Damage Of Composite Under Low-velocity Impact And Application Of Mechanical Equivalent Model

By the rapid development of material science and technology, the compositematerial has been widely used in modern aircraft industry. Even the usage of it hasbeen the standard of the advancement of technology. At the same time, the high speedprogress of computer and commercial software has made FEM analysis to be theimportant method to solve complicated engineering problems.The peculiarity of composite makes its response under low-velocity impactspecial, for example, invisible, buckling of sub-layer under compression. This kind ofspecificity decides its huge threat to practical engineering, such as difficult to detect,sudden failure, so assess of the resistance property of structure under impact and theprediction of damage is very important to the safety of the structure. Due to the highcost of destructive inspection and difficulty of the impact experiment for wholestructure (most researches are based on simple laminates). The FEM simulation hasbecome the useful tool to study the damage under low-velocity impact. In this thesis,we use ABAQUS user subroutine VUMAT, which introduces the criteria of compositedamage and the evolution law after damage into FEM, to simulate the damage ofcomposite laminates under low-velocity impact. The results obtained prove thecomputation model, code and criteria correct.The damage tolerance method is used to design composite structure, which is veryimportant to get the ultimate strength of the composite including damage. In this thesis,we use user defined field subroutines to simulate the ultimate strength of compositelaminates including circular hole under the tension and compression loading. Theresults reflect the features of structure including hole and the damage of compositelaminates. By means of user defined material subroutine, which introduces theprogressive failure of composite into computation, we simulate the failure of thecomposite laminate including the prefabricated delamination under compression. Wefocus on the buckling of sub-layer and the extension of delamination.The simulation of practical engineering with thin thickness structure oftenchooses shell model, but the difference of co-curing structure including continuous chamfering, filling area and traditional composite makes it impossible to simplify themodel into shell model by ignoring some geometrical features. Here, we usemechanical equivalent method to simplify the complicated co-curing model intoshape-like shell model. It can save a lot of computation time on the basis of keepingaccuracy. This method gets good effect in the simulation of experiment for structures.Combining the research about damage of composite under low-velocity impactand mechanical equivalent method, we can simulate the structures. It proves theaccuracy and high efficiency of mechanical equivalent model through comparing withsolid element model. Based on the above, we propose the concept of database ofimpact damage by large numerical simulation. It can help instruct and inspect theexperimental work during the process of maintenance composite structures.