An introduction to fracture mechanics for engineers Part II: Using the stress intensity factor to characterise fracture and fatigue crack growth
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This paper reviews the experimental work on the influence of variable amplitude or random loads on the mechanics and micromechanisms of fatigue crack growth. Implications are discussed in terms of the crack driving force, local plasticity, crack closure, crack blunting, and microstructure. Due to heterogeneity in the material’s microstructure, the crack growth rate varies with crack tip position. Using the weakest link theory, an expression for crack growth rate is obtained as the expectation of a random variable. This expression is used to predict the crack growth rates for aluminum alloys, a titanium alloy, and a nickel steel in the midrange region. It is observed using the present theory that the crack growth rate obeys the power law for small ΔK and that the power is a function of a material constant.
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Abstract The effects of specimen thickness and positive stress ratio on the fatigue crack growth behavior of Ti-6Al-4V are discussed in this paper. Fatigue crack growth rates are shown to increase with increasing stress ratio. Stress ratio is shown to have a strong effect on fatigue crack growth behavior. However, for the range of specimen thicknesses examined in this study, the fatigue crack growth rates are shown to not be strongly affected by specimen thickness. A single intrinsic fatigue crack growth rate curve is obtained when the fatigue crack growth rates are plotted against the effective (closure-corrected) stress intensity factor range. Multiparameter extensions to the Paris law are also presented for the combined assessment of the effects of specimen thickness, stress ratio, and crack closure on the fatigue crack growth behavior of Ti-6Al-4V.
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This article studies plasticity-induced fatigue crack growth retardation in metals caused by crack path bifurcation in the near-tip region. To this end, the finite element method was used to model the crack with its bifurcated tip under several stress intensity factor (SIF) ranges. The results show the appearance of a retardation effect in the fatigue crack growth rate of the bifurcated crack in relation to the growth rate of the fully straight crack.
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Fatigue crack growth tests were carried out on P/M (Powder Metallurgy) aluminum alloys, 7090-T7, Al-12Si-5Fe, Al-15Zn and Al-2C, in air under ΔK-increasing and-decreasing conditions. Crack growth rates and crack closure behavior were investigated in detail by using the unloading elastic compliance method. The crack opening ratio U was found to depend on the roughness of the fracture surface. The fracture ductility of material εf was found to exert a considerable effect on the fatigue crack growth rate of the intermediate region, and the following empirical equation was proposed to account for the effect of material properties on the crack growth rate ; da/dn = 4.87×103(ΔKeff)/(Eε0.2f))2.7 The fracture toughness of the materials was hoped to be increased to obtain the improved fatigue crack growth resistance in the high growth rate regime.
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In the present work the fatigue crack growth in AISI304 specimens is investigated experimentally. In 3D finite element analysis the virtual crack closure technique is applied to calculate distributions and variations of the stress intensity factor along the surface crack front. It is confirmed that the stress intensity factor along the surface crack front varies non-uniformly with crack growth. Crack growth rate is proportional to the stress intensity factor distribution in the 3D cracked specimen. The fatigue crack growth in surface cracked specimens can be described by the Forman model identified in conventional compact tension specimens. For crack growth in the free specimen surface the arc length seems more suitable to quantify crack progress. Geometry and loading configuration of the surface cracked specimen seem to not affect the fatigue crack growth substantially.
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