INVESTIGATION OF FATIGUE CRACK BEHAVIOR ON NEAR THRESHOLD REGION OF AISI 4340 STEELS FOR DIFFERENT HEAT TREATMENT CONDITIONS

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2023-1-27

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Çalışkan, Salim

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Damage tolerant approach is an important phenomenon in engineering practice and must be evaluated at the onset of the design stage for components that are subject to cyclic loading during functioning; if not, devastating fatigue failure may occur. Inaccurate assessments in allowable fatigue data can cause substantial variation in fatigue limit evaluation for components with small crack size. Determining long crack threshold value as a design criterion is not a new research topic; even so, some anomalies arising from different test methods in the literature make this subject suitable for investigation. The fatigue crack growth mechanism takes place in the plastic zone region, which is very small in size; the order is thought to be microstructural units, especially at low stress intensities. Hence, differences in microstructure caused by heat treatment methods are often attributed to changes in monotonic and yield strength, which results in variations in plastic zone size. In this thesis, endurance limit of AISI 4340 steel in annealed and tempered conditions was investigated using the most established staircase and curve fitting approaches, additionally the reliability of data set was also questioned. Moreover, the effect of microstructure around low stress intensities was detailed by crack growth tests. Also, the influence of stress ratio and compression precracking on near-threshold performance were investigated. Eventually, fatigue limit and long crack threshold values were found as 585 MPa and 4.89 MPa√m for tempered specimens, and 481 MPa and 4.01 MPa√m for annealed specimens. Accordingly, endurance limit assessment for components with physically small crack sizes was obtained by correlating the fatigue limit with the threshold data for each microstructure.

Subject Keywords

Fatigue, Threshold, Crack Growth, Steels, Heat Treatment, Reliability

URI

https://hdl.handle.net/11511/101952

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Graduate School of Natural and Applied Sciences, Thesis

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Citation Formats

S. Çalışkan, “INVESTIGATION OF FATIGUE CRACK BEHAVIOR ON NEAR THRESHOLD REGION OF AISI 4340 STEELS FOR DIFFERENT HEAT TREATMENT CONDITIONS,” Ph.D. - Doctoral Program, Middle East Technical University, 2023.