Experimental study of the effect of loading condition on fracture surface contact features and crack closure behavior in a carbon steel

To better understand the crack closure effect in the fatigue process, influence of fatigue stress amplitude and R ratio on the contact features of fracture surfaces in an annealed carbon steel was studied via two special experimental approaches: (i) the collection of the fracture debris fallen from the crack surfaces, and (ii) the direct observation of the contact zones on the fracture surface through an ink dyeing method. The results of this study show that the change of fatigue CMOD value as a function of a/W ratio depends strongly on the loading condition; the fatigue stress amplitude and R ratio are the major factors that determine the contacting status between the mating fracture surfaces; the severity of the fracture surface contact can also be characterized by the dropping rate of the fracture debris particles collected during the fatigue test.