VTechWorks Repository :: Browsing by Author "Morton, John"

Browsing by Author "Morton, John"

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Application of localized hybrid methods of stress analysis to some problems in the mechanics of composites
Tsai, Ming-Yi (Virginia Tech, 1990)
A new method of stress analysis which combines an experimental technique — moire interferometry, and a numerical method — finite element analysis, is presented. In this localized hybrid method, the displacement fields which the moire experiments provide in some local regions of interest are used as input data for finite element stress analyses. Two important and controversial problems in the mechanics of composites are investigated using the localized hybrid method. One is a thermally loaded bimetal plate, and the other involves the Iosipescu shear specimen popularly used to determine the shear modulus and strength of a fiber reinforced composite. Before applying the localized hybrid analysis, the mechanics of the problems are discussed individually, through a numerical study and strength of materials analysis. Based on these fundamental studies, the localized hybrid method is applied to stress or strain analyses of moire experimental results, and special techniques of this method are developed to handle these data. For the thermally loaded plate, several finite element models, simulating 2-D and 3-D mechanics, are used to assess the stress state at the interface near the free surface, and identify a boundary layer. It is shown that high gradients and stress turnaround are documented in stress component normal to the interface, along the surface line crossing the interface, and a boundary layer is identified in the small region near the interface around the free surfaces. These observations are also confirmed by the hybrid analysis of moire experimental results. However, additional variations of the localized hybrid method were needed to capture the three-dimensional nature of the problem. A comparison of numerical results with experimental data resolved an apparent anomaly between experiments and mechanics principles. For the Iosipescu shear specimen, the 3-D mechanics associated with twisting is proposed for accounting for the inconsistent and variable results in the literatures. The results form the localized hybrid analysis indicate that uniformity and purity of shear stress state in the test section cannot be accomplished for each fiber orientation specimens; the 0° specimens suffer from a load proximity effect, the 90° specimens are affected by apparent in- and out-plane bending, and the 0°/90° specimens are midway between those. Several variations of the localized hybrid method of stress analysis have been presented for three-dimensional problems in the mechanics of solids. It is showed that the approach developed not only provides a powerful and efficient technique for the reduction of experimental data, but also gives a good insight into the mechanics of the experimental observations.
An assessment of subscale notched specimens for composites shear property measurement
Budiman, Haryanto Tiara (Virginia Tech, 1991-12-04)
The feasibility study of subscale notched specimens to determine the shear response of composites is presented. The investigation consists of finite element analyses, conventional strain-gaged testing, and photomechanics experiments. Several notch geometries of the subscale specimens are studied, the standard 900 V -notch, U-notch, and circular notch. The investigation is performed on two different material systems, a standard high performance graphite/epoxy (AS4/3501-6) material and an SMC R-28 material reinforced with 28% volume fraction strand glass fiber. The moduli obtained from the subscale specimens are compared with those obtained from the standard specimens. Different degrees of twisting observed in testing the sub scale specimens are discussed. Numerical and experimental results of the SMC R-28 materials are presented. The dependence of the measured shear modulus on the relative orientation of the specimen in the panel is identified. The application of the subscale, circular notched specimens to obtain the shear modulus of the SMC material is discussed.
Design and analysis of a composite wire-socket attachment
Lutz, Ernst (Virginia Tech, 1994-10-15)
A detailed study of the feasibility of anchoring fiber reinforced plastic wires for civil engineering applications is presented. An experimental investigation using tensile testing machines is performed, testing anchorages of 1, 7 and 19 wires. Conventional strain gage and acoustic emission measurement techniques are used. The tests are essential in determining the failure load and failure mode. However, the experimental data alone do not provide enough information about the behavior of the anchorage to be used exclusively in the design process. The results are used to modify the design of the anchorage system. It is shown that for a successful anchorage system the choice of material for the load transfer medium is crucial. A solution is presented to overcome the high stress concentration at the load entry area of the wire into the anchor. A finite element analysis of the anchors for 1 and for 19 wires is used to assess the stress and strain fields inside the anchor, to validate the analytical model, and to determine locations of possible high stress concentrations. Three-dimensional and one-dimensional models, that utilize axisymmetry, are evaluated. The results of the numerical analysis are used to demonstrate the improvement as a result of a change in material choice or design of the anchor. It is shown that the modification of the load transfer medium results in a decrease of 30 % of the average stress level. In the analytical investigation, several common models are introduced that describe the fiber pullout behavior. Based on a recent treatment by C. H. Hsueh, a model is developed that describes the anchorage of a wire in a conical shaped socket using orthotropic materials. This model includes boundary conditions that are similar to the ones observed in the experiments. A parametric study is performed to obtain information on the ideal geometry of the anchor system. The results and predictions of the applied techniques, i. e. analytical description, finite element method and experimental investigation, are compared and contrasted. Based on the analytical, numerical and experimental results, recommendations for improving the design of the anchor system are made. Subsequently, a modified anchor system is proposed that utilizes the properties of a load transfer medium that has a variable stiffness. The inclusion of a pure resin collar and supporting wires is suggested. For a successful completion of this project, ideas are proposed and suggestions made for future work.
Developments in moire interferometry: carrier pattern technique and vibration insensitive interferometers
Guo, Yifan (Virginia Polytechnic Institute and State University, 1989)
Due to the rapid expansion of applications of composite materials, investigations of their properties have greatly increased. Since theoretical and numerical methods have many limitations for anisotropic materials, experimental methods are sometimes the only way to answer the questions. It has been proved that moire interferometry is a powerful technique in the study of composite materials. The high sensitivity and resolution of a measurement technique is the key to determining the properties of a material which has a fine and complicated structure such as fiber reinforced composite laminates. In this paper, a carrier fringe method is introduced to increase the resolution of the fringe gradient in the moire technique. The ability of measurement is extended to the micromechanics region. High strain concentrations and the dramatic displacement variations can be determined by measuring the slopes of carrier fringes. Strain distributions across the plies (with the thickness of 125 μm) in graphite/epoxy composites and strain concentrations in the resin-rich zones (with the thickness of 10 μm) between neighboring plies are revealed by the carrier fringe technique. Three experiments are presented to show the effectiveness of the application of carrier fringes to resolve fringe gradients and obtain strains. The current moire technique is limited to the optical laboratory because it is extremely sensitive to the disturbance of the environment. A vibration with magnitude of 0.2 μm can completely wash out the contrast of a moire fringe pattern. The study has been done in moving moire interferometry off the optical table. Vibration insensitive moire systems are investigated to extend the moire technique to the tests of large structures and using testing machines for loading. Vibration problems are discussed and the new ideas for eliminating vibration effects are presented. Six representative schemes are analyzed and three of these systems are built to perform experiments in rough environments such as on a hydraulic testing machine. The results show the great success of these new systems.
The effect of specimen size on the mechanical response of laminated composite coupons loaded in tension and flexure
Johnson, David Page (Virginia Tech, 1994-04-05)
The effect of specimen size on the uniaxial tensile stress/strain response of sublaminatelevel scaled composite specimens, and the four point flexure load/deflection response of ply- and sublaminate-level scaled composite specimens was investigated. Three laminates were studied in the tensile program, namely [+30/-30/90₂]_ns, [+45/-45/0/90}_ns and [90/0/90/0|_ns, where n = 1, 2, 3, 4. Two material systems were used, namely AS4/3502 graphite/epoxy and APC-2 graphite/PEEK, to investigate the relative effect of resin toughness. Three laminates were also studied in the flexure program, The baseline lay-ups were (±45/0/90}_2ns, [0/90/0/90J_2ns and [±45/±45J_2ns, where n = 1, 2, 4. Ply- and sublaminate-level scaling were used to increase specimen thickness. All flexure specimens were of AS4/3502 graphite/epoxy. Enhanced X-ray radiography and edge photomicroscopy were used to examine damage development in specimens loaded to various fractions of their ultimate stress. This nondestructive examination was coupled with observations of critical events in the stress/strain response to try to correlate scaling effects with the damage development in the specimens. Analytical and numerical methods were employed in order to understand the stresses driving certain damage modes observed. 2-D and 3-D finite element models were used to find delamination stresses in an undamaged laminate, and an approximate clasticity approach was used to find stresses duc to cracks in the 90° plies. It was found that the tensile strength of the [+30/-30/90₂]_ns and [+45/-45/0/90}_ns laminates gencrally increased as n increased. This effect was more pronounced for the matrixdominated [+30/-30/90₂]_ns. Both the [+30/-30/90₂]_ns and the quasi-isotropic [+45/-45/0/90}_ns laminates seemed to be approaching a maximum strength, beyond which the strength scaling either stops, or is reversed. As # increased from 1 to 4, these two laminates exhibited a delay in the onset of certain damage mechanisms, such as delamination and transverse matrix cracking. The [90/0/90/0|_ns laminates showed no tensile strcss/strain response scaling effects, although the stress at which first ply failure occurred was found to increase as 7 increased. (±45/0/90}_2ns and [±45/±45J_2ns flexure specimens showed no strength scaling cffects when sublaminate-level scaling was uscd, but significant decreases in s{rength were found when specimen size was increased using ply-level scaling. [0/90/0/90J_2ns specimens showed no global load/deflection scaling effects.
Energy-absorption capability of composite tubes and beams
Farley, Gary L. (Virginia Polytechnic Institute and State University, 1989)
In this study, the objective was to develop a method of predicting the energy-absorption capability of composite subfloor beam structures. Before it is possible to develop such an analysis capability, an in-depth understanding of the crushing process of composite materials must be achieved. Many variables affect the crushing process of composite structures, such as the constituent materials’ mechanical properties, specimen geometry, and crushing speed. A comprehensive experimental evaluation of tube specimens was conducted to develop insight into how composite structural elements crush and what are the controlling mechanisms In this study, the four characteristic crushing modes, transverse shearing, brittle fracturing, lamina bending, and local buckling were identified and the mechanisms that control the crushing process defined. An in-depth understanding was developed of how material properties, affect energy-absorption capability. For example, an increase in fiber and matrix stiffness and failure strain can, depending upon the configuration of the tube, increase energy-absorption capability. An analysis to predict the energy-absorption capability of composite tube specimens was developed and verified. Good agreement between experiment and prediction was obtained. Sine-wave and integrally stiffened composite beams were evaluated. Composite energy-absorbing beams crush in modes similar to tubular specimens that are made from the same material and have similar geometry. Energy-absorption trends of the composite beams are similar to energy-absorption trends from composite tube specimens. Composite beams are equal or superior energy absorbers to comparable geometry metallic beams. Finally, a simple and accurate method of predicting the energy-absorption capability of composite beams was developed. This analysis is based upon the energy-absorption capability of the beams’ constituent elements.
An evaluation of the Iosipescu specimen for composite materials shear property measurement
Ho, Henjen (Virginia Tech, 1991-07-05)
A detailed evaluation of the suitability of the Iosipescu specimen tested in the modified Wyoming fixture is presented. An experimental investigation using conventional strain gage instrumentation and moire interferometry is performed. A finite element analysis of the Iosipescu shear test for unidirectional and cross-ply composites is used to assess the uniformity of the shear stress field in the vicinity of the notch, and demonstrate the effect of the nonuniform stress field upon the strain gage measurements used for the determination of composite shear moduli. From the test results for graphite-epoxy laminates, it is shown that the proximity of the load introduction point to the test section greatly influences the individual gage readings for certain fiber orientations but the effect upon shear modulus measurement is relatively unimportant. A numerical study of the load contact effect shows the sensitivity of some fiber configurations to the specimen/fixture contact mechanism and may account for the variations in the measured shear moduli. A comparison of the strain gage readings from one surface of a specimen with corresponding data from moire interferometry on the opposite face documented an extreme sensitivity of some fiber orientations to eccentric loading which induced twisting and yielded spurious shear stress-strain curves. In the numerical analysis, it is shown that the Iosipescu specimens for different fiber orientations have to be modeled differently in order to closely approximate the true loading conditions. Correction factors are needed to allow for the non uniformity of the strain field and the use of the average shear stress in the shear modulus evaluation. The correction factors, which are determined for the region occupied by the strain gage rosette, are found to be dependent upon the material orthotropic ratio and the finite element models. Based upon the experimental and numerical results, recommendations for improving the reliability and accuracy of the shear modulus values are made, and the implications for shear strength measurement discussed. Further application of the Iosipescu shear test to woven fabric composites is presented. The limitations of the traditional strain gage instrumentation on the satin weave and high tow plain weave fabrics is discussed. Test result of a epoxy based aluminum particulate composite is also presented. A modification of the Iosipescu specimen is proposed and investigated experimentally and numerically. It is shown that the proposed new specimen design provides a more uniform shear stress field in the test section and greatly reduces the normal and shear stress concentrations in the vicinity of the notches. While the fabrication and the material cost of the proposed specimen is tremendously reduced, it is shown the accuracy of the shear modulus measurement is not sacrificed.
Experimental method of analyzing stress intensity factors and singularity order in rocket motor geometry
Che-Way, Chang (Virginia Tech, 1990)
A series of frozen stress experiments were conducted on surface flaws of varying aspect ratios in pressurized cylinder with star-shaped cutout in order to study the stress intensity factor distribution along the flaw border and to estimate the loss of the inverse square root singularity when the crack border intersects the inner star surface at right angles. By applying a refined optical method, the photoelastic data are converted into classical stress intensity factors resulting from the three dimensional stress state existing at the inner surface and compared with a numerical analysis to indicate the nearly uniform distribution of the stress intensity factor along the crack border. Based upon this result a two dimensional weight function approach is demonstrated to yield accurate values of the maximum stress intensity factor for the motor grain test geometry.
Habitat use and energetics of American black ducks wintering at Chincoteague, Virginia
Morton, John (Virginia Polytechnic Institute and State University, 1987)
The habitat use and energetics of American black ducks (Anas rubripes) wintering at Chincoteague National Wildlife Refuge, Virginia, were investigated. Twenty-two female black ducks were systematically radiotracked on the 25,600 ha study area between 15 December 1985 and 28 February 1986. Diurnal time and energy budgets were constructed by distributing 1,471 scans (collected in 1985-86 and 1986-87) over a time-tide matrix within refuge, saltmarsh, and tidal water habitats. Sixty-four ducks were collected during early, mid, and late winter in 1985-86 to determine changes in carcass composition. The Habitat Suitability Index (HSI) model for wintering black ducks was evaluated. Age affected range and core areas but did not affect habitat selection. Tide, ice, and time of day affected habitat use. Refuge pools were used during the day and saltmarsh was used at night. Subtidal water was used during periods of icing. Black ducks fed least and rested most when in refuge pools but fed most and rested least when in tidal waters. Black ducks curtailed feeding and increased time spent in alert and locomotion behaviors in response to disturbance. Whole carcass analysis indicated that black ducks were at least as fat and heavy in the spring as they were in the fall. Comparisons with similar work in Maine suggested that black ducks wintering in Maine and Virginia expend the same energy at a given temperature. However, because of lower temperatures, black ducks collected at Chincoteague were in relatively better condition than ducks wintering in Maine.
High resolution interferometric measurements of residual strains in composites
Lee, Joosik (Virginia Tech, 1990-12-15)
As composites have been more widely accepted as structural materials, residual stresses in them have become a more serious issue. Various experimental methods have been developed and used to measure residual strains in different kinds and shapes of materials. The anisotropic and heterogeneous nature of composites and the complexity of residual stresses, however, pose limitations on current techniques. Those techniques lack the sensitivity or spatial resolution that is required for the measurement of local deformation of composites on a ply·by-ply basis, or give point-by-point and averaged information. Also they are incapable of resolving the complexity of combined effect of different residual stress components. I n order to measure residual strains more effectively, a new method of measuring then1 is required. Moire interferometry combined with the cut-and-sectioning method has been developed for effective measurement of residual strains in fiber-reinforced composites. This optical technique provided the capability of studying separately the effect of each component of residual stresses. It also allowed the determination of high-sensitivity full-field deformation information. This approach was applied to thick composite cylinders for measuring residual strains. The results showed a strong influence of curing procedure on residual stresses. Also, in order to determine residual strains on a within-the-ply basis, a new high-resolution data reduction procedure has been developed. This procedure enhanced the resolution of the existing data reduction technique without losing qualitative information. The combination of both aforementioned techniques provided an effective tool for measuring residual strains of composite materials. The technique is illustrated in an investigation of the effect of stacking sequence on residual strains in flat composite panels.
Impact damage resistance and tolerance of advanced composite material systems
Teh, Kuen Tat (Virginia Tech, 1993)
Experimental evaluations of impact damage resistance and residual compression strengths after impact are presented for nine laminated fiber reinforced composite material systems. The experiments employ a small scale specimen for assessing the impact damage resistance and impact damage tolerance of these materials. The damage area detected by C-scan is observed to develop linearly with the impact velocity for impact velocities higher than a threshold value. Brittle material systems have lower threshold velocities and higher damage area growth rates than toughened systems. The impact damage resistance of each material system can be characterized with threshold velocity V_c and damage area growth rate C. The residual compressive Strength after impact was observed to decrease linearly with the damage area equivalent diameter. The rate of compressive strength reduction, K_d, has been observed to be independent of the material properties. The impact damage can be simulated from quasi-static indentation test in which the damage due to these two loading conditions are quite similar. The residual compressive strength can also be simulated from specimens with similar damage size resulting from quasi-static indentation load.
Integration and processing of high-resolution moiré-interferometry data
Lin, Shih-Yung (Virginia Tech, 1992-05-05)
A new hybrid method combining moire interferometry, high resolution data-reduction technique, two-dimensional datasmoothing method, and Finite Element Method (FEM) has been successfully developed. This hybrid method has been applied to residual strain analyses of composite panels, strain concentrations around optical fibers embedded in composites, and cruciform composite shear test. This hybrid method allows moire data to be collected with higher precision and accuracy by digitizing overexposed moire patterns (U & V fields) with appropriate carrier fringes. The resolution of the data is ± 20 nm. The data extracted from the moire patterns are interfaced to an FEM package through an automatic mesh generator. This mesh generator produces a nonuniform FEM mesh by connecting the digitized data points into triangles. The mesh, which uses digitized displacement data as boundary conditions, is then fed to and processed by a commercial FEM package. Due to the natural scatter of the displacement data digitized from moire patterns, the accuracy of strain values is significantly affected. A modified finite-element model with linear spring elements is introduced so data-smoothing can be done easily in two dimensional space. The results of the data smoothing are controlled by limiting the stretch of those springs to be less than the resolution of the experimental method. With the full-field hybrid method, the strain contours from moire interferometry can be easily obtained with good accuracy. If the properties of the material are known, the stress patterns can also be obtained. In addition, this method can be used to analyze any two-dimensional displacement data, including the grid method and holography.
An investigation of shear response of composite material systems
Zhang, Yanhong (Virginia Tech, 1994)
An investigation of shear response for various composite material systems is presented. The uniformity of the strain fields is studied experimentally and numerically for different specimen configurations. Conventional strain gage measurements and the moiré interferometry technique are employed to obtain information of actual deformation of the specimen. Based on the contour maps of displacement obtained from moiré tests, the localized hybrid method is used to quantify the magnitude and scale of the nonuniform deformation in the real strain fields. The finite element analysis is also performed for predicting the global nonuniformity of the strain fields. It is shown that the significant nonuniformity in shear deformation observed in experimental results can not be predicted by the existing analytical and numerical models. It is considered that the nonuniformity is primarily at a local level, which is associated with the material inhomogeneity. The implication of the local non-uniform deformation fields on the material property evaluation and failure prediction are discussed. The nonlinearity of shear response is investigated experimentally by performing strain gage and moiré tests. Curve fitting techniques proved to be a convenient and effective tool for characterizing the nonlinear shear response of composites. It is suggested that not only the initial shear modulus but also other coefficients of the fitting function be used for the evaluation of nonlinear shear behavior of a composite. The experimental results show that the nonlinearity has no significant effect on the shear strain distribution, verifying the validity of correction factors in the nonlinear range. Shear stress at a selected shear strain level is suggested as an engineering definition for shear strength of composites. The shear response of some novel composites is also investigated, the test results of which are presented.
Moiré interferometry at high temperatures
Wu, Jau-Je (Virginia Tech, 1992)
The objective of this study was to provide an optical technique allowing full-field in-plane deformation measurements at high temperature by using high-sensitivity moiré interferometry. This was achieved by a new approach of performing deformation measurements at high temperatures in a vacuum oven using an achromatic interferometer. The moiré system setup was designed with particular consideration for the stability, compactness, flexibility, and ease of control. A vacuum testing environment was provided to minimize the instability of the patterns by protecting the optical instruments from the thermal convection currents. Also, a preparation procedure for the high-temperature specimen grating was developed with the use of the plasma-etched technique. Gold was used as a metallic layer in this procedure. This method was demonstrated on a ceramic block, metal/matrix composite, and quartz. Thermal deformation of a quartz specimen was successfully measured in vacuum at 980 degrees Celsius, with the sensitivity of 417 nm per fringe. The stable and well-defined interference patterns confirmed the feasibility of the developments, including the high-temperature moiré system and high-temperature specimen grating. The moiré system was demonstrated to be vibration-insensitive. Also, the contrast of interference fringes at high temperature was enhanced by means of a spatial filter and a narrow band interference filter to minimize the background noise from the glow of the specimen and heater. The system was verified by a free thermal expansion test of an aluminum block. Good agreement demonstrated the validity of the optical design. The measurements of thermal deformation mismatch were performed on a graphite/epoxy composite, a metal/matrix composite equipped with an optical fiber, and a cutting tool bit. A high-resolution data-reduction technique was used to measure the Strain distribution of the cutting tool bit.
A reliability-based method for optimization programming problems
Esteban, Jaime (Virginia Tech, 1992-12-01)
In this study, a method is developed to solve general stochastic programming problems. The method is applicable to both linear and nonlinear optimization. Based on a proper linearization, a set of probabilistic constraints (performance functions) can be transformed into a corresponding set of deterministic constraints. this is accomplish by expanding all the constraints about the most probable failure point. The use of the proposed method allows the simplification of any stochastic programming problems into a standard linear programming problem. Numerical examples are applied to the area of probability- based optimum structural design.
Scale effects in buckling, postbuckling and crippling of graphite-epoxy Z-section stiffeners
Wieland, Todd M. (Virginia Tech, 1991)
Scale model testing can improve the cost-effectiveness of composite structures by reducing the reliance on full size component testing. Use of scale models requires the relationship be known between the responses of the small scale model and full size component. This relationship may be predicted by dimensional analysis or through mechanics formulations. The presence of physical constraints may prevent the complete reproduction of all responses in small scale models. Scaling relationships may not be available at the level necessary to predict all scaled responses. Investigations of the scalability of composite structures are needed in order to evaluate the reliability of small scale model predictions of the responses of full size components. The scaling of the responses of graphite-epoxy laminated composite Z-section stiffeners subjected to uniaxial, compressive loading has been evaluated. The response regimes investigated are prebuckling, initial local buckling, postbuckling and crippling. A mechanistic approach to scaling has been used, in which the scalability of the responses has been judged relative to governing mechanics models. A linked-plate analytical model has been obtained which predicts the buckling loads, and from which two nondimensional load parameters have been obtained. The finite element method has been used for prediction of the buckling and postbuckling responses. The analytical and numerical analyses were used to define an experimental program involving fifty-two specimens of seventeen basic geometrical configurations and three stacking sequences. The buckling, postbuckling and crippling responses were largely determined by the flange-to-web width ratio and both the absolute and relative values of the bending stiffnesses. Buckling loads increased with decreasing flange width and the laminate orthotropy ratio, and increasing flange-to-web corner radii and laminate thickness. The postbuckling load range was the greatest for specimens having wider flanges, but the failure stresses were greatest among the narrower specimens. The crippling mechanisms included flange free edge delamination at both nodal and anti-nodal axial positions, material crushing in the flange-to-web corner at nodal axial positions, and ply splitting in the flange-to-web corner at anti-nodal axial locations. The constraint of the potted end supports of the experimental specimens was not scaled. The effect of displacements within the end supports was manifested by lower prebuckling axial stiffnesses than predicted based on the gage length properties alone. This phenomenon required a post-test adjustment to the data in order to permit comparisons of the experimental and finite element predictions of the response of the gage length on an equivalent basis. Once corrected, the prebuckling stiffnesses were generally observed to have scaled. One of the nondimensional load parameters normalized the buckling loads for specimens of various web widths only. The second parameter normalized the buckling loads for all of the geometric and material variables contained in the model. This parameter also normalized the postbuckling loads, and is, therefore, a general nondimensional parameter for the buckling and postbuckling responses of the Z-section stiffeners. No scale effects were observed in the buckling response. The quality of the postbuckling load predictions degraded with the width of the postbuckling load range. It was not determined whether genuine scale effects were present in the postbuckling response or whether the observed error was a result of inadequate modelling of structural and material nonlinearities or other effects such as damage development in the specimens. Good correlation between experimental and finite element predictions of the out-of-plane displacements and load-axis strains has been demonstrated. Predicted local material strain development has been related to the structural deformation characteristics. Consideration of individual strain values, however, could not predict which of several competing failure modes would determine the actual crippling response. Neither could the strain data provide any quantitative prediction of the crippling loads. Thus, the determination of strength scale effects is hindered by the complex structural-material interaction and the lack of a mechanics-based interactive failure model.
Scaling effects in the static and dynamic response of graphite- epoxy beam-columns
Jackson, Karen E. (Virginia Tech, 1990-05-05)
Scale model technology represents one method of investigating the behavior of advanced, weight-efficient composite structures under a variety of loading conditions. Testing of scale models can provide a cost effective alternative to destructive testing of expensive composite prototypes and can be used to verify predictions obtained from finite element analyses. It is necessary, however, to understand the limitations involved in testing scale model structures before the technique can be fully utilized. The objective of this research is to characterize these limitations, or scaling effects, in the large deflection response and failure of composite beams. Scale model beams were loaded with an eccentric axial compressive load designed to produce large bending deflections and global failure.
Scaling Effects on Damage Development, Strength, and Stress-Rupture Life on Laminated Composites in Tension
Lavoie, J. André (Virginia Tech, 1997-04-04)
The damage development and strength of ply-level scaled carbon/epoxy composite laminates having stacking sequence of [+Tn/-Tn/902n]s where constraint ply angle, T, was 0, 15, 30, 45, 60, and 75 degrees, and size was scaled as n=1,2,3, and 4, is reported in Part I. X-radiography was used to monitor damage developments. First-ply failure stress, and tensile strength were recorded. First-ply failure of the midplane 90 deg. plies depended on the stiffness of constraint plies, and size. All 24 cases were predicted using Zhang's shear-lag model and data generated from cross-ply tests. Laminate strength was controlled by the initiation of a triangular-shaped local delamination of the surface angle plies. This delamination was predicted using O'Brien's strain energy release rate model for delamination of surface angle plies. For each ply angle, the smallest laminate was used to predict delamination (and strength) of the other sizes. The in-situ tensile strength of the 0 deg. plies within different cross-ply, and quasi-isotropic laminates of varying size and stacking sequence is reported in Part II. No size effect was observed in the strength of 0 deg. plies for those lay-ups having failure confined to the gauge section. Laminates exhibiting a size-strength relationship, had grip region failures for the larger sizes. A statistically significant set of 3-point bend tests of unidirectional beams were used to provide parameters for a Weibull model, to re-examine relationship between ultimate strength of 0 deg. plies and specimen volume. The maximum stress in the 0 deg. plies in bending, and the tensile strength of the 0 deg. plies (from valid tests only) was the same. Weibull theory predicted loss of strength which was not observed in the experiments. An effort to model the durability and life of quasi-isotropic E-glass/913 epoxy composite laminates under steady load and in an acidic environment is reported in Part III. Stress-rupture tests of unidirectional coupons immersed in a weak hydrochloric acid solution was conducted to determine their stress-life response. Creep tests were conducted on unidirectional coupons parallel and transverse to the fibers, and on ±45°. layups to characterize the lamina stress- and time-dependent compliances. These data were used in a composite stress-rupture life model, based on the critical element modeling philosophy of Reifsnider, to predict the life of two ply-level thickness-scaled quasi-isotropic laminates.
A study of tension, compression, and shear test methods for advanced composites
Johnson, David Page (Virginia Tech, 1991-05-05)
A study of the literature pertaining to test methods for advanced composite materials has been carried out. Several test methods were discussed and compared for each of three areas of interest. These areas were uniaxial tension, uniaxial compression and in-plane shear. Test methods were selected for tension, compression and shear and guidelines set for the entry of material property data into a comprehensive mechanical property database being undertaken by Virginia Tech's Center for Composite Materials and Structures (CCMS). According to the findings, recommendations for future work were made.
Thermoplastic composite consolidation
Li, Min-Chung (Virginia Tech, 1993-10-05)
Fabrication of high-quality composites from thennoplastic prepregs requires careful selection of the processing cycles so that intimate contact at the ply interfaces is achieved resulting in the formation of strong interply bonds and the process-induced residual stress is minimized to ensure superior mechanical performance. The void formation and the consolidation mechanism were studied experimentally. A refined model was developed to relate the processing parameters of pressure, temperature and time to the interply intimate contact of thermoplastic composites. The model was developed by integrating a prepreg surface topology characterization with a resin flow analysis. Both unidirectional and cross-ply lay-ups were modeled. Two-ply unidirectional laminae fabricated from graphite-polysulfone and graphite-PEEK prepregs and [0/90/0]_T laminates were consolidated using different processing cycles. Optical microscopy and scanning acoustic microscopy were used to obtain the degree of intimate contact data. Agreement between the measured and calculated degree of intimate contact was good. A finite element model was developed to analyze residual stresses in thermoplastic composites by combining a plane-strain elasticity analysis and a temperature-dependent matrix properties. The residual stress model takes into account the mismatch of the thermal expansion coefficients and the crystallization shrinkage of the matrix. [O₁₀/90₆]_T graphite-PEEK laminates were manufactured at different cooling rates to verify the model. The induced residual thermal defonnations were measured by a shadow moire system. The model accurately estimated the out-of-plane displacement of the non-symmetrical laminates.

Browsing by Author "Morton, John"

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