Microstructure effects on transverse cracking in composite laminae by DEM

A particle discrete element method (DEM) was employed to simulate transverse cracking in laminated fiber reinforced composites. The microstructure of the laminates was modeled by a DEM model using different mechanical constitutive laws and materials parameters for different constituents, i.e. fiber, matrix and fiber/matrix interface. Rectangular, hexagonal and random fiber distributions were simulated to study the effect of fiber distribution on the transverse cracking. The initiation and dynamic propagation of transverse cracking and interfacial debonding were all captured by the DEM simulation, which showed similar patterns to those observed from experiments. The effect of fiber volume fraction was also studied for laminae with randomly distributed fibers. It was found that the distribution and volume fraction of fibers affected not only the transverse cracking path, but also the behavior of matrix plastic deformation and fiber/matrix interface yielding in the material.     

Effect of embedded optical fiber sensors on transverse crack spacing of smart composite structures

In this study the effect of the presence of embedded optical fiber sensors on the transverse cracking of cross-ply laminates was investigated. The transverse crack spacing of cross-ply laminates with embedded optical fiber sensors was predicted using modified shear-lag analysis considering the presence of optical fibers and compared with experimental results. The effect of the orientation and quantity of optical fibers was evaluated and the effect of the coating of optical fiber was also investigated. Specimens were made with transparent glass/epoxy prepreg because the transverse crack and other damages such as delamination, splitting and bleeding of laser can be examined directly and visually. It has been found that the transverse crack spacing was not affected significantly by the embedding of optical fibers at low volume fraction of optical fibers. However, the cracks of specimens with embedded optical fibers which were initiated at a slightly lower stress level showed smaller spacing at the same stress level than those of specimens without embedded optical fibers. The theoretical crack spacing evaluated from the shear lag analysis showed good agreements with experimental results.     

Effect of physicochemical structure of natural fiber on transverse thermal conductivity of unidirectional abaca/bamboo fiber composites

To study the effect of the microstructure of natural fiber on the transverse thermal conductivity of unidirectional composite, abaca and bamboo fibers were unidirectionally aligned to fabricate epoxy composites by a resin transfer molding (RTM) technique. The transverse thermal conductivity of these two types of composites was measured in a steady-state platform. X-ray diffractometer and scanning electron microscopy were applied to analyze the microstructure and morphology of both fibers and composites. The results indicated that the transverse thermal conductivity showed two types of tendencies with fiber content increasing: increasing for bamboo fiber composites, and decreasing for abaca fiber composites. The microstructure and theoretical analysis suggest that the lumen structure plays a great role rather than crystal structures and chemical compounds on the transverse thermal conductivity of unidirectional composites, which is useful for further development and design of natural fiber reinforced composites with better thermal insulation property for people’s daily life.     

The transverse compression of PPTA fibers Part I Single fiber transverse compression testing

We report on the transverse compression testing of fine,highly anisotropic polymer fibers. Single KEVLAR 29 fibers were laid on a flat, stiff platen, and compressed by a second, stiff, parallel platen. Test output was a force-deflection curve, from which the effective transverse modulus, apparent strain at yield and the work required to compress the fibers were determined. The effects of specimen aspect ratio was examined experimentally and by finite element simulation for loaded fiber lengths of 1/4 to 7 fiber diameters, and a method proposed to deduce the plane strain response from short aspect ratio tests.     

Wet fiber shear flexibility and its contribution to the overall transverse deformation of fibers

A new method has been developed for measuring not only bending but also shear flexibility of pulp fibers by using confocal laser scanning microscopy. Based on the Steadman and Luner method, a two-stage wet pressing process was used which enabled both bending and shear flexibility and both bending and shear modulii of fibers to be determined with a single test. Three types of fibers, i.e., bleached spruce Kraft Pulp (BKP), aspen bleached chemi-themomechanical pulp (BCTMP), and aspen themomechanical pulp (CTMP), were tested. Results show that the longitudinal elastic modulii of the fibers are in a range of 3–37 GPa, and the transverse shear modulii of them are in a range of 27–103 MPa. It was also found that the shear contribution to the overall fiber deformation ranged from 60% to 90% for the fibers measured. This substantiates the concept of shear contribution to measured fiber flexibility as proposed by Waterhouse and Page.     

基于纤维束/环氧树脂复合材料试验的单向层合板横向拉伸强度预测方法

实验研究表明,纤维束/环氧树脂复合材料试件的横向拉伸强度与工程上常用的单向层合板横向拉伸强度在趋势上具有很好的相关性,但是数值上存在一定差距。本文使用两种碳纤维和两种环氧树脂制备了三种纤维束/环氧树脂复合材料和单向层合板,并分别测量了纤维束/环氧树脂复合材料和单向层合板的横向拉伸强度,以及环氧基体的拉伸强度。在实验基础上,应用Griffith断裂强度理论建立了纤维束/环氧树脂复合材料和单向层合板的横向拉伸强度的关系模型,通过两种复合材料实验的结果拟合了该模型中的参数。利用第三种复合材料实验进行校验,发现该模型预测的单向层合板横向拉伸强度与实测强度之间达到很好的一致性,相对偏差为9%。采用本文提出的方法,可以用较为简单的纤维束/环氧树脂复合材料和环氧基体拉伸试验预测单向层合板的横向拉伸强度。     

The use of boundary strip method (BSM) for evaluation of the transverse mechanical properties of fibrous composites

The boundary strip method (BSM) is applied for evaluation of the transverse mechanical properties of fibrous composites with random and periodical fiber distributions. This special semi numerical method helps find the link between the microscopic behavior of the composite material and its macroscopic response in a rather detailed manner, enabling definition of stress and strain magnitudes at each point of the cross section. Here, specifically statistical model based on the boundary strip method, is used for assessment of the transverse effective moduli of fibrous composites. Random fiber distributions are compared with periodic fiber distributions having square or hexagonal array arrangements. Those are the common models used nowadays and modeled by the finite element or the boundary element. A comparison with the bounds of the polarization extremum principles is conducted too. The influence of the randomly distributed fibers on the transverse effective moduli is investigated and a good correlation is found between the results of the present model and the lower bound of the polarization extremum principles.     

New higher-order bounds on effective transverse elastic moduli of three-phase fiber-reinforced composites with randomly located and interacting aligned circular fibers

A higher-order structure for three-phase composites containing randomly located yet unidirectionally aligned circular fibers is proposed to predict effective transverse elastic moduli based on the probabilistic spatial distribution of circular fibers, the pairwise fiber interactions, and the ensemble-area homogenization method. Specifically, the two inhomogeneity phases feature distinct elastic properties and sizes. In the special event, two-phase composites with same elastic properties and sizes of fibers are studied. Two non-equivalent formulations are considered in detail to derive effective transverse elastic moduli of two-phase composites leading to new higher-order bounds. Furthermore, the effective transverse elastic moduli for an incompressible matrix containing randomly located and identical circular rigid fibers and voids are derived. It is demonstrated that significant improvements in the singular problems and accuracy are achieved by the proposed methodology. Numerical examples and comparisons among our theoretical predictions, available experimental data, and other analytical predictions are rendered to illustrate the potential of the present method.     

Effective transverse elastic moduli of three-phase hybrid fiber-reinforced composites with randomly located and interacting aligned circular fibers of distinct elastic properties and sizes

A higher-order multi-scale structure for three-phase hybrid fiber-reinforced composites containing randomly located yet unidirectionally aligned circular fibers is proposed to predict effective transverse elastic moduli based on the probabilistic spatial distribution of circular fibers, the pairwise fiber interactions, and the ensemble-area homogenization method. Specifically, the two inhomogeneity phases feature distinct elastic properties and sizes. Two non-equivalent formulations are considered in detail to derive effective transverse elastic moduli of three-phase composites leading to new higher-order bounds. Numerical examples and comparisons among our theoretical predictions and other analytical predictions are rendered to illustrate the potential capability of the present method.     

Bending behavior of mortar reinforced with steel meshes and polymeric fibers

The main purpose of this research was to study the effects of combining reinforcing steel meshes with discontinuous fibers as reinforcement in thin walled Portland cement based mortar beams. The term ‘thin’ implies thicknesses of less than about 25 mm. The underlying idea behind this combination is to satisfy the ultimate strength limit state through the steel mesh reinforcement (main reinforcement) and to control cracking under service loads through fiber reinforcement (secondary reinforcement).

An extensive experimental program with bending tests was undertaken. Specimens were 127 × 457 × 12.7 mm. The following variables were investigated: (a) the reference mesh size — 25.4 × 25.4 mm and 50.8 × 50.8 mm; (b) the transverse wire spacing — 25.4 mm, 50.8 mm, and no transverse wires; (c) the type of fibers — polyvinylalcohol (PVA) and polypropylene (PP); and (d) the fiber volume fraction — 1 and 2% for PVA fibers, and 0.5 and 1% for PP fibers.

Some of the main conclusions are: (a) for the same fiber volume fraction, the use of PVA fibers led to a better overall performance than that of PP fibers; (b) an increase in cracking moment and a decrease in crack spacing was observed when 1% PVA, 2% PVA, and 1% PP fibers were used; (c) when 0.5% PP fiber was used, no noticeable change in behavior was observed in comparison to specimens without fibers; and (d) for 1% PVA fibers the transverse wire spacing had little effect on the crack spacing and for 2% PVA fibers, the transverse wire had no influence.     

Transverse plastic deformation of metal-matrix with randomly arranged continuous fibers

Within the framework of continuum plasticity theory, a numerical analysis in this investigation is made of the role of microstructures of fibrous composites against transverse plastic flow by means of the finite element method (FEM). In this way, the effective mechanical properties can be related quantitatively to the micro structures of composites reinforced by randomly arranged fibers. The effects of different cross-sectional geometry, such as the fiber shape (circular, square and lozenge), size, and random fiber distribution on the transverse elastic and plastic deformation of the metal-matrix composites with specific randomly distributed, aligned continuous fibers, are examined. Numerical results show that the overall transverse plastic flow of the composites is rather sensitive to the fiber geometric parameters while the elastic properties exhibit a much lower sensitivity to the fiber distribution. The interference of fibers with flow paths is seen from stress contours analysis to play an important role in the transverse strengthening due to the constraint imposed by the reinforcements. The calculations of the alterations in matrix field quantities in response to controlled changes in the random fiber distribution give valuable insights into the effects of fiber clustering on the transverse tensile properties.     

Electrical resistivity prediction of dry carbon fiber media as a function of thickness and fiber volume fraction combining empirical and analytical formulas

The transverse electrical resistivity of the dry unidirectional carbon fiber preforms was studied experimentally taking into consideration various parameters. The dependency of the electrical resistivity transverse to the fibers was thoroughly experimentally studied as a function of the preform thickness and the fiber volume fraction. Empirical mathematical relations were extracted and combined with a non-linear compaction semi-analytical formula. The extracted formula consolidates the compressibility of the preform material, the preform thickness and the fiber volume fraction or the applied pressure in order to calculate the electrical resistivity of the unidirectional preform material transverse to the fibers. Two electrical resistance measurements, at two different thicknesses and two electrical resistance measurements, at two different pressure levels, are necessary to obtained, in order to predict the full range of the electrical resistivity values of the preform material transverse to the fibers as a function of thickness and fiber volume fraction. Very good agreement between the proposed formulas and the experiments has been obtained.     

Determination of the index profile of optical fibers from transverse interferograms using Fourier theory

A novel formula is proposed to determine the index profile of optical fibers or preforms from transverse interferograms. Neither numerical differentiation nor an Abel transformation of the fringe shift is required. Index profiles can be calculated only from simple algebra using the Fourier coefficients of the fringe shift and a matrix independent of fiber parameters.     

The effects of repeated transverse impact load on the burst pressure of composite pressure vessel

The present experimental study deals with the repeated transverse impact effect on the burst pressure of composite pressure vessels. Filament winding method is used to produce the vessels. Glass fiber reinforced (GFR) vessels are manufactured by using E-glass and epoxy resin. Composite pressure vessel was manufactured from fibers oriented [+55°/−55°/+55°/−55]_2s and the impact energies were chosen as 10, 15, 20, 25, 30 J for empty vessel during the impact tests. In addition, 10, 15, 20, 25 J for water filled conditions at 25 and 70 °C. The transverse impact load was applied in single and three times repeated form. The results show that when the impact load and water temperature increases, the burst pressure decreases.     

Micromechanical modeling of interface damage of metal matrix composites subjected to transverse loading

A three-dimensional finite element micromechanical model was developed to study effects of thermal residual stress, fiber coating and interface bonding on the transverse behavior of a unidirectional SiC/Ti–6Al–4V metal matrix composite (MMC). The presented model includes three phases, i.e. the fiber, coating and matrix, and two distinct interfaces, one between the fiber and coating and the other between coating and matrix. The model can be employed to investigate effects of various bonding levels of the interfaces on the initiation of damage during transverse loading of the composite system. Two different failure criteria, which are combinations of normal and shear stresses across the interfaces, were used to predict the failure of the fiber/coating (f/c) and coating/matrix (c/m) interfaces. Any interface fails as soon as the stress level reaches the interfacial strength. It was shown that in comparison with other interface models the predicted stress–strain curve for damaged interface demonstrates good agreement with experimental results.     

Evaluation of the transverse flexure test method for composite materials

A preliminary evaluation of the transverse flexure test method was conducted, using unsized AU4 and AS4, and epoxy-sized AS4, carbon fiber/EPON 828 epoxy matrix composite materials. The transverse flexure test yielded significantly higher values of the transverse tensile strength of these unidirectional composites than did the standard transverse tensile test. Furthermore, the transverse flexure test was more sensitive to variations in fiber surface treatment and sizing, indicating its potential as a test for characterization of the tensile strength of the fiber-matrix interfacial bond.     

Organoclay effect on transverse compressive strength of glass/epoxy nanocomposites

This research focuses on the fabrication of glass fiber/epoxy nanocomposites containing organoclay as well as understanding the organoclay effect on the transverse compressive strength of nanocomposites. To demonstrate the organoclay effect, three different loadings of organoclay were dispersed, respectively, in the epoxy resin using a mechanical mixer followed by sonication. The corresponding glass/epoxy nanocomposites were produced by impregnating dry glass fiber with organoclay epoxy compound through a vacuum hand lay-up procedure. Unidirectional block specimens were employed for transverse compression tests on a hydraulic MTS machine. Experimental observations indicate that glass/epoxy nanocomposites containing organoclay exhibit higher transverse compressive strength than conventional composites. Furthermore, the failure mechanisms for all tested specimens were found to be fiber and matrix debonding. Therefore, results indicate that the increasing characteristic in transverse failure stress may be ascribed to the enhanced fiber/matrix adhesion modified by the organoclay.     

Micromechanics of piezoelectric composites with improved effective piezoelectric constant

This paper is concerned with the derivation of a micromechanics model of a new type of piezoelectric fiber reinforced composite (PFRC) materials. A continuum mechanics approach is employed to determine the effective properties of these composites. The piezoelectric fibers of these composites are considered to be electroded at the fiber–matrix interface such that the electric fields in the fiber and matrix become equal in the direction transverse to the fiber direction. The model has been verified with the existing models. The present model also predicts that the effective piezoelectric coefficient of these PFRC which accounts for the actuating capability in the fiber direction due to the applied field in the direction transverse to the fiber direction improves over the corresponding coefficient of the material of the piezoelectric fibers if the fiber volume fraction exceeds a critical fiber volume fraction.     

Progressive failure analysis of unidirectional fiber-reinforced polymers with inhomogeneous interphase and randomly distributed fibers under transverse tensile loading

The effect of damage due to interfacial debonding on the post initial failure behavior of unidirectional fiber-reinforced polymers subjected to transverse tension was investigated using numerical homogenization techniques based on the finite element method. Calculations were performed for unit cells containing fibers distributed at random over the transverse cross-section with inhomogeneous interphase layers. The mechanism of progressive failure was examined at both a global and a local level. A detailed analysis of the proposed micromechanics model revealed that it is able correctly to simulate the evolution of damage and to explain the softening mechanism. It was found that the post initial failure behavior of unidirectional lamina under transverse tension is mainly controlled by the interface strength and the interphase stiffness. The present study showed that local fiber array irregularities are a significant contributor to matrix cracking through local stress concentrations and the occurrence of localization.     

The shear properties and deformation mechanisms of porous metal fiber sintered sheets

Porous metal fiber sintered sheets (MFSSs) are a type of layered transversely isotropic open cell materials with low relative density (i.e., volume fraction of fibers), high specific stiffness and strength, and controllable precision for functional and structural applications. Based on a non-contact optical full field strain measurement system, the in-plane and transverse shear properties of SMFFs with relative densities ranging from 15% to 34% are investigated. For the in-plane shear, the modulus and strength are found to depend linearly upon the relative density. The associated deformation is mainly due to fiber stretching, accompanied by the direction change of metal fibers. When the shear loading is applied in the transverse direction, the deformation of the material is mainly owing to fiber bending, followed by the separation failure of the fiber joints. Measured results show that the transverse shear modulus and strength have quartic and cubic dependence upon the relative density respectively and are much lower than their in-plane counterparts. Simple micromechanics models are proposed for the in-plane and transverse moduli and strengths of MFSSs in shear. The predicted relationships between the shear mechanical properties of MFSSs and their relative density are obtained and are in good agreement with the measured ones.