Abstract: Since the current device for geotechnical direct tension tests is limited, a new direct tension device was developed in this study. The testing device consists of four components: sample preparation, loading, control and data acquisition system. With the novel design of “dovetail” groove and a double sliding plate on the device, direct tension tests can be conducted on prepared samples with different lengths. Due to the chosen sampling forms and their corresponding stretching fixture, the problems of relaxation and stress concentration on the ends of the specimen can be solved during the drawing process. The phenomenon of eccentric stress appearing in the drawing process can be avoided using the double rail stretching device. The minimum tensile rate of the device is 0.001 mm/min assisted by the two-stage gearbox design. Therefore, the device can be used to describe the evolution of uniaxial tensile failure and to determine tensile strength and tensile stress-displacement curve in the whole process. Direct tension tests were carried out on clay specimens using the developed device. The results show that the uniaxial tensile failure mode of clay is not purely brittle fracture, but there exists a softening stage after the tensile strength, and at this moment, clay specimens still have the certain bearing capacity. With the increase of the length of stretching section, the tensile strength decreases logarithmically while the peak displacement increases logarithmically. With the increase of tensile rate, tensile strength increases logarithmically while the peak displacement increases linearly. Both tensile strength and the peak displacement increase linearly with increasing compactness. With the growth of moisture content, tensile strength increases initially and decreases afterward, but the peak displacement linearly increases.

Key words: direct tension, test device, stretching section length, failure mode, tensile strength, peak displacement