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Experimental results are presented on subcritical crack growth under sustained and cyclic loads in a HIPed Si3 N4 at 1450°C and a hot–pressed Si3 N4 –10 vol% SiCw composite in the temperature range 1300°–1400°C. Static and cyclic crack growth rates are obtained from the threshold for the onset of stable fracture with different cyclic frequencies and load ratios. Fatigue crack growth rates for both the monolithic and SiCw -reinforced Si3 N4 are generally higher than the crack growth velocities predicted using static crack growth data. However, the threshold stress intensity factor ranges for the onset of crack growth are always higher under cyclic loads than for sustained load fracture. Electron microscopy of crack wake contact and crack–tip damage illustrate the mechanisms of subcritical crack growth under static and cyclic loading. Critical experiments have been conducted systematically to measure the fracture initiation toughness at room temperature, after advancing the crack subcritically by a controlled amount under static or cyclic loads at elevated temperatures. Results of these experiments quantify the extent of degradation in crack–wake bridging due to cyclically varying loads. The effects of preexisting glass phase on elevated temperature fatigue and fracture are examined, and the creep crack growth behavior of Si3 N4 –based ceramics is compared with that of oxide-based ceramics. 相似文献
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Retardation of Fatigue Crack Growth in Ceramics by Glassy Ligaments: A Rationalization 总被引:1,自引:0,他引:1
Upadrasta Ramamurty 《Journal of the American Ceramic Society》1996,79(4):945-952
In high-temperature fatigue crack growth (FCG) experiments on ceramic materials containing amorphous grain boundary phases, the crack growth rates under cyclic loads were observed to be lower than those predicted solely on the basis of crack growth velocities measured under static loads. In this paper, a rationalization was offered for such a behavior by means of a phenomenological glass-bridging model which takes the relaxation behavior of glass into account. In ceramics which exhibit subcritical crack growth through cavitation ahead of the crack tip, the maximum stress intensity factor of the fatigue cycle required to initiate FCG was observed to be always greater than or equal to the threshold stress intensity factor for crack growth under sustained far-field loads. This trend was also explained with the aid of the glass-bridging model and invoking the equivalence between bridging and damage zones. The elevated temperature FCG behavior of nitride-based ceramics which exhibit grain bridging in the wake during crack propagation was discussed and contrasted with oxide-based ceramics which show glass bridging. 相似文献
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The fracture toughness of bulk metallic glasses 总被引:1,自引:0,他引:1
Jian Xu Upadrasta Ramamurty Evan Ma 《JOM Journal of the Minerals, Metals and Materials Society》2010,62(4):10-18
Stiffness, strength, and toughness are the three primary attributes of a material, in terms of its mechanical properties.
Bulk metallic glasses (BMGs) are known to exhibit elastic moduli at a fraction lower than crystalline alloys and have extraordinary
strength. However, the reported values of fracture toughness of BMGs are highly variable; some BMGs such as the Zr-based ones
have toughness values that are comparable to some high strength steels and titanium alloys, whereas there are also BMGs that
are almost as brittle as silicate glasses. Invariably, monolithic BMGs exhibit no or low crack growth resistance and tend
to become brittle upon structural relaxation. Despite its critical importance for the use of BMGs as structural materials,
the fracture toughness of BMGs is relatively poorly understood. In this paper, we review the available literature to summarize
the current understanding of the mechanics and micromechanisms of BMG toughness and highlight the needs for future research
in this important area. 相似文献
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The addition of small amounts of B to Ti–6Al–4V alloy reduces the as-cast grain size by an order of magnitude and introduces TiB phase into the microstructure. The effects of these microstructural modifications on both the high cycle fatigue and cyclic stress–strain response were investigated. Experimental results show that B addition markedly enhances the fatigue strength of the alloy; however, the influence of prior-β grain size was found to be only marginal. The presence of TiB particles in the matrix appears to be beneficial with the addition of 0.55 wt.% B to Ti–6Al–4V enhancing the fatigue strength by more than 50%. Strain-controlled fatigue experiments reveal softening in the cyclic stress–strain response, which increases with the B content in the alloy. Transmission electron microscopy of the fatigued specimens indicates that generation of dislocations during cyclic loading and creation of twins due to strain incompatibility between the matrix and the TiB phase are possible reasons for the observed softening. 相似文献
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U. Ramamurty F. W. Zok F. A. Leckie 《Metallurgical and Materials Transactions A》1997,28(12):2731-2740
The utility of Ti cladding in alleviating the notch sensitivity of both the monotonic and cyclic tensile properties of Ti
matrix composites (TMC) has been examined. Experiments have been conducted on panels with two different clad thicknesses as
well as on the TMC alone. Crack bridging models have been used to describe the composite behavior, incorporating explicitly
the effects of the cladding. It is demonstrated that the notched strength can be raised up to the level corresponding to the
unnotched TMC alone, with a critical clad thickness that depends on the fracture properties of the TMC and the notch size.
The fatigue threshold can be elevated also, though it cannot reach the threshold of the un-notched TMC. The bridging models
have been used to calculate the trends in the strength and the fatigue threshold with the clad thickness. 相似文献
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