous magnetic source defect modelling

Abstract: Defect-induced field perturbations occurring during electromagnetic testing of ferromagnetic material can be modelled as combinations of anomalous eddy current and missing magnetization sources. At low frequencies, such as are used in Remote Field Eddy Current (RFEC) inspection of steel tubes, the anomalous magnetization defect sources dominate and give strong responses to circumferential slits orthogonal to the axial magnetic field. Here we present the results of finite element calculations and computer animations of the time-varying fields from a missing magnetization model of a slit defect. These are compared with the defect-induced anomalous field patterns obtained by phasor vector subtraction of calculations for slit defect and defect-free problems. It is noted that the ferromagnetic material in which the defect is located must be included in the missing magnetization model.

Thin film inspection with millimeter-wave reflectometer

Abstract: Inspection of dielectric thin films (25.4–508 μm) placed directly above a conducting plate is performed with a W-band millimeter-wave network analyzer. The signal launched by a small antenna placed near the sample surface is picked up by a separate receiver. The phase of the received signal can be related to the thickness and permitivity of the thin films under inspection. While the transmitter and receiver are at larger distances from the dielectric surface, the system operates as a conventional reflectometer. But when they both are close to the surface, guided waves will be excited and even may become dominant over the direct reflection. The significance of the guided-wave contribution is controlled by the locations of the guided-wave poles in the complexk p-plane. In this paper we employ a modal approach that previously was developed for geophysical remote sensing to study the effects of the guided waves, and to outline the limits of the conventional reflectometric method for evaluating thin films.

Longitudinal AC Interactions with Axial Slits in Steel Pipes

Abstract: The Remote Field Eddy Current (RFEC) nondestructive inspection technique uses low frequency AC and through wall transmission to inspect pipes and tubes from the inside. In steel pipes, it has generally greater sensitivity to circumferential rather than axial slits because the perturbation of magnetic fields orthogonal to slits dominates. Circumferential AC magnetic fields, generated by passing AC axially along a steel pipe from an external supply, have therefore been tested in order to give greater sensitivity to axially aligned cracks characteristic of stress corrosion cracking in pipe-lines. Anomalous source missing magnetization defect models suggest that, as slit widths are reduced, the importance of magnetic interactions is reduced until eddy current interactions predominate. This suggests that, for very fine axial cracks, true RFEC geometry, which gives circumferential eddy currents, will give stronger signals than circumferential AC magnetic fields.

High frequency ultrasonic investigation for ceramic materials to test the manufacturing process

Abstract: High frequency ultrasonic techniques for nondestructive tests of fine grained ceramic materials are presented. Green and partially sintered are considered. To this purpose, ultrasonic velocities and attenuation were measured and correlated to the porosity degree reached in different phases of the manufacturing process. A physical understanding of the experimental results is achieved by comparing them with the theoretical predictions provided by a self consistent scattering theory introduced in the 1980s. Finally, porosity maps are proposed since they give a comprehensive view of the process quality.

A Numerical Study on the Effects of Line Heating in Layered Anisotropic Carbon Fiber Composites

Abstract: Thermal effects of line heating on layered anisotropic carbon fiber composites were studied numerically Surface temperature distributions were calculated for delaminations of different sizes placed between different fiber layers using finite difference methods The anisotropy of thermal conductivity caused detectable effects on the surface temperature distributions that depend on the scanning direction of line heating and the velocity of the scanning The fiber orientation had a clear effect on the image contrast, whereas the resolution showed only a slight dependency on this The performance of the imaging method was estimated by comparing the size of the delamination defined in the numerical model to the half-measure size or the 1/e level size obtained from the computed surface temperature distributions The defect size obtained by using the half-measure values was found to be slightly smaller and the size defined by the 1/e values larger than the actual one The half measure criterion gave a better estimate compared to the 1/e values However, in practice the observed size was very close to the actual one when either of the criteria is used As expected, the lateral resolution deteriorated when the depth of the delamination was increased