考虑动态应变率效应的混凝土单轴拉伸统计损伤模型

基于准脆性材料细观损伤机制,将损伤概括为"有效受力面积的减小"和"有效受力部位弹性模量的减小"两个方面,可分别利用断裂和屈服2种损伤模式表征;认为材料破坏过程实质上为2种损伤模式累积演化的连续过程。干燥混凝土所表现出的动态本构行为本质上是由于自身的惯性效应引起材料破坏形态以及细观2种损伤累积演化过程的改变。忽略水的黏滞性影响,通过假设不同应变率情况下干燥混凝土损伤演化服从某种相似性规律,建立相应增量形式的单轴拉伸动态损伤本构模型。通过算例和试验结果比较,表明该模型可预测变应变率加载情况下材料均匀损伤阶段的本构行为,等应变率加载可作为其中的一个特例。结合算例从有效应力、名义应力、吸能能力3个角度对材料的动态损伤机制进行探讨。区分开峰值应力状态和局部破坏的临界状态,并建议后者作为本构模型的最终破坏点。此举可充分考虑材料均匀受力阶段的延性,同时避免过多考虑局部破坏阶段本构模型尺寸效应的影响。

STATISTICAL DAMAGE MODEL OF CONCRETE UNDER UNIAXIAL TENSION CONSIDERING DYNAMIC STRAIN-RATE EFFECT

Based on the meso-scale damage mechanism of quasi-brittle materials,the damage can be generalized into two aspects,"the decrease of the effective cross-section"and"the degradation of the elastic modulus of the effective region corresponding to the effective cross-section",which can be represented by rupture and yield damage modes respectively. The essence of failure process is the continuum accumulation and evolution process of the two damage modes. The dynamic behavior behaved by dry concrete can be essentially attributed to the inertia effect itself,which leads to the change of the breakage form and the two damage evolution process of materials. Ignoring the effect of water viscosity,by the hypothesis that the damage evolution rule of material under different strain-rate cases obeys a certain similarity,an increment-format dynamic damage constitutive model under uniaxial tension is proposed. Comparison of the prediction results with experimental data shows that it can accurately reflect and predict the dynamic damage characteristics in the uniform damage phase for concrete under the constant or variable strain-rate cases. Combining with the examples,the dynamic damage mechanism of materials is discussed from the view points of effective stress,nominal stress and the energy absorption capability. Two characteristic states,the peak nominal stress state and the critical state when appears macro-crack are distinguished;and the latter is proposed as the final failure point in the constitutive model. Treated by this way,not only the ductibility of the uniform damage phase of materials can be considered adequately,but also the size effect of constitutive model during the local breakage phase can be avoided to consider.