Observation of short fatigue crack-growth process in SiC-fiber-reinforced Ti-15-3 alloy composite
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I.
INTRODUCTION
TITANIUM matrix composites are now being evaluated as structural materials for applications that require high specific modulus and specific strength. Such applications often require resistance to a repeated mechanical loading condition. Recent articlestt ~Ol show the experimental details of the characteristic fatigue crack growth mechanisms in SiC(SCS-6) fiber-reinforced Ti alloy matrix composites. SiC(SCS-6) fiber-reinforced Ti- 15-3 alloy matrix composite and other composites are known to have good resistance to fatigue damage.I5-~~ The majority of fatigue articles on the SiC fiber-reinforced Ti-15-3 composite deal with a long crack and show the dominant mechanism is fiber bridging behind a tip of the matrix crack. Fatigue behavior under the short crack is not well known. The composite usually contains weak fiber; fracture of a weak fiber generates a stress concentration source during service condition that might act as a short crack for further repeated loading. Thus, as usually recognized in fatigue characteristics of Ti alloys, the short-crack fatigue behavior seems different from that of the long-crack one.
We reported t~ the damage evolution process of unnotched SiC(SCS-6) fiber-reinforced Ti- 15-3 composite and revealed that fiber fracture and matrix fatigue crack growth play important roles in predicting the fatigue life of the composite. The detailed mechanisms of initiation and propagation are not yet clearly understood, however. The objective of the current work was to investigate the microscopic
S.Q. GUO, Graduate Student, Faculty of Engineering, and Y. KAGAWA, Associate Professor, and K. HONDA, Associate Researcher, Institute of Industrial Science, are with the University of Tokyo, Tokyo 106, Japan. Manuscript submitted June 8, 1995. METALLURGICAL AND MATERIALS TRANSACTIONS A
fatigue damage process and short fatigue crack growth behavior of unnotched SiC(SCS-6) fiber-reinforced Ti-15-3 alloy composite at room temperature. The obtained fatigue crack growth resistance of the composite is discussed along with the microscopic damage evolution process of the composite.
II.
EXPERIMENTAL PROCEDURE
A. Composite Material
The composite material used throughout this study was a metastable titanium alloy (Ti-15V-3Cr-3A1-3Sn, hereafter denoted as Ti-15-3) reinforced with unidirectionally aligned continuous SiC fiber (SCS-6 Textron Corp., Lowell, MA). The chemical composition of the Ti- 15-3 alloy was V 15.22 wt pct, Cr 3.26 wt pct, A1 3.12 wt pct, Sn 2.94 wt pct, and the remainder Ti. Some properties of the fiber and matrix are listed in Table I. The SiC(SCS-6) fiber was approximately 140 /xm in diameter and was coated with ~3-/xmthick alternating outer layers of carbon and nonstoichiometric SiC, which protected the fiber from cracking during handling and consolidation.tt2,t3,14IThe Ti-15-3 alloy is a/3 (bcc) phase stabilized by vanadium, and the measured average grain size after fabrication was - 1 0 0 /.~m. Metastability manifests through precipitation of hexagonal oJ and a phase in the
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