The Application of an Integrated Structural Analysis to the Prediction of Reliability

The methods developed in recent years for measuring and characterizing the mechanical behavior of solid propellant and utilizing this information in structural analyses to predict rocket motor structural integrity are reviewed in terms of the applicability of this approach to other materials and applications. It is shown that the recognition of statistical distributions of response and failure behaviors leads to the prediction of the distribution of failure times in actual use. The combining of behavior data with structural analysis to give a cumulative damage prediction of failure is shown to be possible through the use of the computer.

The development of reliability is shown to start with the development of the tests used to evaluate the material, which must measure those properties directly pertinent to the structural requirements. The product structural requirements are converted into test requirements through analytical and experimental stress analyses. Viscoelastic analysis, supported by photoelastic model studies, defines the critical regions of stress and strain in the proposed design, and the correct tests are then selected based on close simulation of the expected stress conditions and failure modes. Estimates of the lower confidence limits of the critical parameters can then be calculated and compared with the corresponding confidence limits from the stress analysis to assess the safety margin, the results indicating those critical points where design revision must be balanced against propellant reformulation to provide adequate safety margins.