Influence of bundle coating on the tensile behavior,bonding, cracking and fluid transport of fabric cement-based composites

The goal of this work was to study the mechanical performance and fluid ingress of fabric cement based components made of epoxy coated and non-coated multifilament carbon fabrics. Direct tensile, pullout, and fluid transport tests were performed. Cracking was observed using four test geometries: (i) tensile tests, (ii) pullout tests, (iii) restrained shrinkage tests, and (iv) wedge splitting tests. The results show that coating multifilament carbon yarns improves mechanical behavior and bonding of the composite when compared with non-coated carbon yarn composites. The non-coated carbon systems may be problematic due to poor bonding as well as their potential to permit fluid ingress along the bundle–matrix interface and through the empty spaces between filaments. In addition, it was also found that fabric with coated bundles reduces crack width and develops dense branched network cracks. However, these additional fine cracks were found to increase fluid ingress into the matrix as compared with the plain cement paste.     

碳纤维织物力学性能和冲压成形实验研究

在实际成形过程中,碳纤维复合材料往往处于复杂的应力状态,开展近于真实载荷环境下的力学试验分析,能够更准确地认识实际应用中材料的成形性能和变形机理.为获得碳纤维织物的基本力学特性,设计了平纹碳纤维织物拉伸试样及成形试样,进行了单轴拉伸、双轴拉伸、镜框剪切试验和方盒冲压成形实验研究,对比了不同双拉比及纱线取向对力学性能及成形性能的影响.研究结果表明:碳纤维织物具有高度的非线性、各向异性和双拉耦合特性,即经纬向纤维的力学性能会相互影响;剪切变形是成形过程中的主要变形模式,当剪切角达到临界锁死角时,织物发生起皱现象;同种织物不同纱线取向试样表现出不同的成形性能,因此可以根据零件几何形状选择合适纤维取向的织物,从而减少缺陷,优化成形零件的力学性能.研究结果为后续建立碳纤维织物本构模型和成形仿真奠定了基础.     

地应力条件下的凿岩爆破数值模拟

为了研究地应力对凿岩爆破的影响,采用DDA方法模拟爆炸应力波作用下考虑地应力条件时的单孔和多孔凿岩爆破破岩过程。模拟发现,随着初始地应力水平的增加,裂纹扩展半径和破岩区域面积减小,裂纹发育主方向趋于地应力的最大主应力方向,初始地应力对裂纹的抑制和引导作用明显;初始地应力水平的增加,对拉伸裂纹的抑制作用更为显著,从而降低了拉伸破坏对爆破破岩的贡献。模拟也表明,在初始地应力存在的条件下,通过对爆破载荷和炮孔布置进行针对性的优化,可以克服地应力带来的影响,并取得预期的爆破效果。本研究对地应力条件下的凿岩爆破工程具有理论和参考意义。     

Hydrogen induced cracking in high strength steel

Initiation of quasicleavage (QC) cracks in delayed fracture of hydrogen charged, quenched and tempered AISI 4340 steel was studied by means of a fractographic technique. The fatal crack leading to the delayed fracture of an unnotched specimen is found not to be caused by intergranular cracking as is commonly found in notched specimens, but by QC cracking. An SEM fractographic examination has revealed that QC cracks, which initiate around non-metallic inclusions and under the effect of hydrogen alone, grow mainly along the rolling direction of the specimen. Furthermore, the tensile component of stress causes a QC crack to grow in the plane of maximum tensile stress. The condition under which brittle fracture finally occurs in delayed fracture can be evaluated by the classical Griffith criterion from two parameters: the QC crack length and the nominal applied stress.

MST/3264     

Effects of crack properties and water-cement ratio on the chloride proofing performance of cracked SHCC suffering from chloride attack

This study aims at developing an SHCC mixture that offers improved workability and high rebar corrosion proofing performance while ensuring moderate tensile ductility. Specimens were made from several mixtures with part of the cement replaced with limestone powder, and with a reduced fiber content. Mechanical properties and corrosion proofing performance under tensile stress were examined to determine the effects of chloride in a simulated situation under uniaxial tensile strain. The results indicated that the fiber content in the mixture had no influence on the crack properties of steel reinforced SHCC under tensile load. It was found that the chloride proofing performance of SHCC with multiple cracks was affected by the crack properties such as the number of cracks and the accumulated crack width. Reducing the water-cement ratio was effective to enhance the chloride proofing performance of SHCC.     

Anisotropy in the fracture properties of apple flesh as investigated by crack-opening tests

The texture of apple flesh is important in assessing the eating qualities of the fruit. Texture is in turn related to the structure of the parenchyma. Crack-opening tests (wedge penetration tests and notch tensile tests) have shown the tissue to have marked anisotropy in its fracture properties. These differences can be detected by sensitive mechanical instruments and also in the mouth. The flesh of the apple is split much more easily along the fruit's radius than, for example, in a direction parallel to the fruit's tangent. This was shown with the fracture tests as well as discriminated by a taste panel. In tangential orientation the wedge, or teeth, have to penetrate to a greater distance exerting a greater force to initiate a free-running crack, and fracture toughness is about 50% greater than in radial orientation. The mechanical behaviour of apple parenchyma is directly related to its structural composition. The radially elongated intercellular spaces ease the passage of radially travelling cracks, i.e. along the direction of the spaces, and act as crack stoppers and crack deflectors to tangentially travelling cracks, i.e. at right angles to the spaces. This increases the energy requirement for crack propagation for tangential cracks hence increasing the fracture toughness in that orientation.     

Distributed cracking and stiffness degradation in fabric-cement composites

Formation of distributed cracking and the associated degradation in the stiffness of fabric-cement composites under tensile loading were studied. Composites made from low modulus woven polyethylene fabric and bonded Alkali Resistant (AR) glass mesh were manufactured by means of pultrusion technique. The influence of fabric type, matrix modification and curing as well as the pressure applied after pultrusion were studied using tensile stress strain response. Three distinct measures of damage including quantitative crack spacing by image analysis, stiffness degradation, and microstructural observation by optical and scanning electron microscopy are evaluated. The evolution of crack spacing as a function of applied strain was correlated with the tensile response as well as with the stiffness degradation for various composites. Also, the microstructure of the different composites was characterized and correlated with their mechanical properties using optical and scanning electron microscopy. It was observed that the mechanical properties as well as crack spacing and composite stiffness were significantly affected by the matrix formulation, curing procedure, and the intensity of the pressure applied after the pultrusion process. The best tensile performance was achieved for glass fabric composites with a high content of fly ash.     

含孔CFRP正交层合板的双轴拉伸力学行为

采用加载臂开槽的中心开孔等厚度十字形试样,实验研究了正交对称铺层碳纤维增强聚合物基复合材料（CFRP）层合板在双轴拉伸载荷作用下的力学行为,分析了3种双轴加载比对其拉伸强度和破坏行为的影响。研究表明:纤维被切断的铺层部分在拉伸作用下容易与其相邻铺层脱粘,导致层合板承载力下降;等双轴加载时,在孔边的被切断纤维与连续纤维间基体在横向拉伸和纵向剪切组合作用下首先开裂;非等双轴加载时,在垂直于快速拉伸方向的铺层中沿孔边应力集中处先出现基体裂纹;随着加载比的增大,快速拉伸方向的细观结构损伤随载荷的增大发展更快,刚度下降更快,破坏时主裂纹的扩展方向更趋于垂直于快速拉伸方向;强度包络线的分析表明快速拉伸方向的拉伸强度随加载比的增大呈缓慢增大的趋势。      

Approximation of curved cracks under mixed-mode loading

The calculation of stress intensity factors or mechanical energy release rate for non-straight cracks can be complicated. Approximation to equivalent crack shapes can simplify calculations considerably, but this requires an understanding of the influence of key shape parameters on crack-tip stresses. A simple analytical model has been developed, based on the concept of a relaxed volume, to predict mechanical energy release rate and deflection angle for a range of crack shapes under mixed-mode loading. Results from this model compared well with those obtained from finite element (FE) simulations, and with predictions from previous analytical models. It was found that the crack length and orientation of the crack-tip with respect to loading direction are the key influences on fracture parameters, whilst curvature near the crack-tip can also affect results.     

虚拟裂缝尖端拉应力大小的确定

针对单边切口的混凝土轴心受拉构件,基于虚拟裂缝模型提出一种计算极限承载力的解析模型,并在此基础上确立了虚拟裂缝尖端拉应力与混凝土轴心抗拉强度之间的关系。结果表明:二者的比值随初始缝高比的增大呈线性增加,但对混凝土强度等级的变化不敏感。其原因是由于所有的混凝土试件都存在初始缺陷,导致截面上存在明显的应力梯度,因而得到的混凝土轴心抗拉强度值是截面应力的平均值,而虚拟裂缝尖端拉应力为截面上的最大应力。很显然,轴心受拉构件的初始缺陷越长,截面的应力梯度越大,虚拟裂缝尖端拉应力与平均应力的比就越大。通常情况下,虚拟裂缝尖端的拉应力大小约为混凝土轴心抗拉强度值的1.22倍,约等于混凝土的抗折强度。     

Similarity rule of crack geometry in statistically homogeneous rock masses

An $\text{nth}$ rank tensor called the generalized fabric tensor is introduced to express crack geometry due to discontinuities like joints and faults in rock masses. Statistically homogeneous rock masses can be regarded as geometrically similar bodies if they are characterized by a common fabric tensor. In order to say that they are also similar in mechanical properties, the crack geometry must be similar in the sense of the generalized fabric tensor. The generalized fabric tensor can be expressed by (1) the number of cracks crossed by a unit length of a scanning line, (2) the number of cracks associated with a unit area of a scanning plane, and (3) the density function $\text{E}$(n) to describe the orientation of crack normal unit vectors n. Since these are all determined by conventional field surveys, one can say that the similarity rule for crack geometry is ready to be used in practical rock mechanics.     

双轴动载作用下脆性裂纹扩展问题的近场动力学建模与分析

在双参数微极近场动力学弹脆性模型基础上,引入反映长程力尺寸效应的核函数修正项以提高定量计算精度和收敛稳定性。通过定量变形计算和双轴动载作用下含中心裂纹准脆性板的裂纹扩展过程模拟,验证了本文模型与算法。应用本文模型能准确反映动载作用下的裂纹起裂、扩展、分叉与二次分叉等现象。进一步分析了双轴动载作用下含不同倾角、不同间距平行双裂纹脆性板的破坏模式、起裂时间与承载能力,以及含不同倾角多裂纹脆性板的破坏机制。结果表明:初始裂纹倾角与间距对脆性板承载能力有显著影响,在平行双裂纹倾角一定时,间距越大脆性板承载能力越强;双轴动载作用下多裂纹扩展时,裂纹之间首先相互连接且无分叉,在初始裂纹连接贯通后向边界扩展时出现分叉及二次分叉。     

Creep fracture parameter C* solutions for semi-elliptical surface cracks in plates under tensile and bending loads

In order to predict and assess creep life for plate structures with semi-elliptic surface cracks under high temperature condition, the accurate calculation of the creep fracture mechanics parameter C* is a critical step. In this paper, the effects of crack sizes, plate geometries, and material creep properties on the parameter C* have been investigated under tensile and bending loads by extensive finite element analyses. Based on the results, the creep influence functions Hc for calculating C* values were obtained and fitted into equations for surface cracks in plates under both loads. The equations have been verified by finite element calculations. The C* solutions were obtained through these equations which are suitable for wide ranges of crack sizes, plate geometries, and materials.     

FATIGUE CRACK GROWTH UNDER MIXED-MODE I AND II LOADING

Abstract— Mixed-mode fatigue crack growth has been studied using four point bend specimens under asymmetric loads. A detailed finite element analysis provides the stress intensity factors for curved cracks under different mixed-mode load conditions. Both fatigue crack growth direction and crack growth rate are studied. The maximum tangential stress and the minimum strain energy density criteria were found to provide satisfactory predictions of the crack growth directions. An effective stress intensity factor was used to correlate the fatigue crack growth rates successfully. It is found that the use of mode I fatigue crack growth rate properties results in a conservative crack growth rate prediction for mixed-mode load conditions.     

拉—拉疲劳下15MnMoVN多裂纹扩展研究

为分析单裂纹或多裂纹在裂纹面承受疲劳拉伸载荷作用下尖端应力强度因子变化规律和裂纹形貌变化以及疲劳寿命情况,以含不同初始长深比的半椭圆单裂纹或双裂纹的薄片试样为研究对象,对试样在应力比R=0.1的疲劳拉伸载荷下单裂纹或双裂纹情况进行了仿真分析。建立含裂纹试样的有限元模型,仿真分析了裂纹在扩展过程中尖端应力强度因子的分布情况,并将单裂纹扩展结果与双裂纹相互作用影响下的结果进行了对比研究;进行含裂纹试样的疲劳实验,分析了含单裂纹或双裂纹的试样的断裂面的形成原因,并验证仿真结果正确性。结果表明,裂纹面之间的相互作用会逐渐影响裂纹的扩展方向、扩展速率以及在扩展过程中尖端应力强度因子的变化趋势;而且初始形貌为半椭圆形的双裂纹在相互作用影响下会逐渐过渡到半圆形。     

Fracture of a piezoelectric material layer bonded by two elastic layers

A piezoelectric material layer bonded by two elastic layers under mechanical and electrical loads is studied. The piezoelectric material layer contains a central crack or two collinear cracks. Both mixed-mode crack and anti-plane crack are considered for the impermeable crack assumption and the permeable crack assumption. The effect of electric boundary conditions on electrical and mechanical field intensity factors are discussed. Some new observations are found.     

EVALUATION ON LIMITATION OF LINEAR ELASTIC FRACTURE MECHANICS FOR SMALL FATIGUE CRACK GROWTH

Abstract— The growth characteristics of small fatigue cracks were investigated under rotary bending in a high tensile strength steel, and the critical crack length above which linear elastic fracture mechanics (LEFM) is applicable was evaluated systematically from the results of the present study and previous studies in which two steels having different microstructures and strengths were used. It is found that the critical crack length extends over two regions each exhibiting different growth mechanisms. These regions of microstructurally small cracks and mechanically small cracks are found to be a unique function of the microstructural unit size and the yield strength of the materials, respectively. Therefore, using these relationships, it is possible to estimate the critical crack length for a given material.     

Growth of cracks under far-field cyclic compressive loads: Numerical and experimental results

Crack growth from a notch tip, under the influence of fully compressive far-field cyclic loads, is investigated both numerically and experimentally in notched specimens of a lower strength steel and an aluminum alloy. Such cracks, emanating from the root of the notch, progressively decelerate until complete crack arrest occurs. Analyses of the stress state along the crack plane indicate that while the extent of the residual tensile stress field diminishes with an increase in crack length, there is a progressive increase in crack closure during crack advance. Approximate numerical calculations of the crack closure loads are in reasonable agreement with experimental results.     

Fracturing behaviour of sandstone specimens with a cavity formed by intersecting excavations under compression: Experimental study and numerical modelling

To investigate the mechanical properties, damage characteristics, and fracturing behaviour of specimens with a cavity formed by intersecting excavations, a series of uniaxial compression tests were conducted incorporating digital image correlation (DIC) and acoustic emission (AE) techniques. PFC^2D modelling was conducted to further study the failure modes and crack evolution under biaxial loading. The results showed that the mechanical properties are significantly weakened by the cavity and influenced by its shape. The failure of the specimens containing a cavity under uniaxial compression can be considered as a progressive process of crack initiation, propagation, and coalescence of different cracks with each other, leading to forming macrofractures, which can be visually displayed by the DIC technique. A new method for determining the crack closure stress is proposed, and the crack initiation stress and the crack damage stress of specimens are also obtained by the AE measurements. The failure mode of the intact specimen changed from the tensile–shear failure mode under the uniaxial compression to the shear‐dominated failure mode under the biaxial compression. Failure of the specimens with a cavity is dominated by shear cracks rather than tensile cracks. Under high confining stresses, almost no macrotensile cracks appeared on the roof or floor of the cavity; instead, several spalling fractures were visible on the two sides of the cavity. The fracturing mechanism is well explained by the evolution of the internal stresses in the specimens.     

60Si_2Mn钢的低周拉扭复合微动疲劳特性

测量60Si_2Mn钢在拉扭复合载荷作用下的低周微动疲劳特性,研究了不同轴向循环拉伸应力幅值对微动疲劳寿命、循环软化特性以及摩擦磨损表面和断口形貌的影响.结果表明,随着循环拉伸应力幅值的提高,60Si_2Mn钢的微动疲劳寿命降低幅度不同,发生循环软化的时期不断提前,完成循环软化的疲劳周期也不断缩短。同时,微动摩擦副产生的氧化物磨屑对微动磨损性能有重要影响,在疲劳前期加剧摩擦磨损,在疲劳后期减轻摩擦磨损。微动疲劳裂纹源形成于试样发生微动摩擦磨损的表面,并出现疲劳台阶。在扭矩产生的切向剪切应力作用下,疲劳裂纹沿着与轴向45°角的方向扩展,最终在断口上留下显著的舌状凸起,拉应力的幅值越大舌状凸起越明显。