Preliminary Application Study On Strain Field Measurement Of Triaxial Specimen

As a digital image measurement method, the sub-pixel accurate location of a corner can be found in digital image by sub-pixel accurate corner locator. The principle based on sub-pixel corner detection in the digital image measurement is applied in geotechnical triaxial test by measuring deformation of the specimen. The discrete deformation test data can be obtained by the application of this method. The triaxial specimen surface strain field can be obtained by a series of mathematical calculations. It contains strain data of each point on the surface of specimen. This Paper does three parts of study on the basis of this system:(1)The end restraint range can be determined by the hoop strain-time curve. Then through the study of different parts of specimen like its end parts, middle part and whole part it finds that: at the initial loading stage, the stress of the end parts and whole part is similar. But at the moment of peak stress, the end parts' stress is more than the whole parts, while the whole part is more than the middle parts. The differences of the three parts' hoop strain are appeared at the initial loading stage. Basically the largest hoop strain is happened in the middle part, then followed by the whole part. The end parts' strain is smallest because of the end restraint. The gap of three parts' hoop strain is larger and larger when going on loading. The end restraint can be neglected when determining the strength parameters of c andφ.(2)The layered sand and layered structure of slit containing fine sand are studied in this paper. From their axial strain field and stress-strain curves, it finds that: The different sands' shear strength is depended on particle size when their void ratio and gradation are similar. The sand of large particle size has more shear strength because of the ability of occlusion.Two local large strain region can be seen on the axial strain field of layered silt. The layered structure silt is separated by the fine sand. A certain layer silt local strain development rate is decreased when it reaches a certain extent. This happens because of the blocking of sand of big deformation modulus.Otherwise the other layer silt local strain development rate is increased. Finally, the local large strain region can be found in both the upper and lower layer.(3)The study of the formation and development of the shear zone is on the basis of the axial strain field. It reveals that the shear zone starts at hardened zone and finally is formed at the softened zone before the residual strength.The strain development mainly concentrates in the shear zone. The strain out of the shear band is small. The single shear band' softening degree is higher than the complicated shear band's. Basically, the final shear band happens in the local large strain region. But some is not because weak point may exist inside of the specimen.