| To understand the physical basis of crack closure on a microscopic scale, utilization of a high resolution nondestructive imaging method under in situ loading conditions is critical to correctly capturing the way in which the nominal stress intensity is reduced to a lower effective level due to "closure". X-ray tomographic microscopy, a high resolution version of medical CAT scanners, is a nondestructive technique that allows in situ examination, and tomography experiments using laboratory and synchrotron X-radiation was performed on several fatigue-crack containing notched tensile and compact tension samples of Al-Li 2090-T8E41. These imaging experiments were the first quantitative measurement of crack opening as a function of position over the entire face of the crack, and the three dimensional volume of each sample containing the fatigue crack was imaged at different loads.; The changes in crack opening at different positions along the fracture surface are followed visually and quantified from reconstructed images as a function of load. The results show that changes in crack opening for non-planar, jagged cracks present in these samples can be measured with sensitivity approaching 1 {dollar}mu{dollar}m. The crack opening measurements are compared for different loads and between different samples and are discussed with respect to the macroscopic crack closure measurements. The measurements of crack opening as a function of position and of applied load reveal that physical crack tip closure precedes the bend in the load-displacement. The three-dimensional pattern of crack opening shows substantial contact on asperity faces behind the crack tip even at 80% of the maximum stress intensity. Notched tensile sample data showed, for example, asperity contact at K = 4.3 MPa(m){dollar}sp{lcub}.5{rcub}{dollar} for K{dollar}sb{lcub}rm max{rcub}{dollar} = 5.4 MPa(m){dollar}sp{lcub}.5{rcub}{dollar} and the bend in the load-deflection curve at K = 1.8 MPa(m){dollar}sp{lcub}.5{rcub}.{dollar} From these results, it appears that increasing asperity contact behind the crack tip is responsible for the apparent stiffening of the sample at intermediate loads, not crack tip closure, and this study illustrates the impact that volumetric high resolution x-ray computed tomography can have on materials research. |
