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The University of Southampton

Research project: Short crack growth and propagation in steels under creep-fatigue cycling

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Failure by low-cycle fatigue is one important consideration in the design of structures used at elevated temperatures. The aim of this project is to investigate the failure mechanisms of 316 stainless steel during creep-fatigue cycling. A programme of tests was conducted to examine short crack growth behaviour in reversed-bending, high strain fatigue cycle that contained a tensile hold- period.

The failures of most engineering components operating at elevated temperature are caused by a complex process. In particular, the design of components for resistance to creep-fatigue cracking has received considerable attention in the past two decades.

A practical investigation and analysis into short crack growth behaviour of 316 stainless steel under creep-fatigue conditions at 550°C were undertaken on a high temperature reverse bending rig. Throughout the tests, surface crack initiation and growth on both tensile and compressive sides were monitored by means of a plastic replication.

The intensive observations and subsequent detailed analyses revealed that with high strain ranges of 0.9-2.5% and 60 minutes hold time, the behaviour of individual initiation and growth of many subcracks in Stage I, and their subsequent coalescence in Stage II are the dominant characteristics for the failure of the specimens.

Calculations of damage due to growth of a single crack cannot effectively predict lifetime under the present creep-fatigue conditions. A new life prediction model, derived from a theoretical analysis, is proposed which incorporates the mechanism of subcrack coalescence. On this basis, satisfactory predictions of creep- fatigue lifetime have been achieved.

of minor crack linkage patterns for forming a major crack on the tensile-hold surface at 1.43% strain range.
Replica studies
of minor cracks at different strain ranges.
Calculated maximium growth patterns


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