Comparison of Interlaminar Fracture Toughness in Unidirectional and Woven Roving Marine Composites

This paper investigates the effect of fibre lay-up and matrix toughness on mode I and mode II interlaminar fracture toughness (G_Ic and G_IIc) of marine composites. Unidirectional and woven roving fibres were used as reinforcements. Two vinyl ester resins with different toughness were used as matrices. Results from both modes showed toughness variation that is consistent with matrix toughness. Values of G_Ic were not significantly influenced by fibre lay-up except at peak load points in the woven roving/brittle-matrix composite. Each peak load point, caused by interlocked bridging fibres, signified the onset of unstable crack growth. For unidirectional specimens, crack growth was stable and G_Ic statistically more reliable than woven roving specimens, which gave fewer G_Ic values due to frequent unstable crack growth. Mode II tests revealed that, except for crack initiation, G_IIc was higher in woven roving composites. This was due to fibre bridging, perpendicular to the crack growth direction, which encouraged stable crack growth and increased energy absorption. Mode II R-curves were obtained for the woven roving specimens. These R-curves provide additional information useful for characterising delamination resistance. The paper concludes that composites with woven roving fibres show similar mode I delamination characteristics to the unidirectional composites; but their mode II delamination characteristics, after crack initiation, are quite different.     

A method to determine the fracture toughness of unidirectional fibre reinforced composites in mode II (forward shear), using a thin tubular specimen

A method has been developed, using thin tubular specimens, to determine the fracture toughness of unidirectional fibre reinforced composites in Mode II. The tubular specimens were loaded under torsion and hence produced pure shear at the crack tips located on the circumference of the tube. The cracks were made parallel to the transverse axis and in the mid-length of the tube. Calibration factors for Mode II were obtained. The stress-intensity factors at instability, K_IIR(INS) were obtained by experiments on thin tubular specimens through a compliance matching procedure. The crack growth resistance at instability and the corresponding initial strain energy release rates were independent of the initial crack in the range of crack length investigated. The stress-intensity factor obtained by the thin torsion tube method is slightly higher than the stress-intensity factor at instability, K_IIR(INS) obtained by the method developed by Giare for end cracked beams [Engng. Fracture Mech. 20, 11–21 (1984)]. This method may be applied to a different geometrical shapes and hence may be useful in determining the fracture toughness of any closed geometrical sections.     

Mixed mode fracture behaviour of steel fibre reinforced concrete

In this paper, results are reported for a series of discrete end hooked and straight fibre pullout tests subjected to mixed mode action with the results compared to that of discrete fibres pulled out in Mode I (tensile) and Mode II (shear) fracture. As has been previously observed from Modes I and II fracture tests, the snubbing effect dominates the behaviour of fibres at large fibre bending angles. At large fibre bending angles, considerable slip and crack separation occurred prior to the fibres being engaged in taking load and fibres that are inclined close to the cracked surface are ineffective in carrying load. The results of the test were compared with the fibre engagement and bond stress models in the Unified Variable Engagement Model (UVEM). A good correlation is observed for the UVEM model with the test data and provides further confirmation of the validity of the UVEM model to predict the mix mode fracture of steel fibre reinforced concrete.     

Microfailure behaviour of randomly dispersed short fibre reinforced thermoplastic composites obtained by direct SEM observation

The microfailure behaviour of thermoplastic polyamide 6,6 composites reinforced with randomly dispersed short glass fibres was studied. Scanning electron microscopy was carried out on the surface of the composites under load to observe directly the behaviour. The microfailure proceeds following the steps (1) interfacial microfailure occurs at the fibre tips, (2) the microfailure propagates along the fibre sides, (3) plastic deformation bands of the matrix occurs from the interfacial one, (4) crack opening occurs in the band and the crack grows slowly through the band, (5) finally a catastrophic crack propagation occurs through the matrix with pulling-out fibres from the matrix. A model for the microfailure mechanism of the composites is proposed and some methods to improve the mechanical properties of the composites are discussed on the basis of the mechanism.     

Compressive maximum shear crack initiation and propagation

A maximum shear crack γ (a Mode II shear crack along the maximum shear direction associated with the crack tip shear displacement) was produced successfully in a so-called compressive maximum shear (CMS) specimen. This specimen was specificially designed to produce a compressive maximum shear failure which is one of two mechanisms widely believed to be responsible for limiting bearing fatigue life in rolling contact. The fracture initiation stress (or crack nucleation stress) σ_c and the upward crack propagation rate (toward the loading surface) $\text{dli}\text{dσi}$ per unit cyclic compressive stress increment were determined for the 52100 steel. These parameters were measured at two cleanness levels (DE and CEVM) [DE: basic electric arc furnace melted plus vacuum degassed. CEVM: Consumable electrode vacuum melted] and two tempered hardness levels, RC61 and 51. The possibility of determining K₁₁ for ith cycle was also elucidated. The formation of tail cracks and parallel multiple cracks as fine structure of CMS cracks can be well expounded by the concept of restoring tensile stresses and the residual shear stress relaxation at the CMS crack tip. The fracture mechanism advanced here can explain the formation of similar tail cracks and parallel multiple cracks frequently observed along the inclined shear cracks existing in the subsurface regions of rolling     

Mode I interlaminar fracture toughness properties of advanced textile fibreglass composites

The Mode I interlaminar fracture toughness properties of vinyl ester-based composites reinforced with fibreglass manufactured by the advanced textile technologies of braiding, knitting, stitching and through-the-thickness weaving are assessed in comparison to a variety of traditional composites made from fibreglass such as unidirectional or woven rovings. The interlaminar fracture toughness (G_Ic) of braided and knitted composites are higher than traditional composites by factors of more than two and four, respectively. Toughening in these textile composites was caused by extensive crack branching as the interlaminar crack was forced to follow a tortuous path through the complex fibre architectures. The G_Ic values of the composites reinforced in the through-thickness direction by weaving or stitching were higher than traditional composites by factors of nearly two and three, respectively, with the main toughening mechanism being crack bridging by the through-thickness binder yarns/stitches. A review of Mode I interlaminar fracture data collected from papers shows that advanced textile techniques are capable of manufacturing composites with substantially improved delamination resistance.     

Influence of fibre surface oxidation treatment on mechanical interfacial properties of carbon fibre/ polyarylacetylene composites

Abstract

In the present work, the mechanical interfacial properties of carbon fibre (CF) reinforced polyarylacetylene (PAA) resin composites were modified through the surface oxidation treatment of carbon fibres by ozone. Both X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR) spectroscopy showed that oxidation treatment could increase the amount of elemental oxygen on the fibre surface markedly by introducing more oxygen groups. Atomic force microscopy (AFM) images indicated that weak surface regions of fibres had been etched and removed, and the degree of fibre surface roughness was increased. The interlaminar shear strength (ILSS) and the interfacial shear strength (IFSS) of CF/PAA composites were both improved notably (no less than 50%). It could be concluded that an improvement of fibre surface chemical activity, better wettability of resin on the carbon fibre surface, and stronger mechanical joining between fibres and resin all resulted in the modification of interfacial properties of carbon fibre reinforced PAA composites. The influences of temperature, ozone concentration, and treatment time on the oxidation results were studied, and optimal treatment parameters determined.     

Influence of residual stresses and interfacial shear strength on matrix properties in fibre-reinforced ceramic matrix composites

Zircon matrix composites, uniaxially reinforced with a variety of SiC fibres were fabricated in order to create composites with different interfacial properties. Interfacial properties were varied by changing the nature of fibre coatings. The effect of changes in interfacial shear strength on important matrix properties, such as hardness and fracture toughness, was studied on a micro-scale using the microindentation technique. In addition, the relative orientation of the indented cracks with respect to the fibres was varied to investigate the existence of anisotropic behaviour of the matrix. The results indicated that the crack growth in the matrix was influenced by the presence of residual radial and axial stresses, such that relatively higher crack lengths were seen in certain directions in the matrix with respect to other directions. This asymmetric nature of the crack formation upon indentation was the reason for the observed anisotropic fracture toughness of the matrix. The residual stresses also led to anisotropic hardness and a critical load for crack initiation in the matrix.     

Effect of interphase on fibre-bridging toughness of a unidirectional FRP composite thin plate

A parametric analysis of the toughening mechanisms in a uniaxially fibre reinforced polymer (FRP) thin plate with a power-law hardening shear interphase is presented. An interfacial shear-lag model is used to analyse the relationship between the crack surface traction exerted by the intact fibres and the crack opening displacement (COD). Numerical solutions of the equations governing bridge-toughening are given. Two special kinds of interphase, i.e. linearly elastic and perfect plastic, are discussed. The results demonstrate that the toughening ratio of the composite thin plate is sensitive to several parameters, e.g. the thickness of the interphase between fibre and matrix, the hardening parameter of the interphase, the interfacial shear properties (stiffness and strength), the fibre radius and the far-field load. The results of this investigation will be beneficial to the selection of constitutive materials, the improvement of mechanical behaviour and the fabrication process of FRP composites.     

Effect of humidity during manufacturing on the interfacial strength of non-pre-dried flax fibre/unsaturated polyester composites

The influence of moisture content in the environment during manufacture of a novel cobalt-free UP matrix reinforced with flax fibres, on the fibre–matrix adhesion was studied. Flax surface energy was experimentally determined by measuring contact angles on technical fibres, using the Wilhelmy technique and the acid–base theory. The mechanical strength of the interface under different humidity conditions was characterized by the critical local value of interfacial shear stress, τ_d, at the moment of crack initiation, which was assessed by single-fibre pull-out tests. Differential scanning calorimetry and X-ray photoelectron spectroscopy analysis gave further insight into the topic. The results suggest that the effect of humidity during manufacturing on the composite interface might be limited. However, longitudinal composite strength decreased somewhat for composites produced in humid conditions, showing that there is some detrimental effect of high levels of moisture during cure on the fibre mechanical performance, likely caused by some fibre degradation.     

On the influence of rubbing fracture surfaces on fatigue crack propagation in mode III

Fatigue crack growth has been studied under fully reversed torsional loading (R = ?1) using AISI 4340 steel, quenched and tempered at 200°, 400° and 650°C. Only at high stress intensity ranges and short crack lengths are all specimens characterized by a microscopically flat Mode III (anti-plane shear) fracture surface. At lower stress intensities and larger crack lengths, fracture surfaces show a local hill-and-valley morphology with Mode I, 45° branch cracks. Since such surfaces are in sliding contact, friction, abrasion and mutual support of parts of the surface can occur readily during Mode III crack advance. Without significant axial loads superimposed on the torsional loading to minimize this interference, Mode III crack growth rates cannot be uniquely characterized by driving force parameters, such as ΔK_III and ΔCTD_III, computed from applied loads and crack length values. However, for short crack lengths (?0.4 mm), where such crack surface interference is minimal in this steel, it is found that the crack growth rate per cycle in Mode III is only a factor of four smaller than equivalent behaviour in Mode I, for the 650°C temper at $\text{ΔK}{}_{\mathrm{III}}\text{= 45 MPa m}\text{1}\text{2}$.     

Mode II interlaminar fracture properties of Moso bamboo

Bamboo is a kind of biological composites reinforced by unidirectional long fiber. Once there exists crack, the propagation of delamination is controlled by the interlaminar fracture toughness instead of by strength. In this paper, the end notched flexure (ENF) beam specimen was used to test the Mode II interlaminar fracture toughness G_IIC along grain of Moso bamboo internode and the fracture surface was analyzed. The results were obtained that the Mode II interlaminar fracture toughness G_IIC calculated by the experiment parameter substitution method was more accurate and the value was 1303.18 J/m² (coefficient of variation = 8.96%) which was about three times higher than the value of Mode I interlaminar fracture toughness; the crack propagation of Mode II interlaminar fracture was mainly self-similar cracking, but the fracture surface was rougher. Ground tissue in the zone of Mode II crack propagation was characterized by hackle shearing deformation. The SEM photos showed that ground tissue separated from fiber along middle lamella under shear stress and as the increasing of the dislocation of upper and lower layer, the thin-walled ground tissue would fracture transversely by tension, while to thick-walled fiber cell, only middle lamella and primary wall were torn then debonded, fragments remained.     

Three techniques of interfacial bond strength estimation from direct observation of crack initiation and propagation in polymer–fibre systems

Three techniques of bond strength determination in micromechanical tests—fibre strain profile analysis by means of Raman spectroscopy, “kink” force determination in a traditional pull-out test, and crack length monitoring in a microbond test—were used for investigation of interfacial debonding in epoxy–glass fibre and epoxy–aramid fibre systems. Crack propagation was characterised by local interfacial parameters—critical energy release rate, G_ic, and ultimate interfacial shear strength (IFSS), τ_ult. The comparison of the results showed good agreement both between different techniques and between stress-based and energy-based failure criteria. Sizing of glass fibres caused more pronounced variations in the IFSS than for aramid fibres due to different interfacial failure patterns. The strength of “real” epoxy–glass composites with sized and unsized fibres correlates well with the bond strength determined from the micromechanical tests.     

Fatigue behaviour related to interface modification during load cycling in ceramic-matrix fibre composites

Ceramics reinforced with continuous fibres exhibit delayed failure under pulsating load. A micromechanical model describing the fatigue effects is proposed. It is based on a decrease in shear stress at the fibre/matrix interfaces, as a result of interfacial wear caused by see-saw sliding. The main features of this model are as follows. During the first load cycle, the material exhibits multiple matrix cracking and some fibre breaks. The system is then a serial set of matrix cracks, each of them bridged by a parallel set of intact or broken fibres. During subsequent cycles the interfacial shear stress decreases, leading to an increase in the failure probability of the bridging fibres. These changes give both a reduction of stiffness and a widening of the hysteresis loops. For a critical fraction of broken bridging fibres, instability occurs and the specimen fails, thus defining the lifetime. The higher the applied load, the higher is the initial damage on the first cycle and the faster the instability condition is reached. For peak stresses that are lower, but still higher than the proportionality limit, the material also changes but no failure occurs (up to 10⁶ cycles), indicating that the interfacial shear stress decreases to a non-zero value; this limiting value controls the fatigue limit in the lifetime diagram.     

Quasistatic thermal crack growth in unidirectionally fiber reinforced composite materials

Micromechanical investigations concerning the quasistatic thermal crack propagation in self-stressed unidirectionally reinforced composite structures with a low-fiber volume fraction have been performed. Thus, in order to gain certain microstructural informations about the thermal shock resistance of those reinforced composites different cracked unit cells of several two-phase composites as well as ensembles of such composite microcomponents are considered which are subjected to thermal loading. Therein the principal facets of composite material failure theories, emphazising matrix cracking, fiber breaking and interfacial debonding, respectively, have been studied. The resulting mixed boundary-value problems of the stationary plane thermoelasticity are solved numerically by using standard finite element programs. Further, the influences of the fiber diameters, different shapes of the fiber-matrix interfaces as well as of the external boundaries of the microcomponents on the crack opening displacement $\text{u}{}_{\mathrm{y}}{}^{\mathrm{c}}$ and the opening mode stress intensity factor $\text{K}{}_{\mathrm{I}}$, respectively, have been investigated. Numerical results are given for an Al₂O₃ matrix/Molybdenum fiber composite by consideration of different crack configurations.     

The influence of thermal treatments on interfacial properties and fatigue crack growth resistance in SCS-6/Ti β21s composites

The mechanical properties of fibre-reinforced composites are dependent on the properties of the fibres, the matrix and the fibre/matrix interface. The latter two were investigated in a metastable β Ti matrix (Ti β21s) reinforced with silicon carbide (SCS-6) fibres by heat treatment for 2 and 6 h at 910°C (solution treatment) and 8 h at 540°C (ageing). Characterization of the microstructure using scanning and transmission electron microscopies revealed that the interfacial reaction layer had become thicker in the solution-treated specimens. This variation did not result in any significant change in interfacial shear strength, determined by a fibre push-out technique, nor in the fatigue crack growth resistance in three-point bending. Conversely, ageing at 540°C resulted in the precipitation of 34% volume fraction of α plates with negligible change to the fibre/matrix reaction layer thickness. This resulted in a large increase in both the interfacial debonding and frictional shear strength, which was consistent with local volume changes in the matrix on precipitation of α. The fatigue crack growth resistance of the peak-aged composite was also reduced under some testing conditions employed. In particular, the number of cycles to failure was decreased markedly after ageing.     

The condition of fatigue crack growth in mixed mode condition

The condition of the initiation of fatigue crack growth in mixed mode conditions has been investigated by using precracked low carbon steel specimens.It is pointed out that, firstly, the critical condition of crack growth should be defined with regard to the modes of fatigue crack growth, i.e. shear mode and tensile mode. Secondly, it is proposed that the critical condition of fatigue crack growth is given by the local tensile stress and shearing stress at the notch tip determined by stress intensity factors $\text{K}{}_{\mathrm{I}}$ and $\text{K}{}_{\mathrm{II}}$, and that this criterion is generally applicable to in-plane-loading conditions, i.e. Mode $\text{I}$, Mode $\text{II}$ and Mixed Mode conditions.     

Comparing single-walled carbon nanotubes and samarium oxide as strain sensors for model glass-fibre/epoxy composites

The study of the interfacial stress transfer for glass fibres in polymer composites through the fragmentation test requires certain assumptions, such as a constant interfacial shear stress. In order to map the local interfacial properties of a composite, both Raman spectroscopy and luminescence spectroscopy have been independently used. Unlike other polymer fibre composites, the local strain state of a glass fibre cannot be obtained using Raman spectroscopy, since only very broad and weak peaks are obtainable. This study shows that when single-walled carbon nanotubes (SWNTs) are added to the silane sizing as a strain sensor, it becomes possible to map the local fibre strain in glass fibres using Raman spectroscopy. Moreover, if this model glass fibre contains a small amount of Sm₂O₃, as one of the components, luminescence spectroscopy can be simultaneously used to confirm this local fibre strain. A combined micromechanical properties study of stress transfer at the fibre–matrix interface using luminescence spectroscopy, together with Raman spectroscopy, is therefore reported. The local strain behaviour of both Sm³⁺ doped glass and SWNTs in the silane coating are shown to be consistent with a shear-lag model. This indicates that Sm³⁺ dopants and SWNTs are excellent sensors for the local deformation of glass fibre composites.     

Influence of interfacial stress transfer on fatigue crack growth in SiC-fibre reinforced titanium alloys

An important damage mechanism during fatigue of unidirectional SiC-fibre reinforced titanium alloys is the formation of matrix cracks transverse to the fibre direction. Due to the relatively low fibre/matrix bond strength these matrix cracks initially do not break the fibres, so that matrix cracks bridged by fibres develop. It is shown experimentally, that the strong drop in fatigue strength is caused by the formation of a bridged crack of a critical size and the crack propagation rate (da/dN) for a single load level has been determined. A prediction of da/dN on the basis of finite element calculation of the stress intensity factor range of the bridged matrix crack ΔK_m and the ΔK_m–da/dN relationship of the used titanium alloy (Timetal 834) has been performed. Calculation of ΔK_m assuming a negligible fibre/matrix bond strength and considering shear load transfer at the fibre/matrix interface due to Coulomb friction (coefficient of friction μ=0.5 and μ=0.9) led to a large discrepancy between the measured and predicted crack growth rate. It can be concluded, that the assumed conditions of stress transfer at the fibre/matrix interface neglecting bonding is the reason for this discrepancy.     

Assessment of interfacial bonding between polymer threads and epoxy resin by transverse fibre bundle (TFB) tests

Two experimental approaches were employed to assess the fibre/matrix adhesion between polymer threads and epoxy resin by transverse fibre bundle (TFB) tests. The first approach was to measure interfacial bonding strength of the fibre/matrix interface in dog-bone-shaped tensile specimens by applying normal stress until failure, simulating the Mode I failure mode. The second approach was to determine the fibre/epoxy interfacial bonding strength in shear (simulating the Mode II failure mode) by means of a V-notched beam shear testing method, i.e. a modified Iosipescu test. In both methods, polymer threads were transversely incorporated in the middle section of the specimens. It was found that both methods were simple, reliable, and sensitive to changes in the fibre/matrix adhesion conditions, though interpretation of the test results was somewhat complex. The two experimental approaches were able to produce consistent results and can thus be adopted as alternative methods for determining the interfacial bonding properties between fibres and matrix in composite systems where conventional micro-mechanical or macro-mechanical testing methods cannot be used.