Tensile properties and creep response of polypropylene fibre composites with variation of fibre diameter

The mechanical properties and morphology of polypropylene (PP) long‐fibre reinforced random poly(propylene‐co‐ethylene) (PPE) composites (50/50 % vol/vol) have been investigated with reference to the fibre diameter with constant length. There is an improvement in the mechanical properties of PPE matrix by incorporation of long PP fibres into the matrix. The elastic modulus of the composite increased with decrease in the fibre diameter to 50 µm, to 0.91 GPa, which was 5 times higher than for pure PPE. However, composite stiffness decreased with decreasing fibre diameter of less than 50 µm and this is discussed in term of the fibre stiffness, packing, stress concentration and aspect ratio. Creep resistance of the composites showed the same behaviour. Morphology of the composites was investigated using scanning electron microscopy. This showed that there was a thin layer of matrix on the reinforcement, which was attributed to good impregnation and wetting of the fibres. Moreover, prediction of tensile modulus using the Cox model correlated well with experimental data. Copyright © 2004 Society of Chemical Industry     

Mechanical and physical properties of ultraoriented polyoxymethylene produced by microwave heating drawing

The properties of ultra-oriented polyoxymethylene tubes produced by drawing under microwave heating have been assessed by mechanical testing, optical microscopy, scanning electron microscopy, X-ray analysis, birefringence and differential scanning calorimetry. The highest Young's modulus of 58 GPa was obtained at room temperature (77 GPa at ?150°C) at a draw ratio of 33. The maximum tensile strength was 1.7 GPa at a draw ratio of 26. The nonuniformity of Young's modulus in a radial direction has been compared with the nonuniformity of the birefringence and heat of fusion.     

Short glass fibre reinforcedpoly(butylene terephthalate) Part 2 –Effect of hygrothermal aging onmechanical properties

Abstract

The mechanical properties of injection moulded poly(butylene terephthalate) (PBT) containing various loadings of short glass fibres (SGF) have been investigated. Properties studied include tensile, flexural, and impact. Effect of hygrothermal aging on the mechanical properties was investigated by immersing the respective specimens in distilled water at 30, 60, and 90°C. All the materials tested showed poor retention in mechanical properties upon exposure to hygrothermal aging. The effect became particularly pronounced at an immersion temperature of 90°C. Fractographic inspection of the fracture surfaces revealed that both PBT and SGF–PBT composites embrittled owing to a hydrolitic degradation process. Hydrolysis not only suppressed the matrix ductility but also reduced the bonding quality between PBT and SGF. Poor interfacial bonding was indicated by the absence of polymer matrix adhering to the fibre surfaces. The decrease in the impact strength of hygrothermally aged SGF–PBT composites provided further evidence that hygrothermal aging at high temperature reduces the contribution of fibre related toughening mechanisms. The residual mechanical properties of both PBT and its composites were not fully recovered after redrying. The permanent damage to these materials was attributed to severe hydrolytic degradation of PBT.     

The effect of diameter on the mechanical properties of amorphous carbon fibres from linear low density polyethylene

Summary The mechanical properties of amorphous carbon fibres, derived from linear low density polyethylene strongly depend on the fibre diameter, which may be attributed to the presence of a skin/core structure in these fibres. High strength carbon fibres could thus be prepared by using thin precursor filaments, that are obtained by a melt-spinning process, in which the spinline is stretched at an elevated surrounding temperature. Careful carbonization of these precursors gives carbon fibres with a strength of 2.16 GPa, a modulus of 130 GPa and a high strain at break of 3%.     

Effect of oxidation and residual stress on mechanical properties of SiC seal coated C/SiC composite

The effect of oxidation and thermal residual stress on mechanical properties of SiC seal coated C/SiC composite at ambient temperature and high temperature were studied. The oxidation of SiC seal coated C/SiC composite at 1300 and 1500 °C resulted in carbon fibres burn area near through thickness micro cracks in the SiC seal coating. With the increase in exposure time, the formation of SiO₂ layer in SiC matrix near carbon fibres burns area was found. Residual mechanical properties of SiC seal coated C/SiC composite after exposure in air show significant degradation. First time, a continuous measurement of Young's modulus with temperature of C/SiC composite was carried out using an impulse excitation technique. The effect of relaxation of thermal residual stress on mechanical properties was observed with the help of continuous measurement of Young's modulus as a function of temperature in an inert atmosphere.     

The effect of fibre diameter on the drawing behaviour of gel-spun ultra-high molecular weight polyethylene fibres

Summary In the continuous drawing of gel-spun UHMWPE fibres, the diameter of the undrawn fibre appears to have a pronounced effect on its drawing behaviour and on the mechanical properties of the resulting hot-drawn fibres. A highly oriented structure is developed more efficiently upon drawing of thinner fibres, which may be attributed to differences in the deformation mechanism between as-spun fibres of various diameters. By drawing of thin fibres, UHMWPE filaments having a strength of 6.0 GPa and a Young's modulus of 222 GPa can be obtained at a relatively low draw ratio of =70.     

Compressive behaviour of rigid rod polymer fibres and their adhesion to composite matrixes

Abstract

The deformation behaviour of the new high performance polymer fibres, poly(p-phenylene benzobisoxazole) (PBO) and polypyridobisimidazole (PIPD) and their adhesion to an epoxy composite matrix have been investigated. Both fibres give well defined Raman spectra, and the deformation micromechanics of PBO and PIPD single fibres and composites were studied from stress induced Raman band shifts. Single fibre stress-strain curves were determined in both tension and compression, thus providing an estimate of the compressive strength of these fibres. It was found that the PIPD fibre has a higher compressive strength (~1 GPa) than PBO (~0·3 GPa) and other high performance polymer fibres, because hydrogen bond formation is possible between PIPD molecules. It has been shown that when PBO and PIPD fibres are incorporated into an epoxy resin matrix, the resulting composites show very different interfacial failure mechanisms. The fibre strain distribution in the PBO-epoxy composites follows that predicted by the full bonding, shear lag model at low matrix strains, but deviations occur at higher matrix strains due to debonding at the fibre/matrix interface. For PIPD-epoxy composites, however, no debonding was observed before fibre fragmentation, indicating better adhesion than for PBO as a result of reactive groups on the PIPD fibre surface.     

Structure and mechanical properties of PGA crystals and fibres

The elastic constants of poly(glycolic acid) (PGA) crystals are reported on the basis of a commercial software package and the published crystal structure of the polymer. Due to the planar zigzag conformation of the molecular chains, very high elastic anisotropy is found with a tensile chain modulus of 294 GPa and a longitudinal shear modulus for a fibre of 6 GPa. A combination of small and wide angle X-ray scattering and differential scanning calorimetry are used to characterise the structure of highly oriented PGA fibres. The combination of long period data, crystal size and crystallinity measurements suggests a structure similar to the Prevorsek model, with alternating crystalline and amorphous regions along the fibre axis, and layers of amorphous material in parallel. A parallel-series Takayanagi model, using the theoretically calculated chain modulus, is shown to give good agreement with the experimental data in a wide temperature range.     

Tensile properties of long aligned double-walled carbon nanotube strands

The mechanical properties of well-aligned double-walled carbon nanotube (DWNT) strands with diameters of 3-20 μm and lengths of ∼10 mm were measured using a stress-strain puller. The average tensile strength and Young’s modulus of the tested strands are 1.2 GPa and 16 GPa, respectively. Deformation and fracture processes of these samples are discussed. The tensile strength and Young’s modulus of an individual DWNT bundle were estimated, with values comparable to those of SWNT bundles. The superior mechanical strengths of our as-prepared DWNT strands are expected to give them potential as a high-strength material and a reinforcement in composites.     

Effects of electron irradiation on physical properties of rubber/cellulose fibre composites

Short term stress–strain properties were investigated for natural rubber reinforced with short cellulose fibres at various fibre loads. Stress–strain measurements were also performed on natural rubber composites containing cellulose fibres, electron irradiated in the presence of butadiene or N-hydroxymethylacrylamide, where the latter monomer produced the greatest improvement in stress–strain properties. Chemiluminescence analysis indicated the existence of a surface layer on the fibres after irradiation treatments, and water absorption measurements showed a decrease in water uptake for composites containing irradiated fibres. The results demonstrate the improvement of mechanical properties with a reduced sensitivity to moisture.     

Mechanical properties of poly(ether-ether-ketone) for engineering applications

An outline of the characteristics of PEEK and the versatility of its compositional forms (micro and macro composites) are given to illustrate its wide potential for success in engineering applications. Although it is necessary to have particular tabulations of mechanical properties for engineering design, these are seldom available and consequently it is argued that an understanding of stiffness, toughness and strength properties are required to fully exploit available manufacturer's data and thus develop the full potential of PEEK and its composites. Stiffness characteristics are considered in terms of a modulus function which is dependent on time under load and temperature. In its composite forms, whether reinforced with short or continuous fibres, stiffness anistropy can be both considerable and complex, but some empirical ground-rules are apparent. For continuous fibre composites even in the form of complex lay-ups, it is also possible to attempt some stiffness prediction from certain pseudo-elastic constants. Toughness of PEEK and its composites is described in terms of both comparative and intrinsic properties. Instrumented falling weight impact data, particularly as a function of temperature enable some insight into ductile-brittle transitions for the unreinforced material, but crack initiation and crack propagation processes for the various fibre reinforced forms. Intrinsic toughness is described in terms of linear elastic fracture mechanics theory. Strength properties are described for static and dynamic loading configurations. In particular, PEEK and its composites are evaluated for increasing test severities for strength characteristics; stress concentration, loading form and test temperature are considered.     

Structure and properties of polypivalolactone

Pivalolactone (α,α-dimethyl-β-propiolactone) can be polymerized to linear polyesters with widely different molecular weights. The polymer has a high degree of crystallinity and a high crystalline melting point. Several other basic properties of the polymer have been determined, such as molecular weight, glass-transition temperature, rheological characteristics, etc., and its possible use in the fibre and plastics field has been extensively investigated. It was found that the polymer, when adequately stabilized, has a high thermal stability and shows hardly any discoloration upon processing.Ultimate products such as fibres and injection-moulded articles based on polypivalolactone have a high resistance to hydrolysis, heat and chemicals, and exhibit excellent weathering properties. A remarkable characteristic of the products is their high elastic recovery, in particular after annealing at high temperatures. Many of the observed properties can be explained by means of the molecular and crystalline structure of the polymer.     

On‐line opto‐viscoelastic analysis of polypropylene fibres using multiple‐beam Fizeau fringes in transmission and a modified creep device

A creep device attached to an automated multiple‐beam Fizeau system in transmission was modified with a designed digital ruler. This device allows on‐line measurements of fibre length during creep experiments in terms of an analogue voltage value. The influence of sustained stress values on creep deformation and optical properties (n^||, n^? and Δn) for polypropylene (PP) fibres was studied interferometrically. The opto‐viscoelastic properties of PP fibres were determined for three different values of constant applied stress of 11.536, 18.717 and 25.905 MPa, at room temperature. Also, the variations of the cross‐sectional area and Poisson's ratio were studied during creep extensions. The compliance curves were obtained as a function of both time and applied stresses. Empirical formulae are suggested to describe the creep compliance curves for PP fibres, and the constants of these formulae were determined and described at each applied stress. A Kelvin chain was used to model the mechanical behaviour of the PP fibres under study. The effect of strain on the mean refractive indices, orientation function density and crystallinity was investigated as a result of the recorded data. Microinterferograms are given for illustration. The modified creep device with the designed digital ruler enables one to obtain instantaneous automatic accurate recording of fibre length values during creep experiments. Calculation of refractive indices, orientation function and crystallinity shows a difference in material behaviour at small stresses from that at higher stresses which may be attributed to different strain rates caused by different stresses. Copyright © 2010 Society of Chemical Industry     

The sintering behaviour,mechanical properties and creep resistance of aligned polycrystalline yttrium aluminium garnet (YAG) fibres,produced from an aqueous sol–Gel precursor

Continuous ceramic fibres are finding applications as reinforcements in ceramic matrix composites, and yttrium aluminium garnet (YAG) is a particularly attractive candidate material on account of its creep resistance at high temperatures. A continuous, aligned, 5·5 μm diameter polycrystalline YAG fibre was manufactured from an aqueous sol–gel precursor which contained chlorine, and compared to a similar nitrate containing YAG precursor fibre we have reported previously. The precursor sol was found to be stable at a higher concentration than the nitrate containing sol, and this resulted in denser gel fibres which demonstrated better sintering at equivalent temperatures, giving a 98·5% sintered YAG fibre at 1550°C with a grain size of only 1 μm. However, on firing in air, the fibres formed fully crystalline YAG between 800 and 900°C, a temperature 100°C higher than the fibres containing nitrate, and they were weakened by the presence of many hemispherical faults. It was shown that both of these features were due to the retention of chlorine until the onset of formation of the crystalline YAG phase, and a series of steaming experiments were devised to remove the halide before this process could occur. It was found that steaming the precursor fibre from 200 to 500°C over 3 h, followed by firing to the required temperature in air, removed the chlorine and the problems it caused in the formation of the YAG phase without any change in the sintering characteristics or grain size. The steamed fibres were of a strength and quality comparable to fibres drawn from organometallic precursors. Empirical friability measurements showed the strength was maintained after firing to 1550°C, although there was a deterioration in apparent strain to break of the aligned blanket product above 1200°C. Conversely, the creep resistance, measured using the BSR test, improved with increase in temperature. The fibres fired to 1550°C were fully relaxed at temperatures 100–150°C below that of coarser, larger YAG fibres previously reported with a 3 μm grain size and 120 μm diameter. However, when allowance was made for grain size, the difference in creep rates was within the range obtained by extrapolating previous data using lattice diffusion and grain boundary effect models. Fibres fired to 1400°C were much finer grained but only slightly inferior to the 1550°C fibre in terms of creep. The alumina sol used in this work contained a significant level of sodium, and this suggests that the creep rates are effected by grain boundary impurities, especially sodium. A sodium free sol has been procured and further work is recommended to clarify the effect of impurities and improve fibre properties.     

Fabrication,properties and usage of single-crystalline YAG fibres

Single crystalline YAG fibres are now produced by an internal crystallization method. Essentially, the method is crystallization of the oxide melt infiltrated into continuous channels made in an auxiliary matrix, normally molybdenum, and then extracting the fibre from the auxiliary matrix by chemical dissolution of it. The method is relatively simple and requires sufficiently small energy input into a real process of the fibre production. Because crystallising a batch of the fibres by using ICM is actually similar to making bulk crystals an expected cost of ICM fibres is of the same order of magnitudes as that of bull crystals. It is shown that usage of ICM–YAG-fibres in a Ni-based matrix yields composites with high fibre/matrix interface strength and, hence, high creep resistance at very high temperatures for Ni-based materials.     

Complete characterisation of BN fibres obtained from a new polyborylborazine

A new polymer was prepared at room temperature from a di-chloroborazine and a reactive aminoborane. It displays borazine rings unambiguously linked through three atoms N–B–N bridges. This connecting mode was evidence by ¹⁵N solid state NMR. This polyborazine was processed into a continuous polymer fibre of about 21 μm diameter, which was subsequently heat-treated under NH₃/N₂ up to 1800 °C for conversion into BN fibres. The achievement of hexagonal boron nitride was confirmed by X-ray diffraction and Raman spectroscopy. Tensile tests were carried out on the ceramic fibres. The average tensile strength is about 1000 MPa and the Young's modulus is close to 200 GPa. Structural characterisation of the BN fibres was undertaken by polarised light and transmission electronic microscopies.     

Morphology,Thermal and Mechanical Properties of Poly(propylene) Fibre‐Matrix Composites

Preparation and properties of poly(propylene)‐poly(propylene) composites have been investigated. Poly(propylene) fibres of varying diameter have been incorporated in a random ethylene co‐poly(propylene). The composites prepared from the same semi‐crystalline polymer in the matrix and reinforcement have lead to inherently strong interfacial bonding between the two phases of the same polymer. The composites demonstrated enhanced stiffness, which increased with fibre diameter. The structure, thermal, static and mechanical properties of poly(propylene) long fibre reinforced random co‐poly(propylene) composites have been studied with reference to the fibre diameter. The matrix and fibre components retained their separate melting temperatures. After melting, the two phases remained separate and showed their individual crystallization temperatures on cooling, and melting temperatures on a second heating. The melting temperature of the poly(propylene) fibres increased after formation of the composites. The compression molding of the composites at a temperature below the melting temperature of the fibres caused annealing of the fibre crystals. By incorporation of long poly(propylene) fibre into random co‐poly(propylene), the glass transition, storage and static modulus have been found to be increasing and composite with the largest fibre diameter shows better properties. Transcrystallization of the matrix poly(propylene) was observed.

Optical microscopy of composites with fibre diameter 68 μm.     

Fiber size effects on mechanical behaviours of SiC fibres-reinforced Ti3AlC2 matrix composites

In this study, different diameters SiC fibres-reinforced Ti₃AlC₂ matrix composites were fabricated via hot-pressing. The mechanical behaviours of the obtained composites were evaluated at room temperature by using 4-point bending testing. The results show that the flexural strength and damage tolerance are significantly higher for smaller diameter fibres-containing composites than for large diameter fibres-containing ones. In addition, the mechanical properties of the composites were improved when smaller diameter fibres partially replaced large diameter fibres and the improvement increased with increase of the amount of smaller diameter fibre. Furthermore, the fracture fashion of the composites depended on fibre diameters used.     

Tensile properties of ultrathin double-walled carbon nanotube membranes

Direct tensile tests of double walled carbon nanotube (DWCNT) membranes with thickness of 40–80 nm were performed using a micro-stress-strain puller. The tensile strength and Young’s modulus are 4.8E2–8.4E2 MPa and 4.4–8.8 GPa, respectively. The deformation and fracture processes were analyzed using the stress vs. strain curves, and SEM observations of the fracture surface of a membrane. The membrane experienced elastic strain and plastic strain during tensile-loading to fracture, and the plastic process is due to the real plastic deformation of the membrane and the slippage between the DWCNT bundles. Cracks occur and spread during the tensile test which causes the membrane to be mangled. With these excellent mechanical properties, the DWCNT membranes can be used in nanotube-reinforced composites.