Single-fibre Model Composites Research Articles

Investigation of Transcrystalline Interphases in Polypropylene/Glass Fiber Composites Using Micromechanical Tests

In composites, a strong interphase between the components is essential for mechanical properties. By using a suitable sizing (i.e., surface modification) of the fiber, the interphase may be varied, e.g., by suppressing or promoting heterogeneous nucleation of a thermoplastic matrix. In the latter case, three-dimensional transcrystallized interphases with properties differing from those of the bulk matrix are formed. Polypropylene-glass fiber composites are prepared as single-fiber model composites with (a) sizings either inducing or suppressing a transcrystalline interphase, (b) different amounts of modifier maleic acid anhydride grafted polypropylene, and (c) different molecular weights of the matrix polymer. These are studied in quasi-static or cyclic load tests. Static tests permit insights in the interfacial characteristics such as critical interface energy release rate, adhesion strength and frictional stress. Cyclic tests on these model composites can be used to study the nature of dissipative processes and the damage behavior. Atomic Force Microscopy (AFM) investigations of the fiber fracture surfaces provide supplementary information. The transcrystalline layer can indeed improve the mechanical parameters (a 70–100% increase of strength and a 25 or 125% increase in toughness, depending on the molecular weight (MW) of the matrix polymer at low modifier concentration). However, the effect is partially neutralized by an opposing effect: high nucleation in the bulk in samples with commonly used concentrations of modifier.

Alkali resistant glass fiber reinforced concrete: Pull-out investigation of interphase behavior under quasi-static and high rate loading

Single fiber model composites of alkali resistant (AR-) glass fibers and a cementitious matrix were used to investigate the pull-out behavior under quasi-static and high speed loading. For fundamental understanding of the effect of the fiber/matrix interphase on the pull-out behavior under impact, differently sized AR-glass fibers were spun. As a first approach, the fiber surface was modified in oppositional ways using the following sizings: one based on a polypropylene (PP, weak) film former and another one based on a styrene-butadiene (strong) film former. Additionally, some of the fibers were kept unsized for comparison. A new ‘alternative’ approach was employed to determine the local interfacial shear strength, τd, and the critical energy release rate, Gic, from the reliable force values of the force–displacement curves. For all fiber surface states, the τd and Gic values for high loading rates appeared to be considerably greater than the corresponding parameters for a quasi-static pull-out test. This can be explained using a model based on Zhurkov's kinetic (thermal fluctuation) theory of the strength of solids, which also enabled to estimate the apparent activation energy for interfacial debonding. Both quasi-static and high-rate pull-out tests on this fiber/matrix pair can be considered as ‘normal’ (slip-dependent interfacial friction was not observed) and their results can be evaluated using the described approaches. The interfacial frictional stress reduced at high-rate pull-out tests for all systems. One of the possible mechanisms responsible for this behavior may be the smoothing of surface asperities upon debonding. As revealed by AFM of fracture surfaces, in the case of unsized fibers or those with the ‘strong’ styrene–butadiene sizing, the interfacial crack occurs through surface layers of the matrix material adjacent to the fiber, but it may propagate through the weak interface when the fiber is sized with ‘weak’ PP film former.

Expanded graphite nanoplatelets as coupling agents in glass fiber reinforced polypropylene composites

The interfacial adhesion between E-glass fibers and various types of nanomodified polypropylene (PP) matrices have been investigated on single-fiber model composites. In particular, an evaluation of the fiber–matrix interfacial shear strength was performed by the fragmentation tests on model composites prepared by using PP matrices containing various amounts (up to 7wt%) of expanded graphite nanoplatelets (xGnP).The presence of xGnP in the polymer matrix resulted in a remarkable increase of the interfacial shear strength values (up to a factor of about 6 for a 7wt% content of xGnP) if compared to neat PP. Moreover, wettability measurements in various liquids evidenced that the work of adhesion of the polymer matrix with respect to glass fiber, was improved by the presence of xGnP.

Investigation of the long term effects of moisture on carbon fibre and epoxy matrix composites

The aim of this work is to investigate the long term effects of moisture on the interface between a carbon fibre and an epoxy matrix. High modulus carbon fibres were used to prepare single fibre model composites based on an epoxy resin. The samples were immersed in the seawater and demineralised water and their moisture uptake behaviour was monitored. The equilibrium moisture content and diffusion coefficients for the samples were determined. DSC has been used to analyse the moisture effects on glass transition temperature and thermal stability of the pure epoxy specimens. These results showed a reduction in the glass transition temperature (Tg) after moisture absorption. Tensile tests were also carried out for the epoxy specimens and a general decrease in the mechanical properties of the epoxy matrix was observed. Raman spectroscopy was used to observe the effects of moisture on the axial strain of the carbon fibre within the composite and stress transfer at the interface as a function of exposure time. The results show that the decrease in the mechanical and interfacial properties of the model composites under the seawater immersion is more significant than under demineralised water immersion.

Synergy of matrix and fibre modification on adhesion between carbon fibres and poly(vinylidene fluoride)

Interfacial properties between carbon fibres and poly(vinylidene fluoride) (PVDF) were tuned by modifying both constituents. Atmospheric plasma fluorination (APF) was utilised to tailor the surface composition of carbon fibres, which resulted in an incorporation of up to 3.7 at.% of fluorine functionalities in to the fibre surfaces. The PVDF matrix was modified by blending pure PVDF with maleic anhydride (MAH) grafted PVDF. Both fibre and matrix modifications act in synergy with improvements of up to 50% in the apparent interfacial shear strength (IFSS) above the level of pure fibre or matrix modification. Modification of both constituents led to the formation of various interactions at the fibre/matrix interface namely dispersive and polar (H-bonds) between (modified) PVDF and the fluorine as well as oxygen functionalities on the fibre surfaces. The apparent IFSS between the modified fibres and matrix reaches a maximum of 42 MPa, which is almost the tensile strength of the pure PVDF. The improvements in apparent IFSS in single fibre model composites for both fibre and matrix modifications translated to a seven times improvement in the interlaminar shear strength of unidirectional composites.

Characterization of interphase properties: Microfatigue of single fibre model composites

Tests on single fibre model composites are particularly sensitive to the properties of the interphase. A self made equipment for measuring hysteresis curves (force as a function of displacement) is introduced and subjected to a feasibility test. End-embedded single fibre model composites were exposed to cyclic tension and compression loading. Long-term, relaxation and progressive load tests are performed on sets of single fibre model composites. Glass and PBO fibre reinforced epoxy, polyamide, polypropylene, and cement matrices were investigated in the virgin state or after water treatment. The data are presented and discussed in terms of force amplitudes, stiffnesses or energies as a function of time or cycle number.

Functional interphases with multi-walled carbon nanotubes in glass fibre/epoxy composites

The interphase between reinforcing fibre and matrix is a controlling element in composite performance. We deposited multi-walled carbon nanotubes (MWCNTs) onto electrically insulating glass fibre surfaces leading to the formation of semiconductive MWCNT–glass fibres and in turn multifunctional fibre/polymer interphases. The deposition process of MWCNTs onto glass fibre surfaces involved both electrophoretic deposition (EPD) and conventional dip coating methods. The EPD coating method produces a more homogeneous and continuous nanotube distribution on the glass fibre surface compared with the dip coating. According to fragmentation test results, the interphase with a small number of heterogeneous MWCNTs in the EPD fibre/epoxy composites, mimicking a biological bone structure, can remarkably improve the interfacial shear strength. We found that the semiconductive interphase results in a high sensitivity of the electrical resistance to the tensile strain of single glass fibre model composites. This material provides a possible in situ mechanical load sensor and early warning of fibre composite damage.

Characterization of Interphase Properties by Frequency-Dependent Cyclic Loading of Single Fibre Model Composites

A device for cyclic loading of single fibre model composites with an end-embedded fibre in the regime of 5–350 Hz is introduced and investigated in this feasibility study. Model composites with different interphases are exposed to subcritical cyclic load in the fibre axial direction. The force amplitude and phase lag between displacement and resulting force are determined as a function of frequency and displacement. The device is optimized for high stiffness and play-free movement. A set of different single fibre model composites is chosen, consisting of epoxy/glass fibre, epoxy/polyamide fibre and four polypropylene/glass fibre composites varying in the molecular weight of the matrix and in the local crystalline structure of the interphase. The differences in the composite interphase properties are reflected in the resulting force amplitude and the phase lag between the displacement and the force signal. It is shown that the force amplitude and stiffness are very sensitive to interphase property differences.

Dependence of Fiber Stresses on Cyclic Changes of Temperature and Water Absorption in Carbon/Epoxy Single-Fiber Model Composites

Dependence of Fiber Stresses on Cyclic Changes of Temperature and Water Absorption in Carbon/Epoxy Single-Fiber Model Composites

Residual Stresses Depending on Changes of Temperature and Water Absorption in Carbon Fiber/Epoxy Single-Fiber Model Composites

The axial stresses of the fiber in single carbon fiber/epoxy resin model composites under change of surroundings, e.g., temperature and water, have been measured by micro-Raman spectroscopy. The specimens were soaked in hot air, water and hot water, and returned to the beginning state to cause cyclic changes of temperature and water absorption : (1) heating and cooling at atmospheric environment, (2) immersing in the water of the room temperature and drying, and (3) immersing in the hot water of 80°C and drying at the room temperature. In the case of (1) and (2), the fiber axial stresses after the cyclic change of the surroundings recover almost the same before the heating or the immersion. On the other hand, the fiber axial stress after the drying is more compressive than that before the hygrothermal exposure in the case of (3). Hygrothermal effects on the residual stress are discussed based on fiber stresses caused at elevated temperature over the glass transition temperature of the epoxy resin.

Interfacial stress transfer in an aramid reinforced thermoplastic elastomer

The interfacial micromechanics of Twaron 2200 aramid fibers in an engineering thermoplastic elastomer (Pebax 7033, polyether amide block co-polymer) has been investigated by determining the distribution of interfacial shear stress along fibers in single-fiber model composites using Raman spectroscopy. The effects of various fiber surface treatments on the interfacial shear stress and fragmentation of the aramid fibers are discussed. The fiber average stress increased linearly with applied matrix stress up to first fracture. Each composite was subjected to incremental tensile loading up to full fragmentation, while the stress in the fiber was monitored at each level of the applied stress. It was shown that the experimental approach allowed us to discriminate between the strengths of the interfaces in the different surface-treated aramid fiber Pebax matrix systems, but also to detect different phenomena (interfacial debonding, matrix yielding and fiber fracture) related intimately to the nature of stress transfer in composite materials. The efficacy of the surface treatments was clear by comparing the maximum interfacial shear stress with the fragment lengths of the modified aramid fibers. The fiber breaks observed using Raman spectroscopy were not clean breaks as observed with carbon or glass fibers, but manifested themselves as apparent breaks by fiber skin failure. The regions of fiber fracture were also investigated using optical microscopy.

Jute/polypropylene composites I. Effect of matrix modification

Natural fibre-reinforced polymers can exhibit very different mechanical performances and environmental aging resistances depending on their interphase properties, but most studies have been focused on fibre surface treatment. Here, investigations of the effect of maleic anhydride grafted polypropylene (MAHgPP) coupling agents on the properties of jute fibre/polypropylene (PP) composites have been considered with two kinds of matrices (PP1 and PP2). Both mechanical behaviour of random short fibre composites and micro-mechanical properties of single fibre model composites were examined. Taking into account interfacial properties, a modified rule of mixture (ROM) theory is formulated which fits well to the experimental results. The addition of 2 wt% MAHgPP to polypropylene matrices can significantly improve the adhesion strength with jute fibres and in turn the mechanical properties of composites. We found that the intrinsic tensile properties of jute fibre are proportional to the fibre’s cross-sectional area, which is associated with its perfect circle shape, suggesting the jute fibre’s special statistical tensile properties. We also characterised the hydrophilic character of natural fibres and, moreover, humidity environmental aging effects. The theoretical results are found to coincide fairly well with the experimental data and the major reason of composite tensile strength increase in humidity aging conditions can be attributed to both improved polymer–matrix and interfacial adhesion strength.

Processing of single polymer composites using the concept of constrained fibers

AbstractThe concept of “overheating” is one of the known methods for manufacturing single polymer composites. This concept is validated on two categories of semi‐crystalline polymers: the drawable, apolar (i.e., isotactic polypropylene [iPP], ultra‐high molecular weight polyethylene [UHMWPE]) and the less drawable, polar ones (i.e., polyethyleneterephalate [PET] and polyamides [PA]). The interchain interactions in apolar polymers are relatively weak and therefore a high degree of drawability can be obtained. Polar polymers on the other hand have relative strong interchain interactions, they are therefore less drawable. A shift higher than 20°C of the melting temperature can be obtained in case of highly extended iPP (draw ratios >14). Ultra‐drawn PE shows only 10°C overheating upon constraining and this is mainly due to the change in chain mobility for PE in the hexagonal phase. In case of PET and PA6, only draw ratios of 4 could be reached; however, temperature shifts of about 10°C for constrained fibers compared to unconstrained fibers could be measured. A proof of principle of the potential of the constraining concept for the manufacturing of single polymer composites is obtained by the preparation of single fiber model composites. The effect of the post‐drawing conditions on overheating is examined in details on the example of iPP. It is concluded that both post‐drawing temperature and ultimate draw ratio have a significant influence on the degree of overheating. POLYM. COMPOS., 26:114–120, 2005. © 2004 Society of Plastics Engineers

The Influence of Internal Stresses on the Microbond Test II: Physical Aging and Adhesion

The present work investigated the effects of physical aging on interfacial adhesion and internal stresses using single-fibre model composites. First, the stress state of the interface and its evolution upon aging were studied for the microbond geometry. This stress analysis was used with experiments to determine an intrinsic interfacial shear strength. Second, this stress analysis, including aging effects, was used in a fracture mechanics or energy release rate analysis of the microbond test, which involves the energy necessary to propagate a crack instead of the minimum stress to initiate a crack. These methods were tested for both freshly cured and aged epoxy droplets on glass fibres, and were shown to yield similar results. It was found that neither the interfacial shear strength nor the interfacial toughness were changed by physical aging of the polymer matrix when the relaxation of internal stresses and shift of relaxation times of the matrix due to physical aging were accounted for. Thus, the intrinsic shear strength and toughness of the fibre– matrix interface were determined.

In situ monitoring of residual strain development during composite cure

AbstractInternal (residual) stresses build up in a thermosetting composite as the matrix shrinks during cure, and again as the composite is cooled to ambient from its elevated processing temperature. These stresses can be significant enough to distort the dimensions and shape of a cured part as well as initiate damage in off‐axis plies, either during fabrication or under the application of relatively low mechanical loads. The magnitude of these stresses depends on a number of factors including constituent anisotropy, volume fraction and thermal expansion, ply orientation, process cycle, and matrix cure chemistry. In this study, embedded strain gauges were employed to follow, in situ, the buildup of residual strains in carbon fiber‐reinforced laminates during cure. The data were compared to those from volumetric dilatometer studies to ascertain the fraction of resin shrinkage that contributed to residual stress buildup during cure. Based on earlier studies with single‐fiber model composites, the process cycle in each case was then varied to determine if the cycles optimized to minimize residual stresses for isolated fibers in an infinite matrix were applicable to the reduction of residual stresses in conventional multifiber composites. The results of these studies are reported here.

Poly(carbazole-co-acrylamide) electrocoated carbon fibers and their adhesion behavior to an epoxy resin matrix

The surface properties of original high strength and preoxidized high modulus carbon fibers were altered by electrocopolymerizing acryl amide and carbazole and therefore depositing a copolymer coating onto the fibers. Scanning electron microscopy and zeta-potential measurements confirmed the presence of a rough but dense and continuous electrocoating with a basic surface character. Therefore, ‘good’ adhesion behavior between the electrocoated carbon fibers and an epoxy resin matrix should be expected. The interfacial adhesion was measured using the single fiber pull-out and single fiber indentation test. It was shown that only ‘intermediate’ adhesion was present between the carbon fibers and the electrocoating, but superior adhesion between the coating and epoxy resin exists. The single fiber model composites always failed at the fiber/electrocoating interface. However, as shown by using the indentation test, the interfacial adhesion between fibers and electrocoating can be significantly improved if preoxidized fibers are used as substrate for electropolymerization. A very high tensile strength for the electrocoating can be expected as derived from the single fiber pull-out tests.

Fibre/matrix stress transfer through a discrete interphase. Part 1: single-fibre model composites

Abstract This paper reports a study of the effect of an interphase on strain development in fibre-fragments. In order to form an interphase, an epoxy resin with known properties was applied to the surface of unsized reinforcing fibres and cured. These were then embedded in a matrix resin coupon, prior to fragmentation testing. The study included an examination of the effect of interphase thickness, by applying multiple coats of one of the resins, and the effect of the interphase properties, by varying the coating resins. It was found that the average fragment lengths at saturation were difficult to distinguish, as a result of the scatter introduced by the statistical distribution of fibre strengths. However, the strain interval between onset of fragmentation and saturation was found to be more sensitive to variations in the interphase properties. A finite element model was used to examine the strain development in the fragments in more detail. The mechanical properties of the fibre, interphase and matrix were accurately incorporated into the model, providing a realistic representation of the state of strain in the experimental samples. The predicted deformations around the fibre-break provided an explanation for the experimental observations.

Morphological investigations of polypropylene single-fibre reinforced polypropylene model composites

Melt-spun isotactic polypropylene (iPP) fibres have been prepared with moduli of about 12 GPa and strength of 730 MPa. Single-fibre model composites are prepared by embedding constrained high-modulus iPP fibres in thin films of a matrix material based on the same isotactic polypropylene grade. The morphology of these composites has been investigated by optical microscopy and low-voltage scanning electron microscopy techniques. After isothermal crystallisation from the melt a transcrystalline layer was found having lamellar crystals grown perpendicular to the fibre axis. The work illustrates that the processing of polypropylene fibre reinforced polypropylene composites as self-reinforced single-polymer composite systems is feasible and that these composites may fulfil the demands for fully recyclable engineering composites.

A New Method to Reduce Cure-Induced Stresses in Thermoset Polymer Composites, Part I: Test Method

The inhomogeneous structure of polymeric composites causes internal stresses to develop due to matrix volume changes during processing. The volume changes occur during cure and during cooldown after the cure is completed. Most of the previous studies on residual stresses concentrated on stress development during cooldown. In this study, a new test method was used to monitor fiber stresses that develop during cure in single fiber model composites. The method was used to study the effect of changing the cure cycle on curing induced fiber stresses. It was seen that changing the cure cycle changes the resulting stress significantly. Also, it was shown that the cure-induced stresses and their contribution to final residual stresses vary for different resins. A cure cycle with almost zero cure-induced stresses is demonstrated. The new cycle was found to satisfy the cure requirements such as glass transition temperature and cure cycle duration.

Characterization of a maleic anhydride-modified polypropylene as an adhesion promoter for glass fiber composites

Isothermal and nonisothermal crystallizations of isotactic polypropylene (iPP), maleic anhydride (MAH)-grafted PP, and MAH-modified iPP were studied by differential scanning calorimetry (DSC), to evaluate the influence of a small amount of MAH-grafted PP in iPP on its crystallization behavior. Isothermal crystallization was followed in the temperature range from 391 K to 403 K, and the rate constant and Avrami exponents were determined. Nonisothermal crystallization was carried out at different cooling rates (1-20 K/min). It was found that the crystallization kinetics of iPP was significantly altered by modification with the MAH-grafted polymer. A decreased equilibrium melting temperature, as well as decreased surface energy of folding and critical dimensions of a growing nucleus, was determined for the MAH-modified iPP, indicating faster growth of lamellae and a higher rate of crystallization. The improved nucleation ability of the modified polymer was shown to cause a shift in the crystallization peak temperature towards higher values (from 393.7 K to 399.6 K, at a cooling rate of 1 K/min), resulting in crystal structures less disposed to recrystallization. Model composites of iPP and MAH-modified iPP with glass fibers were also analysed. The apparent shear strength of single-fiber model composites with MAH-modified iPP was drastically increased compared with homo-iPP.