Burst pressures and fibre stress ratios. For all types of cylinder the minimum actual burst pressure shall not be less than the values given in table 6.3 of this annex. For type CNG-2, CNG-3 and CNG-4 designs the composite over-wrap shall be designed for high reliability under sustained loading and cyclic loading. This reliability shall be achieved by meeting or exceeding the composite reinforcement stress ratio values given in table 6.3 of this annex. Stress ratio is defined as the stress in the fibre at the specified minimum burst pressure divided by the stress in the fibre at working pressure. The burst ratio is defined as the actual burst pressure of the cylinder divided by the working pressure; For type CNG-4 designs, the stress ratio is equal to the burst ratio; For type CNG-2 and CNG-3 designs (metal-lined, composite over-wrapped) stress ratio calculations must include:
(a) An analysis method with capability for non-linear materials (special purpose computer program or finite element analysis program);
(b) Elastic-plastic stress-strain curve for liner material must be known and correctly modelled;
(c) Mechanical properties of composite materials must be correctly modelled;
(d) Calculations must be made at: auto-frettage, zero after auto-frettage, working and minimum burst pressures;
(e) Prestresses from winding tension must be accounted for in the analysis;
(f) Minimum burst pressure must be chosen such that the calculated stress at minimum burst pressure divided by the calculated stress at working pressure meets the stress ratio requirements for the fibre used;
(g) When analyzing cylinders with hybrid reinforcement (two or more different fibre types), the load share between the different fibres must be considered based on the different elastic modulii of the fibres. The stress ratio requirements for each individual fibre type must be in accordance with the values given in table 6.3. of this annex. Verification of the stress ratios may also be performed using strain gauges. An acceptable method is outlined in the informative Appendix E to this annex.
Burst pressures and fibre stress ratios. For all types of cylinder the minimum actual burst pressure shall not be less than the values given in table 6.3 of this annex. For type CNG-2, CNG-3 and CNG-4 designs the composite over-wrap shall be designed for high reliability under sustained loading and cyclic loading. This reliability shall be achieved by meeting or exceeding the composite reinforcement stress ratio values given in table 6.3 of this annex. Stress ratio is defined as the stress in the fibre at the specified minimum burst pressure divided by the stress in the fibre at working pressure. The burst ratio is defined as the actual burst pressure of the cylinder divided by the working pressure; For type CNG- 4 designs, the stress ratio is equal to the burst ratio; For type CNG-2 and CNG-3 designs (metal-lined, composite over-wrapped) stress ratio calculations must include:
(a) An analysis method with capability for non-linear materials (special purpose computer program or finite element analysis program);
