基于数字图像相关的复合材料层间剪切力学性能参数测试方法

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

短梁剪切(short beam shear,SBS)实验结合数字图像相关技术(digital image correlation,DIC)可实现快速识别单向纤维增强树脂基复合材料的多力学性能参数,而且能够获得复合材料单向应力状态下完整的层间剪切应力-应变行为和层间剪切强度,这对建立厚截面复合材料三维应力状态的强度准则至关重要。为研究实验设计对材料层间剪切力学参数识别精度的影响,本研究首次发展了由四台CCD相机组成的两套立体数字图像相关系统,实测SBS实验加载过程中试样前后表面标距区内应变分布情况。结果表明:由于实验夹具工装配合中存在螺纹间隙,工装刚度不足,试样前后表面剪切应变分布不对称,相对偏差可达44%。一方面提出针对试样前后表面剪切应变非对称性分布、材料剪切力学性能参数识别的新方法,通过采用DIC技术和有限元模型修正技术(finite element model updating,FEMU),将试样前、后表面标距区的实测平均剪切应变和有限元模型相应位置处计算应变数据的方差作为目标函数进行本构参数和偏轴角度的识别,可获得材料完整的非线性剪切本构参数,且识别过程对初始参数不敏感;另一方面通过改进实验夹具提高工装刚度,消除非对称性剪切应变分布现象,准确地识别得到了单向层合板完整的层间剪切应力-应变本构关系参数。

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.