Comparison of the growth of individual and average microcracks in the fatigue of Al-SiC composites
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I.
INTRODUCTION
THE need for more cost effective and reliable quality assurance inspections have led to expanded use of fracture mechanics through the introduction of concepts such as unified life cycle engineering. Fracture mechanics is particularly useful for making fatigue crack growth rate predictions when estimating the remaining life of a structure and when scheduling inspections. In such instances, the driving force for long crack* growth in fatigue is taken *Crack sizes in this work will be distinguished by a terminology similar to that set in Ref. 18. Microcracks refer to mechanically and microstructurally small cracks which are less than 100 /~m in length and depth, on the order of the grain size, and of sizes comparable to their own crack tip plastic zones. Short cracks are physically small cracks which are 100 to 500/~m in either the length or depth direction. Long cracks have lengths and depths which are greater than 500/~m and have growth rates which can be described by the Paris equationY]
to be a function of the crack length. This driving force, typically taken in the form of the cyclic stress intensity factor (A/Q, is the product of the applied stress amplitude (Ao-) and crack length to the one-half power. The Paris equationV] is the most common crack growth equation. It is of the form da/dN = CAK m
[ 1]
E.Y. CHEN, Graduate Research Assistant and M. MESHII, John Evans Professor, are with the Department of Materials Science and Engineering, Northwestern University, Evanston, IL 60208. L. LAWSON, Postdoctoral Fellow, formerly with the Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois 60208, can now be reached at 226 Interstate Parkway, Bradford, PA 16701. This article is based on a presentation made in the symposium entitled "Creep and Fatigue in Metal Matrix Composites" at the 1994 TMS/ASM Spring meeting, held February 28-March 3, 1994, in San Francisco, California, under the auspices of the Joint TMS-SMD/ASM-MSD Composite Materials Committee. METALLURGICAL AND MATERIALS TRANSACTIONS A
where d a / d N is the crack growth rate and C and m are the Paris coefficient and exponent, respectively. It adequately describes the growth of long cracks in the linear portion of their sigmoidal log-log-scaled d a / d N vs A Jr relation (Figure 1). It holds for monolithic metals and alloys as well as for some composites. In this linear region, long crack growth rates are empirically fitted to a power law curve. For long cracks, the Paris equation overestimates d a / d N for lower values of AK approaching the long crack growth threshold (z~kKth), [2'3'4] Nevertheless, for short cracks, it is still often extended into and beyond the "thresholded" regime (dashed line in Figure 1). The validity of extending the Paris equation to microcracks is controversial. It is well known that short crack growth rates can deviate in both directions from those predicted by the unthresholded Paris equation governing long cracks in the same material. Since most of the life of a crack is
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