A. Guvenilir

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

Abstract 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 μm 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√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 samples 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.

Damage in aligned-fibre SiC/Al quantified using a laboratory X-ray tomographic microscope

Abstract A first-generation laboratory X-ray tomographic microscope is used to non-destructively ‘section’ a continuous, aligned-fibre SiC/Al metal-matrix composite (MMC). Damage in the MMC associated with mechanical deformation is the principal focus of the study. Two types of deformation are examined: wedge loading (with and without load) and three-point bending. Quantification of crack opening and fibre fracture detection is found to be practical down to one-tenth of a pixel in the reconstructed sections

Application of X-Ray Microtomography in Materials Science Illustrated by a Study of a Continuous Fiber Metal Matrix Composite

The advantages of the use of x-ray microtomography in materials science are discussed, and illustrated by the nondestructive study of the mechanical damage in a continuous fiber SiC/Al composite at a resolution of about 25 μm. A laboratory x-ray source was used, and it was shown that quantitative measurements of the linear absorption coefficient at this resolution are possible, even though the AgKα radiation used is accompanied by a considerable amount of white radiation, provided that the counter system is properly corrected for pulse pile up and dead-time.

Nondestructive Imaging of Materials Microstructures Using X-Ray Tomographic Microscopy

A technique for nondestructively imaging microstructures of materials in situ, especially a technique capable of delineating the time evolution of chemical changes or damage, will greatly benefit studies of materials processing and failure. X-ray tomographic microscopy (XTM) is a high resolution, three-dimensional inspection method which is capable of imaging composite materials microstructures with a resolution of a few micrometers. Because XTM is nondestructive, it will be possible to examine materials under load or during processing, and obtain three-dimensional images of fiber positions, microcracks, and pores. This will allow direct imaging of microstructural evolution, and will provide time-dependent data for comparison to fracture mechanics and processing models. 23 refs., 8 figs.

X-ray microbeam quantification of grain subdivision accompanying large deformations of copper

This work reports the application of X-ray microbeam diffraction to quantifying grain subdivision processes in copper. Polychromatic synchrotron X-radiation was used to study samples in the as-received (low deformation) and 100% torsion strained material. The large range of domain disorientations (within individual grains) observed in the highly strained material agrees with results on other f.c.c. materials obtained by electron beam methods; it is not surprising, therefore, that models of texture development which do not include this effect predict too rapid sharpening of preferred orientation compared to experimental pole figures.

Quantitative Characterization of Damage in a Composite Material Using X-Ray Tomographic Microscopy

A laboratory x-ray tomographic microscope (XTM) is used to nondestructively “section” a continuous, aligned-fiber SiC/Al metal matrix composite (MMC). Damage in the MMC associated with mechanical deformation is the principal focus of the study. Three types of deformation are examined: wedge loading (with and without load), three-point bending and tensile loading.

Impact of X-Ray Tomographic Microscopy on Deformation Studies of a SiC/Al MMC

Damage in a continuous, aligned-fiber SiC/Al metal matrix composite (MMC), e.g. fiber fracture, fiber-matrix interphase microcracking, intra-ply matrix voids and cracks, is examined with synchrotron x-ray tomographic microscopy (XTM). Quantitative three-dimensional measurements of damage are reported in as-fabricated and monotonically loaded SiC/Al. The XTM results indicate that increases in observed macroscopic structural stiffness during the first few fatigue cycles of an MMC coupon correspond to elimination of processing-related matrix porosity and to displacement of the fibers from a somewhat irregular arrangement into a more nearly hexagonal array. The XTM data also show that the carbon cores of the SiC fibers begin to fail at or below 828 MPa, that is, at loads far less than those for fracture of the entire fiber. The implications of these results and of the use of in situ loading for fatigue damage quantification are also discussed for mechanical/thermal modelling.

Computed tomography part III: Volumetric, High-Resolution X-Ray analysis of fatigue crack closure

Although single-slice computed tomography is a useful tool in materials science and engineering, there are some materials problems that are better understood when represented with volumetric data. This article describes the use of a two-dimensional detector system to examine fatigue crack closure in an aluminum-lithium alloy.

Micro-, Meso- and Macro-Texture and Fatigue Crack Roughness in Al-Li 2090 T8E41

The use of synchrotron polychromatic x-ray microbeams in the transmission geometry is described for mapping grain orientation as a function of position and for relating this microtexture to the formation of large asperities on fatigue crack surfaces in Al-Li 2090 T8E41. In common with the centers of rolled plates of many aluminum alloys, Al-Li 2090 T8E41 has a sharp average texture or macrotexture different from that in the outer portions of the plate. The geometry of large asperities in Al-Li 2090 has been related to this macrotexture, and the resulting roughness-induced crack closure is recognized to be responsible for the very low crack propagation rates in certain plate orientations. This report focuses on why asperities form at certain positions and why the crack remains relatively planar elsewhere. The microtexture (i.e., the grain-to-grain orientation variation) seems to be organized into a specific type of mesotexture: multiple adjacent grains have nearly identical orientations and form substantial volumes of near-single-crystal material. Transitions between differently oriented near-singlecrystal volumes or between a near-single- crystal region and more randomly oriented grains appear to bound asperities.