High reliability and operational safety are most important requirements of aerospace products. Structural Safety (Reliability) assessment techniques help to achieve safe designs and also identify effect of uncertainty occur in structural systems of aerospace products. In general the structural safety is evaluated using various statistical methods like Mean Value and Moment Methods. Currently several inverse reliability measures such as Probabilistic Performance Measure (PPM) and Probabilistic Sufficiency Factor (PSF) have been introduced as measures of safety. In the present paper, thin Maraging steel cylindrical pressure vessels which are extensively used in space vehicle applications are considered for safety evaluation using Probabilistic Performance Measure (PPM) and is demonstrated with a practical case.

Structural Safety Evaluation Using Inverse Measure Approach

In this paper, an efficient technique is developed to predict failure probability of three failure modes(case rupture, fracture and bolt breakage) related to solid rocket motor case due to the inner pressure during the mission flight. The overall procedure consists of the steps: 1) design parameters affecting the case failure are identified and their uncertainties are modelled by probability distribution, 2) combustion analysis in the interior of the case is carried out to obtain maximum expected operating pressure(MEOP), 3) stress and other structural performances are evaluated by finite element analysis(FEA), and 4) failure probabilities are calculated for the above mentioned failure modes. Axi-symmetric assumption for FEA is employed for simplification while contact between bolted joint is accounted for. Efficient procedure is developed to evaluate failure probability which consists of finding first an Most Probable Failure Point(MPP) using First-Order Reliability Method(FORM), next making a response surface model around the MPP using Latin Hypercube Sampling(LHS), and finally calculating failure probability by employing Importance Sampling.

Reliability Prediction of Failure Modes due to Pressure in Solid Rocket Case

High reliability and operational safety are most important requirements of an aerospace product. Traditionally, structural safety in the design is ensured by implementing appropriate safety factors to account for shortcomings stemming from lack of knowledge, insufficient data or inherent variability. The factor of safety concept does not provide a quantitative measure of the structural reliability. Even with the implementation of the safety factor, there exists a finite probability of failure of a structure. In this chapter, a thin maraging steel cylindrical pressure vessel is identified for structural reliability and safety analysis. Thin maraging cylindrical pressure vessels are used extensively in aerospace vehicles. Reliability (safety index) analysis is carried out using actual data collected during design, manufacturing and testing. Two distinct failure modes, i.e. fracture failure and plastic yielding, are considered for safety evaluation. The structural integrity is evaluated using an empirical model to predict the burst pressure. Moment methods are used for reliability analysis. This chapter will help pressure vessel designer to assess the structural safety and integrity during preliminary design.

C. M. Chang

Papers

Reliability analysis and quality assurance of rocket motor case considering proff testing