A Semi-analytical Method For Characterizing Themechanical Properties Of Composite Laminates Based On Load-displacement Data

Composite materials have superior mechanical properties,such as high specific strength,high specific stiffness,etc.However,compared with the in-plane mechanical properties of composite materials,the interlayer mechanical properties are weaker,and delamination damage is prone to occur during service.The occurrence and growth of delamination will result in a significant reduction in the stiffness and strength of the composite structure,which will affect the structural integrity and security.Therefore,researchers are required to accurately obtain the mechanical properties of composite materials before structural design,so as to provide a theoretical reference for structural design.Interlaminar fracture toughness G is used to characterize the ability of composite materials to resist crack propagation when subjected to external loading.For the delamination that is significantly affected by fiber bridging,bridging stress is often used to characterize the mechanical behavior of the bridging zone.Accurate determination of fracture toughness and bridging stress distribution is very important for revealing the mechanical properties of laminates.Determining the fracture toughness and bridging stress of laminates through tests requires external equipment to measure the delamination length and the relative displacement at the pre-crack.The test process is cumbersome and costly.In order to solve this problem,this thesis proposes a semi-analytical method for the mode?,mode?and mixed-mode?/?delamination of laminates.Only the experimental load-displacement data are required to obtain the fracture toughness and bridging stress of the laminate.The innovative work of this thesis mainly includes the following points:a)The force analysis of the laminated in the delamination test is carried out to derive the compliance formula of the specimen,and the semi-analytical method for the delamination growth of the laminates is established.The method only needs exprimental load-displacement data to determine the interlaminar fracture toughness and the relative displacement at the pre-crack.Then the J-integral theory is used to determine the bridging stress distribution.b)Based on the R curve and bridging stress obtained by the semi-analytical method.numerical simulation is performed using cohesive zone model in ABAQUS^?software.The numerical results and test results of the load-displacement reponse are consistent,which verifies the accuracy of the semi-analytical method.To sum up,this thesis establishes a semi-analytical calculation method based on the compliance formula of the specimens in delamination tests.Using this method,only the load-displacement data from the standard test are required to obtain the equivalent delamination length,interlaminar fracture toughness and bridging stress distribution of the laminate.The semi-analytical method effectively reduces the test cost,and provides theoretical and technical support for revealing the mode I,mode II and mixed-mode I/II delamination behavior of laminates.