The compressive behavior of continuous fiber composites is not as well understood as their tensile behavior because research and industrial applications have until recently focused on the latter. Furthermore, most theoretical and experimental studies on the compression of composites have examined the case of unidirectional specimens with fibers along the loading direction (0�� fibers). While this is a logical approach since it isolates the failure mode specific to this geometry (kinking), the study of multidirectional laminates is essential because these are used in all practical applications. Few theories model the compressive behavior of multidirectional laminates. None of the theories account for the stress field or the sequence and interaction of the various observed failure modes (kinking, delamination, matrix failure) specific to the multidirectional configuration. The principal objective of this investigation is to construct a realistic theory to model the compressive behavior of multidirectional composites. Compression experiments have repeatedly shown that the initial failure mode was in-plane kinking of 0�� fibers initiated at the edges of the specimens. We decided to base our compressive failure theory upon interlaminar stresses because in multidirectional laminates these are known to exist in a boundary layer along the edges. This required development of an analytical theory giving the amplitude of these stresses at the free edges. We then incorporated these stresses into a new general microbuckling equation for 0�� fibers. The global laminate failure strain was determined through several fiber and matrix failure criteria. Theoretical predictions were compared with experimental results obtained from compression testing of graphite/thermoplastic laminates with the same ply sequence but different off-axis ply angles. The theory correlated well with experiments and confirmed that in-plane kinking was the critical failure mode at low and medium angles, while revealing that out-of-plane buckling was responsible for failure at high angles. Furthermore, the theory correctly predicted the sequence of various fiber and matrix failure modes. / Graduation date: 1997

Graduation date: 1994

No description available.

No description available.

Changes in the stiffness and strength of notched quasi-isotropic graphite-epoxy laminates were recorded and related to the fatigue damage. Two different laminates [0,90,+45,-45]s (type A) and [ +45, 90, -45, 0] s (type B) were considered and the effects of stacking sequence were compared. Nondestructive testing techniques such as X-radiography, moire technique, acoustic emission, deply technique, and stiffness change were performed to observe damage development. Static properties and damage initiation were related to an approximate stress analysis. Results show that the mechanical response and the fatigue damage depend strongly on the stacking sequence of laminates. In general, residual strength increased remarkably for both laminates due to stress redistributions while the continuous stiffness change curve is typical for each laminate and reflects damage characteristics. Buckling effects as well as matrix cracking and delaminations contribute to stiffness changes. / M.S.

The effects of room temperature tensile loading and five thermal loadings upon the development of transverse microcracks (TVM) in Celion 6000/PMR-15 graphite-epoxy laminates were investigated. Microcracks were observed using a replicating technique, microscopy and x-ray. The mechanical or thermal load at which microcracking initiates and the ply residual stresses were predicted using laminate analysis with stress and temperature-dependent materials. The TVM density as a function of tensile load was predicted using the multiple cracking theory proposed by Garrett, Parvizi and Bailey. Reasonably good correlation between theory and experiment was obtained for both the load at initial failure and the TVM density as a function of tensile load. It has been shown that TVM density and the load to initiate microcracking are functions of the type of loading and the laminate configuration. Generally, cross-ply laminates exhibit higher TVM density after thermal loading than do quasi-isotropic laminates, but the converse is true for tensile loading. Cross-ply laminates attain saturation TVM density prior to failure, but the TVM density of quasi-isotropic laminates continues to increase until failure. Edge effects have a significant influence on the development of TVM, and TVM present at the free edge of a laminate extend throughout the entire width of the laminate. <i>[Vita removed Feb. 15, 2011. GMc]</i> / Master of Science

An investigation was conducted to study the constraint effects on flaw growth in composite laminates. Results were presented for the case of a transverse flaw in an interior ply perpendicular to the loading axis. Two orientations of the flawed ply were examined (0 and 90 degrees), and two distinctly different constraint situations were studied (cross-ply constraint and biaxial constraint). Throughout the study, various nondestructive testing methods were employed to evaluate the material response and to determine the damage and damage growth in the specimens. These techniques include replication, ultrasonic c-scan, ultrasonic attenuation, acoustic emission, x-radiography, thermography and stiffness measurement. The effects of constraint on the response of composite materials can be classified in two categories: (a) in-plane effects and (b) through-the-thickness effects. In-plane constraint is the principal contributor to notched strength and changes in notched strength under quasi-static loading. Through-the-thickness constraint controls the pattern and spacing of transverse cracks in the off-axis plies to form a characteristic damage state in the laminates. Out-of-plane stresses produced by constraints are influential on the growth of damage along ply interfaces, especially during cyclic loading. The mode of damage and the extent of damage in constrained notched plies are governed by the stress state in those plies, as determined by the constraining plies, and the relationship of the stress state to the strength state. Maximum constraint on the flawed ply does not produce minimum damage in the laminate; and the lesser degree of damage (in terms of axial splitting and delamination) does not necessarily result in a higher laminate strength or long fatigue lives. In the design of composite structures, a compromise has to be reached with regard to optimizing material parameters such as strength, stiffness, fatigue life, and residual strength. In maximizing one parameter, one might have to sacrifice other requirements on the other material properties in the design. / Ph. D.

No description available.

The focus of this work is the buckling response of symmetrically laminated composite plates having a planform area in the shape of an isosceles trapezoid. The loading is assumed to be inplane and applied perpendicular to the parallel ends of the plate. The tapered edges of the plate are assumed to have simply supported boundary conditions, while the parallel ends are assumed to have either simply supported or clamped boundary conditions. Plates with one end being up to 3 times narrower than the other end, and the plate being up to 3 time longer than the width of the wide end are considered. A semi-analytic closed-form solution based on energy principles and the Trefftz stability criterion is derived and solutions are obtained using the Rayleigh-Ritz method. Intrinsic in this solution is a simplified prebuckling analysis which approximates the inplane force resultant distributions by the forms N_x=P/W(x) and N_y=N_xy=0, where P is the applied load and W(x) is the plate width which, for the trapezoidal planform, varies linearly with the lengthwise coordinate x. The out-of plane displacement is approximated by a double trigonometric series. This analysis is posed in terms of four nondimensional parameters representing orthotropic and anisotropic material properties, and two nondimensional parameters representing geometric properties. With nondimensionalization, the analysis is well suited for parametric studies. The analysis uses standard eigenvalue extraction routines and converges using 5 terms in the out-of-plane displacement series. It appears that this analysis captures the buckling response of plates having tapered planform and should be a useful design tool. For comparison purposes, a number of specific plate geometry, ply orientation, and stacking sequence combinations are investigated using the general purpose finite element code ABAQUS. Comparison of buckling coefficients calculated using the semi-analytical model and the finite element model show agreement within 5%, in general, and within 15% for the worst cases. In addition to the good agreement between the semi-analytical analysis and the finite element results, the finite element model also suggests that the simplified inplane force resultant distribution assumed in the analysis is valid. In order to verify both the finite element and semi-analytical analyses, buckling loads are measured for graphite/epoxy plates having a wide range of plate geometries and stacking sequences. Test fixtures, instrumentation system, and experimental technique are described. Experimental results for the buckling load, the buckled mode shape, and the prebuckling plate stiffness are presented and show good agreement with the analytical results regarding the buckling load and the prebuckling plate stiffness. However, the experimental results show that for some cases the analysis underpredicts the number of halfwaves in the buckled mode shape. In the context of the definitions of taper ratio and aspect ratio used in this study, it is concluded that the buckling load always increases as taper ratio increases for a given aspect ratio for plates having simply supported boundary conditions on the parallel ends. There are combinations of plate geometry and ply stacking sequences, however, that reverse this trend for plates having clamped boundary conditions on the parallel ends such that an increase in the taper ratio causes a decrease in the buckling load. The clamped boundary conditions on the parallel ends of the plate are shown to increase the buckling load compared to simply supported boundary conditions. Also, anisotropy (the D₁₆ and D₂₆ terms) is shown to decrease the buckling load and skew the buckled mode shape for both the simply supported and clamped boundary conditions. / Master of Science

The objective of this study was to investigate the bending strength and failure characteristics of AS4/3501-6 and AS4/1806 graphite/epoxy angles sections as a function of processing induced defects and porosity. The angle sections were removed from 30-inch long angles fabricated at Lockheed Georgia Company with two quasi-isotropic stacking sequences, (± 45/90₂/ ∓ 45/0₂), and (± 45/90₂ ∓ 45/0₂)₃. Various degrees of porosity were introduced into the angles using four processing techniques: a standard lay-up, a solvent wipe during lay-up, moisture introduction between plies during lay-up, and a low pressure cure cycle. Two 2.5-inch wide angle sections, each with a 1.5-inch short leg and a 3.0-inch long leg, were bonded together along their long leg to form a T-shaped specimen. Bending of the T-specimen was introduced by pressing up on the underside of the flanges while holding the base of the specimen fixed. The experimental results have shown a significant effect of the processing induced defects on the failure load and bending stiffness for AS4/3501-6 specimens, but not for AS4/1806 specimens. An anisotropic analysis of the angle curved section was performed using Lekhnitskii's stress function approach. Stress and strain fields were studied and two failure criteria (Dual maximum stress and Tsai-Wu) were investigated in order to predict T-specimen failure load and failure mode. Reasonable correlation between prediction and experiments was found for the AS4/3501-6 (± 45/90₂/ ∓ 45/0₂)₃ T-specimens, but both failure criteria were found to be too conservative in predicting failure for the AS4/3501-6 (± 45/O₂/ ∓ 45/90₂)₃, T-specimens. The predicted failure modes were in good agreement with the experimental observations for both Iaminates. / Master of Science