Evaluating the Impact of Conservatism in Industrial Fatigue Analysis of Life-Limited Components

Abstract

Abstract. This paper presents a review of the conservatism approaches applied by different industrial sectors to the stress-life (S-N) analysis of 'life-limited' or 'safe-life' components. A comparison of the fatigue design standards for 6 industrial sectors identified that the conservatism approaches are highly inconsistent when comparing the areas of variability and uncertainty accounted for along with the conservatism magnitude and method of application. Through the use of a case-study based on the SAE keyhole benchmark and 4340 steel S-N data, the industrial sector which introduces the greatest reduction of a component life-limit was identified as the nuclear sector. The results of the case-study also highlighted that conservatism applied to account for scatter in S-N data currently provides the greatest contribution to the reduction of component life-limits.

A Framework to Implement Probabilistic Fatigue Design of Safe-Life Components

“…Within safe-life fatigue design and analysis, significant resources are expended during material testing and loading spectra development, resulting in a significant amount of data being available for fatigue design (Schijve, 2009). However, especially in the case of generating material design allowables, this data is used to produce a single safe value to be used for design (Hoole, 2018). As probabilistic approaches utilise the complete dataset to generate input probability distributions, this increases the utilisation of data that is already available for design.…”

“…Component safe-life values are computed using 'classical' fatigue analysis methods, such as a stress-life (S-N) approach incorporating Miner's Rule (Braga, 2014). However, safe-life fatigue analysis contains many sources of variability within the design parameters of the analysis process, such as scatter in material properties and uncertainty in loading (Hoole, 2018). This variability propagates through the analysis process into component safe-life values and is currently accounted for using conservatism and safety factors (Hoole, 2018).…”

“…However, safe-life fatigue analysis contains many sources of variability within the design parameters of the analysis process, such as scatter in material properties and uncertainty in loading (Hoole, 2018). This variability propagates through the analysis process into component safe-life values and is currently accounted for using conservatism and safety factors (Hoole, 2018). The use of safety factors is known as a 'deterministic' approach, whereby all design parameters are set to single and safe values.…”

Systematic statistical characterisation of stress-life datasets using 3-Parameter distributions

“…Within the sphere of fatigue design, many components across the aerospace, nuclear, offshore, steel structures and wind energy sectors are designed using a 'safe-life' philosophy (also known as 'life-limited') [1]. The safelife of a component represents the number of duty cycles after which the component must be removed from service [2], regardless of whether fatigue crack propagation is observed.…”

“…The safelife of a component represents the number of duty cycles after which the component must be removed from service [2], regardless of whether fatigue crack propagation is observed. Safe-life components are typically designed using a 'classical' fatigue analysis approach of 'Stress-Life' (S-N) analysis based upon Miner's Rule [1].…”