Research On Impact Damage Of Laminates And Fatigue Life Of Impacted Laminates

The advanced composite materials have been widely used in aeronautic structures with the development of aero-engine. They are susceptible to be impacted during manufacture and in service such as tool drop or equipment bump, and the impacted composite structures usually suffer fatigue loading during working. Thus, the mechanisms of fatigue damage development and destroy are so complex that it is quite difficult to predict the fatigue life of the impacted laminates. It's very important to research on the impact properties and fatigue life of composite structures under impact and subsequently fatigue loading conditions.The main research works include:1. Based on unidirectional ply fatigue theory and the principle of equivalent damage, a gradual material property degradation model is developed for impacted laminates subjected to fatigue loading, and a 3-D progressive fatigue damage analysis method is proposed and established to predict fatigue life of impacted laminates combined with the gradual material property degradation model and Tserpes-type failure criteria of fatigue damage. The equations of virtual displacement are also deduced to analyze stress-strain states of impacted laminates under fatigue loading conditions in this method. This method can predict the fatigue life of the impacted laminates under fatigue loading accurately.2. The concept of whole-process analysis about laminates under impact and subsequently fatigue loading conditions is brought forward. Based on the concept of whole-process analysis and the 3-D progressive fatigue damage analysis method of impacted laminates, and on the assumption that the laminates properties are related to the damage mode and damage situation, and irrespective to the loading conditions which deduced the damage, a 3-D progressive damage whole-process analysis method of laminates under impact and subsequently fatigue loading condition is established. This whole-process analysis method is applicable for different loading conditions of impact and fatigue. A good agreement is found between the results of this analysis and the test data. Based on the software of ANSYS, a parametric modeling analysis software is compiled, which provides a better technique platform for the structure design and damage analysis of composite laminates.3. The impact tests and fatigue tests after impact of T300/BMP-316 composite laminates with different ply orientation angles are accomplished. In impact tests, the rules of impact damage area varied with impact energy are attained. In fatigue test, the rules of stiffen degradation and damage development are studied and the stress-life curves of the impacted laminates are obtained. The influences of ply parameters, tip diameter, impact angle on the impact damage and fatigue property of laminates are analyzed by using this 3-D progressive damage whole-process analysis method.4. The fiber compressive failure mode caused by the crushing stresses of impact surface and fiber tensile failure mode aroused by tensile stresses of impact back surface have been distinguished in the 3-D progressive impact damage analysis method. The initiation and propagation of fiber tensile failure, fiber compressive failure, matrix crushing, matrix cracking and delamination are simulated. A delamination criterion is developed according to delamination damage mechanism under low velocity impact loading, and the influence of through-thickness tensile stresses, interlaminar shear stresses and matrix cracking on delamination damage are taken into consideration.5. The damage initiation and development process of the aero-engine case under the impact loading as well as the fatigue loading after impact are analyzed.