IMPROVEMENT OF ASME NH FOR GRADE 91 NEGLIGIBLE CREEP AND CREEP FATIGUE
|Publication Date:||3 September 2008|
Composite cylinders have been used for over 50 years in commercial, vehicle, defense and aerospace applications. New materials, processes, design approaches and applications have been incorporated during that time. The industry has maintained a high level of safety. The industry has adapted to these changes and has developed new and revised standards to address these changes and to reflect a better understanding of service conditions.
Recommendations are made that the industry:
• Continue to monitor field use and incorporate changes to requirements, standards and codes that reflect knowledge gained for composite pressure vessels,
• Use a failure modes and effects analysis (FMEA) approach to standards, using the knowledge gained from field experience,
• Develop standards for composite pressure vessels that are more performance based to improve both safety and performance,
• Address requirements using performance testing, not by using excessive safety factors,
• Use stress ratios for the various reinforcing fibers that accurately reflect their stress rupture and fatigue characteristics to achieve high reliability, Harmonize testing requirements where practical, Use qualification tests that are appropriate for the application and for the materials and design features of the pressure vessels being used, and
• Consider using fleet leader programs for new materials, designs or applications if there is likely to be a significant safety issue
To support these recommendations, history of use of composite cylinder in aerospace/defense, commercial and vehicle applications is reviewed. This includes review of applications, materials of construction; standards used and field service issues.
The use of performance-based requirements is discussed, as is the background of safety factors used for various reinforcing fibers. Recommendations are made for validation testing of materials and pressure vessels, with consideration for failure modes and effects analysis (FMEA) involving the field use of the vessels.
Cyclic fatigue and stress rupture are discussed, with examples of laboratory testing and correlation from field experience.