PLY-BY-PLY CALCULATING METHOD AND STRENGTH CRITERIA FOR COMPOSITE PRESSURE VESSELS WOUNDED BY SYMMETRIC PAIRS OF LAYERS. Part 2. Simple strength criteria for biaxial tension of angle-ply reinforced tubes
Simple strength criteria are justified for biaxial tension of coaxially wound composite pipes, taking into account directed failure mechanisms. A limiting surface in the stress space is a rectangle, two sides of which are responsible for the destruction of the matrix, and the third side - for the break of the fibers. A limiting surface can also be described by the equation of an inclined ellipse constructed by the values of the strengths at uniaxial axial or circumferential tension and by the strength at the optimal ratio of the applied stresses. To estimate the limiting surface for a pair of symmetrically wound layers, it is sufficient to know only two (or three) values of the strength of the pipes with winding (0), (± 45°) and (90°). The application of the least-squares analysis for tracing a limiting surface in the form of a rectangle or an ellipse with an inclined axis on experimental points is explained. Stresses in pairs of layers are calculated using the ply-by-ply method (part 1 of this article), and then safety margin factors are estimated for each pair of layers. The algorithm of sequential destruction analysis is discussed taking into account degradation of properties.