Evaluation of a Crack-Growth Gage for Monitoring Possible Structural Fatigue-Crack Growth

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Abstract
The results of an experimental and analytical investigation on the use of a precracked coupon for monitoring the effect of service loads upon fatigue crack growth in a structure are discussed. The precracked coupon or “gage” is a simple device which provides a convenient means for determining the potential damage, that is, crack growth, in a structure since crack extension in the gage is a result of loads on the structure which cause the structural damage. Experimental results are reported for gages made from two aluminum alloys and having two types of crack geometries. These gages contained either a center crack or a single crack at the edge of a hole. The effect of load amplitude upon growth of the crack in the gage as a function of the crack growth in the structure was investigated. All tests were conducted under constant-amplitude cyclic load. The theoretical model is developed to predict the correlation between the growth of the cracks in the gage and in the structure. Two areas of major importance in the analysis are the load transfer from the structure through the ends of the crack gage and the crack-propagation law for the crack growth in the gage and in the structure. Comparison of the analyses and the experimental results is made.