The fracture and fatigue properties of a newly developed bulk metallic glass alloy, Zr41.2Ti13.8Cu12.5 Ni10Be22.5 (at. pct), have been examined. Experimental measurements using conventional fatigue precracked compact-tension C(T) specimens (∼7-mm thick) indicated that the fully amorphous alloy has a plane-strain fracture toughness comparable to polycrystalline aluminum alloys. However, significant variability was observed and possible sources are identified. The fracture surfaces exhibited a vein morphology typical of metallic glasses, and, in some cases, evidence for local melting was observed. Attempts were made to rationalize the fracture toughness in terms of a previously developed micromechanical model based on the Taylor instability, as well as on the observation of extensive crack branching and deflection. Upon partial or complete crystallization, however, the alloy was severely embrittled, with toughnesses dropping to ∼1 MPa
 
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 . Commensurate with this drop in toughness was a marginal increase in hardness and a reduction in ductility (as measured via depthsensing indentation experiments). Under cyclic loading, crack-propagation behavior in the amorphous structure was similar to that observed in polycrystalline steel and aluminum alloys. Moreover, the crack-advance mechanism was associated with alternating blunting and resharpening of the crack tip. This was evidenced by striations on fatigue fracture surfaces. Conversely, the (unnotched) stress/life (S/N) properties were markedly different. Crack initiation and subsequent growth occurred quite readily, due to the lack of microstructural barriers that would normally provide local crack-arrest points. This resulted in a low fatigue limit of ∼4 pct of ultimate tensile strength.

Mechanisms for fracture and fatigue-crack propagation in a bulk metallic glass

The current understanding of the underlying reasons behind the load interaction effects in fatigue crack growth under variable amplitude loading is presented. Mechanistic arguments proposed to control the load interaction phenomena are reviewed and evaluated based on their capability to qualitatively explain empirical trends in variable amplitude fatigue crack growth summarized in Part I [Fatigue Fract, Engng Mater. Struct. 1998. 21(8), 987-1006] of the present paper. Mechanisms linked to plastic straining at the crack tip enable an interpretation of the majority of the experimental results. Some observations, however, which cannot be understood in terms of plasticity-induced crack closure, or which are even in contradiction with the crack closure approach, indicate a possible role of other factors. A general conclusion is that conditions under which various phenomena can affect variable amplitude fatigue crack growth and interactions between them are insufficiently recognized.

Load interaction effects during fatigue crack growth under variable amplitude loading—a literature review. Part II: qualitative interpretation

https://estudogeral.sib.uc.pt/bitstream/10316/4272/1/file88bcce917a134cbbb926ff3fee3c83c4.pdf

Evaluation of overload effects on fatigue crack growth and closure

The ambient-temperature subcritical growth behavior of both long and microstructurally small cracks is investigated during cyclic-fatigue loading in a SiC-whisker-reinforced alumina (Al2O3–SiCw) ceramic composite (fracture toughness, Kc∼ 4.5 MPa · m1/2). Based on long-crack experiments using compact C(T) specimens, cyclic fatigue-crack growth rates (over the range 10−11 to 10−5 m/cycle) are found to be sensitive to the applied stress-intensity range and load ratio, and to show evidence of fatigue crack closure. Similar to other ceramic materials, under tension–tension loading the “long”(>3 mm) crack fatigue threshold, ΔKTH, was found to be on the order of 60% of Kc. Conversely, “small”(1 to 300 μm) cracks grown from micro-indents on the surface of cantilever-beam specimens were observed to grow at applied ΔK levels some 2 to 3 times smaller than ΔKTH. similar to behavior widely reported for metallic materials. The observed small-crack behavior is rationalized in terms of the residual stress field associated with the indent, and the restricted role of crack-tip shielding (from mechanisms such as crack bridging, closure and deflection) with cracks of limited wake, analogous to closure effects with small fatigue cracks in metals. Consistent with the lack of zone-shielding mechanisms in Al2O3–SiCw, under variable-amplitude cyclic loading crack-growth rates do not exhibit the marked transient response following block overload sequences as do transformation-toughened ceramics or ductile metallic materials. Possible mechanisms for cyclic crack advance in reinforced ceramic-matrix composite materials are discussed.

Cyclic Fatigue-Crack Growth in a SiC-Whisker-Reinforced Alumina Ceramic Composite: Long- and Small-Crack Behavior

Aluminium-lithium alloys are a class of low density, high strength, high stiffness monolithic metallic materials that have been identified as prime candidates for replacing 2000 and 7000 series aluminium alloys currently used in commercial and military aircraft. In this review, the cyclic fatigue strength and fatigue crack propagation characteristics of aluminium-lithium alloys are reviewed in detail with emphasis on the underlying micromechanisms associated with crack advance and their implications to damage tolerant design and lifetime computations. Compared with traditional aerospace aluminium alloys, results on the fatigue of binary Al-Li, experimental Al-Li-Cu, and near commercial Al-Li-Cu-Zr and Al-Li-Cu-Mg-Zr systems indicate that alloying with Li degrades the lowcycle fatigue resistance, though high-cycle fatigue behaviour remains comparable. The alloys, however, display superior (long crack) fatigue crack growth properties, resulting from a prominent role of crack tip shielding, principa...

C.M Ward-Close

Papers

Mechanisms associated with transient fatigue crack growth under variable-amplitude loading: An experimental and numerical study