基于虚拟裂缝模型的长期荷载作用下混凝土时变断裂过程及其机理研究

Time-dependent fracture in concrete, which happens under long-term sustained loading due to concrete creep and crack progressive development, is closely associated with the safety and durability of concrete structure. The present project focuses on time-dependent fracture issue of concrete and, both experimentally and numerically, investigates its time-dependent fracture behavior through combining time-dependent stress intensity factor with fictitious crack model describing softening property of concrete. Considerable concrete time-dependent fracture tests are planned on three-point bending beams and wedge-splitting tension specimens. Strain guages stuck on two sides of fracture ligament of specimens are utilized to monitor crack propagation corresponding to different load level. Based on the measured crack opening displacement, the analytical formula with the consideration of effect of loading time is subsequently established to work out the length of crack extension. Then, the formulae to evaluate crack extension rate and crack opening rate are analytically constructed. Crack extension resistance R curve considering time-dependency of concrete is computed, in which the contribution of distributed cohesive stress in fracture process zone ahead of propagating crack tip to fracture resistance capacity is determined according to softening relationship, measured crack opening displacement as well as the computed crack extension length determined through the proposed analytical formula. And, the stress intensity factor produced due to external force is also calculated using experimentally measured load and the calculated crack extension length. The time-dependent fracture model is then set up with the help of the calculated stress intensity factor and crack resistance R curve and the time-dependent fracture behavior and the mechanisms related are in depth investigated. Finally, the numerical simulations are attempted by making use of the developed constitutive model of concrete in which both the proposed time-dependent fracture model representing the property in fracture process zone and the viscoelastic mechanical property of concrete describing outside fracture process zone are considered.

服役中的混凝土结构处在长期荷载作用下，与服役时间相关的时变断裂问题直接关系到结构的安全。本项目拟将反映混凝土软化特性的虚拟裂缝模型与时变应力强度因子相结合研究混凝土时变断裂问题。试验研究在三点弯曲梁试件和楔入劈拉试件韧带两侧贴电阻应力片监测裂缝发展，施加不同荷载水平的长期荷载，开展大批量混凝土时变断裂试验。理论分析方面，由测定的裂缝口张开位移建立裂缝时变扩展长度的解析公式，在此基础上获得裂缝时变张开率和时变扩展率的解析公式；由外荷载和裂缝时变扩展长度计算出裂缝端部的时变应力强度因子；由裂缝口张开位移、裂缝时变扩展长度以及混凝土软化本构，确定虚拟裂缝上粘聚力的分布时间相关性，计算裂缝扩展阻力R；将时变应力强度因子与依赖于时间的R阻力耦合建立时变断裂模型，分析时变断裂过程，揭示时变断裂机理。将上述描述混凝土断裂过程区的裂缝时变断裂模型和断裂过程区外混凝土的粘弹性耦合，数值模拟断裂过程。

本课题以服役中的处于长期荷载作用下的大体积混凝土时变断裂问题为研究背景，开展了不同荷载水平下的长期荷载作用下的混凝土时变断裂试验，并将虚拟裂缝模型与时变应力强度因子相结合进行理论分析，对混凝土在长期荷载作用下的时变断裂问题进行了的研究。项目主要研究内容及结论如下:.（1）进行常规的非时变断裂试验，得到混凝土在准静态试验状态下的最大承载力Pmax及相关断裂参数。然后分别施加荷载水平约为0.7Pmax、0.75Pmax、0.8Pmax、0.85Pmax、0.95Pmax的恒定荷载开展大批量混凝土时变断裂试验，测定裂缝发展规律及裂缝口张开位移CMOD（t）。.（2）试验表明，恒定荷载作用下裂缝扩展长度接近于常规断裂试验；而典型的裂缝口张开位移与时间关系曲线（CMOD-t）可分为三个阶段：在恒定荷载加载完成后初始阶段CMOD增长速度逐渐减小，之后在较长时间段内保持较为稳定的速度缓慢增长，第三阶段，也是最短的一个阶段，CMOD-t曲线斜率变化快，在接近破坏时，裂缝口张开速率接近无限大。.（3）依据虚拟裂缝模型，得到了混凝土在不同荷载水平长期荷载作用下的时变等效裂缝长度a(t)、时变裂缝尖端开口位移CTOD(t)，总结了其发展规律。进而计算了时变应力强度因子。.（4）提出了混凝土时变断裂韧度K Ic(P)的概念及依据混凝土时变裂缝开口速率的变化规律确定其计算方法。将时变裂缝开口位移CMOD(t)求导即得到混凝土的时变裂缝开口速率CMOR(t)。CMOR-t曲线表现出显著的三阶段特征。当CMOR经过第二、三阶段的转折点后，裂缝扩展速率进入不可逆的持续增长阶段，即裂缝在此时进入失稳扩展阶段，该转折点始即为临界失稳状态，对应的应力强度因子即为时变失稳断裂韧度。.（5）由时变裂缝尖端开口位移、时变裂缝等效裂缝长度和混凝土软化本构，得出了混凝土的时变黏聚韧度，进而得到了混凝土的时变裂缝扩展R阻力曲线。将时变应力强度因子与时变裂缝扩展R阻力曲线耦合建立了混凝土时变双K 断裂模型。.本项目对长期荷载作用下的混凝土时变断裂问题的认识，裂缝稳定性的评定提供了参考。