Querrißbildung in 0/90/0 CFK-Laminaten

Cross-ply Cracking in 0/90/0 CFRP Laminates Cross-ply cracking in carbon fibre reinforced plastics (CFRP) with thermoset as well as thermoplastic matrix systems was investigated on 0/90/0 laminates with varying 90°-ply thickness. Since in an angle ply laminate the 90°-ply fails first, the higher strength of the other plies cannot be taken advantage of. For this reason efforts have to be done to increase the transverse strength (strain) of fibre reinforced plastics. In the first place thus it is necessary to investigate the influence of the different parameters which contribute to the transverse strength. In this work the influence of matrix (fracture strain), fibre/matrix interface, voids and constraining effect of neighbouring plies is investigated. With the aid of two-parameter Weibull distributions of the 90°-ply fracture strain, which describe the phenomenon of multiple cracking in a specimen, it was found that the constraining effect due to neighbouring plies, improved fibre/matrix interface and matrix ductilty increase, whereas voids decrease the transverse fracture strain.     

A tensile strength model for unidirectional fiber-reinforced brittle matrix composite

A model for the ultimate tensile strength of unidirectional fiber-reinforced brittle matrix composite is presented. In the model, transverse matrix crack spacing and change in debonding length between the fiber and the matrix is continuously monitored with increasing applied load. A detailed approximate stress analysis, together with a Weibull failure statistics for fiber fracture, are used to determine the probability of fiber fracture and fiber fracture location in the composite. Results of the model are consistent with experimental data. It is suggested from the results that the strength and toughness of the composite are significantly influenced by the Weibull modulus of the fiber and the fiber/matrix interfacial shear stress. A higher fiber Weibull modulus results in a lower composite strength while a higher fiber/matrix interfacial shear stress results in a composite with higher strength but lower toughness. A moderate variation in matrix strength and fiber/matrix interfacial shear strength does not significantly affect the strength of the composite.     

On cross-ply cracking in glass and carbon fibre-reinforced epoxy laminates

The transverse tensile strength (or fracture strain) of unidirectional fibre-reinforced materials is an important mechanical property. The transverse fracture strain of a single ply in an angle-ply laminate, however, is not an independent mechanical property as it is influenced by its thickness and neighbouring plies. The present investigation describes the phenomenon of multiple fracture of glass and carbon fibre-reinforced epoxy 90° plies. Based on the model that a 90° ply consists of elements all of which can break, the applicability of Weibull statistics in describing the fracture strain is investigated.     

Experimental study on CFRP-to-steel bonded interfaces

This paper presents an experimental study on the behaviour of CFRP-to-steel bonded interfaces through the testing of a series of single-lap bonded joints. The parameters examined include the material properties and the thickness of the adhesive layer and the axial rigidity of the CFRP plate. The test results demonstrate that the bond strength of such bonded joints depends strongly on the interfacial fracture energy among other factors. Nonlinear adhesives with a lower elastic modulus but a larger strain capacity are shown to possess a much higher interfacial fracture energy than linear adhesives with a similar or even a higher tensile strength. The variation of the interfacial shear stress distribution in a bonded joint as the applied load increases clearly illustrates the existence of an effective bond length. The bond–slip curve is shown to have an approximately triangular shape for a linear adhesive but to have an approximately trapezoidal shape for a nonlinear adhesive, indicating the necessity of developing different forms of bond–slip models for different adhesives.     

Multiple transverse fracture in 90° cross-ply laminates of a glass fibre-reinforced polyester

Specimens of a 90° cross-ply glass-reinforced polyester were tested in tension in a direction parallel to one of the directions of reinforcement. Extensive cracking of the transverse ply occurred at strains much lower than the resin failure strain. These cracks formed in a direction parallel to the transverse reinforcement and showed a remarkably even crack spacing. Results of crack spacing measurements are presented against applied stress for specimens with differing transverse-ply thicknesses. The transverse-crack spacing was found to decrease with increasing applied stress and to increase with increasing transverseply thickness. There was no evidence of debonding between the plies during cracking and a multiple cracking theory in which the plies remain elastically bonded has been presented which can account for the results.     

Temperature dependence of lifetime statistics for single Kevlar 49 filaments in creep-rupture

Experimental data are presented for the strength and lifetime under constant stress of single Kevlar 49 aramid filaments at two elevated temperatures, 80 and 130° C. As seen in previously published work performed at room temperature (21 °C), the strength data could be fitted to a two-parameter Weibull distribution; increasing the temperature caused a decrease in the Weibull scale parameter while the shape parameter remained relatively constant, indicating a decrease in the mean strength but no change in strength variability. Lifetime experiments at both 80 and 130°C were performed at different filament stress levels, ranging from 55 to 92.5% of the Weibull scale parameter for short-term strength at that temperature. These data were fitted to a two-parameter Weibull distribution with large variability (scale parameter values 1), and evaluated using an exponential kinetic breakdown model in the spirit of Eyring and Zhurkov. Using this model, activation energies in the neighbourhood of 80 kcal mol^–1 (3.35 × 105 J mol^–1 ) were obtained, suggesting that scission of the C-N bond plays the dominant role in fibre failure at longer times under constant stress.Kevlar and Kevlar 49 are registered tardemarks of E. I. du Pont de Nemours & Co., Inc.     

Effects of notch length,specimen thickness,ply thickness and type of defect on the fracture behaviour of angle-ply graphite-epoxy composites

An experimental study was carried out on the effects of notch length, specimen thickness, ply thickness and type of defect (centre-notch or hole) on the fracture toughness of graphite-epoxy composites with lay-up sequences of 0/±45°/0 and 0/90°. Three fracture-mechanical concepts were applied: the Waddoups-Eisenmann-Kaminski (WEK) model, the Whitney-Nuismer model (point and average stress cirteria) and the K _R-curve method. An increase of notch length as well as an increase of ply thickness led to a higher toughness, whereas a change in specimen thickness did not noticeably affect the toughness. Furthermore, the hole caused a smaller strength reduction than a notch. The applicability of the above-mentioned concepts is probably restricted mainly because of the fact that they do not, or do only insufficiently, regard the damage zone near the border of the defect, if a large damage zone usually leads to larger material parameters of the investigated concepts, namely the inherent flaw size, the characteristic lengths of the point and average stress criterions and the K _R -value at fracture.     

正交铺设陶瓷基复合材料单轴拉伸行为

采用细观力学方法对正交铺设陶瓷基复合材料单轴拉伸应力-应变行为进行了研究。采用剪滞模型分析了复合材料出现损伤时的细观应力场。采用断裂力学方法、 临界基体应变能准则、 应变能释放率准则及Curtin统计模型4种单一失效模型确定了90°铺层横向裂纹间距、 0°铺层基体裂纹间距、 纤维/基体界面脱粘长度和纤维失效体积分数。将剪滞模型与4种单一损伤模型结合, 对各损伤阶段应力-应变曲线进行了模拟, 建立了复合材料强韧性预测模型。与室温下正交铺设陶瓷基复合材料单轴拉伸应力-应变曲线进行了对比, 各个损伤阶段的应力-应变、 失效强度及应变与试验数据吻合较好。分析了90°铺层横向断裂能、 0°铺层纤维/基体界面剪应力、 界面脱粘能、 纤维Weibull模量对复合材料损伤及拉伸应力-应变曲线的影响。      

The effect of isothermal exposure on the transverse properties of a continuous fibre metal-matrix composite

The effect of isothermal exposure at 500° C on the transverse mechanical properties of 30 and 50 vol % continuous boron-fibre reinforced 1100 aluminium composites has been investigated. Experimental results indicate that the fibre-matrix interfacial reaction gives rise to an increase in the fibre-matrix bond strength. Consequently, the fracture mode undergoes a transition from interfacial debonding to fibre splitting with increasing exposure time. The fracture surfaces and fibre-matrix interfaces have been studied by scanning electron microscopy and the observations coincide with the above interpretation of the mechanical test results. Finally, a new theoretical model using Eshelby's theory is developed to analyse the stress-strain behaviour of a continuous fibre reinforced metal matrix composite subjected to transverse tensile loading.On leave from Pioneering R & D Laboratories, Toray Industries, Inc., 2-1, 3-chome, Sonoyama, Otsu, Shiga 520, Japan.     

Characterisation of Transverse Cracking in a Quasi-Isotropic GFRP Laminate under Flexural Loading

Transverse cracking behaviour in a quasi-isotropic glass/epoxy (GFRP) laminate loaded in flexure is studied experimentally and theoretically. A theory developed for cross-ply laminates is applied to a [0°/90°/–45°/45°] _S quasi-isotropic laminate. An equivalent laminate is introduced to derive the Young's modulus of a cracked transverse ply on the basis of a shear lag analysis. The model predicts the flexural stiffness, the neutral axis position and the residual curvature as a function of the transverse crack density and the in-situ ply stress at first ply failure. Experimental results are obtained with the use of the applied moment – strain data in four-point flexural tests and compared with predictions. Time-dependent behaviour of the residual curvature is also investigated.The theoretical predictions are in reasonably good agreement with the experimental results. It is found that the decrease in the residual curvature after unloading is mainly ascribed to viscoelasticity of the material.     

Mechanical and dielectric failure of BaTiO3 ceramics

Both at room temperature and above the Curie temperature, Weibull plots for the mechanical and dielectric strengths of BaTiO₃ thick films sintered 1300° C to 1400° C showed good a correlation. The present results indicate that the microstructure plays a similar role in both mechanical and dielectric failures and firmly suggest that the fracture origins in both failures are analogous, possibly surface flaws associated with grain size. Specimens sintered at 1450° C showed a bend in the Weibull distribution for mechanical strength. This bimodal Weibull distribution was explained by the assumption that the geometric relation between the ratio of grain sizes to specimen thickness and stress gradient under a three-point bending condition affects the effective tensile stress of the tips of surface flaws.     

Role of hydroxyapatite crystal shape in nanoscale mechanical behavior of model tropocollagen–hydroxyapatite hard biomaterials

Shape of mineral (e.g. hydroxyapatite (HAP) or aragonite) crystals can be a strong determinant of the nanoscale strength of hard biological materials such as bone, dentin, and nacre. This work presents an understanding of the effect of HAP shape variation on nanoscale strength of model TC-HAP biomaterials. For this purpose, 3-dimensional molecular dynamics analyses of direction dependent tensile deformation in two structurally distinct TC-HAP cells with HAP crystals in needle shaped configuration and plate shaped configuration are performed. Analyses point out that the peak interfacial strength for failure is the highest for supercells with plate shaped HAP crystals. In addition, the plate shaped HAP crystals result in the localization of peak stress over a larger length scale indicating higher fracture strength. Peak strength during transverse loading is always found to be lower than that during the longitudinal loading. However, interfacial strength shows a reverse trend. Overall, analyses point out that HAP crystal shape along with the optimal direction of applied loading with respect to the TC-HAP orientation strongly influence biomaterial strength at the nanoscale.     

Matrix crack-induced delamination in composite laminates under transverse loading

Fibre-reinforced multidirectional composite laminates are observed in experiments under transverse static or low-velocity impact loading to suffer considerable delamination damage. The intensity of this damage depends on the difference in the ply angles above and below the interface. In this paper a fracture mechanics model is presented for investigating the role of matrix cracks in triggering delaminations and the influence of ply angles in adjacent plies on delamination cracking. The fracture mechanics analysis shows that for a graphite fibre-reinforced composite laminate containing a transverse intraply crack, the crack-induced largest interfacial principal tensile stress is a maximum when the difference between the ply angles across the interface is 90 °, and it attains a minimum when the difference is 40 °. When the crack tips touch the interfaces, the minimum mode II stress singularity, which is weaker than the usual square-root type, appears when the difference between the ply angles is about 45 ° for one glass fibre-reinforced laminate and three graphite fibre-reinforced laminates. These results are in agreement with the experimental observation that the largest delaminations appear at the interface across which the difference between the ply angles is the largest i.e. 90 °.     

Statistical model of the transverse ply cracking in cross-ply laminates by strength and fracture toughness based failure criteria

Cross-ply laminate subjected to tensile loading provides a relatively well understood and widely used model system for studying progressive cracking of the transverse ply. This test allows to identify material strength and/or toughness characteristics as well as to establish relation between damage level and the composite stiffness reduction. The transverse ply cracking is an inherently stochastic process due to the random variability of local material properties of the plies. The variability affects both crack initiation (governed by the local strength) and propagation (governed by the local fracture toughness). The primary aim of the present study is elucidation of the relative importance of these phenomena in the fragmentation process at different transverse and longitudinal ply thickness ratios. The effect of the random crack distribution on the mechanical properties reduction of the laminate is also considered. Transverse ply cracking in glass fiber/epoxy cross-ply laminates of the lay-ups [0₂/90₂]_s, [0/90₂]_s, and [0/90₄]_s is studied. Several specimens of each lay-up were subjected to uniaxial quasistatic tension to obtain crack density as a function of applied strain. Crack spacing distributions at the edge of the specimen also were determined at a predefined applied strain. Statistical model of the cracking process is derived, calibrated using crack density vs. strain data, and verified against the measured crack spacing distributions.     

Effect of curing stresses on the behaviour of fibre reinforced plastic composites under biaxial loading

Residual stresses are induced in fibre reinforced plastic (FRP) composites during fabrication and environmental exposure. The curing residual stresses induced during fabrication are mainly due to the thermal expansion mismatch of the constituents. The residual stresses can be either microresidual or macroresidual stresses. Macroresidual stresses in 0° plies and 90° plies of [90/0]_s symmetric cross-ply laminates are calculated starting with ply elastic and thermal properties for different material systems. The calculated curing stresses in Kevlar49/Epoxy unidirectional tape plies in the transverse direction are more than the transverse strength of the corresponding ply. First ply failure (FPF) envelopes are plotted using classical lamination theory and Tsai-Wu quadratic failure theory with and without considering the curing residual stresses. There is a significant effect of residual stresses on the FPF envelopes.     

Monitoring local strains in cracked cross-ply composites using an embedded aramid fibre strain sensor

Single aramid fibre strain sensors have been embedded in transparent glass-fibre reinforced epoxy resin cross-ply laminates. The sensors, located at the 0°/90°/0° ply interfaces, were interrogated by a remote laser Raman microprobe which enabled the changes in the longitudinal strain in the 0° plies caused by transverse cracking in the 90° ply to be monitored. Strain magnifications of up to about 6 were measured in the crack plane, 10 m from the 0°/90° ply interface, and it is estimated that the region of enhanced strain extends to a distance of about 40 m from the interface. Crack interactions were seen to occur for crack spacings of less than two transverse ply thicknesses.     

Statistical scatter of fracture toughness in the ductile-brittle transition of a structural steel

The statistical scatter of fracture toughness in the ductile-brittle transition temperature range was experimentally examined on a 500 MPa class low carbon steel. Fracture toughness tests were replicatedly performed at −60 °C, −20 °C and −10 °C. The tests at −60 °C resulted in a single modal Weibull distribution with a shape parameter of 4 for the critical stress intensity factor converted from J-integral, whereas the Weibull distributions of the critical stress intensity factor at −20 °C and −10 °C showed a bilinear pattern with an elbow point, which caused a wider scatter than that at −60 °C. Such scatter transition behavior was discussed with reference to stable crack initiation. A model of the statistical scatter transition has been proposed in this work and the model reasonably explains the experimental results.     

Deformation and fracture behavior of notched and unnotched unidirectional C/C-Mg composite with Young's modulus 520 GPa and tensile strength 1 GPa

Experimental study on tensile fracture behavior of the newly developed C/C-Mg composite, prepared by infiltration of Mg into the pores in the C/C composite heat-treated at 3000°C, was carried out. The volume fraction of the filled Mg was 9–10%. The composite had a specific density 2.1, Young's modulus 520 GPa and Poisson's ratio 0.26. The average tensile strength measured for the specimen with a nominal width 8 mm, gage length 40 mm and thickness 1 mm was 1 GPa. The Young's modulus was improved from 450 to 520 GPa and the strength from 0.9 to 1.0 GPa by Mg-infiltration. The specific Young's modulus and specific strength based on the average measured values were 2.5 × 10⁷ m and 5 × 10⁴ m, respectively, showing high potential as light-weight, stiff and strong structural material. The strength distribution of the composite was described by the two-parameter Weibull distribution function with a shape parameter 7.6 and scale parameter 1060 MPa. Prior to the overall fracture of the composite, the longitudinal cracking arose at the notch tip, due to which the notch tip was blunted and the ligament portion behaved like an unnotched specimen. As a result, the notched strength could be described by the net stress criterion. The apparent critical energy release rate at formation of the longitudinal crack was around 70–90 J/m².     

High-temperature mechanical behaviour of multidirectional Nicalon/CAS-II composite

High-temperature mechanical behaviour of Nicalon/CAS-II composite has been investigated. Oxidation of the exposed interfaces along matrix cracks at 1000 °C lowered the longitudinal unidirectional strength to the stress level at which matrix cracking began to occur. The strength of cross-plied composites was also severely reduced in 800 °C air. Transverse plies cracked prior to 0° ply matrix cracking. However, embrittlement did not occur until the matrix in the 0° plies cracked. It was established that oxidation does not take part in crack growth parallel to the fibres, except adjacent to exposed edges. Neither does oxygen enter 90° ply cracks in cross-plied composites in sufficient quantity to produce oxidation embrittlement, at least up to the 0° matrix cracking strain.The work was established while the author was at the Department of Chemical Engineering at Montana State University.