聚丙烯纤维水泥稳定碎石断裂性能试验研究

通过30个尺寸为100mm×100mm×515mm的聚丙烯纤维水泥稳定碎石和普通水泥稳定碎石三点弯曲试件断裂试验,探讨了聚丙烯纤维对水泥稳定碎石断裂韧度(KIC)、断裂能(GF)、临界裂缝嘴张开位移(CMODC)、临界裂缝尖端张开位移(CTODC)、极限裂缝嘴张开位移(CMODmax)和极限裂缝尖端张开位移(CTODmax)的影响。试验结果表明:聚丙烯纤维的掺入可增大水泥稳定碎石的断裂韧度、断裂能、临界裂缝嘴张开位移、极限裂缝嘴张开位移、临界裂缝尖端张开位移和极限裂缝尖端张开位移;随着聚丙烯纤维体积掺量的增加,断裂韧度、临界裂缝嘴张开位移和临界裂缝尖端位移的变化无明显规律,但断裂能、极限裂缝嘴张开位移和极限裂缝尖端位移基本上呈线性增加的。     

高韧性混凝土拉伸荷载下的裂缝控制能力研究

通过分析纤维在水泥基体中的界面连接应力,建立了高韧性混凝土的多缝开裂模型。基于开裂模型分析了摩擦粘结强度、化学粘结强度、基体开裂强度和纤维弹性模量单独变化时对高韧性混凝土多缝开裂区结束时的应变和裂缝宽度的影响。分析结果表明:随着摩擦粘结强度的增加,多缝开裂区结束时的应变和裂缝宽度减小;随着化学粘结强度的增加,多缝开裂区结束时的应变增加而裂缝宽度减小;随着基体开裂强度的增加,多缝开裂区结束时的应变和裂缝宽度均增大;随着纤维弹性模量的增加,多缝开裂区结束时的应变和裂缝宽度减小。通过对带切口试件的直接拉伸试验,研究了高韧性混凝土在存在初始缺陷情况下的裂缝控制能力。试验结果表明,初始缝高比为0.2时,试件切口处的极限应力为5.3 MPa,非切口处的极限应力为4.3 MPa,整个试件范围内出现了多缝开裂,且裂缝宽度控制在0.1 mm以内。初始缝高比为0.4时,试件切口处的极限应力为5.8 MPa,非切口处的极限应力为3.5 MPa,裂缝面上的纤维未能有效控制裂缝的发展,试件仅在切口附近出现了多条围绕切口的弧形裂缝。     

脆性材料复合型裂纹的断裂准则

实际工程中,脆性材料中的裂纹多处于复合型受力状态,因此,确定脆性材料中的复合型裂纹起裂角和临界荷载有着重要的理论意义和实用价值。以复合型裂纹为研究对象,将裂纹尖端的最小无量纲塑性区尺度ρmin和广义合成偏应力强度理论相结合,建立脆性材料复合型裂纹的断裂准则,预测裂纹起裂角及临界荷载,将其结果与最大周向应力准则和应变能密度因子准则相比较发现,基于该文方法得到的临界荷载曲线大于最大周向应力准则得到的临界荷载曲线,与应变能密度因子准则得到的临界荷载曲线比较接近。因而,表明了用该文的方法来预测脆性材料复合型裂纹起裂角和临界荷载是行之有效的。     

超高强结构钢AF1410的断裂韧度试验研究

平面应变断裂韧度是材料进行损伤容限设计时用到的重要力学性能指标,因此如何通过试验手段获得该指标显得格外重要。本文对超高强结构钢AF1410进行了平面应变断裂韧度和延性断裂韧度的试验研究,结果表明:由于该材料的断裂韧度值较高,使得确保处于平面应变状态的试样尺寸过大,难以在普通低吨位的试验机上完成,而选用合适的试样尺寸,通过延性断裂韧度试验和相关的公式计算,则可以间接地获得超高强结构钢材料的有效平面应变断裂韧度值。     

岩石-混凝土界面拉伸断裂性能的率相关性研究EI北大核心CSCD

该文弯曲断裂试验获得了不同应变率下界面的抗拉强度、荷载-加载点位移曲线、荷载-裂缝口张开位移曲线、起裂荷载和峰值荷载,通过夹式引伸计法和DIC法获得了临界裂缝扩展长度。并计算了界面断裂能及双K断裂参数,分析了不同应变率下界面断裂过程区演化规律及特征长度的变化。结果表明:随应变率的增大,断裂能和起裂韧度增大,临界裂缝长度和失稳韧度先增加后减小,断裂过程区长度及特征长度随应变率的提高而减小。该文从裂缝发展路径、自由水粘性、惯性效应三方面探讨了岩石-混凝土界面断裂参数的率效应。     

新老混凝土粘结面断裂损伤过程区研究

在新老混凝土粘结面断裂性能试验的基础上,建立了新老混凝土粘结面断裂破坏模式,分析讨论了界面层特性及界面微裂缝扩展对新老混凝土粘结面断裂性能的影响。建立了新老混凝土粘结面带状微裂缝断裂过程区模型,根据该模型可求出断裂过程区尺寸。结合裂缝过程区的荷载~应变试验结果,用临界断裂韧度对新老混凝土粘结断裂韧度进行了断裂过程区影响修正。     

混凝土断裂过程区长度计算方法研究

该文基于粘聚裂缝概念,以起裂韧度作为裂缝起裂和扩展的准则,提出了混凝土断裂过程区长度的计算方法。以Ⅰ型裂缝为例,计算了不同初始缝长和起裂韧度情况下的断裂过程区长度值,结合以往大体积混凝土的试验数据对其进行了验证。进而分析了断裂过程区长度的影响因素,结果表明:断裂过程区长度随初始缝长的增大而逐渐增大,随起裂韧度的增大而逐渐减小。     

回火温度对16Co14Ni10Cr2Mo钢断裂韧度和抗应力腐蚀性能的影响

研究了回火温度对１６Ｃｏ１４Ｎｉ１０Ｃｒｔ２Ｍｏ钢断裂韧度ＫＩＣ和抗应力腐蚀性能ＫＩＳＣＣ的影响。发现５１０℃以下回火ＫＩＣ和冲击值ａｋ的变化一致，都表面出回火马氏体脆性，但在回火脆性区内ＫＩＣ不降低。在二次硬化峰之前ＫＩＳＣＣ值很低，这是二次硬化钢不能在硬化峰之前回火使用的重要原因。     

钢纤维对水泥基复合材料抗起裂特性的影响

通过不同钢纤维体积分数及不同试件尺寸的预制缺口三点弯曲梁断裂试验,研究了普通乱向及定向钢纤维增强水泥基复合材料的抗起裂特性。利用试验测得的荷载-裂缝口张开位移曲线,分析了钢纤维对水泥基复合材料断裂性能的影响,并基于线性相关系数陡降法计算了起裂韧度。结果表明,定向钢纤维增强水泥基复合材料的起裂韧度明显高于普通乱向钢纤维增强水泥基复合材料;起裂韧度随钢纤维体积分数的增加而逐渐增大,当钢纤维体积分数达到0.9%左右时,定向钢纤维增强水泥基复合材料的起裂韧度值趋于稳定;在本试件高度范围内(40~100mm),起裂韧度随试件尺寸增加而逐渐增大,且定向钢纤维增强水泥基复合材料的增长趋势较为平缓。此外,从裂缝尖端夹杂改变其应力强度因子的角度解释了钢纤维的掺入及定向对起裂韧度的提高作用。     

小尺寸混凝土试件双K断裂参数试验研究

采用最大尺寸为680mm×160mm×40mm的标准三点弯曲梁试件,利用在初始裂缝两侧粘贴电阻应变片并利用混凝土裂缝扩展到此处时其应变回缩的方法测得了起裂荷载Pini,在此基础上根据Pini及初始缝长a0得到了起裂断裂韧度KIiCni;根据在试验中测得的最大荷载Pmax及对应的裂缝口张开位移CMODC计算了混凝土等效裂缝长度aC,据此计算了失稳断裂韧度KIuCn。结果表明:采用电阻应变片法可准确测定混凝土的起裂荷载Pini,且方法简单。试验结果还表明:在本试验范围内,三点弯曲梁法测得的混凝土双K断裂参数KIiCni、KIuCn与试件高度无关,进一步说明了混凝土双K断裂参数可以作为描述混凝土裂缝扩展的断裂参数。     

多尺度纤维增强水泥基复合材料力学性能试验

基于水泥基材料多尺度的结构特征及破坏过程,设计了一种由钢纤维、聚乙烯醇（PVA）纤维以及碳酸钙晶须构成的多尺度纤维增强水泥基复合材料（MSFRCC）,研究了其抗压强度、抗弯强度、弯曲韧性、多缝开裂形态以及断裂过程等基本力学性能。结果表明：基体材料的强度和韧性均得到了显著提高;MSFRCC在弯曲荷载作用下表现出了硬化行为和多缝开裂模式。扫描电子显微镜和断裂试验结果证实了多尺度纤维在水泥基复合材料破坏过程中发挥了多尺度阻裂作用。研究认为：通过对纤维进行多尺度组合设计,可以显著改善水泥基复合材料的韧性,廉价的碳酸钙晶须可以适量取代钢纤维和PVA纤维。     

陶瓷材料断裂韧性与缺口半径 Ⅱ 断裂韧性估算方法

在陶瓷材料裂纹尖端存在一个断裂过程区，当断裂过程区内平均应力达到断裂强度时，裂纹扩展。本文由理论推导结合实验数据，得到了新断裂过程区的大小是平均晶粒直径的四倍。并由平均应力断裂模型，给出了陶瓷材料断裂韧性和缺口半径及平均晶粒直径之间的关系式，由此关系式可以用宽缺口试件测出的断裂韧性去估算陶瓷材料的本质断裂韧性。     

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。      

A finite fracture mechanics approach to structures with sharp V-notches

Criteria assuming that failure of quasi-brittle materials is affected by the stresses acting over a finite distance from the crack tip are widely used inside the scientific community. For instance, they have been applied to predict the failure load of specimens containing sharp V-notches, assuming as a critical parameter the average stress ahead the notch tip. However, this kind of approaches disregards energy balance considerations, which, as well known, are the basis of linear elastic fracture mechanics (LEFM). In order to overcome these drawbacks, the present paper uses a recently introduced finite fracture mechanics (FFM) criterion, i.e. a fracture criterion assuming that crack grows by finite steps. The length of this finite extension is determined by a condition of consistency of both energy and stress requirements; as a consequence, the crack advancement is not a material constant but a structural parameter. The criterion is applied to structures with sharp V-notches. The expression of the generalized fracture toughness, which is a function of material tensile strength, fracture toughness and notch opening angle, is given analytically. Finally, we provide comparisons with: (i) the experimental data we obtained from testing Polystyrene specimens under three point bending; (ii) some experimental data available in the literature. The agreement between theoretical predictions and experimental results is generally satisfactory and, for most of the cases analyzed, the FFM predictions are better than the ones provided by the simple average stress approach.     

Influence of fibre type on flexural behaviour of self-compacting fibre reinforced cementitious composites

This paper investigates the flexural properties of self-compacting fibre reinforced cementitious composites that contain high fly ash volume. Seven types of fibres were compared at the same volume fraction and in similar matrices containing high-volume fly ash and having a high compressive strength of around 85 MPa at 28 days. Third-point bending test was conducted on beam specimens to obtain their load–deflection curves, and investigate their fracture behaviour, flexural strength, deflection and toughness. The results showed that using straight steel and micro-polyvinyl alcohol fibres produced composites demonstrating stable deflection-hardening with multiple-cracking phenomenon. This behaviour resulted in high flexural strength, along with large maximum deflection and toughness values, which are important for applications in cementitious composites. This study indicates that fibres with both sufficiently high aspect ratio and high tensile strength are necessary for achieving deflection-hardening in self-compacting cementitious composites with high-strength matrices containing high-volume fly ash.     

Effects of interfacial coating and temperature on the fracture behaviours of unidirectional Kevlar and carbon fibre reinforced epoxy resin composites

The enhancement of transverse fracture toughness of unidirectional Kevlar and carbon fibre reinforced epoxy resin composites (KFRP and CFRP) has been studied using polymer coatings on the fibres. The results obtained show a substantial improvement in the impact fracture toughness of both KFRP and CFRP with polyvinyl alcohol (PVAL) coating without any loss of flexural strength; but there is only a moderate increase in impact toughness with other types of coating (i.e. carboxyl-terminated butadiene acrylonitrile (CTBN) copolymer and polyvinyl acetate (PVA)) with some reduction in flexural strength. The dependence of impact fracture toughness of these composites (with and without PVAL coating) on temperature was analysed on the basis of existing theories of toughening mechanisms from measurements of fibre-matrix interfacial properties, debond and fibre pull-out lengths and microscopic observations. The beneficial effect of fibre coating with PVAL on transverse fracture toughness is shown to sacrifice little damage tolerance of the composites against delamination fracture.     

Effect of compressive transverse normal stress on mode II fracture toughness in polymeric composites

Effect of transverse normal stress on mode II fracture toughness of unidirectional fiber reinforced composites was studied experimentally in conjunction with finite element analyses. Mode II fracture tests were conducted on the S2/8552 glass/epoxy composite using off-axis specimens with a through thickness crack. The finite element method was employed to perform stress analyses from which mode II fracture toughness was extracted. In the analysis, crack surface contact friction effect was considered. It was found that the transverse normal compressive stress has significant effect on mode II fracture toughness of the composite. Moreover, the fracture toughness measured using the off-axis specimen was found to be quite different from that evaluated using the conventional end notched flexural (ENF) specimen in three-point bending. It was found that mode II fracture toughness cannot be characterized by the crack tip singular shear stress alone; nonsingular stresses ahead of the crack tip appear to have substantial influence on the apparent mode II fracture toughness of the composite.     

陶瓷的断裂韧性与缺口半径 Ⅰ.断裂韧性测试技术

本文用一种新的简单方法制备了不同半径的尖缺口，用单边切口梁试件测试了不同缺口半径氧化铝陶瓷的断裂韧性.对七种不同测试方法测得的、同一种材料的断裂韧性进行了对比，讨论陶瓷材料断裂韧性的最佳测试方法和测试中对缺口半径的要求.