Fiber Microbuckling Research Articles

Compressive strength of composite laminates with an open hole: Effect of ply blocking

This paper examines the application of a cohesive zone model to predict the open hole compressive strength of an IM7/8552 carbon fibre/epoxy quasi-isotropic multidirectional laminate and investigates the level-ply scaling or ply blocking effect on notch sensitivity. Cohesive zone models have been successfully applied to predict the damage from notches in engineering materials loaded in tension. They have also been used to determine the growth of fibre microbuckling from a hole in composite laminates under compression. The usual strategy is to replace the inelastic deformation associated with plasticity or microbuckling with a line-crack and to assume some form of stress-displacement (σ − δ) bridging law across the crack faces. Here a plastic fibre kinking analysis and a linear reduced (softening) σ − δ relationship are used for the prediction of the unnotched and open hole compressive strength; the theoretical results will be compared with experimental data in Part B of the 3rd World-Wide Failure Exercise.

Effective moduli of multi-scale composites

The effective moduli of a multi-scale composite are evaluated by a bottom-up (hierarchical) modeling approach. We focus on a two-scale structure in which the small scale includes a platelet array inside a matrix, and the large scale contains fibers inside a composite matrix. We demonstrate that the principal moduli of the multi-scale composite can be fine-tuned by the platelet arrangement and orientation. As a case study, we consider the phenomenon of fiber micro-buckling within the multi-scale composite. It is found that the compressive micro-buckling strength can be considerably increased for specific platelet orientations. The multi-scale design approach presented here can be used to generate novel families of composite materials with tunable mechanical properties.

Prediction of Compressive Strength for Unidirectional Fiber Reinforced Plastics in Considering Effect of Strain-Rate Dependency on Mechanical Properties of Constituent Materials

This study presents an analytical model to predict compressive strength of unidirectional FRP. Proposed model considers the effect of strain-rate dependency on mechanical properties of constituent materials. The model is based on the elastic foundation model and the microbuckling model of fiber which has initial misalignment in matrix. Compressive deformation of unidirectional FRP is considered by dividing into fiber microbuckling region and plastic kinking region. Additionally, to take into consideration the change in compressive deformation mode accompanying fiber volume fraction or fiber microbuckling, A mode function is introduced. The predictions from the proposed model are compared with experimental results of unidirectional E-glass/Epoxy and T700SC/Epoxy evaluated by using the conventional split Hopkinson pressure bar method. Incorporating strain-rate dependency on compressive modulus of reinforcement calculated from composite mixture law, the predictions are found to be in good agreement with experimental results of strain-rate dependency on compressive strength. Accuracy of the prediction is improved by changing the mode function.

On the longitudinal compressive strength prediction of unidirectional laminated composites based on an improved model

AbstractThe compressive failure of unidirectional (UD) composite laminates is investigated using an improved model. Most of the existing models to predict the compressive strength of UD composites are either based on microbuckling or kinking of fibers. However, the failure mechanism analysis based on the SEM microscopy observations of failed specimens shows that the two mechanisms are responsible for the failure simultaneously when kinking occurs following the microbuckling of fibers. Therefore, the fiber microbuckling and fiber kinking models are combined to predict the compressive strength of UD T300/QY8911 composite laminate. Furthermore, this study proposes a new model for the stiffness of the foundation determination, making the predicted microbuckling strength closer to its real value. Also the effects of parameters presented in the prediction formula on the compressive strength are discussed, and it is found that the Young's modulus of matrix is the most important parameter. The predicted compressive strength using the improved model with combined modes shows a very good agreement with the experiment measurement. POLYM. COMPOS., 2011. © 2011 Society of Plastics Engineers

Effect of clamping force on the delamination onset and growth in bolted composite laminates

Clamping force is a key element that alters the mechanism and sequence of failure in bolted joints of composite laminates. The mode of failure in bolted joints can be controlled by geometrical parameters and the preferred fail safe mode of failure is ‘bearing’ which generally consists of matrix cracks, delamination and fibre microbuckling. Three-dimensional (3-D) pinned (without clamping force) and bolted (1 kN clamping force) joint models were developed in [0/90] s carbon fibre reinforced plastic (CFRP) laminates to show the clamping force effect on the onset and growth of delamination. It is shown that delamination was resulted from the shear stress components (Mode II & III) at the interface and the contribution of the out-of-plane component (Mode I – opening), so the clamping force, was negligible without modelling the in-plane failure modes and their coupling with delamination, which will be considered in future work.

Preparation of Unidirectional Carbon Fiber Preform for Aluminium Matrix Composites

The unidirectional carbon fiber (CF) preform for carbon fiber reinforced aluminium (CF/Al) composites has been investigated in terms of the fabrication condition and the strength property. The CF preform which consists of CFs and Cu particles was fabricated by spark plasma sintering (SPS) process with different fabrication temperatures. In order to determine the infiltration pressure of molten Al to the CF preform, the compression test was performed to the CF preform. The unidirectional CF preform by SPS process was well formed with the fabrication temperature condition at 1123 K in accordance with the formation of Cu particle bridging between fibers and Cu deposition on fibers. The compression strength of the CF preform increased with increasing fabrication temperature. Besides, the CF preform was deformed such as fiber micro-buckling or fiber kinking phenomena over the maximum compression strength. The infiltration pressure of the molten Al can be decided under the maximum compression strength. [doi:10.2320/matertrans.L-MZ201110]

ACM Special Issue: ICCM-17: Deformation and Fracture of Composites

This ACM issue is a refereed compilation of selected papers presented at the 17th International Conference on Composite Materials (ICCM-17, www.iccm17.org) that was held in Edinburgh, UK, on 27–31 July 2009 in the session of Deformation and Fracture of Composites. The focus of this session was on understanding the initiation, propagation and interaction of failure mechanisms in the form of resin cracking, delamination and fibre microbuckling developed in multidirectional laminated constructions when loaded in tension and/or compression and their effect on strength and stiffness properties. Composite materials have gained popularity in high-performance products that need to be lightweight, yet strong enough to take high loads such as aerospace structures (tails, wings and fuselages), ship construction and racing car bodies. Modern polymers containing long fibres of carbon, glass or Kevlar, contribute more than 50% of the structural mass of the Boeing 787 and the proposed Airbus A350. Present research effort is devoted to analysis and computational simulation of the manufacturing and assembly process as well as simulation of the performance of the structure, since they are all intimately connected and could make design and building with composites more affordable. Prediction of structural performance through modelling is the way forward. Predictive modelling based on the physics of composite material behaviour is wealth generating; by guiding material system selection and process choices, by reducing experimentation and associated high costs. No longer is computer hardware or software a barrier to the deployment of predictive modelling. Instead, with experience and the building of confidence we can identify the right approach and application of appropriate calculations at the appropriate level of sophistication to solve the correctly identified problem. Optimum material microstructures can be designed rather than found by trial and error, whilst maximising structural high performance and sustainable safe life. As a guest editor of this special issue in Applied Composite Materials, I would like to thank all the authors who submitted their papers, reviewers who have helped to maintain the high standards set by the journal and Dr Peter Beaumont, the Editor-in-Chief of ACM for agreeing the publication of this special issue; hope you will enjoy reading them. Appl Compos Mater (2011) 18:1 DOI 10.1007/s10443-010-9183-6

A Graphical Method Predicting the Compressive Strength of Toughened Unidirectional Composite Laminates

The in-plane shear and compressive properties of unidirectional (UD) HTS40/977-2 carbon fibre-toughened resin (CF/TR) laminates are investigated. Scanning Electron microscopy (SEM) and optical microscopy are used to reveal the failure mechanisms developed during compression. It is found that damage initiates by fibre microbuckling (a fibre instability failure mode) which then is followed by yielding of the matrix to form a fibre kink band zone that leads to final fracture. Analytical models are briefly reviewed and a graphical method, based on the shear response of the composite system, is described in order to estimate the UD compressive strength. Predictions for the HTS40/977-2 system are compared to experimental measurements and to data of five other unidirectional carbon fibre reinforced polymer (CFRP) composites that are currently used in aerospace and other structural applications. It is shown that the estimated values are in a good agreement with the measured results.

Fiber-waviness Model in Filament Winding Process

Abstract Fiber waviness is one of the initial defects in the filament winding process, and causes reduction of compressive strength of the composite structure. The mechanism of growth of fiber waviness is, however, not completely clear. In the present study, a model for generating fiber waviness is proposed. It is assumed to be due to local fiber micro-buckling arising from the compression load caused by shrinkage of a metal jig. Three faults are considered as causes of micro-buckling: bonding between metal jig and composite, insufficient cure of the resin, and initial deflection of fibers. Analysis and experiments based on this model have been carried out. Key words : Carbon Fibers; Polymer-matrix Composite, Filament Winding, Fiber Waviness, Curing Defects 1. Introduction Although applications of fiber reinforced composites (FRP) are increasing for aerospace structural components because of their high specific strength and stiffness, FRP is not widely accepted for other structures. One of the reasons is difficulty of processing FRP: many trial and error approaches are indispensable for preventing initial defects. Fiber waviness is one of the important initial defects in the filament winding process, and causes large reduction of the compressive strength of composite structures

Investigating Silica Nanoparticle Effect on Dynamic and Quasi-static Compressive Strengths of Glass Fiber/Epoxy Nanocomposites

The research is aimed to investigate the compressive strengths of glass/epoxy nanocomposites, containing various loadings of spherical silica nanoparticles. Through a sol—gel technique, the silica particles with a diameter of 25 nm were exfoliated uniformly into the epoxy resin. Subsequently, by inserting the silica—epoxy mixture into the unidirectional glass fiber through a vacuum hand lay-up process, the glass fiber/epoxy composite laminates with 10, 20, and 30 wt% of silica nanoparticles were fabricated. Quasi-static and dynamic compression tests were conducted on the brick composite specimens with fiber orientations of 0°, 5°, 10°, 15°, and 90° using a hydraulic MTS machine and a split Hopkinson pressure bar, respectively. Observations on the failure specimens indicated that for fiber orientations less than 15°, the fiber microbuckling is the dominant failure mechanism. On the other hand, for the 90° samples, the out-of-plane shear failure is the main failure mechanism. In addition, it was denoted that as the silica contents increase, the compressive strengths of the glass/epoxy composites are improved accordingly. The enhancing mechanism in the compressive strengths can be properly explicated using the microbuckling model.

Design Analysis of the Outwater-Murphy Single-Fiber Specimen

A design analysis of the Outwater-Murphy (OM) compression test specimen is presented. The objective is to size the dimensions of the OM specimen for interface fracture toughness testing of glass and carbon fibers in a polymer matrix. A simple fracture mechanics analysis is developed based on mechanics of materials principles assuming linear-elastic behavior of the fiber and matrix. A successful test demands fiber/matrix debonding prior to any other failure mechanisms such as net section yielding, fiber microbuckling and global buckling of the specimen. The analysis of the results indicates that the test should be viable for OM specimens with a small cross section and a small central hole. Tests on E-glass/vinylester specimens were successfully conducted providing a fiber/matrix interface toughness of 38 J/m2, in reasonable agreement with interface toughness data for similar systems determined by other test methods.

Fracture mechanisms and failure analysis of carbon fibre/toughened epoxy composites subjected to compressive loading

Abstract This study investigates the failure mechanisms of unidirectional (UD) HTS40/977-2 toughened resin composites subjected to longitudinal compressive loading. A possible sequence of failure initiation and propagation was proposed based on SEM and optical microscopy observations of failed specimens. The micrographs revealed that the misaligned fibres failed in two points upon reaching maximum micro-bending deformation and two planes of fracture were created to form a kink band. Therefore, fibre microbuckling and fibre kinking models were implemented to predict the compressive strength of UD HTS40/977-2 composite laminate. The analysis identified several parameters that were responsible for the microbuckling and kinking failure mechanisms. The effects of these parameters on the compressive strength of the UD HTS40/977-2 composite systems were discussed. The predicted compressive strength using a newly developed combined modes model showed a very good agreement to the measured value.

Modelling open hole compressive strength of composite laminates tested in hot wet conditions

The objective of this paper is to analytically predict the compressive and in-plane shear response of T800/924C carbon fibre epoxy composite laminates (currently available for aerospace structural applications) exposed to hot wet environments. The weight gains, maximum moisture contents and diffusion coefficients of unidirectional and various multidirectional laminates immersed in boiling water (accelerated aging) were measured in earlier work and briefly reported here. Data are also presented on the effects of moisture and temperature on the uniaxial compressive strength/failure mode of unidirectional laminates and multidirectional plates with an open hole. It is observed that the failure in the hot wet specimens always occurs as a result of out-of-plane microbuckling of the 0° plies. This is attributed to the reduction in matrix strength properties and weakening of the ply interface arising from elevated temperatures and environmental conditioning. Test results are compared to theoretical predictions made by the Budiansky fibre microbuckling model and the Soutis–Fleck cohesive zone model for the open hole (notched) compressive (OHC) strength.

Fiber reinforced shape-memory polymer composite and its application in a deployablehinge

This paper investigates the shape recovery behavior of thermoset styrene-based shape-memorypolymer composite (SMPC) reinforced by carbon fiber fabrics, and demonstrates thefeasibility of using an SMPC hinge as a deployable structure. The major advantages ofshape-memory polymers (SMPs) are their extremely high recovery strain, low density andlow cost. However, relatively low modulus and low strength are their intrinsic drawbacks. Afiber reinforced SMPC which may overcome the above-mentioned disadvantages is studiedhere. The investigation was conducted by three types of test, namely dynamicmechanical analysis (DMA), a shape recovery test, and optical microscopic observationsof the deformation mechanism for an SMPC specimen. Results reveal that theSMPC exhibits a higher storage modulus than that of a pure SMP. At/aboveTg, the shape recovery ratio of the SMPC upon bending is above 90%. The shape recoveryproperties of the SMPC become relatively stable after some packaging/deployment cycles.Additionally, fiber microbuckling is the primary mechanism for obtaining a large strain inthe bending of the SMPC. Moreover, an SMPC hinge has been fabricated, and a prototypeof a solar array actuated by the SMPC hinge has been successfully deployed.

A comprehensive explanation of compression strength differences between various CFRP materials: Micro-meso model, predictions, parameter studies

The present work proposes a model to predict the compression strength of CFRP as a function of various material characteristics: fiber properties and volume content, non-linear matrix properties, interface properties, residual strains, fiber misalignment, ply waviness, and interlaminar properties. For this, a three-step FE-based approach is developed considering (i) fiber microbuckling, (ii) wavy ply mesobuckling, and (iii) the mutual influence of these two. This approach is then applied to investigate the relative contribution of the mentioned material characteristics on compression strength deficits of NCF (Non-crimp Fabric) CFRP with respect to prepreg, in order to identify ways to improve NCF performance.

FW成形時の繊維うねりのモデル化

Abstract Fiber waviness is one of the initial defects in the filament winding process, and causes reduction of compressive strength of the composite structure. The mechanism of growth of fiber waviness is, however, not completely clear. In the present study, a model for generating fiber waviness is proposed. It is assumed to be due to local fiber micro-buckling arising from the compression load caused by shrinkage of a metal jig. Three faults are considered as causes of micro-buckling: bonding between metal jig and composite, insufficient cure of the resin, and initial deflection of fibers. Analysis and experiments based on this model have been carried out. Key words : Carbon Fibers; Polymer-matrix Composite, Filament Winding, Fiber Waviness, Curing Defects 1. Introduction Although applications of fiber reinforced composites (FRP) are increasing for aerospace structural components because of their high specific strength and stiffness, FRP is not widely accepted for other structures. One of the reasons is difficulty of processing FRP: many trial and error approaches are indispensable for preventing initial defects. Fiber waviness is one of the important initial defects in the filament winding process, and causes large reduction of the compressive strength of composite structures

3D FE-modeling of surface and anisotropy effects during micro-buckling in fiber composites

A 3D finite element model is proposed and a study is carried out of the surface and fiber anisotropy effects during fiber micro-buckling in fiber-reinforced composites. The cases of both transversal isotropic and isotropic fibers with round and rectangular cross-sections are studied. The model accounts for the distinct difference in mechanical properties of the reinforcement and the matrix. Perfect bonding at the fiber-matrix interfaces is assumed. For simplicity, single fibers in a matrix are considered during the calculations. An eigenvalue analysis of the considered systems fibers-matrix subjected to axial compression is performed. Calculations show that the surface effects result in an increase of the buckling wavelength and in a significant decrease of the critical shortening for both isotropic and anisotropic fibers.

1438 CFRPにおける微視座屈による成形欠陥発生のモデル化(S06-2 複合材料の動向とその加工技術II,21世紀地球環境革命の機械工学:人・マイクロナノ・エネルギー・環境)

Fiber waviness is one of initial defects during processing such as filament windindg. The fiber waviness is made due to the local fiber micro-buckling of the curing process. The fiber waviness causes reduction of compressive strength of the composite structure. The mechanism of developing fiber waviness is, however, not completely clear. In the present study, a model of generating fiber micro-buckling is proposed. The model is based on the compression load caused by shrinkage of a metal jig. Two faults are considered in the model: bonding between a metal jig and a composite and insufficient cure of the resin. The analysis and experiment based on this model are conducted. The new model successfully predicts the initiation of the fiber micro-buckling.

Measuring the notched compressive strength of composite laminates: Specimen size effects

Large fibre reinforced composite structures can give much lower strengths than small test specimens, so a proper understanding of scaling is vital for their safe and efficient use. Small size (scale) specimens are commonly tested to justify allowable stresses, but could be dangerous if results are extrapolated without accounting for scaling effects. On the other hand large factors are sometimes applied to compensate for uncertainties, resulting in overweight designs. The most important variables of scaling effects on the strength of composites with open holes have been identified from experimental tests as notch size, ply and laminate thickness. In this study, these have been scaled both independently and simultaneously over a large range of combinations. The specimens are fabricated from commercially available (Hexcel Composites Ltd.) carbon/epoxy pre-impregnated tapes 0.125 mm thick (IM7/8552). The material is laid up by hand in unidirectional [0 4] ns with n = 2, 3, 4, and 8 (i.e., 2, 3, 4 and 8 mm thick) and multidirectional laminates; two generic quasi-isotropic lay-ups, one fabricated with blocked plies [45 n /90 n /−45 n /0 n ] s and the other with distributed layers [45/90/−45/0] ns with n = 2, 4 and 8 are examined. It is shown that the critical failure mechanism in these laminates is in the form of fibre microbuckling or kinking. The unnotched compressive strength in unidirectional specimens thicker than 2 mm is found to be limited by the stress concentration developed at the end tabs and manufacturing induced defects in the form of ply waviness, fibre misalignment and voids rather than specimen size (scaling). In the open hole specimens, for both lay-ups, the strength reduction observed is due to hole size effect rather than specimen thickness or volume increase. The open hole (notched) compressive strength results obtained compare favourably to predictions by a linear softening cohesive zone fracture model developed in earlier work by the second author.

Longitudinal Compressive Strength of Continuous Fiber Composites

The in-plane longitudinal compressive strength of continuous fiber-reinforced composites is significantly less than their tensile strength, a phenomenon that is still not well understood. Dow and Rosen attempted in 1965 to explain this phenomenon through a fiber microbuckling model, in which each fiber was modeled as a beam-on-an-elastic-foundation. The model, however, grossly overestimates the compressive strength; researchers have since focused on introducing factors that reduce the predicted compressive strength to fit experimentally observed values. These include initial fiber waviness, fiber and matrix properties, the interfacial strength, and interphase properties. While empirical results have demonstrated the influences of some of these factors, an analytical model that can accurately predict the strength while taking into account the underlying failure mechanism is still lacking. The present study treats the compressive strength prediction of continuous fiber-reinforced composites using a different approach. Here, the matrix surrounding the fiber is not assumed to be homogeneous, but is divided into a discrete interphase region adjacent to the fiber and bulk matrix phase beyond the interphase. The three-phase model is shown to provide a more accurate prediction of the compressive strength of typical high-performance composites, such as glass and carbon fiber-reinforced epoxy. The present model's prediction agrees well with the trends shown in the experimental database available in the literature.