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.
A Study of Failure Characteristics in Thermoplastic Composite Material.
Abstract : The recently introduced thermoplastic composite material, graphite polyetherether-ketone (Gr/PEEK) APC-2, promises lower costs, lower part weight, and higher operating temperatures. This new class of organic material has fracture toughness properties superior to those of graphite epoxy. This thesis examines the failure characteristics of Gr/PEEK through an experimental investigation and through the application of a fully nonlinear ply-by ply finite element technique. The experimental investigation of Gr/PEEK APC-2 involved the testing of 34 tension and compression coupons to derive basic material properties for use with the finite element program. This investigation provided further data on the application of tensile loads to Gr/PEEK containing circular discontinuities. This study also proved that a nonlinear finite element program can closely approximate progressive ply failure in a Gr/PEEK laminate. This research reinforced the proposition that the thermoplastic matrix does produce a reliable composite that should be considered for use on aircraft, spacecraft, and space facilities.