| Digital imaging correlation (hereafter referred to as DIC) is an optical testing technical over strain and displacement field. While having the advantage of non-contact, wide testing area, high accuracy, wide specimen dimension range, wide strain rate range and easy testing, DIC is developed very quickly and applied to many areas such as fracture mechanics, biology mechanics, dynamic mechanics, material manufacturing and welding since its appearance in1980s. While widely used in the quasi-static experiments, less application and research happens in dynamic experiments.In this paper, a measurement system consisting of DIC, high speed camera and Hopkinson Bar is established in order to measure strain field in dynamic testing. Reliability of the system is verified by dynamic testing of two kinds of materials, namely aluminum foam and granite. Deformation process is discussed and results show that dynamic deformation is inhomogeneous, changing from homogeneous in the beginning to inhomogeneous at the end. Also, the degree of irregularity contains strain rate effect.This paper firstly introduces the principle of DIC, including principles of every part of the program. Then the program flow part is given. Simulating speckle image is used to verify the validity of the program in this paper, including displacement mode, strain mode and shear strain mode.Secondly, a dynamic strain field measurement system is established, and high-speed camera and external light as main devices are discussed. The quality of natural speckle image and artificial speckle image are evaluated. A method of making artificial speckle field is discussed.Deformation of the aluminum foam under impact compression load is analyzed, and local deformation band is found as main deformation pattern in the loading process. Then impact experiments with different speed are processed, and strain filed shows completely different results. Local deformation band happens mainly at low speeds. At high speeds, local crushing band occurs. Local crushing band occurs in the first and local deformation band in the end at moderate speeds. Finally, wave propagation theory is used to explain the reason that with the speed increases, nonuniformity of the stress at the two ends of the specimen improves.In order to study the mechanical properties of rock under complex stress, dynamic and quasi-static experiments under combined compression and shear loading of granite are carried out with five angles:0°,15°,30°,45°and60°. Mechanical properties are obtained, according to which failure criterion are analyzed. Deformation process images are taken throughout while the experiment, through which dynamic and quasi-static strain field can be get with strain measurement system showed above this paper. Results show that:strain field changes from uniform to nonuniform during dynamic and quasi-static experiments, and the irregularity increases with the speed. The macroscopic crack form is the result of yield in specimen, accompanied by a drop in stress. The crack does not appear suddenly, but with a very long time development before succumbing. Different strain rate experiments show different crack deformation rules. Dynamic full field strain measurement system in this paper can measure the crack location, while critical strain to produce micro crack is estimated, which is of important significance to macro crack prediction and furthermore the study on the mechanism of crack generation and rock yielding. This paper also analyzes the yield strain filed of different angle specimens. The results show that the specimen is indeed at the compression shear coupling loading conditions, with uniform shear strain field, under combined compression and shear loading conditions. The failure mode of the specimens is tensile coupling with shear. Finally, this paper also measures the dynamic slip in the experiment, finding that there is a slip in the45°and60°experiment. This result explains the uneven shear strain field experiment and significant failure stress decrease in the60°experiment. |
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