New direct observations of crack closure processes in Al-Li 2090 T8E41

Earlier, crack opening as a function of position within an optically opaque notched tensile sample (NT-3) of Al-Li 2090 was reported for four loads by Guvenilir and co-workers; these measurements were made with high-resolution X-ray computed tomography. This paper reports the same type of results for a second notched tensile sample (NT-4): the authors believe this to be the second sample whose crack closure was quantified as a function of position. The crack within NT-4, was found to be more planar than, and to close differently to, that in NT-3. in NT-4, the crack 'zipped' shut from the tip; this is in contrast to the earlier results on the asperity-dominated crack in NT-3, where contact occurred primarily behind the open crack tip. During unloading of NT-4, the crack faces came into contact above the closure load defined by the intersection of tangent lines to the upper and lower portions of the load-deflection curve; this is similar to previous observations in sample NT-3. Estimates of d alpha/dN as a function of Delta K for samples NT-3 and NT-4 agree with others' measurements, which suggests that the closure observations of this paper also apply to compact tension samples of Al-Li 2090.