Cracking mechanisms in thermally cycled Ti-6AI-4V reinforced with SiC fibers |
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Authors: | S H Thomin P A NoËl D C Dunand |
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Affiliation: | (1) Ingénieur de l’'Armement, Délégation Générale pour l’Armement, Centro d’'Essais Aéronautique de Toulouse, 31056 Toulouse, France;(2) New Processes and Materials Department, Matra Défense, 78140 Vélizy, France;(3) Department of Materials Science and Engineering, Massachusetts Institute of Technology, 02139 Cambridge, MA |
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Abstract: | A titanium alloy (Ti-6A1-4V) reinforced with continuous SiC fibers (SCS-6) was thermally cycled between 200 ‡C and 700 ‡C
in air and argon. The composite mechanical properties deteriorate with an increasing number of cycles in air because of matrix
cracks emanating from the specimen surface. These cracks also give oxygen access to fibers, further resulting in fiber degradation.
The following matrix cracking mechanisms are examined: (1) thermal fatigue by internal stresses resulting from the mismatch
of thermal expansion between fibers and matrix, (2) matrix oxygen embrittlement, and (3) ratcheting from oxide accumulating
within cracks. Matrix stresses are determined using an analytical model, considering stress relaxation by matrix creep and
the temperature dependence of materials properties. Matrix fatigue from these cycli-cally varying stresses (mechanism (1))
cannot solely account for the observed crack depth; oxygen embrittlement of the crack tip (mechanism (2)) is concluded to
be another necessary damage mechanism. Furthermore, an approximate solution for the stress intensity resulting from crack
wedging by oxide formation (mechanism (3)) is given, which may be an operating mech-anism as well for long cracks.
S.H. THOMIN, formerly Graduate Student, Department of Materials Science and Engineering, Massachusetts Institute of Technology,
Cambridge, MA |
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