Showing papers in "Journal of Composite Materials in 1999"

A progressive Damage Model for Mechanically Fastened Joints in Composite Laminates

Abstract: A three-dimensional finite element model is developed to predict damage progression and strength of mechanically fastened joints in carbon fibre-reinforced plastics that fail in the bearing, tension and shear-out modes. The model is based on a three-dimensional finite element model, on a three-dimensional failure criterion and on a constitutive equation that takes into account the effects of damage on the material elastic properties. This is accomplished using internal state variables that are functions of the type of damage. This formulation is used together with a global failure criterion to predict the ultimate strength of the joint. Experimental results concerning damage progression, joint stiffness and strength are obtained and compared with the predictions. A good agreement between experimental results and numerical predictions is obtained.

Delamination fracture of multidirectional carbon-fiber/epoxy composites under Mode I, Mode II and Mixed-Mode I/II loading

Abstract: The purpose of the present study was to characterize the delamination fracture of continuous carbon fiber/epoxy multidirectional-laminates under Mode I, Mode II and Mixed-Mode I/II loading conditions. The present study considers the variation of the interlaminar failure energy, GC, with the extent of crack jumping, and ensuing fiber bridging, which arises during the growth of the delamination in the multidirectional-laminates under the various modes of loading. The main type of laminate which was studied was a multidirectional fiber composite prepared from 24 ply lay-ups of (-45°/0°/+45°)2S (+45°/0°/-45°)2S. The initial delamination was located at the +45°/-45° mid-plane of the specimen. It has been found that when the values of the interlaminar fracture energy, Gc, are ascertained as a function of the length of the propagating crack, a, then very complex relationships are observed. This was the case for all the different modes of loading, and these observations reflected the complex failure paths which o...

Delamination Onset Prediction in Mechanically Fastened Joints in Composite Laminates

Abstract: A three-dimensional finite element model is created to simulate a mechanically fastened joint in a composite laminate Using the results from the finite element model, a spline approximation for the through-thickness stress is applied in order to determine the stress state at the interfaces between the layers A delamination onset criterion is then applied at each interface After validation, the model is used to assess the effects of stacking sequence and clamping pressure on the delamination onset loads and surfaces It is concluded that "blocked" laminates lead to lower delamination onset loads and larger initial delaminated regions Finger-tight and clamped joints have higher delamination onset loads and smaller initial cracked regions than pin-loaded joints These results are in agreement with experimental observations, but no significant difference is detected between finger-tight and clamped joints It is therefore concluded that the through-thickness pressure has an effect not only on damage initi

Testing and Analysis of Composite Skin/Stringer Debonding Under Multi-Axial Loading

Abstract: Damage mechanisms in composite bonded skin/stringer constructions under uniaxial and biaxial (in-plane/out-of-plane) loading conditions were examined. Specimens consisted of a tapered composite flange bonded onto a composite skin. Tests were performed under monotonic loading conditions in tension, three-point bending, and combined tension/bending. For combined tension/bending testing, a unique servohydraulic load frame was used that was capable of applying both in-plane tension and out-of-plane bending loads simultaneously. Specimen edges were examined on the microscope to document the damage occurrence and to identify typical damage patterns. The observations showed that, for all three load cases, failure initiated in the flange, near the flange tip, causing the flange to almost fully debond from the skin. A two-dimensional plane-strain finite element model was developed to analyze the different test cases using a geometrically nonlinear solution. For all three loading conditions, principal stresses exceeded the transverse strength of the material in the flange area. Additionally, delaminations of various lengths were simulated in two locations where delaminations were observed. The analyses showed that unstable delamination propagation is likely to occur in one location at the loads corresponding to matrix ply crack initiation for all three load cases.

Strain Rate Effects on the Transverse Compressive and Shear Behavior of Unidirectional Composites

Abstract: Methods for dynamic characterization of composite materials were extended and applied to the study of strain rate effects under transverse compression as well as shear. Falling weight impact and Split Hopkinson Pressure Bar systems were developed for dynamic characterization of composite materials in compression and shear at strain rates up to 1800 sO. Strain rates below 10 s5l were generated using a servohydraulic testing machine. Strain rates between 10 sol and 300 sol were generated using the drop tower apparatus. Strain rates above 500 s-l were generated using the Split Hopkinson Pressure Bar.Seventy-two and forty-eight ply unidirectional carbon/epoxy laminates (IM6G/3501-6) loaded in the transverse direction were characterized. Off-axis (15°, 300, 450 and 600) compression tests of the same unidirectional material were also conducted to obtain the in-plane shear stress-strain behavior. Strain rates over a wide range, from 10-4 s-1 (quasi-static) up to 1800 s-1, were recorded. The 90-degree properties,...

Fatigue Strength Prediction under Multiaxial Stress

Abstract: A multiaxial fatigue failure criterion for composite materials is presented in this paper along with an assessment of the capability it offers for design under multiaxial constant or variable amplitude stresses. The applicability of this criterion, based on the well known quadratic failure tensor polynomial criterion for static loading, is validated through comparisons with uniaxial and multiaxial fatigue experimental data. Static and fatigue tests were carried out during this study on glass/polyester specimens cut off-axis from a multidirectional laminate at different angles. The agreement between experimental values and theoretical predictions is good. The proposed criterion is also compared to existing fatigue criteria and an overall assessment of their performance is given. Some theoretical design considerations for the case of irregular stress spectra, introducing the concept of multiaxial Miner coefficient, are finally presented.

Thermal Conductivity of Fiber Reinforced Composites by the FEM

Abstract: Applicability of the finite element method (FEM) in predicting the effective transverse thermal conductivity of fiber reinforced composites is systematically studied. Four different boundary condition combinations representing the periodicity of the temperature field are employed for ideal composites having perfect bond between fiber and matrix. Both circular and square cross-section fibers are studied. Comparisons of present FEM results with available analytical and experimental results reveal that periodicity realized by prescribed temperatures yields most accurate results up to high fiber volume fractions. In composites with interfacial thermal barrier resistance the effective conductivity varies in a wide range depending on the interfacial conductance between fiber and matrix. Best fit with available experimental results is obtained for both circular and square fibers when the dimensionless interfacial conductance is about 30. By employing the modeling practice found successful in the cases for which ...

An impregnation model for the consolidation of thermoplastic composites made from commingled yarns

Abstract: A model for the consolidation of thermoplastic composites based on commingled yarns was developed. The evolution of composite void content during consolidation was related to the processing parameters and the material properties. Additionally, the analysis relied on experimental investigation of the yarn structure, taking into account a distribution in dry fiber bundle sizes. Residual porosity was considered to result from pore closing at a given stage during fiber bundle impregnation. The accuracy of the model was assessed by conducting consolidation experiments using a unidirectional commingled yarn fabric made of polyamide 12 resin and carbon fibers. Excellent correlation was found between predicted and experimental void contents. The model aided prediction of suitable processing conditions and identification of the main physical and geometrical parameters influencing the consolidation rate.

An Examination of the Effects from Surface Texture on the Strength of Fiber Reinforced Plastics

Abstract: Surface texture and process defects resulting from net-shape machining of Fiber Reinforced Plastics (FRPs) are often ignored in a design analysis of component parts. The authors recently proposed a mathematical model which defines the effects of surface texture on the strength of engineering components in terms of an effective stress concentration factor (KT). In this study, a verification of the proposed model for FRP materials is presented using results from a comprehensive experimental analysis on the flexure strength of graphite/epoxy and graphite/bismaleimide laminates. The effects of surface texture resulting from three methods of machining were evaluated using this new approach. The proposed model was also compared with the predictive capabilities of existing theoretical models in the evaluation of the experimental data. It was found that the proposed model successfully predicted the reduction in flexure strength of FRP laminates resulting from net-shape machining over a large range in surface qual...

Advanced Calculation of the Room-Temperature Shapes of Unsymmetric Laminates

Abstract: It is well known that the room-temperature shapes of unsymmetric laminates do not always conform to the predictions of classical lamination theory. Instead of being saddle shaped, as classical lamination theory predicts, the room-temperature shapes of unsymmetrically laminated composites are often cylindrical in nature. In addition, a second cylindrical shape can sometimes be obtained from the first by a simple snap-through action. Since 1981 several models, which are restricted to rectangular plates and sometimes only some special lay-ups, have been developed. The Finite Element Analysis (FEA) has just recently been used for the calculation of the room-temperature shapes of unsymmetric laminates, because more sophisticated finite element codes are now available and the calculations can be made in an acceptable time. Using the FEA there are no restrictions concerning the laminate geometry or the lay-up and the snap-through effect can be modeled. Additionally the edge-effects can also be monitored in the F...

Modeling of Plain Weave Fabric Composite Geometry

Abstract: Woven unit-cell geometry functions are presented for a balanced plain weave fabric composite. Geometry models for polymer and ceramic matrix woven systems are developed. For polymer matrix woven sy...

An Experimental Investigation of High Velocity Impact and Penetration Failure Modes in Textile Composites

Abstract: The dynamic failure evolution of textile composites, which were subjected to impact velocities up to 1100 m/s, was investigated. Specialized machines were used to fabricate composites from combinations of Spectra®, Kevlara®, and Twaron® fibers and two- and three-dimensionally woven, braided, and needle-punched nonwoven fabrics. This control of fabrication and processing enabled us to characterize response as a function of areal density, fabric finish, and consolidation techniques. Failure was categorized in terms of material layers, debris mass, matrix cracking, fiber failure, and shear-plugging. Results indicate that shear-plugging occurs at velocities corresponding to decreases in debris mass.

Prediction of Effective Three-Dimensional Elastic Constants of Translaminar Reinforced Composites

Abstract: A Trans-Laminar-Reinforced (TLR) composite is defined as composite laminate with up to five percent volume of fibrous reinforcement oriented in a trans-laminar fashion in the through-thickness direction. The objective of this work was to examine the effect of important parameters on elastic response. Detailed finite element models of unit cells were used to study the effects of adding TLR on the elastic constants. Parameters investigated included TLR material, TLR volume fraction, TLR diameter, TLR through-thickness angle, ply stacking sequence, and the microstructural features of pure resin regions and curved in-plane fibers. The work was limited to materials with at least one ply interface. Adding a few percent TLR had a small negative effect on the in-plane extensional and shear moduli, Ex, Ey and Gxy but a large positive effect (up to 60 percent) on the thickness direction extensional modulus, E,. The volume fraction and the axial modulus of the TLR were the controlling parameters affecting E,. The ou...

Damping Performance of Cocured Graphite/Epoxy Composite Laminates with Embedded Damping Materials

Abstract: Cocuring viscoelastic damping materials in composites has been shown to be successful in greatly increasing the damping of composite structures. The damping performance, however, is often not as high in cocured composites as in secondarily bonded composites, where the damping material does not undergo the laminate cure cycle. The reason for the discrepancy in damping between the cocured and secondarily bonded samples was found to be resin penetration into the damping material. Samples with a barrier layer between the damping material and the epoxy resin had a 15.7% to 92.3% higher effective loss factor (depending on the frequency) than cocured FasTape™ 1125 samples without the barrier and at least 168% higher effective loss factor than cocured ISD 112 samples without the barrier. These higher damping values are very close to the values achieved by secondarily bonding. Viscoelastic damping materials typically have maximum recommended temperatures below that of the composite cure cycles. The effect of cure ...

Damage Development in Woven Fabric Composites during Tension-Tension Fatigue:

Abstract: Impacted woven fabric composites were tested in tension-tension fatigue. In contrast to results from static testing, the effects of low energy impact damage in a fatigue environment were found to be the critical element leading to failure of the specimen. This difference emphasizes the need to identify and understand the fatigue damage mechanism. A relatively new non-destructive inspection technique using infrared thermography was found to be a very useful tool in detecting damage initiation and growth. Furthermore, this technique supplies valuable information to the characterization of the operating fatigue damage mechanism(s).Fatigue leads to a degradation of material properties. Consequently, in connection with impact induced local stress raisers, fatigue produces continuously changing non-uniform stress fields because of stress redistribution effects. Other models addressing evolution of fatigue damage in composite materials have not been able to simulate evolving non-uniform stress fields. Therefore....

Ductility of Concrete Beams Reinforced with FRP Bars and Steel Fibers

Abstract: A total of 18 concrete beams were tested to study the influence of adding steel fibers (SF) to concrete mix on the ductility of concrete beams reinforced with fiber reinforced plastics bars (FRP beams). The main variables in the study were the type and volume fraction of the steel fiber. The study also investigated the accuracy of an available model, developed originally to predict the flexural strength of concrete beams reinforced with SF and steel bars (SF-steel beams), after modification in estimating the flexural capacity of the fibrous FRP beams (SF-FRP beams).The results indicate that the ductility of FRP beams is less than 50% of that of the respective steel beams. The results also reveal that the ductility of SF-FRP beams is directly related to the fiber content. In addition, the test results show that inclusion of 1% of hooked SF can improve the ductility of FRP beams to be the same as that of the steel beams. Furthermore, comparison between the predicted and measured flexural capacity of SF-FRP ...

An Improved Shear Lag Model for Broken Fibers in Composite Materials

Abstract: A shear lag model is formulated to predict the stresses in a unidirectional fiber reinforced composite. The model is based on assumptions consistent with the finite element method and the principle of virtual work by assuming that the matrix displacements can be interpolated from the fiber displacements. The fibers are treated as one-dimensional springs and the matrix is modeled as three-dimensional finite elements. The resulting finite element equations for the system are transformed into differential equations by taking the discretization length to approach zero. The governing ordinary differential equations are solved using Fourier transformations and an influence function technique. The technique is used to solve for the stresses around a single fiber break in an infinite square or hexagonal array of fibers. The results are compared with previous shear lag models and finite element results. The model predicts stress concentrations that are in good agreement with more detailed finite element analyses.

Modeling and Control of an Injection Pultrusion Process

Abstract: Injection Pultrusion (IP) is a novel process that combines the best features of traditional Resin Transfer Molding (RTM) and wet bath pultrusion for manufacturing polymer composites. Few researchers have dealt with the quality control aspects of this process which is extremely crucial to the plant economics. The main contribution of this paper is the study of the IP process from the point of view of product quality control while maximizing production rates. A mathematical model that includes resin flow and cure, and heat transfer in the part and the die is used to determine sensitivity of the quality variables to important processing variables and parameters. Similarities and differences with traditional pultrusion process are pointed out. Control-relevant features of the process are identified and the requirements necessary for a control system are presented. A cascaded control strategy which uses an on-line process model is proposed.

Axial Compression of Empty and Foam Filled Composite Conical Shells

Abstract: Experimental and theoretical studies of axial compression of both empty and foam filled glass fibre/polyester resin truncated conical shells are presented.An analysis is carried out to determine th...

Fiber-Optic and Ultrasonic Measurements for In-Situ Cure Monitoring of Graphite/Epoxy Composites

Abstract: To produce high-quality composites with high density and low void content, the knowledge of cure process is very important, and the sensors capable of monitoring the cure process are therefore desirable.Since the term of "fully processed" should be a reflection of the ultimate material application, the objectives of the present work are to monitor how material properties have been developed during the cure and justify when "end-of-cure" has been achieved by measuring chemical and mechanical properties of the curing composites. As a reference, differential scanning calorimetry is used to evaluate the degree of cure while fiber-optic sensors are used to measure the evolution of process-induced strains, and ultrasonic sensors are used to monitor the development of viscoelastic properties of the curing composites. Moreover, an ultrasonic cure monitoring system has been developed, by using conventional broadband ultrasonic sensors together with clad buffer rods. The major merits of this system can be summarize...

Stress Transfer from the Matrix to the Fibre in a Fragmentation Test: Raman Experiments and Analytical Modeling

Abstract: The stress transfer properties of the flbre/matrix interface in the single flbre fragmentation test were investigated. Two carbon flbre-resin systems involving epoxy-sized and unsized flbres, were examined. Axial flbre stress data at resolutions of the order of one micron, were obtained with the technique of Remote Laser Raman Microscopy. Subsequent analytical modeling of the data was performed using a Bessel-Fourier Series stress analysis approach. The analysis provides a nearly exact solution for the stress fleld in the fragmentation test and simultane- ously accounts for damaged or imperfect interfaces through the use of an interface parameter Ds. All data were flt using a two zone model in order to account for the propagation of interfacial damage as a function of applied strain. The fltting process was used to determine D (i) s , the interface parameter in undamaged zones, D (d) s , the interface parameter in the damage zones near flbre breaks, and ld, the length of the damage zones, all as a function of applied strain. The interface parameter in the undamaged zones, D (i) s , was independent of applied strain. We claim D (i) s is a good property for comparing flbre/matrix interfaces and the most relevant property for predicting the role to the interface in real laminates.

Studies on Polymer-Wood Interface Bonding: Effect of Coupling Agents and Surface Modification

Abstract: Mechanical properties of composites are a direct function of the interface bonding between strengthening fiber and the matrix. This study investigates the influence of surface modification of wood chips and presence of different interface coupling agents on the interface bonding of wood-polyethylene composites. Single wood-polyethylene interfaced samples were prepared by introducing extruded polyethylene from a twin extruder to bond to the rip surface of a rod-shaped wood specimen. Sodium hydroxide and sulfuric acid were used as surface modifiers for wood. The bond-modifying properties of three silane coupling agents were investigated. The coupling agents used were vinyltrimethoxysilane, 98%, 3-(tri-methoxysilyl)propylmethacrylate, 97%, and 3-glycidoxypropyltrimethoxy silane, 96%. Direct measurement of the interfacial bond strength was carried out using an Instron tensile test machine. Individually, the acidic modifier showed adverse effect on the interface. However, sodium hydroxide improved the interfac...

Accelerated Curing of Adhesives in Bonded Joints by Induction Heating

Abstract: This paper presents the results of a study on the use of induction heating for rapid curing of a commercially available room-temperature curing paste adhesive, with a view to application in the repair of composite structures. The repair of damaged composite structures using adhesively bonded patches is often a very time-consuming process. This is partly due to the long times required for complete and satisfactory cure of the adhesive systems used. While the curing process can be accelerated by application of heat through devices such as heating blankets and lamps, these methods are inefficient since considerable heat is lost to the surrounding material and environment. The method of electromagnetic heating, however, is well suited for rapid and localized heating of the adhesive bondline provided suitable susceptors are used. In this paper, it is shown that induction heating can be successfully used to cure a room-temperature curing paste adhesive. Furthermore, results of single lap shear and double notche...

A Micromechanical Approach to the Tensile Strength of a Knitted Fabric Composite

Abstract: This paper describes a micromechanical approach to estimate the tensile strength of a knitted fabric reinforced composite. In this new approach, the state of stress in each constituent phase of the...

Use of the free vibration spectrum to detect delamination in thick composites

Abstract: A free vibration method was evaluated for damage detection in thick composite plates, whereby vibration was initiated by a single external mechanical pulse and was sensed by piezoelectric patches bonded to the surface of the composite plate. The investigation was conducted on unidirectional fiber glass/epoxy plates approximately 0.387 in (==10 mm) thick, containing controlled delaminations. The free vibration method described in this paper was found relatively insensitive for identifying small to medium delaminations in thick plates. Consequently, several frequency computations using a finite element model were performed to determine the influence of delamination size and location for a single delamination.

Thickness Effects on Pinned Joints for Composites

Abstract: Mechanical joints are commonly used for joining composite laminates, especially thick section composites. In most studies of mechanical joints, the interactions between pin diameter (D) and composite dimensions such as width (W) and distance from hole center to composite end (e), resulting in W/D and e/D ratios, are the primary parameters for joint evaluation since composite laminates are usually very thin. However, as composite thickness increases, the contact interaction between pin and composite alters. Subsequently, the composite joint stiffness and joint strength are affected. Since the contact interaction is strongly affected by pin diameter (D) and composite thickness (H), this study investigated mechanical joints with combinations of various composite thicknesses and pin diameters. Composite material made of woven glass fabric and phenolic matrix was investigated. Sixteen joint configurations based on four composite thicknesses and four pin diameters were examined. Both experiments and finite element analysis were conducted in this study It was found that the ratio of H 2 /D 2 could be used to distinguish the two primary damage modes, i.e. pin bending and bearing failure. Results also showed that thick composites with small pins and thin composites with large pins had Iower efficiencies for joint stiffness and joint strength than those having similar dimensions between pin diameter and composite thickness.

Energy Absorption Characteristics of Stitched Composite Sandwich Panels

Abstract: The energy absorption characteristics of sandwich panels with through-the-thickness stitches, under edgewise compression, was investigated by conducting static crush tests. The effects of stitch row spacing on the sustained crush load were of particular interest. Also, the effects of the stitch spacing on the instability preceding the crush failure mode was investigated using finite element analysis. The analysis indicated that the local instability mode was influenced by the stitch row spacing only when the spacing was less than the semi-wavelength of wrinkles that would develop in the absence of stitches. The static crush tests showed that the average sustained crush load increases with reduced stitch row spacing and thereby increasing the total energy absorbed. The other parameters of interest that were investigated were the load ratio and the load uniformity index.

A Novel Method to Measure Residual Stresses in Laminated Composites

Abstract: A non-destructive method, called the Cure Referencing Method (CRM), was developed to measure the residual stresses in laminated composites. The method incorporates the use of moire interferometry (a laser based optical method) and replication of diffraction gratings on the surface of laminates in the autoclave during cure. This allows the strains due to resin shrinkage and thermal expansion to be referenced back to the stress free state. For thermoset matrix composites, the stress free state exists just prior to matrix solidification. To determine the residual stress in multidirectional laminates, the strain difference from cure temperature to room temperature was first measured. Then, by doing the same for a unidirectional panel, that was cured simultaneously with the multidirectional laminate, the free thermal expansion of the unidirectional lamina could be defined for the multidirectional case. Using laminate theory, the residual stresses in each ply of the multidirectional laminate were calculated acc...

Modeling of Load Frequency Effect on Fatigue Life of Thermoplastic Composites

Abstract: This paper presents a simple prediction scheme correlating fatigue life to the thermal degradation of fatigue strength. Shifting factors similar to the time-temperature shifting in viscoelastic media were employed to account for the effect of temperature on fatigue strength and an iso-strength plot was introduced for fatigue life prediction under non-isothermal conditions. The scheme presented in this paper can predict the load frequency effect associated with hysteretic heating from limiting basic material information. The load frequency effect on the fatigue life of an AS4/PEEK ±45 thermoplastic composite laminate was investigated at 1 Hz, 5 Hz and 10 Hz. Fatigue life prediction for 5 Hz and 10 Hz based on S-N data at 1 Hz was also demonstrated. The predictions agreed reasonably with the experimental data.

A Study of Negative Poisson's Ratio in Randomly Oriented Quasi-Isotropic Composite Laminates:

Abstract: In isotropic materials, it is known that values of Poisson's ratio larger than one half are thermodynamically inadmissible, for such values would lead to negative strain energy under certain loads. Although a negative Poisson's ratio is not forbidden by thermodynamics, it is rare in crystalline solids. With the development of modem fiber reinforced composite materials, the effective Poisson's ratio of laminated fiber reinforced composites shows a peculiar behavior as it becomes larger than one half or less than zero.In this article, a study of negative in-plane Poisson's ratio for a general class of randomly-oriented composite laminates is presented. A simple random statistical analysis has been presented. It is demonstrated that composite laminates with negative in-plane Poisson's ratio could be achieved by using the specific values of independent elastic constants E1, E2, G12, and v12 in each lamina. Also, the influence of the lamina material properties to the negative in-plane Poisson's ratio of the co...