Degree

Doctor of Philosophy (PhD)

Department

Mechanical and Industrial Engineering

Document Type

Dissertation

Abstract

A methodology is proposed to identify the onset of crack initiation that utilizes the material damping. The damping is measured using the impulse excitation technique (IET). The damping is also used to correlate fatigue life of additively manufactured (AM) specimens to their damping characteristics. Results reveal that the damping value is inversely proportional to the fatigue life of the specimens.

To detect the damage accumulation and crack initiation in metals due to cyclic loading, another methodology based on the measurement of the surface roughness parameters is introduced. Results presented reveal that the evolution of the surface roughness parameters can be used to effectively characterize the damage accumulation and crack initiation in the material.

A promising technique to increase the fatigue life of metals by means of polishing the specimens during cyclic loading. The results of an extensive set of experiments reveal that fatigue life can be extended significantly (from 45% to 100%) by polishing the surfaces at appropriate intervals.

To extend the fatigue life of SS316 specimens, the effect of a coating is investigated using both Cr-coated and uncoated specimens. Scanning electron microscopy is used to understand the mechanism of crack initiation in both types of specimens. Results show longer fatigue life for Cr-coated specimens in HCF and longer fatigue life for uncoated specimens tested at LCF.

A methodology for enhancing the fatigue life of SS316 by intermittent heat treating at different temperatures is investigated. The heat-treatment is done at both 400 °C and 600 °C and the results show better fatigue life improvement for specimens heat-treated at 600 °C. Results also show that by heat-treating the specimens close to the crack initiation the fatigue life can be improved by 30%

The intrinsic dissipation terms of metals under cyclic loading is revisited to obtain the fracture fatigue entropy (FFE) accurately. Also, a technique to identify the fatigue limit of metals by measuring their damping value at different stress levels is introduced. Moreover, the temperature dependence of internal friction for SS304 and CS1018 specimens is investigated. The result demonstrates a linear relationship and the existence of a peak.

Date

8-21-2020

Committee Chair

Khonsari, Michael

DOI

10.31390/gradschool_dissertations.5360

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