DAMAGE EVOLUTION AND STIFFNESS DEGRADATION OF THICK WALLED FILAMENT WOUND COMPOSITE PIPES SUBJECTED TO INTERNAL PRESSURE AND AXIAL LOAD

摘要

Thick walled composite pipes loaded beyond first ply failure will show a variation in transverse crack density through the wall thickness. Even in pipes showing global material homogeneity this is likely to occur simply because of the inhomogeneous state of stress through the thickness. In this case, the onset of damage would be at the inner surface. In hybrid composite pipes or pipes with different winding angles, the onset of damage is less predicable, depending on material properties and load condition. In all cases, the loading path could be crucial. In order to get the elastic solution for all kinds of linear elastic materials, lay-ups and load conditions, a finite difference model was used. This method gives a very precise solution of the elastic problem in all radial positions. The 3-dimensional stiffness degradation due to transverse matrix cracking was solved using solutions from a proposed displacement model assuming equally spaced cracks in angle ply structures. The model shows good agreement with finite element results and thin walled pipe experiments. The stress and strain fields of the cracked structure are used with suitable failure criterions for introducing new cracks. From the onset of failure the problem was solved using small load steps to insure all layers to be in consistent state of damage during loading. The general finite difference model applies to all kinds of materials and lay ups. On the other hand, the degradation model is restricted to angle ply regions. Further more, the elastic properties of these regions must be reducible to globally orthotropic material upon homogenisation, with axes coinciding with the global cylindrical co-ordinate system.