Acta Materialia 51 (2003) 1045–1057 www.actamat-journals.com Measurements of ﬁbre bridging during fatigue crack growth in Ti/SiC ﬁbre metal matrix composites M. Preuss a,* , G. Rauchs a , T.J.A. Doel b , A. Steuwer a , P. Bowen b , P.J. Withers a a Manchester Materials Science Centre, University of Manchester, Manchester M1 7HS, UK b Structural Materials Research Centre, School of Engineering, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK Received 17 October 2002; accepted 25 October 2002 Abstract High spatial resolution synchrotron X-ray strain mapping has been used to map the elastic matrix and ﬁbre strains in the vicinity of a fatigue crack in a Ti–6Al–4V/SCS6 SiC ﬁbre composite. A 0.61 mm fatigue crack was initiated and grown in three-point-bending. By using an in-situ loading stage it was possible to map the crack opening (longitudinal) strain distribution at K appl = K max and K appl = 0. In the far ﬁeld region, signiﬁcant thermally induced stresses were evident, being compressive in the ﬁbres and tensile in the matrix. Around the notch and in the wake of the crack tip essentially no residual strain and only small interfacial shear stresses were found in the unloaded case, indicative of a debonded/damaged interface. At K max the maximum tensile stress in the matrix is in the vicinity of the crack tip, whereas for the SiC ﬁbres the maximum stress is in the bridging zone in the wake of the crack. The perturbed zone extends about ±1.5 mm either side of the crack. It was at the boundary of this zone that the maximum interfacial shear stresses (80 MPa) were measured in the loaded stage. A small area of tensile strain in front of the crack tip in the unloaded condition suggests frictional resistance from the bridging ﬁbres acts to keep the crack slightly open. A simple three-dimensional ﬁnite element model has been developed to help interpret the results. The crack is introduced statically by node release and the Coulomb friction law governs the interface strength. The results of the model are compared to the synchrotron strain measurements. This comparison conﬁrms the degradation of the interface strength in the wake of the crack.  2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. Keywords: Metal matrix composite; Finite element modelling; Fatigue crack growth; Fibre bridging; Interfacial shear stress 1. Introduction Metal matrix composites can achieve excellent high temperature properties compared to unre- * Corresponding author. Tel.: +44-161-2008959; fax: +44- 161-2003586. E-mail address: michael.preuss@man.ac.uk (M. Preuss). 1359-6454/03/$30.00  2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. doi:10.1016/S1359-6454(02)00508-6 inforced Ti alloys, making them attractive for the cooler parts of aeroengines. Furthermore, unidirec- tional ﬁbre reinforcement can improve static and fatigue crack growth resistance of both metallic and intermetallic materials. Experiments have shown that crack-tip shielding by ﬁbre bridging can lead to crack arrest [1–3]. The cyclic fatigue of this material is also improved by the addition of unidirectional ﬁbres to the matrix material.