Thermal cycling response behavior of ceramic matrix composites under load and displacement constraints

Thermal cycling response of a two-dimensional carbon fiber reinforced SiC matrix composite (2D C/SiC) to load constraint (LC) and to displacement constraint (DC) in an oxidizing environment was investigated. During thermal cycling between 700 and 1200 °C, a constraint strain with a 0.208% range and a constraint stress with a 180 MPa range were, respectively, generated on the composites in LC and DC. It was found that with increasing cycles, the constraint strain increased in LC and the constraint stress decreased in DC. After 50 cycles, in contrast to the as-received composite materials, the as-cycled composites suffered greater loss in mechanical properties: the residual strength and failure strain are 204 MPa and 0.49% for the LC tested samples, and 223 MPa and 0.64% for the DC tested samples, respectively. Microstructural observations indicated that the LC could develop thermal microcracks and assist in oxidizing the internal fibers, whereas the DC reduced crack propagations and fiber oxidation because of decreasing tensile and increasing compressive stresses.     

碳纤维/环氧树脂预浸带铺放应力场及制品微观结构的模拟与实验

碳纤维/环氧树脂预浸带复合材料在铺放成型时,由于树脂基体与碳纤维之间的热膨胀系数存在差异以及成型时热-力参数作用下由于纤维的变形而导致纤维与基体接触处产生应力集中等原因,在制品材料中会产生热残余应力。针对碳纤维/环氧树脂预浸带复合材料的实际结构特点,利用ABAQUS有限元软件建立含有界面的碳纤维/环氧树脂预浸带复合材料的细观代表性体积单元(Representative volume element, RVE)有限元模型,采用实验研究和有限元仿真分析的方法,研究在温度-压力参数作用下预浸带铺放制品残余应力的分布规律及影响机理。首先,建立预浸带铺放时的温度和压力模型,研究不同温度和压力参数条件下碳纤维/环氧树脂预浸带铺放制品残余应力的分布情况。其次,采用耦合降温法模拟碳纤维/环氧树脂预浸带残余应力随纤维体积含量、铺放压力以及铺放温度的变化规律,并采用扫描电镜对不同工艺参数条件下预浸带铺放制品的微观结构进行分析。通过对模拟结果进行分析比较得到各因素对制品残余应力的基本影响规律;最后进行不同温度和压力等铺放参数对预浸带铺放成型时残余应力影响的实验测试研究。      

高温下短纤维增强金属基复合材料界面的微观结构和热残余应力状态研究

利用透射电镜观察了δ-Al₂O₃短纤维增强Al-5.5Mg合金复合材料界面在不同环境温度下的微观结构特征。 同时, 基于该类复合材料的单纤维模型, 利用弹塑性有限元分析方法, 研究了在不同温度下界面热残余应力的大小和分布情况, 并讨论了热残余应力对界面行为的影响。 最后, 讨论了界面的微观结构和热残余应力特征对复合材料整体性能的影响。 研究表明, 不同环境温度下, 界面具有不同的微观结构和热残余应力特征, 这些特征的变化将引起复合材料整体性能的明显变化。     

Thermal stress development in fibrous composites

In this work, the thermal stress development in anisotropic fiber-reinforced polymer composites is investigated for temperatures below the glass transition temperature of the resin. By applying two independent experimental methodologies, it was found that the initial thermal (residual) strain in the reinforcing fibers is compressive of about − 0.04% at ambient temperatures. This is due to the mismatch of the thermal expansion coefficient between the polymer matrix and fiber, as the material is cooled down from the processing temperature. However, on reheating the composites the compressive stress in the fiber gradually diminishes and becomes zero at 50 °C. Further heating to 100 °C introduces tensile strains in the fiber of maximum of 0.13%. The conformity of these results to analytical models that relate the composite thermal strain to the thermal expansion coefficients of fiber and resin, as well as, the fiber volume fraction, is examined. Finally, the possibility of tailoring the sign (positive, negative or, even, zero) of the composite thermal expansion coefficient of certain advanced composites by simply varying the thermal expansion of the polymer matrix, is discussed.     

Analysis of Residual Performance of UD-CMC in Oxidation Atmosphere Based on a Notch-like Oxidation Model

Experimental observation indicates unidirectional ceramic matrix composites (UD-CMC) will react with oxygen under high-temperature atmosphere inhomogeneous. As a result of the oxidation on fiber surface, fiber shows a notch-like morphology. Stress concentration near by the fiber notch causes a decline of the mechanic performance of UD-CMC. In this paper, the change rule of fiber notch depth is fitted by circular function. Based on this formula the residual strength and modulus of UD-CMC under 400–900 °C atmosphere are derived. The mechanical performance of unidirectional C/SiC composite is simulated by finite element method. The stress distribution of fiber, matrix and interface are obtained. The residual properties of unidirectional C/SiC composite are predicted by theoretical method and finite element method. And the predicting results are compared with the experiment data. The predicting results show a good accordance with experiment data, which means the notch-like oxidation model can analyze the mechanic performance of UD-CMC efficiently.     

More on the effects of thermal residual and hydrostatic stresses on yielding behavior of unidirectional composites

The influence of manufacturing process thermal residual stresses and hydrostatic stresses on yielding behavior of unidirectional fiber reinforced composites has been investigated when subsequently subjected to various mechanical loadings. Three-dimensional finite element micro-mechanical models have been used. The results of this study reveal that the size of the initial yield surface is highly affected by the thermal residual and hydrostatic stresses. It was also found that effects of a uniform temperature change on the initial yield surface in the composite stress space is not equivalent to a solid translation of the surface in the direction of the hydrostatic stress axis. At the micro-level, magnitudes of various stress components within the matrix due to the thermal residual and hydrostatic stresses are different. However, at a macro-level, both temperature change and hydrostatic loading of composites show similar effects on the initial yield surface in the composite stress space. In an agreement with experimental data, results also show that residual stresses are responsible for asymmetric behavior of composites in uniaxial tension/compression in the fiber direction. This asymmetric behavior suggests that the existing quadratic yield criteria need modification to include thermal residual stress effects.     

Stress Concentrations Caused by Embedded Optical Fiber Sensors in Composite Laminates

The fiber optic sensor (FOS) embedded perpendicular to reinforcing fibers causes an `Eye' shaped defect. The length is about 16 times fiber optic radius (R_Fos) and height is about 2R_Fos. The eye contains fiber optics in the center surrounded by an elongated resin pocket. Embedding FOS causes geometric distortion of the reinforcing fiber over a height equal to 6 to 8 R_Fos. This defect causes severe stress concentration at the root of the resin pocket, the interface (in the composite) between the optical fiber and the composite, and at 90° to load direction in the composite. The stress concentration was calculated by finite element modeling of a representative micrograph. The FE results agreed reasonably with analytical and experimental data in the literature for a similar problem. The stress concentration in axial direction was about 1.44 and in transverse direction at the interface was -0.165 and at resin pocket was 0.171. Under tensile loading, the initial failure was by transverse matrix cracking (fiber splitting) at the root of the resin pocket, then that lead to final fracture by fiber breakage. Under compression loading, the failure initiation was by interfacial cracking due to large transverse tensile stress and the final fracture was by compression. Fracture stress calculated from the analysis using the maximum stress criteria agreed reasonably with test data.     

考虑纤维随机分布的复合材料热残余应力分析及其对横向力学性能的影响

建立了考虑纤维随机分布并包含界面的复合材料微观力学数值模型,模拟玻璃纤维/环氧复合材料固化过程中的热残余应力。通过与纤维周期性分布模型的计算结果进行对比,发现纤维分布形式会对复合材料的热残余应力产生重要影响,纤维随机分布情况下的最大热残余应力明显大于纤维周期性分布的情况下。研究了含热残余应力的复合材料在横向拉伸与压缩载荷下的损伤和破坏过程,结果表明:热残余应力的存在显著影响了复合材料的损伤起始位置和扩展路径,削弱了复合材料的横向拉伸和压缩强度。在横向拉伸载荷下,考虑热残余应力后,复合材料的强度有所下降,断裂应变显著降低;在横向压缩载荷下,考虑热残余应力后,复合材料的强度略有下降,但失效应变基本保持不变。由于热残余应力的影响,复合材料的横向拉伸和压缩强度分别下降了10.5%和5.2%。      

Thermal residual stresses in metal matrix composites: A review

Recently, metal matrix composites (MMCs) have generated a considerable interest in the materials field because of their attractive physical and mechanical properties. However, during the fabrication of MMCs, thermal residual stresses are reportedly developed in the matrix as a result of the mismatch of the thermal expansion coefficients between the reinforcement and the matrix. It is well established that these residual stresses have a significant effect on the composite properties. For example, due to the presence of thermal residual stresses, it is almost never possible to achieve the maximum elastic response of the composites. In addition, yield stress and fracture toughness of the composites are significantly affected by thermal residual stresses. In this paper, a critical review of the published literature on thermal residual stresses in MMCs and their effect on composite properties are presented. Also, experimental and numerical techniques that are currently available to measure and estimate thermal residual stresses are reviewed and discussed.     

金属基复合材料的热残余应力力学模型研究进展

金属基复合材料在高温制备的冷却过程中或热处理过程中由于组分间热膨胀系数（CTEs）的差异会产生较大的热残余应力，热残余应力对复合材料的宏观性能有着重要的影响。本文综述了分析金属基复合材料热残余应力的有限元模型和解析模型等理论模型，并指出有待深入研究的问题。     

FEM elasto-plastic analysis of a new manufacturing method for covering the free surface of unidirectional composites

Fiber/matrix debonding at the free surface of unidirectional composites deteriorates their performance and ultimately reduces the working life of these materials. Identifying the source, controlling the propagation and elimination of the damages has been the subject of many research attempts. The results show that the singular radial thermal residual stress at the fiber/matrix interface on the free surface of composites is the main reason for initiation of the debonding. Also, it has been experimentally and numerically proven that a thin layer of matrix-like cover on the free surface could highly help in elimination and/or reduction of the damage. However, the results were limited to the samples, which their free surface is covered during their production process. To extend the results to the pre-made composites, one should be able to deal with the existing free surface debondings and/or the residual thermal stresses. Here, in this study, a new manufacturing method for covering the free surface of metal matrix composites is suggested and it is numerically shown that elimination of the end effects before casting the covering materials on the free surface is necessary. Different manufacturing schemes are considered and numerically tested.     

Finite-element modeling of initial matrix failure in CFRP under static transverse tensile load

The failure of transversely loaded unidirectional CFRP has been investigated by the use of mechanical and thermo-mechanical test methods and finite-element analysis. The case considered here is characterized by a high interfacial strength between fiber and matrix, so that matrix failure governs the fracture process of the composite. On the basis of the experimental results, the parabolic and other failure criteria were applied to the FE calculations. The failure dependence of the resin on the actual stress state could be described. Furthermore, the influence of thermal residual stresses on the initial matrix failure has been investigated, and the actual stiffnesses and thermal expansion changes of the epoxy resins and the composites as a function of temperature have been determined experimentally. The results of the mechanical and thermo-mechanical tests performed on the pure resins and on the composites were incorporated into a finite-element analysis and compared with the transverse tensile properties of the composite laminates. In the FE analysis, the local fiber-volume fraction was varied over a wide range in order to investigate its influence on the thermal residual stresses and transverse composite strength. The results could explain the low strain to failure of transverse laminates under tensile loading.     

Effects of thermal cycling on thermal expansionand mechanical properties of SiC fibre-reinforced reaction-bonded Si3N4 composites

Thermal expansion curves for SiC fibre-reinforced reaction-bonded Si3N4 matrix composites (SiC/RBSN) and unreinforced RBSN were measured from 25 to 1400 °C in nitrogen and in oxygen. The effects of fibre/matrix bonding and cycling on the thermal expansion curves and room-temperature tensile properties of unidirectional composites were determined. The measured thermal expansion curves were compared with those predicted from composite theory. Predicted thermal expansion curves parallel to the fibre direction were between the measured curves for the strongly- and weakly-bonded composites, but those normal to the fibre direction for both bonding cases were similar to that of the unreinforced RBSN. Thermal cycling in nitrogen for both bonding cases resulted in no net dimensional changes at room temperature and no loss in tensile properties from the as-fabricated condition. In contrast, thermal cycling in oxygen for both composites caused volume expansion primarily due to internal oxidation of RBSN. Cyclic oxidation affected the mechanical properties of the weakly-bonded SiC/RBSN composites the most, resulting in loss of strain capability beyond matrix fracture and catastrophic, brittle fracture. Increased bonding between the SiC fibre and RBSN matrix due to oxidation of the carbon-rich fibre surface coating and an altered residual stress pattern in the composite due to internal oxidation of the matrix are the main reasons for the poor mechanical performance of these composites. This revised version was published online in November 2006 with corrections to the Cover Date.     

力、热载荷作用下纤维在复合材料中的界面应力传递行为

采用三维光弹性实验应力分析和有限元计算两种方法,在拉拔载荷和热残余应力联合作用下,对单丝拔出树脂基复合材料三维冻结切片界面剪应力进行了研究。实验结果和计算表明,在单纤维与基体界面的埋入端及埋入末端附近出现界面残余剪应力的极值;力、热载荷作用下纤维界面剪应力呈抛物线分布,单丝埋入端附近是应力的主要传递区域,最先达到危险应力,出现界面脱胶破坏,然后剪应力沿纤维埋入长度由纤维埋入端附近向埋入末端逐渐传递;界面热残余应力对界面剪应力的影响是使纤维埋入末端应力集中程度降低,使界面剪应力最大值增大。      

短纤维增强橡胶复合材料热应力及热-结构耦合的数值研究

基于橡胶材料的非线性和不可压缩特性,建立细观数值模型,采用非线性有限元方法,在细观层面,通过对短纤维增强橡胶复合材料受热载荷和热-结构载荷时的应力传递分析,研究了温度对材料热弹性和失效形式的影响,探讨了对材料热应力影响的细观结构关键因素,揭示了材料的细观破坏机理。研究表明:当受热载荷时,界面处纤维受到的压应力加强了橡胶和纤维的粘合,而纤维的端部容易脱粘;材料受热-拉伸载荷时的应力是热应力及拉伸载荷产生应力的线性组合,且随着温度增大,界面脱粘失效的几率增大,纤维断裂失效的几率减小,温度的升高使复合材料的刚度急剧下降。     

Thermal cracking of multicomponent white cast iron

Abstract

The effect of the volume fraction of eutectic carbides on the thermal fatigue resistance of multicomponent white cast iron has been investigated. Thermal fatigue tests were carried out for 100 and 500 cycles. Nucleation of thermal fatigue cracks took place mostly at the specimen surface, induced by mechanical and metallurgical stress raisers. The crack nucleated in the matrix as well as at the carbide/matrix interface or at the carbide itself. The surface crack density increased slightly for increasing volume fraction of eutectic carbides from 9 to 14%, approximately. Crack propagation took place mostly at the carbide/matrix interface or through the carbide. The propagation rate was affected by the carbide distribution: the higher was the 'carbide continuity/mean free path between carbides' ratio, the higher was the propagation rate. The propagation rate decreased with increasing test time, regardless of the volume fraction of eutectic carbides.     

胶层厚度对碳纤维/双马来酰亚胺树脂复合材料平-折-平混合连接接头力学性能的影响

以碳纤维/双马来酰亚胺(BMI)树脂复合材料平-折-平(FJF)连接接头为对象,通过试验对比分析了特定胶层厚度下碳纤维/BMI树脂复合材料FJF连接接头的静强度和疲劳性能,并探究了胶层厚度对碳纤维/BMI树脂复合材料FJF混合接头力学性能的影响。利用背面应变技术对碳纤维/BMI树脂复合材料FJF混合接头搭接区端部胶层开裂进行监测。利用有限元软件ABAQUS对不同胶层厚度下碳纤维/BMI树脂复合材料FJF混合接头搭接区胶层应力分布进行了分析。结果表明,碳纤维/BMI树脂复合材料FJF混合接头的平均拉伸极限载荷、搭接区端部胶层开裂平均循环次数和平均疲劳寿命均随着胶层厚度在0.1~0.3 mm范围内增加而增大。不同胶层厚度的碳纤维/BMI树脂复合材料FJF混合接头均经历相同的失效阶段,即搭接区胶层端部开裂,胶层沿搭接区断裂扩展,最终靠近加载端孔边拉伸断裂,呈±45°断口。随着胶层厚度在0.1~0.3 mm范围的增加,搭接区端部胶层剥离应力、剪切应力及孔边胶层压缩应力均减小。在胶层厚度为0.1~0.3 mm范围内,剪应力是胶层破坏的控制因素。      

树脂基纤维复合材料的热残余应力数值分析

以环氧树脂R368-1/硼纤维复合材料为研究对象,采用柱体单胞结构,建立了三维有限元分析模型。考虑试样加工制备过程和常温使用时的温度差,对残余应力分布特点和应力水平进行了讨论,给出了应力分布云图和应力沿径向的分布规律。进一步考察了纤维体积分数、温度差和附加界面层对残余应力分布的影响,结果表明,基体主要受拉伸应力作用,纤维主要受压缩应力作用,纤维体积分数增加和附加界面层有助于改善复合材料中残余应力的分布,试样制备温度的升高对纤维中应力的增加具有较大影响。     

Thermisch‐mechanisches Ermüdungsverhalten der partikelverstärkten Aluminiumknetlegierung EN AW‐6061‐T6 und die Entwicklung von Eigenspannungen im Matrixwerkstoff unter thermisch‐mechanischer Beanspruchung

Thermal mechanical fatigue behaviour of particle reinforced EN AW‐6061‐T6 and development of residual stresses in the matrix material by thermal mechanical loading The behaviour of non reinforced and 15 Vol.‐% α‐alumina particle reinforced wrought aluminium alloy EN AW‐6061‐T6 in thermal mechanical fatigue loading was investigated at different maximum temperatures. The tests were performed in strain controlled mode by means of an electro‐mechanical testing machine. Alternating load deformation and life cycle behaviour either materials were compared. It came out, that the reinforcement leads to an decreasing thermal mechanical fatigue life cycle while keeping constant the maximum temperature and mechanical loading. The two materials showed softening behaviour due to high maximum temperatures of 573 K to 673 K. However, there is an intense scatter of the number of cycles to failure of the non reinforced alloy aggravating the interpretation of the results. On the other hand the thermal mechanical life cycle increases in combination with increasing maximum temperatures. Simultaneously the part of plastic deformation in mechanical loading increases for both materials, while for a constant total strain range the effective maximum and minimum stresses are decreasing. Furthermore, the development of residual stresses in the matrix of the reinforced alloy by thermal mechanical fatigue loading was analysed. It was observed that only small absolute values of residual stresses will be obtained for these loads. Nevertheless, tendencies of mounting tensile residual stresses can be identified in the direction of thermal mechanical fatigue loading and subsequently reduction of the residual stresses.     

复合材料跨尺度失效准则及其损伤演化

提出了一种新的基于物理失效模式的复合材料跨尺度失效准则, 从细观层面分别对纤维和基体的失效模式进行了表征, 将纤维失效分为拉伸失效和压缩失效, 将基体失效分为膨胀失效和扭曲失效。建立了相应的失效准则及损伤演化方法。通过正方形和六边形的代表体积单元(RVE)模型, 计算了宏观应力到细观应力的机械应力放大系数和热应力放大系数。以IM7/5250-4复合材料拉伸试验作为算例对失效模型进行了验证。计算结果与试验结果吻合较好, 表明跨尺度失效准则能够准确预测复合材料层合板的破坏。