13th International Conference on Fracture June 16–21, 2013, Beijing, China -4- All of fractures originated from the surface. In low humidity, crack was not observed on the surface of non-fractured specimens tested at the stress level of fatigue limit, σw, which is defined as the strength for a specimen not to fail after being stressed for 107 cycles. In this case, it means that the fatigue limit of the alloys was determined by the resistance to crack initiation. Figure 4 shows the humidity dependence of the fatigue limit. Effect of humidity on fatigue strength was relatively small in the range of low humidity below about RH50%. In the high humidity beyond RH50%, fatigue strength decreased largely with increasing humidity, especially the higher the static strength, the larger the decrease in fatigue strength. For example, the fatigue limit of Steel C in RH85% was less than a half of that in RH25%. The humidity examined in the current study was not particular but normal so that this marked decrease of fatigue strength in high humidity is of extremely importance in the practical applications of the alloys. Figure 5 shows (a) crack growth curves and (b) the relation between crack length and the ratio of number of cycles to fatigue life, N/Nf, in low (RH25%) and high (RH85%) humidity, respectively. Figure 4. Humidity dependence of fatigue limit (a) a-N curves (b) a-N/Nf curves Figure 5. Crack growth behavior 0 20 40 60 80 100 0 100 200 300 400 500 600 700 800 Steel A (HV550) Steel C (HV705) σ w (MPa) Humidity (%) 0 1x10 5 2x10 5 3x10 5 4x10 5 0.01 0.1 1 10 Crack length, a mm Number of cycles, N cycle RH 25% 85% SteelA ( σ a =590MPa) SteelC ( σ a =750MPa) 0.0 0.2 0.4 0.6 0.8 1.0 0.01 0.1 1 10 Crack length, a mm N / N f RH 25% 85% SteelA ( σ a =590MPa) SteelC ( σ a =750MPa)
RkJQdWJsaXNoZXIy MjM0NDE=