Environment effects and surface roughness on fatigue crack growth at negative R-ratios

Fatigue crack growth has been widely studied, since it plays an important role on the damage tolerance analysis of mechanical components and structures. The environment, material properties and stress ratio significantly influence the fatigue crack growth behaviour of materials. Experimental tests were performed in M(T) specimens of a normalized DIN Ck45 steel at constant load ratios for R = 0.7, 0.5, 0, −1, −2, −3, in ambient air and vacuum conditions, using a new and patented chamber of vacuum. Special emphasis is given to the study of environment effects, stress ratios and related effects of crack roughness. Fracture surface roughness and crack closure effect were systematically measured for all tests in order to compare the influence of different environment and R-ratios. Results have shown that fatigue crack growth rates are higher in air than in vacuum and the fracture surface roughness is also higher in air than in vacuum for comparable stress ratios. The effect of the environment on fatigue crack growth rates seems to be more significant than any mechanical contributions such as plasticity, oxide and roughness which can induce the so-called crack closure.