Showing papers in "Experimental Mechanics in 1988"

Process zone and acoustic-emission measurements in concrete

Abstract: It is necessary to study the microlevel failure mechanisms of a material in order to improve its quality and to develop a rational constitutive model to describe the material. Nonlinearity and strain-softening behavior of concrete has to be incorporated into any model which can be implemented into efficient design. Acoustic-emission (AE) techniques are useful for obtaining information pertaining to internal cracking and investigating the applicability of a particular material model.

Visualization of the Abrasive-Waterjet Cutting Process

Abstract: Cutting with abrasive waterjets was visualized in three types of materials: Lexan, Lucite and glass. Movie cameras were used at speeds of 64 and 1000 frames/s to record sequences of the jet penetration in these materials. It was found that the cutting process consists of two basic modes of erosion. The first, known as the cutting-wear mode, occurs at relatively shallow angles of impact. This mode results in a steady-state jet-solid interface. The other mode, the deformation-wear mode, occurs at large angles of impact and results in an unsteady penetration zone. The relative contribution of each of these two modes or mechanisms to material removal depends on the process parameters. The cutting process is cyclic in nature when the deformation-wear mechanism is partially or totally contributing to cutting. Qualitative and quantitative results of these visualization experiments suggest a mechanistic model for the penetration process. The results of this work may also be expanded to explain other ‘stream-like’ cutting-tool processes, such as laser and flame cutting.

Dynamic measurements of initiation toughness at high loading rates

Abstract: An experimental method is described for measuring the dynamic initiation toughness of a sharp stationary crack. A plane specimen is utilized which consists of a central region 50-mm wide and 200-mm long with integral dog-bone ends. The loading is accomplished by the detonation of four small explosive charges which produce two tensile stress waves upon reflection from the dog-bone ends. The stress waves meet at the midpoint of the specimen and reinforce to produce a relatively large, uniformly stressed region with a very high loading rate. The crack is positioned at the midpoint of the specimen at the location where the reinforcing tensile stress waves meet. A series of photoelastic experiments were conducted using Homalite 100 as the model material to observe, in a full-field view, the arrival of the dilatational waves, the subsequent development of the stress field at the tip of the stationary crack and the initiation of the crack. The isochromatic fringe pattern was also used to determine the instantaneous value of the stress-intensity factorK(t) after the characteristic fringe loops developed in the region near the crack tip. Finally,K(t) was measured using a single strain gage positioned and oriented so that its signal output was proportional toK(t) and independent of the next two higher order terms in the series representation of the strain field. A method was developed to determine the instant of initiation from the strain-time trace. Results obtained from the photoelastic and strain measurements of the dynamic-initiation toughnessK ID were consistently higher than the static value ofK IC .

Fracture energy for three-point-bend tests on single-edge-notched beams

Abstract: Three test series of single-edge-notched beams in three-point bending are conducted to evaluate the fracture energy of concrete. The fracture energy is determined from the area under the complete load: load-point deflection diagram. The nonlinear fictitious crack model is implemented in a finite-element analysis showing good agreement with the experimental data. By varying the notch depth and the beam depth it is shown that the fracture energy, traditionally presented as a material property, depends upon the specimen size and configuration. This is atributed to the energy dissipation in the process zone which is not accounted for in the analytical model.

A new technique for ultrasonic-nondestructive evaluation of thin specimens

Abstract: Combining standard FFT methods with conventional ultrasonics, a method has been developed for measuring the phase velocity, the group velocity and the attenuation in ultrathin specimens (submillimeter or subwavelength in thickness). A detailed description of this technique is given. The technique was used on four disparate materials: aluminum, an epoxy, a particulate composite and a graphite-fiber/epoxy composite. The method works equally well for thin or thick specimens, and for dispersive as well as nondispersive media.

Mode II fracture-parameter estimates for concrete from beam specimens

Abstract: At the present time, work on determination of fracture parameters for concrete subjected solely to mode I deformation states has advanced to the point where standard methods of testing and data evaluation have been proposed. Thus, many researchers are now considering the dual problems of (1) determination of fracture parameters for mode II deformation and (2) the need for this information (or put another way—the possible application of this information). In fact, the physical testing arrangements for mode II testing with no influence of mode I are not obvious; compromises must be made which will lead to approximations—perhaps fairly gross—in the estimates of mode II fracture parameters. Herein are presented experimental and analytical results for fracture parameters for beams loaded in antisymmetric four-point bending with a single-edge starter notch located in a region of high-shear and low-bending moment. This study complements another, presented elsewhere, in which beams in three-point bending were used with notches located off center. From this study, which included tests on 18 beam specimens, the following conclusions are reached. The beams must be tested in strain control. Crack initiation started in mode II but quickly changed to a mixed mode (although finite-element calculations indicate mode I is dominant). The fracture energy associated with the onset of unstable crack propagation is much larger than normally obtained for mode I—this is thought to be due to extensive aggregate interlock forces not present in normal bending or direct tension tests. Probably this type of testing arrangement is not suitable for determination of mode II fracture parameters because propagation does not occur under mode II conditions.

Biaxial testing of fiber composites using tubular specimens

Abstract: The failure of advanced continuous fiber laminates is not well understood. Stresses on a ply level are usually multiaxial, and reliable rules for the prediction of laminate failure are not generally available. Experiments to measure laminate-failure properties are difficult to perform and to relate to composite structures, being complicated for example by stress concentrations at the free edges of laboratory specimens. A multiaxial laminate specimen based on internal pressure and axial-force loading of a cylinder has been developed that appears to give reliable response and failure data. The purpose of this paper is to present in detail the design and show the supporting analysis of the specimen. In addition recent results on the failure of carbon/epoxy laminates are reviewed.

An overdeterministic approach for measuring K(I) using strain gages

Abstract: An overdeterministic method for determining the opening-mode stress-intensity factor,K I , from many measurements of the radial strain,∈ rr , is described The method was verified with an experimental study of a compact-tension specimen where strains along the 0, 45 and 90-degree lines were measured using strip gages with ten strain sensors per strip gage The results indicated errors in the range of one to three percent with three or four parameter models of the strain field in the region near the crack tip

Damage assessment based on the structural frequency-response function

Abstract: A method is presented for determining both the amount and position of damage present in members which can be modeled as longitudinally vibrating uniform beams. The method is valid for all boundary conditions, and for simplicity, is applied to the free-free case. It uses the shift in the natural frequencies of vibration, which are determined from the structural frequency-response function, caused by the damage present in the beam. These altered values of frequency are then utilized in a graphical solution technique which predicts the damage location directly and provides a parameter whose value is related to the magnitude of the damage present.

Comparison of the techniques of transmitted caustics and photoelasticity as applied to fracture

Abstract: A comparison of the optical techniques of photoelasticity and caustics as applied to fracture mechanics is presented. Static as well as dynamic experiments were conducted using the two methods with the same specimen geometry and under the same loading conditions. The results for the crack-tip stress-intensity factor as a function of crack length were obtained from both techniques in three different specimen geometries. A comparison of the results shows good agreement under static conditions but large differences for running cracks.

Interferometric-fiber-optic strain sensor

Abstract: An interferometric-fiber-optic sensor and an efficient fringe-detection scheme are described. The fiberoptic interferometer consists of two fibers; they are labeled the reference fiber and the sensing fiber. The reference fiber is arranged in a circular pattern, whereas the sensing fiber is arranged in an ‘S’ pattern. These fibers are exposed to the same strain field and each experiences a strain-induced phase shift. A difference in the phase shift between the two fibers indicates a change in strain.

Crack profiles in mortar measured by holographic interferometry

Abstract: In order to test the accuracy of theoretical fracture models for mortar and concrete, it is necessary to have accurate measurements of the crack profiles. In this study, sandwich holographic interferometry has been used to find the crack profiles in a center-notched plate specimen loaded at the center of the notch. The results have shown that at low load levels with corresponding short crack lengths, there is little difference between the measured crack profiles and elastic crack profiles computed by finite-element analysis. At high loads with long crack lengths there is a large difference between measured and computed elastic crack profiles. The data suggest the presence of a closing pressure at the crack tip and that there may be a limit to crack-tip-opening displacement (CTOD) before the crack propagates.

Interferometric Testing at Very High Temperatures by TV Holography (ESPI)

Abstract: This paper describes how TV-holography or electronic-speckle-pattern interferometry (ESP) can be used to monitor and measure with interferometric sensitivity the surface behavior, vibrations and deformations of objects heated to temperatures up to +3000°C. If problems due to rapid microstructure changes and the object actually melting are disregarded, the temperature limits are determined by the accessible laser illumination due to the increasing background radiantion.

The development of a modified double-cantilever-beam specimen for measuring the fracture energy of rubber to metal bonds

Abstract: Rubber to metal bonds are important in a variety of automotive, tire and marine applications. A new technique is discussed for measuring the strain-energy release rates of these adhesively bonded joints in the presence of harsh environments. Guidelines for design and applications are given.

High-rate decremental-strain-rate test

Abstract: A modified torsional split-Hopkinson bar is intoduced and used to study material response associated with a sudden reduction of stain rate during high-rate plastic deformation. In tests on 1100-0 aluminum iniial deformation at a strain rate of approximately 2400 s−1 is reduced by a factor of 15 after 200 μs of high-rate deformation. After the reduction, the deformation continues at the low rate for additional 550 μs. The change in the strain rate is obtained by using a stepped input bar. The results for 1100-0 aluminum show a decrease in the flow stress following the reduction in the strain rate. A short delay exists between the beginning of the strain-rate reduction and the response of the stress. The magnitude of the drop in the stress agrees with the difference in flow stress expected in constant-strain-rate tests in the corresponding high- and low-strain rates. Following the stress reduction. The stress remains essentially constant with no hardening during the subsequent deformation at the low rate.

The improvement of end-boundary conditions for off-axis tension specimen use

Abstract: Inaccuracies in using the off-axis tension test are caused by the nonuniform stresses produced by the end constraints. The purpose of this study is to show that these nonuniformities can be virtually eliminated by adjusting the amount of tab clamping and selectively locating the point about which the clamp may rotate.

Projection-speckle digital-correlation method for surface-displacement measurement

Abstract: A whole-field projection-speckle technique to measure out of plane displacements of initially flat structures has been presented. The method is based on digital correlation of images obtained from a TV camera. An improved digital-correlation function has been derived and described along with the analysis scheme. An instrumentation system based on a desktop computer has been set up to implement the method. Experimental results from simple static tests have been used to validate the technique.

Measurement of residual stress by the hole-drilling method: General stress-strain relationship and its solution

Abstract: When a hole is drilled at any position with respect to the gages in a thin plate, a general relationship of relieved strain as a function of residual stress is presented. A simple and explicit solution for the principal residual stresses and their directions is also presented. This solution is available for the center or off-center hole-drilling cases and for the case of which the array of gages is arbitrary.

Automated analysis of holointerferograms for the determination of surface displacement

Abstract: A method for using an image digitizing system with a minicomputer to automatically determine surface-displacement fields by evaluating holographic fringe patterns is presented. The problem of determining the sign of the displacement vector as well as the need to produce a monotonic phase change across the surface under observation is solved by utilizing the method of carrier fringes. A set of programs has been written so that the camera/computer system can view the pattern of deformation-modulated carrier fringes and, with a knowledge of the initial carrier-fringe pattern, draw a profile plot of the deformed surface. The procedure is exemplified with a study of a centrally loaded disk.

J Resistance Curves in Aluminum SEN Specimens Using Moire Interferometry

Abstract: TheU y -displacement field obtained by white-light moire interferometry were used to estimate the approximate far- and near-fieldJ-integral values associated with the subcritical crack growths in fatigue precracked 7075-T6 and blunt notched and fatigue precracked 2024-0 and 5052-H32 aluminum, single-edged notch (SEN) specimens. The initial phases of theJ-resistance curves for the somewhat brittle 7075-T6 and the two ductile 2024-0 and 5052-H32 aluminum SEN specimens are presented.

The influence of grating characteristics on moiré fringe multiplication

Abstract: Coherent optical techniques of fringe multiplication can be used to enhance the sensitivity of moire measurements. However, for some coherent optical-processing systems, the significance of the resulting interference pattern is influenced by the characteristics of the gratings being processed. Analysis of one commonly used system for fringe multiplication involving two bar and space amplitude gratings reveals that accuracy of displacements and strains calculated from a fringe pattern is very sensitive to the transmission ratios of the gratings being processed. For variations of the transmission ratio of only 10 percent from the ideal case of 50 percent, the interference patterns produced by optical processing no longer correspond to multiplied moire-fringe patterns.

Magnetoelastic buckling: why theory and experiment disagree

Abstract: This paper does not stand alone; it is directly related to N.S. Christopherson's experimental study1,2 of the magnetoelastic bending of thin steel plates, presented at a recent SEM meeting. It is, in fact, an extension of that study and relies upon some of Christopherson's data.

Residual-Stress Distributions Produced by Strain-Gage Surface Preparation

Abstract: Abrasion of a metallic surface to improve bonding during strain-gage installation is generally thought to produce negligible effect on the measurement of residual stresses by blind hole drilling. However, residual stresses induced by surface abrasion may affect residual-stress measurements in shallow subsurface layers of residual-stress fields produced by processes such as grinding and shot peening.

Transient thermal fracture of cracked plates

Abstract: The effects of a transient thermal load on a cracked plate are studied experimentally using photothermoelasticity The three crack configurations of an edge crack, an interior vertical crack and an interior crack inclined at 45 deg are analyzed In each case, the initially heated plates are subjected to cooling along the edge, while the faces of the plate are either completely insulated, or noninsulated, or in a third case, they are covered with heated transparent Plexiglas plates It is shown that among the three cracks, the largest transient maximum stress-intensity factor occurs for the edge crack The inclined crack is subjected to a mixed-mode loading Among the three cooling conditions, the most severe is the insulated faces case while the noninsulated is the least severe The relative effect of the cooling conditions on the stress-intensity factors for the three crack types is different enough that the results with one cooling condition would not represent those of another one A comparison of the experimental transient stress-intensity factors for the vertical crack cases to the finite-element results shows good agreement

Study of pure and mixed-mode fracture of a brittle material

Abstract: In order to study the behavior of a crack in a linear-elastic material in plane mixed mode (modes I+II), a specimen's shape and loading have been specially adapted. The specimens are first precracked in mode I and then subjected to monotonic loading until instability is reached by an original device which makes it possible to control the nature of the mixed mode applied and which is adjustable from pure mode I to pure mode II. After the specimens are fractured, the lengths of the initial cracks and the kinking angles were measured in the plane-strain area. Then the stress-intensity factorsKI andKII at the moment when crack instability appeared were calculated. This made it possible to apply two criteria: maximum principal stress and maximum stressintensity factorK1* at the onset of kinking. From comparing the calculated values with the experimental values we may note that there is good agreement with respect to the crackkinking directions. However, for the limit load values considerable divergences have been recorded which are analyzed.

An experimental study of wire strains in an undamaged and damaged steel strand subjected to tensile load

Abstract: Wire strains in a wire-rope strand subjected to static and quasistatic tensile load are experimentally examined both before and after the cutting of one of the external wires. These tests are carried out with electrical-resistance strain gages. Significant differences between the most accepted Italian regulations and the measured values, obtained with the strand with one external wire severed, were noted.

Fatigue crack closure of surface flaws loaded in pure bending

Abstract: The objective of this paper is to report fatigue crack growth and closure behavior for surface flaws subjected to pure bending. An optical interference technique is employed with PMMA, a transparent polymer, to determine the three-dimensional crack-surface displacement field. The point-wise measure of crack closure along the crack front obtained from optical interference is compared with an average (bulk) crack-opening load determined via crack-mouth-displacement gage measurements. In addition, the effect of crack-closure loads on the subsequent growth of the surface flaw is examined.

Ultrasonic-image evaluation of microstructural damage accumulation in materials

Abstract: Ultrasonic imaging techniques for portraying and evaluating cumulative internal microstructural damage in engineering materials are described A quantitative delineation of the damage is made in terms of acoustic attenuation obtained from computer analyses of digitized ultrasonic images Acoustic attenuation data are a basic ingredient in previously developed models of damage processes in materials The ultrasonic imaging methodology has been developed using filled polymer (inert solid rocket-propellant) samples subjected to progressive uniaxial tensile strain Successive ultrasonic images taken at various levels of applied strain display dewetting and the evolving microvoid formation/growth which occurs Both initially intact material and that with pre-existing cracks are of interest Changes in acoustic attenuation with strain, derived from the processing of digital images, have provided results as to the degree of preferential damage accumulation at sites of filler particle agglomerations appearing on the ultrasonic images Also, the quantitative extent of an asymmetry in the damage-field distribution near the tips of an extending crack was determined in precracked material Iso-attenuation type contours generated by computer reveal that kidney-shaped damage zones occur in the neighborhood of the propagating crack tips, reminiscent of the plastic-zone shapes near crack tips in ductile metals under strain Ultrasonic images of precracked samples show that before crack extension begins, the material damage in the neighborhood of the crack already extends over a relatively large volume of the specimen

Effect of sequence of combined loading on buckling of stiffened shells

Abstract: The vibration-correlation technique, VCT for definition of real boundary conditions, and the method of repeated buckling were employed for nondestructive generation of improved interaction curves for buckling of stringer-stiffened circular-cylindrical shells subjected to a combined axial compression and external-pressure state of loading. Thirteen shells were tested, five on clamped boundary conditions and eight on norminal simple supports. The study also included an assessment of the influence of the order of loading on the behavior of the shells before and at buckling as a result of the nonlinear interaction. It has been shown that the VCT and repeated buckling approach are feasible for closely stiffened shells and are adequate tools for the derivation of more realistic buckling interaction curves. It appears that the sequence of loading, constant axial compression first and then increasing the external pressure until buckling occurs, or the reverse order of loading, does not influence the buckling loads.