Imaging and strain mapping fibre by fibre in the vicinity of a fatigue crack in a Ti/SiC fibre composite

Abstract

High resolution X-ray micro-tomography has been combined with high spatial resolution diffraction measurements to map the relationship between damage and elastic strain in individual fibres in the bulk of a metal matrix composite containing a matrix fatigue crack in 3D for the first time. Approximately three plies bridge the transverse matrix fatigue crack. The interfacial shear stress distributions in individual bridging fibres have been derived from the respective axial fibre strain profiles. For each fibre these profiles indicate a step jump in interfacial shear stress at the boundary between sliding and perfect bonding. This sudden decrease is taken to be the result of a sharp drop in the sliding stress due to fretting fatigue. Furthermore, the sliding stress appears to decrease towards the matrix crack. The extent of fibre sliding makes a 'V' shape in the crack wake, extending around 1?5 mm either side of the crack for ply 1. There is some evidence supporting stick-slip and a lower reverse sliding stress (about 30-50 MPa) than the forward one (about 50-60 MPa).

Interfacial shear strength behaviour of Ti/SiC metal matrix composites at room and elevated temperature

“…While SiC fibre/Ti metal matrix composites have been identified as promising materials for elevated temperature applications [2,3], perhaps surprisingly, most research has focused on characterizing the interface properties at room temperature (RT) [4][5][6][7][8][9][10][11][12]. Consequently, little is known about how the interfacial shear strength varies at elevated temperatures.…”

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

“…The elastic matrix and fibre strains parallel to the loading axis (z) were subsequently measured by diffraction for the (102) Ti and (108) SiC peaks, respectively, using a procedure developed previously (Sinclair et al 2004(Sinclair et al , 2005. The 100(x) × 40(z) mm incident beam was oriented edge-on to the crack front.…”

“…More recently, we have used synchrotron diffraction to measure the distribution of interfacial strength along bridging fibres in the wake and ahead of a fatigue crack in individual fibres (Sinclair et al 2005) or plies ). This has shown that during fatigue cycling the frictional sliding strength for fibres in the vicinity of a fatigue crack falls dramatically to around 60 MPa at RT and 30 MPa at 300…”

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

“…in Ref. [6]). Thus, in this work we propose an experimental setup that allows both imaging and diffraction to be performed sequentially and in situ during loading, examining composites with reinforcements typical in size for current CMCs.…”

“…Several synchrotron X-ray studies in composites have been performed previously, by diffraction, radiography, tomography [5] or a combination of them [6]. 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].…”