Damage Development and Moduli Reductions in Nicalon—Calcium Aluminosilicate Composites under Static Fatigue and Cyclic Fatigue

Unidirectional and cross-ply Nicalon fiber-reinforced calcium aluminosilicate (CAS) glass-ceramic composite specimens were subjected to tension–tension cyclic fatigue and static fatigue loadings. Microcrack densities, longitudinal Young's modulus, and major Poisson's ratio were measured at regular intervals of load cycles and load time. The matrix crack (0° plies) density and transverse crack (90° plies) density increased gradually with fatigue cycles and load time. The crack growth is environmentally driven and depends on the maximum load and time. Young's modulus and Poisson's ratio decreased gradually with fatigue cycles and load time. The saturation crack densities under fatigue loadings were found to be comparable to those under monotonic loading. A matrix crack growth limit strain exists, below which matrix cracks do not grow significantly under fatigue loading. This limit coincides with the matrix crack initiation strain. Linear correlations between crack density and moduli reductions obtained from quasi-static data can predict the moduli reductions under cyclic loading, using experimentally measured crack densities. A logarithmic correlation can predict the Young's modulus reduction in a limited stress range. A fatigue crack growth model is proposed to explain the presence of two distinct regimes of crack growth and Young's modulus reduction.     

Multiple Cracking of Unidirectional and Cross-PlyCeramic Matrix Composites

This paper examines the multiple cracking behavior of unidirectional and cross-ply ceramic matrix composites. For unidirectional composites, a model of concentric cylinders with finite crack spacing and debonding length is introduced. Stresses in the fiber and matrix are found and then applied to predict the composite moduli. Using an energy balance method, critical stresses for matrix cracking initiation are predicted. Effects of interfacial shear stress, debonding length and bonding energy on the critical stress are studied. All the three composite systems examined show that the critical stress for the completely debonded case is lower than that for the perfectly bonded case. For cross-ply composites, an extensive study has been made for the transverse cracking in 90° plies and the matrix cracking in 0° plies. One transverse cracking and four matrix cracking modes are studied, and closed-form solutions of the critical stresses are obtained. The results indicate that the case of combined matrix and transverse crackings with associated fiber/matrix interfacial sliding in the 0° plies gives the lowest critical stress for matrix cracking. The theoretical predictions are compared with experimental data of SiC/CAS cross-ply composites; both results demonstrated that an increase in the transverse ply thickness reduces the critical stress for matrix cracking in the longitudinal plies. The effects of fiber volume fraction and fiber modulus on the critical stress have been quantified. Thermal residual stresses are included in the analysis.     

A ply-level electrical resistance approach to monitor crack evolution in a laminate SiC/SiC composites

In the aim of providing a reliable technique to monitor the development of damage in 0°/90° melt-infiltrated SiC-fiber reinforced prepreg laminate ceramic-matrix composites, it was hypothesized that the electrical resistivities of different layers of this material were significantly different due to their free Si content and morphology. Three distinct layers: a 0° fiber ply, a 90° fiber ply and a matrix only ply, were distinguished in the composite architecture. Free silicon is the most conductive phase in this composite system; however, the Si content and morphology were different in each of the three types of plies. Unidirectional and [0°/90°]_2s specimens enabled quantification of ply-level resistivities. An electric circuit model was constructed; it consists of parallel resistors where each resistor represents a ply in the composite system. This ply-level electrical model was validated using composites of different vintages which contained different silicon contents. A room temperature stepped fatigue test was conducted and the ply level circuit model was used to discern crack morphology with the support of acoustic emission and digital image correlation.     

Delamination Cracking in a Laminated Ceramic-Matrix Composite

Delamination crack propagation has been investigated in a laminated fiber-reinforced ceramic-matrix composite. The crack growth initiation resistance has been shown to be dominated by the critical strain energy release rate for the matrix. However, the resistance increases with crack extension because of bridging effects associated with intact fibers and, in some cases, intact segments of matrix. The delamination cracks also assume a steady-state trajectory within a 0° layer close to the 0°/90° interface.     

Influence of the interfacial resin layer on delamination initiation in broken ply composite laminates

The present study has investigated the influence of a resin layer on the delamination initiation at the interface of broken and continuous plies in the case of GR/E (graphite/epoxy) laminates with broken central plies. A full three-dimensional (3D) finite element (FE) analysis was performed with each layer of the laminate modelled as homogeneous and orthotropic. The interface between the broken and the continuous plies was modelled with a thin resin-rich layer. Eight-noded isoparametric layered elements were used to model the laminate specimen. Also, 3D contact elements were used to prevent inter-penetration of the delaminated faces at the interface. Based on the results of the 3D FE analysis, strain energy release rates were calculated at the delamination front using Irwin's 'crack closure integral'. Using the concepts of linear elastic fracture mechanics (LEFM), the strain energy release rate was used as a parameter for assessing delamination initiation. The effects of various factors such as resin layer stiffness, resin layer thickness, and fibre orientation at the interface on the three components of the strain energy release rates, namely G_I, G_II and G_III, were studied for laminates with various crack sizes of the broken ply, and the influence of the resin layer in the delamination initiation was established. It was observed that delamination initiation is a mixed-mode phenomenon even in the case of uniaxial loading and the dominance of the mode of delamination is governed by the resin layer stiffness, thickness, and lamina orientation at the interface. The present work also concludes that an increase in the resin layer modulus leads to an increase in the probability of mode I delamination while the probability of mode II delamination decreases. A 0/90 interface exhibits a higher chance of delamination in modes I and II, while mode III delamination is maximum for 0/30 and 0/60 fibre orientation interfaces. It was also observed that the larger the crack width, the greater the probability of delamination initiation at the interface.     

High-Temperature Transverse Fracture Toughness of Nicalon-Fiber-Reinforced CAS-II Glass-Ceramic Matrix Composite

Cracking parallel to the fibers in off-axis plies is usually the initial form of damage in composite laminates. This cracking process has been associated with the (transverse) fracture toughness, defined by the critical strain energy release rate, G _Ic. The measurement of G _Ic provides basic information about the transverse crack resistance. In this study, the utility of the double torsion (DT) test technique to determine _G Ic in a glass-ceramic matrix composite (Nicalon/CAS-II) at temperatures up to 1000°C has been demonstrated. _G Ic did decrease moderately with increasing temperature (as does the bulk matrix); however, no evidence of an interphase oxidizing effect on crack growth (parallel to the fibers) could be found. The inevitable misalignment of fibers in the material was not very efficient at bridging the crack in the DT specimens, in contrast to the significant matrix crack interactions with the fibers reported for other geometries such as double cantilever beam and flexure specimens.     

Fractography of injection molded polycarbonate acrylonitrile-butadiene-styrene terpolymer blends

Fractography has been used in the post-failure analysis of single edge notched specimens of injection molded blends of polycarbonate (PC) and acrylonitrile-butadiene-styrene terpolymer (ABS). The mode of ductile tensile fracture of single edge notched specimens depended on comosition. Plane stress shear tearing was observed in the composition range PC/ABS 90/10 to 70/30 by weight where PC was the continuous phase. Intermediate compositions, PC/ABS 60/40 to 40/60, had a co-continuous or almot co-continuous phase morphology; these blends fractured by mixed mode pop-in, where a tunneling center crack relieved the triaxiality and permitted plane stress shear lips to form near the edges. Herringbone fracture, a plane strain mode characterized by discontinuous crack growth, was observed when ABS was the continuous phase, PC/ABS 30/70 to 10/90. An S-shaped relationship was observed between the ductile-to-brittle transition temperature and the composition. Addition of ABS to PC increased ductility up to PC/ABS 70/30 and 60/40, which were the most ductile compositions. Further addition of ABS decreased the ductility, and the least ductile compositions were PC/ABS 30/70 and 10/90.     

The effect of isothermal aging on transverse crack development in carbon fiber reinforced cross-ply laminates

An investigation into the effect of isothermal aging on the development of transverse cracks in cross-ply laminates of two high temperature composite systems was performed. The composite materials investigated were BASF X5260/640–800 and DuPont Avimid K/IM6. Changes in the glass transition temperature, composite weight loss, crack density, and mode I intralaminar fracture toughness were monitored during isothermal aging in air at 177°C for up to 2232 h. The two laminate configurations used in this study include two variations of the generic cross-ply configuration [0₂/90_n]_s, in which n equals 1 and 2. The results of this investigation show that a layer of degraded material forms at the surface of the X5260/640–800 bismaleimide laminates and that the thickness of the degraded layer increases with aging time. After 744 h of aging, transverse cracks form in the surface plies and an increasing crack density evolves as aging time is increased; however, transverse cracks do not form in the inner 90° ply groups with aging during the time period investigated. The Avimid K/IM6 thermoplastic polyimide laminates, which show evidence of cracking prior to aging, do not exhibit any significant change in crack density with aging. The results of the aging experiments also show that the bismaleimide system exhibits a weight loss of 1.5% and an increase in glass transition temperature from 250°C to 300°C after 2232 h of aging at 177°C, while the thermoplastic polyimide system shows a weight loss of only 0.05% and an increase in glass transition temperature from 280 to 285°C after 2232 h. Changes in the resistance to crack formation are also seen in these materials during aging. The mode I intralaminar fracture toughness, a measure of resistance to transverse crack formation, shows a 50% decrease after aging for 2232 h for the bismaleimide system, while the behavior exhibited by the thermoplastic polyimide shows little evidence of a reduction.     

The Use of Peel Ply as a Method to Create Reproduceable But Contaminated Surfaces for Structural Adhesive Bonding of Carbon Fiber Reinforced Plastics

In the fabrication of fiber-reinforced plastics materials peel plies are commonly used as an additional layer on top of the laminates to sponge up the surplus resin and to create an activated surface for adhesive bonding or coating by peel ply removal. In theory, the peel ply removal results in a new and uncontaminated fracture surface that is activated by polymer chain scission. The peel ply method is often presented as being a good surface treatment for structural bonding.

In this study carbon fiber-reinforced plastics (Hexcel® 8552/ IM7) were produced by the use of five different peel plies and a release foil made of polytetrafluorethylene (PTFE). The peel plies themselves and the surfaces on the CFRP created by peeling were examined by scanning electron microscopy (SEM), x-ray photo electron spectroscopy (XPS), energy-dispersive x-ray spectroscopy (EDX), infrared (IR) spectroscopy, atomic force microscopy (AFM), and contact angle measurements to characterize the surfaces produced. Furthermore, the bond strength of lap shear and floating roller peel samples was determined with and without additional plasma treatment. For bonding, a room temperature-curing two-component-epoxy adhesive (Hysol® 9395) was used to prove the applicability of different peel plies for structural adhesive bonding under repair conditions.     

Mixed-Mode Fracture in Soda-Lime Glass Using Indentation Flaws

Mixed-mode failure of soda-lime glass under inert and fatigue test conditions was studied using Knoop indentation flaws. For annealed cracks (residual stress-free) crack extension (catastrophic or subcritical) is by an abrupt transition from the initial crack plane to a noncoplanar crack plane followed by a reorientation of the crack normal to the applied stress. Although fatigue strength of these inclined flaws increased linearly with respect to orientation of the flaws to the applied stress up to an angle of 60°, this increase was considerably less than what was predicted by existing theories. It is believed that subcritical crack growth causes the crack to be realigned perpendicular to the applied stress before failure for all orientations; hence, fatigue strength does not show the dramatic increase at orientation angles as predicted by theory. For as-indented cracks the contact residual stress causes the crack extension to be less inclined to the initial crack plane than for annealed cracks, but in this case also, the crack realigns itself perpendicular to the applied stress. Again, fatigue strength is relatively insensitive to the orientation angle as predicted by theory and subcritical crack growth is believed to play a primary role in determining this strength dependency.     

Impact behavior of unidirectional polyethylene–glass fibers: Resol/vinyl acetate‐2‐ethylhexylacrylate interpenetrating polymer network systems

Resol was solution blended with vinyl acetate‐2‐ethylhexylacrylate (VAc–EHA) resin in an aqueous medium at a 90‐10 w/w ratio with hexamethoxymethylmelamine (HMMM) as crosslinker. Here we aimed to study the impact behavior of unidirectional laminates cast from (Resol/VAc–EHA/HMMM)/glass fiber (GF), (Resol/VAc–EHA/HMMM)/polyethylene fiber (PEF), and (Resol/VAc–EHA/HMMM)/GF/PEF (hybrid) and to study the role of PEF ply/plies in hybrid laminates toward the impact behavior, as dependent on the relative position of the ply/plies. A brittle failure mode was observed in the GF‐reinforced laminates, which tended to the ductile failure mode, with the incorporation of PEF ply/plies. Again, the impact fracture mode of GF was minimized by the placement of PEF ply/plies at the impacted side of the hybrid laminates. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 339–342, 2004     

Investigation of the microcracking behavior of bismaleimide composites during thermal aging

Samples of 16-ply, quasi-isotropic bismaleimide composites were aged in convection ovens at 150°C, 177°C, and 204°C for up to 16,000 hours. As a measure of degradation, transverse microcrack density was characterized as a function of time, temperature, and ply depth. Times required to reach onset and saturation crack density were delayed on the tool side of the laminate, for lower temperatures, and for deeper ply depths. Saturation crack densities ranged from 35 cracks/cm to 46 cracks/cm, depending on ply depth. Master curves were constructed for each ply level to express all time-temperature data for that ply as a single reference curve. A mass transfer analysis further suggests that diffusion is the controlling mechanism in the degradation process. Finally, a prediction of the degradation behavior in the 120,000-h lifetime of the HSCT aircraft for a sustained temperature of 150°C indicates that ∼13 plies will have reached initiation in a quasi-isotropic laminate.     

Fracture toughness and crack instability in tough polymers under plane strain conditions

The plane strain fracture toughness and fracture mechanisms of several tough engineering plastics have been studied and compared with poly(methyl methacrylate) (PMMA), a relatively brittle polymer. The tough polymers all are observed to form a multiple craze zone at the crack tip, which is shown to be the primary source of plane strain fracture toughness in these materials. The multiple craze zone is retained during slow crack growth but is metastable, and at a critical stress intensity and associated crack velocity, the system passes through a transition to a greatly accelerated single craze mode of unstable propagation.     

Slow Crack Growth in Sapphire Fibers at 800° to 1500°C

Sapphire fibers with (near) c -axis orientation were tested in tension over a range of strain rates (10⁻⁵ to 0.5 min⁻¹) at elevated temperatures (800° to 1500°C). The strength of the fibers was dependent on the strain rate. Slow crack growth was confirmed as the degradation process by direct inspection of the fracture surfaces and estimation of fracture stresses from measured flaw sizes. The slow crack growth parameter, N , decreased with increasing temperature. At 1400°C a threshold in strength was observed; the threshold stress intensity factor was estimated to be ≅ 1 MP·m^1/2 at 1400°C. Thermally activated bond rupture (e.g., lattice trapping model) is postulated as the mechanism responsible for the slow crack growth.     

Static behavior and the effects of thermal cycling in hybrid laminates

The influence of hybridization with stacking sequence variation on static stiffness, strength, ultimate strain and residual properties after thermal cycling of graphite/Kevlar 49/ epoxy and graphite/S-glass/epoxy angle-ply laminates was investigated. Tensile stress-strain curves to failure and uniaxial tensile properties were determined for all laminates. Theoretical predictions of modulus, Poisson's ratio and ultimate strain were made, based on linear lamination theory, constituent ply properties and measured strength. Reasonably good agreement was found. Stacking sequence variation showed no significant systematic influence on the measured results. Specimens containing only two 0-degree Kevlar or S-glass plies behaved linearly to failure. Specimens containing four 0-degree Kevlar or S-glass plies displayed characteristic non-linearity. One group of laminates was subjected to a tensile load and to 100 thermal cycles between room temperature and 280°F, and another group to a tensile load and 100 thermal cycles between room temperature and ?100°F. All surviving specimens were tested statically to failure to determine residual properties which were compared with the properties of uncycled specimens. Specimens containing two Kevlar or two S-glass plies behaved linearly to failure. Residual moduli in these specimens were lower than for uncycled specimens but residual strengths and ultimate strains were higher. Specimens with four Kevlar or four S-glass plies showed some nonlinear behavior. No significant differences were found between residual and uncycled values for modulus and strength.     

Crack Growth in Soda–Lime–Silicate Glass near the Static Fatigue Limit

The atomic force microscope (AFM) was used to explore the nature of features formed on the surfaces of cracks in soda–lime–silicate glass that were held at stress intensity factors below the crack growth threshold. All studies were conducted in water. Cracks were first propagated at a stress intensity factor above the crack growth threshold and then arrested for 16 h at a stress intensity factor below the threshold. The stress intensity factor was then raised to reinitiate crack growth. The cycle was repeated multiple times, varying the hold stress intensity factor, the hold time, and the propagation stress intensity factor. Examination of the fracture surface by optical microscopy showed surface features that marked the points of crack arrest during the hold time. These features were identical to those reported earlier by Michalske in a similar study of crack arrest. A study with the AFM showed these features to be a consequence of a bifurcation of the crack surface. During the hold period, waviness developed along the crack front so that parts of the front propagated out of the original fracture plane, while other parts propagated into the plane. Crack growth changed from the original flat plane to a bifurcated surface with directions of as much as 3° to 5° to the original plane. This modification of crack growth behavior cannot be explained by a variation in the far-field stresses applied to the crack. Nor can the crack growth features be explained by chemical fluctuations within the glass. We speculate that changes in crack growth direction are a consequence of an enhancement in the corrosion rate on the flank of the crack at stresses below the apparent crack growth threshold in a manner described recently by Chuang and Fuller.     

Modelling stiffness degradation due to matrix cracking in angleply composite laminates

Abstract

When a multidirectional composite laminate is subjected to in plane static or fatigue tensile loading, matrix cracks parallel to the fibres appear in the off axis plies long before catastrophic failure. Matrix cracking significantly reduces the laminate stiffness properties and triggers development of other harmful resin dominated damage modes such as delaminations. Concurrent matrix cracking in the adjacent off axis plies is an extremely complex problem to model analytically and has been analysed mostly using finite element methods.

The present paper is concerned with the theoretical modelling of stiffness reduction in cracked angleply [θ₁ /θ₂ ]_s laminates subjected to multiaxial in plane loading. A new approach based on the equivalent constraint model of the damaged ply and an improved two-dimensional shear lag method has been applied to model matrix cracking in angleply [θ₁ /θ₂ ]_s laminates. Stresses in the cracked ply are determined from a system of two coupled ordinary differential equations and used to calculate the in situ damage effective functions, which describe the stiffness loss. For angleply [±θ ]_s laminates with a cracked midlayer, it is found that the reduction due to matrix cracking of the laminate axial and transverse moduli is more significant than in crossply laminates, while for the shear modulus, the opposite is true. Matrix cracking in such laminates can result in an increase in the Poisson's ratio – a phenomenon not observed in crossply [0/90]_s laminates. In addition, matrix cracking in angleply [±θ ]_s laminates introduces coupling between extension and shear.     

High-Temperature Failure of Polycrystalline Alumina: II, Creep Crack Growth and Blunting

Creep crack growth in fine-grain alumina is measured by using surface cracks. A narrow power-law crack growth regime occurs at both 1300° and 1400°C, wherein the power-law exponent and activation energy are comparable to steady-state creep values. Asymptotic crack velocity behavior is exhibited near both the critical stress intensity factor, K_C , and the crack growth threshold, K_th . The threshold occurs near 0.4 K_1C at both 1300° and 1400°C and is associated with a transition in the size and distribution of damage. Displacement measurements indicate that crack tip damage exerts a strong influence on the displacement field, as predicted by recent theories. Furthermore, use of the stress intensity factor as a loading parameter does not produce adequate correlation with displacement measurements and is, therefore, not strictly suitable for nonlinear creeping ceramic poly crystals.     

Flexural behavior of stepped graphite/epoxy laminates

Cross-ply tapered/stepped laminates with taper angles of 1 and 2° between the top and bottom surfaces were fabricated using T300/943 graphite/epoxy by compression molding. Ply terminations were done internally within the laminate and externally on the surface of the laminate at various cross-sections in order to obtain the taper. Equivalent cross-ply specimens with internal and external ply terminations, respectively, were tested in three point bending. The bending stiffness of the equivalent corss-ply are nearly equal, while the failure modes are significantly different. The tapered laminates with internal ply terminations failed due to a series of delaminations originating at the step corners of the 0° plies. Macroscopically, the specimens with internal ply term nations failed by tensile fracture of the outer plies at relatively higher loads.     

The application of the essential work of fracture methodology to the plane strain fracture of ABS 3-point bend specimens

The applicability of the EWF methodology to 3-point bend (SEB) specimens under conditions other than plane stress has been assessed experimentally. Different fracture conditions, pure plane strain and plane strain/plane stress transition, were obtained by varying the specimen thickness and testing temperature (20 and 80 °C). Post-mortem fracture surfaces appeared always completely stress-whitened, indicating ductile fracture. The load-line displacement plots are similar over a well-defined range of ligament lengths for which the application of the EWF methodology was in principle possible. Nevertheless, in experiments conducted at room temperature, crack growth was observed to initiate before maximum load and complete ligament yielding. This behaviour was confirmed through plastic collapse analyses. A critical ligament length was found, over which the total specific work of fracture was dominated by edge effects. Below this critical ligament length, EWF methodology was still applicable and it was possible to extrapolate reliable w_Ie values.