The damage-tolerance behaviors including fatigue crack propagation and fracture toughness were investigated in TC4ELI (extra-low-interstitial) alloy.
Crack tip plastic zone (CTPZ) in two kinds of microstructures, lamellar microstructure and short-bar microstructure, was studied to investigate the propagation behavior of fatigue crack. The effects of CTPZ on fatigue crack propagating path and crack growth rates were analyzed in the two microstructures. Results show obvious differences of CTPZ between the two microstructures. In lamellar microstructure, large CTPZ and intensive slip influenced by grain orientation lead to flexuous crack path and high crack closure level which will both reduce fatigue crack growth (FCG) rates. In short-bar microstructure, small CTPZ and flat crack path lead to high FCG rates. Meanwhile the investigations of energy dissipated in CTPZ also reveal the difference of CTPZ between the two microstructures.
The turning points in Paris region of FCG rates were studied in lamellar microstructure. Result indicates that the appearance of the turning point is induced by CTPZ size growing larger than β grain size.
The low FCG rates in lamellar microstructures are more suitable for the damage-tolerance design. FCG rates of lamellar microstructure with various size parameters were tested to investigate the key microstructure factors which could affect FCG rates in different ΔK stages. Analysis of crack propagating path in lamellar structure helps to understand the effect of microstructure on FCG rates. FCG rate tests reveal that the thickness of α lamella is the main factor to affect FCG rates in near ΔKth stage, however the FCG rates in Paris region are influenced by grain size, colony size and lamellar thickness.
Long-focus microscopy and SEM micrography were used to investigate short fatigue crack initiation and propagation in lamellar and short-bar microstructure. Short fatigue crack growth rates in the two microstructures were tested and compared with long crack at three load ratios. In lamellar microstructure, short crack initiates at PSB position with short initiating life; In short-bar microstructure, short crack initiates at the largest stress concentration region with long initiating life. Difference of notch plastic zone in the two microstructures is considered to be the key factor to determine the different initiating styles. Short fatigue crack propagates faster than long cracks at the same ΔK in both microstructures, but only in the lamellar microstructure can short fatigue crack propagate below long fatigue crack threshold ΔKth.
Fracture toughness of TC4ELI alloy with different microstructures was tested. Results show high KIC value in lamellar microstructure, and the KIC value increases with the decreasing of yield stress. The CTPZ was introduced to interpret the variations of KIC.
修改评论