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The effect of cooling rate and stacking sequence on fatigue behavior was analyzed for notched quasi-isotropic APC-2 laminates. The fatigue behavior of fast (475° F/min) and slow (1° F/min) cooled specimens of the following two layups was studied: Layup A of (-45/0/45/90), and Layup B of (45/90/-45/0),. All specimens were subjected to a load controlled, Tension - Tension fatigue loading with a stress ratio R = 0.1 at a frequency of 5 Hz. Parameters such as strain, temperature rise across the notch and number of cycles fatigued were continuously monitored during the fatigue tests. Damage was monitored by the reduction in modulus, penetrant enhanced X-ray radiography, and Scanning Acoustic Microscopy (SAM). Post failure analysis of the specimens was carried out by Scanning Electron Microscopy (SEM). A quasi-3D Finite Element Analysis was performed to compare the differences in the interlaminar stresses arising around the notch in the specimens of the two layups under study. The ultimate static strengths did not show any appreciable dependence on either cooling rate or stacking sequence. The maximum load in the fatigue cycle was selected as a fraction of the ultimate notched static tensile strength in each case. The fatigue lives showed appreciable difference between the two cooling rates in layup A when tested at the lower load levels. The fatigue behavior was vastly different between the two cooling rates for specimens of layup B. Also, specimens of layup B, both fast and slow cooled, had longer lives than their counterparts from layup A. A model, based on a constant strain-to-failure criterion, was developed for life prediction and the predicted lives are in good agreement with the experimental values. Fast cooled specimens of layup A showed a gradual degradation in the modulus till failure while slow cooled specimens of the same layup showed a more drastic reduction as they approached failure. No such distinguished behavior was observed in the specimens of layup B. Scanning Electron Micrographs of the fast cooled specimens indicate better fiber/matrix bonding conditions and more matrix plasticity as compared to the slow cooled specimens. A rotated stacking sequence technique was used for the calculation of the interlaminar stresses around the notch. No single stress seems to control failure but it is likely that failure occurs by the interaction of the different stresses since a three-dimensional stress state exists at the notch. Based on this reasoning, effective stresses were calculated at all those interfaces where one of the interlaminar (normal or shear) stresses has a maximum value. A comparison of the effective stresses calculated showed the layup A to be 1.7 times more prone to delamination than layup B. Damage analysis of the fatigued specimens by X-ray radiography and Scanning Acoustic Microscopy shows the specimens of layup A to be dominated by delaminations as compared to those of layup B. The interfaces predicted to be critical by FEA agreed well with the experimental observations, in general. / Master of Science

Thesis (Sc.D.)--Massachusetts Institute of Technology, Dept. of Aeronautics and Astronautics, 1981. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND AERO. / Includes bibliographical references. / by Olivier Andre Bauchau. / Sc.D.

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Within the past several years, researchers have detected the presence of a third “phase” between the bulk fiber phase and bulk matrix phase in a polymeric matrix composite. This finite-thickness region — termed the interphase — possesses mechanical, physical, and chemical properties that are distinct from the fiber and matrix constituents. Thus, the interphase embodies the characteristics of the fiber/matrix bond, including the strength and stiffness of the bond. In essence, the interphase represents the composite system, since it defines the level of synergistic interaction that occurs between the load-carrying fibers and the binding matrix material. Recent interest in the interphase has spawned international conferences and a technical journal devoted to its study. Despite this spate of research, some very fundamental questions about the interphase have remained unanswered. One such question is: “What is best for the performance of a composite, a strong or weak or intermediate-strength interphase?” It is surprising that this question is even asked, since, until recently, it had been assumed that the stronger the fiber/matrix bond, the better the composite behavior. It is now known that this adage is far from true. Two formidable challenges await those who wish to correlate the strength of the interphase to the mechanical performance of polymeric matrix composite materials. First, one seeks to systematically alter the interphase in order to exploit this variable. In this study, fourteen material systems representing permutations of four carbon fibers, three matrix systems, percentages of fiber surface treatment, and three sizing conditions have been examined. Secondly, one needs to quantitatively characterize the properties of the resultant interphase in order to correlate the bond condition to the composite’s mechanical behavior. This investigation utilizes two techniques, the Continuous Ball Indentation Test and transverse flexure testing, as a means of interrogating the strength of the interphase. The influence of the interphase on the tensile and compressive strength and modulus of crossplied laminates possessing a center hole is investigated. Unnotched angle-ply ([±45]_ns) laminates are also tested in order to assess the role of the interphase in the strength of a “matrix-dominated” laminate. Fully-reversed (R =-1), axial fatigue of notched cross-plied laminates from each of the fourteen material systems 1s performed. During fatigue testing several data are monitored, including cycles to failure, dynamic modulus, and notch temperature. The tension-tension (R= 0.1) fatigue response of the unnotched angle-ply laminates is also studied. Results from X-ray radiography of fatigue-damaged specimens help to explain the relationship between the interphase and the initiation and propagation of life-critical damage mechanisms. Having observed the formative role played by the interphase in the performance of these laminates, an attempt is made to introduce variables representing the interphase into micromechanical models of composite behavior. / Ph. D.

A change in laminate thickness due to terminating internal plies acts as a stress riser for both intralamina and interlaminar stresses. This laminate configuration is referred to as a ply drop. The linear elastic, three-dimensional stress distributions in the vicinity of a ply drop are determined for a graphite-epoxy laminate subject to axial tension and compression by a finite element analysis. It is shown that the interlaminar stresses have a maximum magnitude at the ply drop-off, and decrease proceeding away from the drop-off. Two modes of failure initiation are analyzed. In the pure resin regions surrounding the dropped plies, the maximum stress criterion is assumed to govern failure. The Tsai-Wu criterion is used for intralamina failure prediction. The influence of two laminate lay-ups and a variety of ply drop geometries on the response and failure are presented. / M.S.

The characteristics of thermally-induced transverse cracks in T300/5208 graphite-epoxy cross-ply and quasi-isotropic laminates were investigated both experimentally and analytically. The formation of transverse cracks and the subsequent crack spacing present during cooldown to -250°F (116K) and thermal cycling between 250 and -250°F (116 and 394K) was investigated. The state of stress in the vicinity of a transverse crack and the influence of transverse cracking on the laminate coefficient of thermal expansion (CTE) was predicted using a generalized plane-strain finite element analysis and a modified shear-lag analysis. It is shown that a majority of the cross-ply laminates experienced transverse cracking during the initial cool-down to -250°F whereas the quasi-isotropic laminates remained uncracked. All cross-ply laminates and the [0/±45/90]_s quasi-isotropic laminate exhibited transverse cracking following 20 thermal cycles. The uniformity of crack spacing increased with an increasing number of thermal cycles. The cross-ply laminates exhibited a rather sharp drop in CTE at crack densities less than 50 cracks/in. (19.7 cracks/cm) whereas the quasi-isotropic laminates exhibited a smaller decrease in CTE. The in situ transverse strength of the 90° layers was more than 1.9 times greater than the transverse strength of the unidirectional 90° material for all laminates investigated. / M.S.

Carbon Fiber / Epoxy Laminates are increasingly being used in the primary structure of aircraft. To make effective use these materials, it is necessary to consider the ability of a laminate to resist damage, as well as material strength and stiffness. A possible means for improving damage tolerance is the use of non-traditional composite laminates, in which the longitudinal 0 plies are replaced with 5 or 10 plies. The main objectives of this collaborative Georgia Tech / Boeing research was the characterization of these non-traditional laminates, and the determination of appropriate lamina-level analytical techniques that are capable of predicting the changes caused by the use of slightly off-axis longitudinal plies. A quasi-isotropic [45/90/-45/theta/45/90/-45/-theta]s and hard [45/theta/-45/theta/90/45]s lay-up, where theta =0,5 or 10, were tested in open hole tension, filled hole tension, open hole compression, single shear bearing, and unnotched tension. These coupon level tests illustrated the effects of lay-up, notch constraint, and load type on traditional and non-traditional laminates. Die penetrant enhanced in-situ radiography was performed to determine the extent of damage suppression. The use of non-traditional laminates was found to reduce longitudinal ply cracking and delamination, with significant effect on the stress distribution around the notch. The use of non-traditional laminates also resulted in a 15%-20% improvement in bearing strength of the traditional laminates. Several predictive techniques were implemented to evaluate their ability to predict the effect of slight changes in ply orientations. A progressive damage model was written to compare Tsai-Wu, Hashin, and Maximum Stress unnotched strength criterion. Additionally, several semi-empirical failure theories for notched strength prediction were compared with linear and bi-linear cohesive zone models to determine applicability to non-traditional laminates.