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.
Fatigue effects on the coordinative pattern during cycling: kinetics and kinematics evaluation.
The aim of the present study was to analyze the net joint moment distribution, joint forces and kinematics during cycling to exhaustion. Right pedal forces and lower limb kinematics of ten cyclists were measured throughout a fatigue cycling test at 100% of PO(MAX). The absolute net joint moments, resultant force and kinematics were calculated for the hip, knee and ankle joint through inverse dynamics. The contribution of each joint to the total net joint moments was computed. Decreased pedaling cadence was observed followed by a decreased ankle moment contribution to the total joint moments in the end of the test. The total absolute joint moment, and the hip and knee moments has also increased with fatigue. Resultant force was increased, while kinematics has changed in the end of the test for hip, knee and ankle joints. Reduced ankle contribution to the total absolute joint moment combined with higher ankle force and changes in kinematics has indicated a different mechanical function for this joint. Kinetics and kinematics changes observed at hip and knee joint was expected due to their function as power sources. Kinematics changes would be explained as an attempt to overcome decreased contractile properties of muscles during fatigue.