The fracture behaviour of short glass fibre-reinforced polyoxymethylene

This paper describes an examination of the fracture behaviour of hydrostatically extruded polyoxymethylene. Unfilled and glass-filled extrudates were tested, both with a nominal draw ratio of 8, the highest obtainable for this material using the extrusion process. The compact tension geometry was used for the fracture tests. Samples were loaded perpendicular and parallel to the extrusion direction and tests were carried out at +20 and −50°C. The glass-filled extrudate showed a significant improvement in fracture behaviour compared with the unfilled extrudate, with an increase in the fracture toughness and stable rather than unstable crack propagation. Most interestingly, the crack propagation was still stable at − 50°C.     

Failure and fracture of short glass fibre-reinforced nylon composites

The failure and fracture behaviour of a range of reinforced nylon compounds have been studied using a number of different techniques. Toughness was measured by an instrumented falling weight impact method; fracture toughness was monitored by measurements of yield stress and a critical value for stress field intensity factor; deformational mechanisms were monitored using a volume strain/axial strain function together with the usual tensile stress/strain plot. Materials used in this work differed in type of matrix (nylon 6, nylon 6.6 and a toughened nylon 6.6), type of reinforcement (glass fibres mainly, but also spheres), fibre content (which was varied in the range 0–50 weight %) and type of interfacial bond between fibre and matrix. It is apparent that whilst an individual technique might be adequate for monitoring performance, a combination of methods are needed to provide an understanding.     

Mechanisms of fatigue fracture in short glass fibre-reinforced polymers

Fatigue-crack profiles and fracture surfaces of several short glass fibre-reinforced polymers were examined to gain insight into the mechanisms of cyclic damage and fatigue-crack propagation in these materials. Several distinctly different features were noted between fracture surfaces generated by stable fatigue crack growth and those produced by monotonic or unstable fracture. Among the most significant differences were the higher degree of single and multiple fibre fracture generally observed on stable fatigue-crack growth fracture surfaces, and the variations in the interfacial failure site in well-bonded systems. While the former effect is attributed to the occurrence of crack closure and the build-up of compressive stresses in the crack-tip damage zone during unloading, the differences in the interfacial failure mode are related to the adverse effect of fatigue loading on the interfacial bond strength. No features could be identified that would allow a quantitative correlation between the applied stress intensity factor level or the crack growth rates and characteristic fracture surface details.     

Micromechanisms of compressive failure in a glass fibre-reinforced amorphous thermoplastic

Compressive failure of a 0°/90° glass fibre-reinforced amorphous thermoplastic has been characterized. It was found that the critical event is the nucleation within 90° laminates of multiple shear crazes, which become shear microcracks, transition to axial cracks, and permit the specimen to fail by the flexure of 0° elements. It is shown that the apparent kinetics of this process provide a rationale for the dramatic strain-rate strengthening of these composites at high loading rate.     

Fracture mechanics of short glass fibre-reinforced nylon composite

The fracture behaviour of injection-moulded short glass fibre-reinforced, thermoplastic nylon 6.6 plaques has been studied under static loading using compact tension specimens and under impact loading using single-edge notched charpy specimens. The influences of specimen position as taken from the plaque mouldings, notch direction, notch sharpness and the rate of testing on the fracture toughness of this composite system were investigated. Results indicated that the fracture toughness is highest for the cracks perpendicular to the mould fill direction and is lowest for cracks parallel to the mould fill direction. A single fracture parameter, K _c, seems to be inadequate for fracture toughness characterization. Evaluation of the fracture toughness as a function of notch sharpness indicated that for notches perpendicular to the mould fill direction the fracture toughness is not affected by the sharpness of the initial notch. However, for cracks in the mould fill direction, sharpness of the initial notch had a significant effect upon the measured value of the fracture toughness. Results also indicated, that the fracture toughness is rate insensitive over the crosshead speed ranging from 0.5–50 mm min^–1. Finally, the specimen position, as taken from plaque mouldings, had no significant effect on the measured value of the fracture toughness.     

Structure and mechanical behaviour of short glass fibre-reinforced ethylene-tetrafluoroethylene copolymers

A copolymer of ethylene, tetrafluoroethylene, hexafluoropropylene and perfluoropropylvinylether has been investigated. Uniaxial tensile tests at small strains showed improved mechanical properties, when the copolymer was filled with short glass fibres. This is due to a change in superstructure, as can be seen with small-angle X-ray scattering (dynamic measurements during heating with synchrotron radiation). This results in some kind of bonding between matrix and fibres: scanning electron micrographs of fracture surfaces obtained at temperatures between –192 and +160 °C show the bonding as well as the nucleating influence of the fibres on the matrix in their surroundings. Wide-angle X-ray scattering up to 170 °C (made with a Guinier camera) shows that the crystalline structure itself was not influenced by the fibres and that the crystallites in the copolymer were longitudinally disordered to a high degree.     

Interfacial shear stress in a short glass fibre-reinforced polypropylene sheet

The interfacial shear stress, τ, in an extruded short glass fibre-reinforced polypropylene sheet having good fibre alignment is evaluated using two approaches. The first approach considers the matrix to be elastic and results in τ increasing linearly with composite strain. The second approach assumes that the interfacial shear stress is directly proportional to the composite stress. On this scheme, up to 0·8% composite strain, τ is quantitatively identical to that predicted by the first model and it also increases linearly with composite strain. At higher strains, however, the rate of increase of τ is smaller. It is observec that the dependence of τ on composite strain is very similar to the tensile stress-strain behaviour of the homopolymer extruded sheet. The possible implications of this observation are briefly discussed.     

An experimental analysis of fracture mechanisms of short glass fibre reinforced polyamide 6,6 (SGFR-PA66)

In the present work, toughness of unfilled polyamide 6,6 (PA66) and short glass fibre reinforced polyamide 6,6 (SGFR-PA66) was investigated. Digital image correlation (DIC) was used with a single camera for in-plane displacement field measurement and then strain computation. The results allowed to extract the resistance curve for the PA66 and critical stress intensity factors, K_Ic, for the SGFR-PA66 with three glass fibre contents (15%, 30% and 50% (wt)) and under room temperature (20 °C). The tests were carried out on single edge notched tension (SENT) specimens. The DIC technique allowed to precise the spatial distribution of the local strains in a defined region including the crack tip at different steps of the loading. Scanning electron microscopy observations illustrated different damage mechanisms occurring in the studied composites: matrix crack, fibre–matrix interface failure and fibres pull out.     

Multiaxial fatigue of a short glass fibre reinforced polyamide 6.6 – Fatigue and fracture behaviour

The multiaxial fatigue behaviour of a short glass fibre reinforced polyamide 6.6 (PA66-GF35) is investigated on hollow tubular specimens in the range of fatigue lives between 10² and 10⁷ cycles. Fatigue experiments included pure tension, pure torsion, combined tension–torsion at different biaxiality ratios and phase shifting angles between the stress components. Tests were carried out with load ratio R = 0 and R = −1 at room temperature as well as at 130 °C. The influence of biaxiality ratio, phase angle between load components and load ratio is discussed.An extensive analysis of the fracture behaviour is performed on the specimens to recognise the crack nucleation and propagation mechanisms; failure modes were evaluated via optical and scanning electron microscopy.     

The strength of the fibre-polymer interface in short glass fibre-reinforced polypropylene

A method for the determination of the interfacial bond strength in well-aligned short glass fibre-reinforced polypropylene samples is discussed. The method takes into account the variation of the interfacial shear stress during the deformation process; consequently, it yields very consistent results at all values of the composite strain. The influence of the fibre orientation with respect to the load axis is appropriately considered using macro-mechanical analyses for stiffness and strength of the composite. The method is compared with two other methods reported in the literature.     

Interlaminar shear fracture of chopped strand mat glass fibre-reinforced polyester laminates

The interlaminar shear fracture of chopped strand mat glass fibre-reinforced polyester laminates has been studied both experimentally and analytically. Lap shear (double-grooved) specimens were used to measure the interlaminar shear strength and the cracking mechanism was studied using photomicrography. The finite element method was used to calculate the stress distribution along the shear surface and the mixed-mode stress intensity factors K_I and K_II. The length of the shear surface was found to have a significant effect on the results. Based on the experimental and analytical results, the validity of the British Standard for GRP pressure vessels (BS4994, 1973) was evaluated and the critical stress intensity factors K_Ic and K_IIc for this material were estimated.     

Microstructure and fracture behaviour of short and long fibre-reinforced polypropylene composites

Microstructure and fracture mechanical behaviour of injection-moulded, longer glass fibrereinforced polypropylene (Verton* aspect ratio 320) were studied as a function of fibre volume fraction and compared to that of shorter fibre-filled polypropylene (aspect ratio 70). Toughness was measured using instrumented notched lzod and falling weight impact tests, as well as compact tension specimens. It was found that the addition of longer fibres generally increased the toughness of the material, although more significant increases were seen in the impact tests than were seen in the compact tension test. For the latter results, a correlation between toughness improvement and microstructural details was performed on the basis of the microstructural efficiency concept, a semi-empirical approach of the formK _c,C = (a* +nR)K _c,M, where,K _c,C andK _c,M are the fracture toughnesses of the composite and the matrix, respectively,_a* is a matrix stress correction factor,n is a scaling parameter andR is a fibre reinforcement effectiveness factor. The latter corrects for differences in the composite microstructures, and incorporates effective fibre orientation factors, layering of injection moulded parts, and fibre volumes in the different layers.Nomenclature a crack length - a ^* matrix toughness correction factor - A cross-sectional area - B thickness of the sample plaques - C thickness of the composite core regions - E _peak energy adsorbed up to the maximum force in the impact load-displacement curve - E _t tensile modulus - F _max maximum force in impact force-displacement curves - f _p fibre orientation factor - f _pe effective orientation factor - f _pe,C effective orientation parameter, core region - f _{pe, s} effective orientation parameter, surface region - F critical load in the tensile test load-displacement curves - K _c critical stress intensity factor/fracture toughness - K _L fracture toughness of the composite materials - K _d dynamic fracture toughness - K _L fracture toughness of the matrix - L test with crack parallel to the mould filling direction - M microstructural efficiency factor - n scaling parameter for reinforcement effectiveness factor (energy absorbtion ratio) - R reinforcement effectiveness factor - S thickness of the composite surface regions - T test with crack perpendicular to the mould filling direction - V _f fibre volume fraction - V _m matrix volume fraction (= 1 —V _f) - W specimen width - W _f fibre weight fraction - W _m matrix weight fraction (= 1 —W _f) - X _n number average fibre length - X _v volume average fibre length - Y(a/ W) polynomial correction for compact tension specimens - variable in effective orientation factor formula - variable in effective orientation factor formula - _B strain to break - _c density of the composite - _f fibre density - _m matrix density - _F fracture strength - fibre angle with respect to a reference direction     

Prediction of failure properties of injection-molded short glass fiber-reinforced polyamide 6,6

This study simulates the tensile failure of injection-molded short glass fiber-reinforced polyamide 6,6 (GF/PA66). Tensile tests of unreinforced PA66 are first conducted and the material properties are obtained by fitting a simulated stress–strain curve to the experiment result. Using the obtained material properties, failure simulations of GF/PA66 composites are performed for four types of specimens with various fiber lengths and fiber orientation distributions. In the simulations, multiscale mechanistic model, which can simulate micromechanical damage, and Micromechanics Model (MM), which has very low computational cost, are adapted and the results are compared with experiments. Both models reproduce the experiment results well. Considering the computational cost, MM is the better model for predicting the failure properties of GF/PA66 composites.     

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.     

Recycled carbon fibre-reinforced polypropylene thermoplastic composites

Comingled carbon fibre (CF)/polypropylene (PP) yarns were produced from chopped recycled carbon fibres (reCF) (20 mm in length, 7-8 μm diameter) blended with matrix polypropylene staple fibres (60 mm in length, 28 μm diameter) using a modified carding and wrap spinning process. Microscopic analysis showed that more than 90% of the reCF were aligned along the yarn axis. Thermoplastic composite test specimens fabricated from the wrap-spun yarns had 15-27.7% reCF volume content. Similar to the yarn, greater than 90% of the reCF comprising each composite sample made, showed a parallel alignment with the axis of the test specimens. The average values obtained for tensile, and flexural strengths were 160 MPa and 154 MPa, respectively for composite specimens containing 27.7% reCF by volume. It was concluded that with such mechanical properties, thermoplastic composites made from recycled CF could be used as low cost materials for many non-structural applications.     

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.     

The stiffness and strength of a polyamide thermoplastic reinforced with glass and carbon fibres

It has been established that the optimum degree of mechanical property enhancement by fibre reinforcement of a typical thermoplastic material (polyamide 6.6) is achieved if comparatively long fibres are used, the fibre length required being determined by the properties of the interface between the fibre and the thermoplastic matrix. The extent of stiffness improvement at low strains is described by simple modifications to the law of mixtures to allow for fibre orientation and length. The strength enhancement is limited by an embrittlement effect which reduces the strain to fracture as the stiffness of the composite is improved. The cause of this effect has been identified as matrix crack formation at the ends of the reinforcing fibres. At strains of between 0.5% and 1.0%, according to fibre type, length andV _f, cracks form at the tips of the longest fibres aligned in the straining direction. Subsequently as the strain is increased more cracks form progressively at the ends of shorter and/or more misaligned fibres. It has been shown that initially this cracking can be accommodated by load transfer to adjacent fibres which bridge the cracked region. Final failure occurs when the extent of cracking across the weakest section reaches a critical level when the surrounding fibres and matrix can no longer support the increasing load.     

Experimental study of size effect on quasi-static strengths for short glass fibre-reinforced plastics

The present paper examines size effect on the strength of short glass fibre-reinforced phenolic resin (SGP) composites made by press moulding with different loading modes and specimen shapes. Three- and four-point flexural tests and tension–torsion combined tests were conducted at room temperature in order to evaluate the influence of V_f and loading mode on fracture strength. The obtained uniaxial strength data were analysed using the Weibull statistical theory. The relationship between fracture strength and effective volume was investigated based on the Weibull statistical theory and agreed well with the effective volume theory (EVT), regardless of specimen size, dimensions or loading mode. The experimental results revealed that the tension–torsion multiaxial SGP strength was in agreement with the Tsai–Hill failure criterion. The EVT was also applied to the Tsai–Hill failure criterion to consider the size effect, and the validity of the proposed method was confirmed experimentally.