Evolution of crack-bridging and crack-tip driving force during the growth of a fatigue crack in a Ti/SiC composite

Abstract

High spatial resolution diffraction and imaging using synchrotron X-rays are combined to monitor the incremental growth of a fatigue crack through the matrix of a Ti-6Al-4V/SCS-6 SiC monofilament metal matrix composite. X-ray tomography is used to quantify the crack opening displacement (COD) and diffraction to measure the crack-tip stress field in each phase, the wear degraded interfacial strengths, as well as the crack face tractions applied by the bridging fibres, at maximum (K max ) and minimum (K min ) loading as a function of crack length. In this way, it has been possible to quantify the crack-tip driving force (the stress intensity range effective at the crack-tip) in three ways: from the COD, the bridging stresses and the crack-tip stress field. The fibre stresses act to prop open the crack at K min and shield the crack at K max such that the change in COD is small over the fatigue cycle. Consequently, the effective stress intensity range at the crack tip remains around 10 MPa √ m as the crack lengthens, as more and more fibres bridge the crack despite the normally applied stress intensity rising to 60 MPa √ m. The implications of the derived fracture mechanics parameters are assessed and the wider potential of X-ray diffraction and imaging for crack-tip microscopy is discussed.

Quantitative X-ray tomography

“…Stress intensity factor: the PT and DVC methods can be used to provide a measure of the total displacement/ strain field around a fatigue crack. 256,314 This method was first used by Toda et al 306 to derive the local mode I, II and III crack driving forces K I , K II , K III from the local displacement vector u(x,y,z)…”

“…In this case image stitching strategies 184 were employed to allow a sample, 4 mm in size, to be viewed using a 1?4 mm pixel size. 256 Often, 3D imaging is best considered as part of a multi-scale imaging strategy. For example, micro-CT has been powerfully complemented by FIB serial sectioning and electron tomography to characterise the pore structure in catalysts across four orders of magnitude 257 as well as clays.…”

Study of short cracks under biaxial fatigue

“…Subsequently 20 grey values were added the minimum value to account for small intensity differences occurring within the crack. This information was employed for binarising the image, as has been done in previous works [13]. The binarised image was then analysed with an edge-finding routine to detect the edges which coincide with the crack edges ( Fig.…”

In situ imaging and strain determination during fracture in a SiC/SiC ceramic matrix composite

“…Most attention has been devoted to unidirectional metal matrix composites containing aligned ceramic monofilaments (>100 lm), since the diameter of the reinforcing phase is well over the typical spatial resolution in microdiffraction setups allowing for determination of strain on individual monofilaments [7,8], as well as fiber bridging and sliding stresses [9,10]. Current generation CMCs, alternatively, are reinforced by yarns of fibers $10 lm in diameter woven at right angles, and strain determination in a statistically significant number of individual fibers is impossible with current setups.…”