考虑广义边界条件的混凝土应变软化

CONCRETE STRAIN SOFTENING CONSIDERING GLOBAL BOUNDARY CONDITIONS

混凝土试验结果往往具有很大离散性,造成这种离散性的原因不仅是"材料的离散性"也存在"边界条件的离散性"。除试件与试验机加载板和试件接触面(即所谓的"狭义边界条件")以外,试验机刚度、试件尺寸和加载方式均系统地影响试验结果,特别在量测混凝土的应变软化方面,这个问题不可忽视。目前这些问题如果不能得到澄清,将严重影响对混凝土真实力学性质的认识。根据通常的实验室条件定义了一种"广义边界条件",以量测比较困难的混凝土单轴拉伸应变软化试验为例,来探索这种"广义边界条件"的影响。为此,建立了一个新的系统模型,将试验机与试件置于同一个系统中考虑,利用能量原理得出量测的稳定条件,并利用系统的平衡协调关系定量求出了应变软化条件下的稳定平衡路径。最后,通过在实验室完成的一组混凝土单轴拉伸试验验证了这个系统模型的合理性。同样的做法可以推广到其他更容易量测的混凝土加荷试验中,这将有助于正确认识混凝土试验结果的离散性。

It is well known that test data of mechanical behavior of concrete show great scatterness, which is caused by the material scatterness and also the scatterness of boundary conditions. In addition to the interfaces between test machine and specimen, which is so-called the local boundary condition, the stiffness of test machine, specimen dimensions, and loading conditions can also effect the test results systematically, especially during the strain softening measurement of concrete. The latter has become a baffle for researchers to recognize the real mechanical behavior of concrete. Considering very popular experimental conditions in structural laboratories, a concept of global boundary condition is defined; and some uniaxial tension tests of concrete, which usually are difficulty to be measured, are used to investigate the influences of global boundary conditions. Therefore, a new system model is introduced, in which the test machine and specimen are all considered in a unique system. Using energy principle, a stability criterion of the system for uniaxial tension test of concrete is derived; and using equilibrium and compatibility equations of the system model, a stable equilibrium path for strain softening of concrete under uniaxial tension is obtained quantitatively. Finally, the new system model is validated by a set of uniaxial tension test data of concrete, which had been performed by authors in the laboratory. The presented method can also be easily extended to other loading conditions, which may more easily be measured. The presented results should be very helpful for recognizing the scatterness of test data of mechanical behaviors of concrete.