Direct observation of crack opening as a function of applied load in the interior of a notched tensile sample of Al-Li 2090

Results of in situ high resolution X-ray computed tomography are reported for a notched tensile sample of Al-Li 2090 T841. The fatigue crack within the interior of the sample is imaged with 6 mu m voxels as a function of applied load, and the crack face morphology is found to be similar to that observed in compact tension samples of this alloy. The loads and approximate stress intensities at which the tomography data were obtained were 82, 50, 25 and 5 kg and 7.1, 4.3, 2.2 and 0.4 MPa root m, respectively. Crack openings measured during unloading as a function of position show that physical closure at portions of the crack tip and at positions behind the crack tip precedes (during unloading) the bend in the sample's load-displacement curve. The three-dimensional pattern of crack opening shows substantial mixed mode I-III contact on the faces of asperities behind the crack tip, even at the maximum load of the fatigue cycle. Mixed mode I-II contact is also observed at loads above the bend in the load-displacement curve. The fraction of voxels open remains nearly constant for the loads immediately above and below the nominal closure load, as determined from the load-displacement curve, of 41 kg; and this suggests that these mixed mode I-III surfaces begin to carry significant load at the point where the load-displacement curve starts to deflect, and is the source of the apparent stiffening of the sample at loads below the nominal closure load. (C) 1997 Acta Metallurgica Inc.