K Sayad - Msc of Design of Composite Structures, Research Center of Structures, Aerospace Organization

Composite grid stiffened cylindrical shell (CGSCS) usually is fabricated by winding from fiber (carbon or glass) reinforced matrix (epoxy). The CGSCS consists of a system of ±j (with respect to the shell axis) helical ribs and circumferential ribs, and has no skin. Modes of failures under axial compressive load in a CGSCS are: 1.failure due to stress in helical ribs exceeds material strength, 2. failure due to local buckling of helical ribs, 3. failure due to general buckling of shell. A CGSCS is specified with four independent design variables consist of thickness of shell H, angle of helical ribs j, spacing of helical ribs ah and width of helical and circumferential ribs b. The Mass of grid shell depends on the above variables. In this paper the mass equation of CGSCS and 3 analytical design equations under axial compression are derived (the helical ribs strength formula, local buckling formula for helical ribs, general buckling formula for shell). The objective function of optimization is mass function and the optimization constraints are the above analytical design equations. The graphical method is used for optimization of problem. This method determines the allowable range of design variables for three constraints; then, by using mass curve, optimum design variables are obtained for minimum mass.

Composite, Grid stiffened cylinder, axial compressive load (ACL), Design variables,Optimization, Minimum mass, Graphical method