Unidirectional Glass Fiber-reinforced Composites Research Articles

X-ray computed tomography study of compressive failure in a notched unidirectional glass fiber-reinforced composites

An experimental study on the axial compressive failure of a notched unidirectional (UD) glass fiber-reinforced composites has been performed to better understand the failure mechanisms in three dimensions (3D). A sequence of failure initiation and propagation has been monitored based on X-ray computed tomography (CT) images of the sample during failure. The CT images have shown that fiber kinking failure is the main failure mechanism, and there are two different types of kink bands, classified as type 1 and type 2 by final geometric form. The type 1 kink band, which has an X shape, extends outward from the notch along the width direction before finally creating a shear type 2 kink band at a distance from the notch. The novelty of this work involves observing the nucleation and propagation of fiber kink bands in three-dimensional space, as well as defining and quantifying kink band variables.

An experimental and computational investigation of embedded wrinkle’s impacts on the compressive responses of thick unidirectional glass fiber-reinforced composites

In thick unidirectional glass fiber-reinforced composites, the out-of-plane embedded wrinkle flaws have a major impact on compressive failure, which should be taken into account and assessed during the design, manufacturing, and inspection phases. The thick unidirectional coupons with embedded wrinkles of various severities were manufactured and tested. The deformation of the wrinkle region was captured by the Digital Image Correlation (DIC). As the wrinkle angle increased, the compressive failure stress decreases and the modulus does not change significantly, while the wrinkle amplitude has a negative effect on both the compressive failure stress and elastic modulus. The linear texture image regression approach is used to account for the path of fibers. The high-fidelity three-dimensional finite element (FE) models considering the real paths of the fibers are established. To reveal the interplay and impact of wrinkle severity parameters on the compressive failure behavior of composites, the progressive damage analyses and cohesive elements are also incorporated. Compared with the experiments, the FE studies accurately identify the dominant failure modes and failure stress values for various wrinkle severity parameters.

Dynamic effects of single fiber break in unidirectional glass fiber-reinforced composites

In a unidirectional composite under static tensile loading, breaking of a fiber is shown to be a locally dynamic process that leads to stress concentrations in the interface, matrix and neighboring fibers that can propagate at high speed over long distances. To gain better understanding of this event, a fiber-level finite element model of a two-dimensional array of S2-glass fibers embedded in an elastic epoxy matrix with interfacial cohesive traction law is developed. The brittle fiber fracture results in release of stored strain energy as a compressive stress wave that propagates along the length of the broken fiber at speeds approaching the axial wave-speed in the fiber (6 km/s). This wave induces an axial tensile wave with a dynamic tensile stress concentration in adjacent fibers that diminishes with distance. Moreover, dynamic interfacial failure is predicted where debonding initiates, propagates and arrests at longer distances than predicted by models that assume quasi-static fiber breakage. In the case of higher strength fibers breaks, unstable debond growth is predicted. A stability criterion to define the threshold fiber break strength is derived based on an energy balance between the release of fiber elastic energy and energy absorption associated with interfacial debonding. A contour map of peak dynamic stress concentrations is generated at various break stresses to quantify the zone-of-influence of dynamic failure. The dynamic results are shown to envelop a much larger volume of the microstructure than the quasi-static results. The implications of dynamic fiber fracture on damage evolution in the composite are discussed.

Study on Cutting Mechanism of UD-GFRP Based on Fracture Mechanics

This document explained and demonstrated the crack growth of unidirectional glass fiber-reinforced composites (UD-GFRP) using the fracture mechanics theory during the process of machining. The models of fracture mechanics were established at antidromic cutting and direct cutting. The result is obtained that mode-I crack and mode-II crack transform each other during cutting and the complex crack is come into being. The propagation of complex crack makes the material fracture. The quality of machined surface at direct cutting is better than the quality at antidromic cutting.

Influence of filler/reinforcing agent and post-curing on the flexural properties of woven and unidirectional glass fiber-reinforced composites

The aim of this study is to investigate the reinforcing effect of woven and unidirectional glass fibers and the effect of post-curing on the flexural strength and flexural modulus of glass fiber-reinforced composites. A series of composites containing 2,2-bis[4-(2-hydroxy-3-methacryloyloxypropoxy)-phenyl]propane and triethyleneglycol dimethacrylate matrices and different reinforcements of unidirectional or woven glass fibers were prepared. The samples, 25 × 2 × 2 mm, were cured with a halogen curing lamp, followed by additional curing by thermal treatment at 135 ± 5 °C temperature and 60 psi pressure. Samples were tested before and after post-curing in order to determine the flexural strength and flexural modulus. The degree of reinforcement with glass fibers was varied between 14 and 57 wt% or 7.64 and 38.44 vol% by changing the number of unidirectional bundles or woven glass fiber bands in the composites, respectively. The obtained flexural strength values were in the range of 95.20–552.31 Mpa; the flexural modulus ranged between 2.17 and 14.7 GPa. The highest flexural strength and flexural modulus values were recorded for samples with unidirectional glass fibers. The mechanical qualities of the glass fibers-reinforced composites increased after post-curing treatment. Increasing of the glass fiber amount in the experimental composites improves both flexural strength and modulus. SEM micrographs of fractured composites indicate a strong interfacial interaction between the glass fibers and the polymer matrix.

Study of Cracked Unidirectional Glass Fiber-reinforced Composites by Digital Speckle Correlation Method

Deformation and fracture behavior of glass fiber-reinforced composites with single-edge crack under tensile loading is studied by means of the traditional stress measurement method combined with digital speckle correlation method. After capturing the deformation image of the crack tip in different loading processes, the horizontal and vertical displacement fields are obtained. At the same time, using a three-element strain gauge, the strain responses near the crack tip during the whole damage and fracture process of the tensile specimen are recorded. The width of the local deformation region at the crack tip of the glass fiber-reinforced composite materials is determined. Then the analyses of deformation and fracture in a unidirectional fiberreinforced composite are performed. These results play an important role in understanding the deformation and fracture mechanism of the unidirectional glass fiber-reinforced composite materials.

The influence of short-term water storage on the flexural properties of unidirectional glass fiber-reinforced composites

Objective: The aim of this study was to determine flexural properties of unidirectional E-glass fiber-reinforced composite (FRC) with polymer matrices of different water sorption properties. Methods: Rhombic polymer and FRC test specimens made of three commercially available diacrylate resin (Sinfony Activator, Triad Gel, 3M Scotchbond Adhesive) and different volume fractions of fibers were tested with three-point bending test according to ISO 10477 after storing in water for 30 days. Water sorption of specimens was also measured. Results: Flexural strength of specimens with 45 vol% fraction E-glass fibers varied from 759 to 916 MPa in dry conditions. Water-stored specimens showed flexural strengths of 420–607 MPa. ANOVA analysis revealed that the fiber-volume fraction and the water sorption of the polymer matrix had a significant effect ( p<0.001) on the flexural properties. Dehydration of specimens recovered the mechanical properties. Decrease of flexural properties after water immersion was considered to be mainly caused by the plasticizing effect of water and the decrease depended on water sorption. Significance: Use of polymers with low-water sorption seems to be beneficial in order to optimize the flexural properties of FRC.

Variation of glass transition temperature with direction in unidirectional glass fiber-reinforced composites

A theoretical expression of the value of the glass-transition temperatureTg at any direction of orientation, defined by the angle ϑ subtended between this direction and the average axis of fibers; was introduced. Dilatometric, as well as dynamic tests were executed for a type of unidirectional composite in order to compare the respective values ofTg derived from these tests. The experimental results were also compared with theoretical values derived from the theoretical expression. A satisfactory agreement between, theory and experiment was established.