The embedded single-fibre dynamic load test — a new non-destructive interphase investigation tool

Embedded single-fibre tests are commonly used to investigate fibre/matrix adhesion. However, unstable crack propagation renders these destructive tests hard to interpret. Most of the relevant theories apply an energy or a strength criterion to indirectly deduce interphase properties from the obtained results. A new method, the embedded single-fibre dynamic load (SFD) test, was developed to measure interphasial viscoelastic properties directly in a non-destructive way. Using different fibres in combination with poly(phenylene sulfide) in a comparative study, it is shown that results from the SFD test correlate well with those from single-fibre pull-out (SFP) tests. The morphology is established as a key link for an understanding of the fracture behaviour as well as the dynamic-mechanical behaviour of the interphase in fibre-reinforced composites.     

Analysis of the single-fibre pull-out test by means of Raman spectroscopy: Part II. Micromechanics of deformation for an aramid/epoxy system

The single-fibre pull-out test has been analysed for Kevlar-49 fibres in a cold-cured epoxy resin by using both a conventional pull-out experiment and Raman spectroscopy. The interfacial shear strength (ISS) has been estimated from the pull-out force for fibres with a range of embedded lengths. Raman spectroscopy has been used to analyse the distribution of fibre strain in the pull-out test by mapping the variation of strain along an aramid fibre undergoing pull-out from the epoxy resin matrix. At low strains the behaviour follows elastic shear-lag analysis but, as the fibre strain is increased, debonding takes place at the fibre/matrix interface. It is found that this debond propagates along the interface until the entire fibre is debonded. The fibre is then pulled out of the resin matrix by a frictional pull-out process. It is shown that the conventional pull-out experiment produces only an apparent value of ISS and that through a partial-debonding model it is possible to use the interfacial parameters obtained from the Raman analysis to predict the data from the conventional test.     

Fracture mechanisms in glass-reinforced plastics

Model glass fibre/polyester resin composites have been made in the form of double cantilever beams and the effect of a small number of fibres on quasi-static crack propagation has been studied by simultaneous plotting of load/deflection curves, measurements of crack length, and observation of the progress of fibre/resin debonding and fibre pull-out. By varying the condition of the fibre surface and the arrangement of the fibres to a limited extent and carrying out subsidiary experiments on single-fibre samples of identical character it has been possible to make direct measurements of all of the important parameters required for an analysis of the macroscopic behaviour in terms of established models of fibre/matrix interaction. Agreement between experimental and calculated fracture energies for these model composites is not highly satisfactory, but it seems clear that the fracture energy of grp is likely to be determined very largely by work done against friction between fibres and matrix after the debonding process has occurred. This conclusion opposes the currently-held view which attributes the largeγ _F values of grp to the fibre/resin debonding mechanism.     

Interface toughness of carbon nanotube reinforced epoxy composites

Traditional single-fiber pull-out type experiments were conducted on individual multiwalled carbon nanotubes (MWNT) embedded in an epoxy matrix using a novel technique. Remarkably, the results are qualitatively consistent with the predictions of continuum fracture mechanics models. Unstable interface crack propagation occurred at short MWNT embedments, which essentially exhibited a linear load-displacement response prior to peak load. Deep embedments, however, enabled stable crack extension and produced a nonlinear load-displacement response prior to peak load. The maximum pull-out forces corresponding to a wide range of embedments were used to compute the nominal interfacial shear strength and the interfacial fracture energy of the pristine MWNT-epoxy interface.     

Micromechanics of multiple cracking Part II Statistical tensile behaviour

A computational model for fibre-reinforced brittle materials in tension is developed. The model includes multiple cracking and strain-hardening processes, as well as single fracture and strain softening. The composite behaviour is derived from a single-fibre analysis by integrating over all possible fibre locations and orientations. The single-fibre analysis is based on symmetry fibres satisfying the equilibrium condition. The result is a complete constitutive relation: stress–strain or stress–crack width curve, and a prediction of crack spacing. The model is an extension of the ACK theory by Aveston, Cooper and Kelly, as it can be used with discontinuous fibres with different distributions, as well as for analysing hybrid composites. Fibre orientation introduces additional phenomena, which are taken into account with simple models. It was seen that matrix spalling at the fibre exit point may have a considerable effect on the composite strain and the crack width. The effect of fibre aspect ratio on the failure mode was studied, and it was found that with an intermediate fibre diameter the composite fails by fibre pull-out in a multiple-cracking stage, resulting in a strain-hardening material with a high ductility. The proposed model was verified against experimental results of a strain-hardening material, called an engineered cementitious composite. The model can be used in tailoring new materials to meet certain requirements, or in studying the effects of micromechanical properties on the composite behaviour, including the crack width, crack spacing, post-cracking strength, ultimate strain, and ductility. The derived constitutive relationship can further be used in finite element analyses defining the behaviour perpendicular to the crack. © 1998 Kluwer Academic Publishers     

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.     

Enhancement of delamination fatigue resistance in carbon nanotube reinforced glass fiber/polymer composites

We show that the addition of small volume fractions of multi-walled carbon nanotubes (CNTs) to the matrix of glass–fiber composites reduces cyclic delamination crack propagation rates significantly. In addition, both critical and sub-critical inter-laminar fracture toughness values are increased. These results corroborate recent experimental evidence that the incorporation of CNTs improve fatigue life by a factor of two to three in in-plane cyclic loading. We show that in both the critical and sub-critical cases, the degree of delamination suppression is most pronounced at lower levels of applied cyclic strain energy release rate, ΔG. High-resolution scanning electron microscopy of the fracture surfaces suggests that the presence of the CNTs at the delamination crack front slows the propagation of the crack due to crack bridging, nanotube fracture, and nanotube pull-out. Further examination of the sub-critical fracture surfaces shows that the relative proportion of CNT pull-out to CNT fracture is dependent on the applied cyclic strain energy, with pull-out dominating as ΔG is reduced. The conditions for crack propagation via matrix cracking and nanotube pull-out and fracture are studied analytically using fracture mechanics theory and the results compared with data from the experiments. It is believed that the shift in the fracture behavior of the CNTs is responsible for the associated increase in the inter-laminar fracture resistance that is observed at lower levels of ΔG relative to composites not containing CNTs.     

Effect of plastic coating on fibre-matrix interface debonding

A theoretical model including the effect of plasticity of the coating material on the single-fibre pull-out test has been developed. Both hardening and perfectly plastic coatings were modelled and the calculations have provided much information on the debond and maximum pull-out stresses. For both SiC fibre-glass and carbon fibre-epoxy systems, a stiff coating reduces substantially the partial debonding stress, but a soft coating increases it markedly. Although a higher coating yield stress increases the partial debonding stress slightly, the maximum pull-out stress is independent of the coating.On leave at the Department of Mechanical Engineering, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong.     

陶瓷基复合材料（SiCw／Y—TZP）在循环压应力下的疲劳裂纹扩展

研究了碳化硅晶须（ＳｉＣｗ）增强，Ｙ２Ｏ３稳定的ＺｒＯ２四方多晶体（Ｙ－ＴＺＰ）复合材料（ＳｉＣｗ／Ｙ－ＴＺＰ）在循环压应力作用下的疲劳特性，单边缺口弯曲试样在纵向循环压应力作用下缺口根部产生垂直于压应力的Ｉ型裂纹，类似于金属材料，在室温下循环应力导致Ｉ型裂纹的稳定扩展。压应力在缺口根部产生的不可逆损伤区在循环卸载过程中形成较大的残余拉伸应力场，使裂纹萌生并长大，同时，裂纹面产生的碎粒及晶须拔出导     

Evaluation of interface parameters in push-out and pull-out tests

Push-out and pull-out tests with single-fibre model composites are performed to evaluate the frictional parameters of the interface. Analysis of the experimental data according to models based on the assumption of an ideal cylindrically shaped fibre does not satisfy all observed phenomena. Thus, a model is proposed to include the effects of roughness interaction during fibre sliding. The predictions of the model fit the experimental results very well.     

Characterization of the aramid/epoxy interfacial properties by means of pull-out test and influence of water absorption

Single fiber pull-out tests were carried out to investigate the influence of water absorption on the interfacial properties of aramid/epoxy composite. The fiber/matrix interfacial strength was severely decreased between 4 and 7 week immersion time in deionized water at 80 °C, and thereafter showed a plateau. This change with immersion time did not correspond with that of the water gain of the pull-out specimens, because the water gain did not reflect the one in the fiber/matrix interface. As a result of the degradation of the fiber/matrix interfacial strength, the pulled-out fiber surfaces of 7, 10 and 13 week wet specimen were smooth. In situ observations of interfacial crack propagation by a video microscope and an analysis of acoustic emission (AE) signals showed that AE signals obtained during the pull-out process were classified into four types according to fracture modes. AE signals detected at final unstable crack propagation and fiber breakage had high amplitude and long duration.     

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.     

Analysis of anchor bolt pull-out tests by a two-domain boundary element method

Mixed-mode crack propagation in anchor bolt pull-out tests of concrete is analysed, based on the maximum circumferential tensile stress criterion of linear elastic fracture mechanics. Analysis is carried out by implementing a two-domain boundary element method for the plane stress condition. Automatic remeshing of the boundary is accommodated with the increment of crack growth, and the prediction of crack trajectory is accomplished by generating new boundaries. In a parametric study, results are presented for five such cases as combinations of two embedded depths of the anchor bolt, different distances from the edge of the anchor bolt head to the reaction supports, and two lateral support conditions. A simplified formula for the estimation of pull-out capacity is derived from the analytical results, as a function of given geometric and material parameters.     

Deformation micromechanics in model carbon fibre-reinforced composites

Raman spectroscopy has been used to study the deformation micromechanics of the single-fibre pull-out test for a carbon fibre/epoxy resin system using surface-treated and untreated versions of the same type of PAN-based fibre. It has been possible to determine the detailed strain distribution along embedded fibres and it has been found that it varies with the level of strain in the fibre outside the resin block. The variation of interfacial shear stress along the fibre/matrix interface has been determined using the balance of forces equilibrium and this has been compared with the single values of interfacial shear strength determined from conventional pull-out analyses. It has been demonstrated that it is possible to identify situations where the interface is well-bonded, partially debonded or fully debonded and also to follow the failure mechanisms in detail. It has been found that the level of interfacial adhesion is better for the surface-treated fibre and that, for the untreated fibre, interfacial failure takes place by the cohesive failure of a weakly-bonded surface skin that appears to be removed by the surface pretreatment process.     

Inverse End Loaded Split test configuration for stable mode II crack propagation in bonded joint: macroscopic analysis—effective crack length approach

Mode II crack propagation along a bonded joint is investigated using newly proposed Inverse-End Loaded Split experimental configurations. This test configuration allows stable crack propagation all along the crack propagation path. The specimen compliance and strain energy release rate for the new experimental arrangement are derived. An experimental data reduction procedure, based on the effective crack length approach, is also proposed. Two series of experiments are performed to assess the stable nature of the crack propagation and data reduction scheme associated to this new experimental arrangement. In addition to stable crack growth, the experiment may prove his worthiness in the study of crack onset under mode II loading.     

A round-robin programme on interfacial test methods

A round-robin programme has been undertaken to assess the compatibility in the micromechanical techniques used to evaluate the interfacial shear strength of the fibre/matrix bond in composite materials. The tests selected for evaluation were the single-fibre pull-out test, the microdebond test, the fragmentation test and the micro-indentation test. Twelve laboratories were invited to participate in this programme. Each laboratory was supplied with Caurtaulds XA fibre in the untreated condition and with a standard surface treatment, and a quantity of epoxy resin, hardener and catalyst, all from the same batch. Some laboratories were supplied with composite bars made with the same materials. A common cure cycle was chosen for sample preparation. Each laboratory conducted the tests to its own procedures. The results showed that the scatter within each laboratory was acceptable but the scatter between laboratories for a particular test was high. The results are discussed and possible explanations are presented for these observations. The indications are that the fundamental procedures used in each laboratory are sound. The results also suggest that there is great potential for achieving standard procedures and reducing the inter-laboratory scatter. A further round-robin programme is proposed to generate test protocols.     

Consideration of radial dependences of axial stresses in the shear-lag model for fibre pull-out

The shear-lag model has been used extensively to analyse stress transfer during single-fibre pull-out. To achieve analytical solutions, the radial dependences of the axial stresses in the fibre and the matrix are generally ignored in the shear-lag model. The present study considered these radial dependence in the shear-lag model. The differences between the predictions obtained by ignoring these radial dependences, considering the radial dependence of the axial stress in the fibre only, considering the radial dependence of the axial stress in the matrix only, and considering both radial dependences, have been addressed.     

Mechanical properties Of Si3N4 cerarnics reinforced with SiC whiskers and SiC platelets

Silicon nitride ceramics reinforced with SiC whiskers and SiC platelets were fabricated by hot pressing and their mechanical properties were studied. They showed higher fracture energy than conventional composites, particularly when they were consolidated by gas-pressure hot pressing at high temperature. A high fracture toughness (10.7 MPa m^1/2) which was measured by the single-edge pre-cracked beam method was achieved. Furthermore, a unique method to observe the crack propagation behaviours directly in a scanning electron microscope with loading devices was developed. As a result, much bridging and pull-out of the whiskers and the elongated Si₃N₄ grains, and crack deflection along the platelets, were observed behind the crack tip. This means that these grains are effective in enhancing the fracture resistance during crack propagation.     

动焦散线在爆炸裂纹扩展试验研究中的应用

爆炸裂纹的动态扩展问题是爆炸力学研究的热点。本文建立了爆炸加载的动焦散线测试系统,解决了试验中的有效光学图片获取、精确同步控制等问题,为进一步定量分析爆炸裂纹扩展规律提供了一种新的有效实验方法。采用爆炸加载的透射式动焦散实验系统,初步研究了爆炸载荷下的裂纹起裂、发展和止裂的动态扩展行为,分析其破坏模式和动态应力强度因子、裂纹扩展速度的变化,并研究了爆炸裂纹穿过有机玻璃板的人工层理的规律。     

动焦散线在爆炸裂纹扩展试验研究中的应用

爆炸裂纹的动态扩展问题是爆炸力学研究的热点。本文建立了爆炸加载的动焦散线测试系统,解决了试验中的有效光学图片获取、精确同步控制等问题,为进一步定量分析爆炸裂纹扩展规律提供了一种新的有效实验方法。采用爆炸加载的透射式动焦散实验系统,初步研究了爆炸载荷下的裂纹起裂、发展和止裂的动态扩展行为,分析其破坏模式和动态应力强度因子、裂纹扩展速度的变化,并研究了爆炸裂纹穿过有机玻璃板的人工层理的规律。