Showing papers in "Experimental Mechanics in 1989"

Digital image correlation using Newton-Raphson method of partial differential correction

Abstract: Digital image correlation is finding wider use in the field of mechanics. One area of weakness in the current technique is the lack of available displacement gradient terms. This technique, based on a coarse-fine search method, is capable of calculating the gradients. However the speed at which it does so has prevented widespread use. Presented in this paper is the development and limited experimental verification of a method which can determine displacements and gradients using the Newton-Raphson method of partial corrections. It will be shown that this method is accurate in determining displacements and certain gradients, while using significantly less CPU time than the current coarse-fine search method.

Impact testing of concrete using a drop-weight impact machine

Abstract: A detailed description of the instrumented dropweight impact machine is presented. The instrumentation, the calibration, the inertial loading correction, and the dynamic analysis of a concrete beam specimen undergoing three-point impact flexural loading are described. Some results, using such an impact testing machine, obtained from tests done on plain concrete, fiber-reinforced concrete, and conventionally reinforced concrete are presented. It is concluded that the use of such a testing machine may be successfully made in order to test cementitious materials under impact.

On the relationship between impact energy and delamination area

Abstract: Thin glass/epoxy plates fabricated from 3M prepreg tape were subjected to low-velocity impact. Delamination in the impacted composite plates was measured by edge replication and was identified as the major damage mode. The effects of fiber orientation, thickness, and lamination on the delamination resistance were investigated. Experimental results verified three previous findings: (1) a linear relationship holds between delamination area and impact energy, (2) the mismatch of bending stiffness between adjacent laminae can be correlated with the delamination area on the interface, and (3) the behavior of a thin composite plate under low-velocity impact is very similar to that caused by global bending. In addition, based on the calculation of impact energy per unit delamination area, the dynamic fracture energy and the energy of dissipation in the thin glass/epoxy plates can be examined.

A novel impact three-point bend test method for determining dynamic fracture-initiation toughness

Abstract: A novel impact three-point bend test method has been developed for determining the dynamic fracture-initiation toughness,K Id, over the range of loading rates $$10^5 MPa\sqrt m /s \leqslant K_I \leqslant 10^5 MPa\sqrt m /s$$ . The split-Hopkinson pressure-bar technique is used to measure dynamic loads applied to a bend specimen with a fatigue precrack. The stress-intensity-factor histories for the bend specimen are evaluated by means of a dynamic finite-element technique and the standard formula (ASTM E 399-83) based on the measured dynamic loads. The time of crack initiation is determined using a strain gage mounted near a crack tip. The results for 7075-T6 aluminum alloy and Ti−6A1−2Sn−4Zr−6Mo alloy indicate that the reliableK Id data can only be obtained by evaluation procedures which take the inertial effects into account. It is shown that the novel impact bend test method in conjunction with dynamic finite-element analysis provides an effective means of characterizing the dynamic fracture-toughness parameterK Id.

Determining transverse impact force on a composite laminate by signal deconvolution

Abstract: Dynamic impact forces on a composite structure were recovered by using experimentally generated Green's functions and signal deconvolutions The signal processing is straightforward Extra windowing and filtering the recorded signals are unnecessary The Green's functions account for boundary conditions, material properties and structure geometry This approach can be applied to linearly elastic structures with different boundary conditions It is realistic and convenient to use for the recovery of impact force on anisotropic or isotropic solid structures

Determination of thermal strains by moiré interferometry

Abstract: An existing method is extended to measure thermal strain distributions on an absolute basis. Free thermal expansion and stress-induced deformations are separated, allowing the determination of coefficients of expansion, stress-induced strains, normal stresses and shear stresses. The method is applicable to many steady-state and transient thermal-strain problems.

Determining fracture parameters with full-field optical methods

Abstract: In order to increase the accuracy of stressintensity-factor measurements and to obtain data on additional parameters which may influence fracture behavior (such as crack branching and crack curvature) a technique for fullfield fringe-pattern analysis (referred to as the local collocation method) has been developed. This method removes the restriction of limiting the data analysis region to the near-field region by including additional nonsingular terms in the algorithm. In this paper the theory of the method is developed and sample results using photoelastic, holographic and moire full-field patterns are provided.

Two-dimensional moiré method and grid method using Fourier transform

Abstract: Formerly, the authors presented one-dimensional strain analysis by a moire method using a Fourier transform. In the present work, the method is extended to two-dimensional strain analysis. The analysis is completely automated by introducing digital image processing. All of the laborious and subjective procedures required in the classical and conventional moire method, such as separation of the two gratings, fringe-sign determination, fringe ordering and fringe interpolation, are completely eliminated; and objective, fast and accurate analysis can be made.

Automated measurement of birefringence: Development and experimental evaluation of the techniques

Abstract: Traditional photoelasticity has started to lose its appeal since it requires a well-trained specialist to acquire and interpret results. A spectral-contents-analysis approach may help to revive this old, but still useful technique. Light intensity of the beam passed through the stressed specimen contains all the information necessary to automatically extract the value of retardation. This is done by using a photodiode array to investigate the spectral contents of the light beam. Three different techniques to extract the value of retardation from the spectral contents of the light are discussed and evaluated. An experimental system was built which demonstrates the ability to evaluate retardation values in real time.

Transient bending waves in plates studied by hologram interferometry

Abstract: Propagating bending waves are studied in plates made of aluminum and wood. The waves are generated by the impact of a ballistic pendulum. Hologram interferometry, with a double pulsed ruby laser as the light source, is used to record the out of plane motion of the waves. Elliptic-like fringes visualize differences in wave speed for different directions in the anisotropic plate and circular ones are obtained for the isotropic plate. The experimental data for the isotropic plate compare favorably with analytical results derived from the Kirchhoff-plate equation with a point impact of finite duration. A similarity variable is found when starting conditions are modeled as a Dirac pulse in space and time, that brings new understanding to the importance of specific parameters for wave propagation in plates. A formal solution is obtained for a point force with an arbitrary time dependence. For times much larger than the contact time, the plate deflection is shown to be identical to that from a Dirac pulse applied at the mean contact time. A method for determining material parameters, and the mean contact time, from the interferograms is hence developed.

Strain-field analysis acquired through correlation of X-ray radiographs of a fiber-reinforced composite laminate

Abstract: The strain fields can be determined on the surface of fiber-reinforced composites by correlation analysis of X-ray radiographs of the specimen. One radiograph is taken of the specimen in an unloaded state and another radiograph of the same specimen is taken in a deformed state. Each radiograph is then imaged by a video camera and digitized using a Matrox digitization board. The displacement map is obtained from the two radiographs by dividing one image into subsets of 20 pixels by 20 pixels and using a correlation algorithm to identify the corresponding subsets in the second image. The correlation is completed between patterns of the grey-level intensities for each subset. The appropriate discrete derivatives can then be evaluated by first difference taken to form the in-plane strains. The X-ray radiographs correlation analysis strain-field method outlined above was applied to a uniaxially loaded [90, 0] s Gl-Ep coupon. Simultaneously, an extensometer measured the average longitudinal strain over a 2.54-cm (1-in.) gage length. The error between the two methods was less than three percent of the applied strain of 1.3 percent. The same specimen was impacted and re-examined. A radical shift in the strain field was observed when the specimen was reloaded. Further investigation showed the method reliable down to 0.5-percent strain.

Computerized fringe analysis in photomechanics

Abstract: A generalized method is presented for analyzing the fringe patterns frequently encountered in experimental mechanics By utilizing digital-image-processing and computer-graphics techniques, a set of menu-driven software is developed for interactively implementing the fringe processing Tests of this software on the images obtained experimentally by photoelasticity, holographic interferometry and speckle interferometry demonstrate its usefulness Good agreement between the experimental and theoretical results is established

The magic of carrier fringes in moiré interferometry

Abstract: Practical applications in which carrier fringes are used with moire interferometry for strain measurements are presented. Examples illustrate how moire carrier fringes are applied to obtain the desired data in complex laminated composite specimens. In many cases, carrier fringes permit extraction of much more detailed information, with procedures that are easier and more accurate than those using loadinduced fringes alone. The fringe vector for carrier fringes is introduced and its application to the interpretation of fringe patterns is explained. In moire interferometry, the carrier fringes are produced easily by adjustments of optical elements that control the virtual reference grating.

Determination of plastic strains at notches by image-proceessing methods

Abstract: An image-processing method was developed to quantitatively extract the level of plastic deformation in metal specimen made of 304 stainless steel under remote tensile loading. The effective strain distribution around the notch tip was obtained and compared with the finite-element results. An exponential decay of the plastic strain concentration with distance from the notch tip was observed.

A new bending test method of advanced composites

Abstract: This paper deals with a new bending test method of advanced composites. Although the conventional three- or four-point bending is convenient to obtain a bending strength, it has several disadvantages especially for advanced composites. A newly proposed method is based on a bending by means of axial compression and this method overcomes the above disadvantages. A set of testing devices was designed and machined, and various kinds of composites were tested. It is made clear that the new method is suitable especialy for high performance composites such as T800/epoxy.

Thermographic analysis of stress concentrations in a composite

Abstract: This paper examines the stress concentrations in the knit yarn/fiber intersections in a unidirectional carbon/epoxy composite. Stress-concentration factors for four different knit yarn spacings are determined by thermographic stress analysis (TSA). This method is also known in the technical community by several other names. For example the SPATE method (referring to commercially available infrared equipment), and thermoelastic stress analysis. The latter term is not sufficiently descriptive since it is also used to describe purely analytical studies of thermal-stress problems of different phenomenology. Because of these and other technical considerations, TSA is the most accurate and specific terminology for the method at hand.

Experimental determination of stress-intensity factors using caustics and photoelasticity

Abstract: The optical method of caustics has been used with considerable success in recent years for determining stress-intensity factors in both static and dynamic problems. However the midplane analysis explaining the formation of transmission caustics has certain approximations that need to be examined. In this paper it is demonstrated that the midplane analysis is in good agreement with the numerical solution of the exact geometrical optical equations for mapping the light-ray path in a cracked plate. Since both these analyses are obtained by imposing a two-dimensional crack-tip stress field the sensitivity of the method to the deviations from the imposed stress field is examined next. The implications of this examination on the photoelastic technique are then discussed.

Transient bending waves in anisotropic plates studied by hologram interferometry

Abstract: Propagating bending waves are studied in plates made of glass-fiber reinforced polyester. The waves are generated by the impact of a ballistic pendulum. Hologram interferometry, with a double pulsed ruby laser as light source, is used to record the out of plane motion of the waves. The interferograms have an elliptic-like symmetry for an orthotropic plate, while the wave pattern for a symmetric angle-ply reinforced plate has a symmetry about the axes of reinforcements. Experimental data are compared on one hand to analytical results obtained by assuming that the orthotropic plate can be described as if isotropic along the main axes, and on the other hand to numerical results from calculations using the finite-element method. The effective Young's modulus raised to power 1/4 is shown to be an important parameter for the description of the dispersive wave pattern. A defect in the plate alters the wave pattern in the interferograms significantly. This may have technical use.

Whole-field experimental displacement analysis of composite cylinders

Abstract: The lack of a quick, nondestructive method of flaw detection in composite materials is a hindrance to their use. Optical methods of determining displacements hold promise as a method of detection of these flaws. This paper illustrates the use of digital-image-processing equipment to determine the displacement fields in an internally pressurized composite cylinder. Included are results of actual experiments performed by the authors on both a Plexiglas and a composite cylinder.

Impact of small-crack effects on design-life calculations

Abstract: The potential impact of the observed small-crack-effect in 2024-T3 aluminum alloy on calculated crack-growth lives is described. The significance of the small-crack effect is assessed by comparing lives computed using only large-crack data to those computed using a combined small- and large-crack data base for constant-amplitude (R = -1 and 0) and fighter-wing spectrum loading (FALSTAFF) conditions. Based on the life calculations, the small-crack effect would have no impact on life analyses that assume an initial crack larger than about 0.3 mm; only a small impact on life analyses that assume an initial crack of 0.1 mm; but a large impact on life analyses that assume an initial crack of about 0.01 mm. When small-crack effects are taken into account, fatigue life analyses based solely on crack growth may provide a viable alternative to traditional two-part initiation-plus-crack-growth life analyses.

Dynamic crack propagation along a weakly bonded plane in a polymer

Abstract: Dynamic crack propagation speeds along the weakly jointed interfaces of PMMA and Homalite-100 were determined experimentally. These speeds were found to be highly dependent on the bonding strength and on the magnitude of the applied impulsive loads. As applied loads increase, the maximal speed was found to approach asymptotically the Rayleigh wave velocity of the material.

Three-dimensional effects in beams: Isodyne assessment of a plane solution

Abstract: The stress distribution in a homogeneous beam subjected to three-point bending is investigated using the method of optical isodynes. The three stress componentsσ xx,σyy andτ xy acting in the planes formed by the longitudinal and vertical axes of the beam are determined in three planes situated at different through the thickness locations with respect to the beam's midplane. The experimental results are subsequently correlated with the two-dimensional elasticity solution. It is illustrated that at locations sufficiently removed from the centrally applied concentrated load, good correlation between theory and experiment is obtained. In the regions where high stress gradients exist however, differences are observed in the in-plane stress distributions in the different planes. These differences are explained by the presence of the out of plane normal stressσ zz using the relations of optical isodynes. Greatest differences between theory and experiment are obtained for the in-plane shear stress componentτ xy.

Transmission of pressure waves in partially saturated soils

Abstract: A split-Hopkinson pressure bar (SHPB) was used to experimentally determine stress-transmission coefficients and wave speeds for dry and moist 50/80 silica soil specimens. Results show that for a constant input pressure the transmitted pressure and wave speed increase to a maximum at approximately five- to ten-percent moisture content. Then, both wave speed and transmission ratio decrease with increasing moisture content down to the approximate values of dry soil. Preliminary analysis indicates that these trends can be explained by the effects that saturation and soil capillarity have on effective stress in the soil.

Fatigue life of dovetail joints: Verification of a simple biaxial model

Abstract: A biaxial fatigue specimen, designed to model the state of stress of the blade to disk joint in a typical gas turbine, has been analyzed and tested The method of analysis was shown to predict the location of the worst damage and of crack initiation This paper describes the analysis and testing of a simplified model that may be used in preference to the previous one

An optimal cam profile design considering dynamic characteristics of a cam-valve system

Abstract: In this work, an optimal cam profile design method for an OHV-type cam-valve train is studied considering the dynamic characteristics of the valve system. When designing a cam profile for an internal combustion engine, it is desirable to make the valve lift area be as large as possible and the valve peak acceleration and seating velocity be as low as those can be within the cam-event angle. But, as we know, those features conflict with one another. An optimal design must strike a compromise between the two. Another important factor in valve train design is avoiding abnormal valve motions such as jump and bounce. It is known that jump and bounce are closely related to valve train dynamic characteristics. In this paper, a two-step optimization technique to design an optimal cam profile is proposed. In the first step, an attempt was made to maximize valve lift area without causing abnormal valve motions while satisfying all the given constraints such as cam-event angle, maximum valve acceleration, and cam displacements at both ends of the cam-event angle. Then, in the second step, minor modifications of the cam developed in the first step were made in order to reduce the cam acceleration while maintaining the maximized valve lift area and satisfying constrains obtained in the first step. In order to prove the effectiveness of the optimization method, the valve motion driven by the optimized cam was not only simulated with a four degree of freedom model but was also tested experimentally. It was found that the measured valve motions agree quite well with the simulation results. Comparing the valve motions of the optimized cam with those of the original cam, it was found that the optimized cam can increase the valve-lift area by 8.6 percent while reducing the peak cam acceleration by 28.7 percent. Also, it was noted that the optimized cam increases the cam-valve train operating speed at which jump and bounce occur.

Stress-optic law in a single crystal and its application to photo-anisotropic elasticity

Abstract: A general stress-optic law in a plate made of a single crystal with birefringence is developed, the plate has an arbitrary crystallographic direction. From the general stressoptic law, a condition for obtaining stress distributions in the plate under a plane stress state is derived. Some optical and mechanical properties when the plate is used as a photoanisotropic model are also explained. Experiments on silicon beams in pure bending are performed by using an infrared photoelastic method. Experimental results show that the stress-optic law is valid. The optical and mechanical properties of silicon beams are shown in tables.

Moiré interferometry for simultaneous measurement of U, V W

Abstract: A simple setup for simultaneous measurement of in-plane and out-of-plane displacement components is demonstrated. Since the in-plane and out-of-plane components are recorded at separate positions, no additional processing of the patterns is required. Suggestions are made for further simplification by proper choice of specimen and reference gratings.

A new technique for the determination of stress-optical constants using the shadow spot method

Abstract: A new technique has been developed to determine both the transmitted and the reflected stress-optical constants. The reflected optical constant is associated with the light reflected from the rear face of a transparent material. The stress-optical constant related to the light reflected from the front surface is not considered in this paper since it is equal to Poisson's ratio divided by Young's modulus. In this new method, a monochromatic light beam emitted from a He−Ne laser impinges on the area surrounding a circular hole under load in a plate. A digital image-analysis system, EYE-COM III, is used to determine the points of the highest light intensity of the caustic that results from the light transmitted or reflected from the rear face of the plate. The effect of the hole size in optically isotropic materials is investigated.

Plastic strain analysis of the machined surface region using fine grid etched by photoresist technique

Abstract: The photoresist technique was used to generate a mesh of fine grid pattern to determine the plastic strain distribution in the subsurface of machined workpieces. For this purpose rectangular grids measuring 20 μm by 20 μm with a line thickness of 5 μm were etched on workpieces of inconel-718 nickel-base superalloy and then machined under plane-strain condition. Subsurface plastic strain distribution was determined from distorted grids after machining using standard analytical methods. Using this technique, it was possible to generate a finer rectangular grid size of 5 μm by 5 μm with a 2-μm line thickness.

The maximum-distortion-energy ellipse as a biaxial fatigue criterion in view of gradient effects

Abstract: The maximum-distortion-energy ellipse is shown to be a well-defined boundary for certain high-cycle fatiguelimit data having gradient-induced scatter. In connection with this, the nonzero gradients are seen to have only a beneficial effect on fatigue behavior. The work is limited to fully reversed biaxial-stress components which are either in-phase or 180 degrees out-of-phase with each other. A means to account for anisotropy in fatigue properties is tentatively suggested.