Showing papers in "Experimental Mechanics in 1993"

Accurate measurement of three-dimensional deformations in deformable and rigid bodies using computer vision

Abstract: Recently, digital-image-correlation techniques have been used to accurately determine two-dimensional in-plane displacements and strains. An extension of the two-dimensional method to the acquisition of accurate, three-dimensional surfacedisplacement data from a stereo pair of CCD cameras is presented in this paper. A pin-hole camera model is used to express the transformation relating three-dimensional world coordinates to two-dimensional computer-image coordinates by the use of camera extrinsic and intrinsic parameters. Accurate camera model parameters are obtained for each camera independently by (a) using several points which have three-dimensional world coordinates that are accurate within 0.001 mm and (b) using two-dimensional image-correlation methods that are accurate to within 0.05 pixels to obtain the computer-image coordinates of various object positions. A nonlinear, least-squares method is used to select the optimal camera parameters such that the deviations between the measured and estimated image positions are minimized. Using multiple orientations of the cameras, the accuracy of the methodology is tested by performing translation tests. Using theoretical error estimates, error analyses are presented. To verify the methodology for actual tests both the displacement field for a cantilever beam and also the surface, three-dimensional displacement and strain fields for a 304L stainless-steel compact-tension specimen were experimentally obtained using stereo vision. Results indicate that the three-dimensional measurement methodology, when combined with two-dimensional digital correlation for subpixel accuracy, is a viable tool for the accurate measurement of surface displacements and strains.

Electron beam moiré

Abstract: A method of writing very high frequency line and dot pattems, in excess of 10,000 lines/mm, is described. This method uses a very small diameter, 10 to 20 nm, beam of electrons to sensitize a 100-nm thick layer of electron resist. The line and dot patterns are produced by etching the sensitized resist. Moire fringe patterns occur when the line arrays are observed in the scanning electron microscope. Moire fringes with excellent contrast have been produced at magnifications as high as 1900x. This capability permits e-beam moire to be employed in micromechanics. Examples of line arrays, dot arrays and moire fringe patterns on a brass disk and on a tensile specimen fabricated from glass-fiber-reinforced plastic are demonstrated to introduce the possibilities for micromechanics applications.

The effect of residual stresses on hardness measurements

Abstract: The RockwellC hardness,RC, was measured as a function of position on steel rings with different residual-stress profiles through the thickness. An experimental correlation between residual stress andRC was obtained. A relationship between the average pressurep of a spherical indenter, the yield strengthSy and the residual stress of the material was conceived and used in fitting the experimental data.

Micromechanical fatigue testing

Abstract: This paper describes the design, modeling, and experimental test results of a single crystal silicon micromechanical device developed to evaluate fracture and fatigue of silicon based micromechanical devices. The structure is a cantilever beam, 300 microns long, with a large silicon plate and gold inertial mass at the free end. Torquing and sensing electrodes extend over the plate, and with associated electronics, drive the structure at resonance. Fatigue crack propagation is measured by detecting the shift in the natural frequency caused by the extension of a preexisting crack introduced near the fixed end of the cantilever. Experimental data are presented demonstrating time-dependent crack growth in silicon. Crack extensions of 10 to 300 nm have been measured with a resolution of approximately 2.5 nm, and crack tip velocities as low as 2.1×10−14 m/s. It is postulated that static fatigue of the native surface silica layer is the mechanism for crack growth. The methodology established here is generic in concept, permitting sensitive measurement of crack growth in larger fatigue specimens as well.

New test method for obtaining softening response of unnotched concrete specimen under uniaxial tension

Abstract: A new method was developed to obtain the complete stress-deformation response of an unnotched concrete specimen. This method employs a digitally controlled closed-loop testing machine and five control channels: stroke LVDT and four LVDTs mounted on the specimen. The test portion of the specimen was fully spanned by these four LVDTs. The outputs of these control channels and the load cell were monitored during the test by a computer equipped with LabTech's Notebook software. A ‘C’ language program was written to enable quick switching of the mode of control from one LVDT to another. The problem of uncertainty in the location of the major crack was tackled by the said LVDT arrangement and the computer program. It was demonstrated that it is always possible to obtain stable post-peak response provided one ensures that at any time during the test, the feedback used is the LVDT that exhibits, at that time, the largest slope of the response-time curve. An acoustic emission (AE) measurement system of six channels was also used in the experiments. Monitoring of signals from the AE transducers provided valuable information which helped in making the decision to switch control.

The application of interferometric techniques to the nondestructive inspection of fiber-reinforced materials

Abstract: A method for increasing the sensitivity of dynamic materials evaluation (DME) to localized damage in fiber-reinforced composites was examined. To obtain this improved sensitivity, different aspects of DME were examined. These included an increase in the frequency used to evaluate the dynamic properties, utilization of mode-shape information and different procedures for evaluating the experimental data.

Force identification from dynamic responses of a bimaterial beam

Abstract: The dynamic response due to impact of a two-dimensional bimaterial beam system is described in terms of a waveguide model involving two displacements and a rotation. Coupled with spectral analysis, this allows measured signals to characterize the waves propagating in the system; this in turn, is used to predict responses at any locations. In particular, the impacting force can be reconstructed. The results are demonstrated for an aluminum/expoxy system.

A Kolsky bar: Tension, tension-tension

Abstract: The present paper introduces a new technique which combines rotation disk and traditional Kolsky bar (often termed as split-Hopkinson bar). This technique can be employed to study the tension stress-strain relations and tension-unloading-tension strain-rate history effects of materials in the strain rate range from 102–103s−1. The rise time of the incident wave is as short as 15 μs because of the particular design. An attempt is made to estimate strain error caused by the thread connection between the specimen and the bars, and stress error due to the mismatch of the cross section of the specimen and bars. A short rise-time incident wave appears to be most advantageous in view of maintaining the accuracy of the stress-strain curve obtained near the initiation. Preliminary tests are performed on the instrument. Comments are made for this design configuration.

Scanning moire at high magnification using optical methods

Abstract: Methods of employing scanning moire at high magnification are developed and demonstrated. Modern lithographic techniques for producing custom moire gratings with a frequency up to 250l/mm are described. On a probing station equipped with a video system, pseudo-color moire fringes are produced using the scannning lines of the color charge-coupled-device (CCD) camera. Fringe multiplication from 1 to 5 is possible with correct combinations of magnification and grating pitch. An analysis is given to show that strain sensitivity depends only on the number of scanning lines used to record the image. The grating pitch and the magnification are important because they reduce the gage length of the strain measurement. The high-magnification scanning moire was used to study plastic- strain fields in an aluminum tensile specimen. Local disturbances in the strain field were observed at 2 to 2.5 percent applied strain. These discontinuities became more significant at higher levels of applied strain.

Grip pressure distribution under static and dynamic loading

Abstract: The dynamic response of a vibrating handarm system is strongly related to the grip force. While the relationship between total grip force and vibration characteristics of the hand-arm system has been extensively studied, no attempts have been made to investigate the distribution of grip pressure at the hand-handle interface. The local grip-pressure distribution may be more closely related to the finger blood flow, fatigue and loss of productivity than total grip force. In the present study, distribution of static and dynamic forces at a hand-handle interface is investigated using a grid of pressure sensors mounted on the handle. The pressure distribution is acquired for different values of static and dynamic grip forces in the range of 25–150 N. The dynamic measurements were conducted at various discrete frequencies in the 20–1000 Hz range with peak acceleration levels of 0.5 g, 1.0 g, 2.0 g and 3.0 g. The grip-pressure distribution under static loads revealed a concentration of high pressures near the tips of the index and middle fingers, and the base of the thumb. This concentration of high pressures shifted towards the middle of the fingers under dynamic loads, irrespective of grip force, excitation frequency and acceleration levels. These local pressure peaks may be related to impairment of blood flow to finger tips and the possible causation of vibration white finger.

Numerical and experimental investigation into the fatigue behavior of plain concrete

Abstract: Fatigue of concrete has received considerable attention in the last 15 years. The investigations, however, have been mainly phenomenological. Therefore, the knowledge about the cause and mechanism of concrete fatigue is still limited. This paper reports about a research project in which fatigue of concrete is studied by applying nonlinear fracture mechanics. With a description of the crack cyclic behavior of concrete, crack growth could be studied numerically. Qualitatively promising results have been found. A number of fatigue experiments are described in which the development of deformations with the number of cycles is followed as accurately as possible. For that purpose, a data-acquisition system was specially designed.

Phase-measuring Profilometry Using Sinusoidal Grating

Abstract: When a sinusoidal amplitude grating is projected on an object, the surface-height distribution of the object is translated to a phase distribution of the deformed grating image. In this paper, two algorithms developed for phase acquisition of such images are presented and compared. The phase-acquisition algorithms are sufficiently simple that high-resolution phase maps using a highresolution area detector array can be generated in a short time. The average detection error is within 30 mm, which can be reduced further by changing the period of the projected grating and the angle offset between the projection and the observation optics.

Geometric methods in determining rigid-body dynamics

Abstract: This research develops a measurement system using linear accelerometers to determine the three-dimensional, six degrees of freedom, impact response of an anthropomorphic test device (dummy). A procedure using spherical geometric analysis (SGA) was developed. It uses three triaxial accelerometer clusters for determining angular velocity, angular acceleration, and linear acceleration. SGA differs in its calculation of angular velocity from other procedures which determine rigid-body motion. Unlike procedures which use linear accelerometers to determine angular velocity by integration of angular acceleration, SGA uses the topology of the sphere to obtain both angular acceleration and angular velocity through algebraic manipulation of the output from the linear accelerations. The validation of SGA is accomplished by the use of hypothetical as well as experimental data.

Thermal residual stresses in thick graphite/epoxy composite laminates—Uniaxial approach

Abstract: Thermal residual stresses have been known to be very large in laminates of continuous-fiber-reinforced polymer composites. When the thickness of the laminate is large, however, the measurement of the residual stresses raises questions on the accuracy of the conventional methods. A novel concept of layer separation is developed to measure quantitatively and precisely the tensile residual stresses in thick plates with layered distribution of residual stresses. It is applied to thick [O2/9O4]13s, AS/3501-6 graphite/epoxy laminates. The test specimens were mechanically modeled into the thin strips for the application of the new concept of layer separation. The tensile residual stresses measured in the 90-deg layers of these laminates are nonuniform throughout the specimen, and vary from 55.6 MPa to 71.4 MPa. It is very interesting to compare these values with the transverse strengthF 2 tu of AS/3501-6 unidirectional composites, which is 65.4 MPa.

Mixed-mode crack-tip deformations studied using a modified flexural specimen and coherent gradient sensing

Abstract: An optical mapping of deformation fields and evaluation of fracture parameters near mixed-mode cracks in homogeneous specimens under elastostatic conditions is undertaken. A modified edge notched flexural geometry is used in the study and its ability in providing a relatively wide range of mode mixities is demonstrated. A full-field, optical shearing interferometry called ‘coherent gradient sensing’ (CGS) is used in the study. Crack-tip parameters such as stress-intensity factors, mode mixity and energy-release rate are measured from the interference patterns. The patterns are analyzed using Williams' mixed-mode, asymptotic expansion field. An expression for energy-release rate for the specimen is also derived using beam theory. The theoretical stress-intensity factors are then obtained using a mode-partitioning method based on moment decomposition. Experimental measurements and theoretical predictions are found to be in good agreement. Limitations of the mode-partitioning method used in the investigation are also pointed out.

On the effects of through-thickness thermal conduction on stress measurement by thermoelastic techniques

Abstract: Stress measurement by means of the thermoelastic technique has been growing in popularity since the mid-1980s. This paper looks in detail at the effects of thermal conduction on such measurements. It is shown that for accurate quantitative stress determination, on all but the simplest of geometries, thermal-conduction effects may have to be considered. Such effects can be experimentally observed and modifications which can be made to an experiment affected by thermal conduction are suggested.

Polarimetry for spatiotemporal photoelastic analysis

Abstract: This paper describes a new photoelastic technique for the spatiotemporal stress analysis. In a polarimeter developed, an elliptically polarized signal beam of light, modulated in state of polarization by two-dimensional principal-stress distributions interferes with a reference beam of light consisting of orthogonal linearly polarized two components. A time-sequential series of two-dimensional interference patterns are received one after another by a MOS video camera, followed by a computer. Of the elliptically polarized signal beam, the orthogonal field components along the directions of the principal stresses in a two-dimensional photoelastic sample can be computed from a recorded interference pattern, which offer the data needer for mapping the spatiotemporal principal-stress distribution over the sample. Not only each of the two orthogonal principal stresses but also the principal-stress difference are mapped in a time-sequential diagram. No use of any movable polarization element such as a rotating analyzer allows us to follow a rapid change in stress distribution within the maximum frame rate 2066 s−1 of the MOS video camera.

Automated fringe pattern analysis for moiré interferometry

Abstract: The theory of an automated fringe-pattern analysis using temporal phase stepping method for moire interferometry is presented. The automated method provides a fast and accurate strain measurement for moire interferometry. Measurements on a tapered composite specimen with a dropped ply are shown to highlight the capability of this technique to obtain the strain distribution, particularly around the region with a dropped ply.

Characterization of shock accelerometers using davies bar and strain-gages

Abstract: This paper proposes a novel method for evaluating the dynamic characteristics of shock accelerometers under high acceleration levels and a wide frequency bandwidth. High accelerations of 103∼105m/s2 can be generated by the reflection of an elastic wave pulse propagating in a metal bar known as the Davies bar. The elastic wave pulse is produced by the collision of a projectile against one end of the bar, and is detected by straingages. The accelerometer to be characterized is attached to the other end of the bar. The one-dimensional theory of elastic waves enables the derivation of an input acceleration to the accelerometer from the measured strain. The dispersion of the elastic waves caused by the lateral inertia of the bar is compensated for by using a two-dimensional analytical solution. This method was validated by an experiment characterizing a piezoelectric-type accelerometer within the frequency band approximately 1 kHz∼70 kHz.

Filtering of the experimental data in plane stress and strain fields

Abstract: The paper presents a filtering algorithm which corrects the results of measurements of strain and stress fields in order to satisfy the fundamental equations of the continuum. It is proved that the algorithm is very efficient and requires small computational time. The developed filter can be used to correct the measuring data from any experiments provided some additional information about the measured system is available.

Nondestructive evaluation of model adhesive joints by PVDF piezoelectric film sensors

Abstract: Metallized poly(vinylidene fluoride) (PVDF) films can be etched into nondestructive evaluation (NDE) sensor devices. Since these sensors are relatively inexpensive, thin and lightweight, they can be attached permanently to adhesively bonded joints, laminated composites, and other structures to measure structural integrity. The present study has addressed techniques to design, attach, and utilize such sensors for adhesive joint and laminated composite applications. PVDF sensors have been successfully used as NDE transducers in pulse-echo, through-transmission, and acousto-ultrasonic techniques to monitor curing, and to detect porosity and crack propagation in different model joint geometries. Feasibility of several applications has been demonstrated, although several problems remain. The potential of using these techniques for practical bonded structures is also suggested.

Enhancement of the shadow-moiré method through digital image processing

Abstract: Surface topography can be conveniently investigated by a well-known shadow-moire technique. It is relatively free from stringent requirements on stability of instrumentation and quality of the light source. The main unresolved problem of the shadow moire is its relatively low resolution. The digital-image-analysis enhanced shadow-moire technique described here is based on the marriage of two well-known and developed methods: classical shadow moire and contemporary digital image analysis. This combination allows a significant increase of the sensitivity of the classical shadow moire by utilizing the ability of the digital-image-analysis system to acquire wholefield light-intensity information and process this information to yield the related displacement field. The suggested approach was utilized for a three-dimensional curved surface. The obtained topography of the three-dimensional specimen was compared to the one measured by a high precision dial gage and the differences were on average less than three percent between the two sets of data. The described combination of the shadow moire and digital image analysis may prove as a useful tool for on-line quality control in a variety of manufacturing processes.

Fiducial Grid for Measuring Microdeformation Ahead of Fatigue Crack Tip Near Aluminum Bicrystal Interface

Abstract: Fiducial grids have been applied to measure micro-deformation near interfaces in aluminum bicrystals and to study mechanisms of fatigue crack transfer across the interface under loading in a scanning electric microscope (SEM). Three types of aluminum bicrystals were designed to allow the primary slip to induce different incompatible plastic strain at the interface. An inclined notch was spark cut along the primary slip band on one edge of each specimen to initiate a crack smoothly. This paper also describes the means to produce fiducial grids of evaporated gold with 7-μm square mesh size and 0.3-μm line width in a place of interest on a specimen surface, usually ahead of a crack tip near the interface. Interface cracking and width increase of persistent slip band (PSB) induced by an approached transvese crack have been identified by comparing two photos of fiducial grid patterns taken, respectively, at maximum and minimum cyclic stress in a SEM. Grids were also used to measure both cyclic shear strain at a crack tip and crack growth rate, which decreases as the crack approaches an incompatible interface.

Experimental investigation of phase-strain-temperature models for structurally embedded interferometric fiber-optic sensors

Abstract: Analytical models relating the optical phase change in a structurally embedded optical fiber sensor to the strain and temperature state integrated along that sensor path is investigated in this paper. Generalized plane strain elasticity solutions are combined with two phase-strain-temperature models to predict the thermomechanically induced optical retardation of the light propagating in the fiber sensor. These predictions are compared with experimental data obtained from Mach-Zehnder and Fabry-Perot sensors embedded in transverse compression and uniformly heated specimens respectively.

Geometric moire methods with enhanced sensitivity by optical/digital fringe multiplication

Abstract: A robust fringe-shifting and image processing scheme is applied to geometric moire experiments. High quality contour maps of U, V and W displacement fields are obtained with sensitivity enhanced by a factor of ten. The method is compatible with complicated intensity distributions, variable bar-to-space ratios of gratings, and optical noise. It is applicable to large fields for both in-plane moire and shadow moire measurements.

Mechanical testing using direct control of the inelastic strain rate

Abstract: A methodology is described for carrying out mechanical tests under direct control of the inelastic strain rate. The method depends on the use of a servohydraulic testing machine and suitable electronic-control circuitry. The technique permits the use of abrupt changes of inelastic strain rate as well as maintenance of constant values of inelastic strain rate. The application of the method to several modes of testing is described.

Holographic interferometry measurements of transient bending waves in tubes and rings

Abstract: Propagating bending waves are studied in a tube of steel and in a ring of aluminum. The waves are generated by the impact of a ballistic pendulum. Holographic interferometry, with a double-pulsed ruby laser as light source, is used to record the waves. A conical mirror is placed axially inside the tube. Axial illumination and axial observation directions, make it possible to view all sides of the tube simultaneously with a high sensitivity to radial deformation. The interferograms, which have an unusual perspective, are captured with a CCD-camera and then spatially transformed into an unwrapped strip of the tube wall. This makes the interpretation of the measurements simpler. The geometry of the tube causes the wave pattern to propagate with different speed and amplitude along and across the tube, even when the material itself is isotropic. A finite-element simulation of the impact is compared to the corresponding experiment. An impact on a ring with a defect is performed in order to study the effect on the wave pattern. The proposed method could be used in nondestructive testing of pipes.

Energy-dissipation mechanisms associated with rapid fracture of concrete

Abstract: A hybrid experimental-numerical procedure, involving moire interferometry and dynamic finite-element analysis, was used to analyze rapid crack growth in an impact loaded three-point-bend concrete specimen with an offset straight precrack. The dissipated energy rates in the fracture process zone (FPZ), which trails the rapidly extending crack, and in the frontal FPZ ahead of the crack tip, the kinetic-energy rate and energy-release rate were computed. The results showed that while the trailing FPZ was the dominant energy dissipation mechanism, much of the released energy was converted to kinetic energy in the fracturing concrete specimen.

An experimental study of interfacial crack kinking

Abstract: The growth of a crack located at the interface between two linearly elastic solids is investigated experimentally. It is found that the crack may advance by kinking into either of the adherends or by propagating along the interface itself, depending on the applied loading conditions. The experimentally observed kinking behavior is compared with analytical results. Agreement between these results is improved by suitable manipulation of a presumably material characteristic crack-extension length, the origin of which is rooted in the linearized analysis. The influence of material rate effects on the crack-kinking behavior is also investigated.

Optimal orientation of flakes in oriented strand board (OSB)

Abstract: Panelized housing can reduce both the cost and construction time of residential housing. The panels, generally comprised of rib-reinforced OSB sheets, are assembled into floor, wall, and root structures. Irrespective of application, commercially available OSB sheets incorporate flakes oriented generally in the longitudinal direction along the faces and in the cross direction in the core. This often results in directional material properties that are not ideally suited for the specific application, requiring excessive sheet thickness and flake density in order to provide the necessary strength. It is proposed that through proper orientation of flakes, material properties of OSB can be tailored to a specific application, resulting in appreciable improvement in load-bearing capacity over commercially available OSB of equal density. In this paper we present a method for determining the near-optimal orientation of flakes in OSB sheets under specific loading. Using three-point bending and rupture tests, we experimentally verify that OSB with optimally oriented flakes is significantly superior to commercially available OSB of equal density.