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.
Initiation and Propagation of Delamination in a Centrally Notched Composite Laminate
Interlaminar stresses occur at the free edge and notch to cause composite laminates to delaminate. Actually, delamination is an important failure mode and affected, for example, by fiber orientation and stacking sequence as discussed. In analysis, our de veloped mathematical model yields the interlaminar edge stresses and singularities around the hole to predict the onset of delamination. Furthermore, our work mainly aims to inves tigate experimentally the initiation and propagation of delamination subjected to tension and cyclic tension-tension (T-T) tests in centrally notched T300/5208 graphite/epoxy com posite laminates, including four layups: (a) orthotropic [=±45]4s, [90/0] 4s and (b) quasi- isotropic [0/90/±45]2s, [90/0/±45]2s. Experimental result and observation associated with the obtained maximum stresses and the adopted failure criterion are in good agree ment with our predicted delamination onset. Other important findings are highlighted as follows. The failure mode of each layup recorded by X-ray radiograph can be closely cor related to the prediction of our model. The delamination growth rate has also been mea sured. An approximately linear relationship between delamination area and stiffness re duction for the centrally notched quasi-isotropic lammates is established. The critical load for delamination can be estimated by the Hashin-Rotem failure criterion satisfactorily. As a benefit of nondestructive testing, we finally find that the residual strength of the laminate first increases as applied cycles increase and then decreases during fatigue tests. However, residual stiffness always decreases monotonically with the increase of applied cycles.
On the use of a multilayer higher-order theory for the stress analysis around a circular hole of laminates under tension
Many single layer and multilayer higher-order theories were recently proposed for the analysis of the three-dimensional stress field that usually occurs in composite laminates especially in critical areas such as free edges. In this paper a finite element analysis based on a multilayer theory is performed in order to predict the complicated pattern of the stress field around the hole of laminates under tension. The numerical results are compared with the ones obtained with other modelling approaches, including single layer higher-order theories. The analysis shows that multilayer-theory-based finite elements can be effectively used in the prediction of the stress response of composite structures in cases of practical interests.