钢-剑麻混杂纤维再生混凝土断裂性能研究

为改善再生混凝土(RAC)的断裂性能，通过三点弯曲断裂试验，研究了钢纤维、剑麻纤维及钢-剑麻混杂纤维对RAC试件断裂性能的影响。同时，采用数字图像相关(DIC)技术测得RAC试件的裂缝扩展全过程。结果表明：未掺纤维的RAC试件断裂性能较差，而掺入纤维后的RAC试件断裂性能明显提升；单掺钢纤维时，试件的起裂韧度与纤维掺量无关；单掺剑麻纤维时，最佳体积掺量为0.15%,其试件的起裂荷载较未掺纤维RAC试件提高了67%。单掺纤维和混掺纤维均可提高失稳韧度和断裂能，但混掺纤维效果更佳。当体积掺量为1.0%的钢纤维和体积掺量为0.30%的剑麻纤维混杂时，其起裂韧度、失稳韧度和断裂能较未掺纤维的RAC试件分别提高了83.92%、575.86%和1 244.05%。     

PVA-ECC材料的力学与收缩性能研究

以水泥砂浆为基体,聚乙烯醇(PVA)纤维为增强材料制备PVA-ECC材料,研究PVA纤维掺量、水胶比对其抗拉强度、抗压强度、应变-时间曲线及收缩性能的影响。结果表明:随着PVA纤维掺量的增加,PVA-ECC材料出现小幅度的收缩应变减小现象,各条应变-时间曲线基本重合。随着水胶比的降低,PVA-ECC材料的收缩均减小。水胶比对PVA-ECC材料前期的收缩影响较大,对14～28 d收缩影响较小。PVA纤维的掺入会降低PVA-ECC材料的抗压强度,可以通过减小水胶比来提高其抗压强度。且随着PVA纤维掺量的增加,试样抗拉强度呈上升趋势,即使加入少量纤维也可较大提高基材的抗拉强度。     

大掺量矿物掺合料纤维增强轻骨料混凝土的抗疲劳性能

为研究纤维增强轻骨料混凝土抗疲劳性能，开展了恒应力循环压缩试验，对疲劳应力-应变响应进行了研究。试验采用质量分数为20%的粉煤灰和50%的粒化高炉矿渣部分替代水泥，变量为单掺或混掺不同掺量的钢纤维和聚乙烯醇(PVA)纤维。结果表明：随着循环加载次数的增加，钢纤维混凝土的宏观裂纹数量比PVA纤维混凝土多，试件的破坏形态表现为轻骨料的破裂和纤维的渐进拔出(钢纤维)或断裂(PVA纤维);钢纤维混凝土的疲劳应变及残余应变均最大，而混杂纤维混凝土的最小；在同一应力水平下，混杂纤维混凝土的疲劳寿命最长，而钢纤维混凝土的最短；钢纤维混凝土的极限疲劳损伤高于PVA纤维混凝土和混杂纤维混凝土，且随最大应力水平的降低，该差异逐渐缩小。     

高掺量粉煤灰PVA-ECC的力学性能研究及粉煤灰作用机理分析

主要研究了高掺量粉煤灰对PVA-ECC力学性能的影响,对粉煤灰掺量分别为40%、50%、60%情况下PVA-ECC的抗压强度、薄板四点弯曲性能等进行试验研究,并分析了粉煤灰在ECC中的作用机理.结果表明:随着粉煤灰掺量的增加,ECC的抗压强度呈降低趋势,四点弯曲试验中ECC试件的初裂荷载和峰值荷载降低,跨中挠度增加,且裂缝宽度减小;高掺量粉煤灰下,纤维长度对ECC抗压强度和延性有较大的影响,掺12 mm纤维的PVA-ECC在保证强度的同时,可获得较高的延性和韧性.     

不同纤维增强对PZT/PVA复合材料性能影响的研究

压电陶瓷复合材料较低的力学性能限制其应用。为进一度探究纤维种类及掺量对锆钛酸铅(PZT)/聚乙烯醇(PVA)复合材料性能影响规律,对不同纤维掺量下碳纤维/PZT/PVA、芳纶纤维/PZT/PVA复合材料开展电学及力学性能测试,分析纤维种类、掺量及PZT/PVA体积比对试件介电、压电性能及弯曲、拉伸性能的影响规律,并结合电镜扫描(SEM)分析纤维掺量对试件性能的影响。结果表明,少量的纤维掺量能够提高复合材料的电学性能,过量的纤维使得复合材料结构孔隙结构增加,介质常数和压电常数均减小,介质损耗增大。7%芳纶纤维/0.5PZT/0.5PVA、5%芳纶纤维/0.6PZT/0.4PVA、3%芳纶纤维/0.7PZT/0.3PVA复合材料的介电性能和压电性能较高。芳纶纤维掺量为9%时,芳纶纤维/0.5PZT/0.5PVA、芳纶纤维/0.6PZT/0.4PVA、芳纶纤维/0.7PZT/0.3PVA、芳纶纤维/0.8PZT/0.2PVA复合材料的弯曲强度分别提高了10.72%、14.49%、13.85%及16.78%,拉伸强度分别提高了24.04%、23.94%、27.00%和28.52%。纤维在试件内...     

端钩型钢纤维UHPC抗压强度的试验研究

本文对5组超高性能混凝土(UHPC)立方体试件进行了轴心受压试验,观察不同纤维掺量及不同尺寸UHPC试件的破坏过程及破坏形态,研究了端钩型钢纤维及不同尺寸对UHPC受压性能的影响,比较各纤维体积掺量立方体试块的荷载-位移曲线,给出了纤维约束系数,分析了其对UHPC立方体抗压强度及压缩耗能的影响,建立了UHPC立方体抗压强度的预测模型。结果表明:与未掺纤维UHPC试件相比,掺入端钩型钢纤维的试件,在载荷达到极限荷载的40%左右时,试件开始发生损伤,在载荷接近极限荷载时,试件内部发生持续的快速断裂声响;掺入端钩型钢纤维的UHPC试件最终破坏呈多条斜向裂缝,且最终破坏时试件仍能保持完整形态,呈现“裂而不碎”的状态;随着端钩型钢纤维体积掺量的增加,试件的受压峰值荷载增加,且伴随着试件的变形增大;与未掺纤维UHPC试件相比,随着纤维掺量的增大,尺寸效应对UHPC的受压性能的影响逐渐减小。基于纤维约束指数,建立了UHPC立方体抗压强度的预测模型,预测结果与试验结果吻合度较高。     

聚乙烯醇纤维掺量对水泥胶砂力学性能的影响

为了研究聚乙烯醇(PVA)纤维的掺量对水泥胶砂力学性能的影响,采用硫铝酸盐水泥、粉煤灰、硅灰、聚乙烯醇纤维等材料制备40 mm×40 mm×160 mm胶砂试件,通过流动度试验、抗折试验、抗压试验测试7组试件的力学性能。试验结果表明,随着纤维掺量的增加,胶砂的流动度逐渐降低。抗折、抗压试验中,试件的抗折强度均比不加纤维的有不同程度提高,掺加质量分数0.20%的PVA纤维的试件3、7、28 d的抗折强度均最大;试件的抗压强度和抗折强度的变化规律相同。韧性数值表明,试件在龄期为3 d的测试结果呈上升趋势,龄期为7 d和28 d呈下降趋势。经综合比较,PVA纤维质量分数为0.20%时,试件的力学性能最佳。     

钢-PVA混杂纤维增强工程水泥基复合材料弯曲性能研究

为了弥补现有钢-聚乙烯醇(PVA)混杂纤维增强工程水泥基复合材料造价过高、工程应用面狭窄的缺陷,本文通过使用廉价的国产PVA纤维部分替代日产PVA纤维制备出一种新型的多元混杂纤维增强工程水泥基复合材料(MFECC)。研究MFECC材料薄板试件的弯曲性能和破坏形态,对试件的弯曲韧性和性价比进行评价,并通过SPSS软件的多元非线性回归法建立极限弯曲性能预测模型。结果表明,引入国产PVA纤维后MFECC薄板的应变硬化力学行为和多缝开裂现象相较于仅掺日产PVA纤维时有所降低,但仍具有较高的强度与延性。当钢纤维、日产PVA纤维和国产PVA纤维体积掺量分别为0.2%、0%和2.0%时,MFECC的极限拉伸应变为4.4%,抗压强度为46.39 MPa,极限抗弯挠度可达12.697 mm,性价比最高。建立的MFECC薄板试件的极限弯曲性能预测模型对试验值的拟合度良好。     

纳米粒子和石英砂对PVA纤维水泥基复合材料单轴拉伸性能的影响

为研究纳米粒子种类和掺量以及石英砂粒径对聚乙烯醇纤维(PVA纤维)水泥基复合材料单轴拉伸性能的影响,通过单轴拉伸试验测得了试件的极限拉应变和极限拉应力,并得到了试件应力-应变关系曲线.PVA纤维的体积掺量为0.9％,选择纳米SiO2质量掺量和石英砂粒径各四种.结果 表明,纳米SiO2的掺加对PVA纤维水泥基复合材料抗拉伸性能有一定的提高,随着纳米SiO2掺量从0％增大到2.5％,试件极限拉应变和极限拉应力整体上呈逐渐增大趋势.相对于纳米CaCO3,纳米SiO2对PVA纤维水泥基复合材料抗拉伸性能的增强效果更明显.石英砂的粒径对PVA纤维水泥基复合材料抗拉性能影响较大,石英砂的粒径越小,PVA纤维水泥基复合材料的极限拉应变和极限拉应力越低.     

PVA纤维对水泥基材料性能的影响研究

通过对不同纤维掺量的PVA-ECC试件进行抗压强度和抗折强度试验以及干缩试验,探究PVA纤维对水泥基复合材料基本力学性能和干缩性能的影响。结果表明:PVA纤维的掺入,可以提高水泥基复合材料的抗折强度,在0～3.6kg/m~3的掺量区间内,PVA纤维掺量越大,P水泥基复合的抗折强度越大;PVA纤维的掺入可以提高水泥基材料的抗压强度,但是不显著;PVA纤维掺量1.8kg/m~3时,其对水泥基材料的干缩有抑制作用。     

Mechanical properties and prediction of fracture parameters of geopolymer/alkali-activated mortar modified with PVA fiber and nano-SiO2

Properties of fly ash (FA) and metakaolin (MK) based geopolymer/alkali-activated mortar modified with polyvinyl alcohol (PVA) fiber and nano-SiO₂, including workability, compressive strength, flexural performance, elastic modulus and fracture property were tested in this study. PVA fiber content varies from 0 to 1.2%. Nano-SiO₂ content is 0 and 1%. Adaptive neuro-fuzzy interfacial systems (ANFIS) method was used to establish the artificial intelligence (AI) model to predict the fracture parameters of geopolymer/alkali-activated mortars. The inputs of ANFIS models include PVA fiber content, nano-SiO₂ content, compressive strength, flexural strength, elastic modulus, critical crack mouth opening displacement, crack load and peak load. The outputs of ANFIS model include critical effective crack length, initiation fracture toughness, unstable fracture toughness, and fracture energy. Experiment results showed that PVA fiber addition enhanced the mechanical properties especially the compressive strength and fracture performance, but decreased the workability. 0.8%–1.0% was considered as the optimal content of PVA fiber. Addition of 1% nano-SiO₂ shows a slight improvement on both workability and mechanical properties of the mortar no matter how much fiber is added. Based on the ANFIS algorithm and 42 sets of experimental data, the trained models were proved to have high accuracy with root mean square error (RMSE) under 0.15, mean absolute error (MAE) under 0.01, and coefficient of determination (R²) over 0.85. The ANFIS model established in this study combined the fracture properties with the basic mechanical properties of geopolymer/alkali-activated composites, which can provide a new method to assess the fracture performance of geopolymer/alkali-activated mortars modified with PVA fiber and nano-SiO₂ in the future.     

PVA纤维表面改性对水泥基复合材料弯曲性能的影响

采用有机硅柔软剂对国产聚乙烯醇(PVA)纤维进行表面改性,并制备了纤维增强水泥基复合材料(PVA-ECC)。采用扫描电子显微镜研究了有机硅柔软剂改性对PVA纤维表面结构的影响,用三点弯曲试验研究了有机硅柔软剂改性的PVA纤维对PVA-ECC复合材料弯曲性能的影响。研究结果表明:随着有机硅柔软剂含量的增加,PVA-ECC的极限弯曲强度和极限跨中挠度均先增加再减小,当有机硅柔软剂质量分数为7%时,极限弯曲强度和极限跨中挠度达到最大值,分别为5.627 MPa和2.123 mm;用ASTM C1609标准分析PVA-ECC三点弯曲韧性,当有机硅柔软剂质量分数为7%时,弯曲韧性达到最大值。     

粉煤灰掺量对PVA-ECC性能的影响

采用粉煤灰替代部分水泥(40%、50%、60%和70%(质量分数))进行聚乙烯醇纤维增强水泥基复合材料(PVA-ECC)的力学性能、韧性、抗渗性能研究;建立了PVA-ECC抗压强度与其极限拉伸强度、弯曲荷载、弯曲韧性指数、挠度及抗渗性能之间的相关性;同时还进行了PVA-ECC渗水高度的统计数据概率分布分析。结果表明,随粉煤灰含量增加,强度逐渐下降,极限拉伸应变逐渐增大,韧性指数、能量吸收能力及跨中挠度逐渐增大,各组别抗渗性能相对于粉煤灰40%含量组别均有所提升。建立抗压强度与其余指标之间相关性,得到一套完整的强度、韧性、抗渗性的相关性理论体系。对PVA-ECC的渗水高度的频率分布直方图采用正态分布进行统计分析,得到渗水高度概率分布,同时计算出平均渗水高度95％置信区间。     

Effect of Subcritical Crack Growth on Fracture Toughness and Work-of-Fracture Tests Using Chevron-Notched Specimens

A correlation between the plane strain stress intensity factor K_I , load, and crack extension has been analyzed for constant displacement and constant loading rate experiments, using chevron-notched, four-point-bend specimens. It is assumed that at the beginning of the experiment the chevron triangle tip is not ideally sharp. As loading continues, the crack initially moves with velocity v_t at K_I equal to a threshold value K_t . Maximum crack velocity is reached at K_I= K_IC , the fracture toughness. Depending on the type of material tested, a specific displacement or loading rate must be used to correlate the maximum load with K_Ic . An error in K_IC calculation is estimated if different displacement rates are applied. Repeated loading-unloading work-of-fracture (WOF) experiments generate values related to the resistance of the material to fracture initiation, K_t , only when the crack length approaches 100% of the specimen width. Values related to material's fracture toughness, K_IC are not generated in WOF tests.     

增强纤维对超高性能混凝土改性效果及优化研究

为探索增强纤维对混凝土性能影响的规律,选用聚乙烯醇(PVA)纤维、超高相对分子质量聚乙烯(UHMWPE)纤维、玄武岩纤维和玻璃纤维增强混凝土制备超高性能混凝土(UHPC)。通过性能测试,选出UHPC改性最佳纤维——PVA纤维和UHMWPE纤维。采用Design-Expert专业实验数据分析软件,对PVA和UHMWPE混杂纤维设计进行理论模拟,针对目标纤维进行掺量优化设计,得出当12 mm长度的PVA纤维的体积分数为0.3%、6 mm长度的UHMWPE纤维的体积分数为0.9%时,目标UHPC的抗折强度、抗压强度与流动度达到最优化设计目标。     

Fracture Toughness Determination of A12O3 Using Four-Point-Bend Specimens with Straight-Through and Chevron Notches

Fracture toughness of a sintered A1₂O₃ was determined with four-point-bend specimens having either straight-through or chevron notches. For the straight-through notched specimens, measured K _Ic decreased with decreasing notch width. For the smallest notch width (66 μm) K _Ic= 3.42±0.13 MN m^−¾. For specimens with chevron notches, a crack initiates and extends from the tip of the notch under increasing load. K _Ic is calculated from the maximum load without measuring crack length, under the assumption that the derivative of the compliance is the same as that for a specimen with a straight-through crack. A refined calculation accounts for the truncated chevron crack shape at maximum load using Bluhm's slice model. For the chevronnotch configuration, a value of K _Ic= 3.49±0.11 MN m^−¾ was measured, which appears to be independent of the initial notch length a ₀ (distance from the crack mouth to the tip of the triangular notch). An effect of a ₁ (length of the chevron notch at the surface) on K _Ic was observed, independent of whether the calculation of K _Ic was based on the straight-through crack assumption or on the slice model.     

Notch Strength and Fracture Behavior of 2-D Carbon-Carbon Composites

This study examined Mode I in-plane fracture in two-directional carbon-carbon composites. Linear elastic fracture mechanics (LEFM) provided good predictions for the failure loads of the compact tension specimens, and consistent values of K _IC were calculated. However, values for the strain energy release rate G _IC were inconsistent. Analysis of loading/unloading cycles applied to the specimens showed that significant inelastic behavior occurred prior to fracture. Hence the J -integral method is not thought to be appropriate for fracture prediction. It was also found that the real crack tip stresses in carbon-carbon composites are much lower than theoretical predictions. An interesting discovery is that cracks in carbon-carbon composites appear to be naturally blunt. The lower limit on the crack tip diameter corresponds with the spacing of the woven fiber bundles. A significant conclusion from this study is that the material behavior can be modeled as the sum of two contributions: (i) a linear elastic solid, which fails according to LEFM, and (ii) an independent damage mechanism, which absorbs energy but does not alter the ultimate fracture load. Possible candidates for the damage mode are fiber pullout and inelastic deformations due to shear stresses.     

A micro/macro approach to fracture in composites

The results of an integrated microscopic/macroscopic finite element analysis of fracture in fiber-reinforced composites are presented. A macroscopic analysis of a composite double-cantilever-beam (DCB) fracture toughness test specimen was carried out using a singular finite element method. The effects of fiber layup angle on strain energy release rate are discussed. Results from this analysis were input as boundary conditions to a microscopic model used to calculate J-integral values in the crack tip region. Nonhomogeneity in this region causes the elastic strain energy release rate to vary with crack tip location and geometry. Elastic-plastic calculations showed that significant matrix plasticity occurs near fibers away from the crack tip region. The constitutive equation chosen for the matrix plasticity was shown to have an important effect on the J-integral value. The results show how the microscopic J-integral is related to the macroscopic strain energy release rate.