Browsing by Author "Moffatt, JE"

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    Effect of frequency on high-temperature fatigue crack growth in a silicon carbide reinforced silicon nitride composite
    (Elsevier, 2013-02-01) Moffatt, JE; Fitzpatrick, ME; Edwards, L
    A detailed study on a silicon nitride reinforced with silicon carbide whiskers, Si3N4SiCW, has been undertaken at elevated temperature during static and dynamic loading at increasing K and ΔK respectively. It is shown that cyclic sub-critical crack growth rates are lower than static crack growth rates. The increased crack growth rate during static far field loading is attributed to the stress relaxation of the inter-granular glass phase which allows time-dependent processes to occur ahead of the crack tip which lead to enhanced sub-critical crack growth rates. During cyclic fatigue the glass phase has insufficient time to relax and glassy ligaments are able to bridge the crack wake thereby shielding the crack tip from the full force of the applied load. Also, at particular temperatures, bridging between the surfaces of the crack wake by the inter-granular glass phase results in increased strength and fatigue retardation. The extent of ‘crack wake healing’ is shown to be time and temperature dependent. The viscosity of the glass phase is directly related to the temperature and the bonding force associated with glass phase bridging is observed to reduce with increasing temperature. The results from a previous study at room temperature are compared to those found during this investigation. © 2012, Elsevier Ltd.
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    Fatigue and fracture of a 316 stainless steel metal matrix composite reinforced with 25% titanium diboride
    (Elsevier, 2013-03-01) Bacon, DH; Edwards, L; Moffatt, JE; Fitzpatrick, ME
    Fatigue and fracture mechanisms have been studied in a steel-based metal matrix composite (MMC),comprising a 316L austenitic matrix reinforced with 25 wt.% particulate titanium diboride (TiB2). The fracture toughness was determined in the as-HIPped condition as being slightly below 30 MPapm. Fatigue crack growth rates have been determined, and corrected for the effects of crack closure. The fracture surfaces have been studied to determine the mechanisms of damage during crack advance, which are determined as matrix fatigue, reinforcement particle fracture, and ductile rupture of the matrix. We show that the occurrence of damage mechanisms during fatigue of the material is linked to Kmax, rather than to DK. This is rationalised in terms of a semi-cohesive process zone within the monotonic plastic zone ahead of the crack tip. © 2012, Elsevier Ltd.