Experimental Study On Short Fatigue Crack Growth Behavior In 2A12 Aluminum Alloy Under Complex Stress State

With the rapid development of material science in recent years, the technology of metal smelting and casting have improved a lot which makes a variety of high quality metal materials available in manufacturing industry and aerospace science. Because of its high strength, low density, good toughness and resistance to corrosion,2A12 aluminum alloy has been widely used in aerospace industry. Study of the fatigue crack growth behavior of 2A12 alloy and pattern of fatigue crack initiation, propagation, interaction and coalescence can provide valuable theory support to the design of 2A12 alloy components.In order to study the effect strain amplitude has on fatigue life and choose suitable strain amplitudes for fatigue experiment, tests on Ⅱ-notch specimens with strain amplitude of 0.40%,0.50%,0.60% were conducted. Then fatigue experiment was conducted on 2A12 aluminum alloy specimens with three different sizes of annular notch under strain amplitude of 0.52%,0.56%,0.60%. Interruption treatment was-used in this experiment based on the notch size and strain amplitude in order to observe and record the fatigue crack information with the help of electron metallographic microscope. By analyzing the photos fatigue crack initiation, propagation, interaction and coalescence of 2A12 aluminum alloy were studied as well as the effect notch size and strain amplitude have on fatigue crack.Through the experiment we found that fatigue crack in 2A12 alloy initiated at different sites which included:(1) Second particles such as large black impurity Al6 (FeMn); (2) Undissolved particles from 9(CuAl2) phase or S (Al2CuMg) phase; (3) Tiny defects in the surface of the alloy. Initiation usually happened at the early and middle stage of fatigue life and was rarely seen at the last stage. After comparison of fatigue crack growth behavior with different notch size under same amplitude, it was found that notch size had significant influence on the crack growth rate, the smaller the notch size, the more obvious the stress concentration state and the faster the crack growth rate. The crack growth behavior with same notch size under different strain amplitude was also analyzed and the result indicated that high strain amplitude could lead to high level of surface fatigue damage.At the middle and last stage of 2A12 alloy material fatigue life, interaction and coalescence of fatigue cracks were observed. Interaction usually happened when two or more cracks were not far from each other and effect of interaction was very slow crack growth rate or coalescence of adjacent cracks.