Fracture mechanism in short fibre reinforced thermoplastic resin composites

The properties of two types of short carbon fibre (CF) reinforced thermoplastic resin composites (CF-PPS and CF-PES-C), such as strength (_y). Young's modulus (E) and fracture toughness (K _1c), have been determined for various volume fractions (V _f) of CF. The results show that the Young's modulus increases linearly with increasingV _f with a Krenchel efficiency factor of 0.05, whereas _y andK _1c increase at first and then peak at a volume fraction of about 0.25. The experimental results are explained using the characteristics of fibre-matrix adhesion deduced from the load-displacement curves and fractography. By using a crack pinning model, the effective crack tensions (T) have been calculated for both composites and they are 57 kJ m^–1 for CF-PPS and 4.2 kJ m^–1 for CF-PES-C. The results indicate that the main contribution to the crack extension originates from localized plastic deformation of the matrix adjacent to the fibre-matrix interface.     

Fracture toughness evaluation for short glass fibre reinforced composites

Valid plane-stress fracture toughness evaluation of short fibre reinforced composites relies essentially on the successful separation of the energy absorbed in the localized crack-tip region out of the total energy absorbed by the cracked material body at large. Three different experimental techniques, all stemming from the energetic interpretation of theJ integral, are utilized and their relative merits in the characterization of fracture initiation in short glass fibre reinforced injection-moulded nylon 6.6 examined. Various theoretical aspects concerning these experimental methods are outlined. The rationale behind using a single-edge-notched tension type specimen for theJ _c test is presented. TheJ _c value obtained from the compliance calibration method and the quasistatic energy method agree closely and can be considered to be independent of pre-crack length and specimen geometry when the pre-crack length to specimen width ratio (a/w) is larger than 0.45. The extrapolation method fails nevertheless to yield a physically consistentJ _c value, possibly due to its questionable theoretical representation. As no constraint on boundary conditions is necessitated during the course of crack extension, the quasistatic energy is physically more appealing.     

Fracture toughness of 2-D carbon fibre reinforced carbon composites

The fracture toughness of 2-D woven carbon fibre reinforced carbon laminate has been evaluated by linear elastic fracture mechanics (LEFM),R-curve andJ-integral analysis using the single edge-notched bending (SENB) specimen of edge and flatwise geometries. The edgewise specimens failed by a small extension of the self similar crack whereas the flatwise specimens failed by delamination. The surface damage developing from the tip of the initial crack was revealed by the brittle lacquer coating technique and the zone shape varied with the specimen geometry, i.e. the loading axis relative to the woven layers. Acoustic emission (AE) was also used to monitor crack growth, and the total ring down count of AE was observed to increase as the initial crack length was decreased. Both the damage zone size and total AE counts were found to increase in two linear stages as a function of the square of the stress intensity factor,K.     

Fracture characterization of short glass fibre reinforced thermoplastic polyester by theJ-integral

Fracture initiation of short glass fibre reinforced thermoplastic polyester was characterized by theJ-integral measurement based on the energy release rate interpretation ofJ. The criticalJ value (J _c) is shown to be a fracture characterizing parameter for the onset of the crack initiation in the injection moulded short glass fibre reinforced composite material. TheJ _c value of the composite is estimated by be 6.0kJ m^–2. This value is in good agreement with the linear elastic strain energy release rate (G _c), since the composite exhibited a fairly linear stress-strain relationship. The estimated ratios ofJ _c to the total energy absorbed per unit uncracked area are in good agreement with the analytically obtained values after the remote energy dissipation due to fibre and matrix interaction away from the crack tip has been subtracted from the total energy.Nomenclature J J-integral - J _c The criticalJ - G Elastic strain energy release rate - G _c The criticalG - K _l Opening mode stress intensity factor - The criticalK _l - P Applied load - x Load-point displacement - B Specimen thickness - E Young's modulus - v Poisson's ratio - F Force - Y Central deflection - a/W Ratio of the crack length to the specimen width - _y Yield stress - U _t Total strain energy in loading a specimen - U _d Remotely dissipated strain energy after unloading - U _t–d U _t–U _d - _t Ratio ofJ _c toU _t per unit uncracked ligament - _t–d Ratio ofJ _c toU _t–d per unit uncracked ligament.     

Fracture behaviour of collimated thermoplastic poly(ethylene terephthalate) reinforced with short E-glass fibre

The fracture behaviour of thermoplastic poly(ethylene terephthalate) reinforced with short E-glass fibre was investigated using fractography and a fracture mechanics approach. The observed microstructures, crack propagation and the stress-rupture lifetime data indicate a sudden breakdown induced by far-field effectS. The critical damage appears to be correlated with a ductile-to-brittle transition of matrix fracture. The calculation of fracture toughness for various fibre orientations indicates that the fibre pull-out energy is the dominant term in the case in which the fibre orientation is perpendicular to the notch tip.     

Chemically-assisted fracture of thermoplastic PET reinforced with short E-glass fibre

The study of the stress-rupture lifetime of a PET/glass fibre system by means of fracture mechanics methods indicates the degradation of lifetime under an aggressive environment (10% HCl solution). The SEM-EDAX analysis reveals the depletion of calcium and aluminium elements from the fibre, and this is believed to be the cause of multiple fibre fractures. The fracture toughness is decreased because the role of fibre pull-out, which is the significant energy-consuming process, is negligible. A statistical analysis, from which the lifetime behaviour can be predicted, is carried out.     

复合处理碳纤维增强聚酰亚胺复合材料力学性能

采用浓硝酸氧化和聚酰亚胺(PI)包覆复合方法对短切碳纤维(CF)进行表面改性,提高CF增强热塑性聚酰亚胺复合材料(CF/TPI)力学性能。采用比表面积及孔容分析、原子力显微镜、扫描电子显微镜、热重分析仪研究了CF表面处理前后结构和形貌的变化。结果表明：CF经浓HNO₃处理后比表面积增加144.2%,CF表面沟壑加深;复合处理后有PI层包覆在 CF表面;包覆处理后CF耐热性能提高。力学性能测试表明,经过包覆处理后CF/TPI复合材料的拉伸强度比未处理的提高11.34%,弹性模量提高109.2%,弯曲强度提高18.78%,冲击强度提高74.15%。     

Fracture toughness of unidirectional fibre reinforced composites in MODE II

Experimental investigations have been performed on unidirectional glass fibre reinforced/epoxy composites in Mode II (Forward shear) with the presence of crack parallel to the fibres direction through the use of end-cracked beam. A concentrated load at the Centre of the beam produced bending-induced shear deformation at the crack tip. Calibration factors for Mode II have been obtained. The stress-intensity factor at instability $\text{K}$_IIR(INR) is obtained by experiments on a small end cracked beam through a compliance matching procedure. The crack growth resistance at instability and the corresponding critical strain energy release rate are independent of initial crack in the range of crack length investigated. In composite materials, fibre-matrix interfacial shear stress play an important role in load transfer mechanism: hence Mode II study may be very useful to analyse the interfacial mechanisms and to understand the fracture behaviour of unidirectional fibre reinforced composites in Mode I when load is applied in the direction of the fibres.     

Fracture characterisation of short bamboo fibre reinforced polyester composites

In the study, fracture behaviour of short bamboo fibre reinforced polyester composites is investigated. The matrix is reinforced with fibres ranging from 10 to 50, 30 to 50 and 30 to 60 vol.% at increments of 10 vol.% for bamboo fibres at 4, 7 and 10 mm lengths respectively. The results reveal that at 4 mm of fibre length, the increment in fibre content deteriorates the fracture toughness. As for 7 and 10 mm fibre lengths, positive effect of fibre reinforcement is observed. The optimum fibre content is found to be at 40 vol.% for 7 mm fibre and 50 vol.% for 10 mm fibre. The highest fracture toughness is achieved at 10 mm/50 vol.% fibre reinforced composite, with 340% of improvement compared to neat polyester. Fractured surfaces investigated through the Scanning Electron Microscopy (SEM) describing different failure mechanisms are also reported.     

Plasma-sprayed hydroxylapatite coating on carbon fibre reinforced thermoplastic composite materials

Plasma-spraying of metallic impiant surfaces is an established method for the application of hydroxylapatite (HA) coatings. Carbon fibre reinforced thermoplastics show different thermal and mechanical properties, compared with titanium substrates. In this paper first results of the influence of the established coating method on carbon fibre reinforced thermoplastics are presented. First investigations of the tensile adhesion strength, tested with a newly developed testing device, showed that the adhesion between the HA coating and the carbon fibre reinforced polyetheretherketone (PEEK) composite is very low. Macromechanical bending tests showed a change to initial tensile instead of compression failure of the coated composite substrate. Micromechanical bending tests in a scanning electron microseope (SEM) hot tensile stage (Raith GmbH) revealed crack propagation within the ceramic coating and in the coating-substrate interface before the total failure of the composite substrate occurred.     

Influence of weathering on unreinforced and short glass fibre reinforced thermoplastic polyester

The influence of weathering on fracture toughness, Jc, yield strength, y, local ultrasonic velocity, VR, and microhardness, Hu, in unreinforced and a short glass-fibre-reinforced polyester-based thermoplastic Xenoy has been investigated. Unreinforced material weathered for 11 months outdoors in Perth, West Australia, exhibited a significant decrease in VR and Jc, whereas little change was shown in y, Hu and the fracture surface morphology. Irradiation for 1000 h by artificial ultraviolet rays upon the unreinforced material caused a considerable increase in Hu and only a slight deterioration in VR of the surface layer. Filling with the short fibres induced an improvement in y and a large reduction in Jc for the unreinforced material. The natural weathering of the reinforced material caused a small reduction in Jc but a large degradation in the slope of the R-curve. It was concluded that the measurement of Jc and the slope of R-curve in combination with VR and Hu was an effective way to study the effects of weathering on engineering plastics. © 1998 Chapman & Hall     

Fracture toughness of thermoset composites reinforced by perfectly bonded impenetrable short fibers

The fracture toughness of brittle thermoset resins could be improved significantly by perfectly bonded tough, short fibers through both crack trapping and bridging effects. In this paper, the crack trapping effect was studied through the analysis of the change of strain energy associated with the crack propagation across a regular array of fibers, and the bridging effect was discussed based on the Andersson–Bergkvist model. The fracture resistance increases with the fiber volume fraction, and is independent of the elastic properties of the matrix, the crack length, and the cross-sectional diameter of the fibers.     

Resistance welding of carbon fibre reinforced thermoplastic composite using alternative heating element

The focus of this work is the use of a metal mesh as an alternative heating element for the joining of carbon fibre fabric reinforced polyetherimide composite laminate. A more homogeneous temperature distribution was generated by the metal mesh at the bonding surface. Glass fibre fabric reinforced PEI (GF/PEI) was used as an electrical insulator between the heating element and adherend laminates. Experimental results show that the GF/PEI prepreg could effectively prevent current leakage and enlarge the welding area. Welding parameters, such as input power level, welding time and pressure, were optimized according to the results of mechanical and microstructure characterization. Mechanical performance of composite specimens joined using metal mesh, in terms of lap shear strength and Mode I interlaminar fracture toughness, was equivalent to that of compression moulded benchmarks. Fracture surfaces of welded specimens showed mostly cohesive failure or intralaminar failure, indicating that good bonding between the PEI matrix and metal mesh was achieved.     

Interfacial bonding and the toughness of carbon fibre reinforced glass and glass-ceramics

The work of fracture of four different carbon fibre reinforced glass and glass-ceramic composites has been measured to determine the effects of the different properties of the components on fracture behaviour. Differences in fracture energies can be explained in terms of the fibre pull-out model and differences in the fibre-matrix interfacial shear bond. The work of fracture of the glass-ceramic is independent of crack velocity while that of the Pyrex matrix composite decreases with increasing velocity at low velocities, the decrease stopping at higher velocities. Work of fracture values agree well with linear elastic fracture mechanics toughness values.     

Improved fracture toughness of carbon fibre/epoxy composite laminates using dissolvable thermoplastic fibres

This paper reports on a novel toughening concept based on dissolvable phenoxy fibres, which are added at the interlaminar region in a carbon fibre/epoxy composite. The composites were prepared by resin infusion of carbon fibre fabric with the phenoxy introduced as a chopped fibre interleaf between the carbon fibre plies. The thermoplastic phenoxy fibre dissolved in the epoxy during curing at elevated temperatures and a phase separated morphology with phenoxy-rich secondary phase was formed upon curing. It was found that the average Mode-I fracture toughness value, G_1c increased tenfold with only 10 wt.% (with regard to the total matrix content) phenoxy fibre added. Other properties such as Young’s modulus, tensile strength and thermal stability were not adversely affected. The mechanical and thermal properties of the neat epoxy–phenoxy blends were also studied for comparison.     

A high-temperature polyimide reinforced with silica fibre

Poly-[N,N-(4,4 oxydiphenylene) pyromellitimide] has been reinforced with unidirectionally aligned silica fibre. A partial pre-cure of the basic laminate before lay-up in the mould successfully prevented porosity and strength degradation in the final composite structure.Samples heated for periods up to 100 h and temperatures up to 400° C in circulating air retained 50 to 90% of their original strength. For all the samples, the dynamic modulus of elasticity was in the range 4.55 to 4.95×10⁵ lb/in² (3.20 to 3.48×10⁵ kg/cm²), the fibre content being 43 to 47.5% by volume. This material incurred a 5% weight loss on heating in air at 350° C for 100 h.     

Fracture toughness of short-fibre reinforced thermoplastics

The failure process of short-fibre reinforced thermoplastics is characterized by different energy dissipation mechanisms, especially by mode II debonding along the fibre/matrix interface, sliding of debonded regions, brittle or ductile matrix fracture and pull-out. It is assumed that these failure processes are acting within a certain zone ahead of the notch tip— the dissipation zone. The modes of energy dissipation are mainly affected by the matrix fracture mode (brittle or ductile) which is mainly determined by the loading rate or temperature conditions. On the basis of an energy principle and relationships for the different energy dissipation mechanisms, we propose theoretical expressions for the static and dynamic fracture toughness and we compare these with experimental results.     

Creep properties of fatigued short carbon fibre reinforced nylon 6 plastics

The effect of low-cycle fatigue on the creep properties of a fibre-reinforced thermoplastic has been examined from the standpoint of the stress dependence and cycle ratio, which is the fraction of the mean life cycles to fracture. The time dependence of the strain and the strain rate of a virgin specimen can be described by a straight line on a logarithmic scale, therefore the deformation process seems to be ruled by a flow law. In addition the time dependence of the strain rate does not indicate the stress dependence for each stress level, which shows the same value as in the gradient of these relations. In the case of fatigued specimens, however, a knee point which divides the creep process into a primary region and a secondary one appears in the creep relation. The gradient in the primary region is less than that for the virgin material, and the magnitude and the timing of its appearance show either a stress dependence or a cycle ratio dependence. A weak stress dependence of the relation for log against logt can also be found; however, the results do not indicate a dependence on the cycle ratio. The reason for these results and the relationship of the structural changes of the materials are discussed using a McLean model.