In situhigh resolution synchrotron x-ray tomography of fatigue crack closure micromechanisms

Abstract

Fatigue crack closure has been identified as an important factor in determining crack growth rates. However, the methods of measurement of crack closure remain the subject of ongoing controversy. To date, computed finite element models, analytical models and widely established compliance-based experimental methods have offered limited micromechanical insight and/or direct information on the active crack tip region within bulk material. To understand the absolute contributions of crack closure mechanisms, such as plasticity-induced and roughness-induced closure, to fatigue properties, an internal, three-dimensional insight into crack behaviour during loading and unloading is clearly of value. In this work, synchrotron radiation x-ray microtomography is carried out at a high resolution of 0.7 µm to provide unique three-dimensional in situ observation of steady state plane strain fatigue crack growth in a 2024-type Al alloy (Al-Cu-Mg-Mn). Using such high resolution imaging (additionally exploiting the phase contrast effect in interface imaging), the details of fatigue cracks are readily observed, along with the occurrence of closure. A novel microstructural crack displacement gauging method is used to quantify the mixed mode character of crack opening displacement and the closure effect. A liquid gallium grain boundary wetting technique is used in conjunction with the microtomography to visualize the correlation between the three-dimensional structure of the grains and fatigue crack behaviour. Subsequently, electron backscattering diffraction assessment of the grain orientation on the samples provides a uniquely complete 3D description of crack-microstructure interactions.

Quantitative X-ray tomography

“…In recent years considerable attention has been focussed on using propagation phase contrast 292 to enhance the detectability of cracks. 293,294 While this does aid crack detection, because of the Fresnel edge contrast, it can be more difficult to quantify the opening of the crack. 295 Wright et al 103 report a crack sensitivity of 14% of a pixel for polymer composites with phase contrast.…”

“…298,299 Subsequent work 283 substantiates this showing that in many cases loss of crack face contact occurs gradually up to the maximum load rather than at a single definable opening level. For AA, Khor et al 294 20 In contrast to digital volume correlation (DVC), discrete particle tracking (PT) does not treat the material as a continuum and so can identify individual particle rotations for example. Here approx.…”

Stochastic Virtual Tests for High-Temperature Ceramic Matrix Composites

“…Information concerning matrix voids, individual fibers, and fiber coatings also emerges for even smaller scales, down to 1 μm (10,11,72). Figure 3a shows typical 2D image slices of a 3D woven carbon/SiC composite, and Figure 3b shows a 3D image of a unit cell.…”

Application of Diffraction-Amalgamated Grain Boundary Tracking to Fatigue Crack Propagation Behavior in High Strength Aluminum Alloy

“…By adding pencil-beam X-ray diffraction (XRD) measurement, Diffraction-Amalgamating Grain Boundary Tracking (DAGT), 4) which provides accurate information about grain morphologies and individual grain orientation in fourdimensions (4D), has been developed. Khor et al 5,6) utilized the electron backscattering diffraction (EBSD) assessment combination with CT for the study of crack closure processes associated with crystallographically rough long crack path in commercial airframe aluminum alloy 2024-T351. The results revealed that the fatigue crack appears crystallographic deflections and the region of crack closure associated with a specific orientated ridge.…”