Single-fiber Composite Fragmentation Test Research Articles

A three-phase model of the single-fiber composite fragmentation test to evaluate viscoelasticity, stress distribution and interface fracture toughness

ABSTRACTA three-phase model of the single-fiber composite fragmentation test is proposed to evaluate viscoelasticity, stress distributions and interface fracture toughness. Based on the three-parameter standard linear solid model, a unit cell is established under periodic boundary conditions to study viscoelasticity considering the interface effect. Under the thermal residual stress and the tensile load, a three-dimensional (3D) theoretical cylinder model is utilized to predict the stress distributions in radial, hoop and axial directions for fiber, interface and matrix. The results are in good agreements with finite element method (FEM) results. Especially, we discuss the effects of the fiber shape on the mechanical behavior. Classic shear-lag theories are compared to modify the axial stress distributions. For the fragmentation test, two basic failure modes are studied in the proposed model. Based on accurate stress distributions, the energy-balance scheme takes into account all strain energies, the failure consumption, the friction consumption and the applied load work to calculate interface fracture energy release rate, results of which is consistent with other models. Furthermore, we study the effects of friction coefficients and radial debonded locations. Using the softening and failure model of the interface, the debonding process is simulated for zero-thickness and thick interfaces, respectively.

Influence of cryogenic treatment on mechanical and interfacial properties of carbon nanotube fiber/bisphenol-F epoxy composite

Abstract The effects of cryogenic treatment (−196 °C) on properties of the carbon nanotube (CNT) fiber/epoxy composite are investigated using a temperature controlled cooling process with low cooling rate. The CNT fiber/epoxy single fiber composite fragmentation test is performed in conjunction with electrical resistance measurements to compare the interfacial properties at ambient and cryogenic temperatures. Weibull distribution for CNT fiber strengths shows a decreased variation (the shape parameter increases from 4.98 to 6.93) for cryo-treated fibers although their mechanical properties are not significantly influenced by the treatment. The electrical resistance measurement shows an increased electrical resistance changing rate with strain for the cryogenic treated sample, indicating better fiber/matrix interaction. In addition, the cryogenic treatment also improves interfacial bonding strength by 31% owing to the effect of differential thermal shrinkage for the CNT fiber and epoxy. An asymmetrical interfacial stress distribution is observed corresponding to the asymmetrical birefringence patterns of the fragmented fiber in the single fiber composite, which could be due to the twisted structure of the CNT fiber.

Fracture behavior of carbon nanotube/carbon microfiber hybrid polymer composites

Growing carbon nanotubes (CNTs) on the surface of fibers has the potential to modify fiber–matrix interfacial adhesion, enhance composite delamination resistance, and possibly improve toughness. In the present study, aligned CNTs were grown upon carbon fabric via chemical vapor deposition. Continuously monitored single-fiber composite fragmentation tests were performed on pristine and CNT-grafted fibers embedded in epoxy, and single-laminate compact-tension specimens were tested for fracture behavior. A significant increase (up to 20 %) was observed in the interfacial adhesion, at the cost of a decrease in the fiber tensile strength. As a result, the maximum load of the composite was decreased, but its residual load-bearing capacity more than doubled. The likely sources of these effects are discussed, as well as their implications.

Increasing the interfacial strength in carbon fiber/epoxy composites by controlling the orientation and length of carbon nanotubes grown on the fibers

Multi-walled carbon nanotubes (MWCNTs) were grafted onto carbon fibers (CFs) using an injection chemical vapor deposition method. The orientation and length (16.6–108.6 μm) of the MWCNTs were controlled by the surface treatment of the CFs and the growth time, respectively. The interface between the MWCNTs and the CFs indicated the grafted CNTs were immobilized by embedding catalyst on CFs. Two orders of magnitude increase in the specific surface areas of CFs was obtained by grafting the MWCNT. MWCNT–CF hybrids exhibited good wettability with the epoxy resin due to the surface roughness and capillary action. Single-fiber composite fragmentation tests revealed an remarkable improvement of interfacial shear strength (IFSS) controlled by the orientation and length of MWCNTs. MWCNTs with an perpendicular alignment and long length showed a high IFSS in epoxy composites due to better wettability and a large contact interface between the hybrids and the resin. Hybrids with an optimum length (47.2 μm) of aligned MWCNTs showed a dramatic improvement of IFSS up to 175% compared to that of pristine CFs.

Effect of Atmospheric Plasma Treatment on Carbon Fiber/Epoxy Interfacial Adhesion

In this work the effect of atmospheric plasma treatment on carbon fiber has been studied. The carbon fibers were treated for 1, 3 and 5 min with a He/O2 dielectric barrier discharge atmospheric pressure plasma. The fiber surface morphology, surface chemical composition and interfacial shear strength between the carbon fiber and epoxy resin were investigated using atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS) and the single fiber composite fragmentation test. Compared to untreated carbon fibers, the plasma treated fiber surfaces exhibited surface morphological and surface composition changes. The fiber surfaces were found to be roughened, the oxygen content on the fiber surfaces increased, and the interfacial shear strength (IFSS) improved after the atmospheric pressure plasma treatment. The fiber strength showed no significant changes after the plasma treatment.

Numerical simulation of the fiber fragmentation process in single-fiber composites

This paper attempts to simulate the process of fiber break in single-fiber composite fragmentation test (SFCF). The ensued stress redistribution in fiber, matrix and interface after the fiber break is also researched. A new simulation method based on the user subroutine: “User subroutine to redefine field variables at a material point (USDFLD)” in the general finite element method (FEM) software ABAQUS is proposed. The subroutine is used for the definition of the fiber material constitutive model and is programmed in FORTRAN. It is called by ABAQUS. So the damage mode of the fiber break is simulated by the method. The forms of stress redistribution in fiber, matrix and interface are also obtained in the simulation. Then the T300/epoxy single-fiber composite is fabricated and the single-fiber composite fragmentation test is done. The simulation method and results are proved to be appropriate by the comparative analysis with the experiment.

Effect of carbon nanotubes on the interfacial shear strength of T650 carbon fiber in an epoxy matrix

The interfacial shear strength of carbon nanotube coated carbon fibers in epoxy was studied using the single-fiber composite fragmentation test. The carbon fibers were coated with carbon nanotubes (CNT) on the fiber surface using thermal chemical vapor deposition (CVD). The CVD process was adjusted to produce two CNT morphologies for the study: radially aligned and randomly oriented. The purpose of the CNT coating was to potentially produce a multifunctional structural composite. Results of the single-fiber fragmentation tests indicate an improvement in interfacial shear strength with the addition of a nanotube coating. This improvement can most likely be attributed to an increase in the interphase yield strength as well as an improvement in interfacial adhesion due to the presence of the nanotubes.

Parameters regulating interfacial and mechanical properties of short glass fiber reinforced polypropylene

The performance of thermoplastic composites is known to depend on the intrinsic properties of the two composite components, the quality of the fiber-matrix interface, and the crystalline properties of their matrix. The objective of this work is to characterize the effect of the addition of modified polypropylene (PP) and silane coupling agent on the mechanical and interfacial properties of short fiber reinforced PP composites. Differential scanning calorimetry (DSC), single fiber composite fragmentation tests (SFC), and mechanical testing are used to understand the different parameters regulating the interfacial properties of composites. No influence of the modified PP on the level of crystallinity is observed. Some differences in the size of the spherulites are observed for acrylic acid grafted PP (PP-g-AA). Those samples also show lower mechanical properties in spite of good interfacial interactions. Maleic anhydride grafted PP (PP-g-MAh) leads to better mechanical performances than PP-g-AA. A high MAh content PP-g-MAh grade with low viscosity is the best polymeric additive used in the present work.

Fiber–matrix adhesion from the single-fiber composite test: nucleation of interfacial debonding

The level of fiber–matrix interfacial adhesion in composites is traditionally evaluated by means of a stress-based parameter. Recently, it was suggested that an interfacial energy parameter might constitute a valid alternative. From an overview of the literature regarding the single-fiber composite fragmentation test, it appears that energy-based approaches have already been proposed in the past, but were either not successful, or not fully developed. Our recent energy balance scheme, proposed for the analysis of the initial interface debonding which occurs at fiber breaks during a fragmentation test, is presented and expanded here. The effects of thermal residual stress in the fiber, and of friction in the debonded area, are now incorporated in the energy balance model. We use a different shear-lag parameter proposed by Nayfeh, rather than the commonly used Cox parameter. New, extensive single-fiber fragmentation data regarding the interface crack initiation regime is presented, using sized and unsized E-glass fibers embedded in UV-curable or epoxy polymers. Some data for unsized carbon in epoxy is also presented. Fiber fragmentation is forced to take place entirely in the linear elastic region of the stress–strain curve, by means of pre-stressed single fibers. The importance of this procedure is discussed. Future work will focus on the interface crack propagation regime.

Toughness of interfaces from initial fiber-matrix debonding in a single fiber composite fragmentation test

We present new theoretical and experimental results which demonstrate that the degree of fiber-matrix bonding can be quantified by means of the interface energy for the initiation of debonding, rather than by using a stress-based interfacial parameter. A one-dimensional model for the energy necessary to initiate/nucleate an interfacial crack from its associated transverse fiber break during a single fiber fragmentation test is proposed. The interface energy for the initiation of debonding is shown to be a function of the fiber and matrix geometrical and material characteristics, and of the initial debonding length. The validity of the approach is demonstrated in the case of fragmentation of sized and unsized E-glass fibers embedded in an UV-cured polymeric matrix.