Predicting dwell fatigue life in titanium alloys using modelling and experiment.

Area of Science:

• Materials Science
• Mechanical Engineering
• Computational Modeling

Background:

• Fatigue is a critical engineering challenge involving multi-scale material behavior.
• Cold dwell fatigue in titanium alloys causes significant reductions in cyclic life, leading to service failures.
• Understanding localized plasticity at the dislocation and microstructure level is key to predicting fatigue.

Purpose of the Study:

• To predict fatigue lifetimes for titanium under cold dwell conditions.
• To investigate the micro-scale mechanisms driving cold dwell fatigue.
• To validate computational models against experimental observations.

Main Methods:

• Discrete dislocation plasticity modeling was employed.
• Transmission electron microscopy (TEM) was used for experimental validation.
• Analysis focused on 'worst case' microstructures relevant to jet engine tests.

Main Results:

• Fatigue loading above a threshold stress causes slip in soft grains, creating dislocation pile-ups at hard grain boundaries.
• Pile-up stresses are sufficient to nucleate basal dislocations in hard grains, consistent with experimental findings.
• Reduced cyclic loads and temperature excursions during cycles decreased dislocation densities.

Conclusions:

• Discrete dislocation plasticity modeling can accurately predict cold dwell fatigue lifetimes.
• Microstructural features and dislocation behavior under stress holds are critical factors in titanium fatigue.
• The study provides insights into mitigating cold dwell fatigue in titanium components.