| Summary: | 博士 === 國防大學中正理工學院 === 國防科學研究所 === 89 === ABSTRACT
The purpose of this study was to investigate the crack closure phenomenon and crack retardation due to the extra material existed inside the fatigue crack.
There are two parts in this study. The first part investigated the thickness and distribution of oxide occurred inside the crack of the 4130 low alloy steel in 3.5% NaCl solution. Moreover, we explored the influence of oxide morphology on the fatigue crack growth rate in air and in 3.5 % NaCl solution environments. The findings tested in air showed that the larger crack opening and higher solution temperature induced the greater oxide thickness, and therefore resulted in the stronger crack retardation. The crack growth retardation was less obvious when the specimens were tested in the NaCl solution. It was probably due to the oxide dissolution and its gelatinous morphology. The Paris law and the rigid-wedge model were used to predict the crack growth rates due to oxide-induced crack closure, and the theoretical crack growth approached to the experimental data.
In the second part of this work, cracks in AISI 4130 low alloy steel specimens were artificially filled with closure materials through plating on the crack faces. The closure materials included plating metals such as electroless and electroplated nickel, and electroless copper. Premature crack closure occurred to retard the subsequent crack extension. This work investigated how the mechanical properties and in-crack distribution of these plating metals affected crack retardation. The extent to which specimen thickness, crack prop-opening load and sucker site affected crack retardation was also studied. Experimental results indicated that the strength of plating metal and the deposit thickness T2 affected the post-plate crack propagation the most, while deposit volume was the next most influential factor. In the case of satisfactory crack face plating, crack growth rate decreased substantially and even caused crack arrest. Finally, the elastic-wedge model can accurately predict crack development after the infiltration of electroless nickel plating.
|
|---|
