bluuebunny/crossref_metadata_2025_split huggingface dataset:
Fatigue Damage and Fracture Mechanism of Notched Woven Laminates
Authors: Xiao Jiayu , Bathias Claude 
Published: April 2011

Three E-glass/epoxy notched woven laminates (two orthotropic and one quasi-isotropic) were studied under tensile fatigue loading with a stress ratio of 0.1 and a frequency of 20 Hz (sine wave form). At a fixed ratio of maximum fatigue stress to ultimate tensile strength (UTS) (0.5), the apparent secant stiffness changes with the cycle number were measured and the fatigue damage mechanism was investigated. The S-N curve and notched fatigue strength (at 1,000,000 cycles) were determined. All results obtained were compared with those for unnotched specimens. The results show that the normalized apparent secant stiffness change can be divided into three stages. During the first and second stage, damage may be presented in the form of microcracks within the interlacement structure of the woven plies. But they have not been observed and verified with traditional experimental methods such as radiography and microscopy. The last stage corresponds to initiation and evolution of damage observed directly on the specimen surface with a video-camera system. The fatigue life at a same ratio of maximum fatigue stress to UTS for the three laminates is very slightly influenced by the notch. The quasi-isotropic laminate has a lower fatigue strength and an earlier first visible damage than the two orthotropic laminates. However, for the two orthotropic laminates, the fatigue strength and the appearance of the first visible damage are very slighly influenced by the stacking sequences, but, the normalized secant stiffness curve appears to be different from one to another. The notched specimens have the same ratio of the fatigue strength to UTS as the unnotched ones, almost equal to 0.4. The ratio of the notched fatigue strength to the unnotched fatigue strength is amost equal to the static strength reduction factor (SRF) which is equal to the ratio of the static notched strength to the static unnotched strength.
Analytical Characterization of the Anisotropy and Local Heterogeneity of Short Fiber Composites: Fiber Fraction as a Variable
Authors: Pan Ning 
Published: April 2011

A statistical analysis to characterize the system anisotropy and the local heterogeneity of short fiber composites is presented in this report. Firstly, using a fiber orientation probability density function (pdf), the statistical mean values and variations of the fiber cut ends and the parameters of the matrix material distribution on an arbitrary cross section of a composite are derived. The fiber area fraction Af, which is shown to be a function of the fiber orientation and the direction of the cross section, is used to calculate the composite elastic properties such as the tensile modulus associated with the cross section so that the direction-dependence or the anisotropic nature of the composite properties is predicted. However, to study other important system properties such as the strength, the fiber-matrix interaction, failure process and impact resistance, neither the overall fiber volume fraction Vf nor the fiber area fraction Af at cross sections is proved to be adequate. Local fiber volume fraction Vaf is hence defined in this work to specify the local heterogeneity of the composite, i.e., the non-uniformity of the distribution of the fibers and the matrix material at local level. This local fiber volume fraction can be inferred from the statistical features such as the radius r and the free length I of the spaces occupied by the matrix material. The allowable local minimum and maximum fiber volume fractions, two concepts from the existing theories, as well as the allowable variances of the radius r and the free length 1 of the matrix material can be used to predict in composites the weakest locations and the weakest cross sections in anisotropic cases. The theory has been applied to two special cases with hypothetic fiber orientation pdf functions, and the effects of the important variables involved are investigated and illustrated.
A Self-Correcting, Thermal-Curvature-Stable Bending Element
Authors: Wetherhold Robert C. , Wang Jlanzhong 
Published: April 2011

Under a through-thickness temperature variation, thin structural elements such as beams, plates, and shells tend to bend. This bending deformation can distort the elements of a structure and cause thermal stresses. In this paper, the requirements for a laminate which is "self-correcting" are given. These thermal-curvature-stable (TCS) laminates exhibit zero bending curvature under steady-state heat transfer conditions. The TCS effect is independent of the actual temperatures, within the context of linear theory. This effect is demonstrated for an aluminum/composite laminate, where the solution appears as the ratio of composite lamina to aluminum lamina thicknesses. The example laminated beams demonstrate a reduced net in-plane thermal expansion when compared to mono-lithic aluminum beams. The efficiency of the iCS laminate is shown to be more sensitive to lamina thickness than to lamina properties. An additional design parameter for TCS laminates is the fiber volume fraction (Vf), which affects the composite modulus, thermal expansion, and transverse thermal conductivity. It is shown that at higher Vf, the TCS effect is relatively insensitive to Vf. At lower Vf, the in-plane thermal expansion can be further reduced at the expense of increased composite layer thickness.
An Accumulative Damage Model for Tensile and Shear Failures of Laminated Composite Plates
Authors: Shahid Iqbal , Chang Fu-Kuo 
Published: April 2011

A progressive failure model was developed for predicting the accumulated damage and the effect of such damage on the in-plane response of laminated composite plates subjected to tensile and shear loads. Internal damage in composites induced by matrix cracking under in-plane tensile and shear loads was the primary concern. Free edge-induced damage was not considered. The model assumes that the damage would accumulate uniformly in a laminate when it is subjected to uniformly distributed loads. The progressive failure model consists of two parts: constitutive modeling and damage accumulation prediction. Constitutive equations were established for relating the effective material properties of a damaged ply in a laminate to the extent of damage. Damage accumulation criteria were proposed by modifying the existing failure criteria for estimating the accumulated damage as a function of the applied loads. A finite element analysis, designated as "PDCOMP," was developed based on the model. In order to verify the model, predictions from the analysis were compared with the experimental data available in the literature. Additional tests were also conducted to substantiate the model. Overall, the predictions agreed with the data quite well.
Model Based Control of Voids and Product Thickness during Autoclave Curing of Carbon/Epoxy Composite Laminates
Authors: Joseph Babu , Hanratty Frieda Wang , Kardos John L. 
Published: April 2011

In this article we present a novel feedback control strategy for the autoclave curing of carbon/epoxy laminates. The strategy utilizes an on-line model of the process to control the quality by adjusting the cure cycle. On line measurements are used to correct for modeling errors. The quality variables of interest here are the thickness of the product laminate and the maximum void size present in the laminate. Artificial neural networks are utilized to regress past operational data to generate nonlinear models for predicting the product quality. These models utilize properties of the raw material prepreg and secondary measurements taken while the process cycle is in progress. Some results of a control study done using a simulated model of a process for autoclaving a carbon fiber/epoxy resin system are also reported. The main contribution of this paper is the successful demonstration of a neural network model based feedback control strategy for autoclave curing. While discussion in this paper is limited to control of the autoclave process, the methodology suggested is also equally applicable to other composite manufacturing processes.
Life Predictions of Metal Matrix Composite Laminates under Isothermal and Nonisothermal Fatigue
Authors: Gao Zhanjun , Zhao Huixia 
Published: April 2011

A remarkably simple model is proposed in this paper to predict the fatigue life of titanium matrix composite (TMC) laminates under thermomechanical fatigue. It is shown that the maximum applied stress, together with the associated temperature, represents a critical state of loading, of the fatigue process. This critical state of loading dictates a unique state of damage represented by stiffness reduction of the off-axis plies and viscoplastic deformation of the total laminate. As far as the fatigue life prediction is concerned, it is not necessary to know how the damage evolves, and what the stress-strain response is in each loading cycle. After a relatively small number of loading cycles, the stress of 00 fibers saturates to a stable level, and further damage is dominated by the 0° plies of the laminate. The fatigues life is determined by the saturated stiffness, cumulated nonelastic strain and the maximum applied stress. The model is based on a firm understanding of the fundamental damage mechanisms in the TMC laminates under various mechanical and thermal loading. Damage mechanisms and fatigue lives of isothermal, in-phase and out-of-phase fatigue are unified and examined using the concept of critical state of loading. The predictions of fatigue life are shown to be in a good agreement with experimental data of [0/90]2 s, [02/±45] s and [0/±45/90] s made of SCS-6/Titanium for room temperature fatigue, as well as in-phase and out-of-phase thermomechanical fatigue. The most important feature that separates the present research from the existing approaches is its simple and clear representation of the physics involved in the fatigue damage and failure of TMC. The key to the success of the model is the simple fact that failure is controlled by fibers in 0° plies for any laminates containing 00 plies. The data needed to predict the life of any laminates, containing 0° and off-axis plies, under various thermomechanical fatigues are minimal and obtained essentially from a unidirectional laminate. No fatigue life data for the laminates themselves are used to calibrate the model or to determine curve fining parameters. The model makes it possible to establish a data base for different unidirectional laminate and predict the fatigue life of a laminate based on the data of its laminate and the laminate stacking sequence.
Analytical Models for Low Cost Manufacturing of Composite Components by Filament Winding, Part II: Inverse Kinematics
Authors: Mazumdar S. K. , Hoa S. V. 
Published: April 2011

There are no available techniques for the prediction of fiber angle distribution for various mandrel shapes under different conditions of mandrel's rotations and delivery point motions. In the present paper a novel approach based on the geometric and trigonometric relations is developed for predicting the fiber laydown path for a given equation of motion for the delivery point. This is important during the start-up and reversal periods of the filament winding operation. During the start-up period, the delivery point accelerates from a velocity of zero to a final value in some interval of time, whereas during the reversal period the delivery point decelerates to zero velocity and then accelerates to a final velocity. Equations of motion for the delivery point are presented to have smooth start-up and reversal periods. Closed form solution are developed to compute the fiber winding angle distribution on various types of cylindrical mandrels with axisymmetric and non-axisymmetric cross-sections, whereas numerical solutions are presented for non-cylindrical mandrels. Fiber slackening phenomena during a reversal period is discussed. Based on the current approach, an interactive computer program INVERSEKIN is developed to determine fiber winding angle distributions for a given equation of motion, mandrel shape geometry and experimental set-up conditions. Effects of winding parameters on fiber position and fiber winding angle distributions are discussed. Experimental verification of the model is done on a McClean Anderson filament winding machine for a conical mandrel for various equations of delivery point motion.
Influence of Void Shape, Void Volume and Matrix Anisotropy on Effective Thermal Conductivity of a Three-Phase Composite
Authors: Krach Ann , Advani Suresh G. 
Published: April 2011

By using high conductivity carbon fibers in a graphitizable carbon matrix, very high values for the thermal conductivity of a composite can be obtained in the direction parallel to the fibers. The carbon matrix of this material exhibits anisotropy and is oriented in the direction of the fibers. Thermal conductivity of the matrix material is of the same order as that of the carbon fibers. The processing of such materials invariably introduces voids up to 25% giving rise to a three phase system of fibers, matrix and voids. The void volume, shape and distribution will have a significant impact on the overall or effective thermal conductivity of the composite. Current empirical or simplified models do not show clearly this effect and experimental measurements are non-existent. Hence, we have adopted a numerical approach consisting of a unit cell to explore the influence of void volume and shape on the effective conductivity of a unidirectional sample. The effective conductivity of a composite consisting of spherical, oval and crack-type voids up to 25% in the direction transverse and parallel to the fiber is calculated and compared with electric circuit analogies and a generalized dispersion formulation for effective thermal conductivity. The results clearly show that the effect of porosity on thermal conductivity cannot be described solely by void volume. The shape and the distribution of the voids affect the effective thermal conductivity which is not captured by any existing models. The finite element models developed should prove useful for the estimation of effective thermal conductivity and hence provide a tool for evaluating analytical continuum models.
Bearing Failure of Bolted Composite Joints. Part I: Experimental Characterization
Authors: Wang Hong-Sheng , Hung Chang-Li , Chang Fu-Kuo 
Published: April 2011

An investigation was performed to study the bearing failure of mechanically fastened fiber-reinforced laminated composite joints. Only double-lap metal/composite/metal bolted joints were investigated. The results of this study will be presented in two parts: experimental characterization and analytical prediction. This paper summarizes the experimental work. The major focus of the experiments was to characterize the bearing failure mechanism and mechanics, and to evaluate the effect of clamping pressure on the bearing response and bearing strength of bolted joints. The bearing damage was characterized either as pure bearing, which had no lateral supports, or as bolt bearing, which contained lateral supports with various degrees of clamping pressure. A specially designed semi-circular notched specimen was proposed to characterize pure bearing damage. For bolt bearing, a load cell was designed and manufactured which was mounted on the fastener to monitor the bolt clamping pressure as a function of the applied load. T800/3900-2 graphite/epoxy prepregs were selected to fabricate the specimens. All the specimens were x-rayed and sliced at different load levels to examine internal damage. Based on the experiments, it could be concluded that lateral support is crucial for bolted laminated composite joints. Bearing damage can be catastrophic if there is no lateral support. Shear cracks induced by accumulated compression failure appeared to be the primary failure mode of the bearing damage. Lateral supports could suppress the shear crack propagation and change failure from a catastrophic to a progressive failure mode. Clamping pressure could increase bearing strength.
Strength Envelope of Bolted Composite Joints under Bypass Loads
Authors: Hung Chang-Li , Chang Fu-Kuo 
Published: April 2011

An investigation was performed to study the strength and failure of bolted composite joints subjected to bypass loads. The damage accumulation models developed previously by the authors were implemented in a nonlinear finite element code, designated "BJSTAT," for predicting failure and response of bolted composite joints. Utilizing the code, numerical simulations were conducted to generate the strength envelope for a bolted composite joint under multi-axial bypass loads. The predictions of bolted joints subjected to an axial bypass load were compared with available experimental data. The prediction agreed with the data very well. The envelope can provide information on the strength as well as the failure type of the joint as functions of the bypass loads and, therefore, is very useful in joint design.
Durability of Closed-End Pressurized GRP Pipes under Hygrothermal Conditions. Part I: Monotonic Tests
Authors: Ghorbel I. , Spiteri P. 
Published: April 2011

Environmental factors are of extreme importance with respect to the ser-vice life of composite components based on polymeric engineering matrices. Part I deals with the role of matrix ductility and chemical environmental degradation on the composites mechanical properties. Part II is concerned with the lifetime prediction of GRP filament wound pipes under low pressure levels and hygrothermal conditions. There composites are an E glass fiber reinforced polyester resin and an ECR glass fiber reinforced vinylester resin. The mechanical behavior of the pipes before and after exposure to water for different duration is studied through internal pressurizing tests with closed end procedure. The results indicate that in the hoop direction, fibers carry the complete load while in the axial direction there is a substantial contribution from the matrix. The use of elasticity theory appears as an appropriate method to describe the mechanical behavior of the pressurized pipes under monotonic loading conditions. In the hoop direction the behavior is governed by the fibers one whereas in the axial direction damage must be accounted for in the elastic law. Chain scission effects on the mechanical properties are investigated. Experimental results show that the mechanical properties, the damage mechanisms and the failure mode of the pipes are changed by water uptake. Relations between the mechanical properties and the exposure duration based on physical considerations are assessed, allowing for the definition of a damage parameter related to hygrothermal degradation. Environmental effects (swelling and chain scission) mainly affect the yielding pressure and the pressure to failure, whereas load condition and initial defects affect the axial modulus. This leads to the definition of two independent damage parameters.
Durability of Closed-End Pressurized GRP Filament Wound Pipes under Hygrothermal Aging Conditions. Part II: Creep Tests
Authors: Ghorbel I. 
Published: April 2011

The creep and damage behavior of GRP filament wound pipes are investigated as a function of the resin ductility. To predict the life time of pipes under low pressure levels and hygrothermal conditions, different methods based on the experimental data and approximations have been used. All these methods involve assumptions that have been discussed and examined carefully. Under creep tests the response of preconditioned pipes by immersion in water at 60°C follows a nonlinear viscoelastic behavior. A damage creep law is, also, assessed followed by microscopic observations to identify the creep damage mechanisms. Two stages are observed: the first describes the initiation of cracks and the second is related to the crack's propagation. Decohesions between the mechanical part of the pipe and the liner followed by microcracking of the liner lead to the failure. Also, the pipes' life is governed by the critical creep strain to failure of the liner. From microscopic observations and physicochemical analysis it appears that pipes' failure results for low pressure levels from mechanical damage and chemical bond rupture, otherwise cracking and delamination lead to the failure. Experimental data show that the use of the creep damage law give the more realistic creep lifetime although this criterion is the most pessimistic one. Predictions based on the Eyring-Zhurkov approach and the nonlinear viscoelastic creep law over-estimate the lifetime because assumptions are not appropriate. Moreover it appears using both microscopic observations and extensometric measurements, that the lifetime, the damage kinetics and the failure mechanisms of the pressurized pipes depend strongly on the matrix ductility.
Experimental and Finite Element Analysis of Pultruded Glass-Graphite/Epoxy Hybrids in Axial and Flexural Modes of Vibration
Authors: Nori Chandrasekhar V. , Mantena P. Raju , McCarty Tyrus A. 
Published: April 2011

The effects of hybridization on the extensional and flexural dynamic properties of pultruded composite materials are reported in this paper. The composite materials considered were made up of unidirectional glass, graphite fibers in an epoxy matrix, and hybrids of glass-graphite/epoxy produced by the pultrusion manufacturing process. The dynamic storage modulus and loss factor of the rectangular and circular cross-section samples were first evaluated using the nondestructive impulse-frequency response vibration technique. The long and slender flat specimens were analyzed in both extensional and flexural modes of vibration in a free-free test configuration, whereas the thicker round specimens were analyzed only in the axial mode of vibration. The properties of the monofiber type glass/epoxy and graphite/epoxy composites were used as input to the finite element model for predicting and experimental validation of various pultruded (and hypothetical) glass-graphite/epoxy hybrid combinations. The modal strain energy method was used for computing the structural loss factors of the hybrid specimens based on the element stiffness matrices and estimated mode shapes in the fundamental mode of vibration. Results of this investigation showed excellent agreement between experimental data and numerical predictions for the dynamic extensional properties of both flat and round specimens. The numerical results for dynamic flexural properties of flat specimens were found to have a similar trend as that obtained from the experimental technique. Small variations in flexural properties could be attributed to the irregular shapes (which were not taken into account in the finite element model) of layers in the hybrid combinations formed during the pultrusion manufacturing process. Results also demonstrated that while the extensional properties were independent of the fiber location and fiber packing geometry, the flexural properties were highly dependent upon these two factors. Previously reported data (which was obtained by testing these flat hybrid specimens in the flexural mode of vibration in a clamped-free boundary condition) were re-analyzed using the free-free configuration and modal strain energy method, yielding more reasonable results.