Fatigue Loading Considerations for a Composite Cylindrical Shell with a Square Cutout
To determine strength allowables for cylinders–such as aircraft fuselages–made of fiber-matrix composites, practical analytical tools are needed to determine damage extent and residual strength around cutouts. A higher-order shell theory is used to analyze compressive and tensile loads on a graphite/epoxy laminated cylinder containing a square cutout. Failure in the fiber, matrix, or lamination is determinated by the Hashin failure criterion and modeled by reducing the appropriate stiffness terms. This results in a redistribution of stress, leading to further failure. Under fatigue loads, graphite/epoxy will lose stiffness as cycles increase. To model this effect of cyclic loading, the stiffness matrix is further reduced at the beginning of each cycle. Therefore, the static analysis can be used to predict damage under fatigue loading by representing several thousand fatigue cycles in a single computational cycle. In compression loading, the reduction in stiffness ultimately leads to geometric instability, or collapse. In tension, the reduction is stiffness–to account for fatigue–increases the displacements, but not the stress. Therefore, a strain-based criterion is added to induce macromechanical failure from the cyclic loading. The material damage propagates until failure of the curved panel occurs. The objective is not to model the micromechanical failure in the structure, but rather to determine the damage extent, stress redistribution, and residual strength of composite cylinders under fatigue loading.
The fatigue damage mechanics of a carbon fibre composite laminate: I—development of the model
Abstract The mechanics of fatigue damage of a carbon fibre composite laminate is developed. In this system, damage consists of a delamination front, with associated matrix cracking, which propagates inwards from the sample edges. The elastic stiffness of the laminate is related to the current level of damage, and is used to measure it. The damage growth rate is a power function of the stress amplitude and of the mean stress, and is independent of damage when cycling is at constant stress amplitude. Failure occurs when the damage reaches a critical level which depends on the maximum stress seen in the loading cycle. The results are applied to life prediction in Part II of this work.
The Assessment of Fatigue Damage in CFRP Laminates
Results of an investigation into damage in an un-notched carbon fibre-epoxy composite laminate subjected to tensile fatigue loading are presented. The progress of damage, such as cracking in off-axis plies and delamination, was monitored using optical microscopy scanning electron microscope, x-radiography and ultrasonic C-scan.