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。     

环氧沥青混凝土裂纹起裂与失稳扩展的临界特征

为了获取环氧沥青混凝土的裂纹起裂与失稳扩展临界点,重构了环氧沥青混凝土非均质(集料、砂浆和空隙)多层次(矿料级配)结构三维虚拟试件,采用离散元方法实施了单边切口小梁虚拟三点弯曲试验,借助数字摄像法捕捉了室内小梁试件表面裂纹发展情况,分析了裂缝发展过程中裂缝尖端张开位移(CTOD)和裂缝口张开位移(CMOD)之间的变化关系。研究结果表明:裂尖张开位移δ₂₅参数的使用为理解实际流入断裂带用于裂缝扩展的那部分能量开辟了一条新思路;裂缝尖端张开位移和裂缝口张开位移关系曲线的两个转折点描述了裂缝起裂和临界失稳扩展状态,环氧沥青混凝土断裂过程中裂缝的扩展经历了起裂、稳定扩展和失稳扩展3个阶段;δ₂₅-CMOD曲线分析法可以作为研究沥青混凝土材料断裂行为的辅助手段。     

基于弥散与分离裂缝模型的混凝土开裂比较研究

裂缝扩展是影响混凝土结构非线性响应的重要因素,其扩展深度与张开位移是评价结构安全性的重要指标。基于弥散裂缝框架,该文建立了等效裂缝张开位移与损伤因子、断裂带宽度之间的函数关系。分别采用弥散、分离两类裂缝模型模拟I型断裂和I-II混合型断裂试验,二者在结构承载力、裂缝扩展和裂缝张开位移方面均获得了基本一致的结果。数值算例表明弥散裂缝模型与分离裂缝模型计算精度接近但其效率更高。     

采用峰值荷载法确定全级配水工混凝土断裂参数

该文采用峰值荷载法研究了全级配水工混凝土试件的断裂参数。通过有限元方法,得到了适用于全级配水工混凝土的非标准楔入劈拉试件的应力强度因子K、裂缝嘴张开口位移CMOD计算公式以及裂缝张开位移COD与CMOD比值COD/CMOD的相关曲线。基于相关断裂试验,通过该文所提公式得到了全级配水工混凝土的等效断裂韧度与临界裂缝尖端张开口位移CTOD_c等断裂参数。研究结果表明:峰值荷载法可应用于截面尺寸不同而缝高比相同的全级配水工混凝土试件,峰值荷载法只需实测各试件的峰值荷载,就可方便确定全级配水工混凝土的断裂参数。     

水泥混凝土路面裂缝成因及裂缝应力分析

分析水泥混凝土路面裂缝生成与扩展的原因机理,了解裂缝对路面使用寿命的危害,提出相应预防控制措施。利用ANSYS软件模拟水泥混凝凝土路面带圆孔面板,结合断裂力学原理,分析带圆孔板尖端处的应力及位移分布规律。     

反分析法确定钢纤维水泥砂浆拉应力与裂缝张开位移关系

在数值模拟钢纤维混凝土结构或构件和用基于断裂力学理论的设计方法设计钢纤维混凝土结构或构件时,钢纤维混凝土材料σ-w关系是一个重要的材料参数。该文根据三点弯曲缺口梁的荷载与裂缝张开位移(CMOD)曲线用反分析法确定了钢纤维水泥砂浆的σ-w关系。试验制作了五种不同体积含量的钢纤维水泥砂浆单轴拉伸试件、圆柱体压缩试件和梁试件,钢纤维体积含量分别为0.5%、1.0%、1.5%、2.0%和2.5%。试验发现,当钢纤维体积含量较小时,三点弯曲缺口梁的荷载与裂缝张开位移曲线呈现CMOD软化特性;而当钢纤维体积含量较大时,三点弯曲缺口梁的荷载与裂缝张开位移曲线呈现CMOD强化特性。对有CMOD软化特性的钢纤维水泥砂浆,可用三折线软化模型来模拟钢纤维水泥砂浆的σ-w关系;对有CMOD强化特性的钢纤维水泥砂浆,可用应力跌落-常残余强度模型来拟合。     

动荷载-水-冻融共同作用下混凝土宏观裂缝扩展与演变的研究进展

混凝土是土木工程建设的重要材料.由于设计、施工、服役环境及其材料自身等原因,混凝土在投入使用时就存在部分非结构裂缝.工程实践发现,在长期动荷载作用下,混凝土裂缝会在长度、宽度和深度方向出现扩展和演变,而动荷载和环境共同作用将会加快裂缝的扩展,加速混凝土材料的劣化,导致混凝土力学和耐久性能降低,严重影响其结构安全.动荷载的长期作用将导致混凝土裂缝尖端附近的应力场和位移场出现复杂的变化,从而引起应力集中和应变能释放,当裂缝尖端应力强度因子大于材料断裂韧度时,裂缝将发生失稳扩展.动荷载对裂缝中的水产生动水压,动水压作用下水对裂缝内壁产生反复冲刷和溶蚀,导致裂缝内壁的集料和水化产物流失,从而加速了裂缝的扩展.低温冻融过程中,水在裂缝内壁反复结冰溶解,在裂缝内产生了冻胀应力,而动荷载作用使裂缝发生的体积变化增大了冻胀应力,同时冻融对裂缝内壁的集料和水化产物产生了剥蚀作用,加速了混凝土裂缝的扩展.动载-水-冻融共同作用对混凝土宏观裂缝的扩展演化更加复杂,目前尚无系统研究.本文归纳了目前对混凝土宏观裂缝在动载-水-冻融共同作用下扩展演变研究的最新进展,梳理了动荷载作用下水和冻融对裂缝扩展演化的加速机制、裂缝扩展与演化的计算方法和有效预测模型,并提出了该研究方向需要进一步解决的问题,以期为进一步掌握混凝土结构的长期服役性能、完善混凝土结构损伤理论及养护维修技术奠定基础.     

基于非接触式观测技术的再生骨料混凝土断裂性能分析

采用非接触式观测技术(DIC)和传统电测法,对不同再生粗骨料取代率(0％、30％、50％、70％、100％)的混凝土进行三点弯曲梁断裂性能试验,得到双K断裂参数、裂缝口张开位移(CMOD)、裂缝尖端张口位移(CTOD)、裂缝扩展长度(△a)以及裂缝扩展路径,验证DIC非接触观测技术的有效性.结果表明:当再生骨料取代率为...     

基于虚拟裂缝模型的混凝土等效断裂韧度

本文采用虚拟裂缝模型,将临界裂缝尖端张开位移ＣＴＯＤｃ作为控制参数,利用三点弯曲梁试件通过迭代求得了混凝土裂缝亚临界扩展量的临界值△ａｃ,据此求得了混凝土起裂断裂韧度ＫｉｎｉＩｃ、等效断裂韧度ＫｕｎＩｃ值。计算结果表明,随着试件尺寸的增大,△ａｃ增大,但ＫｉｎｉＩｃ、ＫｕｎＩｃ值却是与试件尺寸无关的断裂参数。这表明线弹性断裂韧度准则可应用于混凝土结构的裂缝评定。     

虚拟裂缝模型的接触有限元法实现

沿裂缝可能开展路径设置接触点对,将求解接触问题的有限元混合自由度法进行扩展,以实现混凝土断裂力学虚拟裂缝模型的数值模拟。把作用在结构上的力系分解为外力和接触缝面上的接触力,以结构的位移为基本未知量,而以缝面局部坐标系下的结点接触力为迭代变量,将非线性接触迭代收缩在可能接触缝面上进行,提高了计算效率。计算过程中缝面接触力和开口位移可以显式地求出,故可方便地引入各种类型的混凝土软化本构关系,实现混凝土断裂力学虚拟裂缝模型的数值模拟。首先给出了有限元混合自由度法求解接触问题的基本思路,然后引入混凝土软化本构关系,最后以数值算例说明了该文方法的正确性和有效性。     

温度对结构钢材裂纹尖端张开位移(CTOD)的影响分析

裂纹尖端张开位移(CTOD)与钢材的弹性模量、屈服强度、几何条件和荷载大小等因素有关。结构钢材的相关力学指标随着温度发生变化,因此温度变化也会改变CTOD的数值。从含穿透缺口无限大受拉平板的Dugdale-Barrenblett模型(D-B模型)出发,推导出了温度对裂纹尖端张开位移影响规律的基本公式,并把分析进一步扩展到含跨中裂纹的三点弯曲试样。并对带跨中裂纹的三点弯曲试样进行了有限元分析和低温试验,其结果和公式进行了对比。结果表明,CTOD值随着温度降低而减小,温度对CTOD的影响程度与裂纹尖端的塑性发展程度有关。     

Simplified equations for determining double‐K fracture parameters of concrete for 3‐point bending test

The paper presents simplified polynomial equations for determining the double‐K fracture parameters of concrete for 3‐point bending beams with variable strengths and material properties of concrete. The derived equations avoid complexities involved in computations of fracture parameters using existing analytical methods. The input data required for systematic computation in the study for deriving the nondimensional fracture parameters are obtained using a fictitious crack model. It is inferred that for a relative size of initial crack length, critical load and corresponding crack opening displacement maintain a linear relationship in their nondimensional forms. The value of critical mouth opening displacement can also be determined for known value of peak load using the derived nondimensional equation, thus avoiding the measurement of the crack mouth opening displacement in the experiment. Further, the derived polynomial equations predict the double‐K fracture parameters of concrete with negligible error as compared to those obtained based on experimental results.     

Object Grating Method Application in Strain Determination on CTOD Tests

Abstract:  Standard fracture toughness tests require standard specimens with the presumption that mechanical properties are uniform in the crack growth direction. Standards for crack tip opening displacement (CTOD) fracture tests prescribe remote crack mouth opening displacement, which can lead to inadequate results in the case of heterogeneous materials properties. This paper describes the application of an object grating method (OGM) on the fracture behaviour of a heterogeneous specimen. Fracture behaviour is described by measuring deformation on the surface of a specimen, in terms of CTOD and, consequently, by strain determination. An OGM is advantageously used when measuring modified CTOD tests on two specimens with an initial crack in a macroscopic heterogeneous welded joint. Results significantly show that fracture behaviour depends on the material in the vicinity of the crack tip concerning the direction of crack propagation.     

表面裂纹弹塑性起裂的V准则

本文提出了一个新的断裂准则——裂纹咀张开位移率准则及新的材料断裂韧性参量V(?),并用实验验证了该准则的有效性。该准则特别适用于表面裂纹弹塑性起裂载荷的确定。V(?)可以用单试件法测定,并给出了 V_(1c)与 K_(1(?)),σ_c,J(?)的相互换算关系式。     

RELATIONSHIPS BETWEEN THE J-INTEGRAL AND THE CRACK TIP OPENING DISPLACEMENT FOR STATIONARY CRACKS IN WELDMENTS AT PLASTIC COLLAPSE

Abstract— Slip line field and finite-element analyses for J and crack tip opening displacement, δ_t, at plastic collapse are performed for an idealised weldment. The relationship between these two quantities is shown to depend on the nature of the loading (tension or bending), the ratio of yield stresses in weld and base materials, and on the width of the weld compared to the remaining ligament ahead of the crack. Cases where the weldment behaves as a single weld material are determined. However, it is also shown that there is a significant range of cases where the base material yield stress governs the relationship between J and δ_t, at collapse, even when the crack is located in weld metal.
It is shown that the relationship between J and δ_t, at plastic collapse only occurs at loads very close to the collapse load unless the crack is located close to the weld/base boundary or the weld width is small compared to the remaining ligament ahead of the crack. More generally, the weldment behaves as a single weld material in terms of the relationship between J and δ_t, and conventional fracture assessments using J or δ_t, are equivalent.     

A new expression for crack opening stress determined based on maximum crack opening displacement under tension–compression cyclic loading

The maximum crack opening displacement is introduced to investigate the effect of compressive loads on crack opening stress in tension–compression loading cycles. Based on elastic–plastic finite element analysis of centre cracked finite plate and accounting for the effects of crack geometry size, Young's modulus, yield stress and strain hardening, the explicit expression of crack opening stress versus maximum crack opening displacement is presented. This model considers the effect of compressive loads on crack opening stress and avoids adopting fracture parameters around crack tip. Besides, it could be applied in a wide range of materials and load conditions. Further studies show that experimental results of da/dN ? ΔK curves with negative stress ratios could be condensed to a single curve using this crack opening stress model.     

Mechanisms and modeling of low cycle fatigue crack propagation in a pressure vessel steel Q345

The fatigue process near crack is governed by highly concentrated strain and stress in the crack tip region. Based on the theory of elastic–plastic fracture mechanics, we explore the cyclic J-integral as breakthrough point, an analytical model is presented in this paper to determine the CTOD for cracked component subjected to cyclic axial in-plane loading. A simple fracture mechanism based model for fatigue crack growth assumes a linear correlation between the cyclic crack tip opening displacement (ΔCTOD) and the crack growth rate (da/dN). In order to validate the model and to calibrate the model parameters, the low cycle fatigue crack propagation experiment was carried out for CT specimen made of Q345 steel. The effects of stress ratio and crack closure on fatigue crack growth were investigated by elastic–plastic finite element stress–strain analysis of a cracked component. A good comparison has been found between predictions and experimental results, which shows that the crack opening displacement is able to characterize the crack tip state at large scale yielding constant amplitude fatigue crack growth.