Fatigue Propagation of Fibre-Bridged Cracks in Unidirectional Polymer-Matrix Composites

In order to improve the fatigue resistance of polymer-matrix composites by materials design, or to conceive micromechanics based models for life predictions, the underlying micromechanisms must be understood. Experimental investigations have revealed fibre-bridged cracking as a toughening micromechanism that retards further fatigue crack growth in a unidirectional 0° carbon-fibre-reinforced epoxy. The bridging fibres exert a closing traction on the crack surfaces, thereby reducing the driving force for crack growth. In this study, the growth of bridged cracks has been quantified by a surface replication technique. The da/dN–K curve defined in terms of nominal stress-intensity factors shows a crack retarding behaviour. The crack growth curve can be replotted in terms of the effective stress-intensity factor where the contribution of the cohesive crack surface forces from the bridging fibres are taken into account. This curve falls somewhat closer to that of the neat matrix material, but the difference is still considerable, and it shows a decelerating propagation. Therefore, there must be other active toughening mechanisms besides fibre bridging, that slow the crack propagation down, and account for the fatigue resistant behaviour of the tested material. Ways by microstructural design to promote the fatigue resistant mechanisms are discussed.     

Effect of Crack Propagation Directions on the Interlaminar Fracture Toughness of Carbon/Epoxy Composite Materials

The interlaminar fracture toughness of carbon/epoxy composite materials has been studied under tensile and flexural loading using width-tapered double cantilever beam (WTDCB) and end-notched flexure (ENF) specimens. This study experimentally examines the effect of various interfacial ply orientations, (0°, 45° and 90°) and crack propagation directions, (0°, 15°, 30° and 45°) in terms of the critical strain energy release rate. Twelve differently layered laminates were investigated. The fracture energy is deduced from the data according to the compliance method and beam theory. Beam theory is used to analyze the effect of crack propagation direction. The geometry and lay-up sequence of specimens are designed to probe various conditions such as skewness parameter and beam volume. Results show that fiber bridging occurred due to non-midplane crack propagation; this causes the difference in fracture energy calculated by both methods. For the construction of safer and more reliable composite structures, we obtain the optimal stacking sequence from the initial fracture energy in each mode.     

Analysis of a compressive shear test for adhesion between elastomeric polymers and rigid substrates

A compressive shear test for investigating adhesion between an elastomeric polymer and a rigid substrate has been studied. The test consists of loading a specimen comprising of a 3-ply laminate: substrate/polymer/ substrate, in compression and shear at a specified angle to the loading direction. Under displacement control and when adhesion is sufficiently low, an interfacial crack nucleates at one interface early during loading and propagates stably up to a critical load at which unstable propagation with an associated load drop ensues. The case of an isothennal hyperelastic material has been analyzed by computing the energy release rate for an interfacial crack as a function of crack length. The analysis shows that for a range of initial crack size interfacial crack propagation is stable until crack length reaches a critical size at which unstable propagation ensues. The energy release rate at this instability is relatively insensitive to angle of loading, strain, and hyperelastic parameters, which allows one to extract an interfacial toughness, ₀, from overall measurement of stress and strain. The analysis has been extended to consider combined hyperelasticity and viscoelasticity by using a cohesive zone model for crack propagation implemented as a cohesive finite element. The energy release rate and cohesive zone analyses give identical results for an hyperelastic material. For a viscoelastic-hyperelastic material, the cohesive zone approach allows the viscous losses in the bulk polymer to be estimated separately from the value of interfacial fracture toughness. Both analyses have been applied to experiments on glass/polyvinyl butyral (Butacite®)/glass laminate specimens. The intrinsic interfacial toughness, consisting of contributions from bond rupture and a near-tip process zone, is found to be rate-dependent and lies in the range 50–200 J m^–2.     

FATIGUE CRACK GROWTH BEHAVIOUR OF FIBRE-METAL LAMINATE GLARE-1 AND METAL LAMINATE 7475 WITH DIFFERENT BLUNT NOTCHES

Abstract— The fatigue crack growth behaviour of the fibre metal laminate "GLARE-1" has been investigated for different blunt notches in Constant Amplitude (CA) tests. In order to investigate the influence of the fibres, the same laminate material but containing no fibres (Laminate 7475) was also tested. The fatigue crack growth properties of GLARE-1 are superior to those of Laminate 7475. GLARE-1 shows lower crack growth velocities at the same K_nom values and in addition the crack growth rates decrease with increasing crack length. The Laminate 7475 shows typical metal behaviour for single crack propagation and accelerating crack growth with increasing crack length. In GLARE-1, multiple crack propagation takes place. The cracks propagate independent of each other and have similar crack growth rates, in part due to closure effects caused by the unbroken fibre layers.
The crack growth rates of specimens having a small root radius are higher in both materials than in specimens with a large notch radius. In GLARE-1, the superiority of a larger notch radius is more pronounced than in the Laminate 7475 and is attributed to a stronger crack closure effect owing to fibre bridging. The reason for the higher bridging capability in specimens containing larger notches is that less fibres are broken or damaged in the notch vicinity.     

Fracture and fatigue of discontinuously reinforced copper/tungsten composites

The strength, toughness and resistance to cyclic crack propagation of composites consisting of copper reinforced with short tungsten wires of various lengths have been studied and the results compared with the behaviour of continuously reinforced composites manufactured by the same method, i.e. by vacuum hot-pressing. It has been found that whereas the resistance to fatigue crack growth of continuously reinforced composites is very similar to that of continuous Al/stainless steel composites reported elsewhere, the addition of short fibres completely changes the mode of fracture, and no direct comparisons are possible. In effect, short fibres inhibit single crack growth by causing plastic flow to be distributed rather than localized, and although these composites are much less strong than continuous fibre composites, they nevertheless have much greater fatigue resistance. The fracture toughness of the composites is thought to be derived simply from the separate contributions of matrix and fibre plastic flow and, in composites containing fibres near to the critical length, from the very substantial work of fibre pull-out.     

Short thermal fatigue crack propagation behavior of alumina short fibre reinforced aluminum matrix composites

Thermal fatigue resistance is one of the most important parameters to design engine materials. The thermal fatigue crack growth behavior of alumina short fibre (V _f = 18 vol.%) reinforced AlSi12CuMgNi aluminum alloy composite has been investigated under thermal cycling condition between room temperature and 280 °C. Initiation and propagation of thermal fatigue crack have also been discussed. The results show that in the range of short crack, the fibres play an important role in the path of thermal fatigue crack, and the crack propagation rate of composites is much larger than that of the matrix alloy.     

Fatigue behaviour of carbon fibre-reinforced aluminium laminates

A new hybrid composite (CARALL), consisting of thin layers of carbon fibre/ epoxy prepreg sandwiched between aluminium sheets, has been developed. It is shown that this class of materials offers higher modulus, higher tensile strength and lower density than 2024-T3 alloy in the longitudinal direction. Under tension-tension fatigue loading, the hybrid laminates showed superior fatigue crack propagation resistance in the longitudinal direction, which may be attributed to the bridging effect imposed by the intact fibres in the crack wake. It has also been shown that the effectiveness of fatigue crack growth reduction increases with the thickness of the carbon fibre/epoxy layer. The resistance to fatigue crack propagation can be further improved by introducing compressive residual stresses in the aluminium layer by postcure stretching the laminate in the plastic region of the aluminium alloy.     

The influence of stress intensity and microstructure on fatigue crack propagation in ferritic materials

New and published fatigue crack growth data for a wide range of steels have been categorized in terms of different growth mechanisms, namely striation formation, microcleavage, void coalescence and intergranular separation. General principles emerged concerning the influence of mean stress, specimen thickness, flow stress and toughness on rates of fatigue crack propagation through their effect on growth mechanism.

Crack propagation rates associated with striation formation were insensitive to changes in mean stress (except at very low stress intensities) and specimen thickness. Increase in flow stress resulted in a small decrease in growth rate, although the path of a crack through complex structures like welds was, nevertheless, strongly influenced by plastic relaxation. Crack propagation rates increased when deformation led to net-section yielding (general yielding) and the increase was related to specimen thickness and geometry. It has been shown that simple relationships between the rate of propagation and alternating stress intensity are adequate for describing fatigue crack growth by the striation mechanism.

Departures from exclusively striation formation to include micro-cleavage, void coalescence or intergranular separation were found to result in accelerated growth rates. Where growth occurred by combined striation formation and microcleavage, the increase in fatigue crack growth rate was dependent on the maximum tensile stress and hence on the mean stress and specimen thickness. Similarly, when fatigue involved the void coalescence mechanism the rate was increased by raising the mean stress. The role of microstructure and fracture toughness in promoting the different growth mechanisms is discussed. Modification of the simple growth law is necessary in order to describe the observed results.     

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.     

A MODEL TO PREDICT THE FATIGUE LIFE OF FIBRE-REINFORCED TITANIUM MATRIX COMPOSITES UNDER CONSTANT AMPLITUDE LOADING

A model based on micro-mechanical concepts has been developed for predicting fatigue crack growth in titanium alloy matrix composites. In terms of the model, the crack system is composed of three zones: the crack, the plastic zone and the fibre. Crack tip plasticity is constrained by the fibres and remains so until certain conditions are met. The condition for crack propagation is that fibre constraint is overcome when the stress at the location of the fibre ahead of the crack tip attains a critical level required for debonding. Crack tip plasticity then increases and the crack is able to propagate round the fibre. The debonding stress is calculated using the shear lag model from values of interfacial shear strength and embedded fibre length published in the literature. If the fibres in the crack wake remain unbroken, friction stresses on the crack flanks are generated, as a result of the matrix sliding along the fibres. The friction stresses (known as the bridging effect) shield the crack tip from the remote stress, reducing the crack growth relative to that of the matrix alone. The bridging stress is calculated by adding together the friction stresses, at each fibre row bridging the crack, which are assumed to be a function of crack opening displacement and sliding distance at each row. The friction stresses at each fibre row will increase as the crack propagates further until a critical level for fibre failure is reached. Fibre failure is modelled through Weibull statistics and published experimental results. Fibre failure will reduce the bridging effect and increase the crack propagation rate. Calculated fatigue lives and crack propagation rates are compared with experimental results for three different materials (32% SCS6/Ti-15-3, 32% and 38% SCS6/Ti-6-4) subjected to mode I fatigue loading. The good agreement shown by these comparisons demonstrates the applicability of the model to predict the fatigue damage in Ti-based MMCs.     

The effect of constraints due to fibres on the delamination of composites

Stress criteria and energy criteria for crack propagation are examined. It is shown that attempts to improve the delamination resistance of composites are inevitably hampered by fibre constraints on matrix yield and flow. The toughness of the matrix in the absence of fibres is very roughly equal to twice the product of the matrix yield stress, the strain in the plastic zone, , and the thickness of the yield zone at the crack faces, t. For ductile matrices with a given yield stress, then, it is probable that toughness is very roughly proportional to t (i.e. ductile matrices are expected to have fairly uniformly high values of ). However, when fibres are present (e.g. in a laminate) they severely restrict t, and hence the resistance to delamination. While t is less than the interfibre spacing, making the matrix tougher through increasing t directly affects composite toughness. Hence the resistance of the composite to delamination is directly proportional to matrix toughness. However, when t becomes so large as to equal the interfibre spacing, the development of matrix toughness is inhibited, and composite delamination resistance is little affected by further increases in matrix toughness. At this stage, additional increases in delamination resistance depend on increasing the matrix yield stress, rather than increasing the toughness of the matrix.     

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.     

Fracture resistance of a glass-fibre reinforced rubber-modified thermoplastic hybrid composite

Toughening mechanisms in a hybrid amorphous thermoplastic composite containing both distributed rubber particles and rigid glass fibres have been investigated. Tensile properties were measured for a range of materials with varying rubber particle and glass-fibre contents, and different rubber particle sizes. Fracture toughness was characterized by separating the overall fracture into its initiation and propagation components. Deformation and fracture modes at crack tips were optically characterizedin situ during loading. The results indicate that both initiation and propagation toughness are enhanced by rubber particle additions to the glass-fibre reinforced composite. Synergistic effects between glass fibres and rubber particles are identified: for example, glass fibres inhibit crazing at rubber particles, and rubber particles tend to promote crazing at fibre/matrix interfaces and also void initiation at fibre ends. Toughening mechanisms are discussed in the light of available models.     

Interlaminar fatigue crack growth of cross-ply composites under thermal cycles

The fatigue growth of a fiber reinforced composite laminate was characterized under thermal cycling using a combined experimental and computational investigation. Twenty-four ply composite laminates ([0°₁₂/90°₁₂]) are fabricated with a pre-existing delamination, and subjected to thermal cycling in an environmental chamber. The large mismatch in the coefficients of thermal expansion is used to grow an interlaminar crack at the interface of the 0° and 90° laminae. This thermal fatigue crack growth behavior is investigated for different amplitudes of temperature change (ΔT = 30–140 °C). The inspection of fracture surfaces, after completion of the fatigue tests, reveals an angled or kinked crack front growth with greater propagation distances near the free-surfaces/edges. Due to the non-uniform crack growth across the specimen thickness, three-dimensional finite element analyses are performed to investigate the fatigue growth mechanisms under thermal load. From the analysis, the energy release rate as well as the mixed-mode stress intensity factors is calculated and the variations of these fracture parameters are found to be consistent with the observed crack front configuration. Using the computed results, the experimentally measured crack growth rates are also correlated with the amplitude of energy release rate, and a power law form of the fatigue law is established. The relevant coefficients as well as the threshold energy release rate are also determined. The present analysis is useful for not only understanding the fatigue delamination mechanisms under thermal cycling but also for estimating the threshold temperature variation that is needed to drive crack growth.     

Toughness and fatigue of encapsulation-processed silicon carbide-polymer matrix particulate composites

A new process for composite fabrication was developed which improves distribution of the particulate reinforcing phase by polymer encapsulation of the particulate prior to consolidation. The effect of such processing on the fatigue-crack propagation and fracture toughness behaviour of particulate thermoplastic composites was investigated. Composites of several particulate size ranges were fabricated into disc-shaped, compact tension specimens and tested under cyclic and monotonie loading conditions. For comparison, a composite was also fabricated using a standard casting technique. The observed fatigue-crack growth rates spanned three orders of magnitude (10^–11 to 10^–9 m per cycle) over an applied stress intensity range, K, of 0.3 to 1.1 MPa m^1/2. The measured fracture toughness values ranged from 0.69 to 2.95 MPa m^1/2. Comparison of the two processing techniques indicated that encapsulation processing increased the fracture toughness of the composite by approximately 33%; however, the fatigue-crack growth behaviour was unaffected. In addition, a trend of increasing crack growth resistance (toughness) with increasing reinforcement particle size was observed. These results are discussed in the light of crack shielding and bridging models for composite toughening.     

Interlaminar Fracture Toughness of CF/PEI and GF/PEI Composites at Elevated Temperatures

An experimental study has been conducted to assess temperature effects on mode-I and mode-II interlaminar fracture toughness of carbon fibre/polyetherimide (CF/PEI) and glass fibre/polyetherimide (GF/PEI) thermoplastic composites. Mode-I double cantilever beam (DCB) and mode-II end notched flexure (ENF) tests were carried out in a temperature range from 25 to 130°C. For both composite systems, the initiation toughness, G _IC,ini and G _IIC,ini, of mode-I and mode-II interlaminar fracture decreased with an increase in temperature, while the propagation toughness, G _IC,prop and G _IIC,prop, displayed a reverse trend. Three main mechanisms were identified to contribute to the interlaminar fracture toughness, namely matrix deformation, fibre/matrix interfacial failure and fibre bridging during the delamination process. At delamination initiation, the weakened fibre/matrix interface at elevated temperatures plays an overriding role with the delamination growth initiating at the fibre/matrix interface, rather than from a blunt crack tip introduced by the insert film, leading to low values of G _IC,ini and G _IIC,ini. On the other hand, during delamination propagation, enhanced matrix deformation at elevated temperatures and fibre bridging promoted by weakened fibre/matrix interface result in greater G _IC,prop values. Meanwhile enhanced matrix toughness and ductility at elevated temperatures also increase the stability of mode-II crack growth.     

ELASTIC BRIDGING FOR MODELING FATIGUE CRACK PROPAGATION IN A FIBER-REINFORCED TITANIUM MATRIX COMPOSITE

Abstract— A model based upon linear elastic bridging and fiber crack tip shielding is proposed for predicting fatigue crack growth in a SCS-6/Ti-6–4 composite. The model is characterized by the fiber/matrix debond length rather than the fiber/matrix interfacial frictional shear strength used in most current fatigue models. Finite elements combined with fracture mechanics are applied for computing the local stress intensity. The local stress intensity in the matrix is then utilized to predict crack growth in the composite via comparison to monolithic fatigue crack propagation data for a similar Ti-6–4 matrix material.     

Dynamic crack propagation in rubber-modified composite models

The dynamic crack propagation behaviour of several rubber-modified composite models has been studied. In all cases the method of high speed photography along with the method of dynamic caustics was used. Results of crack propagation mode observation, fracture toughness and crack propagation velocity measurements are presented here. Especially in the case of two complex inclusions it was found that the crack propagation mode is highly rate dependent. At low test rates the crack growth tends to follow an almost straight crack path while an increase in strain rate in general results in the formation of a kink in the interparticle area. In the same area a crack propagation delay, and in some cases arrest, was observed while both the crack propagation velocity v and dynamic stress intensity factor K _i ^d showed an intense variation. For the sake of comparison, specimens with one and/or two press-fitting inclusions as well as with two holes were fractured under dynamic loads. In all cases both qualitative and quantitative results were obtained.     

The molecular weight dependence of fatigue crack propagation in polycarbonate

Fatigue crack propagation has been studied in polycarbonate as a function of specimen thickness and molecular weight. It was found useful to estimate the relative contribution of the shear lips and the plane strain crazing mode to fatigue crack propagation. In addition to measuring the width of the shear lips, the craze in the central region of the crack tip was examined in an optical microscope. The shear lip contribution was found to be particularly important at high values of the range of the stress intensity factor K, and was appreciable in all but the thickest specimens and the lowest values of K.As in the case of fracture toughness, the fatigue crack propagation behaviour of polycarbonate is greatly affected by the molecular weight of the polymer. This is due to changes in both the shear lip contribution and the plane strain craze contribution. Because of the complicated nature of the failure mode it is suggested that the application of the Paris equation to the fatigue crack propagation of polycarbonate will be of limited significance.