三向碳碳材料的非线性双模量力学模型和强度准则

为了在平面十字形试件上实现拉拉、拉压、压压和拉剪的两向应力状态试验,设计并制造了双向加力装置和大剪切变形的测量仪,在试验基础上建立了三向碳碳材料的应力应变关系和强度准则.用纤维所受应力的正负号来决定两个方向里正应力异号时的模量(E_拉≠E_压)应该用哪一个.假定应变势存在,以保证弹性系数矩阵的对称性.非线性来自剪切变形.强度准则用二次型函数拟合实验数据,理论预报与实验吻合. 

NONLINEAR BIMQDULUS MODEL AND STRENGTH OF 3D CARBON CARBON MATERIAL

To establish the stress-strain relationship and strength criterion of 3D carbon-carbon rriaterial,it is imperstive to acliievcari experimental technique to realize the combined state of stresses.The conventional,thinwalled tulle specimen cannot.be used since the fiber bundles of 3D,carbon-carbon material are woven in three otrhogonal directions and to fabricate a tube will cut the fiber bundles and cause a teduction in specimen strength and change the stress-strain relationship.In view of sepcific mechauical properties of 3D carbon-carbon material,the cruciform specimen was employed.In order to ensure the failure of the material will occur in the central gage area,the thickness of central gage area was milled thinner without injuring the remaining fiber bundles in that reg ion.On the cruciform specimens,the vertical force was exerted from a testing machine while the horizontal force was brought to bear by a specially designed loading setup hung freely on a frame with pulleys with negligible friction.To achieve Tension-shear testings,a modified Iocipescu setup was comployed.The shear stear of 3D carbon-carbon material is so large as to amount to scores of 103 με.To measure such a large strain,a specially designed measuring device was fabricated.The experimental result show that the stress-strain relationships under tension-tension,tension-compression and compression compression arc linear.The nonlinearity stems only from shear,In the case of combined stress state with tension stress in one direction and compression in the another,to determine whether the tension or compression modulus should be used,this paper suggests that the fiber bundle stresses instead of principal stresses will doffiinate,The symmetry of the elastic coefficient matrix is attained by assuming the existence of strain potential.Strength data are fitted with quadratic function separately in each quadrant of the stress plane,Both the constitutive equations and strength criteria thus established agree with the experimental results fairly well.