On account of the defects and shortcomings of the existing geotechnical tensile instrument, a set of simple axial tensile testing instrument is designed and improved. By the design of adjustment bolts on the four corners and fixtures, the frictional resistance and end stress concentration are eliminated, and the control of axial line prevents sample eccentric tension to fulfill axial tensile tests of rectangular specimens of different size. As there is no unified standard currently for sample size, a tensile device is designed test the samples with different cross section and slenderness ratio so as to determine specified sample size and analyze the influence of sample size on the tensile properties,. The test results are as follows:when ω=18. 0%, ρd =1. 30 g/cm3 , the average tensile strength value is between 14. 2 ~14. 6 kPa and the average displacement limit value is 8 ×10-2 ~9 ×10-2 mm of samples with different cross section;when ω=22. 0%,ρd =1. 45 g/cm3 , the average tensile strength value of the samples with different slenderness ratio is between 14. 4~14. 9 kPa and the relation between the average displacement limit value and slenderness ratio can be expressed as b=9. 706 7 × ( l/a )-6. 97 . The analyses results show that: when the sample physical conditions are the same, tensile strength values are not affected by cross section and the slenderness ratio whether the moisture content is less or greater than the omega plastic limit, and when the water content is below the plastic limit, the limit tensile displacement doesn’t change with change of the section of specimen; when the water content is higher than plastic limit, the correlation of the limit tensile displacement with slenderness ratio tends to be increasing linear. The calculation of the poor and the standard deviation of tensile strength of each group’s samples and the analysis of the discreteness data suggest that the reasonable specimen size is 2. 5 cm for section length and 3 for slenderness ratio with respect to the difficult degree of actual sample preparation and the amount of soil, which may provide reliable basis for future research on tensile strength of soil.%针对现有土工抗拉仪存在的缺陷与不足,设计改良出一套简易轴向拉伸试验仪。通过四角调节螺栓和夹具设计,消除摩擦阻力和端部应力集中,通过轴心线的控制避免试样偏心受拉。可以完成不同尺寸长方体试样的抗拉试验。由于目前试样尺寸没有统一标准,为了得到试样的规范尺寸并分析试样尺寸对抗拉特性的影响,利用设计的拉伸装置,对不同截面和长细比试样进行试验。得到的试验结果有：ω=18.0%,ρd=1.30 g/cm3时,不同截面试样平均抗拉强度值在14.2~14.6 kPa,平均极限位移值在8×10-2~9×10-2 mm；ω=22.0%,ρd=1.45 g/cm3时,不同长细比试样平均抗拉强度值在14.4 ~14.9 kPa,平均极限位移值与长细比的关系式可表示为 b=9.7067×( l/a)-6.97。分析表明：只要试样的物理状态(含水率ω和干密度ρd )相同,不论含水率ω是否小于或大于塑限值,抗拉强度值均不受截面和长细比的影响；含水率ω低于塑限值时,极限拉伸位移不随试样的截面变化；含水率高于塑限值时,极限拉伸位移与长细比呈递增的线性关系。计算每组试样抗拉强度值的极差和标准差并分析数据的离散性,结合制样难易程度和用土量等方面综合考虑,提出截面边长2.5 cm,长细比3的试样尺寸较为合理,为今后开展土体抗拉强度研究提供可靠依据。
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