Accurate experimental characterization of the crack closure process is important for a number of reasons, the most important of which may be for its use in developing materials and structures that are resistant to the growth of fatigue cracks. This paper describes the results of an investigation aimed at quantifying the point of crack closure. An α/β titanium alloy, Ti-6242S, was chosen for this investigation because its microstructure can be manipulated to produce large variations in the level of asperity-induced crack closure and is thus an ideal model alloy for understanding this important crack closure mechanism. Comparisons of two “global” compliance techniques (backface strain and crack mouth opening displacement) and a near crack-tip surface technique (two-stage replication) were made. In tests conducted at low stress intensities, all three methods gave nearly identical measurements of crack closure. For the two compliance techniques, different numerical procedures were used to determine the inflection point in the load-displacement curve. The magnitude of and variations in crack closure determined using these compliance methods were found to be strongly dependent on the numerical procedure used. Studies of crack-tip opening showed that in α/β-processed Ti-6242S the crack tip is tightly closed even at positive loads, indicating that plasticity induced closure is the predominant form of closure. In contrast, measurements in β-processed material showed that the crack tip is propped open even when no load is applied to the sample, indicating that closure in this microstructure is asperity-induced.

In recent years, the dominant role of fatigue crack closure in determining fatigue crack growth behavior in metals has been recognized. Attention is now turning to more precise quantification of this process and to the application of standardized measurement techniques. This paper reviews the various techniques which have been used to measure the crack opening stress intensity KOP. These include thickness-averaging compliance methods, indirect methods, various techniques which measure crack closure on the surface near the crack tip, and techniques for measuring crack closure in the interior of thick specimens. The practical and fundamental advantages and limitations of these methods are discussed. Of these techniques, the thickness-averaging compliance techniques (back-face strain and crack mouth opening displacement methods) appear to provide reproducible and practical global measurements of the average closure response and are the most logical choices for standardization. The interferometric displacement gage and direct scanning electron microscope measurements although difficult to standardize, allow more detailed studies of local crack-tip closure processes. The increased use of techniques which measure closure in the specimen interior is important for understanding the three-dimensional nature of crack closure. Finally, this paper reviews selected crack closure data measured using thickness-averaging compliance techniques in several different alloy systems. Even when such “standard” techniques are used, significant variations in crack closure are observed, and general patterns for crack closure are difficult to discern. This points to the need for further standardization of measurement techniques and numerical data analysis methods.

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The development of a material model to estimate the S-N response of a cast aluminum alloy is described. An important requirement of such a model is sensitivity to processing effects such as solidification time and subsequent material characteristics such as secondary dendrite arm spacing (SDAS), strength, and porosity. It has been speculated that the fatigue properties of aluminum castings are dominated by the propagation of cracks which initiate from microshrinkage pores ranging in size from ∼ 10 to 1000 μm. A model was developed that calculates fatigue life based on small crack growth rates measured in cast 319 aluminum specimens with two different solidification times. Results from this small crack model are compared with predictions from a traditional LEFM model using long-crack data and measured thresholds.

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