Combined Mode I and Mode II Fracture of Monolithic Ceramics

The mode I, mode II, and combined mode I–mode II fracture behaviors of a coarse-grained silicon nitride, a fine-grained silicon nitride, and an alumina were investigated. These ceramics were fractured from two types of fracture initiating flaws: small surface flaws and large single edge precracks. The small surface flaws were introduced by Knoop indentation in flexural samples at various angles to the tensile stress direction and fractured in four-point bending. The samples with large precracks were fractured in the asymmetric four-point-bend geometry. The mixed-mode fracture toughness values obtained from the two flaw configurations were in good agreement with each other. All three ceramics displayed very similar mixed-mode fracture behavior, although their microstructures were not similar. Comparison of experimental data to mixed-mode fracture theories revealed that the minimum strain energy density theory best described the mixed-mode fracture behavior of all three ceramics.     

Fractographic and Fracture Mechanical Investigations of Refractories under Different Loading Conditions

Crack initiation and propagation have been investigated under tensile and shear loading in ceramically and carbon bonded refractories.A wedge splitting test procedure and a modified shear test have been applied.Test results have been used for material characterization especially with respect to brittleness.Furthermore a microscopic fractographic test procedure was developed and applied on fractured test specimens.In order to explain brittleness dependence on structure properties correlation of fractographic and fracture mechanical results has been evaluated.Frequently brittleness reduction is achieved by a lower amount of transgranular crack propagation associated with a strength decrease while maintaining specific fracture energy unchanged.Deviations from pure linear fracture mechanics increase with decreasing brittleness and contribute to specific fracture energy.Shear specimens may show two generations of cracks,a first one initiated by tensile loads （stable propagation） and a second one by shear loads （unstable propagation）.     

自养护路面混凝土抗盐冻性能及疲劳特性

为有效增强路面混凝土耐久性能,基于盐冻试验、盐冻前后的断裂性能试验及弯拉荷载疲劳试验,探索了高吸水性聚合物(SAP)自养护路面混凝土抗盐冻性能及疲劳特性随SAP掺量、粒径的变化规律,并结合自养护水泥浆体孔隙参数、微观形貌及骨料-水泥石界面过渡区(ITZ)特征,揭示了性能影响机理。结果表明:小粒径SAP形成的残留孔洞能有效释放拉应力,降低结冰点,细化孔结构,从而增强路面混凝土抗盐冻性能;当SAP粒径为100目(150 μm),掺量为0.145%(质量分数)时,路面混凝土在冻融30次时的断裂韧度损失率、断裂能损失率分别比基准组降低了25.25%、10.51%;小粒径SAP对疲劳寿命的提升程度随应力水平的提高而增大,当应力水平为0.80时,自养护组的疲劳寿命相比基准组提升了2.65倍;SAP能够有效提升水泥混凝土结构内部密实度,吸持ITZ区域部分水分,增强水泥石和骨料之间的粘结性,从而改善混凝土抗盐冻性能和疲劳特性。     

Experimental study on AE characteristics of three-point-bending concrete beams

In this research, acoustic emission (AE) characteristics of three-point-bending concrete beams were investigated during the entire loading period. It was found that the relative notch depth significantly influenced AE characteristics. The occurrence of AE events decreased greatly with an increase of the relative notch depth. The influences of different fibers in concrete on AE characteristics were investigated as well. The experimental results indicated that the Weibull function can be used to describe quantitatively the influences of the relative notch depth and fibers on AE characteristics, fracture characteristics, and brittleness of concrete. The two parameters, θ and m, of the Weibull function depended on the geometry of the concrete specimens and the brittleness of concrete, respectively.     

自密实橡胶混凝土性能研究综述

自密实橡胶混凝土是将废弃轮胎加工成橡胶颗粒并掺加到自密实混凝土中制成的新型建筑材料,相比于常规混凝土,其在硬化后具有脆性低、变形能力强、节能环保的显著优点.为了全面了解自密实橡胶混凝土的最新研究现状,从自密实性能(流动性、粘聚性、间隙通过性、抗离析性)、力学性能(抗压强度、劈裂抗拉强度、弹性模量、抗折强度、抗冲击性能、...     

Interfacial Interpretation of Autohesion of Ethylene/1-Octene Copolymers by Atomic Force Microscopy

The T-peel fractured surfaces of bonded films of ethylene/1-octene copolymers with different 1-octene contents were characterized using atomic force microscopy (AFM) and analyzed by fractal analysis. The AFM images showed strong dependence on the bonding temperature, peel rate, and the 1-octene content visually. This dependence has been demonstrated quantitatively by the fractal analyses which quantified an irregular surface by fractal dimensions and characteristic sizes. Two regimes showing fractal features were identified for each surface. In Regime I (higher magnifications) the welding and the following T-peel fracture procedures did little to change the fractal dimensions compared with the original surfaces before welding. But there were significant changes in Regime II (lower magnification) before welding and after T-peel fracture tests. The length scale that separated these two regimes is of the same order as that of polyethylene lamellar crystal structures. This suggests that the amorphous chains interdiffused across the interface while unmelted interfacial crystal structures remain essentially unaltered during the autohesion process. A “stitch-welding” autohesion mechanism was proposed to describe the bonding process in which only chains in the amorphous portions could interdiffuse. During the T-peel fracture tests, a crystal structure on the interface is either pulled over to the other side of the interface due to the interdiffused chains, remains unchanged, or is used as an anchor to pull a crystal structure from the other side of the interface. The characteristic sizes at which the fractal characteristics emerge were shown to be larger for the surfaces fractured at higher peel rates, which corresponds to higher fracture energy. This suggests that the appearance of fractal behavior at larger scales requires higher fracture energies. The characteristic sizes and fractal dimensions were also shown to depend on the molecular structure.     

3D surface image analysis for fracture modeling of cement-based materials

The texture and exposed phases of a fracture surface are direct evidence of the mechanical behavior of a cement-based material. Deflection, microcracking and bridging are toughening mechanisms involved in fracture of brittle matrices that affect surface roughness. This study measures crack deflection and branching using 3D surface measurement techniques with confocal laser microscopy of mechanically fractured mortar prisms and 3D stereo pair microscopy of mechanically fractured plain concrete prisms. Image analysis techniques were used to identify phase composition and out-of-surface crack branching from profiles of cracks intruded with a low melting-point alloy. The resulting data was the basis for a micromechanical model to relate surface and phase data and the measured fracture energy to the increase in energy with respect to fracture of the matrix independent from the composite behavior.     

BSF复合矿物掺合料对路面混凝土力学性能的影响

本文借助扫描电子显微镜、压汞测试仪和力学性能测试手段对BSF复合矿物掺合料的使用对路面混凝土力学性能的影响进行了系统研究。结果表明：（1）使用BSF复合矿物掺合料可在一定程度上提高混凝土的28 d抗折强度,大幅降低混凝土的压折比,有效改善混凝土的脆性;（2） BSF复合矿物掺合料可有效改善混凝土的界面过渡区,使浆体更加密实,并且减少有害孔的数量;（3）在水化过程中,BSF复合矿物掺合料中的砖粉和矿渣微粉会反应生成对改善混凝土抗折强度有益的纤维状晶体。     

高贝利特水泥混凝土的断裂韧性

为研究高贝利特水泥混凝土的断裂性能,通过三点弯曲梁试验,比较了高贝利特水泥混凝土与硅酸盐混凝土的双K断裂参数。结合载荷–裂口张开位移曲线分析,探讨了高贝利特水泥混凝土的断裂能。结果表明：高贝利特水泥混凝土载荷–裂口张开位移曲线下覆盖的面积大于硅酸盐水泥混凝土,混凝土对载荷能量有较高的吸收能力,断裂能大,断裂韧度大;高贝利特水泥混凝土的起裂韧度和失稳韧度分别是硅酸盐水泥混凝土的1.17倍和1.24倍。高贝利特水泥混凝土的断裂性能优于硅酸盐水泥混凝土的。     

Interfacial Interpretation of Autohesion of Ethylene/1-Octene Copolymers by Atomic Force Microscopy

The T-peel fractured surfaces of bonded films of ethylene/1-octene copolymers with different 1-octene contents were characterized using atomic force microscopy (AFM) and analyzed by fractal analysis. The AFM images showed strong dependence on the bonding temperature, peel rate, and the 1-octene content visually. This dependence has been demonstrated quantitatively by the fractal analyses which quantified an irregular surface by fractal dimensions and characteristic sizes. Two regimes showing fractal features were identified for each surface. In Regime I (higher magnifications) the welding and the following T-peel fracture procedures did little to change the fractal dimensions compared with the original surfaces before welding. But there were significant changes in Regime II (lower magnification) before welding and after T-peel fracture tests. The length scale that separated these two regimes is of the same order as that of polyethylene lamellar crystal structures. This suggests that the amorphous chains interdiffused across the interface while unmelted interfacial crystal structures remain essentially unaltered during the autohesion process. A “stitch-welding” autohesion mechanism was proposed to describe the bonding process in which only chains in the amorphous portions could interdiffuse. During the T-peel fracture tests, a crystal structure on the interface is either pulled over to the other side of the interface due to the interdiffused chains, remains unchanged, or is used as an anchor to pull a crystal structure from the other side of the interface. The characteristic sizes at which the fractal characteristics emerge were shown to be larger for the surfaces fractured at higher peel rates, which corresponds to higher fracture energy. This suggests that the appearance of fractal behavior at larger scales requires higher fracture energies. The characteristic sizes and fractal dimensions were also shown to depend on the molecular structure.     

锂渣细度对锂渣混凝土早期抗裂性能影响及分形评价

锂渣作为一种工业废渣利用到混凝土中,可发挥锂渣的潜能,提高混凝土的性能。试验利用了锂渣优良的可磨性,通过平板法开裂试验研究了不同细度锂渣对混凝土塑性收缩、自缩收裂缝的影响,并应用分形理论测量并计算不同细度锂渣混凝土裂缝的分形维数。结果表明:锂渣混凝土早期裂缝的最大宽度、总长度以及总面积随锂渣细度的增大而增大,同时裂缝复杂化程度提高。     

Fatigue enhancement of concrete beam with ECC layer

The pseudo strain-hardening behavior of Engineered Cementitious Composites (ECC) is a desirable characteristic for it to replace concrete to suppress brittle failure. This widespread use of ECC in the industry is, however, limited by its high cost. To achieve higher performance/cost, ECC can be strategically applied in parts of a structure that is under relatively high stress and strain. In this paper, layered ECC-concrete beams subjected to static and fatigue flexural loads were investigated by experiments. The static test results showed that the application of a layer of ECC on the tensile side of a flexural beam increased its flexural strength and the degree of improvement increased with the thickness of ECC applied. In addition, the layer of ECC enhanced the ductility of the beam and the failure mode changed from brittle to ductile. Under four-point cyclic loading, the ECC layer significantly improved the fatigue life of the beam. Moreover, in comparison to plain concrete beams, layered ECC beams could sustain fatigue loading at a larger deflection without failure. The great improvement in fatigue performance was attributed to the effectiveness of ECC in controlling the growth of small cracks. The experimental findings reflect the feasibility of using ECC strategically in critical locations for the control of fatigue crack growth.     

Fabrication and mechanical properties of nacre-like alumina with addition of silicon nitride

How to deal with the brittleness of ceramic materials is always one of the hot topics in the field of materials science. Design of layered ceramics with textured structure is one of the effective methods to improve their fracture toughness. The introduction of additives as interlayer phases can balance fracture toughness and flexural strength. However, the research about addition of interlayer phases and their mechanisms in the layered ceramics is still limited. In this work, nacre-like alumina ceramics were successfully fabricated by freeze casting followed by hot pressing. Silicon nitride was incorporated as the interlayer phase, and the effect on the mechanical properties of the nacre-like alumina was investigated. The addition of silicon nitride was beneficial to improvement of interlayer bonding between the alumina layers due to formation of sialon phase, leading to increase of flexural strength but decease of fracture toughness. When the content of silicon nitride was 3.3 wt%, flexural strength and fracture toughness of the sample was 468 MPa and 6.2 MPa m^1/2, respectively. Compared with the sample without silicon nitride, the flexural strength was enhanced significantly. Additionally, both flexural strength and fracture toughness were improved as compared the sample without any additives. This work can provide available references for design and fabrication of high-strength and high-toughness ceramics by properly tuning the layer structure and interlayer phase composition.     

锈蚀后体外加固梁疲劳性能分析

针对12根由实际工程缩尺,并按不同腐蚀率进行处理后的模型梁,采用体外预应力钢绞线加固方式,分别进行静载试验和等幅疲劳试验研究,进一步了解模型梁在不同锈蚀程度下,体外预应力加固对结构疲劳性能的影响.结果显示:体外预应力钢绞线加固方式可以改善试验梁的抗腐蚀疲劳性能,有效防止严重锈蚀梁发生脆性破坏,使锈蚀梁的疲劳寿命得到明显提高;随着锈蚀程度的不断增加,体外预应力加固对构件的承载力和抗弯刚度的提高幅度不断增大;通过引入锈蚀与疲劳作用的刚度修正系数,提出了疲劳荷载作用下,锈蚀后混凝土梁经过体外预应力加固的跨中挠度计算公式,为实际工程的应用提供参考.     

基于核磁共振技术分析生活垃圾焚烧灰渣代砂混凝土的微观结构与抗压强度

孔结构是混凝土微观结构中重要的组成部分,影响着混凝土的宏观性能,为了研究生活垃圾焚烧灰渣代替天然砂对混凝土孔结构和抗压强度的影响,通过核磁共振技术对生活垃圾焚烧灰渣代砂混凝土微观孔结构进行测试,分析了孔隙率和孔隙分布变化规律,并根据分形理论研究了孔隙分形特征,获得各孔径区间的分形维数与整体分形维数,并探讨了各孔径占比、孔隙率和分形维数与抗压强度之间的灰熵关联度。结果表明:随生活垃圾焚烧灰渣代砂率的增加,混凝土孔隙率增加,无害孔占比减小,抗压强度降低;分形维数随着生活垃圾焚烧灰渣代砂率的增加而减小,由于生活垃圾焚烧灰渣具有潜在的水硬性,随龄期的增长,分形维数呈增加趋势,同时孔隙结构得到了优化。灰熵关联度分析发现生活垃圾焚烧灰渣代砂混凝土整体分形维数和无害孔占比对抗压强度影响最大。     

Fracture behavior of magnesia refractory materials under combined cyclic thermal shock and mechanical loading conditions

To investigate the effect of cyclic thermal shock and mechanical loading on the fracture behavior of magnesia refractories showing different brittleness, the as-received and cyclic thermally shocked specimens are subjected to monotonic and cyclic wedge splitting test. The whole duration of test is monitored by digital image correlation and acoustic emission. Both thermal and mechanical fatigue resistance increase with the reduction of brittleness. Repetitive thermal shock results in pronounced reduction of strength. However, the specific fracture energy and nonlinearity increase after exposure to thermal shock due to the expanded micro-crack network inducing the development of a significant fracture process zone. Periodic loading mainly leads to the decrease of strain bearing capacity, as the fatigue loads favor the extension of crack tip instead of fracture process zone expansion. The combined application of periodic thermal shock and mechanical loads gives a new insight into the progressive damage behavior of refractory under critical conditions.     

Characterization of fractured basement reservoir using statistical and fractal methods

This study presents a characterization of fractured basement reservoir by using statistical and fractal methods with outcrop data, seismic data, as well as FMI log data. In the statistical method, fracture intensity and length have been calculated from various outcrop data. The optimum statistical distribution functions of fracture length for outcrops have been identified with the use of discriminant equation derived from Crofton’s theory. The Fisher distribution constant, representing the fracture orientation, has been computed from FMI log data. With the statistical values and distribution functions, a 3D fracture network system has been generated. The result shows that there is no distinction in orientation of the fracture network system, and it excellently matches with the outcrop data. In the fractal method, fractal dimensions of fracture length and strike for the seismic fracture network in areal distribution were calculated; a greater value in fractal dimension means that the fracture network system has intensive fractal characteristics. Meanwhile, vertical distribution and dip angle of the fracture system have been evaluated from FMI log data. The resulting 3D fracture system presents that the overall strike and distribution of the fracture system are excellently matched with those of seismic data.     

Strength development of mortars containing ground granulated blast-furnace slag: Effect of curing temperature and determination of apparent activation energies

The strength development of mortars containing ground granulated blast-furnace slag (ggbs) and portland cement was investigated. Variables were the level of ggbs in the binder, water-binder ratio and curing temperature. All mortars gain strength more rapidly at higher temperatures and have a lower calculated ultimate strength. The early age strength is much more sensitive to temperature for higher levels of ground granulated blast-furnace slag. The calculated ultimate strength is affected to a similar degree for all ggbs levels and water-binder ratios, with only the curing temperature having a significant effect. Apparent activation energies were determined according to ASTM C1074 and were found to vary approximately linearly with ggbs level from 34 kJ/mol for portland cement mortars to around 60 kJ/mol for mortars containing 70% ggbs. The water-binder ratio appears to have little or no effect on the apparent activation energy.     

纤维混凝土梁研究综述

对纤维增强混凝土梁的国内外研究进展进行了归纳总结,从试验分析方面阐述了纤维对梁力学性能的影响;在理论研究方面对纤维混凝土梁的抗弯和抗剪理论及抗剪承载力计算公式进行了总结并对公式的实用性做了进一步的阐述;在数值模拟方面介绍了纤维混凝土梁数值计算使用的的软件及成果.最后,基于现有的研究成果,提出了目前存在的不足,探讨了未来纤维混凝土梁的发展方向.     

Very-high-performance concrete with ultrafine powders

The compactness and fluidity of binary and ternary compound paste systems containing ultrafine powders such as pulverized fly ash (PFA), pulverized granulated blast furnace slag (PS) and silica fume (SF) were quantitatively studied with the relative density (d/d₀) index. Through optimization of the proportions of compositions and applying heat treatment to specimens, a very-high-performance concrete (VHPC) including large quantities of ultrafine powders has been made successfully, which offers compressive strength up to 200 MPa. Two methods of overcoming the brittleness of VHPC were investigated.