Fiber-matrix Interphase Research Articles

Interface modification and the influence on damage development of flax fibre – Epoxy composites when subjected to hygroscopic cycling

The moisture sensitivity of natural fibre composites limits their use in structural applications. Degradation of the fibre–matrix interface is likely to occur when the material is subjected to variable moisture conditions. This study investigates the damage development in unidirectional flax fibre reinforced epoxy composites during hygroscopic cycling. It is expected that the insertion of a silicone interphase increases interfacial toughness and therefore postpones or avoids interface degradation. Various silicones and a silane coupling agent were used in an attempt to enhance the long-term behavior of the composite. Hygroscopic cycling was performed by varying the relative humidity between approximately 25 and 80% at 80 °C. When examining the flax-epoxy reference composite, regression analysis showed a significant decline of the transverse strength during six consecutive wet-dry cycles. The degradation of the mechanical properties is attributed to fibre–matrix debonding and to cohesive failure in the fibre bundles. Both damage mechanisms are visualized in a unique three-dimensional model. Contrary to the hypothesis, the insertion of a silicone interphase led to an accelerated decrease of the transverse strength. Computed tomography analysis confirmed the weakening of the fibre–matrix interphase. Although the damage development was altered, the composites having a thin silicone interphase did not show a significant decrease in longitudinal stiffness and strength. This result is promising as it suggests that stress transfer is not significantly hindered.

Explorative Study on the Use of Curauá Reinforced Polypropylene Composites for the Automotive Industry.

The automotive industry is under a growing volume of regulations regarding environmental impact and component recycling. Nowadays, glass fiber-based composites are commodities in the automotive industry, but show limitations when recycled. Thus, attention is being devoted to alternative reinforcements like natural fibers. Curauá (Curacao, Ananas erectifolius) is reported in the literature as a promising source of natural fiber prone to be used as composite reinforcement. Nonetheless, one important challenge is to obtain properly dispersed materials, especially when the percentages of reinforcements are higher than 30 wt %. In this work, composite materials with curauá fiber contents ranging from 20 wt % to 50 wt % showed a linear positive evolution of its tensile strength and Young’s modulus against reinforcement content. This is an indication of good reinforcement dispersion and of favorable stress transfer at the fiber-matrix interphase. A car door handle was used as a test case to assess the suitability of curauá-based composites to replace glass fiber-reinforced composites. The mechanical analysis and a preliminary lifecycle analysis are performed to prove such ability.

Effects of Hygrothermal Ageing on the Interphase, Fatigue, and Mechanical Properties of Glass Fibre Reinforced Epoxy

Reliability and cost-effectiveness represent major challenges for the ongoing success of composites used in maritime applications. The development of large, load-bearing, and cyclically loaded structures, like rotor blades for wind or tidal energy turbines, requires consideration of environmental conditions in operation. In fact, the impact of moisture on composites cannot be neglected. As a result of difficult testing conditions, the knowledge concerning the influence of moisture on the fatigue life is limited. In this study, the impact of salt water on the fatigue behaviour of a glass fibre reinforced polymer (GFRP) has been investigated experimentally. To overcome the problem of invalid failure during fatigue testing, an improved specimen geometry has been developed. The results show a significant decrease in fatigue life for saturated GFRP specimens. In contrast, a water absorption of 50% of the maximum content showed no impact. This is especially remarkable because static material properties immediately decrease with the onset of moisture absorption. To identify the water absorption induced damage progress, light and scanning electron microscopy was used. As a result, the formation of debondings and cracks in the fibre–matrix interphase was detected in long-term conditioned specimens, although no mechanical loading was applied.

Long-Term Hydrolytic Degradation of the Sizing-Rich Composite Interphase

Glass fiber-reinforced composites are exposed to hydrolytic degradation in subsea and offshore applications. Fiber-matrix interphase degradation was observed after the matrix was fully saturated with water and typical water absorption tests according to ASTM D5229 were stopped. Due to water-induced dissolution, fiber-matrix interphase flaws were formed, which then lead to increased water uptake. Cutting sample plates from a larger laminate, where the fibers were running parallel to the 1.5 mm long short edge, allowed the hydrolytic degradation process to be studied. The analysis is based on a full mechanistic mass balance approach considering all the composite’s constituents: water uptake and leaching of the matrix, dissolution of the glass fibers, and dissolution of the composite interphase. These processes were modeled using a combination of Fickian diffusion and zero-order kinetics. For the composite laminate studied here with a saturated epoxy matrix, the fiber matrix interphase is predicted to be fully degraded after 22 to 30 years.

Stochastic micromechanical modeling of transverse punch shear damage behavior of unidirectional composites

Punch shear in unidirectional composites is induced by transverse shear loading that progressively perforates the laminate within a narrow shear annulus. At lower micromechanical length scales, punch shear loading creates unique micromechanical damage mechanisms dominated by transverse fiber shear failure, fiber–matrix interphase debonding and large inelastic deformation and cracking of the matrix. A new punch shear experimental method has been developed to test unidirectional S glass/DER353 epoxy composite ribbons at sub-millimeter length scale. The experimental data consist of a statistical measurement of the continuum response (load-deformation and punch shear strength) and the characterization of micromechanical damage modes. A simplified 2D micromechanical finite element model incorporating Weibull fiber strength distribution has been developed and correlated with the experimental data. The 2D micromechanical finite element model can simulate the punch shear failure of the ribbon incorporating mixed mode fiber fracture, and fiber–matrix debonding mechanisms using zero thickness cohesive elements. Results from stochastic simulations of punch shear experiments show that an equivalent 2D micromechanical finite element model can predict the micromechanical damage mechanisms and the statistical distribution of punch shear strength of the continuum with favorable correlation with the experiments. This paper presents a combined experimental and computational approach in simulating the stochastic non-linear progressive punch shear behavior of unidirectional composites for the first time in the literature.

Near–fiber effects of UV irradiation on the fiber–matrix interphase: A combined experimental and numerical investigation

Characterization of the interphase region in carbon fiber reinforced polymer (CFRP) is challenging because of the length scale involved. The interpretation of measured load-displacement curves using indentation is affected by the lack of analytical solutions that account for the fiber constraint effect. A combination of AFM (Atomic Force Microscopy) based indentation and FE (Finite Element) simulations showed a gradient in the elastic modulus of the interphase evaluated along a radial line from the fiber. 3D FEA (Finite Element Analysis) indicated that fiber constraint effect is significant in the region less than 40 nm away from the fiber. Nonetheless, the apparent rise in elastic modulus due to fiber constraint is limited when compared to the gradient in the elastic modulus of the interphase. Additionally, this technique is used to demonstrate that UV irradiation causes a rapid decrease in the modulus of the region near the fiber due to photocatalytic degradation of carbon fiber but subsequently increases due to high cross-linking. Whereas, the modulus of the matrix at 8 mm away from the fiber decreased by 32% after 24 h of UV irradiation. This indicates that the response of epoxy to UV irradiation is influenced by the proximity to the reinforcement.

Effect of seawater immersion on the explosive blast response of a carbon fibre-polymer laminate

Explosions are an ever-present risk to laminates used in naval ships, submarines and offshore oil/gas platforms that are immersed in seawater. This study determines whether the absorption of water by a carbon fibre laminate changes its deformation response and damage resistance when impulsive loaded by an explosive blast. The stiffness and strength properties of the laminate were reduced with increasing immersion time in seawater up to and beyond the point of saturation. Explosive blast tests of increasing shock wave impulse were performed on the laminate before immersion and when in the saturated and beyond saturated conditions. Softening and weakening of the laminate caused by absorbed water reduced the resistance against deformation and damage when subjected to an explosive blast. The amount of blast-induced damage to the laminate increased with the immersion time in seawater due to plasticisation of the polymer matrix and weakening of the fibre–matrix interphase region.

Multi-layered sensor yarns for in situ monitoring of textile reinforced composites

In this contribution, the characteristic of yarns that have intrinsically conductivity as well as such with coaxial conductive coatings acting as in situ strain sensors are described. The objective of the based research projects is the real-time in situ sensing of both global stresses acting on fibre reinforced plastic (FRP) components and the detection of resulted local microscopic damages due to creep, delamination and micro-cracks in the fibre-matrix interphase of glass fibre (GFRP) and carbon fibre (CFRP) composites. Sensor materials similar to the particular FRP and its mechanical behaviour have been chosen. In the first approach, GF- and aramid-based sensor yarns have been developed with multiple tailored silver layer coating system capable to distinguish multiple scaled damage mechanism due to these effects globally and locally. The second approach bases on the piezoresistive effect of CF rovings for their usage as in situ strain sensors. In the next step, suitable fibre and polymer film-based cleading have been tested and evaluated, granting sufficient electrical isolation to avoid shortcircuits between the conductive sensor layers itself or between the sensor and intrinsically conductive CFRP respectively. Initially, the sensor performance of global strain measurement, means the accumulated strain along the integration length of the sensor yarn, has been evaluated during tensile stressing of FRP with integrated suchlike functionalised sensor yarns.

Short-fibre hybrid polypropylene composites reinforced with PET and Rayon fibres – Effects of SSP and interphase tailoring

This research focused on manufacturing of light-weight hybrid polypropylene composites reinforced with short fibres from polyterephthalate and rayon-viscose yarns. The aim of hybridisation of reinforcements was to improve some aspects of mechanical performance of these composites and to reduce their cost. For the first reason also three methods of fibre-matrix interphase tailoring and solid state polycondensation of PET fibres were applied and investigated in terms of influence on their strength, moduli and impact behaviour of composites. Lower density of PET and rayon-viscose fibres in comparison to glass fibres had to a large degree compensated the specific properties of manufactured hybrids. The same level of drop-weight impact energy absorption was achieved for hybrids as for their PP-GF counterpart, while merely 25% lower specific flexural strength was obtained. The HDT was only ∼15 degrees lower. Therefore the developed composites may substitute a standard PP-GF in some particular lightweight applications. The results give also insight into properties and mechanical behaviour of applied fibres and how they can be altered during processing.

Silica–silane coupling agent interphase properties using molecular dynamics simulations

In this paper, strength of the interphase between silica and glycidoxypropyltrimethoxy silane (GPS) coupling agent has been studied using molecular dynamics (MD) simulations. Silica–GPS interphase model is created by coupling the hydroxylated silica surface with monolayer-hydroxylated GPS molecules. The interphase model is subjected to mode-I (normal), mode-II (shear) and mixed-mode (normal–shear) mechanical loading to determine the interphase cohesive traction–separation (T–S) response (i.e., cohesive traction law). In MD simulations, atomic interactions are modeled with the reactive force field ReaxFF. Effects of interphase thickness and GPS bond density on the T–S response are studied. Simulation results indicate that interphase strength decreases with increase in the interphase thickness before attaining a plateau level at higher thickness. For a particular thickness, strength improves significantly with increase in the GPS bond density with the silica surface. Damage mode is adhesive at the silica interface at lower thickness and transitions to mixed mode and cohesive failure within the silane interphase at higher thickness. Mixed-mode T–S responses are bounded by the mode-I and mode-II responses. Characteristic parameters of the continuum-level potential-based cohesive zone model (PPR–CZM) are determined by fitting the MD-based mode-I and mode-II T–S responses with PPR–CZM functional. Development of the PPR–CZM parameters enables bridging length scales from the MD to the continuum scale for fracture modeling of the fiber–matrix interphase in composites subjected to mixed-mode loading. Results on mode-I and mode-II unloading are also presented.

A numerical micro-mechanical study of the influence of fiber–matrix interphase failure on carbon/epoxy material properties

A finite element micromechanical study of unidirectional carbon–epoxy system is performed in order to investigate the role of fiber–matrix debonding in the degradation of mechanical properties and in the onset of failure for this class of composite materials.The presence of interphase flaws, that can be induced during the manufacturing processes, into micro-scale FE models is obtained by means of an original damage injection technique developed by the authors. The fibers are considered as transversally isotropic solids and the matrix is modeled as an isotropic, elasto-plastic, material with damage.The effect of fiber–matrix debonding is analyzed by means of a quasi 3-D unitary cell with a single fiber, with periodic boundary conditions, for different loading cases. Subsequently, multi-fiber representative volume elements are investigated with the same boundary and loading conditions. Finally, the effect of a 3-D debonding propagation is studied via single fiber model with an increased fiber-wise depth.

Analytical investigation of thermomechanical stresses in graded interphase between the fiber and the matrix

ABSTRACTIn this article, the thermomechanical stresses in the graded interphase between the fiber and the matrix in fiber-reinforced composites are investigated. First, the general theoretical analysis of two-dimensional steady-state mechanical and thermal stresses in a graded cylinder along the radial direction with the Power Law and Exponential Law distribution of the material properties is developed. Then the theoretical solution is applied to investigate the mechanical stresses and the thermal stresses of the fiber-matrix interphase, and the effects of material graded constant on the mechanical stresses and thermal stresses are discussed. Finally, the stress transfer mechanisms in the graded interphase are investigated which indicates that the total stresses in the fiber-matrix interphase increased with the increasing of the interphase thickness if the material graded constant varied with the radius in Power Law, but decreased in Exponential Law.

The effect of accelerated aging on the interface of jute textile reinforced concrete

In this work the degradation mechanisms of concrete reinforced with natural jute textile are discussed. Double-sided pullout tests were performed on specimens reinforced with polymer-coated and uncoated jute fabrics. To obtain a composite with an enhanced durability performance a matrix with partial replacement of the Portland cement by metakaolin was used. Before testing, the specimens were subjected to accelerated aging conditions. After a curing period of 28 days in water the samples were exposed to a temperature of 40 °C and a relative humidity of 99% over 28, 56, 90, 180 and 365 days. Microstructural analyses were performed to evaluate the degradation of the jute yarn and the fiber-matrix interphase using an environmental scanning electron microscope. Thermogravimetric analysis was carried out in order to evaluate the calcium hydroxide content. The pullout results showed that coated fabrics formed a stronger bond than did the uncoated. For ordinary Portland cement matrix the maximum fiber pullout force decreased up to 85% after six months of accelerated aging. In the MK matrix the degradation process was retarded substantially. Polymer coatings improved the bond between fiber and matrix and reduced fiber degradation.

Fabrication and characterization of in-situ grown carbon nanotubes reinforced SiC/SiC composite

Nickel nanoparticles, to be used as catalysts for carbon nanotube (CNT) growth, were deposited on SiC fabric using four different methods ­ vacuum impregnation, homogeneous precipitation, electrodeposition and autocatalytic plating. Results showed that suitable catalyst particles were produced by the vacuum impregnation process. The growth of CNTs was performed via a thermal chemical vapor deposition (CVD) process. A SiC/SiC composite was then fabricated by polymer impregnation and pyrolysis (PIP), by using the fiber fabric with in­situ grown CNTs. The CNTs did not exert a marked influence on the density, porosity and thermal conductivity of the SiC/SiC composite. However, its mechanical properties were improved significantly, due to the weakening of the fiber-matrix interphase and the CNT reinforcement.

Fiber-matrix interphase in applied short glass fiber composites determined by a nano-scratch method

Abstract The fiber-matrix interphase in composites is defined as the intersection region between fibers and the matrix material. It shows altered matrix material properties. In dependency of the matrix material or the fiber coating, this phase is created by interdiffusion processes at the macromolecular scale driven by thermodynamic forces. Especially for FE simulations of composites and the validation of multi-scale material modeling approaches, the information about the existing interphase becomes important. Thus, the present study analyses the interphase in applied short glass fiber reinforced thermoplastics. For the identification of the interphase thickness, the nano-scratch method provides admissible results. This methodology is improved and adapted to the use in short fiber composite specimens. Thereby, the measured range of interphase thickness represents the inhomogeneity of the interphase. To assure that the measured interphase is not mainly constituted by the sizing of the glass fiber, incineration tests are performed additionally. The comparison of the interphase and the sizing thickness shows a significant thicker interphase.

Processing and Testing of RE2Si2O7 Fiber–Matrix Interphases for SiC–SiC Composites

Rare‐earth disilicates (RE2Si2O7) are investigated for use as oxidation‐resistant alternatives to carbon or BN fiber–matrix interphases in ceramic matrix composites (CMC). Dense α, β, γ‐Y2Si2O7, and γ‐Ho2Si2O7 pellets were formed at 64 MPa and 1050°C–1200°C for 1 h using the field‐assisted sintering technique (FAST). Pellet modulus was measured using nanoindentation, and Vickers hardness was measured at loads of 100, 500, and 1000 g. The sliding stress of SCS‐0 SiC fibers incorporated in α‐, β‐, and γ‐RE2Si2O7 matrices were measured by fiber push‐out. Deformation of RE2Si2O7 after indentation and after fiber push‐out was characterized by TEM. Implications of the results for use of RE2Si2O7 as a fiber–matrix interphase in CMCs are discussed.

Optimization of interfacial microstructure and mechanical properties of carbon fiber/epoxy composites via carbon nanotube sizing

Repetitious sizing treatment was used to modify the carbon fiber (CF) surface with carbon nanotubes (CNTs) for improving interfacial properties of CF/epoxy composites. Interlaminar shear and flexural results showed that mechanical properties of composites were significantly depended on the dispersion state and contents of CNTs in interfacial regions. Increases of 13.45% in interlaminar shear strength and 20.31% in flexural strength were achieved in quintuple sized-CF/epoxy composites, whereas excessive CNTs led to decrease of interfacial performance due to defects induced by agglomerated CNTs. Energy dispersive X-ray spectroscopy and force modulation atomic force microscope were used to detect the structure of interfacial phase and results indicated that gradient interfacial structure with various thicknesses was formed due to CNT incorporation. This means that such a simple and efficient method to improve interfacial performance of composites via regulating the fiber–matrix interphase structure was developed and showed great commercial application potential.

Effect of fiber surface texture on the mechanical properties of glass fiber reinforced epoxy composite

The effect of fiber sizing and surface texture on the strength and energy absorbing capacity of fiber reinforced composites has been evaluated at two length scales using the macromechanical quasi-static punch shear test and the micromechanical microdroplet test methods. E-Glass/SC-79 epoxy composite laminates with four different fiber sizing formulations with various degrees of chemical bonding and surface texture have been investigated. The failure modes during perforation and different energy dissipating damage mechanisms were identified and quantified. The punch shear strength and the total energy absorption per unit volume of composite with hybrid sizing have increased by 48% and 100% over the incompatible sizing. These results showed linear correlations with the interphase properties reported earlier by the authors (Gao et al., 2011) and provided a methodology for developing new sizing by tailoring chemical bonding and the fiber surface texture at the fiber–matrix interphase for improving both strength and energy absorption of composites.

Mechanical behaviour and practical adhesion at a bamboo composite interface: Physical adhesion and mechanical interlocking

Physical adhesion was experimentally determined by measuring contact angles with different liquids on bamboo and glass fibres, using the Wilhelmy technique, and by applying the acid–base theory for calculating the surface energy components and the theoretical work of adhesion. The mechanical strength of the interfaces was assessed by single fibre pull-out tests. In order to consider the real mechanisms of interfacial failure of natural fibre composites, the fibre matrix interfacial bond strength was characterised by the critical local value of interfacial shear stress, τd, and the radial normal stress at the interface, σult, at the moment of crack initiation. Both interfacial parameters are used for correlating thermodynamic work of adhesion and practical adhesion. Pull-out tests (taking into account friction), XPS, and profilometry techniques were used to study the influence of rough natural fibre surfaces on the interface between the fibre and a thermoplastic matrix, by comparing the mechanical behaviour at the interface of a smooth optical glass fibre with that of rough natural fibres. The results suggest that the physical and chemical compatibility between the bamboo fibre and the matrix does not improve substantially the composite performance if compared with glass composites. The relatively low off-axis strength of the bamboo fibres is suggested as the main reason for the low stress transfer capability at the fibre–matrix interphase. Furthermore, the pull-out process may be friction-dominated in bamboo fibre systems.

Fiber bias effect on characterization of carbon fiber-reinforced polymer composites by nanoindentation testing and modeling

Fiber bias effect on characterization of carbon fiber-reinforced polymer composites by nanoindentation testing has been investigated by computer modeling and validated by experiments. IM7/PEEK composite was selected as a case study material. Nanoindentation tests using Berkovich indenter were carried out on fiber longitudinal direction and in the near fiber region of the matrix to attempt to determine the nanomechanical properties of the fibers and the interphase. A 3D finite element analysis model for simulating nanoindentation was developed, taking into account the carbon fibers’ transversely isotropic properties. A fiber bias effect inducing a gradient in modulus in fiber–matrix interphase region was observed from tests and confirmed by modeling, and this property transition thickness induced by the fiber bias is about 1–2 µm depending on the indentation depth, which suggests an inherent difficulty when attempting to determine the true interphase properties experimentally using the nanoindentation testing.