The Measurement of Crack Closure During Fatigue Crack Growth

Abstract

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.

Thermoelastic stress analysis of fatigue cracks subject to overloads

“…The phenomenon of crack closure was first studied and documented by Elber 1,2 in the 1970s when he studied the effects of crack tip plasticity on a fatigue crack in an Al 2024‐T3 sheet under cyclic tensile loading. Since then various methods including direct observation, measurement of compliance, and indirect (non‐compliance) thickness averaging techniques have been developed to study the phenomenon of crack closure 3 . Stanley and Chan 4 were probably the first researchers to consider using thermoelastic stress analysis (TSA) for studying fatigue cracks.…”

Fatigue crack growth resistance and crack closure behavior in two aluminum alloys for aeronautical applications

“…An example of this dependence is presented in Figure 1, which shows schematic variations of crack closure for different closure mechanisms 7 . During the past decades, Al-Li heat-treatable alloys have been widely investigated [8][9][10] because of their superior specific strength and modulus compared with those of the conventional aluminum alloys.…”

Methods for crack opening load and crack tip shielding determination: a review

“…The use of AE or PD methods involves different set of problems 2,3 . The reason for this is that the asperities may cause acoustical and/or electrical short circuits and both methods would register unrealistically high values of P op 2 , 100 .…”

“…The use of AE or PD methods involves different set of problems. 2,3 The reason for this is that the asperities may cause acoustical and/or electrical short circuits and both methods would register unrealistically high values of P op . 2,100 Thus, an acoustically or electrically closed/open crack is not equivalent to a mechanically closed/open crack, especially at the threshold region where the crack opening is relatively small.…”