Title

The mechanisms of fatigue crack growth at low growth rates

Date of Completion

January 1997

Keywords

Engineering, Metallurgy|Engineering, Materials Science

Degree

Ph.D.

Abstract

The thesis described the mechanisms of crack growth at low growth rates in 1018 steel. For a comparison purpose a 2519 aluminum was also investigated. Further, a detailed study of the striation formation process at high growth rates where striations were readily seen was carried out for the later comparison with that at low growth rates. The environment effect, such as air vs vacuum, was also undertaken. Sawtooth type striations are found on the fracture surface at high growth rates and they are formed cycle by cycle, indicating the crack growth is continuous in this range. "Striation-like" markings are found on the fracture surface at low growth rates and they are actually the reflection of underlying dislocation structures. Fine striations are also found between the "striation-like" markings and they are the continuation of the striations at high growth rates. Crack proceeds along (011) planes through the underlying subgrains cycle by cycle and form the fine striations on the crack surface. The environment, such as vacuum, changes the fracture features at high growth rates to some extent but does not change those at low growth rates. Striations in 2519 aluminum are found to have underlying dislocation bands, which is similar to the situation in 1018 steel. ^