Intrinsic Fatigue Crack Propagation Research Articles

Determination of fatigue crack propagation thresholds using small-scale specimens

The damage tolerance approach is widely used in the design and estimation of inspection intervals of safety-relevant metallic components subject to fatigue loading. The approach relies on the knowledge of the fatigue crack propagation characteristics, wherein a relevant role is played by the fatigue crack propagation threshold. Nevertheless, the use of material data determined by testing on conventional specimens is not straightforward in the case of thin-walled components such as turbine blades or additively manufactured parts, in which the local variation of material properties in highly stressed regions must be considered. In these cases, the possibility of investigating the fatigue crack propagation properties on a limited portion of material is crucial. For this purpose, a new test procedure has been developed for small-scale specimens which allows the determination of the intrinsic fatigue crack propagation threshold and the near-threshold regime. The validity and limitations of the method are demonstrated on the high strength steel S960QL, along with a comparison with data determined by testing on conventional geometries.

Determination of fatigue crack growth in the near-threshold regime using small-scale specimens

A complete description of the experimental procedure for characterizing the intrinsic fatigue crack propagation threshold (ΔKth,eff) as well as the fatigue crack growth rate (FCGR) in the near-threshold regime using small-scale specimens is presented. A comparative study is carried out on the high strength steel S960QL considering different single edge notch bend (SENB) specimen geometries. On one hand, the reference dimensions of 6 mm thickness (B) and 19 mm width (W) are analysed and referred to as conventional specimens. On the other hand, small-scale specimens with B = 3 mm and W = 4 and 6 mm are also considered. Several loading configurations (3-, 4- and 8-point bending) are used and a load ratio R = 0.8 is applied to avoid crack closure effects. The direct current potential drop (DCPD) technique is used to monitor the crack length. The reduced dimensions of the small-scale specimens imply the necessity of implementing a modified testing procedure compared to the recommendations of current standards for the generation of FCGR data. The results show a good agreement between tests conducted on different specimen sizes, which opens new perspectives in the use of small-scale specimens for characterizing the fatigue crack growth properties in metallic materials. Recommendations and limitations of the procedure are provided and discussed.

Metallurgical and fatigue assessments of welds in cast welded hydraulic turbine runners

Decades of hydraulic turbine operation around the world have shown one undeniable fact; welded turbine runners can be prone to fatigue cracking, especially in the vicinity of welds. In this regard, three factors are essential to consider in runner fatigue assessments: (1) the runner's design, which can induce stress concentrations in the fillets, (2) the casting process, which inherently creates defects such as shrinkage cavities and (3) the welding process, which induces significant residual stresses as well as a heat affected zone in the cast pieces near the interface with the filler metal. This study focuses on the latter, the welding process, with emphasis on the influence of the heat affected zone on the runner's fatigue behavior. In a recently concluded study by a large research consortium in Montreal, the microstructure and fatigue crack propagation properties of a CA6NM runner weld heat affected zone were thoroughly investigated to find if this zone deteriorates the runner's resistance to fatigue cracking. The main results showed that this zone's intrinsic fatigue crack propagation resistance is only slightly lower than the unaffected base metal because of its somewhat finer martensitic microstructure leading to a less tortuous crack path. However, it was also confirmed that weld-induced residual stresses represent the dominant influencing factor regarding fatigue crack propagation, though post-weld heat treatments are usually very effective in reducing such residual stresses. This paper aims to further confirm, through a case study, that the weld-induced heat affected zone does not compromise the reliability of welded turbine runners when its fatigue crack propagation properties are considered in fatigue damage models.

The effect of notch plasticity on the behaviour of fatigue cracks emanating from edge-notches in high-strength β-titanium alloys

The present paper is aimed at investigating the behaviour of fatigue cracks emanating from edge-notches for two different microstructures of the Ti-6246 alloy, produced by two specific thermo-mechanical treatments and defined as β-annealed and β-processed, respectively. Pulsating four point bending tests were performed on double-edge-notched specimens. The initiation and early propagation of fatigue cracks were investigated at two relatively high nominal stress levels corresponding to 88 and 58% of the 0.2% material’s yield stress. Plastic deformation at the notch tip initially produced a local stress redistribution followed by elastic shake down due to the high cyclic strain hardening rates exhibited by both microstructures, as confirmed by finite element modelling. Crack closure effects, measured by an extensometric technique, and variations in crack aspect ratio were considered in the Δ K calculation. The obtained crack growth rate data were compared with those of long cracks measured on standard C(T) specimens as well as of microcracks measured on round, unnotched S–N type of specimens to evaluate the intrinsic fatigue crack propagation resistance of the two microstructures. The contribution of notch plasticization to crack closure was estimated by finite element modelling.

Effect of the discrete nature of plasticity on fatigue crack propagation

At cyclic plastic crack tip opening displacements smaller than 100 Burgers vectors per cycle the plasticity is significantly constrained by its discrete nature which is the reason for the intrinsic threshold behavior of fatigue cracks. In this paper the effect of grain boundaries and duplex microstructure on this constraint is investigated and the consequences for the interpretation of the intrinsic fatigue crack propagation behavior in ductile metals are discussed.

Effects of aging and passivation on corrosion fatigue crack propagation of an Al-Li-Zr alloy

Fatigue cracking of an Al-Li-Zr alloy has been investigated by measuring crack closure as a function of ageing stage and applied anodic potential in 0.5 M Na 2SO 4 and 3.5 wt% NaCl solutions with an unloading elastic compliance technique. The present work involves complementary anodic behaviour of the Al-Li-Zr alloy in both solutions by the potentiodynamic polarization and potentiostatic current transient experiments. The Na 2SO 4 solution raised more the intrinsic fatigue crack propagation (FCP) rate for the overaged specimen than that for the peak-aged one. The higher intrinsic FCP rate of the overaged specimen is due to the higher anodic dissolution rate at the crack tip region as compared to the peak-aged one. The intrinsic FCP rates under unstable passivation potentials in both solutions were significantly larger than those obtained under stable passivation potentials. The difference between the intrinsic FCP rates under stable and unstable passivation potentials is discussed in terms of environment-assisted crack-tip damage processes based on the repassivation behaviour.

PASSIVATION EFFECTS ON CRACK CLOSURE IN THE FATIGUE CRACK PROPAGATION OF A PEAK‐AGED Al‐Li‐Zr ALLOY IN NaCl AND Na2SO4 SOLUTIONS

Abstract Fatigue cracking of a peak‐aged Al‐Li‐Zr alloy was investigated by measuring crack closure as a function of applied anodic potential in 0.6 M NaCl and 0.5 M Na2SO4 solutions with an unloading elastic compliance technique, and by comparison with crack closure in dry air. The present work involves complementary anodic behaviour of the Al‐Li‐Zr alloy in both solutions by potentiodynamic polarization and potentiostatic current transient experiments. From the repassivation rates in the passivation potential range in both solutions, it is indicated that a more stable passive film is formed at lower applied anodic potential than at higher applied anodic potential. The intrinsic fatigue crack propagation (FCP) rates under unstable passivation potential in both solutions were significantly larger than those obtained in dry air. Under stable passivation potential in both solutions, however, the intrinsic FCP rates in the low ΔKeff range were slightly lower than those obtained in dry air. The crack closure in the low ΔKeff range increased under stable passivation potential, in dry air and under unstable passivation potential. The high crack closures appearing in the low ΔKeff range were characterized by a tortuous fracture surface in dry air, and the occurrence of various crack paths such as rolling plane delamination under unstable passivation potential. The difference between environmental crack closures under stable and unstable passivation conditions is discussed in terms of environment‐assisted crack‐tip damage processes.

AGEING EFFECTS ON FATIGUE CRACK CLOSURE OF AN Al–Li–Zr ALLOY IN Na2SO4, SOLUTION

Abstract— Fatigue of an Al–Li–Zr alloy has been studied as a function of ageing stage by measuring crack closure in Na2SO4 solution with an unloading elastic compliance technique and comparing results in dry air and oxygenated solution. Anodic behaviour of peak‐aged and overaged alloy specimens in the Na2SO4 solution has been investigated by potentiodynamic polarisation and potentiostatic current transient experiments. The Na2SO4 solution increased the intrinsic fatigue crack propagation (FCP) rate for the overaged specimen in dry air compared to that for the peak‐aged sample. The Na2SO4 solution inhibited the development of crack closure for the peak‐aged specimen in dry air, but aided it in the overaged condition. The result of the environmental crack closure study is discussed in terms of the more enhanced through‐thickness tortuosity of the overaged specimen in the Na2SO4 solution when compared to that of the peak‐aged specimen. The oxygen dissolved in the Na2SO4 solution slightly increased the environmental intrinsic FCP rate, which seems to be due to the reduced repassivation rate as compared to that in the N2‐purged solution. The anodic dissolution rate from the bare surface of the overaged specimen in the Na2SO4 solution was higher than that from the peak‐aged sample. The difference between environmental FCP rates and crack closures for the peak‐aged and overaged specimens is discussed in terms of environment‐assisted crack‐tip damage processes involving anodic dissolution.

Structure-property relationships in AlLiCuMgAgZr alloy X2095

Although aluminum-lithium alloys showed initial promise for aerospace applications, implementation has not proceeded swiftly. In this study, efforts were made to design and develop microstructures with good fracture and fatigue crack propagation resistance to achieve a better balance of mechanical properties in the high strength alloy X2095. Lower aging temperatures were employed, resulting in precipitation of shearable δ' (Al 3Li) particles and reduced subgrain boundary T 1 precipitation. Although fracture toughness was not significantly altered in the 1.6 Li variant, improvements approaching 50% were achieved in the 1.3 Li alloy. Intrinsic fatigue crack propagation resistance was also slightly improved due to reduced environmental interactions. These improvements were made without altering the 660 MPa yield strength.

Intrinsic Fatigue Crack Growth Rates for Al-Li-Cu-Mg Alloys in Vacuum

The influences of microstructure and deformation mode on inert environment intrinsic fatigue crack propagation were investigated for Al-Li-Cu-Mg alloys AA2090, AA8090, and X2095 compared to AA2024. The amount of coherent shearable δ (Al3Li) precipitates and extent of localized planar slip deformation were reduced by composition (increased Cu/Li in X2095) and heat treatment (double aging of AA8090). Intrinsic growth rates, obtained at high constantK max to minimize crack closure and in vacuum to eliminate any environmental effect, were alloy dependent;da/dN varied up to tenfold based on applied ΔK or ΔK/E. When compared based on a crack tip cyclic strain or opening displacement parameter (ΔK/(σys E)1/2), growth rates were equivalent for all alloys except X2095-T8 which exhibited unique fatigue crack growth resistance. Tortuous fatigue crack profiles and large fracture surface facets were observed for each Al-Li alloy independent of the precipitates present, particularly δ, and the localized slip deformation structure. Reduced fatigue crack propagation rates for X2095 in vacuum are not explained by either residual crack closure or slip reversibility arguments; the origin of apparent slip band facets in a homogeneous slip alloy is unclear. Better understanding of crack tip damage accumulation and fracture surface facet crystallography is required for Al-Li alloys with varying slip localization.

Environmental effects on crack closure in the corrosion fatigue of an aluminium-lithium alloy

The corrosion fatigue cracking of an aluminium-lithium alloy has been investigated in dry air, 3.5 wt% NaCl and 0.5 M Na 2SO 4 solutions by measuring the crack length and closure, simultaneously during the crack propagation with a personal computer-aided unloading compliance technique. Fatigue tests were carried out on centre-cracked tension peak-aged alloy specimens (L-T orientation). In the low ΔK ranges, apparent fatigue crack propagation rates in aqueous solutions were similar to those in dry air. However, the plots of da/dN vs ΔK eff indicated that in the low ΔK eff ranges intrinsic environment-induced fatigue crack propagation rates significantly exceeded intrinsic fatigue crack propagation rates in dry air. The high crack closures appearing in the low ΔK ranges under dry air and environments were characterized with the tortuous fractured surface and the occurrence of various crack paths such as rolling plane delamination etc., respectively. The rise in intrinsic environmental crack propagation rates under the active conditions was accompanied by the increment of the environmental crack closure relative to the passive conditions. The difference between environmental crack closures under active and passive conditions has been discussed in terms of the combined action of the localized corrosion and environment-assisted crack-tip damage processes.

Stage II intrinsic fatigue crack propagation: Petit, J. and Henaff, G. Scr. Metall. Mater.25 12 (Dec 1991) 2683–2687

Stage II intrinsic fatigue crack propagation: Petit, J. and Henaff, G. Scr. Metall. Mater.25 12 (Dec 1991) 2683–2687

Environmental influence on the near-threshold fatigue crack propagation behaviour of a high-strength steel

The near-threshold fatigue crack propagation behaviour of a high-strength low-alloy steel has been investigated in ambient air and in vacuum so as to determine the role of the environment precisely. The analysis of the results is conducted by taking crack closure effects into account. It is concluded that fatigue crack growth rates measured in ambient air depend upon three processes: intrinsic fatigue crack propagation as observed in vacuum, adsorption of water vapour molecules on freshly created rupture surfaces, which enhances crack propagation, and a subsequent step of hydrogen-assisted cracking. The appearance of intergranular ruptures and oxide layers on rupture surfaces in ambient air is also discussed.

Environmental fatigue of an Al-Li-Cu alloy: part I. Intrinsic crack propagation kinetics in hydrogenous environments

Deleterious environmental effects on steady-state, intrinsic fatigue crack propagation (FCP) rates(da/dN) in peak-aged Al-Li-Cu alloy 2090 are established by electrical potential monitoring of short cracks with programmed constant ΔK andKmaxI loading. Such rates are equally unaffected by vacuum, purified helium, and oxygen but are accelerated in order of decreasing effectiveness by aqueous 1 pct NaCl with anodic polarization, pure water’ vapor, moist air, and NaCl with cathodic polarization. Whileda/dN depend on ΔK4.0 for the inert gases, water vapor and chloride induce multiple power laws and a transition growth rate “plateau.” Environmental effects are strongest at low ΔK. Crack tip damage is ascribed to hydrogen embrittlement because of acceleratedda/dN due to parts-per-million (ppm) levels of H2O without condensation, impeded molecular flow model predictions of the measured water vapor pressure dependence ofda/dN as affected by mean crack opening, the lack of an effect of film-forming O2, the likelihood for crack tip hydrogen production in NaCl, and the environmental and ΔK-process zone volume dependencies of the microscopic cracking modes. For NaCl, growth rates decrease with decreasing loading frequency, with the addition of passivating Li2CO3 and upon cathodic polarization. These variables increase crack surface film stability to reduce hydrogen entry efficiency. Small crack effects are not observed for 2090; such cracks do not grow at abnormally high rates in single grains or in NaCl and are not arrested at grain boundaries. The hydrogen environmental FCP resistance of 2090 is similar to other 2000 series alloys and is better than 7075.