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ulti-Axial Fatigue Initiation Testing and Specimen Characterization of 304 Stainless Steel

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Current fatigue assessments for the fatigue life of a plant component are usually based on methodologies that use uniaxial fatigue test data (i.e. ASME Section III, and are intended to be conservative for design and fitness-for-purpose assessments when applied to plant components and loading. This data is generated through cyclic loading of specimens at a constant amplitude, and failure is usually defined as when there is a load drop of 25% from steady state stress under strain-controlled conditions (or specimen separation for stress control). The corresponding number of cycles is then used as the definition of fatigue life for a particular strain amplitude. It is known that there are differences between fatigue behaviour in an idealised laboratory setting and in-service components which can contribute to excessive conservatism in plant assessments.

Product Number: ED22-18372-SG
Author: Eleanor M Grieveson, Peter Gill, Paul Onwuarolu, Ben Coult, Alistair Garner
Publication Date: 2022
$20.00
$20.00
$20.00

A specimen design and testing methodology have been developed that enables multi-axial fatigue tests to be performed on standard uniaxial testing rigs. This specimen with multiple arms, when subjected to a uniaxial load, causes a biaxial stress state to form in the central region; a central dish acts as a controlled crack initiation site.
Following calibration trials, tests are now being performed on 304 stainless steel specimens in high temperature air. The cracking patterns and depths in these tested specimens have been investigated using a variety of methods including striation counting, Direct Current Potential Drop, Focused-Ion-Beam sectioning and high resolution X-ray Computed Tomography. These tests have demonstrated the validity of the specimen design and are providing insight into the cracking patterns formed under multi-axial loading.

A specimen design and testing methodology have been developed that enables multi-axial fatigue tests to be performed on standard uniaxial testing rigs. This specimen with multiple arms, when subjected to a uniaxial load, causes a biaxial stress state to form in the central region; a central dish acts as a controlled crack initiation site.
Following calibration trials, tests are now being performed on 304 stainless steel specimens in high temperature air. The cracking patterns and depths in these tested specimens have been investigated using a variety of methods including striation counting, Direct Current Potential Drop, Focused-Ion-Beam sectioning and high resolution X-ray Computed Tomography. These tests have demonstrated the validity of the specimen design and are providing insight into the cracking patterns formed under multi-axial loading.

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