Mechanical Behaviour of 3D Multi-layer Braided Composites: Experimental,Numerical and Theoretical Study

Mechanical properties and failure mechanisms of a newly designed 3D multi-layer braided composites are evaluated by experimental, numerical and theoretical studies. The microstructure of the composites is introduced. The unit cell technique is employed to address the periodic arrangement of the structure. The volume averaging method is used in theoretical solutions while FEM with reasonable periodic boundary conditions and meshing technique in numerical simulations. Experimental studies are also conducted to verify the feasibility of the proposed models. Predicted elastic properties agree well with the experimental data, indicating the feasibility of the proposed models. Numerical evaluation is more accurate than theoretical assessment. Deformations and stress distributions of the unit cell under tension shows displacement and traction continuity, guaranteeing the rationality of the applied periodic boundary conditions. Although compression and tension modulus are close, the compressive strength only reaches 70% of the tension strength. This indicates that the composites can be weakened in compressive loading. Additionally, by analysing the micrograph of fracture faces and strain-stress curves, a brittle failure mechanism is observed both in composites under tension and compression.     

金属基复合材料拉伸断裂过程及其分析模型

本文对金属基复合材料拉伸断裂过程及其分析模型进行了述评。内容包括纤维增强复合材料中直力分布、复合材料中裂缝萌生与扩展的微观机制、关于断裂纤维附近应力集中的分析方法、界面结合状态及界面反应对复合材料断裂特性的影响等。并提出了分析模型的发展趋势。     

A design approach for the mechanical properties of polypropylene discontinuous fiber reinforced cementitious composites by extrusion molding

Polypropylene discontinuous fiber reinforced cementitious composites were prepared by extrusion molding and tested in uniaxial tension to determine the mechanical properties such as ultimate composite strength and strain, and the critical volume fraction for multiple cracking. It was shown that the experimentally determined critical fiber volume fraction reasonably agreed with the theoretical value predicted by a micromechanics model. The extruded fiber composites yielded the ultimate composite strength of 9.0 MPa and composite strain of 0.55% at the fiber volume fraction of 7.4%. Our experimental results suggest that there is an optimal fiber aspect ratio and fiber volume fraction for enhancing the fracture properties.     

混杂纤维增强应变硬化水泥基复合材料的拉伸本构关系

钢纤维与聚乙烯醇纤维混杂增强应变硬化水泥基复合材料(SF-PVA/SHCC)的力学性能研究是近年来的热点问题之一,但目前依然欠缺能够完整描述SF-PVA/SHCC拉伸本构关系的理论模型。本文基于混凝土断裂力学和细观力学理论,通过考虑拉伸应力-应变曲线软化段及SF-PVA混杂纤维对SHCC拉伸性能的影响,提出一种新的可适用于SF-PVA/SHCC材料的单轴拉伸本构模型。为了验证模型的有效性,开展了SF-PVA/SHCC单轴拉伸性能试验,分析了纤维种类和掺量对SHCC拉伸强度、拉伸应变及拉伸韧性的影响。通过与试验数据对比发现,本文所提出的拉伸本构模型可以较好地预测SF-PVA/SHCC的拉伸应力-应变关系。      

Modeling the effect of reinforcement discontinuity on the tensile strength of UD flax fiber composites

To exploit the potential of natural fibers as reinforcement of polymer matrix composites, aligned bast fiber composite materials are being produced and studied. Bast fiber reinforcement is discontinuous due to the limited length of natural fibers, which needs to be reflected in predictive models of mechanical properties of composites. The strength in tension in the fiber direction of an aligned flax fiber-reinforced composite is modeled assuming that a cluster of adjacent fiber discontinuities is the origin of fracture. A probabilistic model of tensile strength, developed for UD composites containing a microdefect, is applied. It follows from the theoretical analysis that the experimental tensile strength as a function the fiber volume fraction can be described with acceptable accuracy assuming the presence of a cluster of ca. 4 × 4 elementary fiber discontinuities.     

Study on coexistence of brittle and ductile fractures in nano reinforcement composites under different loading conditions

Experimental study on high volume fraction of metallic matrix nano composites (MMNCs) was conducted, including uniaxial tension, uniaxial compression, and three-point bending. The example materials were two magnesium matrix composites reinforced with 10 and 15% vol. SiC particles (50 nm size). Brittle fracture mode was exhibited under uniaxial tension and three-point bending, while shear dominated ductile fracture mode (up to 12% fracture strain) was observed under uniaxial compression. The original Modified Mohr–Coulomb (MMC) fracture model (Bai and Wierzbicki in Int J Fract 161:1–20, 2010; in a mixed space of stress invariants and equivalent strain) was transferred into a stress based MMC (sMMC) model. This model was demonstrated to be capable of predicting the coexistence of brittle and ductile fracture modes under different loading conditions for MMNCs. A material post-failure softening model was postulated along the damage accumulation to capture the above two different failure modes. This model was implemented to the Abaqus/Explicit as a material subroutine. Numerical simulations using finite element method well duplicated the material strength, fracture initiation sites and crack propagation modes of the Mg/SiC nano composites with a good accuracy. The proposed model has a good potential to predict fracture for a wide range of material with strength asymmetry and coexistence of brittle and ductile fractures modes.     

Effective temperature rise during propagation of shock wave and high-speed deformation in metals

Two theoretical models are developed for the calculations of temperature rise during high-speed deformation and shock wave propagation. In the first model the calculations of the temperature distribution in metals during high-speed deformation are based on a model where the stationary high-speed deformation is considered as a propagation of shock wave with some fixed velocity in these metals. In this model the self-consistent system of equations describing the equation of state of metals and the conservation laws for momentum, energy and flow of energy is used for the determination of the temperature profile in the front of shock wave. The numerical calculations of the temperature distribution profile in shock wave front have been performed using the microscopic Thomas–Fermi–Dirac model for such metals as Al, Cu and Fe. In the second theoretical model the process of high-speed deformation is considered as an adiabatic process where a fraction of plastic deformation is converted into heat. The results of the numerical calculations of temperature rise during high-speed deformation in the dependence of strain to fracture are presented for metals: Al, Cu, Ni and Fe. It was shown that using these models the temperature during high-speed deformation can increase in different metals up to 1000 K.     

Mercerization of sisal fibers: Effect of tension on mechanical properties of sisal fiber and fiber-reinforced composites

Sisal fibers were mercerized, under tension and no tension, to improve their tensile properties and interfacial adhesion with soy protein resin. Mercerization of fibers under tension is known to minimize fiber shrinkage and to lower the microfibrillar angle by aligning them along the fiber axis. Mercerization improved the fracture stress and Young’s modulus of the sisal fibers while their fracture strain and toughness decreased. Mercerized sisal fiber-reinforced composites with soy protein resin showed improvement in both fracture stress and stiffness by 12.2% and 36.2%, respectively, compared to the unmercerized fiber-reinforced composites. Scanning electron microscope (SEM) photomicrographs of the composite fracture surfaces showed shorter fibrils protruding in the mercerized fiber-reinforced composites resulting in better sisal fiber/soy adhesion. Changed fiber surface properties were also responsible for better adhesion.     

定向钢纤维水泥基复合材料断裂理论分析及试验

基于非线性铰模型研究了定向钢纤维水泥基复合材料的裂缝断裂全过程理论分析方法,结合不同尺寸试件的三点弯曲梁断裂试验对本文方法进行了验证。进而利用该方法预测了大尺寸三点弯曲梁试件的裂缝断裂全过程,并研究了试件尺寸对名义强度的影响。通过理论分析与试验结果对比,表明本文方法可较好地预测定向钢纤维水泥基复合材料的裂缝断裂全过程;此外,定向钢纤维水泥基复合材料的名义强度存在一定的尺寸效应,但尺寸效应表现不明显。     

Phosphate glass-ceramic-titanium composite materials

Phosphate glass-ceramic- titanium particulate composites have been prepared by hot-pressing and their thermal, elastic and mechanical properties have been measured. Results have been then explained using various theoretical models for thermal properties, elasticity and fracture mechanics of particulate composites. It is shown that the thermal and elastic mismatches between glass-ceramic matrix and titanium could produce a microcracking of materials. This microcracking could explain both fracture characteristics and discrepancies between theoretical and experimental values of elastic moduli.     

Phosphate glass-ceramic-cobalt-chromium composite materials

Phosphate glass-ceramic-cobalt-chromium particulate composites have been prepared by hot-pressing and their thermal, elastic and mechanical properties have been measured. Results have then been explained using various theoretical models on thermal properties, elasticity and fracture mechanics of particulate composites. It is shown that the evolution of average thermal expansion coefficients as a function of cobalt-chromium volume fraction agrees well with Turner's model [25]. In the same way, Young's moduli lie within Hashin and Shtrickman's upper and lower bounds [33], which means both a close contact between matrix and particles and no microcracking of materials. Strength and fracture toughness do not show significant evolutions with metal volume fraction. Fracture energy continuously decreases which produces both a nearly planar surface fracture and a poor bonding between matrix and particles.     

Mechanical Properties, Damage and Fracture Mechanisms of Bulk Metallic Glass Materials

The deformation, damage, fracture, plasticity and melting phenomenon induced by shear fracture were investigated and summarized for Zr-, Cu-, Ti- and Mg-based bulk metallic glasses （BMGs） and their composites. The shear fracture angles of these BMG materials often display obvious differences under compression and tension, and follow either the Mohr-Coulomb criterion or the unified tensile fracture criterion. The compressive plasticity of the composites is always higher than the tensile plasticity, leading to a significant inconsistency. The enhanced plasticity of BMG composites containing ductile dendrites compared to monolithic glasses strongly depends on the details of the microstructure of the composites. A deformation and damage mechanism of pseudo-plasticity, related to local cracking, is proposed to explain the inconsistency of plastic deformation under tension and compression. Besides, significant melting on the shear fracture surfaces was observed. It is suggested that melting is a common phenomenon in these materials with high strength and high elastic energy, as it is typical for BMGs and their composites failing under shear fracture. The melting mechanism can be explained by a combined effect of a significant temperature rise in the shear bands and the instantaneous release of the large amount of elastic energy stored in the material.     

Tensile fracture strength of 2124Al–10 vol.% SiCp composite compact tension specimens

This paper examines the tensile fracture behavior of 2124Al–10 vol.% SiCp composites by generating the fracture data from the tensile as well as compact tension specimens. The composites were produced through squeeze casting process. For matrix strengthening and improving ductility, the specimens were heat-treated through solutionizing and ageing. The modified inherent flaw model is utilized for fracture strength evaluation. Fracture strength estimations are found to be in good agreement with test results. The modified relationship between the notched strength and the inherent flaw length will be useful for accurate prediction of the notched tensile strength of composite laminates.     

Microscopic failure mechanisms of fiber-reinforced polymer composites under transverse tension and compression

The mechanical behavior of unidirectional fiber-reinforced polymer composites subjected to tension and compression perpendicular to the fibers is studied using computational micromechanics. The representative volume element of the composite microstructure with random fiber distribution is generated, and the two dominant damage mechanisms experimentally observed – matrix plastic deformation and interfacial debonding – are included in the simulation by the extended Drucker–Prager model and cohesive zone model respectively. Progressive failure procedure for both the matrix and interface is incorporated in the simulation, and ductile criterion is used to predict the damage initiation of the matrix taking into account its sensitivity to triaxial stress state. The simulation results clearly reveal the damage process of the composites and the interactions of different damage mechanisms. It can be concluded that the tension fracture initiates as interfacial debonding and evolves as a result of interactions between interfacial debonding and matrix plastic deformation, while the compression failure is dominated by matrix plastic damage. And then the effects of interfacial properties on the damage behavior of the composites are assessed. It is found that the interfacial stiffness and fracture energy have relatively smaller influence on the mechanical behavior of composites, while the influence of interfacial strength is significant.     

单边切口薄板研究聚乙烯醇纤维增强水泥基复合材料断裂韧性

利用单边切口薄板对配制的聚乙烯醇纤维增强水泥基复合材料进行单轴直接拉伸试验研究,得到硬化的荷载-裂缝张开位移全曲线。通过试验观察到缺口尖端处出现呈发散状的多条微小裂纹,部分试件在远离切口处还有多条裂缝出现,并不像混凝土或水泥净浆这类半脆性、脆性材料只有单条裂缝并沿着单条路径开裂,因此,适用于应变软化材料的双K断裂理论以及断裂能理论不能直接用在应变已经发生假硬化的材料中。鉴于上述原因,该文提出起裂断裂韧度和耗散能两个韧性评价指标。     

Effect of particle morphology and polyethylene molecular weight on the fracture toughness of hydroxyapatite reinforced polyethylene composite

Fracture toughness testing has been performed on hydroxyapatite-polyethylene composites. Sintered and unsintered grades of hydroxyapatite and two grades of high-density polyethylene were used to make 40 vol % hydroxyapatite composites. Compact tension testing was performed at both room temperature and at 37 degrees C and at three strain rates. The effect of increasing the loading rate from 2 to 200 microm s(-1) was to increase the fracture toughness. Increasing the testing temperature or decreasing the surface area of the reinforcing particles also increased the fracture toughness. However, using a lower molecular weight, injection moulding, grade of polyethylene reduced the fracture toughness. Thus for higher fracture toughness, a low surface area sintered hydroxyapatite in a high-molecular weight polyethylene is required.     

Direct observation of the fracture of CAS-glass/SiC composites

The fracture behaviour of a CAS-glass/SiC-fibre-reinforced composite was observed by dynamic in situ scanning electron microscopy (SEM). In a companion paper [1], tests on common delamination geometries are described and the basis of micromechanics models is critically evaluated. Flexure geometries and also the unnotched tensile response of ceramicmatrix composites (CMCs) have received considerable attention, both theoretically and experimentally. The effect of through-thickness notches on the tensile fracture of CMCs has however been relatively neglected. Previous work on polymer-matrix composites demonstrates the strong influence of subcritical damage on the fracture behaviour. In this paper we examine failure of notched CAS-glass/SiC composites in tension, under static- and fatigue-loading conditions, using a combination of in situ and conventional test methods. The subcritical damage which forms is compared with that in polymer-matrix composites, and the consequences for prediction of the notched strength are discussed.     

Multiscale analysis of stiffness and fracture of nanoparticle-reinforced composites using micromechanics and global–local finite element models

A combined micromechanics analysis and global–local finite element method is proposed to study the interaction of particles and matrix at the nano-scale near a crack tip. An analytical model is used to obtain the effective elastic modulus of nanoparticle-reinforced composites, then a global–local multi-scale finite element model with effective homogeneous material properties is used to study the fracture of a compact tension sample. For SiO₂ particle-reinforced epoxy composites with various volume fractions, the simulation results for effective elastic modulus, fracture toughness, and critical strain energy release rate show good agreement with previously published experimental data. It is demonstrated that the proposed parametric multi-scale model can be used to efficiently study the toughness mechanisms at both the macro and nano-scale.     

Crack profile in RC,R/FRCC and R/HPFRCC members in tension

The theoretical approaches used for the evaluation of crack width in reinforced concrete (RC) structures, are generally based on the hypothesis of parallel crack surfaces. In this way, crack width measured on the concrete cover should be equal to that on the bar surface. The results of several experimental analyses, developed during the past years in many Research Institutes, do not justify this assumption. On the contrary, even in RC members under tensile actions, crack width appears wider on external surface than on rebar–concrete interface. To better define the effective crack profile of RC structures, a new model, able to analyze the whole structural response of RC ties, is here presented. In the proposed approach, all the physical phenomena involved in the cracking process are taken into account: the bond-slip behavior between steel rebar and tensile concrete, the nonlinear fracture mechanics of concrete in tension, and the mechanism of aggregate interlock. Crack profiles computed with this model seem to be in accordance with those experimentally measured in RC elements in tension. A good agreement between numerical results and experimental data is also found both in case of steel rebar and ordinary fiber reinforced cementitious composites (R/FRCC), and in case of steel rebar and high?performance fiber reinforced cementitious composites (R/HPFRCC).     

Damage evolution of novel 3D textile-reinforced composites under fatigue loading conditions

For the simulation of the material degradation process during multiaxial fatigue loading of 3D textile-reinforced composites a new physically based damage model is developed based on the fracture mode concept (FMC) of CUNTZE and the continuum damage mechanics. For the damage analysis and the model parameter identification cyclic tests under superposed tension/compression–torque loading are performed.