Abstract
Fatigue crack growth has been investigated in Ti-6Al-4V alloy at ambient and elevated temperatures, i.e. 200, 400 and 500°C. Special attentions were paid to the nucleation and the morphology of secondary cracks. The roughness of fracture surface was evaluated quantitatively using three dimensional reconstruction of fracture surface based on stereo matching technique. The results indicated that the crack growth rates at the elevated temperatures became lower than that at room temperature with increasing stress intensity factor range, ΔK. This behavior was attributed to the nucleation of secondary cracks, which led to the increased roughness of fracture surface and the reduction of crack driving force. The nucleation of the secondary cracks was observed more markedly with increasing test temperature. On the other hand, the crack growth rate at 500°C increased compared to that at room temperature in the region of ΔK≤20MPa√m. This enhanced crack growth may be due to the reduced elastic modulus with increasing temperature. Therefore, the crack growth characteristics at elevated temperatures were controlled by the competing effects between the nucleation of secondary cracks and the reduction of elastic modulus.
