Showing papers in "Journal of Nondestructive Evaluation in 1993"

Recovery of microcrack parameters in mortar using quantitative acoustic emission

Abstract: A three-dimensional quantitative acoustic emission (AE) analysis of microcracking in unreinforced mortar beams was conducted. In order to facilitate the analysis of the large amounts of data generated by an AE test, a simplified method for the inversion of AE signals was developed. By applying the theoretical Green's function for an infinite space, the multichannel deconvolution normally required of AE data inversion reduces to a nonlinear curve-fitting problem. Using this procedure, microcracking in a mortar specimen was evaluated using a seismic moment tensor representation. Source-time functions for the microcracks were also recovered. The locations of the AE events were calculated, and damage localization was observed. The moment tensor analysis showed the dominant mode of microfracture to be mode II, with a limited number classified as mixed mode. A microstructural mechanism for this behavior is presented.

Synchronized reference updating technique for electronic speckle interferometry

Abstract: In this paper, a video-based speckle interferometric method using a continuous reference updating technique is presented. Unlike conventional ESPI techniques, this methodsynchronizes the optical interferometric detection system with the acoustic stressing of the test object, and includes continuous renewal of the reference image. It is shown that the susceptibility of the method to environmental noise caused by vibration, temperature gradients, or thermal currents is substantially lower than that of conventional techniques. The application of this technique to the detection of defects in adhesively bonded structures is demonstrated.

The transverse elastic impact response of thick hollow cylinders

Abstract: The two objectives of the studies in this paper were to determine the transient elastic impact response of thick-walled hollow cylindrical structures and to determine how this response was affected by the presence of flaws in the cylinder. These studies were aimed at determining the feasibility of using the impact-echo method for detecting flaws in the cylindrical concrete structures, such as pipes. Three-dimensional finite element models and laboratory specimens containing various types of flaws at known locations were used in the studies. It is shown that, for a hollow cylinder having a length greater than about eight times its outer diameter, the impact response as measured at points close to the impact point is composed of a number of resonant frequencies caused by cross-sectional (flexural) modes and a thickness frequency caused by dilatational waves reflected between the inner and outer wall surfaces. Equations are presented which relate the response of a hollow cylinder to the frequency of the fundamental cross-sectional (flexural) mode of a solid circular bar. It is shown that the location of cracks, areas of reduced wall thickness, and honeycombing as well as the depth of surface-opening cracks can be determined.

Crack Determination from Boundary Measurements-- Reconstruction Using Experimental Data

Abstract: In this work we assess the effectiveness of Electrical Impedance Tomography for determining the presence and the location of an interior crack from boundary measurements. Electrical Impedance Tomography uses boundary voltages and currents to image the interior of a region. We collect the data needed for this nondestructive evaluation technique by laboratory experiments and apply two numerical inversion algorithms to the data. Our experiments show that the data collected are sufficient to give good estimates of crack locations and crack sizes.

An improved method for magnetic identification and localization of cracks in conductors

Abstract: A SQUID magnetometer can be used to measure the magnetic field produced by flaws in a two-dimensional, conducting plate carrying a current. Identification of the flaw-induced magnetic field is difficult because of the large magnetic field associated with the edges of the plate and the current in the leads that connect the plate to the power supply. We have developed a technique by which the wire and edge fields can be cancelled prior to mapping the magnetic field. In this technique, a similar unflawed conducting sheet is placed adjacent to the flawed plate, with a connection between the sheet and the plate at one edge, and with the opposite edges of the sheet and of the plate connected to the two conductors of a coaxial cable. Thus, an applied current will flow along one conductor of the cable, across the cancelling sheet, cross into the flawed plate, return along the plate, and then return to the power supply along the other conductor of the coaxial cable. As a result of this geometry, there is no magnetic field from the lead-in wires because they are coaxial, and the magnetic field due to the edges of the plate is cancelled by the opposing magnetic field of the edges in the adjacent sheet. The extent of cancellation is determined primarily by the separation between the plate and the cancelling sheet, by the thickness of the plate, and by macroscopic inhomogeneities in their electrical conductivities.

On-line measurement of plastic strain ratio of steel sheet using resonance mode EMAT

Abstract: We have developed an on-line system for the measurement of plastic strain ratio of steel sheet using a thickness resonance EMAT that measures the thickness resonance frequencies of three different modes of bulk ultrasonic waves. The effective size of the EMAT is small, about 20 mm in diameter. A good correspondence was obtained between the in-plane average Young's modulus measured by this method and the in-plane average plastic strain ratio measured by tensile testing, with a standard deviation smaller than 0.04 in units of plastic strain ratio. The effects of temperature, lift-off, tensile stress, speed of the moving sheet and zinc coats were experimentally and theoretically analyzed. We show that none of them have a sizable effect on the measurement accuracy.

Industrial on-line texture determination in rolled steel strips

Abstract: In the manufacture of sheet metal, it is ofgreat importance that the quality should be homogeneous over the entire length of a strip. This can only be monitored using a continuous, on-line measuring method. A new X-ray transmission measuring technique has been developed for the nondestructive determination of texture-dependent technological data of rolled strip. It can be applied on-line for both hot and cold rolled metal strip, especially steel strip. The paper describes the measuring principle and gives information on the application for measuring ther m -values and the earing-values of cold rolled steel strip. With the data from these values, the user can control the deep drawing characteristics of his strips. The extension of the basic measuring principle to a condition-free determination system will be demonstrated with its application for measuring texture dependent technological data of hot and cold rolled steel strip. Long term experience at Hoesch Stahl AG, typical results measured over the total strip length, integration in quality assurance systems, questions of maintenance, etc. will be discussed.

Automated algorithm for the nondestructive detection of subsurface cavities by the IR-CAT method

Abstract: An algorithm is presented to automate the detection of irregular-shaped subsurface cavities within irregular shaped bodies by the IR-CAT method. The algorithm is based on the solution of an inverse geometric steady state heat conduction problem. Cauchy boundary conditions are prescribed at the exposed surface. An inverse heat conduction problem is formulated by specifying the thermal boundary condition along the inner cavities whose unknown geometries are to be determined. An initial guess is made for the location of the inner cavities. The domain boundaries are discretized, and an Anchored Grid Pattern (AGP) is established. The nodes of the inner cavities are constrained to move along the AGP at each iterative step. The location of inner cavities is determined by using the Newton Raphson method with a Broyden update to drive the error between the imposed boundary conditions and computed boundary conditions to zero. During the iterative procedure, the movement of the inner cavity walls is restricted to physically realistic intermediate solutions. A dynamic relocation of the AGP is introduced in the Traveling Hole Method to adaptively refine the detection of inner cavities. The proposed algorithm is general and can be used to detect multiple cavities. Results are presented for the detection of single and multiple irregular shaped cavities. Convergence under grid refinement is demonstrated.

A new technique for time-domain ultrasonic NDE of thin plates

Abstract: A new time-domain ultrasonic NDE technique is reported in this paper for the measurement of the thickness (given the wavespeed) or wavespeed (given the thickness) of thin plates; by thin we mean the thickness is less than the wavelength. By introducing a retrieve function the incident field can be reconstructed from the transmitted (reflected) field by a two-term (three-term) summation. A systematic sensitivity analysis of this technique has been carried out. The new technique has been used to measure the thickness or the wavespeed of aluminum plates with thickness ranging from 0.089 to 6.426 mm using (low frequency) 1-MHz transducers: 0.014≤(thickness/wavelength)≤1.0. The measurement error was found to be about 1% for thickness and 3% for the wavespeed.

Measurement of texture and formability parameters with a fully automated, ultrasonic instrument

Abstract: A fully automatic, ultrasonic instrument to measure texture and formability parameters on metal sheet is described. Arrays of EMAT transducers are used to transmit and receiveS o Lamb waves propagating at 0°, 45°, and 90° with respect to the rolling direction. By analyzing the frequency dependence of the phase of the received signals, the long wavelength limit of the velocities is obtained. Included is a discussion of this algorithm, and subsequent processing steps to predict the ODC'sW400,W420, andW440. On steel, the prediction of drawability parametersr and Δr based on a correlation developed previously by Mould and Johnson is also discussed. Results of blind field trials at facilities of three suppliers/users of steel sheet for automotive applications and one supplier of aluminum sheet for beverage can production are reported. The former confirmed the Mould-Johnson correlation for lowr material but indicated that refinements are needed for modern steels with highr. The aluminum data suggest a correlation between W440 and the degree of four-fold earing.

Microwave sensitivity maximization of disbond characterization in conductor backed dielectric composites

Abstract: A swept frequency microwave nondestructive technique for detection of disbond in a dielectric composite backed by a conducting plate is discussed. The theoretical development is based on an incident plane wave illuminating such a medium, and then formulating the characteristics of the reflected wave. This way the effective reflection coefficient of the medium is found. The phase of this reflection coefficient changes in the presence of a disbonded medium. With the determination of an optimally sensitive frequency range using the theoretical model and the known properties of the composite dielectric material, it is shown that disbonds on the order of a few microns can be accurately detected at relatively low microwave frequencies. Several experimental results are also presented.

On-line measurement of steel sheetr-value using magnetostrictive-type EMAT

Abstract: An ultrasonic on-line system to measurer values in cold rolled steel sheets has been developed with electromagnetic acoustic transducers (EMATs). These EMATs are composed of meanderline coils and electromagnets and operate with the magnetostrictive mechanism. The EMAT instrument measures the propagation times of the fundamental symmetrical Lamb (S o ) wave at a low frequency and relates them to ther values through a calibrated regression curve. Preliminary tests indicate that the on-line monitoring ofr is quite feasible with a standard deviation of 0.07 for various low carbon steel sheets; the thicknesses range from 0.5 mm to 2.5 mm. The measuring time is 20 msec per data. The liftoff is allowed to 5 mm with 2 mm thick sheet. Ther evaluation is independent of the line speed up to 325 m/min. These promising results promote installation of ultrasonicr-value measurement systems in steel production lines.

On-line determination of texture-dependent materials properties

Abstract: Macroscopic properties of polycrystalline materials may strongly depend on crystal orientation distribution, i.e., the texture of the material. This applies to all kinds of crystallographically anisotropic volume and boundary properties. The necessary texture parameters can be determined from a low number of intensity values measured with a fixed-angle, X-ray texture analyzer which is particularly suited for on-line determination. Alternatively, the texture-property relationship can be used to calculate the texture parameters from property measurement in different sample direction. On-line measurement of the texture can also be used as an indicator for other materials properties such as recrystallization or fatigue.

Broadband holography applied to eddy current imaging by using signals with multiplied phases

Abstract: In this paper an eddy current imaging method for nondestructive testing purposes is presented which utilizes the concept of broadband holography. An eddy current coil which is used simultaneously as an antenna for eddy current generation and as a probe for detection of response of interaction between eddy currents and flaws, respectively, is moved along a synthetic aperture during the imaging procedure generating synthetic eddy current pulses by scanning a certain frequency range. In terms of wave propagation phenomena the penetration depth (range) of eddy currents in conducting media is small compared to the equivalent wavelength of this type of fields. Therefore, adequate resolution can only be obtained in the reconstructed cross-sectional images by phase multiplication of received multifrequency signals, which is equivalent to a fictitious reduction of wavelengths. Experimental results verify the imaging capability of this method with improved resolution compared to conventional eddy current testing methods.

The Eddy Current Probe as a Holographic Transducer

Abstract: Experimental work with the application of holography to eddy current imaging has previously been performed. This paper presents a theoretical analysis of the holographic imaging characteristics of the eddy current probe. A specially shaped probe design has been developed during this analysis for enhanced imaging performance. The phase multiplied holographic imaging process is explained and demonstrated. Experimental data are presented confirming the theoretical analysis and thus the eddy current probe as a viable holographic transducer.

Methods to improve the accuracy of on-line ultrasonic measurement of steel sheet formability

Abstract: We report here on an ultrasonic system which quantifies factors which could degrade the resolution of the plastic strain ratio, orr-value, in sheet steel. We also present means to suppress these artifacts. We have developed a moving sheet device (MSD) to be used as a test bed to demonstrate the feasibility of on-line measurement ofr-value in a steel mill. The device can move specimens at speeds comparable to those in industrial practice. An automated velocity measurement system has also been developed and integrated with the MSD. This allows ultrasonic measurements to be made with an array of transducers. Measurements were made in both static and dynamic mode. Artifacts due to sheet motion were small, and should not significantly degrader-value resolution.

Ultrasonic beam propagation through a bimetallic weld: a comparison of predictions of the Gauss-Hermite beam model and finite element method

Abstract: In this paper, predictions of two models for the propagation of ultrasonic beams through a two-dimensional, bimetallic weld geometry are compared. The finite element method can predict beam propagation through quite general geometry with high accuracy. This model, however, requires significant computational time. On the other hand, the approximate Gauss-Hermite model offers considerably greater computational speed, but has lower accuracy in certain regions and cannot treat the most general geometries and inhomogeneities in material properties. This paper compares the performances of the two models for the case of the two-dimensional, bimetallic weld consisting of multiple layers, some of which have anisotropic properties. It is found that the results of the two models are in good agreement in the vicinity of the central ray, and that the deviation increases as one moves away from the axis. Also, as the beam propagates through multiple interfaces, the accuracy of the Gauss-Hermite solution decreases.

Pulsed laser energy through fiberoptics for generation of ultrasound

Abstract: Laser pulses are an effective, noncontacting technique for generating ultrasound in materials. However, for this approach to be practical, a versatile and safe method of delivering the laser pulses must be developed that eliminates exposed beams steered by mirrors and focused by lenses. Investigations by several researchers using fiberoptic delivery systems indicate that fiberoptics may be a viable method for the delivery of laser energy to generate acoustic energy. The main problem experienced with the fiberoptic delivery systems has been the inability to deliver high-energy, short-duration pulses via a fiber for thousands of pulses with no fiber damage and with constant energy output. This paper presents a technique for laser generation of sound using fiberoptics that continuously delivers sustained 20 ns pulses at a pulsing rate of 30 Hz from a doubled, Q-switched Nd:YAG laser operating at 532 nm with output energy from the fiber-optic system up to 26 mJ/pulse. The delivery system is used to excite ultrasound in a molten weld pool as part of a research effort to develop a noncontacting sensing system for real-time weld inspection.

Crack detection in conducting materials using SQUID magnetometry

Abstract: A highly sensitive multichannel SQUID gradiometer, optimized uniquely for spatial resolution, was used to map in two dimensions the magnetic field perturbations from circular holes and thin slots in conducting plates and strips, respectively. Holes as small as 0.75 mm radius in electrically conducting plates were measured directly using a single channel. Edge slots as short as 0.5 mm long in conducting strips were detected, using a multiple-channel measurement method which eliminated the effects from the edge of the strip and the current leads. Clear magnetic signatures from these simulated cracks were measured using low current densities. Numerical computations of the theoretical field distributions are presented which confirm the experimental results. Calculations indicate that much smaller defects should be detectable with an optimization of test parameters.

Development of methods for on-line texture analysis

Abstract: This work presents results obtained recently as part of the ongoing design of laboratory-scale equipment which will be used to develop and test methods for on-line texture analysis. The equipment is in the process of being built. A range of cold rolled and annealed IF deep-drawing steels were fully characterized using standard X-ray diffraction texture methods. They were then analyzed to obtain information which will be used to help optimize the measurement configuration of the laboratory-scale equipment for both a transmission and a reflection X-ray diffraction geometry. Texture measures in both geometries were then identified which are sensitive to texture changes produced by both cold rolling and annealing. The measures were then used to attempt some correlation between them and plastic properties calculated from the coefficients of the orientation distribution function (ODF). Satisfactory agreement between the texture measures and the average plasticity,R, were obtained for both the transmission and reflection geometries.

Crystallography-based prediction of plastic anisotropy of polycrystalline materials

Abstract: The on-line prediction of metal sheet formability requires that both material characterization (texture identification) and yield loci predetermination be done in very shor time intervals. Of two applicable approaches, i.e., continuum mechanics and crystallography-based methods, only the latter are suitable for this purpose. Several models of plasticity of a polycrystalline material were reviewed, and their applicability to the prediction of plastic anisotropy of face-centered cubic (FCC) metals was evaluated. A tailored set of cold-rolled copper alloy samples was designed and manufactured, representing the wide spectrum of textures and cold work levels typical for the sheet metal industry. The texture was quantitatively described in the form of the orientation distribution functions derived by the inversion of four incomplete pole figures. The Taylor-Bishop-Hill model was applied in order to calculate the planar variation of the plastic strain ratio. The continuum mechanics of textured polycrystals approach was also used for the prediction of the plastic strain-rate ratio for the same set of deformed materials. The theoretical predictions were compared with the plastic strain ratios measured in tensile tests using strain gauges. The applicability of the models for prediction of the plastic anisotropy of FCC metals was discussed in view of the operating deformation mechanisms and other factors such as strain hardening sensitivity and grain size/shape effects.

Ultrasonic prediction of R-value in deep drawing steels

Abstract: The textures of five types of deep drawing steels were measured and analyzed using the series expansion method. Electromagnetic acoustic (EMAT) techniques were employed to determine the elastic anisotropy in terms of the angular variation of the ultrasonic velocities. The series expansion formalism was employed for predicting the elastic and plastic anisotropies from texture data. Comparison with the experimental measurements of Young's modulus indicates that the elastic energy method can accurately reproduce the elastic anisotropy if the single crystal elastic constants are appropriately chosen. The angular variation of ther-value in the rolling plane was calculated from the ODF coefficients by means of the pancake relaxed constraint model using an appropriate CRSS ratio for glide on the {112} 〈111〉 and {110} 〈111〉 slip systems. The fourth order and first sixth order ODF coefficients were calculatednondestructively from the anisotropy of the ultrasonic velocities. The calculated pole figures based on the ODF coefficients obtained in this way are similar to those derived from complete X-ray data. It is shown that the plastic properties of commercial deep drawing steels are predicted more accurately when the 4th-and 6th-order ODF coefficients are employed than when only the 4th-order ones are used.

Application of nondestructive techniques for the prediction of elastic anistropy of a textured polycrystalline material

Abstract: Effect of processing parameters including cold work and grain size on texture development in copper and 68:32 α-brass thin sheets is investigated. The preferred orientation of grains (texture) is represented quantitatively by the orientation distribution function (ODF) as derived either by X-ray diffraction or by ultrasonic velocity measurements. The texture-induced elastic moduli are derived in the sheet plane at various orientations to the rolling direction. The predicted values of elastic moduli are compared with the mechanically measured ones. An attempt is made to empirically correlate ODF coefficients to the mechanically measured elastic modulus and plastic strain ratio (r-value). The sources of discrepancies such as the limitations of the measuring techniques and the models applied are discussed.