Reliability Prediction for Naval Shafting Under Cyclic Loads and Determination of Inspection Intervals

Extending the time between drydocking availabilities would simultaneously increase operational readiness of the submarine fleet and reduce its maintenance costs. Mandatory propulsion shaft inspection and overhaul intervals are a key driver of existing maintenance schedules. Extending these intervals is therefore desirable for the current fleet and mandatory for the Ohio Replacement Program’s maintenance objectives. To that end, the Navy has funded a program at MIT with the following key objectives:Objective #1: The development of a probabilistic model for the prediction shaft life for current submarine shafting systems. The assumed degradation mode will be corrosion fatigue crack initiation from pits and growth. The overall model will be developed as a software tool that captures the history of the shaft system. The output of the model will be a probability of failure (POF) due to pitting, followed by crack initiation, followed by crack propagation. The POF will include uncertainty estimates. Objective #2: To explore the possibility that the failure scenario can be interrupted (mitigated or eliminated) through design or material changes.Success for Objective #1 will enable the Navy to evaluate existing and future standards for shaft inspection and maintenance intervals, providing a tool to maximize those periods within the limits of desired safety.

Success of Objective #2 will allow the project team to identify areas where selective design changes may result in reduction in uncertainty, greatly extending the inspection interval or, ideally, eliminating the issue as a concern.

Success for Objective #2 will also provide guidance for future development efforts to eliminate corrosion fatigue as a failure mechanism for an arbitrary inspection interval.