A New Concept for the Design of Filament Wound Composite Pressure Vessels
- Paper number
IAC-06-C2.P.1.12
- Author
Dr. R. Ramesh Kumar, Indian Space Research Organisation (ISRO), VSSC, India
- Coauthor
Mr. Rajesh P. Nair, India
- Year
2006
- Abstract
Filament wound composite pressure vessels that are widely used in Inertial Upper Stages (IUS) of expendable launch vehicles are designed based on netting theory. According to this theory, fibre alone takes the load. Generally a filament wound pressure vessel has a cylindrical portion at the middle with domes on either side of it fitted with metallic end bosses. Geodesic winding of the pressure vessel with unequal opening to accommodate the end bosses introduces slip proportional to the increase in the opening ratio due to the end fittings. The filament winding technique provides basically the polar reinforcements. Hoop layers in the cylinder region are provided based on the design for the ultimate pressure capability. Dome reinforcements in its hoop direction are known as doily generally provided between each polar layers. In the design, it is assumed that when hoop stress in the cylinder region reaches a value equal to the tensile strength of the unidirectional laminate at ultimate design pressure failure occurs. Simple design formula can be used to predict the failure hoop stresses with a good accuracy. This is the general practice followed in industrial and aero space applications. In variably such an assumption holds good, only if polar layer stresses are benign. Intricacy in the design of filament wound composite pressure vessel is identified as an inward movement of the dome end against the direction of pressure causing kink or bending in the dome profile under internal pressure. It was reported in literature such behaviour is due to the insufficient hoop stiffness of the domes. Experimental methods to validate mathematical model are highly expensive and time consuming and also composite pressure vessels can be safely and reliably designed using simple relationship based on the hoop stress in the cylinder region. Due to these reasons attempts were not reported beyond 1980s to offer a design for dome regions without kinks or excessive bending. As on today the coupling between hoop and polar fibre stress on dome bending is not fully understood. Geometric non linear analysis of the composite pressure vessel based on the theoretical winding angle and thickness in the domes with actual number of polar layers in the domes and cylinder region with the number hoop layers in the cylinder region is modelled and displacements and strains are predicted and compared with vast test data. A good agreement in radial dilation and strains in the cylinder is established and a reasonably good correlation for the polar domes with the test data (as expected). A comparison of the deformed configuration and fibre strains in the pressure vessel shows that an increase in the stiffness of the doily alone brings down the bending strains by as much as 40 percentage without any kink in the dome profile. It is concluded that for a ratio of equivalent stiffness along hoop to meridional directions of the dome of about 1.0, mid dome bending is found to reduce considerably. The new design concept is proven that eliminate the dome kink based on the reduced dome fibre stresses due to the required coupling between hoop and polar layers of the dome. Mass savings of 20 percentage is estimated.
- Abstract document