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.