Effects of dwell fatigue, temperature, frequency and environment in a titanium alloy

Download Statistics

Downloads

Downloads per month over past year

Mills, Georgia (2020). Effects of dwell fatigue, temperature, frequency and environment in a titanium alloy. University of Birmingham. Ph.D.

Preview

Mills2020PhD.pdf
Text - Accepted Version
Available under License All rights reserved.
Download (9MB) | Preview

Abstract

This project concentrates on dwell fatigue crack growth, together with a detailed study on fatigue thresholds, fatigue crack growth resistance curves and the influence of environment in a titanium alloy Ti 6246.

When a dwell period is introduced crack growth rates increase by a small but significant amount compared to baseline with the crack growth mechanism remaining transgranular. Striations are observed on the fracture surfaces of some tests, particularly at high ΔK values. When present, striation spacings match crack growth rates obtained experimentally. Conditions influencing the effect of the dwell period included dwell time, environment and temperature. When the dwell time was one hour, the crack growth rate saw an increase of up to 3.5 times compared to baseline crack growth. However, when the dwell time was reduced to two minutes, the maximum increase in dwell crack growth rate over baseline crack growth was only 1.5 times. Furthermore, the environment also influenced the effect of dwell on the fatigue crack growth rate. For example, a test carried out in air conditions had a maximum increase in crack growth rate of 3.5 times when the dwell period was introduced whereas a vacuum test saw an increase of 4.5 times in crack growth rate when the dwell period was introduced. A one hour dwell test carried out at 575oC experienced a rapid increase in crack growth rate, as shown by the significant elevated “m” value obtained of 6.1. The maximum increase in crack growth rate compared to baseline loading experienced for this test was approximately by a factor of 15 times. Even at this increased temperature, the crack growth mechanism remained transgranular, however, creep damage becomes much more dominant.

Near threshold crack growth rates and ΔKth values were assessed for a range of extrinsic operating parameters. Despite differences in near threshold crack growth rates, altering the temperature from room temperature to 500oC, R ratio from 0.1 to 0.5 and frequency from 10 to 0.25 Hz was found to have only very modest effects on ΔKth values, which were all found to be very similar, approximately between 3 and 4 MPa√m. However, when R ratio was significantly increased to 0.87, ΔKth was reduced significantly to 1.5 MPa√m.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):