Fatigue crack growth and the plastic zone in air and in vacuum

Abstract

1. It has been observed that the fatigue crack growth macrorate dl/dN in Cu−7.5% Al is related to the depth of the microscopic plastic zone h by an exponential relationship of the form $$\frac{{dl}}{{dN}} = Bh^p $$ where B and p are constants in a certain range of zone depth dependent upon medium. The critical zone depth hcr corresponding to the break on the crack growth rate curve is 2.5–3 grain diameters in both media. 2. In the initial period of crack growth, when the depth of the plastic zone has not reached hcr, the microrate exceeds the macrorate, i.e., a striation is formed not in each load cycle but in the last of a series of cycles. This is characteristic of a fatigue crack in both media but in vacuum the differences between micro- and macrorates are greater than in air. When the plastic zone exceeds the value of hcr the macrorate becomes equal to the microrate, i.e., a striation occurs in each load cycle. 3. Fatigue crack growth in Cu−7.5% Al alloy in vacuum occurs by the formation of striations which are just as clear and regular as in air. In AMg6 alloy distinct striations appear in vacuum only at low temperature. This confirms the fact that the formation of fatigue striations is not the result of action of the air medium. 4. A hypothesis is stated that the formation of striations in fatigue crack growth occurs with a limited plastic zone size and is dependent upon the mechanism of their formation during relaxation of stresses requiring a certain degree of elastic restraint of plastically deformed microvolumes.