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。     

钢纤维增强聚合物水泥基复合材料界面特性

采用聚丙烯酸酯乳液(PAE)、苯乙烯-丙烯酸共聚乳液(SA)、聚乙烯醇缩甲醛水溶液(PVFO)对钢纤维增强水泥基复合材料进行改性,发现聚合物提高了复合材料的宏观性能如抗弯强度、韧性,对纤维与水泥基体界面粘结强度有很大程度的提高.红外光谱分析表明,聚合物加入使其活性官能团与水泥或集料间发生化学键合,DSC放热曲线表明,加入聚合物后混合物的放热峰值提高,验证了红外光谱的结果.通过聚合物与水泥石之间的化学键合强化了钢纤维与水泥基体间的界面层,形成强度、韧性很好的具有"互穿网络"的钢纤维增强复合材料.     

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。     

炭纤维水泥基复合材料eebeck效应

发现特制纤维水泥具有Ｓｅｅｂｅｃｋ效应，研究结果表明，它对温差有较高的灵敏度，试块上下表面浊国差△ｔ与浊国差电动热呈线性关系。     

水泥基复合材料中倾斜钢纤维细观桥联模型

主要从细观尺度研究水泥基复合材料中倾斜钢纤维的桥联行为.对纤维在基体中的埋入部分采用杆元近似,基体之外的部分,根据其弹性或塑性状态分别作悬臂梁或塑性铰近似,从而简化为两种构形.结合这两种构形和杆元近似,并采用经典的描述轴向行为的Naaman剪滞模型,同时引入纤维的侧向有效承载面积来描述基体局部失效效应,推导了描述高模量倾斜钢纤维桥联行为全过程的解析模型.模型预测的桥联行为与实验有较好的一致性.     

碳纤维水泥基复合材料的阻-温特性

研究了碳纤维水泥基复合材料的阻-温特性。发现当温度低于某一特定值时，电阻率随温度的升高而单调降低(NTC效应)。超过此值时，则表现为电阻率随温度升高而大幅度单调增加(PTC效应)。普通水泥基材料不具有相应的阻-温特性．     

植物纤维增强水泥基复合材料的研究进展与应用

纤维在抑制混凝土裂缝发展中具有重要的作用,研究开发植物纤维增强水泥基复合材料不仅能降低该复合材料的造价,而且有利于环保和可持续发展,具有深远的意义.综述了植物纤维的性能与增强作用、在混凝土应用中的研究进展以及发展前景和存在的一些问题.     

铁氧体水泥基复合材料的电磁特性研究

材料电磁特性的研究对调整和优化材料电磁参数从而达到良好吸收起着至关重要的作用.采用XRD和IR分析了铁氧体与水泥基之间的相互作用,讨论了铁氧体掺量、粒径大小对复合材料电磁参数、反射率的影响,并运用电磁媒质混合定则和传输线理论对其进行了评价.结果表明,铁氧体与水泥基体间存在相互作用,且此作用对材料的电磁参数有直接影响;粒径的变化有利于磁导率虚部的提高,但更显著的是对材料吸收频段的移动作用;铁氧体水泥基复合体系并不完全服从电磁媒质混合理论,微扰理论在该体系内并不适用.     

无宏观缺陷的水泥基复合材料

本文对无宏观缺陷(Macro-Defect-Free MDF)的水泥基复合材料进行了研究。在改变不同水灰比及不同添加剂含量的条件下,对各类试样的主要力学性能进行了测定。结果表明,只有当水、灰及添加剂的含量控制在适当配比时,才能获得最佳的力学性能。MDF 水泥的抗弯强度比普通水泥的强度提高约10—20倍。还采用扫描电镜观察了微观组织的孔隙尺寸、孔隙密度以及试样的断口形貌。对不同情况下的孔隙率和孔隙尺寸对力学性能的影响以及添加剂的作用机理进行了讨论。     

Toughness enhancement and equivalent initial fracture toughness of cementitious composite reinforced with aligned steel fibres

Based on theoretical analysis and numerical simulation, the impact of steel fibres on the stress intensity factor (SIF) at the crack tip for cementitious composite was studied. The enhanced toughness of steel fibre reinforced cementitious composite (SFRC) in resisting cracks was explained by the decrement of SIF caused by steel fibre inclusions at the crack tip of the composite. The equivalent initial fracture toughness was used to characterize the crack initiation of SFRC. A simplified method for determining the of SFRC was proposed based on a linear regression method. Fracture tests were conducted on three‐point bending notched beams with different steel fibre volume fractions and specimen sizes to study the crack initiation behaviour of aligned steel fibre reinforced cementitious composite (ASFRC). of ASFRC was calculated, and the size effect of was analysed. The results showed that slightly increased with the steel fibre volume fraction and gradually became stable. For the tested specimens, whose heights varied between 40 and 100 mm, the specimen size had little impact on the .     

聚乙烯醇纤维增强钢渣粉-水泥复合材料基本力学性能及微观结构

通过掺加钢渣粉来制备聚乙烯醇（PVA）纤维增强钢渣粉-水泥基复合材料,从宏微观两个方面研究了这种复合材料的性能。考虑了基体材料的水胶比（0.25和0.35）、不同钢渣粉质量分数（0、30wt%、60wt%、80wt%）,采用抗压强度试验、薄板四点弯曲试验研究了PVA纤维增强钢渣粉-水泥基复合材料的基本力学性能变化规律及其在弯曲荷载作用下的裂缝控制能力,采用扫描电镜观测了破坏后试样的微观结构。结果表明,水胶比和钢渣粉掺量均可明显影响PVA纤维增强钢渣粉-水泥基复合材料的基本力学性能,在低水胶比条件下（水胶比为0.25）,钢渣粉掺量达到80wt%时,试样表现出较高的韧性指数和良好的裂缝控制能力,基本满足工程所需强度要求,水胶比为0.35时钢渣掺量不宜超过60wt%;同时,从节能减排的角度考虑,利用钢渣粉制备PVA纤维增强钢渣粉-水泥基复合材料是可行的。      

Controlling fracture toughness of matrix for ductile fiber reinforced cementitious composites

An experimental study was carried out to find material parameters for making fiber reinforced cementitious composites (FRCC) more ductile. One of the dominant factors to control the ductility might be hidden in fracture property of matrix as well as the interface property between fiber and matrix. Therefore this study varied air content and water-binder ratio as the parameters to change the fracture property of matrix and experimentally examined their influence on the ductility of FRCC by three-point bend test with notched beams. As a result, it is concluded that fracture toughness of the matrix could be one of key parameters to control the ductility of FRCC. In case of a polyethylene fiber used in this study, the optimum value of the fracture toughness (critical strain energy release rate): G_IC of the matrix was obtained to be 7.5-8.0 N/m.     

试件厚度对超高韧性水泥基复合材料弯曲性能的影响

通过不同厚度超高韧性水泥基复合材料（UHTCC）试件的四点弯曲试验,研究了厚度对其弯曲性能的影响,并通过理论量纲分析做了进一步解释。结合ASTM C 1609标准,提出了以单位塑性铰区体积的能量消耗Tv为参数的韧性评价方法。试验结果表明：不同厚度试件的名义弯曲应力-弯曲应变曲线与裂缝宽度变化曲线几乎重合;挠度随厚度增大...     

Dynamic crack initiation toughness of 4340 steel at constant loading rates

Determination of fracture toughness for metals under quasi-static loading conditions can follow well-established procedures and ASTM standards. The use of metallic materials in impact related applications requires the determination of dynamic crack initiation toughness for these materials. There are two main challenges in experiment design that must be overcome before valid dynamic data can be obtained. Dynamic equilibrium over the entire specimen needs to be approximately achieved to relate the crack tip loading state to the far-field loading conditions, and the loading rate at the crack tip should be maintained near constant during an experiment to delineate rate effects on the values of dynamic crack initiation toughness. A recently developed experimental technique for determining dynamic crack initiation toughness of brittle materials has been adapted to measure the dynamic crack initiation toughness of high-strength steel alloys. A Kolsky pressure bar is used to apply the dynamic loading. A pulse shaper is used to achieve constant loading rate at the crack tip and dynamic equilibrium across the specimen. A four-point bending configuration is used at the gage section of the setup. Results are presented which show a monotonically increasing rate dependence of crack initiation toughness for 4340 high-strength steel.     

超高韧性水泥基复合材料碳化与渗透性能试验研究

对超高韧性水泥基复合材料(UHTCC) 进行了快速碳化、预裂后的快速碳化、渗透性、快速氯离子渗透试验及自由氯离子含量测定, 研究了不同龄期UHTCC 的抗碳化性能和渗透性能。试验结果表明, 在无裂缝状态下, UHTCC 的抗碳化性能与同强度普通混凝土相当, 但在相同荷载预裂后, UHTCC 裂缝处的碳化深度仅为对比混凝土的30 %～40 %; UHTCC 抗渗性能优于同强度普通混凝土, 且随着龄期增长优势更明显, 56 天龄期的渗透系数约为对比混凝土的35 %; 快速氯离子试验和自由氯离子含量测定得到的氯离子渗透系数均表明, UHTCC 具有明显优于普通混凝土的抵抗氯离子渗透性能。      

不同纤维掺量下聚乙烯醇纤维增强工程水泥复合材料梁剪切韧性试验

为研究聚乙烯醇纤维增强工程水泥复合材料（PVA/ECC）无腹筋梁的剪切韧性,基于5组PVA/ECC梁受剪破坏试验结果,以剪切韧性指数和斜裂缝综合指数为指标,对不同纤维掺量下PVA/ECC梁的斜截面剪切韧性进行了研究与评价。结果表明:PVA纤维的掺入能改善梁的开裂性能,明显提高梁受荷全过程的变形能力及斜截面承载力,从而提高构件的剪切韧性;PVA纤维体积分数在0~2vol%范围内时,其值越大,加载过程中消耗的能量越多,斜截面抗剪承载力越高,破坏之前的总变形越大,梁的剪切韧性越好。      

Numerical techniques for the analysis of crack propagation in cohesive materials

The aim of the paper is the development, assessment and use of suitable numerical procedures for the analysis of the crack evolution in cohesive materials. In particular, homogeneous as well as heterogeneous materials, obtained by embedding short stiff fibres in a cohesive matrix, are considered. Two‐dimensional Mode I fracture problems are investigated. The cohesive constitutive law is adopted to model the process zone occurring at the crack tip. An elasto‐plastic constitutive relationship, able to take into account the processes of fibre debonding and pull‐out, is introduced to model the mechanical response of the short fibres. Two numerical procedures, based on the stress and on the energy approach, are developed to investigate the crack propagation in cohesive as well as fibre‐reinforced materials, characterized by a periodic crack distribution. The results obtained using the stress and energy approaches are compared in order to evaluate the effectiveness of the procedures. Investigations on the size effect for microcracked periodic cohesive materials, and on the beneficial effects of the fibres in improving the composite material response, are developed. Copyright © 2003 John Wiley & Sons, Ltd.     

Pull-out work of steel fibers from cementitious composites: Analytical investigation

The main objective of this study is to provide a parametric evaluation of the pull-out work of smooth steel fibers embedded in cementitious matrices. The various parameters controlling the behavior of the bond stress versus slip relationship are analyzed; their effects on the entire pull-out load versus slip response and the corresponding pull-out energy up to full debonding and/or up to total pull-out are investigated. Also discussed are the effects of the fiber geometric parameters such as fiber diameter, length, and aspect ratio. Finally, a brief section addresses the relation between pull-out work and the critical strain energy release rate G_c, a fracture mechanics variable, which can be calculated if the P-Δ curve is known.