The influence of fibre length and concentration on the properties of glass fibre reinforced polypropylene: 7. Interface strength and fibre strain in injection moulded long fibre PP at high fibre content

The mechanical performance of injection moulded long glass fibre reinforced polypropylene with a glass fibre content in the range 0–73% by weight has been investigated. The composite modulus exhibited a linear dependence on fibre content over the full range of the study. Composite strength and impact resistance exhibited a maximum in performance in the 40–50% by weight reinforcement content range. The residual fibre length, average fibre orientation, interfacial shear strength, and fibre strain at composite failure in the samples have been characterised. These parameters were also found to be fibre concentration dependent. The interfacial shear strength was found to be influenced by both physical and chemical contributions. Theoretical calculations of the composite strength using the measured micromechanical parameters enabled the observed maximum in tensile strength to be well modelled.     

Experimental and theoretical studies on the properties of injection moulded glass fibre reinforced mixed plastics composites

Recycled mixed post-consumer and post-industrial plastic wastes consisting of HDPE, LDPE and PP were injection moulded with short glass fibre (10–30% by weight) to produce a new generation composite materials. Intensive experimental studies were then performed to characterise the tensile, compression and flexural properties of glass fibre reinforced mixed plastics composites. With the addition of 30 wt.% of glass fibre, the strength properties and elastic modulus increased by as much as 141% and 357%, respectively. The best improvement is seen in the flexural properties due to the better orientation of the glass fibres in the longitudinal direction at the outer layers. The randomness and length of the glass fibre were accounted to modify the existing rule of mixture and fibre model analysis to reliably predict the elastic and strength properties of glass fibre reinforced mixed plastics composites.     

The influence of fibre length and concentration on the properties of glass fibre reinforced polypropylene. 6. The properties of injection moulded long fibre PP at high fibre content

The results of an investigation of the mechanical performance of injection moulded long glass fibre reinforced polypropylene with a glass fibre content in the range 0–73 wt% are presented. The composite modulus exhibited a linear dependence on fibre content over the full range of the study. Composite strength and impact resistance exhibited a maximum in performance in the 40–50 wt% reinforcement content range. The residual fibre length and fibre orientation in the samples has also been characterised. These parameters were also found to be fibre concentration dependent. Modelling of the composite strength using the measured fibre length and orientation data did enable a maximum in strength to be predicted. However, the position and absolute level of the predicted maximum did not correlate well with the experimental data. Further analysis indicated that deeper investigation of the dependence of the interfacial shear strength and fibre stress at composite failure on the fibre content are required to fully elucidate these results.     

Optimising industrial hemp fibre for composites

The optimisation of New Zealand grown hemp fibre for inclusion in composites has been investigated. The optimum growing period was found to be 114 days, producing fibres with an average tensile strength of 857 MPa and a Young’s modulus of 58 GPa. An alkali treatment with 10 wt% NaOH solution at a maximum processing temperature of 160 °C with a hold time of 45 min was found to produce strong fibres with a low lignin content and good fibre separation. Although a good fit with the Weibull distribution function was obtained for single fibre strength, this did not allow for accurate scaling to strengths at different lengths. Alkali treated fibres, polypropylene and a maleated polypropylene (MAPP) coupling agent were compounded in a twin-screw extruder, and injection moulded into composite tensile test specimens. The strongest composite consisted of polypropylene with 40 wt% fibre and 3 wt% MAPP, and had a tensile strength of 47.2 MPa, and a Young’s modulus of 4.88 GPa.     

Effects of fibre length on tensile strength of carbon/glass fibre hybrid composites

The tensile strength of epoxy resin reinforced with random-planar orientation of short carbon and glass fibres increased as the length of the reinforcing fibres increased, and the increase in tensile strength remained almost unchanged after the fibre length reached a certain level. The tensile strength of composites at any fibre length could be estimated by taking the strain rate and temperature dependence of both the yield shear strength at the fibre-matrix interphase and the mean critical fibre length into consideration. The tensile strength of the hybrid composite could be estimated by the additive rule of hybrid mixtures, using the tensile strength of both composites.     

Predicting the tensile strength of natural fibre reinforced thermoplastics

The tensile strength of short natural fibre reinforced thermoplastics (NFRT) was modeled using a modified rule of mixtures (ROM) strength equation. A clustering parameter, requiring the maximum composite fibre volume fraction, forms the basis of the modification. The clustering parameter highlights that as fibre loading increases, the available fibre stress transfer area is decreased. Consequently, at high volume fractions this decrease in stress transfer area increases the brittleness of the short fibre composite and decreases the tensile strength of the material. A key parameter, the interfacial shear strength, was determined by fitting the micromechanical strength model to tensile strength data at low fibre loading (10 wt%) where there is minimal fibre clustering.To test the modified ROM strength model, compression molded specimens of high-density polyethylene (HDPE) reinforced with hemp fibres, hardwood fibres, rice hulls, and E-glass fibres were created with fibre mass fractions of 10–60 wt%. The modified ROM strength model was found to adequately predict the tensile strength of the various composite specimens.     

Fracture behaviour of long fibre reinforced thermoplastics

This paper deals with the fracture performance of injection moulded long glass fibre composites based on polybutylene terephthalate (PBT) and polypropylene (PP) matrices. The tensile behaviour of these composites is analysed using the shear lag theory taking into consideration the interfacial shear strength, fibre length distribution and fibre orientation in the mouldings. The fracture performance is investigated using the post yield fracture mechanics approach. The crack growth resistance of the PP and PBT long fibre composite was found to increase with increasing fibre volume content up to 35%. Above 35% a plateau in the fracture performance was observed. A combination of high fibre degradation and a change in the fibre orientation pattern of the moulded pieces is found to be responsible for the plateau region in the performance of the high concentration system. In fact, the dependence of the maximum crack growth resistance of the composites on fibre length and fibre orientation is also controlled by testing temperature. The competition between fibre-induced matrix deformation and the fibre pull-out determines the ability of the composites to resist crack propagation.     

Effect of fibre volume on tensile properties of real unidirectional fibre-reinforced composites

A simple theoretical model is proposed to account for the effect of high fibre loading on the tensile properties of real, unidirectionally reinforced fibre composites. Based on the reduction of interfacial surfaces due to fibre-fibre interaction, a modification to the rule of mixtures is proposed. The experimentally observed non-linear variation of tensile strength with fibre volume can be predicted by the modified rule. Experimental data from the literature are used to validate and verify the model and show good agreement with the predictions.     

The effects of the biaxial stretching of leather on fibre orientation and tensile modulus

Leather has been subjected to different degrees of equal biaxial strain (up to 20%) during drying and its tensile modulus has been measured when dry. The collagen fibre orientation distribution in the dried leather has been assessed using wide angle X-ray diffraction. It was found that drying under biaxial strain caused the tensile modulus to increase markedly (by up to 400% at 20% biaxial strains) but with a dependence on the angle of test axis in relation to the principal axes of biaxial strain. The fibre orientation distribution in planes parallel to the surface was affected less by biaxial strain than in planes perpendicular to the surface and it is concluded that the latter type of fibre reorientation is the main factor responsible for the observed increases in tensile modulus.     

Analysis of fibre orientation distribution in short fibre reinforced polymers: A comparison between optical and tomographic methods

The fibre orientation distribution in a material sample of short fibre reinforced polyamide extracted from an injection moulded notched plate was analysed using two different methods, one based on micro-computed tomography and the Mean Intercept Length concept and the other based on the classical optical section method. The two methods were compared in terms of the preferred fibre orientation at a chosen position, and the agreement was found to be excellent provided the correct section plane was chosen for the optical method. The optical method was applied to different section planes to ascertain the best choice. Comparisons with the optical method, which can provide the full fibre orientation distribution, confirm that the analysis based on the MIL concept is capable of capturing important information about the fibre orientation.     

The influence of fibre length and concentration on the properties of glass fibre reinforced polypropylene: 5. Injection moulded long and short fibre PP

We present results of a step by step comparison of the mechanical performance of injection moulded ‘long’ (LF-PP) and ‘short’ (SF-PP) glass fibre-polypropylene compounds. The study allows direct comparison of the mechanical performance of long and short fibre systems in the same resin at the same fibre diameter, and the effect of fibre diameter in short fibre compounds. Furthermore, the comparison of these three systems has been made over the 0–40 wt% fibre content range. At the same fibre diameter and fibre content LF-PP gives significant improvements in room temperature tensile and flexural strength, notched and unnotched impact resistance. The improvement in impact resistance is higher still at lower test temperature. LF-PP also gives increasingly higher modulus over SF-PP as the strain is increased. The effect of lowering the fibre diameter in SF-PP has been shown to increase both strength and unnotched impact, but not to the levels obtained with LF-PP at higher fibre diameter. Notched impact and modulus of SF-PP were relatively unaffected by reduction of the fibre diameter. The relative mechanical data are shown to conform well to available models. The results are discussed in terms of the relevant micro-mechanical parameters of these materials.     

The dimensional stability of glass–fibre reinforced polyamide 66 during hydrolysis conditioning

Injection moulded glass–fibre reinforced polyamide 66 composites based on two glass fibre products with different sizing formulations and unreinforced polymer samples have been characterised both dry as moulded and during conditioning in a water–glycol mixture at 70 °C for a range of times up to 400 h. The results reveal that hydrothermal ageing in water–glycol mixtures causes significant changes in the weight and dimensions of these materials. All conditioned materials showed a time dependent weight increase which could be characterised as pseudo-Fickian. The weight change could be well modelled by a Fickian diffusion process with a time dependent diffusion coefficient. It was not apparent that changing the glass fibre sizing affected the dimensional stability of the composites. There was a strong correlation between the swelling of these samples and the level of fluid absorption. The composites exhibited different levels of swelling depending on direction. These effects were well in line with the influence of fibres on restriction of the matrix deformation in the fibre direction. These differences correlated well with the average fibre orientation with respect to the various direction axes.     

Experimental investigations on bond strength between coconut fibre and concrete

Effect of fibre embedment lengths, diameters, pretreatment conditions and concrete mix design ratios on the bond strength between single coconut fibre and concrete is investigated. Fibres are prepared and categorised manually. Fibre diameters are measured by a stereomicroscope. Fibre and concrete properties are also determined experimentally. The simplified equations are proposed for estimating the fibre tensile stress, elastic modulus and toughness. Single fibre pull-out tests are carried out to determine load–slippage curves with the help of an Instron tensile machine having load cell. Bond strength and energy required for fibre pullout are calculated from the experimental data. The results show that fibres have the maximum bond strength with concrete when (i) embedment length is 30 mm, (ii) fibres are thick, (iii) treated with boiling water, and (iv) concrete mix design ratio is 1:3:3. Similar effects are observed for energy required for fibre pullout. With the obtained knowledge, empirical equations are also developed to determine the bond strength and energy required for fibre pullout.     

Evaluation of continuous alumina fibre reinforced composites based upon pure aluminium

Abstract

Composites of super purity aluminium unidirectionally reinforced with Altex or Nextel 610 continuous alumina basedfibre have been made by liquid metal infiltration. The composites were well consolidated, with fibre volume fractions V_f of 0.4 and 0.6 for the Altex composites and 0.7 for the Nextel composite, the higher values being obtained where preforming involved the use of sized fibre tows. Matrix porosity was very low and there was no evidence of any deleterious reaction product having formed at the fibre/matrix interface. Monotonic longitudinal tensile tests of the composites gave Youngs modulus values between 125 and 250 GPa, in line with rule of mixtures (ROM) predictions and evidence of effective load transfer between fibres. The onset of yielding in longitudinal composites was commensurate with the yield stress of unreinforced super purity aluminium for V_f = 0.4 (～20 MPa), but increased to 225 MPafor V_f =0.7. The tensile strengths of the Altex composites were 760 and 930 MPa, values in accord with ROM predictions based upon equal load sharing of fibres up to the mean filament failure stress. Although the Nextel composite had a higher tensile strength of 1250 MPa, this was significantly lower than the ROM value of 1650 MPa and was better described by fibre ‘bundle’ theory. Predictions of the accumulation of fibre damage, by statistical analysis, indicated that filament breakage commenced at an applied stress of ～50 MPa for the Altex composite and ～ 500 MPa for the Nextel composite. Despite damage at the lower stress, however, the Altex composites were able to tolerate many more ‘double’ fibre breaks than the Nextel composite, the failure of which coincided with the onset of the first double break. Transverse tensile tests of the composites gave Young's modulus values between 80 and 170 GPa, in line with ROM predictions. The yield stress increased with increasing V_f, from 10 to 60 MPa, this behaviour being attributed to plane strain deformation caused by the virtually non-deformable fibres constraining matrix flow. The tensile strengths showed a similar trend, with 84 MPa for V_f =0.4 increasing to 168 MPa for V_f = 0.7.     

Carbon fibre reinforced glass matrix composite tension and flexure properties

Carbon fibre reinforced borosilicate glass matrix composites have been fabricated to determine their mechanical properties in tension and flexure. Composite tensile stress-strain properties, including elastic modulus, proportional limit and ultimate tensile strength, have been measured as a function of fibre content. Composite tensile properties were also obtained at temperatures of up to 625° C through the testing of 0/90 cross-plied specimens. Composite short-beam shear strength was found to depend on specimen orientation and also on the composition of the glass matrix. This compositional dependence was associated with an independent measurement of the fibre-matrix interfacial shear strength and was related to the degree of fibre-matrix reaction taking place during composite fabrication.     

Structure and properties of mesophase pitch carbon fibre with a skin-core structure carbonized under strain

Coal tar mesophase pitch fibres stabilized at 270° C to different extents were carbonized under strain by the constant load or constant length, using different heating rates, and further graphitized at 2500° C. Shallow and moderate stabilization provided a skin-core structure in the resultant fibres which exhibited higher orientation, tensile modulus, and better graphitizability after calcination at 1300° C and graphitization at 2500° C than deep stabilization. The tensile strength and modulus of the graphitized fibre was significantly improved through the strained carbonization when the stabilization was performed to a moderate extent. The strain tended to give an onion-like alignment in the fibre to improve the preferred orientation of carbon planes. Larger load and more rapid heating during carbonization modified the structure and properties of resultant fibres through a significant longitudinal elongation. The stabilization extent of pitch fibres governs the mobility or fusibility of mesogen molecules at the carbonization which allows their better alignment by the strain.     

Temperature and weldline effects on tensile properties of injection moulded short glass fibre PC/ABS polymer composite

The effect of weldline and temperature on tensile properties of injection moulded PC/ABS blend reinforced with different concentration levels of short glass fibres was investigated between 23 and 100 °C. The weldline was formed in the moulded specimens by direct impingement of two opposing melt fronts. It was found that weldline had no significant effect on tensile modulus with weldline integrity factor in the range of 1–0.98. Tensile modulus for both weld and unweld specimens increased linearly with increasing volume fraction of fibres, ϕ_f, and decreased linearly with increasing temperature. Tensile strength of both weld and unweld tensile specimens increased non-linearly with increasing ϕ_f reaching a maximum at ϕ_f≈ 0.16 for specimens without weldline and ϕ_f≈ 0.12 for specimens with weldline. A linear dependence with respect to ϕ_f was found for both weld and unweld tensile strengths for fibre volume fractions in the range 0–0.1. It was found that below the glass transition temperature of the matrix, tensile strength of the composite with and without weldlines decreased linearly with increasing temperature. The weldline integrity factor for tensile strength decreased with increasing ϕ_f, but showed no significant variation with respect to temperature.     

The fracture energy of hybrid carbon and glass fibre composites

The fracture energy of a model carbon fibre/glass fibre/epoxy resin hybrid composite system has been evaluated as a function of the carbon fibre/glass fibre ratio. Work of fracture measurements were less than a rule of mixtures prediction and a pronounced negative synergistic effect was observed at high carbon fibre and high glass fibre contents. Fibre debonded lengths and fibre pull-out lengths for the carbon and glass fibres were accurately measured using a projection microscope technique. Models of microscopic fracture behaviour, together with these measurements, were successful in quantitatively describing the observed fracture behaviour of the hybrid fibrous composites. It was found that post-debond friction energy provided a major contribution to the fracture energy of the glass fibres. The post debond sliding mechanism was also shown to be primarily responsible for the non-linear behaviour of the work of fracture of the hybrid composite.     

Mechanical properties of glass fibre reinforced polypropylene disks produced by rotating,expansion and compression injection moulding

A special mould (RCEM, rotation, compression and expansion mould) intended for inducing complex stress fields during the filling stage of injection moulding was used to manipulate the microstructure of a short fibre reinforced polypropylene. Centred gated discs were injection moulded with different filling sequences (stationary, expansion, compression, expansion with continuous rotation and compression with continuous rotation). The mechanical behaviour of the mouldings was characterized in tensile and flexural loadings on specimens cut at different locations along the flow length. Complete discs were also tested in 4-point support flexural test at low velocity and at impact. The respective results are analysed and discussed in terms of the developed fibre orientation morphology.     

Effects of fluctuation of fibre orientation on tensile properties of flax sliver-reinforced green composites

Plant-based natural fibres are often used as a reinforcing material for environmentally friendly green composites. Especially, the form of slivers of natural fibres is anticipated for increasing their stiffness and strength. However, the sliver structure has fluctuations in fibre orientation, which decreases their mechanical properties. This paper describes the effects of such fibre orientation fluctuation on tensile properties of fibre-reinforced fully green composites. The composites were reinforced with slivers of high-strength flax fibres, for which a fabrication method called ‘direct method’ was applied. To quantify the morphology of the fibre orientation, fibre orientation angles were measured on fine segments, which were divided into 1 mm × 1 mm squares on a photograph of the whole composite surface. Although it is well-known that tensile strength of unidirectional composites decreases with increasing fibre orientation angle, the tensile strength obtained here did not show any appreciable relation to the statistical properties of measured fibre orientation angles such as average and standard deviation. The concept of two-dimensional (2D) autocorrelation was used in the present study to express the degree of similarity between fibre orientation angles in two different local areas. Results show that, if high 2D autocorrelation coefficients occupy more area on a composite surface, then this composite possesses more regular fibre orientation and tends to exhibit higher tensile strength. This tendency is stronger in the composites close to on-axis alignment, whereas it became weak in the off-axis composites angled more than 15° because of shear fracture.