Showing papers in "Journal of Nondestructive Evaluation in 1996"

The excitation of Lamb waves in pipes using dry-coupled piezoelectric transducers

Abstract: The development of a dry-coupled piezoelectric transducer system for the detection of corrosion in chemical plant pipework using cylindrical Lamb waves is described. It is shown that the axisymmetricL(0,2) mode at a frequency of about 70 kHz is an attractive mode to use for longdistance propagation. The results show that a ring of piezoelectric length-expander elements can be used to excite theL(0,2) mode and to suppress all the nonaxisymmetric modes. Tests have been carried out both with the piezoelectric elements bonded to the pipe and fabricated into a simple transducer which was clamped against the pipe. The performance of the dry-coupled system was very similar to that of the bonded elements. In pulse echo tests, the noise floor obtained with the dry-coupled system was less than 1% of the amplitude of the propagatingL(0,2) mode. The drycoupled transducers provide a simple, light, readily detachable system for the long-range inspection of pipework.

Benchmark problems for defect size and shape determination in eddy-current nondestructive evaluation

Abstract: A set of four benchmark problems is presented for verification of theoretical calculations of defect size and shape in eddy-current nondestructive evaluation. The benchmark problems are based on careful measurements of the change in coil impedance as a function of frequency for a circular air-cored coil which is scanned along the axis of an electrodischarge machined slot in a thick aluminum alloy plate. Slots of (i) semi-elliptical, and (ii) double-peaked profiles are considered. Deviations from ideal coil behavior are identified and corrected so that the final impedance data and experimental parameters can be directly used to verify theoretical inversion algorithms.

Embrittlement of austenitic stainless steel welds

Abstract: The microstructure of type-308 austenitic stainless steel weld metal containing {gamma} and {delta} and ferrite is shown. Typical composition of the weld metal is Cr-20.2, Ni-9.4, Mn-1.7, Si-0.5, C-0.05, N-0.06 and balance Fe (in wt %). Exposure of austenitic stainless steel welds to elevated temperatures can lead to extensive changes in the microstructural features of the weld metal. On exposure to elevated temperatures over a long period of time, a continuous network of M{sub 23}C{sub 6} carbide forms at the austenite/ferrite interface. Upon aging at temperatures between 550--850 C, ferrite in the weld has been found to be unstable and transforms to sigma phase. These changes have been found to influence mechanical behavior of the weld metal, in particular the creep-rupture properties. For aging temperatures below 550 C the ferrite decomposes spinodally into {alpha} and {alpha}{prime} phases. In addition, precipitation of G-phase occurs within the decomposed ferrite. These transformations at temperatures below 550 C lead to embrittlement of the weld metal as revealed by the Charpy impact properties.

Nondestructive evaluation of cementite content in steel and white cast iron using inductive Barkhausen noise

Abstract: A nondestructive testing method for the determination of the cementite content in iron-carbon steel and white cast iron is presented. The method is based on micromagnetic measuring parameters derived from inductive Barkhausen noise measurements taken under room temperature and with temperatures above the Curie temperature. The influence of different cementite contents and cementite modifications on the micromagnetic measuring quantities for steels with ferritic, pearlitic, martensitic annealed and martensitic soft annealed microstructure states and for white cast iron was determined. Cementite actively produces its own inductive Barkhausen noise and also influences the Barkhausen noise of the iron matrix both as a foreign body and by its stress fields of the second kind. Each influence has a different effect on steel and on white cast iron. It was shown that parameters derived from the Barkhausen noise can be used for determining the cementite content if the microstructure state is known. Moreover, when the steel grade is known, the microstructure state can be detected from the pattern of the Barkhausen noise curve by means of simple pattern recognition.

Nondisruptive material sampling and mechanical testing

Abstract: The aging and inservice degradation of industrial equipment has underscored a need for efficient and reliable evaluation of the suitability of such equipment for continued service. The structural components of traditional energy production facilities, such as fossil- and nuclear-fueled electric power plants, are prime examples of aging equipment for which integrity during extended service is of major concern. The paper describes a recently developed nondisruptive miniature material sample removal and test approach that is being applied to a range of operating electric power plant components from turbine generators to pressure vessels, and to petrochemical plant reactor vessels for inservice integrity assessment. Thein situ removal of a thin wafer-like sample (less than 25 mm in diameter and 2.5 mm in thickness) from the component surface generally has no effect on component integrity. The miniature specimen small punch (disk bend) test has been developed to mechanically test the as-removed material, and is being used to estimate the material tensile behavior and fracture properties (ductile-to-brittle transition temperature and fracture toughness) required for a reliable component integrity assessment.

Degradation of Structural Ceramics by Erosion

Abstract: Materials wastage by solid-particle erosion can be severe and limit lifetimes. This paper will review the theoretical description of solid-particle erosion in brittle materials, which is well-developed for monolithic ceramics. The models can usually account for effects from the principal projectile properties: size, impact velocity, and impact angle. Materials parameters such as fracture toughness and hardness can be included. Steady-state erosion measurements on a wide variety of ceramics, ranging from Si single crystals to SiC-whisker-reinforced A12O3 are reviewed and compared to the models. It is believed thatR-curve behavior and/or particle fragmentation is responsible for discrepancies between the theory and experimental results for composite ceramics. Additionally, the theories make no attempt to describe threshold or incubation effects.

Nondestructive evaluation of pavements using spectral analysis of surface waves in the frequency wave-number domain

Abstract: The potential of using Spectral analysis of surface waves (SASW) for nondestructive testing of pavement and soil sites has been demonstrated in several publications. The usual SASW field testing and data analysis procedures and the assumption of a single propagation mode yield accurate results when there are no sharp changes in the stiffness of the site with depth. When this is not the case, however, difficulties arise in the interpretation of experimental data due to the participation of more than one mode in the wave field. In an earlier study, the multiple filter/crosscorrelation technique was suggested for analyzing multi-mode SASW signals. This technique, however, may not yield accurate results when propagation modes are closely spaced. A more general method using spectral analysis in the frequency wave-number domain is presented in this paper. Formulas are presented for determining a reasonably small number of surface measurements, and the accuracy of this method in resolving experimental dispersion curves is demonstrated for a pavement site.

Laboratory nondestructive evaluation technology for materials characterization

Abstract: This paper summarizes advances in ultrasonic and magnetic technologies for materials characterization that have recently occurred in the research laboratory and appear to have the potential to impact technology gaps of concern to the energy production industry. Included are discussions of the possibilities that one can characterize structure, such as grain size and dislocations, properties, such as fracture toughness, degradation mechanisms, such as creep, fatigue, and hydrogen attack, as well as the stresses which drive many of the degradation mechanisms. Throughout the discussions, careful attention is given to differentiating those quantities which are fundamentally related to the nondestructive evaluation response, and those which are related by empirical correlations. Included is a discussion of future work needed to further develop these technologies.

An integrated approach to evaluating the fracture toughness of irradiated nuclear reactor pressure vessels

Abstract: Current methods of predicting the inservice fracture toughness of nuclear reactor pressure vessels subject to irradiation embrittlement are briefly reviewed, and a new and integrated approach is proposed. This approach is based on the use of a wide variety of information, including the rapidly emerging understanding of the fundamental mechanisms of fracture in the ductile to brittle transition region as well as the microstructurally-mediated processes leading to embrittlement. However, the focus is on advanced, nonintrusive characterization methods for measuring composition, coarse and fine scale microstructure, and mechanical properties using small sample biopsies from operating vessels.

Some technology gaps in the detection and prediction of hydrogen-induced degradation of metals and alloys

Abstract: One hundred years of study and thousands of individual investigations have led to some understanding of the deleterious effects of hydrogen on the mechanical behavior of metals and alloys. This understanding includes a technical basis to: (a) evaluate the potential for hydrogen-induced degradation through mechanistic, phenomenological, and/or empirical models of the embrittlement process(es), (b) categorize the type of damage in terms of the active or dominant degradation mechanism, (c) select materials to minimize the susceptibility to hydrogen-induced degradation, and (d) avoid the potentially adverse effects of hydrogen exposure through processing and service. The emergence of understanding has not been accompanied by the emanation of nondestructive evaluation technologies to detect the early stages of degradation. Moreover, the ability to predict the susceptibility of new engineering alloys to hydrogen embrittlement is very limited. Data and/or analyses to describe the effects of hydrogen on welds and other as-cast structures are also lacking. This paper provides a brief description of the various hydrogen embrittlement processes and discusses the technology gaps in the development of nondestructive evaluation technologies to predict and/or monitor hydrogen-induced degradation of metals and alloys.

The application of two-refractive-index contouring on the contour difference of two compared specimens

Abstract: The development of comparative holographic Moire interferometry is accelerated by the need for nondestructive evaluation. It is based on a comparison of a master and test objects and a visualization of the difference in the mechanical responses under identical loading condition. In this paper, the hybrid method of comparative holographic Moire interferometry and the two-refractive-index contouring technique are used to generate contour difference for two compared specimens. The interference pattern of the master and test specimens are superimposed by a reconstruction beam. Moire fringes, produced between the two interference patterns, outline the contour differences in the compared specimens. The proposed experimental approach provides an advantage that full knowledge of the optical arrangement during construction and reconstruction of the hologram does not have to be accurately available. A partial-spherical object and its rotated status were simulated as the master and test objects.

Digital image processing of crack growth using a modified caustic set-up

Abstract: A digital image processing method to analyze the fringes of a modified caustic is described. The experiment is easy to perform and simple, and the image processing technique and detecting system can operate automatically, precisely, and quickly. Recording with two video cameras two sets of caustic images during crack growth, the computers process data with a specified fringe counting algorithm. We obtained two complete sets of data, one providing a full field view of the distribution of stress and the other providing the stress intensity factor.

Degradation of components and structures by fatigue

Abstract: This note examines briefly fatigue as a degradation mechanism and how engineers compensate for the fatigue phenomenon. Design is generally based on the stress-lifetime concept, but this concept is not very useful for lifetime assessment of operating components; thus, fatigue crack initiation and the growth of fatigue cracks become important. The numerous factors related to fatigue crack initiation and growth are briefly mentioned, and the importance of including the growth of small fatigue cracks is emphasized through the use of an illustration.

Some characteristics of early stages of stress corrosion cracking

Abstract: Environment-induced crack growth generally progresses through several stages prior to component failure. Crack initiation, short crack growth, and stage I growth are early stages in crack development that are summarized in this paper. The implications of these stages on component reliability derive from the extended time that the crack exists in the early stages because crack velocity is low. The duration of the early stages provides a greater opportunity for corrective action if the short cracks can be detected. Several important factors about the value of understanding short crack behavior include: (1) component life prediction requires a knowledge of the total life cycle of the crack including the early stages, (2) greater reliability is possible if the transition between short and long crack behavior is known since component life after this transition is short, and (3) remedial actions are often more effective for short than long cracks.

The experimental determination of the neutron cross section for selected solvents

Abstract: An experiment was conducted in order to determine the macroscopic neutron cross section of various solvents useful in neutron radiographic inspection. These solvents are used to dissolve the salts of high cross-section elements in order to formulate a fluid of maximal neutron cross section. The resulting fluid could then be used as a contrast enhancing agent in neutron radiographic inspection. This experiment was planned using statistical experimental design techniques. The results of this experiment provided a quantitative measure of the mean and standard deviation of the macroscopic neutron cross section for the nine fluids investigated. A control fluid was found to be in close agreement with published values.

Current ISI and monitoring technologies in European energy industries

Abstract: This paper reviews available information on the state of practice of inservice material property monitoring performed in European energy industries. Current practice includes a range of standard nondestructive testing and monitoring methods, improved remote sampling and surveillance specimen testing, and new monitoring techniques which are emerging from ongoing development programs. These new technologies include ultrasonic, positron annihilation, neutron diffraction, and magnetic techniques.