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.     

Mechanisms for rate effects on interlaminar fracture toughness of carbon/epoxy and carbon/PEEK composites

The objective of this study was to investigate strain-rate dependent energy absorption mechanisms during interlaminar fracture of thermosetting (epoxy) and thermoplastic (PEEK) uni directional carbon fibre (CF) composites. A simple model addressing the translation of matrix toughness to mode I and mode II interlaminar toughness of the composite is presented, in conjunction with a fractographic examination of the fracture surfaces and the fracture process. The observed rate dependency of composite fracture toughness is attributed to the rate dependent toughness of the viscoelastic matrix and the size of the process zone around the crack tip. Other important factors identified are the roughness of the fracture surface and fibre bridging.     

A combined experimental-numerical investigation of crack growth in a carbon–carbon composite

A combined experimental–numerical investigation of crack growth in a carbon–carbon composite is reported. In this material, both matrix fracture and fibre bridging contribute significantly to toughness. Crack growth experiments were performed using side‐notched DCB specimens with doublers. A digital image correlation method was used to measure displacements fields on the specimen surfaces, crack extension and crack opening profiles. An effective cohesive zone law was determined from the experimental data. The effective cohesive zone law is subsequently separated into the individual contributions from matrix cracking and fibre bridging. Numerical simulation of crack growth based on this cohesive zone law and experimental data are in good agreement. Special focus of the numerical study is on the investigation of the discontinuous nature of crack growth.     

Electrical crack length measurement and the temperature dependence of the Mode I fracture toughness of carbon fibre reinforced plastics

The conventional optical crack length measurement in fracture toughness testing is unsuitable for tests carried out in a chamber, like temperature tests. An electrical method has therefore been developed which determines the crack length on the basis of a change of electrical resistivity. Compared with optical measurements this method has proved to be very accurate. With this method the Mode I fracture toughness of a carbon fibre reinforced epoxy composite was determined over the temperature range from −55°C to 120°C. It was found that the fracture toughness rises with increasing temperature. At low temperatures the values were constant or increased slightly, with minimum at about 0°C.     

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.     

Evaluation of the electrical potential drop technique in the determination of crack growth resistance-curves of Carbon/Carbon composites and carbon bonded refractories

Electrical potential drop (EPD) and compliance techniques are compared as techniques for crack length measurement in determining the crack growth resistance-curve (R-curve) of two Carbon/Carbon (C/C) composites and two carbon-bonded oxide-graphite refractories. The two C/C composites differ in the strength of the fibre/matrix interaction, resulting from the use of untreated and surface treated carbon fibres. The refractories differ in the volume fraction of graphite flakes. R-curve measurements on the C/C composites were made on specimens with chevron notches whilst straight-through notches were used for carbon bonded refractories. In the EPD method, the instantaneous crack length was determined from the instantaneous electrical potential across the notch plane, which was recorded in line with load and displacement data, and experimental calibration data. In the compliance method, the instantaneous crack length was determined analytically using the instantaneous load and displacement data. From the EPD technique smaller crack lengths were calculated than from the compliance technique in the regions of fracture where the composites had well developed process zones, and for the whole region in the refractories. The EPD technique underestimates the actual crack length, due to current conduction in the wake zone by bridging fibres/grains, and as a result the R-curves are different from those reported by the compliance technique, which are considered to be more reliable. The compliance-based results are used to establish the effects of fibre surface functionality and graphite flake content on crack growth resistance in the two systems.     

Fracture toughness of short carbon fibre reinforced thermoplastic polyimide

The fracture toughness testing of short fibre reinforced thermoplastic materials were performed. Materials tested were the polyimide resin and also that reinforced with 20 wt% or 30 wt% short carbon fibre. For introducing the initial crack, the tapping method, the sliding method and the bridge indentation method were examined. Among them, the sliding method was found to be effective for every case. The fracture tests were conducted by the three-point bending test with several loading rates. Stable crack growth was observed for the neat material while unstable fracture occurred for the reinforced materials. The critical values of the stress intensity factor at crack initiation were greater for the reinforced materials than for the neat resin. The fracture toughness of the 30 wt% reinforced material was independent of loading rate while that of 20 wt% reinforced material increased with loading rate. In order to investigate the fracture mechanisms, fractographic observations were also performed.     

The role of secondary carbide precipitation on the fracture toughness of a reduced carbon white iron

The fracture toughness of a high chromium, reduced carbon white cast iron was measured using the K_Ic fracture toughness test. The toughness was found to increase with increasing heat treatment temperature for the temperature range of 1273–1423 K. Increases in the fracture toughness were due to crack deflection into the dendritic phase. Cracking in the dendrites was promoted by the presence of secondary carbides which formed during the high temperature heat treatment employed. The characteristic distance for brittle fracture as calculated by the Ritchie–Knott–Rice model correlated well with the centre to centre mean free path of the secondary carbides on the fracture plane.     

Conditions for matrix crack deflection at an interface in ceramic matrix composites

This paper describes a numerical approach developed to simulate the mechanism of matrix crack deflection at the fibre/matrix interface in brittle matrix composites. For this purpose, the fracture behaviour of a unit cell (microcomposite) consisting of a single fibre surrounded by a cylindrical tube of matrix was studied with the help of a finite element model. A fracture mechanics approach was used to design a criterion for deflection at the fibre/matrix interface of an annular crack present in the matrix. The analysis of the fracture behaviour of SiC/SiC and SiC/glass ceramics microcomposites shows that the introduction of a low modulus and low toughness interfacial layer at the fibre/matrix interface (e.g. a carbon coating) greatly favours matrix crack deflection at the interphase/fibre interface.     

Stacking sequence effects on fracture of notched carbon fibre/epoxy composites

The purpose of this study was to investigate the ability of the so-called damage zone model (DZM) to predict the influence of stacking sequence on the strength of notched carbon fibre/epoxy composites. The DZM is in essence based on the unnotched tensile strength, σ₀, and the apparent fracture energy, G_c^*, and the damage zone is modelled as a crack with cohesive forces acting on the crack surfaces. The DZM predicts fracture loads for three-point bend (TPB) specimens and specimens with circular holes quite accurately. As an attempt to explain the difference in strengths, the damage zone extension in the TPB specimens with different stacking sequence was examined.     

The environmental fatigue behaviour of carbon fibre reinforced polyether ether ketone

The fatigue behaviour of carbon fibre/PEEK composite is compared with that of carbon/ epoxy material of similar construction, particularly in respect of the effect of hygrothermal conditioning treatments. Laminates of both materials were of 0/90 lay-up, and they were tested in repeated tension at 0° and at 45° to the major fibre axis. The superior toughness of the polyether ether ketone and its better adhesion to the carbon fibres results in composites of substantially greater toughness than that of the carbon/epoxy material, and this is reflected in the fatigue behaviour of the carbon fibre/PEEK. The tougher PEEK matrix inhibits the development of local fibre damage and fatigue crack growth, permitting a 0/90 composite with compliant XAS fibres to perform as well in fatigue as an epoxy laminate with stiffer HTS fibres. Hygrothermal treatments have no effect on the fatigue response of either material in the 0/90 orientation. The fatigue response of a cross-plied carbon/PEEK laminate in the ±45° orientation is much better than that of equivalent carbon/epoxy composites, again because the superior properties of the thermoplastic matrix.     

Stable crack growth,instability, fatigue and fracture of a 50/50 blend of poly(2,6-dimethyl-1,4-phenylene oxide) and polystyrene

The fracture behaviour of a 50/50 blend of poly(2,6-dimethyl-1,4-phenylene oxide)/poly-styrene has been studied. The crack propagation behaviour is strongly influenced by the temperature, crack driving force and the nature of the crack tip craze zone. A fracture map outlining the regions of stable crack growth as a function of temperature, crack velocity and crack driving force has been determined. At high temperatures and low crack growth velocities, stable crack propagation proceeds through a single-craze crack tip damage zone, while at lower temperatures and high crack velocities, a multiple-craze crack tip zone is observed. Corresponding behaviour can be observed under fatigue loading conditions. An instability leading to very high-speed fracture occurs at a critical crack velocity, thus limiting the stable crack propagation regime to lower velocities. The various reported measures of fracture toughness, such as those based on crack initiation, peak load and the onset of crack instability, are discussed.     

Influence of silane coupling agents on interlaminar fracture in glass fibre fabric reinforced unsaturated polyester laminates

The relationship between the adhesive properties of the interphase of glass fibre/resin and the resultant composite Mode I delamination fracture toughness in glass fibre fabric laminate (GFFL) was studied. The Mode I interlaminar fracture toughness of GFFL was obtained by using a double cantilever beam (DCB) specimen. The delamination resistance of GFFLs which have two silane coupling agents and three concentration finishes is discussed on the basis of interlaminar fracture toughness. The crack propagation behaviour of DCB testing was mainly divided into stable and unstable manners. The fracture toughness and the crack propagation behaviour were dependent on the types and concentration of silane coupling agents.     

Effects of thickness and environmental temperature on fracture behaviour of polyetherimide (PEI)

The fracture behaviour of a polyetherimide (PEI) thermoplastic polymer was studied using compact tension (CT) specimens with a special emphasis on effects of specimen thickness and testing temperatures on the plane strain fracture toughness. The results show that the valid fracture toughness of the critical stress intensity factor, K _IC, and strain energy release rate, G _IC, is independent of the specimen thickness when it is larger than 5 mm at ambient temperature. On the other hand, the fracture toughness is relatively sensitive to testing temperatures. The K _IC value remains almost constant, 3.5 MPa in a temperature range from 25 to 130°C, but the G _IC value slightly increases due to the decrease in Young's modulus and yield stress with increasing temperature. The temperature dependence of the fracture toughness, G _IC, was explained in terms of a plastic deformation zone around the crack tip and fracture surface morphology. It was identified that the larger plastic zone and extensive plastic deformation in the crack initiation region were associated with the enhanced G _IC at elevated temperatures.     

Process and mechanical properties of carbon/carbon–silicon carbide composite reinforced with carbon nanotubes grown in situ

A hierarchical C_f/C–SiC composite was fabricated via in situ growth of carbon nanotubes (CNTs) on fiber cloths following polymer impregnation and pyrolysis process. The effects of CNTs grown in situ on mechanical properties of the composite, such as flexural strength, fracture toughness, crack propagation behavior and interfacial bonding strength, were evaluated. Fiber push-out test showed that the interfacial bonding strength between fiber and matrix was enhanced by CNTs grown in situ. The propagation of cracks into and in fiber bundles was impeded, which results in decreased crack density and a “pull-out of fiber bundle” failure mode. The flexural strength was increased while the fracture toughness was not improved significantly due to the decreased crack density and few interfacial debonding between fiber and matrix, although the local toughness can be improved by the pull-out of CNTs.     

Effects of interfacial coating and temperature on the fracture behaviours of unidirectional Kevlar and carbon fibre reinforced epoxy resin composites

The enhancement of transverse fracture toughness of unidirectional Kevlar and carbon fibre reinforced epoxy resin composites (KFRP and CFRP) has been studied using polymer coatings on the fibres. The results obtained show a substantial improvement in the impact fracture toughness of both KFRP and CFRP with polyvinyl alcohol (PVAL) coating without any loss of flexural strength; but there is only a moderate increase in impact toughness with other types of coating (i.e. carboxyl-terminated butadiene acrylonitrile (CTBN) copolymer and polyvinyl acetate (PVA)) with some reduction in flexural strength. The dependence of impact fracture toughness of these composites (with and without PVAL coating) on temperature was analysed on the basis of existing theories of toughening mechanisms from measurements of fibre-matrix interfacial properties, debond and fibre pull-out lengths and microscopic observations. The beneficial effect of fibre coating with PVAL on transverse fracture toughness is shown to sacrifice little damage tolerance of the composites against delamination fracture.     

Effect of residual stress on cleavage fracture toughness by using cohesive zone model

This study presents the effect of residual stresses on cleavage fracture toughness by using the cohesive zone model under mode I, plane stain conditions. Modified boundary layer simulations were performed with the remote boundary conditions governed by the elastic K‐field and T‐stress. The eigenstrain method was used to introduce residual stresses into the finite element model. A layer of cohesive elements was deployed ahead of the crack tip to simulate the fracture process zone. A bilinear traction–separation‐law was used to characterize the behaviour of the cohesive elements. It was assumed that the initiation of the crack occurs when the opening stress drops to zero at the first integration point of the first cohesive element ahead of the crack tip. Results show that tensile residual stresses can decrease the cleavage fracture toughness significantly. The effect of the weld zone size on cleavage fracture toughness was also investigated, and it has been found that the initiation toughness is the linear function of the size of the geometrically similar weld. Results also show that the effect of the residual stress is stronger for negative T‐stress while its effect is relatively smaller for positive T‐stress. The influence of damage parameters and material hardening was also studied.     

Interlaminar fracture (model II) of commingled yarn-based GF/PP composites

A 5050 wt % mixture of commingled glass/polypropylene fibre system was selected to study the correlations between the morphological details, mode II interlaminar fracture toughness and corresponding failure mechanisms. Mode II interlaminar fracture tests were performed by using the end-notched flexure test procedure. Compared to conventional composite laminates, mode II interlaminar crack extension in these commingled yarn-based composites was very stable, and extensive fibre nesting occurred along the main crack plane. Crack jumping and non-broken matrix links were observed.R-curve behaviour for these materials was identified and the toughness for initiation was much lower than that for propagation. Compared to mode I interlaminar fracture toughness, similar trends in effects of cooling rates and isothermal crystallizations on mode II interlaminar fracture toughness were observed. However, the effects were not as significant as those found for mode I interlaminar fracture toughness.Alexander von Humboldt Fellow.     

Slow crack growth in cellulose fibre cements

Slow stable crack growth is a prominent feature of the fracture behaviour of cellulose fibre cements. It is shown that this characteristic can be described by crack growth resistance against crack extension curves based on linear elastic fracture mechanics. Double-cantilever-beam specimens with side grooves are used to obtain such crack resistance curves for a commercial cellulose cement containing approximately 8% mass fraction of bleached fibres. Both dry and wet samples are tested. Compliances measured during slow crack growth by the unloading/reloading technique at successive crack increments are less than those obtained for saw-cut notches with similar crack lengths. Residual displacements due to either mismatch fracture surfaces or a large inelastic process zone at the crack tip are also observed at zero load. A modified elastic potential energy release rate (G _R ^* ), and hence its equivalentK _R ^* [= (EG _R ^* )^1/2], must be used to include this residual displacement effect in order to yield the true crack growth resistance curves. This is found to be necessary for the wet samples due to their large residual displacements. The crack growth resistances of the wet samples are superior to those of the dry samples: this is explained in terms of the improved ductility and toughness of the wet cellulose fibres.     

Toughness of aromatic polymer composites reinforced with carbon fibres

This experimental study focuses on the toughness of a thermoplastic composite, namely, poly ether-ether-ketone (PEEK) reinforced with 60% by volume of continous carbon fibres (APC 2). Toughness is assessed using both comparative and intrinsic techniques and a critical discussion of the two approaches is presented.The comparative toughness of cross-ply and quasi-isotropic sheets of APC 2 is studied using a damage tolerance test (compression after impact) and by using an instrumented falling weight impacr test over a range of temperatures. Intrinsic toughness is discussed by applying fracture mechanics techniques to unidirectional laminates. Double cantilever beam and three-point flexure tests are used, the latter being performed in six different crack directions. Fracture toughness results are presented for APC 2 and unreinforced PEEK.An ultrasonic C-scan on impacted specimens and scanning electron microscopy on fracture surfaces are used to explore further the mechanisms of fracture, e.g. delamination, fibre breakage and matrix cracking.