Evaluation of fibre bridging stress of short fatigue cracks in SCS‐6/Ti‐15‐3 composite

Abstract

A B S T R A C T Single-edge notched specimens of a unidirectional SiC long fibre reinforced titanium alloy, were fatigued under four point bending. The propagation behaviour of short fatigue cracks from a notch was observed on the basis of the effects of fibre bridging. The branched fatigue cracks were initiated from the notch root. The fatigue cracks propagated only in the matrix and without fibre breakage. The crack propagation rate decreased with crack extension due to the crack bridging by reinforced fibres. After fatigue testing the loading and residual stresses in the reinforced fibres were measured for the arrested cracks by the X-ray diffraction method. The longitudinal stresses in the reinforced fibres were measured using high spatial resolution synchrotron radiation. A stress map around the fatigue cracks was then successfully constructed. The longitudinal stress decreased linearly with increasing distance from a location adjacent to the wake of the matrix crack. This region of decreasing stress corresponded to the debonding area between the fibre and the matrix. The interfacial frictional stress between the matrix and the fibre could be determined from the fibre stresses. The bridging stress on the crack wake was also measured as a function of a distance from a notch root. The threshold stress intensity factor range, corrected on the basis of the shielding stress, was similar to the propagation behaviour of the monolithic matrix. Hence the main factor influencing the shielding effect in composites is fibre bridging.

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

“…Indeed their experimental work shows crack growth until ultimate arrest at DK App = 10 MPa √ m for Ti-6Al-4V/Sigma SiC fibre composite. Akiniwa et al (2007) found fatigue threshold values below 10 MPa √ m for Ti-15-3/SCS-6 SiC composites. It is reassuring that the three methods for evaluating the crack-tip stress intensity range deliver similar conclusions; however, there are still outstanding questions.…”

Determination of SIFs, crack‐tip coordinates and crack angle of anisotropic materials

“…1) [a] = compliance matrix (strain-stress relationship) C = cross-correlation coefficient defined in Eq. (37) D = the magnitude of a search distance to find crack-tip coordinates [D] and [E] = matrices obtained from Eqs (9) to (12) for finding displacement E 11 , E 22 = Young's modulus in the axes of material symmetry (axes 1 and 2 shown in Fig. 1) E a and E b = the errors of the crack-tip coordinates and angle b defined in Eq.…”

“…1 x * , y * = deformed coordinates at selected points for the image-correlation method z i (i = 1,2) = x + μ i y α = the crack angle in specimens from the global X axis to the notch x axis as shown in Fig. 2 β = vector of the crack constants determined by the geometry and applied forces {ε} and {σ } = strain and stress vectors μ 1 and μ 2 = the distinct roots of Eq. (4) ν 12 = Poisson's ratio in the axes of material symmetry 1 and 2 = complex stress functions…”

“…The odd terms of a series solution was deduced to find SIFs with acceptable accuracy. Akiniwa et al 12 performed the fatigue test of single‐edge notched specimens of a unidirectional fibre reinforced titanium alloy, and after fatigue tests, the loading and residual stresses in the reinforced fibres were measured for the arrested cracks by the X‐ray diffraction method.…”

“…The experimental images were analysed by a boundary value method. Seif 12 performed the fatigue test of single-edge notched specimens of a unidirectional fibre reinforced titanium alloy, and after fatigue tests, the loading and residual stresses in the reinforced fibres were measured for the arrested cracks by the X-ray diffraction method. In this study, the crack-tip coordinates, the axes of measurement and the SIFs for a cracked anisotropic plate are determined using image-correlation experiments.…”