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.
A Study of Failure Characteristics in Thermoplastic Composite Laminates Due to an Eccentric Circular Discontinuity
Abstract : The purpose of this thesis was to determine (both experimentally and analytically) the initiation and progression of failure, stress-strain response, and the failure loads of Graphite/Polyetheretherketone (Gr/PEEK) laminates, incorporating an eccentric 0.4 inch circular discontinuity, loaded in axial tension at room temperature. For each ply lay-up, three values of eccentricity were considered (the three values of eccentricity were determined by the hole location within each specimen). In addition, experimentation was conducted to study the effects of boundary conditions on the failure characteristics of the Gr/PEEK laminates; this was accomplished through the use of a special mounting fixture which allowed in-plane rotation of the specimens. Finally, experimentation was conducted, using photoelasticity, to verify the gross stress states of the Gr/PEEK laminates predicted by the analytical study. Analytically, a nonlinear material finite element program was used to predict the initiation and progression of failure, stress-strain response, and the failure loads of the Gr/Peek laminates. In addition, the effects of boundary conditions on the failure modes of the Gr/Peek laminates was studied analytically. And finally, the gross stress states of the Gr/PEEK laminates were considered in the analytical portion of this thesis.
Preparation and performance evaluation of swine manure bio-oil modified rubber asphalt binder
Abstract In order to study the effect of the preparation process of swine manure bio-oil modified rubber asphalt binder on the rheological properties of the binder, the matrix asphalt and asphalt rubber were used as the control group, and swine manure bio-oil modified rubber asphalt binders were obtained by three preparation processes. Then the rheological properties of five binders were evaluated by rotational viscosity, DSR, MSCR and BBR tests. The results show that the addition of swine manure bio-oil to asphalt rubber is beneficial to the workability, but cause negative effect on its high temperature performance, fatigue performance and low temperature crack resistance. The preparation method of premixing bio-oil and rubber powder can reduce the degradation of high-temperature performance of asphalt rubber caused by addition of swine manure bio-oil to a certain extent. The fatigue properties of the three bio-binders are close, and the critical temperature is less than 25 °C. The process of premixing bio-oil and rubber powder will increase the creep stiffness and reduce the low-temperature performance. However, the low-temperature performance of the binder obtained by this process is still better than that of the matrix asphalt and can meet the requirements of use. Therefore, it is recommended to use the preparation process of premixing bio-oil and rubber powder to prepare swine manure bio-oil modified asphalt rubber.