Showing papers in "Experimental Mechanics in 1996"

High-temperature deformation measurements using digital-image correlation

Abstract: The ability of the computer-vision technique of digital-image correlation to measure full-field in-plane surface deformations at elevated temperatures was evaluated by a series of experiments. Samples were subjected to pure translation, free thermal expansion and uniform tensile loads. Results are presented which show that the digital-image-correlation technique remains fully capable of accurate measurement of the displacements and strains on the surface of a planar object at temperatures up to 650°C.

Use of the Kolsky method for confined tests of soft soils

Abstract: A modification of the well-know Kolsky method for dynamic tests is described for investigating the dynamic compressibility of soft soils. The modification consists of placing an axial compression soil specimen into a rigid steel jacket that confines its radial strain. Using this modification, experimental results for plasticine and clay are discussed. An automated system for processing the experimental data in order to plot stress-strain curves is also described.

Investigation of stresses in he orthogonal cutting of fiber-reinforced plastics

Abstract: A photoelastic study was conducted to examine the stress fields in the cutting process of fiber-reinforced plastics (FRPs). Force measurements were made and used in the analysis of the stress fields. Machined surfaces of workpieces with fibers oriented away from the cutting direction showed that the fibers were machined by shearing and tensile fracture; and when fibers were inclined towards the cutting tool, the fibers failed by shearing and bending. In addition, fiber-matrix debonding was observed to be maximum for fibers oriented at 45 deg towards the tool's path. Fiber orientation was shown to have an influence on the machining forces and stresses.

Experimental wavelet analysis of flexural waves in beams

Abstract: The wavelet transform (WT) is applied to the time-frequency analysis of flexural waves in beams. The WT with the Gabor wavelet decomposes a dispersive wave into each frequency component in the time domain, which enables one to determine the traveling time of a wave along the beam at each frequency. By utilizing this fact, a method is developed to identify the dispersion relation and impact site of beams.

Full-field separation of principal stresses by combined thermo- and photoelasticity

Abstract: The combined use of thermoelastic stress analysis and full-field reflection photoelasticity based on the phase-stepping technique has been developed for twodimensional problems. The first method determines the sum of the principal stresses, the latter evaluates the difference of the principal stresses. Thus the principal stresses were separated at each point in the field of view without reference to neighboring points. An evaluation of this approach has been performed using a tensile plate with a central circular hole. The results show that the analysis carried out combining thermo- and photoelasticity incurred errors no larger than those of each system working independently.

Experimental snap-through boundaries for acoustically excited, thermally buckled plates

Abstract: This paper presents some recent experimental results on the dynamic snap-through behavior of a clamped, rectangular plate subject to thermal loading and intense acoustic excitation. The likelihood of snap-through oscillations is characterized in terms of boundaries separating regions of snap-through and no snap-through in the parameter space. Two scenarios are considered. First, using tonal inputs, the regions of snap-through are mapped in the sound pressure level—input frequency domain ((SPL, ω) plane). Second, random acoustic inputs are used, and the effect of varying the overall sound pressure level and frequency bandwidth are investigated ((SPL,\(\omega _{center} + \bar \omega \)) plane). Several nonlinear characteristics are evident and discussed.

An experimental technique for imposing dynamic multiaxial-compression with mechanical confinement

Abstract: A new experimental technique for imposing controlled lateral confinement on specimens subjected to dynamic uniaxial compression has been developed. A description of the experimental technique and experimental results on a ceramic are presented. The axial compression is applied by a split Hopkinson pressure bar modified to subject the specimen to a single loading pulse during the experiment. The specimen is confined laterally by a shrink-fit metal sleeve. The results show that the failure occurs by fragmentation due to axial splitting under uniaxial stress condition, whereas failure occurs by localized deformation on faults under moderate lateral confinement. The compressive failure strength of the ceramic increases with increasing confinement pressure.

Determination of elastic-plastic stresses and strains from measured surface strain data

Abstract: A rigorous approach founded in the fundamental principles of plasticity is used to develop an accurate numerical algorithm for the determination of stresses and elastic and plastic strains from total strain data measured on a structure surface. The approach used to develop the algorithm and its relationship to both the flow theory of plasticity and recent advances in tangent stiffness-based numerical solution procedures for elastic-plastic boundary value problems are presented. Verification of the method for plane stress problems is demonstrated. A discussion of how the method can be used with measured surface displacement data is proved.

Experimental study for reliability of electronic packaging under vibration

Abstract: An automated fatigue-testing system was developed for an experimental study on the integrity of the electronic packaging subjected to mechanical vibration. The fatigue-testing machine utilized the electromagnetic device as an actuator. A data acquisition system was developed for the fatigue test of the electronic board. The fixture for the specimen was designed to be suitable for measuring the fatigue life of a typical module/lead/card electronic system subjected to vibration. With this automated fatiguetesting machine, the mechanical integrity of surface mount component with the spider gullwing leads has been studied by a mechanical flexural fatigue test. An experimental method was developed to measure the changes in electrical resistance in the lead, which is used to indicate a fatigue failure. Finally, a relationship between the loading force and the fatigue life of high-cycle region was discussed for the lead of spider gullwing type surface-mounted component. With the relation, the fatigue life of the surface-mounted component (SMC) subjected to vibration environment was predicted successfully.

Noncontacting strain measurements during tensile tests

Abstract: The application of an innovative noncontracting Doppler laser extensometer is presented. True axial strain has been measured during tensile tests conducted on stainless-steel metal sheets over a range of strain rates (from 10−4 to 102 1/s) and temperatures (from −40°C to 400°C). The laser radiation scattered at the surface of the specimen is recorded during the duration of the experiment. The signals are then used to determine the evolution of the axial strain, which is subsequently combined with the load signal to construct the stress-strain curve for the material. Excellent agreement has been obtained between the total elongation predicted by the laser measurements and the actual values measured from the specimens. This technique offers several advantages over traditional strain-measuring technologies.

Acoustic emission in fiber reinforced concrete

Abstract: This investigation is aimed at diagnostic studies on the behavior at different interfaces in fiber reinforced concrete: between hardened cement paste and sand, between mortar and coarse aggregate and between concrete and fibers. Other types of internal failure include aggregate crushing, fiber rupture, and so on. Only cylindrical specimens, with varying volume percentage and aspect ratio of fibers, have been tested in compression. For quite some time additives like latexes, plasticizers, fibers and silica fume are incorporated in concrete and mortar to improve one quality or the other of concrete, but one single common objective is delaying or arresting interface cracks. This investigation attempts to relate external behavior to internal signals of distress through acoustic emission. This helps to identify critical internal distress vis-a-vis external level of stress. Further, spectral analysis of acoustic signals has been attempted in frequency domain in order to establish which types of interface failure are predominant at different stages of stress. Such understanding could enable material scientists to decide on the parameters of additives with greater confidence when developing composites with desired external response. Of interest are instrumentation and on-line data processing in order to record (a) cumulative acoustic activity and (b) spectral analysis in frequency domain at different stages of progressive loading.

The contact stress in a bolted joint with a threaded bolt

Abstract: The contact stress of a bolted joint, in which a clamped part was fastened to a body with a threaded bolt, was measured using sensitive pins, sensitive films and ultrasonic waves. The contact stress distribution was analyzed as a contact problem by an axisymmetrical theory of elasticity. Numerical calculations were performed. The experimental results were in fairly good agreement with the analytical results.

2D and 3D separation of stresses using automated photoelasticity

Abstract: A procedure for the separation of full-field photoelastic images for use with an automated polariscope is described. Regions of background in the image are identified thus producing the boundary of the model. The shear difference method is used to calculate the components of stress along all raster lines in the image using photoelastic parameters at the boundary points to calculate the initial values of stress. Algorithms were also used to evaluate the stress components along raster lines which did not contain boundary points. A plastic template was used to evaluate the efficiency of the boundary routine. It was found that it was able to identify edges to within approximately one pixel on screen. The complete procedure for stress separation was evaluated using a stress frozen disc in compression and a turbine slot. The values of stress found using the automated polariscope with the stress-separation procedure were found to agree well with theory and with results determined using the method of Tardy compensation and manual analysis. The automated polariscope was also used to analyze three-dimensional stress components along arbitrary lines of a 3D model. A two-model slicing regime was used to analyze a strut subjected to a vertical load. This work was compared to results obtained by Frocht and Guernsey on an identical model machined from Fosterite and subjected to a higher load. Good agreement was found between the results for points away from the region of loading. Significant differences were found near to the load point, however. A finite element analysis of the same problem suggested that this was due to the effects of plasticity.

Three-dimensional Stress Relief Displacement Resulting from Drilling a Blind Hole in Acrylic

Abstract: Analyses of optically based, hole-drilling stress measurements require accurate knowledge of the three-dimensional relaxation displacements induced by the drilling of a blind hole into the surface of a stressed object. These displacements are calculated using two closed-form solutions proposed earlier and a numerical finite element technique. Double exposure holographic fringe patterns calculated from the analytic displacements are in poor agreement with those observed in a controlled laboratory calibration experiment on a block of acrylic subject to a known uniaxial compressive stress. However, the fringe positions predicted by the finite element modeling match those obtained from the observed fringe pattern using image-processing procedures, although some drilling-related discrepancies remain near the stress-relieving hole. The stressstrain behavior of acrylic is extremely temperature sensitive; the discrepancies near the stress relief hole may result from drilling induced heat. Despite these near hole disagreements between the predicted and observed fringe patterns, the overall correspondence indicates that the finite element method adequately provides the desired three-dimensional relaxation displacements necessary for determination of stress magnitudes in some blind hole drilling measurements employing coherent optical recording.

Hybrid moiré-numerical stress analysis around cutouts in loaded composites

Abstract: A hybrid experimental-numerical method is presented for evaluating the stresses around holes or notches in orthotropic composites. Moire-measured displacements and hybrid finite elements are employed. The geometric discontinuity of interest is surrounded by a hybrid element, which is a subregion within the physical structure. Measured displacements provide input information at nodal locations on the external boundary of the hybrid element. Little experimental data are needed, and the approach is accurate and effective. The method is demonstrated for a uniaxially loaded orthotropic laminate containing a central circular hole.

An Experimental Investigation of Composite Repair

Abstract: This study investigates the repair of a glass/vinylester composite material with damage caused by impact loading and bending. The repair technique is based on the “standard procedures” established in a previous study. In addition to the damage due to bending, the repair of composite plates with straight cutting-line damage is also investigated due to its similarity to the bending fracture and good repeatability for evaluation. It is found that two glass reinforcing patches of plain weave—that is, (0, 90)2—on each side of the specimen can restore the original load-bearing capability of the composite material of concern. The investigation of the cutting-line damage can also be viewed as a study of bond-line angles for composite joining. It is concluded that a bond-line angle greater than 60° can restore the undamaged composite strength. In maintaining a large bond-line angle as well as a large bonding surface, various bond-line configurations are presented. Results from the five joints of V, W, WW, U, and UU shapes further verify this conclusion.

Temperature and rate effects on stable crack growth in a particulate composite material

Abstract: In this study, edge cracked sheet specimens made from polybutadiene rubber embedded with hard particles were used in crack propagation tests. Crack propagation tests were conducted under two crosshead speeds (2.54 mm/min and 12.7 mm/min) at three temperatures (−53.9°C, 22.2°C and 73.9°C). The experimental data were analyzed and the crack growth resistance curves and the crack growth rate versus the Mode I stress intensity factor were plotted. Based on these experimental results, the effects of temperature and loading rate on the crack growth behavior were investigated and the results are discussed.

A simple method for measuring surface strains around cracks

Abstract: A simple system has been developed to measure surface strains that occur during in situ deformation of mechanical test specimens. The system uses photolithographically deposited displacement markers and computer image recognition routines to determine in-plane displacements and strains from digital images. The strain calculating routines are integrated into a simple mouse-driven software package that facilitates the transformation from digital images to useful strain field information. Additional routines have been developed to determine crack tip stress fields and J integrals. Crack tip stress intensities have been calculated from strain maps obtained for traction-free cracks in stainless steel. The J integrals were found to be independent of contour and consistent with applied stress intensities. Crack tip stress intensities were calculated for bridged cracks in lamellar TiAl. The toughening effect of the bridging zones was determined by including the bridged region in the contours. Resistance curves generated from strain maps were consistent with those measured during mechanical testing.

Collapse of tubular beams loaded by a wedge-shaped indenter

Abstract: This paper presents experimental studies and analyses on mild steel tube specimens that are indented by a rigid but blunt wedge-shaped indenter at the midspan. Three different support conditions are investigated: simply supported ends, fully fixed ends and the whole tube lying on a flat surface. A test program was carried out to determine the collapse loads and energy-absorbing capabilities of tubes of different sizes. Based on a dimensional argument and the experimental plots, empirical closed-form expressions have been established for the collapse loads and energy. Generally, good agreement has been obtained between the empirical equations and the experimental data. The empirical equations can provide an important first-hand guide for design engineers and analysts in this subject area.

Measurement of Energy Release Rates for Delaminations in Composite Materials

Abstract: Conventional measurements of energy release rates,GI andGII, for delaminations in composite materials, generally utilize loads, crack lengths and simple standard specimen geometries. In this work, a more widely applicable measurement method, using phase shifting moire and the J integral, is presented. The experimental technique described requires only fringe-pattern information and the elastic constants for the measurements—thus it can be used when the standard methods are inapplicable. Using conventional double-cantilever beam and end-notched flexure specimens, the energy release rate has been measured simultaneously by the moire method and the standard methods, with good agreement found between the two. This development will for the first time permit the experimental validation of new finite-element routines as they are developed.

Under load strain partition of a ceramic matrix composite using an ultrasonic method

Abstract: A method for carrying out strain partition under load is described. An ultrasonic device and a suitable extensometer are used simultaneously. Applied to a ceramic matrix composite, it allows one to evaluate the contribution of the various damage mechanisms on its highly nonlinear behavior.

Characterization of the Panoramic Annular Lens

Abstract: The panoramic annular lens (PAL) consists of a single piece of glass, with spherical surfaces, that produces a flat annular image of the entire 360-deg surround of the optical axis of the lens. This paper describes the attributes of the PAL and shows that the lens maps elements from object to image space via a constant aspect ratio polar mapping. A panoramic video system (PVS) is described to illustrate how the characterization can be applied in experimental mechanics for cavity inspection and measurement.

A compact polariscope/shearing interferometer for mapping stress fields in bimaterial systems

Abstract: A compact optical device, which is a combination of a polariscope and a shearing interferometer (PSI), is described for mapping stress fields in bimaterial systems. The PSI device uses commercially available wave plates, and a calcite crystal for beam duplicating. It can operate in a variety of modes, including as a circular polariscope and as a shearing interferometer. In its polariscope mode, it can be used on birefringent materials such as Homolite and epoxy; in its shearing mode, it can be used on optically isotropic materials such as polymethylmethacrylate (PMMA) and glass. As an example, the device is used to obtain full-field maps of stress fields in the vicinity of an interfacial crack in an epoxy-PMMA bimaterial plate.

Stress analysis of an orthotropic material under diametral compression

Abstract: The diametral compression test is commonly used to determine the tensile strength of brittle materials. For isotropic materials a simple relation based on specimen geometry and the applied load at failure is used to calculate the tensile strength. Previous to this work the effect of material orthotropy and material orientation on the specimen stress state had not been completely determined. In this study, both isotropic and orthotropic specimens were analyzed using a finite element analysis and experimentally verified by strain gage and photoelastic measurements. Further, this work investigated the effect of the applied load area on the specimen stress state. Results of this work show that there is a significant difference between the theoretical calculations based on the assumption of material isotropy when compared to an orthotropic material. This difference can be as much as 45 percent depending on the degree of orthotropy and the orientation. It was also determined that the applied load area and material orientation significantly influence the specimen stress state. An applied load area of 8 percent of the circumference was found to reduce the stresses in the applied load region.

Direct measurement of microscopic strain distribution near a crack tip

Abstract: A dot map method was used for direct measurement of the microscopic strain field near a crack tip in a rubber-toughened epoxy. Results indicate that very large viscoelastic strains (37 percent) are present at the crack tip, though elongation in a tensile test is only 5.1 percent. Most of the strain is recovered immediately on unloading, and all of the remaining strain is recovered after annealing above the glass transition temperature.

Experimental verification of the steady-state behavior of a beam system with discontinuous support

Abstract: This article deals with the experimental verification of the long-term behavior of a periodically excited linear beam supported by a one-sided spring. Numerical analysis of the beam showed subharmonic, quasi-periodic, and chaotic behavior. Further, three different routes leading to chaos were found. Because of the relative simplicity of the beam system and the variety of calculated nonlinear phenomena, an experimental setup is made of this beam system to verify the numerical results. The experimental results correspond very well with the numerical results as far as the subharmonic behavior is concerned. Measured chaotic behavior is proved to be chaotic by calculating Lyapunov exponents of experimental data.

Acoustoelasticity : Ultrasonic stress field reconstruction

Abstract: Based on the theory of acoustoelasticity, a new ultrasonic stress reconstruction method—the generalized acoustic ratio (GAR) technique—is developed for locally plane structures and orthotropic materials. For given transit times of the three wave modes and the shear wave polarization angle, the local plane stress tensor is uniquely determined. The GAR technique yields accurate stress estimates with relatively small temperature sensitivity. Based on calibration constants from three uniaxial specimens, the entire stress field in a compact tension specimen is reconstructed. The results are in very good agreement with stress predictions from an elastoplastic finite element analysis. To further improve the measurements, a numerical technique, the stress field approximation (SFA) technique, is developed. The SFA technique uses a smooth local bicubic spline approximation and aims at improving the overall stress field estimate by enforcing the equilibrium equations, the stress boundary conditions and symmetry conditions. Numerical results show that both the average error and its spread are indeed reduced.

Precision strain rate sensitivity measurement using the step-ramp method

Abstract: Current interest in modeling metal processing using constitutive relations is reliant on precision determination of materials testing parameters. One parameter, the strain rate sensitivity, is extensively quoted, but means to measure it precisely are generally unavailable. The best method to measure this parameter is by intermittent strain rate change tests whereby, in theory, the internal structure is kept constant during the rate change. Conventionally, it is difficult to achieve this constant structure requirement, since the load frame's elastic interaction with the specimen, as well as the specimen's elastic compliance, causes inelastic transients. These transients can be nullified by using the step ramp method and a highly responsive servohydraulic testing system. The implementation of such a method and the evaluation of the measured thermodynamic response is described.

Experimentally separating fluid and matrix contributions to polymeric foam behavior

Abstract: A new experimental procedure to determine the loads carried by the fluid (air) and matrix components of a polymeric foam is presented. Testing is carried out in a sealed chamber equipped with a differential pressure transducer to measure changes in the chamber air pressure and a load cell to measure the load applied to the specimen. Multiexposure photographs are used to determine lateral specimen expansion at various degrees of compression. From these data the amount of air trapped and compressed within the foam can be determined. Theoretical analyses suggest and tests confirm that for the strain rates used here the trapped air undergoes isothermal compression. By treating compression of the air trapped in the specimen as an isothermal process, an equivalent volume-average pore pressure can be determined, and the load carried by the fluid phase calculated. The load carried by the polymer matrix component is the difference between the total response and the fluid component. The energy input into each phase during compression can then be calculated.

A system for measuring biaxial creep strains over short gage lengths

Abstract: This paper presents a new system for measuring biaxial strains over gage lengths of 150 micrometers at temperatures up to 300°C under constant loading for times as long as 1300 hours. This system has been used to measure axial and lateral strains at notch roots with a radius of 1 millimeter in zirconium specimens. A complete description of the techniques and procedures as well as an overview of the results is given in this paper.