Concept of interactive determination of safe service life for composite cylinders by destructive tests parallel to operation

• The scatter values of materials used for pressure vessels have been studied.
• Thus results in impracticality of fatigue tests of cylinders without metal liners to CFRP rupture.
• Statistic evaluation of cycle tests is not possible – neither for virgin nor for operated cylinders.
• Additionally conventional burst tests have been detected as not being appropriate, too.
• A test concept of residual strength tests parallel to operation has been elaborated.

There is evidence that composite materials degrade under static loads. Consequently there is no way to design composite gas cylinders for unlimited service life. Yet this fact is not reflected by design standards for composite receptacles approved for the transport of dangerous goods or for stationary application. Unnecessary cost and weight result from this.Due to the degradation aspects of composites it is necessary to provide appropriate retesting procedures, which allow rejection of used vessels as is common practice for metallic receptacles. The current procedure of hydraulic tests appears to be an expensive procedure with serious doubts regarding its usefulness for composite cylinders.Consequently, an efficient method is needed, which allows limiting the life time to its safe maximum. It is thus recommended to combine retesting with the assessment of residual life time. Since relevant non-destructive methods are not yet sufficiently matured and standardized enough, a concept featuring destructive tests on composite cylinders parallel to operation should be considered. An alternative concept introduced here, proposes to substitute the hydraulic testing of 100% of a population by a probabilistic evaluation of destructive tests on samples parallel to operation. Intensified pre-fill inspections should be an additional component of such a concept.