Some Topics On The Fatigue And Fracture Of Aircraft Structures

The topics of the dissertation can be broadly categorized as three parts. The first part deals with the damage models and fatigue life prediction of composite laminates. The second part concerns about the fracture analysis of cracked metalic structures. The last part investigates the behavior of impact dents on aircraft windshield glass and their possibility to be analyzed using fracture mechanics.In Chapter 1, a review is made to the resent developments of cumulative damage models and fatigue life prediction methodology of composite laminates. Based on the concept of a modified fatigue modulus, a cumulative strain damage model, which is able to consider the influence of in-plane multi-axial stresses and mean stress to the fatigue behavior of unidirectional laminates, is proposed. With the typical on-axis fatigue tests to get the relative S-N curves of unidirectional laminates, executed at certain stress ratio, it is possible to predict the fatigue life of multidirectional laminates subjected to cyclic stresses of different stress ratios. This would tremendously reduce test costs for laminate structures. An analysis scheme and the corresponding computer codes have been developed to evaluate the residual strength and fatigue life of multidirectional laminates. In Chapter 2, for T300/QY8911 Carbon/Bismalimide composite laminates, three kinds of basic tests (fatigue of [0]i6 tension-tension, [90] ie tension-tension and [0/90]4S shear-shear) were firstly performed to obtain S-N curves for unidirectional laminates, which were then used as input for the computer codes. Both tests and calculations were performed to get the fatigue lives for six kinds of multidirectional laminate coupons with different tensile cyclic stress applied. Both predictions and experiments agree well for those with less free-edge interlamina stresses. For any laminate which exists an obvious deviation between the estimation and experiment of static strength, maximum loads used in fatigue life predictions were obtained by multiplying the maximum loads used in tests with the ratios of analytical strength to test strength, so as to make the calculation feasible. By means of the calculation results of interlaminar stress distributions, the influence of delamination growth on the resultant fatigue lives is qualitatively discussed, realizing that a morecomprehensive analytical model is essential to include the effect of stacking sequence and the arosing interlamina stresses on fatigue behaviors of laminates. The recommended scheme was also used to calculate the fatigue lives of specimens with central holes subjected tensile-compressive cyclic loadings and has got comparable estimations with experimental results.Multi-directional FRP composite laminates has been used, for some cases to carry shear loads, such as aircraft wing boxes. It is therefore necessary to investigate the fatigue resistance of the materials to the shear fatigue loadings. In Chapter 3, the above cumulative damage model, together with finite element stress analysis, was used to evaluate the fatigue lives of multidirectional laminates under cyclic shear loadings. An losipescu-type of fixture, which had been previously used for the static shear strength tests of unidirectional laminates or metal materials, was proved useful for the shear static and fatigue tests of multidirectional laminates. Each specimen was made of a rectangular piece of T300/QY8911 laminates with double side V notches. Results from the fatigue test and stress analysis indicate that [0/90]s crossply laminates are most appropriate to characterize the fatigue behavior of unidirectional laminates. The cumulative damage modes and their distribution regions on 7t/4 quasi-isotropic laminates were reasonably simulated and the predicted fatigue lives agree well with the test results.Residual strength evaluation is playing a significant role in damage tolerance analysis of structures. Chapter 4 describes a simple procedure for the evaluation of residual strength of metallic structures with cracks developed b...