Fatigue Growth of Initially Physically Short Cracks in Notched Aluminum and Steel Plates

Much of the fatigue life of engineering structures is spent initiating cracks at a notch and propagating one (or more) of these cracks into and through the notch stress field. Experimental results and empirical analysis suggest that early in this process the physically small crack sometimes propagates in a manner inconsistent with analysis based on linear elastic fracture mechanics (LEFM). Available empirical adjustments for this apparent aberration postulate that the data can be made consistent with LEFM by the addition of a constant, with length dimensions, to the current crack length, resulting in a pseudo-crack. The present paper introduces and analyzes an extensive data set pertinent to this so-called short-crack problem. Included are results for notched plates made from two aluminum alloys and a steel. These plates, which contain either circular or one of two different elliptical notches, have been tested under load or displacement control and encompass both confined flow and gross-section yield. Results for crack lengths as small as 20 μm (0.0008 in.) are reported. Data presented and analyzed do indeed show trends which differ from those of LEFM analysis of so-called long cracks. More significantly, they show that the so-called short-crack behavior does not occur only for physically short cracks. Results presented indicate that cracks as large as 2.5 mm (0.1 in.) in the aluminum alloys and 1.25 cm (0.5 in.) in the steel also exhibit aberrations in their behavior as compared with longer-crack trends. Also, the results presented suggest that the data cannot be made consistent with the LEFM trend through the addition of a constant pseudo-crack length to the current crack size as has been suggested in one of the currently popular empirical models. It is postulated that the aberrations observed are a consequence of the failure of the LEFM-based analyses to recognize that crack growth in the plastic field of the notch is dominantly displacement-controlled local to the crack tip. Other relevant aspects of the problem are also discussed.