Date of Award

Spring 2015

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Materials and Infrastructure Systems

First Advisor

Henry E. Cardenas

Abstract

Fatigue is responsible for at least 50% of all mechanical and 90% of all metallic failures. Fatigue cracks often start at stress concentrations, and without timely and appropriate remediation, tend to exhibit relatively fast propagation that leads to property damage and sometimes serious accidents.

The objective of this research was to develop a new method of fatigue crack treatment in steel structures and estimate its efficiency and limitations. The method was based on placing fatigue cracks under compression by depositing nickel onto the surfaces of the cracks. The proposed method was applied to ASTM E399 compact-tension specimens machined from ASTM A36 steel. This study found that the method was able to arrest fatigue crack propagation. Fatigue crack arrest period varied from 2,000 to 30,000 cycles. The fatigue life of the specimens was extended for up to 55,000 cycles. In many cases the re-initiation life of fatigue cracks after treatment was similar to the crack initiation life obtained from the V-shaped starter notches. A power law relationship was developed that successfully correlates the fatigue crack arrest life and the stress intensity factor range applied for post-treatment load cycling. Fatigue crack packing with nickel resulted in significant reduction of stress concentration factors of the cracks.

Spectroscopic analysis confirmed the presence of nickel in electrochemically treated fatigue cracks. The amount of nickel deposited was found to be non-uniform along the length of the cracks. This study found that an elastic finite element analysis (FEA) supported the notion of compressive stresses being developed at the crack tip and of a significant reduction in the stress concentration factor of the fatigue crack due to application of this treatment method. FEA has also supported the expectation that increasing the dosage of crack packing material would tend to result in a longer crack re-initiation period. The proposed method of electrochemical fatigue crack treatment was found to be beneficial in terms of improved corrosion resistance of treated specimens as long as the treatment was uniform and continuous. The average general corrosion rate of nickel-plated ASTM A36 steel specimens that were uncracked was 80% lower than that of the non-plated cases. The discontinuous deposition observed within cracks was expected to promote localized corrosion of the A36 base metal. Future work will examine the use of deposition candidates with less of a tendency toward dissimilar metal corrosion.

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