MicroCT (microtomography) quantification of microstructure related to macroscopic behaviour: Part 2 – Damage in SiC–Al monofilament composites tested in monotonic tension and fatigue

Abstract

The present paper reports results of monotonic tensile and of fatigue testing of an aligned monofilament composite SiC-Al. Macroscopic measures of damage (changes in unloading compliance and in unrecovered strain) correlate with noninvasive X-ray microCT (microtomography) quantification of microstructure (alteration of fibre separation, fibre misorientation relative to the load axis and amount of fibre fracture). Monotonic loading beyond yield (188 MPa) alters fibre arrangement and reduces fibre misorientation. Fracture of fibre carbon cores begins at 828 MPa and increases rapidly with rising stress concomitant with marked unloading modulus decreases and permanent strain increases. Fatigue at 621 MPa produces gradually increasing modulus and unrecovered strain with no evidence of fibre damage. At low cycles, the modulus, permanent strain and fraction of fractured C cores increase while fibre misorientation decreases; above 16 cycles, modulus decreases with gradual increases in the other quantities. Fracture in SiC could only be observed after the specimen broke.

Quantitative X-ray tomography

“…294 observed the crack surface topology, bifurcation and tip geometry using gallium to delineate the grain boundaries (which are usually invisible to CT) to understand the interaction of the crack with the grain structure. A number of crack closure studies have followed for Al–SiC 300 and Ti–SiC composites, 301 cast iron 302,303 and AAs. 304 It is important to remember that closure is believed to influence crack growth though the extent to which it shields the crack-tip from the full range of the stress intensity range.…”

In situ X-ray microtomography characterization of damage in SiCf/SiC minicomposites

“…As reviewed by Stock [14], microtomography has been successfully used in material research. In particular, it has been used to study damage or fatigue cracks [15] in several materials such as fiber reinforced metal [16,17,18] or polymer [19,20,21] matrix composites, aluminium alloys [22,23] or polymers (PMMA) [24]. However, tomography applied to SiCf/SiC composites has been limited to porosity observations [25,26,27], crack observations requiring a very high resolution because openings are smaller than 1 µm.…”

Imaging of voids due to deformation in alloy steel using micro-focus X-ray beam

“…Lately these facilities (e.g. ID15A beam line in ESRF [16]), have been augmented with state of the art components/sub-systems such as suitably designed in situ deformation devices, high efficiency phosphorous screen, reflecting microscope objective, CCD detector with high energy wiggler source, all of which combine to enable the tomographic imaging in dense structural materials within realistic time frames during the real time application of deformation [4][5][6][7][8][9][10][11][12][13][14][15][16][17]. Further the additional benefits of combining tomography with diffraction to yield additional information on the evolution of voids along with recording the attendant microstructural changes such as texture, dislocation density, local internal stresses, strains etc.…”

“…As a result various phenomenological relations predicting degradation of functional properties of structural materials such as steels depend on the ability of sample preparation technique to preserve the integrity of void structure developed in the material as a result of damage process. X-ray micro-tomography (l-CT) has emerged in recent times as a tool to evaluate non-destructively in a quantitative manner the internal volume structure of materials [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18]. The availability of third generation synchrotron radiation beam facilities, having lateral coherence of 100 lm and monochromaticity of about 0.5 Å, [3][4][5][6][7] have led to an upsurge in the investigations into the fundamental processes governing the onset of de-cohesion damage (as for example by nucleating voids and their coalescence) [8][9][10][11][12][13] and progression of crack growth (as for example effect of crack closure) [14,15] in light metal alloys.…”