Measurement method of interlaminar shear mechanical parameters of thick section composite materials based on digital image correlation

Short beam shear (SBS) test combined with digital image correlation (DIC) can quickly identify the multiple mechanical properties of unidirectional fiber reinforced resin matrix composites. In addition, the complete interlaminar shear stress-strain behavior and interlaminar shear strength of composite materials under uniaxial stress state can be obtained, which is very important to establish the strength criterion of three-dimensional stress state of composite materials with thick section. In order to study the influence of experimental design on the identification accuracy of interlaminar shear mechanical parameters of materials, two stereo digital image correlation systems composed of four CCD cameras were developed for the first time in this paper, and the strain distribution in the front and back surface gage regions of the sample during loading in SBS test was measured. The experimental results show that the shear strain distribution on the front and back surfaces of the sample is asymmetrical, and the relative deviation is up to 44%, because of the thread clearance of fixture and insufficient fixture stiffness. On the one hand, a new method is proposed to identify the shear mechanical properties of materials according to the asymmetrical distribution of shear strain on the front and back surfaces of the sample. By using DIC technology and finite element model updating (FEMU), the measured average shear strain in the distance region of the front and back surfaces of the specimen and the variance of the calculated strain data at the corresponding position of the finite element model are used as the objective function to identify the constitutive parameters and off-axis angles. The complete nonlinear shear constitutive parameters can be obtained, and the identification process is not sensitive to the initial parameters. On the other hand, by improving the test fixture to improve the tool stiffness, the phenomenon of asymmetric shear strain distribution is eliminated, and the complete interlaminar shear stress-strain constitutive relationship parameters of unidirectional laminates are accurately identified.