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In conducting a failure analysis on a fastener, measuring fatigue strength or endurance limit is often the time-limiting factor in the analysis. The classical approach to measure the fatigue strength, which often takes as much as 3–4 months to 3–4 years using several machines, cannot be tolerated from an operational cost perspective, especially if the failure closes down production.
This paper presents an accelerated method of testing for the fatigue strength of a fastener based on measuring the threshold stress for fatigue crack initiation at the root of a thread. The method builds on the work of Prot and Corten in combination with a Rising Step Load™-fatigue testing protocol. Using this new approach, the fatigue strength can be measured in less than one week with one machine.
A case history is presented to illustrate the use of this new method in fatigue to quantify the effect of surface conditions at the root of the threads on the life of a large 3.5-in.-diameter (8.9 cm) bolt. Life analysis is based on a Goodman Diagram modified within the framework of fracture mechanics.
failure analysis, fatigue strength, threshold, endurance limit, fasteners, screws, bolt, life analysis
Principal consultant, L. Raymond & Associates, Newport Beach, CA