Abstract
Fatigue crack growth tests of copper, very low-carbon steel, mild steel, stainless steel and aluminum alloys were conducted by using center-cracked specimens under elastic, elastic-plastic and gross yielding conditions. The growth rate of a fatigue crack was correlated to the ranges of J integral ΔJ and crack opening displacement Δφ250, where ΔJ was evaluated from the loading portion of hysteresis. loops of load against crack opening displacement at the crack center and Δφ250 was measured at the position 250μm behind the crack tip.
The growth rate da/dN was expressed as a unique power function of ΔJ or Δφ250 for each material. The variance in the da/dN-ΔJ relation with materials was found to be minimum when da/dN was correlated to ΔJ/E(E=Young's modulus). The variance was even reduced if Δφ250 was taken as a parameter. The exponent in the da/dN-ΔJ relation tended to decrease from 2.3 to 1.4 as the yield strength of the material increased. In the region of rates between 3×10-7 and 5×10-5m/cycle, the fracture surfaces of copper, very low-carbon steel and stainless steel were covered almost entirely by ductile striations, whose spacing s coincided with the macroscopic growth rate da/dN. Since s was not proportional to ΔJ, the simple crack tip blunting model proposed previously was not enough to derive the fatigue crack growth law. Some discussion was made for a further refinement of the mechanics of crack growth by plastic blunting mechanism.
