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 Δ
J and crack opening displacement Δφ
250, where Δ
J was evaluated from the loading portion of hysteresis. loops of load against crack opening displacement at the crack center and Δφ
250 was measured at the position 250μm behind the crack tip.
The growth rate
da/dN was expressed as a unique power function of Δ
J or Δφ
250 for each material. The variance in the
da/dN-Δ
J relation with materials was found to be minimum when
da/dN was correlated to Δ
J/E(E=Young's modulus). The variance was even reduced if Δφ
250 was taken as a parameter. The exponent in the
da/dN-Δ
J 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 Δ
J, 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.
抄録全体を表示