Thermal expansion of morphologically textured short-fiber composites

We studied, both experimentally and theoretically, the thermoelastic response of short-fiber composites with a preferred orientation of the short fibers, i.e., a morphological texture. Our theoretical efforts are general, being applicable to any composite system with constituents that exhibit linear thermoelastic response. We propose a relatively simple micromechanics model to predict the thermal expansion coefficient (CTE), and give simple, easily used, results for orientation distributions of practical significance. We also present a convenient approach to represent the effects of texture on thermal expansion of short fiber composites, namely, a texture map. Our experimental efforts focus on a series of extruded SiC/Al short-fiber composites that we have fully characterized. This includes measurement of the complete set of elastic constants, the volume fraction, and the fiber orientation distribution function (ODF) using neutron diffraction. We measured the axial and transverse components of the overall CTEs of these composites using a quartz-rod dilatometer. Predictions are in good agreement with measurements.     

Field Investigations of Cracking on Concrete Pavements

This paper presents the results of several investigations to identify the underlying causes of longitudinal cracking problems in Portland cement concrete (PCC) pavement. Longitudinal cracking is not intended and detrimental to the long-term performance of PCC pavement. Longitudinal cracking problems in five projects were thoroughly investigated and the findings indicate that longitudinal cracking was caused by: (1) late or shallow saw cutting of longitudinal joints; (2) inadequate base support under the concrete slab; and (3) the use of high coefficient of thermal expansion (CTE) aggregates. When the longitudinal cracks were caused by late or shallow saw cutting of longitudinal joints, cracks developed at a very early stage. However, when there was adequate base support, the longitudinal cracks remained relatively tight even after decades of truck trafficking. Another cause of longitudinal cracking was inadequate base support, and cracking due to this mechanism normally progressed to rather wide cracks. Some cracks were as wide as 57?mm. Evaluations of base support by dynamic cone penetrometer in areas where longitudinal cracks were observed indicate quite weak subbase in both full-depth repaired areas and surrounding areas. This implies that the current requirements for the subbase preparation for the full-depth repair are not adequate. Another cause of longitudinal cracking was due to the use of high CTE aggregate in concrete. Large volume changes in concrete when coarse aggregate with high CTE is used could cause excessive stresses in concrete and result in longitudinal cracking. To prevent longitudinal cracking, attention should be exercised to the selection of concrete materials (concrete with low CTE) and the quality of the construction (timely and sufficient saw cutting and proper selection and compaction of subbase material).     

Thermal transient stresses due to rapid cooling in a thermally and elastically orthotropic medium

This paper reports the residual thermal transient stresses that develop within a thermally and elastically orthotropic medium with a rectangular boundary. The material is rapidly cooled from the steady-state thermal environment under prescribed surface temperatures down to room temperature. Based upon the solution of transient temperature field, the thermal stress analysis is performed by using an elastic displacement potential approach. The two-dimensional temperature and stress fields have been obtained in series expansion forms. Numerical calculations are performed for a typical unidirectional fiber composite. The results demonstrate that the residual thermal stresses exhaust a significant portion of the tensile strength of the composite material. Formerly Visiting Scholar at the Center for Composite Materials, University of Delaware     

纵筋锈蚀无腹筋混凝土梁抗剪性能细观数值研究

以钢筋混凝土梁为研究对象,考虑钢筋非均匀锈蚀膨胀效应,建立三维钢筋混凝土梁剪切破坏分析的数值分析模型。通过多阶段分析方法(钢筋锈蚀阶段,构件性能退化阶段)探索锈蚀对结构力学行为的影响。钢筋的非均匀锈蚀膨胀以施加非均匀径向位移的方式模拟,获得保护层的破坏状态,并以此“最终状态”作为之后混凝土梁静载试验的“初始条件”输入,进而模拟构件的力学行为。在验证了多阶段数值模型合理性的基础上,分析了纵筋锈蚀、剪跨比对无腹筋混凝土梁抗剪性能的影响规律。结果表明,纵筋锈蚀使混凝土梁产生明显的纵向裂缝。纵筋锈蚀率增大,保护层开裂区域增加,梁的抗剪承载力下降严重。另外,剪跨比对梁的抗剪承载力产生影响,剪跨比对未锈蚀梁的影响明显大于对锈蚀梁的影响程度。最后,基于模拟结果对相关设计规范中的抗剪承载力计算公式进行了讨论,发展建立了考虑锈蚀影响的无腹筋混凝土梁抗剪承载力计算方法。      

Thermal stress fracture of refractory lining components: Part III. Analysis of Fracture

The fracture behavior of refractory components heated from one end is simulated using a twodimensional constant heating rate thermoelastic model and the maximum principal tensile stress fracture criterion. Dimensionless graphical relationships that can be used to predict location of fracture and orientation of cracking are presented. Dimensional analysis and the finite element numerical method are used to develop a general solution for the total strain energy. Based on the premise that extent of crack propagation is directly related to available strain energy at fracture and inversely related to the surface energy per unit area, the solution for total strain energy is used to derive a damage resistance parameter useful for the design and selection of refractory components that accounts for material properties, geometry, and heating and cooling rate. Model predictions of location of fracture, orientation of cracking, and extent of crack propagation are in general agreement with experimental results previously reported in the literature. Limitations of the two-dimensional thermoelastic model are discussed.     

Micromechanical Modeling of Filament Wound Cement-Based Composites

A theoretical model to predict the response of laminated cement-based composites is developed. The micromechanical model simulates the mechanical response of a multilayer cement-based composite laminate under uniaxial, biaxial, and flexural loading modes. Tsai-Wu Criterion is used for each lamina and the stacking sequence is utilized to obtain the overall stiffness matrix. The effect of distributed cracking on the stiffness degradation of the cross ply layers under tensile loading is measured using a scalar damage parameter that is empirically related to the apparent strain. The model is calibrated by predicting the load versus deformation response of unidirectional, cross ply, and angle ply laminates under tensile and flexural loading. Results are then compared to the experimental results cross ply and angle composites with various stacking sequences.     

Coefficients of thermal expansion of metal-matrix composites for electronic packaging

Finite element analyses of the effective coefficient of thermal expansion (CTE) of metal-matrix composites are presented, with a focus on composites with potential for use in electronic packaging applications. The analyses are based on two-dimensional plane strain and axisymmetric unit-cell models. The brittle phase is characterized as an isotropic elastic solid with isotropic thermal expansion. The possibility of plastic deformation, described by an isotropic-hardening flow rule, is allowed for in the ductile phase. A wide range of reinforcement volume fractions is considered. The effects of phase geometry, phase contiguity, ductile phase material properties, processing-induced residual stresses, and brittle particle fracture are considered. The CTE is found to be much less sensitive to phase distribution effects than is the tensile stiffness. The results show that there is a significant dependence of the overall CTE on the phase contiguity (i.e., on whether the brittle or the ductile phase is continuous).     

Transverse Thermal Expansion of FRP Bars Embedded in Concrete

Fiber reinforced polymers (FRPs) have a thermal expansion in the transverse direction much higher than in the longitudinal direction and also higher than the thermal expansion of hardened concrete. The difference between the transverse coefficient of thermal expansion of FRP bars and concrete may cause splitting cracks within the concrete under temperature increase and, ultimately, failure of the concrete cover if the confining action of concrete is insufficient. This paper presents the results of an experimental investigation to analyze the effect of the ratio of concrete cover thickness to FRP bar diameter (c/db) on the strain distributions in concrete and FRP bars, using concrete cylindrical specimens reinforced with a glass FRP bar and subjected to thermal loading from ?30?to?+80°C. The experimental results show that the transverse coefficient of thermal expansion of the glass FRP bars tested in this study is found to be equal to 33 (×10?6?mm/mm/°C), on average and the ratio between the transverse and longitudinal coefficients of thermal expansion of these FRP bars is equal to 4. Also, the cracks induced by high temperature start to develop on the surface of concrete cylinders at a temperature varying between +50 and +60°C for specimens having a ratio of concrete cover thickness to bar diameter c/db less than or equal to 1.5. A ratio of concrete cover thickness to glass fiber reinforced polymers (GFRP) bar diameter c/db greater than or equal to 2.0 is sufficient to avoid cracking of concrete under high temperature up to +80°C. The analytical model, presented in this paper, is in good agreement with the experimental results, particularly for negative temperature variations.     

Comprehensive Properties of 400 MPa Grade Corrosion-Resistant Rebar

 The corrosion resistance of the developed 400 MPa grade rebar was evaluated by a series of experiments, including cycles of corrosion-accelerating tests in the simulated concrete pore solution and reinforced concrete cube corrosion-accelerating tests and in-situ exposure experiments in chloride ions condition. In addition, the tensile and bending properties and the connection adaptability of the developed rebar were investigated. The results verify that the comprehensive properties of the corrosion-resistant rebar are excellent. The tensile and bending properties of the rebar are up to the standard of GB1499-2007. The common welding method and the mechanical connection technology of knob-cut rolled parallel thread splicing are suitable for the rebar.     

铝热-离心法制备不锈钢内衬复合钢管残余应力有限元分析

用有限元方法计算了铝热离心法制备不锈钢内衬复合钢管过程中的残余热应力分布。分析了钢管壁厚、复合钢管半径及冷却条件对残余热应力分布的影响。结果表明,缓慢冷却到室温后,不锈钢层中有很大的周向残余拉应力,已超过不锈钢的抗拉强度。钢管壁厚及复合钢管半径对残余周向热应力分布的影响不明显。复合钢管半径地径向残余热应力的影响较明显,复合钢管半径越大,径向残余压力越小。复合钢管制备时,在高温状态下直接在复合钢管外表面喷水能在不锈钢层表面造成压应力状态,有利于防止裂纹生成。     

Prediction of the Failure Time of Glass Fiber Reinforced Plastic Reinforced Concrete Beams under Fire Conditions

A model is proposed to predict the time to failure of reinforced concrete beams in a fire. The model is developed specifically to predict the lifetime of beams reinforced with glass fiber reinforced plastic rebar, but is applicable to beams with any form of reinforcement. The model is based on the calculations for flexural capacity and shear capacity of beams embedded within ACI design codes where time and temperature dependent values for rebar modulus and strength and concrete strength replace the static design values. The base equations are modified to remove safety factors and where necessary the temperature induced reductions in strength for concrete and steel are derived using the equations presented by EUROCODE 2. In order to validate the model it was used to predict the failure times of steel rebar reinforced beams that had been documented in the literature. There was excellent agreement between the model and the reported lifetimes for these conventional beams. The model was applied to predict the lifetimes of two beams that had been manufactured and tested for destruction in a fire by the research group. The model predicted that the failure mode of the beams would be because of rebar rupture as opposed to the design condition of concrete crushing and this was confirmed by the experimental test results. The model provided reasonable agreement with experimental results with a lifetime of 108?min predicted based on flexural failure and 94 and 128?min observed in the experiments.     

The effect of thermal history on the axial tensile behavior of an aligned fiber composite

The axial tensile behavior of an aligned tungsten fiber-copper composite has been studied in conjunction with plastic accommodation to thermal expansion mismatch. When the composite is subjected to a large change in temperature, the thermal expansion mismatch between matrix and fiber is accommodated by plastic deformation of the matrix. In contrast to the case of perfectly elastic accommodation (matrix and fiber both elastic), the composite undergoing plastic accommodation is in one of two distinct mechanical states at a given temperature, one of which is obtained on heating and the other on cooling. This results in that the subsequent tensile behavior of the composite is essentially dependent on the thermal history. A simple analysis based on three dimensional continuum mechanics is presented to explain the observed phenomenon.     

Thermal expansion of metals reinforced with ceramic particles and microcellular foams

The thermal expansion of three isotropic metal-matrix composites, reinforced with SiC particles or microcellular foam, is measured between 25 °C and 325 °C. All three composites show initial co-efficient of thermal expansion (CTE) values in agreement with the Turner model predictions, and near Schapery’s lower elastic bound for CTE. At higher temperatures, the CTE of foam-reinforced Al decreases, while that of the two particle-reinforced composites increases. These observations are interpreted as resulting from the presence of a very small fraction of microscopic voids within the infiltrated composites. This interpretation is confirmed with finite-element simulations of the influence of voids, cracks, and reinforcement convexity in two-dimensional (2-D) composites featuring an interconnected reinforcement of SiC surrounding isolated Al phase regions, thermally cycled from an elevated processing temperature and deforming in generalized plane strain.     

A model for the formation and solidification of grain boundary liquid in the heat-affected zone (HAZ) of welds

The rapid thermal cycle experienced in most welding operations can promote the constitutional liquation of precipitates in certain alloy systems. Grain boundary liquid films form in the subsolidus portion of the heat-affected zone (HAZ) as a consequence of constitutional liquation. Rapid cooling limits the extent of solute diffusion into the matrix from the grain boundary liquid and hence extends its solidification temperature range. Liquation cracking can occur in the HAZ if the grain boundary film exists at a time when the local thermal stresses become tensile. Hence, in order to predict the liquation cracking susceptibility of an alloy under a given welding condition, both the microstructural evolution which centers around grain boundary liquation and the stress generation have to be modeled. This article addresses the issue of microstructural evolution and attempts to present a model for the formation of grain boundary liquid during the heating cycle and its solidifiction during the cooling cycle. The variables which increase the life of the transient grain boundary liquid during the thermal cycle are identified. The onedimensional (1-D) model presented here is an important first step toward the ability to predict liquation cracking susceptibility of an alloy during welding.     

The effect of thermal history on the axial tensile behavior of an aligned fiber composite

The axial tensile behavior of an aligned tungsten fiber-copper composite has been studied in conjunction with plastic accommodation to thermal expansion mismatch. When the composite is subjected to a large change in temperature, the thermal expansion mismatch between matrix and fiber is accommodated by plastic deformation of the matrix. In contrast to the case of perfectly elastic accommodation (matrix and fiber both elastic), the composite undergoing plastic accommodation is in one of two distinct mechanical states at a given temperature, one of which is obtained on heating and the other on cooling. This results in that the subsequent tensile behavior of the composite is essentially dependent on the thermal history. A simple analysis based on three dimensional continuum mechanics is presented to explain the observed phenomenon.     

Effects of bond coat misfit strains on the rumpling of thermally grown oxides

One factor governing the durability of thermal barrier systems is the concurrent thickening and elongation of the thermally grown oxide (TGO) upon temperature cycling. The elongation can cause cyclic rumpling of the TGO: influenced by oxide growth, bond coat phase transformations, substrate-bond coat interdiffusion, and constituent strengths. The individual effects of these phenomena cannot be understood by experiment alone. In the current study, simulations are conducted to isolate the effects of the misfit strains between the bond coat and substrate. These strains originate from thermal expansion mismatch, phase transformations, and bond coat swelling. For each calculation, the response of the system throughout an individual thermal cycle is linked to the stresses in the bond coat and TGO. Results obtained for representative misfit strains indicate that all three sources promote rumpling during the early stages, while phase transformations and thermal expansion mismatch are more prevalent upon extended cycling. These misfits also induce tensile stresses in the oxide large enough to cause cracking at high temperature. Further analysis has been used to assess the benefits of developing bond coats having lower phase transformation temperature, higher strength, and a more closely matched coefficient of thermal expansion.     

混凝土包裹砂岩复合结构变形破坏特征分析

矿山工程中针对不稳定矿柱,对其进行混凝土包裹处理,是一项行之有效的加固措施.国内外对混凝土包裹岩石复合结构的研究相对较少.通过混凝土包裹砂岩复合结构单轴抗压强度试验,对其变形破坏特征进行分析.结果表明:混凝土包裹砂岩复合结构在单轴荷载作用下,其破坏形态以“X”型渐进式为主,裂纹主要为“X”型贯穿包裹体;经混凝土包裹处理的砂岩,其极限荷载与残余强度明显提高.从能量的角度研究了单轴压缩条件下混凝土包裹砂岩复合结构的力学行为,分析了混凝土包裹砂岩复合结构与未包裹砂岩在变形破坏过程中能量吸收与极限荷载之间的关系,进而从能量的角度说明混凝土包裹措施对提高矿柱极限荷载具有较大贡献.      

镀钛金刚石增强玻璃基复合材料的性能

现代电子封装迫切需要开发新型高导热陶瓷（玻璃）基复合材料.本文在对镀钛金刚石进行镀铜和控制氧化的基础上,利用放电等离子烧结方法制备了金刚石增强玻璃基复合材料,并观察了其微观形貌和界面结合情况,测定了复合材料的热导率和热膨胀系数.实验结果表明：复合材料微观组织均匀,Ti/金刚石界面是复合材料中结合最弱的界面,复合材料的热导率随着金刚石粒径和含量的增大而增加,而热膨胀系数随着金刚石含量的增加而降低.当金刚石粒径为100 μm、体积分数为70%时,复合材料热导率最高达到了40.2 W·m^-1·K^-1,热膨胀系数为3.3×10^-6K^-1,满足电子封装材料的要求.     

Copper alloy-impregnated carbon-carbon hybrid composites for electronic packaging applications

Porous carbon-carbon preforms, based on three-dimensional networks of PAN (Polyacrylonitrile)-based carbon fibers and various volume fractions of chemical vapor-deposited (CVD) carbon, were impregnated by oxygen-free, high-conductivity (OFHC) Cu, Cu-6Si-0.9Cr, and Cu-0.3Si-0.3Cr (wt pct) alloys by pressure infiltration casting. The obtained composites were characterized for their coefficient of thermal expansion (CTE) and thermal conductivity (K) along the through-thickness and two in-plane directions. One composite, with a 28 vol pct Cu-0.3Si-0.3Cr alloy, showed outstanding potential for thermal management applications in electronic applications. This composite exhibited approximately isotropic thermal expansion properties (CTE=4 to 6.5 ppm/K) and thermal conductivities (k≥260 W/m K).     

Infiltration of fiber preforms by a binary alloy: Part I. Theory

During infiltration of a fiber preform by a binary hypoeutectic alloy, solid metal can form in the composite because of cooling at the fibers or at the mold wall. Contrary to the case of an unalloyed matrix, temperature, composition, and fraction solid may vary in the composite. This results in macrosegregation and microstructural heterogeneity within the composite casting. It is shown that solid metal that forms because of cooling at the fibers grows gradually behind the infiltration front, while the local temperature increases. Metal superheat, when present, serves to progressively remelt solid metal in the composite during infiltration and increases compositional and microstructural heterogeneity within the composite. General expressions are derived to describe heat, mass, and fluid flow during the infiltration process. In the case of unidirectional adiabatic infiltration driven by a constant applied pressure, a similarity method can be used to reduce the mathematical complexity of the problem. Numerical solution of the resulting equations then allows us to predict temperature, fraction solid, and composition profiles within the composite. With the further assumption of negligible thermal conduction, the problem lends itself to an analytical solution. The analysis is performed for the case of unidirectional adiabatic infiltration under constant applied pressure of 24 vol pct δ-alumina preforms by Al-4.5 wt pct Cu. Results indicate that there is significant latitude for control of macro-segregation and microstructure within cast fiber-reinforced alloys.