改进巴西试验:从平台巴西圆盘到切口巴西圆盘

巴西试验是测试混凝土、陶瓷、岩石等脆性或准脆性材料拉伸强度的标准方法,在材料科学和土木工程中有广泛的应用,研究该方法的改进在科学和工程界都受到日益增长的关注.我们对巴西试验的原始试样即巴西圆盘(Brazilian disc,BD)提出一种新的构型:切口巴西圆盘(grooved Brazilian disc,GBD),利用GBD加载直径两端的窄而浅的切口,消除完整巴西圆盘对径压缩加载时可能非中心起裂的缺点,这一点与我们过去提出的平台巴西圆盘(flattened Brazilian disc,FBD)相同.GBD的构型更普适,因为它涵盖了巴西圆盘和平台巴西圆盘.对于求解GBD切口顶点及其正前方近邻的应力,提出两个近似解析模型,分别求出对应的压应力和拉应力,用叠加原理最后推出的公式能够定性地预测应力的变化趋势,以及试样几何参数的影响.近似解析公式,有限元数值计算,对比实验的结果都证实,对径压缩圆盘的几何与表面边界条件的一个微小的改变,对巴西试验试样的几何稳定性和断裂过程产生显著和有利的影响.利用推出的切口应力公式进行试样几何参数的优化选择,可以使GBD比BD和FBD更具优越性.

IMPROVEMENT OF BRAZILIAN TEST: FROM FLATTENED BRAZILIAN DISC TO GROOVED BRAZILIAN DISC

Brazilian test, a standard method for measuring the tensile strength of concrete, ceramics, rock, and other brittle or quasi-brittle materials, has been widely applied in materials science and civil engineering, research on its improvement attracted ever-growing attention in scientific and engineering communities. We ideate a new specimen configuration to improve the Brazilian test, i.e. grooved Brazilian disc (GBD), with a pair of narrow and shallow groove produced at opposite ends of the loading diameter, GBD relieves the shortcoming of non-central crack initiation possibly inherent to the original complete Brazilian disc (BD) under diametrically compression, having the same merit pertaining to our previously-proposed flattened Brazilian disc (FBD). GBD is also more general in specimen configuration, as it represents both BD and FBD. More advantages may be obtained if the geometric parameters of GBD are suitably chosen by referring to the analytical stress formulas, derived by superposition of two models we proposed for solving the compressive stress and tensile stress respectively. The derived two simple stress formulas can be used to predict qualitatively the stress evolution trend, both at and near the front of the apex of the groove, and also the influencing geometric parameters of GBD, which are also confirmed by quantitative numerical calculations with finite element method. The analytical formulas, numerics and comparative experimental result jointly indicate: a slight change in the geometry and surface boundary condition of a disc-type specimen under diametral compression turns around a substantially favorable change for the stability and fracture process of the specimen, making the GBD more favourable in the Brazilian test.