Interaction of conservative design practices, tests and inspections in safety of structural components

Structural safety is achieved in aerospace application and other fields by using conservative design measures like safety factors, conservative material properties, tests and inspections to compensate for uncertainty in predicting structural failure. The objective of this dissertation is to clarify the interaction between these safety measures, and to explore the potential of including the interaction in the design process so that lifetime cost can be reduced by trading more expensive safety measures for less expensive ones. The work is a part of a larger effort to incorporate the effect of error and variability control in the design process. Inspections are featured more prominently than other safety measures.; The uncertainties are readily incorporated into the design process by using a probabilistic approach. We explore the interaction of variability, inspections and structural sizes on reliability of structural components subjected to fatigue damage growth. Structural sizes and inspection schedule are optimized simultaneously to reduce operational cost by trading the cost of structural weight against inspections to maintain desired safety level.; Reliability analysis for fatigue cracking is computationally challenging. The high computational cost for estimating very low probabilities of failure combined with the need for repeated analysis for optimization of structural design and inspection times makes combined optimization of the inspection schedules and structural design prohibitively costly. This dissertation develops an efficient computational technique to perform reliability based optimization of structural design and inspection schedule combining Monte Carlo simulation (MCS) and first-order reliability method (FORM). The effect of the structural design and the inspection schedule on the operational cost and reliability is explored. Results revealed that the use of inspections can be very cost effective in maintaining structural safety.; Inspections can be made more effective if done at critical locations where likelihood of failure is maximum and the information obtained from inspections can be used to improve failure prediction and update reliability. This aspect is studied by developing a probabilistic model for predicting locations of maximum corrosion damage in gas pipelines. Inspections are done at these locations and failure probabilities are updated based on data obtained from inspections.