定向钢纤维增强水泥基复合材料断裂特性模拟分析

利用钢纤维随机生成算法建立了钢纤维增强水泥基复合材料有限元模型,基于粘聚裂纹模型模拟了定向钢纤维水泥砂浆三点弯曲断裂全过程.将数值模拟得到的荷载-绕度曲线与已有试验结果进行对比,验证了数值模型的可靠性.研究了砂浆基体不同粘聚律对定向钢纤维水泥砂浆断裂全过程的影响.计算分析了不同尺寸试件模型的断裂全过程,研究了试件尺寸对定向钢纤维水泥砂浆断裂特性的影响.结果表明:本文建立的定向钢纤维水泥砂浆有限元模型的数值结果与试验结果对比较好,粘聚律的变化对断裂全曲线影响较小;随着试件尺寸的增大,定向钢纤维水泥砂浆的名义强度存在一定的尺寸效应,本文建立的细观模型可有效研究钢纤维增强水泥基复合材料的细观断裂机理.     

混杂钢纤维对钢纤维–超高性能混凝土界面黏结性能的影响

通过单根钢纤维拉拔试验,研究了2种钢纤维(平直型和端钩型钢纤维)在混杂钢纤维超高性能混凝土(UHPC)中的黏结性能。基于实测钢纤维拉拔荷载–位移曲线,分析了钢纤维混杂配比、钢纤维体积掺量对钢纤维–UHPC基体界面黏结性能的影响,以及钢纤维–UHPC基体界面黏结性能与UHPC抗压强度的关系,阐明了钢纤维在混杂钢纤维UHPC中的拔出过程,并建立了平直型和端钩型钢纤维拔出过程的黏结–滑移模型。指出UHPC基体内平直型和端钩型钢纤维体积掺量均为1%(体积分数)时,钢纤维混杂对钢纤维–UHPC基体界面黏结性能的增强效果最佳。     

水泥砂浆基体中玄武岩纤维的拔出试验研究

纤维与水泥砂浆间界面粘结性能是影响纤维混凝土宏观力学性能的重要因素.通过一系列单根玄武岩纤维拔出试验,考虑了三种不同纤维埋置深度(6 mm、9 mm、12 mm)和三种不同水灰比的水泥砂浆基体(0.40、0.49、0.65)两个因素的影响,得到了玄武岩纤维从水泥砂浆基体被拔出时的荷载位移曲线,确定了埋置深度为12 mm时界面粘结最强.最后通过宏观力学性能试验研究了12 mm长度下玄武岩纤维的掺量对混凝土宏观力学性能的影响.     

钢纤维混凝土界面粘结强度的研究

本文采用拔出试验法研究了水灰比、丙烯酸共聚乳液、减水剂、纤维外形、纤维直径、纤维埋深及纤维走向等因素的变化对界面粘结强度的影响。实验表明:减小界面水膜层的厚度,加强界面过渡环都能提高界面粘结强度和钢纤维增强效果。聚合物水泥砂浆与双肢钢纤维的粘结强度最大。 本文还通过显微硬度试验测得界面过渡环的性状及范围。实验表明:在钢纤维与基体的界面过渡环中,最弱点不是在紧贴纤维表面处,而是在距纤维表面为25～35μm处,水灰比越低,过渡环最弱点的最微硬度值越高,最弱点与纤维表面的距离越小。通过能谱分析得知,在紧贴纤维表面的水泥石中,铁离子浓度最大,并随着与纤维表面的距离增加而逐渐减小。在距离为40μm处,铁离子浓度已很小,并逐渐趋于恒定。铁离子的存在可能会提高界面密实度。此外,还采用扫描电子显微镜,观察和研究了界面形貌。     

纤维增强树脂基复合材料界面粘结强度测试方法探讨

基体和增强材料界面的粘结性能直接影响到复合材料的力学性能，如何测量复合材料界面的粘结强度是界面研究的关键问题之一。本文侧重回顾了目前使用的复合材料界面粘结强度测试方法，如微脱粘、单纤维复合材料断裂、单纤维拔出和压出法，并对相关的计算理论进行了扼要介绍。     

环氧树脂对玻璃纤维/水泥复合材料界面性能的影响

在细观单纤维拔出实验基础上,利用ANSYS有限元程序,结合生死单元方法,对环氧树脂接枝处理的玻璃纤维/水泥基复合材料纤维拔出行为进行数值模拟,获得了界面理想结合/界面非理想结合下的拔出荷载-位移曲线。讨论了环氧树脂光滑接枝与粗糙接枝对玻璃纤维/水泥基复合材料界面强度的影响,研究表明,环氧树脂接枝不仅具有良好的界面结合性,还具有良好的机械嵌合性。     

纤维与水泥砂浆界面黏结性能研究

本文采取“8字”型试件进行单根粗纤维拉拔试验,通过混合正交试验研究了纤维类型、纤维直径、纤维埋置长度及砂浆基体水胶比对纤维-砂浆界面黏结强度显著性优先次序,以及纤维与砂浆间最佳组合的影响。同时得到了纤维最大拔出荷载及荷载-滑移曲线,系统分析了纤维与水泥砂浆的界面黏结性能。试验结果表明,直径为0.6 mm、埋置长度为20 mm的聚丙烯粗纤维,水胶比为0.51的砂浆基体的界面黏结强度最大,其中平均黏结强度为7.71 MPa,等效黏结强度达到13.25 MPa。直径为0.6 mm、埋置长度为10 mm的聚乙烯醇(PVA)纤维,水胶比为0.41的砂浆基体的界面黏结强度次之。四种因素对界面黏结强度的影响优先次序为纤维种类、纤维直径、砂浆水胶比、纤维埋置长度,其中纤维种类对界面平均黏结强度的影响较为显著,纤维直径对界面等效黏结强度的影响较为显著。     

冻融循环作用下嵌入式BFRP筋与混凝土粘结性能研究

为探究高寒地区冻融循环作用对嵌入式FRP加固混凝土界面粘结耐久性能的影响,针对玄武岩纤维增强复合材料(BFRP)筋嵌入式加固混凝土结构,通过采用环氧树脂胶(EP)和高韧性纤维增强水泥基复合材料(ECC)作为粘结材料的嵌入式FRP筋-从混凝土中拉拔试验,分析冻融循环作用下嵌入式FRP-混凝土相邻层之间的破坏模式和粘结退化机理,研究冻融循环次数、粘结材料、混凝土基体性能对粘结性能的影响。结果表明:不同侵蚀条件下试件破坏模式基本表现为胶基体与混凝土剪切破坏、胶基体劈裂破坏、毗邻混凝土-胶界面混凝土薄层剪切破坏、ECC基体与混凝土剪切破坏、FRP与ECC界面粘结破坏五种破坏模式,试件的极限粘结承载力随着循环次数的增加而呈不同程度的下降,50次循环内对试件粘结性能的影响较小,ECC试件相比环氧树脂胶试件,粘结承载力低,但表现出良好的延性。     

渗浆法钢纤维增强磷酸镁水泥基复合材料的力学性能

采用磷酸镁水泥(MPC)制备渗浆钢纤维增强材料(SIFCC),并与普通硅酸盐水泥(OPC)基体进行比较,研究钢纤维掺量对SIFCC抗压强度、抗弯强度、弯曲应力应变曲线和断裂韧性的影响,根据单根钢纤维拔出试验和断裂面微观形貌,分析MPC-SIFCC的增强机制。结果显示:MPC制备的SIFCC抗弯强度和断裂韧性提高显著,抗压强度在纤维掺量6%以上时提高明显;与同强度等级的OPC-SIFCC(28d)相比,MPC-SIFCC抗弯强度及断裂韧性提高幅度更大,钢纤维掺量10%的7 d抗弯强度可达77.4MPa。单根纤维拔出试验与微观形貌观察结果显示,与OPC相比,MPC与钢纤维有更高的黏结力。考虑MPC浆体在低水胶比下的良好流动性,可以认为MPC用于制备SIFCC具有较为明显的优势。     

单根聚丙烯纤维在植被混凝土中的拉拔特性及临界长度研究

纤维加筋技术在复合土和混凝土等工程加固领域应用广泛,但在生态护坡领域的应用则鲜有报道.通过自主设计的试验装置,研究了单根聚丙烯纤维在不同含水率、不同干密度和不同龄期植被混凝土中的拉拔特性,分析了拉拔试验前后纤维的显微特征,并确定了临界加筋长度经验公式.研究表明:纤维拉力-位移曲线呈显著的多峰特征,主要与植被混凝土中存在水泥水化产物有关;1d试样界面抗剪强度在最优含水率时存在极小值,主要原因是试样初期的筋-土界面锚固力提高值相较于摩擦力减小值要低;随着干密度和龄期的增长,界面抗剪强度和残余抗剪强度都显著提高.通过多元线性回归分析,得出植被混凝土中纤维临界加筋长度的经验公式,对提升植被混凝土在高寒、强降雨、坡面高陡等恶劣环境下的工程耐久性有一定指导意义.     

Relation of interfacial adhesion in Kevlar®/epoxy systems to surface characterization and composite performance

A central problem in composite materials is the poorly understood relation between the nature of the surfaces at the fiber/matrix interface, the actual interfacial bond strength, and interface-sensitive composite properties, in this study on the Kevlar®/epoxy composite system, the interface was varied chemically by fiber sizings. The sized and unsized fiber surfaces and the cured matrix surface were characterized by contact angle measurements. The interfacial shear strength was directly measured by single-filament pull-out tests of sized and unsized fibers in epoxy matrix. The shear strengths of the composites made with sized and unsized fibers were measured. The results from surface analysis, interfacial shear tests, and composite shear tests were consitent. This suggests that surface-contact-angle analysis and single-filament pull-out tests may be helpful in screening strength of the composite.     

钢纤维高强水泥基复合材料的界面效应及其疲劳特性的研究

通过综合分析提出，钢纤维与水泥基体、集料与水泥浆的界面层不仅可以改善和强化，而且经有效地调整界面区的组成和结构，界面层及其薄弱特征完全能够消失，并且还有丙强化的可能，再强化程度则与界面区组成结构密切相关，这是扩大纤维效应范围、强化空间随机叠加、配制高性能水泥基复合材料的理论基础，本工作还研究了界面效应与高强混凝土，钢纤维高强水泥基复合材料在反复荷载作用下疲劳特性的关系，实验结果表明，因钢纤维与硅灰     

The interfacial bond stress in steel fiber cement composites

In fiber cement composites most fibers are in a state of partial bond due to internal stresses arising from moisture migration during fabrication and subsequent volume changes in the matrix. A wide variation in the computed interfacial bond strength therefore occurs depending upon the type of test or when derived from phenomena such as crack spacing. In practice debonding of the fibers occurs in flexural tension in the presence of a strain gradient. This paper presents further data on steel fiber mortar and concrete to confirm the validity of the composite mechanics approach to predict the composite flexural strength. It is shown that the composition of the matrix and its strength properties influence the fiber-matrix interfacial bond stress and the relative contributions of the matrix and the fibers to the composite flexural strength.     

The effect of matrix composition on fibre/matrix interfacial bond shear strength in fibre-reinforced mortar

The effects of factors associated with the composition of the matrix, i.e. curing conditions and time and mix proportions, on the shear strength of the interfacial bond between steel fibres and a cementitious mortar matrix have been examined experimentally using a single-fibre pull-out test technique. The experimental results indicate that bond shear strength increases significantly with an increase in matrix curing time and, for specimens with the fibre axis perpendicular to the direction of casting and compaction of the matrix, with a decrease in the proportion of water by weight in the matrix mortar. This latter effect is attributed to bleed water gain under the embedded fibre, as it is not observed in specimens with the fibre axis parallel to the direction of casting and compaction of the matrix. Furthermore, the results indicate that there is no correlation between interfacial bond shear strength and matrix mortar compressive strength.     

A theory for the flexural strength of steel fiber reinforced concrete

A theory is presented to predict the flexural tensile strength of concrete reinforced with short, discontinuous steel fibers randomly oriented and uniformly dispersed in a cement-based matrix. The theory is based on a dual criterion of crack control and composite mechanics. The first crack in the fibrous composite occurs due to bond slip. The fracture process consists of progressive debonding of fibers during which slow crack propagation occurs. Final failure occurs due to unstable crack propagation when fibers pull out and the interfacial shear stress reaches the ultimate bond strength. The theory is supported by test data on fiber reinforced concrete, mortar and paste.     

Interfacial properties of kevlar-49 fiber-reinforced thermoplastics

A single-filament pull-out test was used to study adhesion of Kevlar-49 fibers to thermoplastic polymers. The test involved pulling a partially embedded fiber out of a polymer film. Kevlar-49 fibers with three different surface treatments were used with five thermoplastic materials. The test resulted in the measurement of two properties, an interfacial bond strength and a frictional shear strength. The interfacial bond strength is an essential factor in determining the critical aspect ratio of discontinuous fibers in a composite. The frictional shear strength was found to correlate with the tensile strength of discontinuous fiber composites which fail by fiber pull-out. Scanning electron microscopy was used to examine the fiber pull-out specimens after testing. Observations of the fiber showed that the failure mode at the fiber–matrix interface was complex. The predominant failure mode was fracture at the interface (or in some weak boundary layer). In some cases, cohesive failure of the fiber surface was observed, with the result that strips of material were torn from the fiber surface.     

硅烷偶联剂对复合水泥砂浆性能的影响

通过红外光谱分析及砂浆强度测试，研究了硅烷偶联剂对不同种类复合水泥砂浆的稠度、分层度、抗折强度及抗压强度的影响。结果表明，加入硅烷偶联剂能提高普通水泥砂浆、苯丙胶乳改性水泥砂浆的抗折强度和抗压强度；当硅烷偶联剂质量分数为0．5％时，普通水泥砂浆的抗折强度和抗压强度达到极大值，提高约10％；当硅烷偶联剂质量分数为1％时，苯丙胶乳改性水泥砂浆的抗折强度和抗压强度达到极大值，提高约20％；同时，硅烷偶联剂还能增大普通水泥砂浆和苯丙胶乳改性水泥砂浆的稠度，但砂浆的分层度略有增大。加入经硅烷偶联剂处理的钢纤维，能够提高普通水泥砂浆及苯丙胶乳改性水泥砂浆的抗折强度和抗压强度；当钢纤维用硅烷偶联剂质量分数为1％的硅烷偶联剂水溶液处理时，钢纤维增强砂浆的抗折、抗压强度达到极大值，提高10％以上。     

Use of a Crack-Bridging Single-Fiber Pullout Test to Study Steel Fiber/Cementitious Matrix Composites

The fracture process of steel fiber/cementitious matrix composites has been studied using a single-fiber pullout test that permits detailed measurements of the load-crack opening displacement relationship during fiber debonding and unloading. Using a suitable analytical model, the interfacial fracture energy and interfacial sliding friction have been calculated for composites incorporating steel fibers with cement paste or mortar matrices. Comparison of theoretical debonding curves with the experimental data show that the model accurately represents the fiber debonding process, except for a decrease in interfacial sliding friction due to wear of matrix asperities at the interface. Differences between the calculated interfacial properties of several specimens are associated with changes in the interfacial microstructure.