Micromechanics modeling of fatigue failure mechanisms in a hybrid polymer matrix composite

Initiation of fatigue damage for a hybrid polymer matrix composite material was studied via 3-Dimensional viscoelastic representative volume element modeling in order to gain further understanding. It was found that carbon fiber reinforced composites perform better in fatigue loading, in comparison to glass fiber reinforced composites, due to the fact that the state of stress within the matrix material was considerably lower for carbon fiber reinforced composites eliminating (or at least prolonging) fatigue damage initiation. The effect of polymer aging was also evaluated through thermal aging of neat resin specimens. Short-term viscoelastic material properties of unaged and aged neat resin specimens were measured using Dynamic Mechanical Analysis. With increasing aging time a corresponding increase in storage modulus was found. Increases in the storage modulus of the epoxy matrix subsequently resulted in a higher state of predicted stress within the matrix material from representative volume element analyses. Various parameters common to unidirectional composites were numerically investigated and found to have varying levels of impact on the prediction of the initiation of fatigue damage.     

2.5D机织复合材料经向和纬向振动疲劳行为对比

2.5D机织复合材料抗分层、耐冲击,在航空发动机结构上具有巨大的应用前景。本文对一种2.5D机织碳纤维增强双马树脂基复合材料经向和纬向试件,开展了不同名义应力水平下的一阶弯曲共振疲劳试验。试验结果表明：经向试件的振动疲劳性能优于纬向试件,随着应力水平的提高,经向和纬向试件的寿命明显缩短,而固有频率下降百分比增加,试件内部的损伤严重程度和损伤扩展速度都随之提高。2.5D机织复合材料经向和纬向试件在共振疲劳试验过程中的主要失效模式是纱线与基体之间脱粘造成的结构完整性丧失,从而导致试件的刚度持续下降。试件内部损伤的三维电子计算机断层扫描(Computerized tomography,CT)重构图像表明,损伤散布于试件工作段区域,应力水平越高,2.5D机织复合材料经向和纬向试件内部损伤范围越大,损伤程度越高,而且纬向试件内部损伤状态比经向试件严重。利用双对数线性寿命模型,对经向和纬向试件在不同名义应力水平下的共振疲劳试验数据进行拟合,得到2.5D机织复合材料经向和纬向试件共振疲劳应力-寿命(Stress-life,S-N)曲线的数学模型,得到的S-N曲线可用于预测2.5D机织复合材料的寿命。     

Hybrid nanoreinforced carbon/epoxy composites for enhanced damage tolerance and fatigue life

Hybrid nano/microcomposites with a nanoparticle reinforced matrix were developed, manufactured, and tested showing significant enhancements in damage tolerance properties. A woven carbon fiber reinforced polymer composite, with the polymer (epoxy) matrix reinforced with well dispersed carbon nanotubes, was produced using dispersant-and-sonication based methods and a wet lay-up process. Various interlaminar damage tolerance properties of this composite, including static strength, fracture toughness, fatigue life, and crack growth rates were examined experimentally and compared with similarly-processed reference material produced without nanoreinforcement. Significant improvements were obtained in interlaminar shear strength (20%), fracture toughness (180%), shear fatigue life (order of magnitude), and fatigue crack growth rate (factor of 2). Observations by scanning electron microscopy of failed specimens showed significant differences in fracture surface morphology between the two materials, related to the differences in properties and providing context for understanding of the enhancement mechanisms.     

A generalized damage model for woven ply laminates under static and fatigue loading conditions

A generalized non-linear cumulative damage model for woven ply laminates subjected to static and fatigue loading is developed in this paper. The damage, consisting of small cracks running parallel to the fibers, leads to a loss of stiffness in the warp, weft and shear directions. The model presented here describes the evolution of the damage up to failure of the first ply. By replacing the woven ply by two stacked unidirectional plies corresponding to the warp and weft thicknesses, this general model is extended to cover a broad range of plies, from quasi-unidirectional to balanced woven plies. A continuum damage approach (CDM) is then used to define the behaviour of the two virtual unidirectional plies under static and fatigue loading conditions. The model is applied here to an unbalanced woven ply with glass reinforcement and the results of the simulations are compared with experimental data.     

Influence of matrix material on flexural fatigue performance of unidirectional composites

A deflection-controlled flexural fatigue study of unidirectional glass fiber reinforced epoxy and vinyl ester composites was undertaken. Damage initiation and growth for various deflection levels were evaluated. Also, quantitative assessment of damage was made by monitoring stiffness loss in the composites as a function of fatigue cycles. Results show that the glass/epoxy composite has better performance compared with the glass/vinyl ester composite, especially at low deflection amplitudes. Fatigue behavior of the composites at low deflection amplitudes is found to be primarily influenced by matric and fiber-matrix interfacial damage in the form of longitudinal splitting.     

碳纤维和SiO2纳米颗粒增强环氧树脂复合材料的压缩性能

纤维增强聚合物复合材料的压缩性能与聚合物基体力学性质密切相关。本文利用连续碳纤维（CF）和含有均匀分散的SiO₂纳米颗粒改性的环氧树脂基体,制备了CF-nano SiO₂/Epoxy微纳米多相复合材料单向层合板,并对其轴向压缩性能进行了系统的研究。试验表明,将纳米颗粒引入基体能够有效提高纤维增强聚合物基复合材料的压缩强度,占nano SiO₂/Epoxy体积为8.7%的纳米颗粒可将复合材料的压缩强度提升约62.7%。基于单向层合板的弹塑性微屈曲模型对纳米颗粒的增强效应进行了理论分析。根据含纳米颗粒的环氧树脂在压缩过程中的损伤行为,提出了一套基于加卸载试验建立纳米复合材料基体压缩本构关系的方法。将模型获得的基体本构关系与经典复合材料弹塑性微屈曲模型耦合,能够较为准确地预测本研究制备的微纳米多相复合材料的压缩强度。经试验检验,预测结果与实测数值达到很好的一致性。      

单向纤维增强聚合物复合材料压缩渐进破坏单向纤维增强聚合物复合材料压缩渐进破坏

复合材料结构在承压时破坏如何演化,是其强度破坏分析的基础和核心任务。本文提出了基于连续介质损伤力学（CDM）的单向纤维增强聚合物复合材料压缩破坏渐进损伤分析（PDA）模型。建模中考虑了材料非线性行为、失效判断及损伤演化中材料性能退化等基本问题,分别对应于拉压不对称弹塑性本构关系、Puck准则、LaRC05准则及考虑破坏面方向的刚度退化方法。该模型通过用户材料子程序接口VUMAT引入到有限元软件ABAQUS中实现了有限元求解。对文献中提供的纵向、横向及偏轴压缩案例进行了数值计算并与试验数据对比。数值分析结果与试验数据吻合较好,证明了该方法的合理性和有效性,对开展多向层合板压缩破坏分析富有参考价值。      

The effect of temperature on fatigue damage of FRP composites

The present study intends to investigate the effect of temperature on cumulative fatigue damage (D) of laminated fibre-reinforced polymer (FRP) composites. The effect of temperature on fatigue damage is formulated based on Ramkrishnan–Jayaraman and Varvani-Farahani–Shirazi residual stiffness fatigue damage models. The models are further developed to assess the fatigue damage of FRP composites at various temperatures (T). This task is fulfilled by formulating the temperature dependency of Young’s modulus (E) and ultimate tensile strength (σ_ult) as the inputs of the models. Temperature-dependant parameters of Young’s modulus and ultimate tensile strength are found to be in good agreement with the experimentally obtained data when used for unidirectional, cross-ply and quasi-isotropic FRP laminates. The proposed fatigue damage model is evaluated using six sets of fatigue damage data. The proposed temperature-dependent model was also found promising to predict the fatigue damage of unidirectional (UD) and orthogonal woven FRP composites at different temperatures.     

Bending reinforcement of timber beams with composite carbon fiber and basalt fiber materials

This work shows a study based on data obtained experimentally using bending tests of pine timber beams reinforced with composite materials. Fibers used for the execution of the reinforcement are basalt and carbon. Basalt fiber composites are applied in different grammages, whereas with carbon composites, unidirectional and bidirectional fabrics are used. The behavior of the beams was analyzed regarding the reinforcement variables applied, and the results are compared with those of the tested beams without reinforcement. This work proves the good behavior of fiber reinforce plastic (FRP) with basalt fiber when applied to timber beams, and that of bidirectional carbon fabrics as opposed to the unidirectional ones.     

Non-destructive assessment of the fatigue strength and damage progression of satin woven fiber reinforced polymer matrix composites

The aim of this study is to utilize infrared thermography to assess the critical damage states, and to capture the evolving damage processes, of 5HS and 8HS woven carbon fiber/epoxy composites subjected to uniaxial in-plane tensile quasi-static and fatigue loading. Quasi-static test results revealed that the dominant damage mechanisms were matrix cracks contained within the weft yarns, which initiated at the thermally-detected material thermoelastic limit and were confirmed through SEM observations. An established thermographic technique was also used to confirm the existence of a high cycle fatigue limit, which may in fact be a characteristic of all fabric reinforced polymeric composites. Temperature profiles captured during cyclic testing directly correlated with corresponding stiffness degradation profiles, providing support for thermography as an accurate fatigue damage metric. The infrared camera was able to detect the evolution of weft yarn cracking during the initial stage, as well as the initiation and growth of interply delamination cracking during the final stage of three-stage cyclic damage evolution. The reported results and observations provide an important step in the validation of thermography as a powerful non-destructive tool for assessing the development of damage, as well as predicting the critical damage states of fiber reinforced polymeric composite materials.     

Fatigue Life Prediction of Carbon Fiber-Reinforced Ceramic-Matrix Composites at Room and Elevated Temperatures. Part I: Experimental Analysis

This paper presents an experimental analysis on the fatigue behavior in C/SiC ceramic-matrix composites (CMCs) with different fiber preforms, i.e., unidirectional, cross-ply and 2.5D woven, at room and elevated temperatures in air atmosphere. The experimental fatigue life S???N curves of C/SiC composites corresponding to different stress levels and test conditions have been obtained. The damage evolution processes under fatigue loading have been analyzed using fatigue hysteresis modulus and fatigue hysteresis loss energy. By comparing the experimental fatigue hysteresis loss energy with theoretical computational values, the interface shear stress corresponding to different peak stress, fiber preforms and test conditions have been estimated. It was found that the degradation of interface shear stress and fibres strength caused by oxidation markedly decreases the fatigue life of C/SiC composites at elevated temperature.     

考虑温度和纤维含量影响的单向层合板材料的退化模型

根据复合材料的疲劳损伤机理,重新定义了疲劳损伤因子.根据这个疲劳损伤因子,提出了一种考虑纤维的含量和温度影响的单向纤维增强复合材料剩余刚度和剩余强度的模型;进而根据室温剩余刚度-剩余强度关联模型引入温度修正参数得到了一定温度下的剩余刚度-剩余强度关联模型,并进一步得到了与剩余刚度相关的剩余强度模型.于是,在建立剩余强度...     

三维编织碳/环氧复合材料力学性能测试及破坏机制

通过宏观拉压试验, 研究了三维正交编织碳/环氧复合材料的拉伸和压缩力学性能。对试验过程进行了声发射分析, 对断口进行了扫描电镜观察分析, 给出了该类材料的拉伸和压缩破坏机制。结果表明: 三维正交编织碳/环氧复合材料有良好的拉伸和压缩力学性能; 三维正交编织复合材料在拉伸和压缩载荷作用下的断裂均为脆性断裂, 拉伸试验的主要破坏现象是纤维断裂拔出, 而压缩试验则是纤维剪切破坏; 通过声发射参数分析可以基本判定该类材料损伤过程中的损伤类型。     

Fatigue tensile behavior of carbon/epoxy composite reinforced with non-crimp 3D orthogonal woven fabric

An experimental study of the in-plane tension-tension fatigue behavior of the carbon fiber/epoxy matrix composite reinforced with non-crimp 3D orthogonal woven fabric is presented. The results include pre-fatigue quasi-static test data, fatigue life diagrams, fatigue damage progression, and post-fatigue quasi-static test data for the warp- and fill-directional loading cases. It is revealed that the maximum cycle stress corresponding to at least 3 million cycles of fatigue life without failure, is in the range of 412-450 MPa for both loading directions. This stress range is well above the static damage initiation threshold and significantly above the first static damage threshold (determined by the onset of low energy acoustic emission). The second static damage threshold, determined by the onset of high energy acoustic emission and related to the appearance of local debonds and intensive transverse matrix cracking falls within this range. The established correlation between a 3000,000 cycle fatigue stress limit on one side and the second static damage threshold stress on the other is of a high practical importance, because it will significantly reduce the amount of future fatigue tests required for this class of composites. Surprisingly, for equal maximum cycle stress level, the fatigue life under fill-directional loading appears about three times shorter than that under warp-directional loading. The 100,000 cycle, 500,000 cycle and 1000,000 cycle fatigue loading with 450 MPa maximum cycle stress has resulted in so high variations of post-fatigue static modulus, strength and ultimate strain, that no consistent and statistically meaningful trends could have been established; further extensive experimental studies are required to reliably quantify this effect.     

Effect of span length on micromechanics of fracture under flexure load in CFRP composites

Unidirectional (UD) and bidirectional (BD) woven carbon fiber reinforced plastic (CFRP) composites were tested under three-point flexure at different span lengths ranging from 20 to 60 mm. Expectedly, flexure load decreases with increasing span length. A sharp change in flexure load is observed at a span length of 30 mm with a transition in failure mechanism. Analysis suggests that crack initiation occurs at compression side and tension side, respectively, in UD and BD woven composites. In case of UD composites, at a span length of 20 mm, the failure was governed by kink band initiation (two fractures per fiber) followed by tension side delaminations, and at other span lengths, microbuckling (multiple fiber fractures) together with compression side delaminations led to failure. On the other hand, the failure in BD composites at a span length of 20 mm is controlled predominantly by thickness crack propagation with tensile fiber pullouts, whereas for other span cases the failure occurs due to the development of delaminations at interlace regions. The stress analysis by finite element modeling (FEM) also supports the experimental observations in an attempt to correlate the failure mechanisms under flexure loads in CFRP composites.     

A unified fatigue life model based on energy method

A new unified fatigue life model based on the energy method is developed for unidirectional polymer composite laminates subjected to constant amplitude, tension–tension or compression–compression fatigue loading. This new fatigue model is based on static failure criterion presented by Sandhu and substantially is normalized to static strength in fiber, matrix and shear directions. The proposed model is capable of predicting fatigue life of unidirectional composite laminates over the range of positive stress ratios in various fiber orientation angles. By using this new model all data points obtained from various stress ratios and fiber orientation angles are collapsed into a single curve.

The new fatigue model is verified by applying it to different experimental data provided by other researchers. The obtained results by the new fatigue model are in good agreements with the experimental data of carbon/epoxy and E-glass/epoxy of unidirectional plies.     

碳纤维/玻璃纤维混织布及其环氧复合材料

碳纤维和玻璃纤维混合增强树脂基复合材料目前都用单丝铺放工艺。为了改善单向预浸料的工艺性能,开发了碳纤维和玻璃纤维混织布。用它们制备复合材料,操作方便,可用手糊或模压工艺。根据设计需要,可调整混织布的经纬比例和布的构造,以获最佳性能/价格比。本文通过六种混织布/环氧复合材料的力学性能测定和估算,表明其拉伸和弯曲的强度和弹性模量符合“混合律”。在碳纤维占总纤维的体积分量为26%时出现最低的复合材料强度,而碳纤维的体积分量超过53%时才对复合材料的强度和弹性模量部有提高。      

碳纤维增强树脂基复合材料组分疲劳强度表征

对微观力学失效（Micro-mechanics of failure,MMF）理论的应用做了扩展,将其用于分析连续纤维增强树脂基（FRP）复合材料的三维复杂结构的疲劳强度。基于MMF理论,建立了连续FRP复合材料层合板疲劳强度表征方法。分别对碳纤维/树脂（UTS50/E51）复合材料单向层合板进行静载和疲劳试验,得到层合板的基本力学性能和宏观强度指标;对UTS50/E51层合板组分疲劳强度进行了表征,得到了纤维和树脂的拉伸、压缩MMF疲劳特征参量S-lgN曲线,为MMF方法应用于连续纤维增强复合材料层合板结构的疲劳强度分析提供了判断依据。使用建立的方法对UTS50/E51多向层合板的拉伸疲劳强度进行了分析,并将预测结果与试验结果进行对比。     

Fatigue performance characterisation of resistance-welded thermoplastic composites

This study investigates the fatigue performance of resistance-welded thermoplastic composites. Lap shear specimens consisting of unidirectional carbon fibre/poly-ether-imide (CF/PEI), unidirectional carbon fibre/poly-ether-ketone-ketone (CF/PEKK) and 8-harness satin weave fabric glass fibre/poly-ether-imide (GF/PEI) composites were resistance-welded using a metal mesh heating element. The specimens were fatigue-tested at various percentages of their static lap shear strengths at a load ratio R = 0.1 and frequency f = 5 Hz. The fatigue performances of resistance-welded semi-crystalline (PEKK) and amorphous (PEI) composites were compared and the failure modes of the specimens were described. The stiffness degradation was monitored during the tests in order to evaluate the damage accumulation in the specimens. Linear stress-life (S–N) curves were obtained for all three materials when plotted on a semi-log scale. Interlaminar failure modes, involving tearing of the heating element and damage to the adherends were observed. The indefinite fatigue lives of CF/PEKK and CF/PEI welded specimens were obtained at 25% of their static lap shear strengths. The indefinite fatigue life of the GF/PEI welded specimens was obtained at 20% of the static lap shear strength.