The crack development on the micro- and mesoscopic scale during the pyrolysis of carbon fibre reinforced plastics to carbon/carbon composites

Investigations were conducted to get an insight into the crack development during the carbonization of CFRP components and to understand the mechanisms ruling this cracking. It was found that the major crack types are micro-cracks caused by fibre–matrix-debonding, transversal cracks and partial delaminations. Cracking starts with the onset of pyrolysis as the reaction gases are captured in the still compact material leading to internal pressures and consequently to a channel network. At 490 °C (in the case of a heating rate of 10 K/min) the first micro-cracks evolve due to fibre–matrix-debonding resulting in a homogenisation of the stress distribution. Driven by the high shrinkage in this temperature regime the initiation of transversal cracking takes place soon afterwards at 515 °C resulting in a steep increase in the transversal crack density up to approximately 550 °C. Partial delaminations develop from 520 °C onwards as the crack deflection of the transversal cracks. The development of the latter interrupts the stress transfer between warp and weft fibres, so that the further cracking is mainly driven by local stresses inside the C/C segments between the transversal cracks. Thus the further cracking is dominated by fibre–matrix-debonding occurring in all sizes from the submicro- to the microscopic scale. Although the coarse crack pattern mainly develops up to approximately 570 °C, fibre–matrix-debonding at higher temperatures and especially during cool down plays the major role in terms of crack activity as monitored by acoustic emission.     

Effect of carbon fibres on the static and fatigue mechanical properties of fibre metal laminates

It is crucial to understand the characteristic fatigue crack initiation and its growth mechanisms, as well as the relationship between the mechanical properties and the fatigue damage evolution in fibre metal laminates (FMLs). Two types of FML were studied in this work: a polyacrylonitrile‐based carbon fibre epoxy matrix composite sandwiched by Ti‐6Al‐4V (Ti‐alloy) sheets (IMS60‐Ti) and a pitch‐based carbon fibre epoxy matrix composite sandwiched by Ti‐alloy sheets (K13D‐Ti). The static and fatigue mechanical properties of IMS60‐Ti and K13D‐Ti were investigated. The increased failure strain of the FML was greater than that of carbon fibre‐reinforced polymer (CFRP) matrix composites. The fatigue life of IMS60‐Ti was much longer than that of K13D‐Ti. The fatigue damage process in IMS60‐Ti was related to the fatigue creep behaviour of the Ti‐alloy face sheet and mode II cracking at the CFRP/Ti‐alloy interface, and the damage in K13D‐Ti was related to the K13D CFRP laminate.     

Mikrofraktographische Analyse des Delaminationswachstums in zyklisch belasteten Kohlenstoffaser/Duroplastharz-Verbundwerkstoffen

Microfractographic Analysis of Delamination Growth in Fatique Loaded – Carbon Fibre/Thermosetting Matrix Composites Carbon-fibre-reinforced plastics (CFRP) are known to be considerably less sensitive to fatigue loading than aluminium (Al) alloys, for instance. However, even in the presence of small delaminations, the damage tolerance of structural components may be considerably reduced. The scope of the present contribution is to investigate fatigue phenomena in CFRP materials (with thermosetting matrix) by means of microfractography. The microgfractographic features of the fracture surfaces mirror the processes of deformation and fracture at the delamination front. The fatigue fracture behaviour of a CFRP laminate subjected to cyclic mixed-mode loading is determined by matrix-controlled failure mechanisms. Under pure mode-II loading conditions, rollers in addition to fatigue striations appear in the fibre imprints whose formation mechanism was explained by means of high-resolution field-emission scanning electron microscopy (FE-SEM). The ratio between the local tensile and shear stress components influences the propagation direction of secondary cracks originating at the fibres. The local fracture propagations in these secondary cracks can be recognised through the fatigue striations appearing on the surface of the matrix. A comparison with static mixed-mode loading reveals that in both cases the crack propagation follows the path of the local maximum main stress. Applying mathematical relationships derived from the theory of elasticity permitted developing a mixed-mode loading model which makes it possible to predict the crack processes and hence to explain the formation of typical fracture-morphological features.     

Effects of CFRP bond locations on the Mode I stress intensity factor of centre‐cracked tensile steel plates

The fatigue life of cracked steel members can be greatly extended by externally attached carbon fibre reinforced plastics (CFRP), which reduces the stress intensity factors (SIFs) at the crack tip. Access to cracks is sometimes limited and the CFRP has to be attached away from the cracks. There is a lack of knowledge on SIFs for such strengthening scheme. This paper presents the effects of CFRP bond locations on the Mode I SIF of centre‐cracked tensile (CCT) steel plate. The Mode I SIF at the crack tip is calculated using the finite element (FE) models. A correction factor is introduced as a function of CFRP bond location and crack length. The FE results are compared and agree well with experimental tests conducted by the authors. By combining with another two factors (one considering CFRP mechanical properties and the other considering CFRP bond width) derived previously by the authors, SIF formulae are proposed for CFRP reinforced CCT steel plates.     

The mechanical properties of carbon fibre reinforced Pyrex glass

The mechanical properties of carbon fibre reinforced Pyrex glass are discussed in terms of the volume fraction of fibre, the orientation of the fibres, fibre damage during fabrication, matrix porosity, matrix critical strain, interface properties and the mode of failure in bend tests. The stress at which matrix cracking occurs increases with fibre concentration indicating that the critical strain of the matrix increases as the fibre separation decreases. The ultimate strength of the composite is considerably greater than the stress at which the matrix begins to crack. Preliminary stress cycling experiments at stresses above that at which matrix cracks are formed suggest that propagation of these cracks is inhibited by the fibres.     

A probabilistic approach for transverse crack evolution in a composite laminate under variable amplitude cyclic loading

This paper presents a theoretical approach for predicting transverse cracking behavior in a cross-ply laminate with a thick transverse ply under variable amplitude loads for which the cracks grow instantaneously, or very quickly, across the specimen width. The transverse crack density was derived on the basis of the slow crack growth (SCG) concept using the Paris law in conjunction with the Weibull distribution for a brittle material subjected to multi-stage cyclic loading. A fracture criterion obtained was related with the empirical rules by Miner and Broutman & Sahu. Next, the probabilistic SCG model was applied to transverse cracking in a cross-ply laminate under multi-stage cyclic loading. The two-stage fatigue tests with various loading sequences and amplitudes were conducted for carbon fibre reinforced plastic (CFRP) cross-ply laminates in addition to single-stage fatigue tests for various maximum stresses. The experiment results were compared with the predictions to verify the validity of the model.     

偏心受拉作用下预应力CFRP筋-型钢混凝土构件抗裂试验

为解决我国型钢混凝土桁架转换层拉杆及低层角柱在正常使用阶段易出现大面积拉裂缝的问题,以轻质高强、防腐的碳纤维增强树脂复合材料(CFRP)筋为预应力筋,提出可有效控制裂缝的预应力CFRP筋-型钢混凝土结构体系,并对其偏心受拉作用下的抗裂性能进行系统研究。以预应力水平、偏心距、纵筋直径及型钢翼缘厚度为主要参数制作11个构件,通过自行研发的拉-压转换桁架实现偏拉加载。结果表明:引入CFRP筋后CFRP筋-型钢混凝土构件抗裂度大幅提升,相较于普通偏拉构件,预应力大偏拉构件开裂荷载提高了64.8%~102.3%,预应力小偏拉构件提高了61.7%~117%,其抗裂性能与预应力水平、纵筋直径和型钢翼缘厚度正相关,与偏心距负相关。参照组合结构设计规范,提出构件开裂阶段中和轴的三种位置分布,并推导出开裂荷载公式,与试验值比较吻合度较高,可为其他复合材料筋在预应力偏拉体系的应用提供参考。      

Matrix crack detection of CFRP using electrical resistance change with integrated surface probes

For the cryogenic tanks of next generation reusable launching vehicles, the laminated composite tank is one of the key technologies. For composite fuel tanks made from laminated carbon fibre reinforced polymers (CFRP), matrix cracking is a significant problem that may cause fuel leakage. In the present paper, an electrical resistance change method with integrated probes on a single side of the surface of a CFRP composite structure is adopted to detect the matrix cracking of the laminated composites. For a fuel tank structure made of a CFRP laminate, we cannot mount electrical probes on the end of structure or on the inside of the tank structure. We have to mount all probes only on the outside surface. The present method used finite element analyses (FEA) to search for the best placement of probes for matrix crack detection using a rectangular plate. To simulate the tank structure, all probes are placed on a single surface of the CFRP plate specimen. The present study adopted a four-probe method for measuring the electrical resistance change. The FEA revealed that the electrical resistance increases linearly with increase in the number of matrix cracks inside of the probes. By means of thin CFRP cross-ply laminate, the method was experimentally confirmed to be useful for detecting matrix crack density between the probes. Residual electrical resistance at the completely unloaded condition increased with increase in matrix crack density. Measurements of the residual electrical resistance enabled us to detect the matrix crack density without loading.     

Networks originating from the multiple cracking of different scales in rocks and swelling soils

The objective of this work is not a fracture prediction or prevention. We are interested in modeling the crack network geometry in rocks and swelling soils and in the application of the model to rock volume fragmentation or preferential flow in swelling soils. Natural and explosion-induced rock fragmentation is important in geophysics and mining. Preferential flow in swelling soils is important in agricultural and environmental engineering. The presentation gives a brief review of the authors' work in this area. A concentration criterion of crack connection and effective independency of cracks in the case of multiple cracking are a basis for the modeling of a crack network. This basis enables one to introduce a condition of fragment formation at crack connection and a number of relevant concepts (an average cracking – an average crack number of x dimension in volume; crack connection probability of x dimension; fragment formation probability; average and maximum fragment dimensions; crack connectedness – a ratio connected to the total number of cracks; and crack network tortuosity), as well as to suggest quantitative relations between the concepts. In the frame of an application, the average fragment dimension and crack connectedness (or the maximum fragment dimension and fragment formation probability) can depend on the spatial coordinates and parameters specific for the application. The simplest application relates to the block-dimension distribution of a rock mass for statistically homogeneous conditions. The second application relates to the granulometric composition of a blasted rock mass in quarries. In this case the specific parameters are the preliminary rock disturbance (including natural cracking), charge construction, blasting scheme and others. The third application relates to the shrinkage crack network geometry in swelling clay soils, the spatial coordinate being the soil depth. The specific parameters are an upper layer thickness (of a few tens of centimeters) of intensive cracking and the maximum crack depth (its boundary being the depth of the water table level). Crack width, depth, spacing, volume, and tortuosity of the crack network are estimated by using the shrinkage curve of the soil and a water content profile. The fourth application relates to the hydraulic properties of capillary crack networks in swelling soils as compared to those of the soil matrix. For all the applications considered, comparison between the model prediction and available data shows good agreement.     

Measurement of the mechanical properties of a fibre-reinforced composite on the submicrometre scale

Ultramicrotomy is being used routinely as a sample preparation technique for transmission electron microscopy (TEM). TEM study of thin sections of a diketone-bis-benzocyclobutene composite reinforced with Celion (trademark of BASF Co.) carbon fibre revealed two types of periodic crack in the fibre. Coarse cracks were due to bending at knife tip in the early stage of the ultramicrotomy. Also, very fine cracks were observed near the fibre–matrix interface and believed to have been induced by the shear lags between the fibre and the matrix. A simple analysis indicated that the coarse and fine crack spacings could be used to obtain the compressive strength of the fibre and the ultimate shear stress at the fibre–matrix interface, respectively. The combination of ultramicrotomy and TEM provides a useful tool to explore the mechanical properties of a composite material on the submicrometre scale, in addition to the other microstructural and compositional information accessible by TEM. This revised version was published online in November 2006 with corrections to the Cover Date.     

Fatigue crack growth and delamination mechanisms of Ti/CFRP fibre metal laminates at high temperatures

Ti/CFRP (titanium/carbon fibre reinforced polymer) fibre metal laminates (FMLs) are composed of titanium sheets and carbon fibres reinforced PMR (polymerization of monomeric reactants) type polyimide resin. Due to the outstanding heat resistance of the material, it can be used in hypersonic aircraft applications. Fatigue cracks in the metal layer and delamination at metal/fibre interface may occur in long‐term high‐temperature use processes. However, the behaviour of the fatigue failure at high temperatures has not been investigated. A temperature‐dependent equation has not been presented to predict the crack growth behaviour at high temperatures. In this study, to investigate the crack propagation and delamination behaviours, fatigue crack growth rate tests using tension‐tension loads at 25°C, 80°C, 120°C, and 150°C were conducted in accordance with ASTM E647‐15e1. The results indicated that the variation in fatigue crack growth rate could be described by a modified temperature‐dependent Paris equation. Interfacial strength and tensile strength may influence fatigue failure at high temperatures. Hence, these strength values were also obtained to analyse the mechanism of fatigue behaviour. The delamination area increased exponentially with temperature due to the weakening of the Ti/CFRP interface, and delamination was invariably generated on the microcracks of the titanium layers.     

The role of matrix cracks and fibre/matrix debonding on the stress transfer between fibre and matrix in a single fibre fragmentation test

The single fibre fragmentation test is commonly used to characterise the fibre/matrix interface. During fragmentation, the stored energy is released resulting in matrix cracking and/or fibre/matrix debonding.Axisymmetric finite element models were formulated to study the impact of matrix cracks and fibre/matrix debonding on the effective stress transfer efficiency (EST) and stress transfer length (STL). At high strains, plastic deformation in the matrix dominated the stress transfer mechanism. The combination of matrix cracking and plasticity reduced the EST and increased STL.For experimental validation, three resins were formulated and the fragmentation of an unsized and uncoupled E-glass fibre examined as a function of matrix properties. Fibre failure was always accompanied by matrix cracking and debonding. With the stiff resin, debonding, transverse matrix cracking and conical crack initiation were observed. With a lower modulus and lower yield strength resin the transverse matrix crack length decreased while that of the conical crack increased.     

Mechanism of fracture of short glass fibre-reinforced polyamide thermoplastic

The mechanism of fracture of short glass fibre-reinforced polyamide 6.6 thermoplastic was studied by means of optical and electron microscopy and acoustic emission methods. It was found that there were three stages in the failure, i.e. initiation of the interfacial cracks at fibre ends, propagation of the interfacial cracks along fibre sides, and propagation of the crack into the matrix leading to the failure of the composite. On the fracture surface, fibres were almost pulled-out from the matrix, not broken. The close correspondence between the crack initiation and propagation and the amplitude of AE signals was observed. The AE signals of lower amplitude occurring under a relatively low stress were considered to be made in association with the initiation and propagation of the interfacial cracks. The AE signals of higher amplitude observed prior to the failure of the composite were considered to be made in association with the occurrence of the matrix cracks. Furthermore, in order to analyse the effect of the stress state in the composite on crack occurrence and propagation, the stress levels in matrix, fibre and interface were estimated for the composite stressed to the failure stress. The calculation was based on the equivalent inclusion method proposed by Eshelby and on an assumption of a perfect bond between the matrix and the fibres. The result was found to be consistent with the mechanism of the fracture, the occurrence of the interfacial cracking in the initial stage and the matrix cracking in the final one.     

Some aspects of crack growth and failure in fibre reinforced composites

A theoretical analysis, previously developed to deal with the machanics of matrix cracking in unidirectional composites and with transverse ply cracking in cross ply laminates, has been developed further to deal with the tensile failure of unidirectional fibrous composites in with the fibres have a known distribution of strengths. It is proposed that, under the application of a tensile load, stable transverse cracks are formed which originate from regions of initial damage and which become unstable at some critical strain value. The model takes account of various parameters including the interfacial fibre/matrix debonding energy, the residual frictional shear strength of the debonded interface and the elastic properties of fibres and matrix. Comparisons are made between the predictions of the model and the observed failing strains of the 0° plies in carbon fibre polymer matrix laminates. The relevance of the model to the study of delayed fracture in fibrous composites is discussed. The modification of this model, previously developed to describe crack growth in the transverse plies of 0°/90° laminates, is used to predict the initial cracking strains for a wide range of CFRP laminate geometries and initial crack sizes. Some aspects of the mechanics of crack extension across interply interfaces are discussed.     

Quantification of flexural fatigue life and 3D damage in carbon fibre reinforced polymer laminates

Carbon fibre reinforced polymer (CFRP) laminated composites have become attractive in the application of wind turbine blade structures. The cyclic load in the blades necessitates the investigation on the flexural fatigue behaviour of CFRP laminates. In this study, the flexural fatigue life of the [+45/−45/0]_2s CFRP laminates was determined and then analysed statistically. X-ray microtomography was conducted to quantitatively characterise the 3D fatigue damage. It was found that the fatigue life data can be well represented by the two-parameter Weibull distribution; the life can be reliably predicted as a function of applied deflections by the combined Weibull and Sigmodal models. The delamination at the interfaces in the 1st ply group is the major failure mode for the flexural fatigue damage in the CFRP laminate. The calculated delamination area is larger at the interfaces adjacent to the 0 ply. The delamination propagation mechanism is primarily matrix/fibre debonding and secondarily matrix cracking.     

Effects of nonlinear viscoelastic behaviour and loading rate on transverse cracking in CFRP laminates

The progressive multiplication of matrix transverse cracks in cross-ply laminates made of long carbon fibre reinforced polymer (CFRP) is addressed in this study. Monotonic tensile tests performed on [0₃/90₃]_S laminates at 120 °C have shown a marked dependence of cracking development on loading rate. This paper aims to assess the impact of the material nonlinearity on the loading rate sensitivity of the damaging process. A “shear-lag” damage analysis, using the nonlinear correspondence principle and appropriate failure criteria, is carried out to numerically predict the cracking evolution. This work shows that, though important, the material nonlinearity of the undamaged material does not significantly enhance the loading rate sensitivity of the cracking process and it cannot explain alone the phenomenon. On the other hand, taking into account the loading rate dependence of the critical strength, together with the R-curve effect, which gives good predicted cracking curves, suggests that the observed rate effect pertains to the viscoelastic character of the damaged material in the process zone close to crack fronts.     

Piezoresistivity of unidirectional carbon/epoxy composites for multiaxial loading

The applied strain of carbon fibre reinforced plastics (CFRPs) is measurable by their electrical resistance changes. For damage monitoring of laminated CFRPs, piezoresistivity strongly affects the measured electrical resistance change through residual strain relief attributable to delamination cracks. Although several studies of CFRP laminates’ piezoresistivity have been published, this study uses single-ply CFRP for specific piezoresistivity measurements in four directions. A review of the theory of in-plane piezoresistivity reveals orthotropic properties of CFRP piezoresistivity. In the present study, piezoresistivity of multiaxial loading is derived, and the unsymmetrical piezoresistivity matrix is calculated using the measured piezoresistivity here. Effects of multiaxial loading in a misaligned unidirectional laminate are also discussed here. The misaligned laminate causes large shrink in the transverse direction during tensile tests; poor electrical contacts at electrodes increases the electric current in the transverse direction; these two effects cause decrease of electrical resistance for the poor electrical contact specimen with large fibre misalignment.     

Experimental characterization of microscopic damage behavior in carbon/bismaleimide composite—effects of temperature and laminate configuration

Microscopic damage behavior of high temperature CFRP, carbon/BMI (bismaleimide) under tensile loading was investigated experimentally. To clarify effects of laminate configuration and temperature on the microscopic damage behavior, 10 kinds of laminate configurations were tested at both 25 and 180 °C. Damage initiation at the free edge and progress in the width direction were observed using optical microscopy and soft X-ray radiography, respectively. Damage mechanics analysis was used to predict matrix cracking in 90° plies based on both the energy and average stress criteria. It is clarified that the critical energy release rate and critical average stress associated with matrix cracking in BMI based CFRP are larger than in epoxy-based CFRP, which certify the high crack resistance of carbon/BMI composites.     

Chemical stress cracking of acrylic fibres

The generation of periodic microscopic transverse cracks in oriented acrylic fibres immersed in hot alkaline hypochlorite solution is described in detail and shown to be a variety of chemical stress cracking. It is greatly accelerated by external tensile stress, high fibre permeability, moderate fibre orientation, and water-plasticization. The proposed mechanism for bond cleavage involves cyclization of nitrile groups (similar to the prefatory reaction in pyrolysis of acrylic fibres), followed immediately by N-chlorination and chain scission. Mechanical retractile forces (internal or external) then cause chain retraction and crack growth. Despite the remarkable regularity of the crack pattern, which typically resembles a series of stacked lamellae, the process is independent of any such underlying fibre morphology. The cracking process does, however, appear to be a sensitive indicator of residual latent strain in the fibre, which may persist even after high-temperature annealing.     

无粘结预应力CFRP筋钢纤维轻骨料混凝土梁受弯性能试验研究

建立无粘结预应力FRP筋张拉锚固体系,对8根以CFRP筋为非预应力筋的无粘结预应力CFRP筋轻骨料混凝土梁与1根普通混凝土对比试件进行两点对称加载,观察其破坏过程与破坏形态,分析了混凝土种类、预应力度和净跨长度对开裂弯矩、弯矩-跨中挠度曲线、裂缝宽度等受弯性能的影响.从等效轴向刚度思想出发,修正了现有的以钢筋为非预应力...