Research On Failure Criteria For Aircraft Structural Materials

Aluminum alloy,titanium alloy,alloy steel and advanced composite material are known as the four major structural materials in modern aircraft structures.Accurate prediction of material strength can help reduce the weight of aircraft structures,improve the structural efficiency,reduce the test cost and shorten the development period.Aluminum alloy,titanium alloy and alloy steel are all metals,which are isotropic materials in the macroscopic view.Unidirectional fiber reinforced composites are transversely isotropic materials.Therefore,it is of great theoretical significance and practical value to carry out the research of failure criteria for these materials on the basis of understanding the failure mechanism of isotropic and transversely isotropic composite materials.The main contents of this thesis are as follows:（1）The research status of material failure criteria is reviewed,and the failure criteria for isotropic materials and transversely isotropic composites commonly used in engineering are summarized.（2）A unified failure criterion for isotropic materials is proposed based on the theory of stress invariants.The failure criterion takes the uniaxial tensile strength T,uniaxial compressive strength C,and pure shear strength S as the basic strength parameters,and distinguishes the different effects of hydrostatic tension and compression on material failure.The effectiveness of the criterion is verified by the experimental data of different types of isotropic materials.（3）A unified isotropic material failure criterion is proposed based on Mohr’s failure plane theory.The failure criterion takes the uniaxial tensile strength T,uniaxial compressive strength C,and pure shear strength S as the basic strength parameters,and distinguishes the different effects of the normal tensile and compressive stresses on material failure.By comparing with the experimental data,it is shown that the proposed failure criterion is better than the classical linear Mohr-Coulomb criterion.（4）Some modifications of Hashin’s fiber tensile failure criterion and matrix tensile failure criterion are put forward.The rationality of using the maximum stress criterion instead of Hashin’s criterion to predict the fiber tensile failure mode is physically explained,and the smooth condition atσ₂+σ₃=0 is imposed to make the matrix failure function smooth rather than just piecewise smooth.The experimental data of various unidirectional composites under different stress states verify the effectiveness of the improved criteria.（5）Puck’s original inter fiber failure（IFF）criteria are extended from intrinsically brittle materials to all types of unidirectional composite materials.The values of the two parameters in Puck’s inter fiber failure criteria,namelyR_?^At and p_??,are determined by the transverse tensile strength Y_T and transverse shear strength S₂₃.Theoretical and experimental assessment demonstrates the reasonableness of the extension of Puck’s inter fiber failure criteria.（6）Failure criteria applicable to thermoset composites are formulated based on the further understanding of Mohr’s failure plane theory.The coupling interaction between matrix-dominated and fiber-dominated failure modes is considered.All the unknown coefficients in the proposed criteria are determined by the basic strength values of unidirectional composites as well as the transversal compressive fracture angle?^c_fp.The validity of the criteria is verified by comparing with experimental data.（7）A general method for constructing failure criteria based on Mohr’s failure plane assumption is proposed.Unlike the traditional method of expanding the failure function into the power series of stress invariants,the proposed method expands the failure function into the power series of stress components on the potential failure plane.