Fatigue-Life Prediction Method Based on Small-Crack Theory in an Engine Material

A B S T R A C T It is observed that the short fatigue cracks grow faster than long fatigue cracks at the same nominal driving force and even grow at stress intensity factor range below the threshold value for long cracks in titanium alloy materials. The anomalous behaviours of short cracks have a great influence on the accurate fatigue life prediction of submersible pressure hulls. Based on the unified fatigue life prediction method developed in the authors' group, a modified model for short crack propagation is proposed in this paper. The elastic-plastic behaviour of short cracks in the vicinity of crack tips is considered in the modified model. The model shows that the rate of crack propagation for very short cracks is determined by the range of cyclic stress rather than the range of the stress intensity factor controlling the long crack propagation and the threshold stress intensity factor range of short fatigue cracks is a function of crack length. The proposed model is used to calculate short crack propagation rate of different titanium alloys. The short crack propagation rates of Ti-6Al-4V and its corresponding fatigue lives are predicted under different stress ratios and different stress levels. The model is validated by comparing model prediction results with the experimental data.Keywords crack propagation threshold; fatigue crack propagation rate; short fatigue cracks; titanium alloy; unified fatigue life prediction method. N O M E N C L A T U R Ea = the crack length A = a material-sensitive and environmentally sensitive constant of dimensions in the crack growth rate model A 0 , A 1 , A 2 , A 3 = the coefficients defined to calculate the crack opening function f op d = THE intrinsic crack length da/dN = Fatigue crack growth rate f op = a crack opening function defined as the ratio K op /ΔK k = a material constant that reflects the rate of crack closure development with crack advance K cf = the fracture toughness of the material K max = the maximum stress intensity factor K min = the minimum stress intensity factor m = a constant representing the slope of the corresponding fatigue crack growth rate curve dimensions in the crack growth rate model n = the index indicating the unstable fracture in the crack growth rate model r e = an empirical material constant of the inherent flaw length of the order of 1μm R = the stress ratio defined by σ min /σ max Y = a geometrical factor to calculate the stress intensity factor α = a parameter used to calculate the 'virtual strength' of the material α ′ = the plane stress/strain constraint factor ΔK effth = the effective range of the stress intensity factor at the threshold level

Section: Fatigue Crack Propagation Rate Under Different Stressmentioning

confidence: 98%

Section: The Short Fatigue Crack Propagation Rate Under Different Strmentioning

confidence: 99%

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Prediction of short fatigue crack growth of Ti‐6Al‐4V

Wang

Cui

et al. 2014

“…These studies along with other works such as the NASA-CAE report (RP-1309) 29 have demonstrated that small-crack behaviour consumes the major part of the nucleation cycles from microstructural features (inclusion particles, voids, and micromachining marks). 30,31 Crack-growth-based modelling appears to be a promising method for analyzing the fatigue life of AM materials since the cracks already exist as the process-induced voids in the parts fabricated by the current state of AM. 5,21,[32][33][34] However, a systematic application of damage-tolerance principles and defect-based modelling of fatigue-life for materials fabricated by AM is still missing.…”

Section: Introductionmentioning

confidence: 99%

Fatigue‐life prediction of additively manufactured material: Effects of heat treatment and build orientation

Yadollahi

Elwany

Doude

et al. 2020

Self Cite

In this study, the crack‐growth approach is used to predict the fatigue life of 17‐4 precipitation hardening (PH) stainless steel (SS) fabricated via an additive manufacturing (AM) system in different orientations (ie, vertical and horizontal) before and after heat treatment. To perform fatigue‐life calculations, the effective stress intensity factor as a function of crack‐growth rate was obtained from testing modified compact specimens with different crack orientations in as‐built and heat‐treated conditions. The plasticity‐induced crack closure model, FASTRAN, was used to calculate fatigue lives based on the size of process‐induced defects. Results indicated that in the presence of large voids (ie, lack‐of‐fusion defects), the total fatigue life of AM 17‐4 PH SS in as‐built and heat‐treated conditions is dominated by crack growth. Effect of build orientation on fatigue life of AM 17‐4 PH SS was also captured based on the size of defects projected on a plane perpendicular to the loading direction.

“…persistent slip bands, until the remaining material can no longer carry the load. 28 Considering that the cracks (ie, process-induced voids) already exist in the AM materials, fabricated by the current state of AM, crack growth-based modelling of fatigue appears to be a promising technique for analyzing the life of AM materials under cyclic loads. The purpose of this paper is to use the crack growth approach for predicting the fatigue life of AM materials.…”

Section: Introductionmentioning

confidence: 99%

Fatigue life prediction of additively manufactured material: Effects of surface roughness, defect size, and shape

Yadollahi

Mahtabi

Khalili

et al. 2018

179

In this paper, the effects of process‐induced voids and surface roughness on the fatigue life of an additively manufactured material are investigated using a crack closure‐based fatigue crack growth model. Among different sources of damage under cyclic loadings, fatigue because of cracks originated from voids and surface discontinuities is the most life‐limiting failure mechanism in the parts fabricated via powder‐based metal additive manufacturing (AM). Hence, having the ability to predict the fatigue behaviour of AM materials based on the void features and surface texture would be the first step towards improving the reliability of AM parts. Test results from the literature on Inconel 718 fabricated via a laser powder bed fusion (L‐PBF) method are analysed herein to model the fatigue behaviour based on the crack growth from semicircular/elliptical surface flaws. The fatigue life variations in the specimens with machined and as‐built surface finishes are captured using the characteristics of voids and surface profile, respectively. The results indicate that knowing the statistical range of defect size and shape along with a proper fatigue analysis approach provides the opportunity of predicting the scatter in the fatigue life of AM materials. In addition, maximum valley depth of the surface profile can be used as an appropriate parameter for the fatigue life prediction of AM materials in their as‐built surface condition.