The effect of dwell loading on the strain accumulation behavior of titanium alloys. Ph.D. Thesis
A research program to study the effects of various loading wave forms on the strain accumulation behavior of a titanium alloy, IMI 834, is presented. Baseline and dwell loading wave forms were applied, and the associated strain accumulation rates were determined. The baseline wave form used in this experimental program was a 10 Hz haversine. The `dwell` wave form had similar ramp up and ramp down times as the haversine wave but was held constant for 15 or 30 seconds at the maximum load, depending upon the particular test. The fatigue experiments were conducted in a vacuum (approximately 10(exp {minus}5) torr) at 25 C in a scanning electron microscope (SEM) chamber. The material chosen for this study is typically referred to as a near-alpha alloy and is used in the manufacture of turbine engine compressor discs. The standard (disk) microstructure for the alloy system tested is referred to as near-alpha and consists of equiaxed alpha in a transformed beta matrix. The second material was from the same disc forging but had been reheat treated and aged to obtain a martensitic alpha prime microstructure. The fatigue data showed that the martensitic specimen was much less susceptible to the dwell time at maximum load compared to the disk material. In addition, the cyclic strain accumulation rate between the baseline and dwell wave forms varied significantly from the martensitic material to the disk material. From the data presented, it was concluded that a probable mechanism related to the fatigue life reduction due to the dwell time at maximum load involves a dislocation locking process. Once the strain reaches a saturation level, microcracks nucleate due to microstructural constraint, and subsequent increases in fatigue crack propagation rate are associated with the release of shear stress and, possibly, microcrack linking. The concept of a saturation strain is supported by the similarities in data from the 15 and 30 second hold tests.
- Research Organization:
- Utah Univ., Salt Lake City, UT (United States)
- OSTI ID:
- 147168
- Resource Relation:
- Other Information: TH: Ph.D. Thesis; PBD: 1994
- Country of Publication:
- United States
- Language:
- English
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