Stiffness and strength of flax fiber/polymer matrix composites

Flax fiber composites with thermoset and thermoplastic polymer matrices have been manufactured and tested for stiffness and strength under uniaxial tension. Flax/polypropylene and flax/maleic anhydride grafted polypropylene composites are produced from compound obtained by coextrusion of granulated polypropylene and flax fibers, while flax fiber mat/vinylester and modified acrylic resin composites are manufactured by resin transfer molding. The applicability of rule‐of‐mixtures and orientational averaging based models, developed for short fiber composites, to flax reinforced polymers is considered. POLYM. COMPOS. 27:221–229, 2006. © 2006 Society of Plastics Engineers     

Interfacial shear strength of reduced graphene oxide polymer composites

Interfacial shear strength (IFSS) between particle and matrix in particulate polymer composites is a critical property in determining the mechanical behaviors since it is directly related to not only their Young’s modulus or specific strength, but also energy absorbing capability. However, the conventional techniques often present a technical challenge to accurately measure the IFSS between fillers and matrix in the composites. This is more apparent in graphene particulate composites due to their nano-scale dimensions as well as the platelet-shaped geometry. Here, the focus of this study is to use a semi-empirical approach to determine the IFSS of graphene particulate composites by combining experiments with finite element (FE) modeling. The materials of interest are reduced graphene oxide (RGO) and polycarbonate (PC). The tensile testing was performed to characterize the mechanical properties, while simultaneously monitoring the acoustic emission events in order to measure the global debonding stress (GDS) in the composites. By coupling thermal stress analysis and deformation analysis with the GDS as input to a FE model, the IFSS of the RGO particulate PC composites was successfully estimated by about 136 MPa, avoiding unnecessary assumptions and uncertainties which are seem to be inevitable with the conventional techniques for the IFSS measurement.     

Characterization of fibre/resin bonding in composites using a pull-out test

From the early use of glass fibres in an organic matrix to that of high strength and high modulus fibres such as carbon and Kevlar, the subject of fibre/matrix bonding has received considerable attention. This paper gives details of a pull-out test has been used to measure the adhesion between fibres and matrix. A summary of results obtained with the test technique, over a period of some ten years, is also given. Despite operational difficulties the method gave interesting results regarding the mechanical aspects of the interaction between fibres and matrix.     

Interfacial characterisation of Al2O3-Ni composites

In the present study, the interfacial structure of nickel particle-reinforced alumina composites prepared by a gas reduction process and by reactive sintering is examined by analytical electron microscopy. Amorphous carbon films at the interface and graphite inclusions within the nickel inclusions can be found in products from gas reduction and from reaction sintering, respectively. The microstructural variation results in different amounts of toughness enhancement. Possible explanations for the resulting microstructure and toughness differences are proposed.     

Interfacial shear strength of flax fibers in thermoset resins evaluated via tensile tests of UD composites

A method of interfacial shear strength evaluation, based on the length distribution of fibers pulled out from the tensile fracture surface of an oriented flax-reinforced composite, is applied to composites with vinyl ester and acrylated epoxidized soy oil resin matrices. Two approaches for characterizing the strength of fibers with modified Weibull distribution, fiber fragmentation tests and fiber tension tests, are compared in the analysis of pull-out data. Interfacial shear strength is found to increase by a few percent when loading rate is increased from 1.33% to 8%/min.     

Nanoscale characterisation of thickness and properties of interphase in polymer matrix composites

An overview is presented of the properties and effective thickness of the interphase formed between fibres and polymer matrices. Chemical and physical characterization of the interphase is discussed to portray molecular interactions comprising the interphase layers in silane-treated glass-fibre composites. The gap between physico-chemical investigation on one side and bulk material testing on the other side is bridged by implementation of novel techniques, such as nanoindentation, nanoscratch tests, and atomic force microscopy (AFM), which have been successfully used for nanoscopic characterization of the interphase in the past few years. Salient differences are identified between the major findings of these studies in terms of hardness/modulus of the interphase relative to the bulk matrix material. While there is a significant "fibre stiffening" effect that may cause misinterpretation of the interphase hardness very close to the fibre, the formation of both a softer and a harder interphase is possible, depending on the combination of reinforcement, matrix, and coupling agent applied. This is explained by different interdiffusion behaviour, chemical reactions, and molecular conformation taking place at the interphase region in different composite systems. The effective interphase thickness is found to vary from as small as a few hundred nanometers to as large as 10 µm, depending on the constituents, coupling agent, and ageing conditions.     

A comparative investigation of interfacial adhesion behaviour of polyamide based self-reinforced polymer composites by single fibre and multiple fibre pull-out tests

Abstract

Interfacial adhesion of composite materials plays an important role in their mechanical properties and performance. In the present investigation, analysis of the interfacial properties of self-reinforced polyamide composites by using microbond multiple fibre pull-out test is emphasised. Microbond specimens prepared through thermal processing are tested for their interfacial properties by multiple fibre bundle pull-out tests and compared with that of traditional single fibre pull-out test specimens. Multiple fibre pull-out addresses the volume fraction as well as eliminates the possibility of fibre breakage before matrix shear. Higher scatter in the data in the samples is addressed in the present studies. FTIR and Fractographic studies are carried out for deep understanding of the post pull-out interfacial adhesion.     

Interfacial rheology of uniform polymer dispersions

The effects of high oscillatory shear on the viscosity of a model polymer dispersion of solid particles was investigated. A dispersion of glass beads in poly(dimethyl siloxane) (PDMS) is uniform and free of any agglomeration. When sheared at low stress levels, the dispersion remains uniform. At high stresses under oscillatory shear, two processes can take place. One is spatial ordering (threading) of the spherical beads that does not occur in steady shear. The other is molecular interfacial slip due to high interfacial stress in samples containing fluoro-silane treated glass beads, leading to a lower suspension viscosity than that of samples containing bare non-treated beads.     

Preparation and characterisation of a carbon adsorbent from flax shive by dehydration with sulfuric acid

A carbon adsorbent has been prepared from flax shive by treatment with sulfuric acid. Several factors have been considered in the preparation: reaction time, temperature, and the amount and concentration of sulfuric acid. The optimised conditions of preparation were based on the cation exchange capacity, yield and sorption performance for the metals cadmium and mercury(II). These two metals behave differently when adsorbed on the carbon. Thus cadmium sorption follows the cation exchange capacity data and the optimum conditions for preparing the carbon for this and other metals sorbed by an ion exchange mechanism were: 12 moldm⁻³ sulfuric acid at a ratio of 1 : 9 (weight flax shive : volume acid), 25–30 min reaction time at 200 °C. Similar preparative conditions were found to give the optimum results for mercury sorption except that in this case, the temperature was 160 °C. The sorbent shows a much higher uptake for mercury than cadmium, which suggests that the sorption mechanism for mercury sorption is not only ion exchange. The sorbents retained a high level of moisture (>80%), and elemental analysis shows very low amounts of sulfur (∼1%). Scanning electron microscopy shows that the carbon retains the fibrous texture of the original shive and X‐ray diffraction shows that the carbons are amorphous. The presence of carboxyl, carbonyl and hydroxyl (or phenolic) groups is confirmed by infra‐red spectroscopy. © 1999 Society of Chemical Industry     

基础内预埋锚杆抗拔试验研究

通过对建筑基础筏板内预埋锚杆进行抗拔试验检测其抗拔承载性能,对锚杆周围混凝土受抗拔力破坏形式进行研究。研究表明：在埋入锚杆最底端加拧1个螺母,起扩大头作用,增大锚杆侧阻力,较好地固定锚杆,抗拔试验破坏时,断裂部位发生在千斤顶施加荷载作用点附近,而并非将锚杆整体拔出;加载至锚杆材料屈服破坏点对应荷载739 kN附近,荷载-位移曲线呈线性变化,加载量大于该值后,锚杆塑性变形逐渐发展,抗拔过程主要是锚杆杆体自身形变的过程,前半阶段主要为弹性变形,后半阶段主要为塑性变形,直至锚杆被拉断;从筏板内锚杆取芯样来看,抗拔对锚杆周边混凝土产生一定拉裂,裂缝不大,主要在上部300 mm处。     

Characterization of polymer composites by electron microprobe analysis

This paper describes the use of an electron microprobe for characterizing filler and polymer dispersios in polymer composites. Examples described are: filler distribution in sisal reinforced polyester; aggregation of titanium dioxide in glassreinforced polyester; mixing behavior in a blend of silicone rubber, natural rubber, and titanium dioxide; and distribution of silica in a low-gloss paint on an ABS plastic surface. The limitations of electron microprobe analyses for qualitative and quantitative studies, the use of back scattered electrons for surface topography, and techniques for preparing specimens are mentioned.     

Interfacial properties of two SiC fiber–reinforced polycarbonate composites using the fragmentation test and acoustic emission

Interfacial shear strengths (IFSS) between the fiber and the matrix in two SiC fiber–reinforced polycarbonate (PC) composites (TFC) were investigated through the fragmentation method and the acoustic emission (AE) technique. Statistical analysis of SiC fiber tensile strength was performed mainly in terms of a Weibull distribution. The tensile strength and elongation for SiC fiber decreased with increasing gauge lengths, because of the heterogeneous distribution of flaws on the fiber surface. Using an amino-silane coupling agent, the IFSS showed significant improvement, in the range of 150%, under dry conditions. On the other hand, in the aspect of the environmental effect, the IFSS was improved about 170% under wet conditions (immersed in hot water at 85°C for 75 min). This is probably due to chemical and hydrogen bonds in the two different interphases in the SiC fiber/silane coupling agent/PC matrix system. In-situ monitoring of AE during straining of microspecimens showed the sequential occurrence of two distinct groups of AE data. The first group may result from SiC fiber breakages, and the second probably results from mainly PC matrix cracking. Characteristic frequencies coming from the failures of the fiber and the PC matrix were shown via fast Fourier transform (FFT) analysis. By setting an appropriate threshold level, a one-to-one correspondence between the number of AE events and fiber breakages was established. This AE method could be correlated successfully to the IFSS via the fragmentation technique, which can also applied to nontransparent specimens.     

Interfacial properties of PP/PP composites

All‐polypropylene (PP/PP) composites, consisting of homoisotactic PP fibers and a propylene–ethylene random copolymer matrix, were manufactured. The interfacial morphologies and mechanical deformation of the composites were investigated. It was found that a transcrystalline layer was formed in the matrix depending on the molding conditions. This layer is composed of two regions with different morphologies: In one region close (<300 nm) to the fibers, lamellae exist very densely and the c‐axes of the crystal lattices are oriented along the longitudinal direction of the fiber. In the other region, a little away (>300 nm) from the fiber, a cross‐hatched structure was observed. The well‐developed transcrystalline layer could be highly elongated, and no debonding was observed at the fiber/matrix interface by the tensile stress in the direction perpendicular to the fiber. It is conceivable that this is the reason for the observed improvement of the transverse tensile properties in the PP/PP composite with a transcrystalline layer. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 2875–2883, 2003     

Interfacial grafting and crosslinking by free radical reactions in polymer blends

Relevant developments in polymer blends based on immiscible products have shown how efficient the amphiphilic species are in improving mechanical properties and morphological stabilization throughout successive processing steps. However, it appears that the amount of compatilizer that has to be introduced for reaching the expected level of global properties is largely higher than the calculated concentration based on the interfacial area. In order to obtain the required concentration of amphiphilic copolymer at the interface, it seems of interest to test the possibility of creating these species by in situ reactivity at the interface. As most of the polymers are hydrocarbon compounds, they are subjected to free radical reactivity by hydrogen abstraction on the different hydrocarbon sites, which may lead to crosslinking or grafting. Therefore, this work deals with reciprocal free radical reactivity of two immiscible semicrystalline homopolymers, namely, low density polyethylene (LDPE) and polyamide-11 (PA-11). The reaction has to occur mainly at the interface, where the resulting grafted copolymer has to be anchored for final stabilization of the biphasic system. Different analytical techniques help in characterizing the reacted blend and determining the level of interfacial grafting.     

Physical and mechanical properties of kenaf/flax hybrid composites

This research investigates the physical and mechanical properties of hybrid composites made of epoxy reinforced by kenaf and flax natural fibers to investigate the hybridization influences of the composites. Pure and hybrid composites were fabricated using bi-directional kenaf and flax fabrics at different stacking sequences utilizing the vacuum-assisted resin infusion method. The pure and hybrid composites' physical properties, such as density, fiber volume fraction (FVF), water absorption capacity, and dimensional stability, were measured. The tests of tensile, flexural, interlaminar shear and fracture toughness (Mode II) were examined to determine the mechanical properties. The results revealed that density remained unchanged for the hybrid compared to pure kenaf/epoxy composites. The tensile, flexural, and interlaminar shear performance of flax/epoxy composite is improved by an increment of kenaf FVF in hybrid composites. The stacking sequence significantly affected the mechanical properties of hybrid composites. The highest tensile strength (59.8 MPa) was obtained for FK2 (alternative sequence of flax and kenaf fibers). However, FK3 (flax fiber located on the outer surfaces) had the highest interlaminar shear strength (12.5 MPa) and fracture toughness (3302.3 J/m²) among all tested hybrid composites. The highest water resistance was achieved for FK5 with the lowest thickness swelling.     

Investigations of the flax fiber/thermoplastic polyurethane eco‐composites: Influence of isocyanate modification of flax fiber surface

In this study, alkali and isocyanate surface modifications were applied to flax fiber (FF) to improve its adhesion to bio‐based thermoplastic polyurethane (TPU) matrix. In addition to these treatments, isocyanate treated FF was subjected to curing process. TPU/FF composites were prepared at a constant 30 wt% loading of the total by using melt‐blending method. Their mechanical properties, modulus of elasticity, melt‐flow, water uptake and morphological properties were investigated. All of the surface modifications resulted in better mechanical properties with respect to untreated FF. Cured isocyanate treated FF loaded composite exhibited the best results in the case of tensile strength, Young's modulus and storage modulus. Isocyanate treatments caused reduction in melt flow rate due to enhancement in interfacial interactions between phases. It was observed from the SEM micrographs that surface treated fibers dispersed more homogeneously in the TPU matrix. Results confirmed that surface modifications improved the adhesion of FF to TPU matrix. POLYM. COMPOS., 38:2874–2880, 2017. © 2015 Society of Plastics Engineers     

Interfacial shear strength of C/C composites

Fiber-bundle push-out, single-fiber push-in, and single-fiber push-out tests were conducted in order to examine the applicability of these methods for determining the interfacial shear strength of carbon-carbon composites. The fiber-bundle push-out test resulted mostly in fractures along the fiber/matrix interface but created a small amount of fractures in the matrix. Hence, the evaluated strength was regarded as an approximate value. In order to precisely evaluate the interfacial strength, push-in and push-out tests for a single fiber were performed using a micro-Vickers indentation tester. In these tests, the load has to be placed within a target fiber, and the indentation should not extend to the matrix. This condition restricted the load that could be applied to a carbon fiber. Within this limit, a single carbon fiber could not be pushed-in. For the sake of load reduction, single-fiber push-out tests were conducted using thin specimens. The thickness appropriate for a single-fiber push-out specimen was estimated based on the interfacial shear strength obtained by the bundle push-out tests. Below this thickness, single-fiber push-out tests could be successfully performed.     

膨胀石墨填充SBS复合材料的界面相互作用

膨胀石墨（ＥＧ）填充量小于２５％时,ＥＧ／ＳＢＳ复合材料的拉伸强度随ＥＧ充量的增加而增加,最大拉伸强度达２７．８ＭＰａ;而在ＥＧ填充量为１０％时,复合材料的扯断伸长率达到最大值（７８５％）,随后开始下降。用二甲苯作溶剂,在１４５℃抽提ＥＧ／ＳＢＳ复合材料２４ｈ后,复合材料中形成了稳定的交联结构,同时,凝胶含量随ＥＧ填充量的增加而增加。由此说明复合物中ＥＧ与ＳＢＳ之间产生了强烈的界面相互作用。ＤＭＡ