Fatigue Crack Growth Mechanisms Research Articles

The Role of Extrusions and Intrusions in the Initiation and Intergranular Growth of Fatigue Cracks

The role of extrusions and intrusions in the initiation and growth of intergranular fatigue cracks is studied by performing strain‐controlled fatigue experiments on polycrystalline copper and by analyzing the development of intergranular crack initiation and resulting fracture surfaces. Fatigue cracks initiate either in the persistent slipbands within a grain or on the grain boundaries. The grain boundary initiation mechanism is due to the production of extrusions and intrusions on the grain boundaries that decrease the grain boundary cohesion forming submicroscopic crack nuclei. The alternating extrusions and intrusions are found also on the grain boundary facets of the fracture surface of a growing fatigue crack. Up to three slip systems are identified on the facets of cracked grain boundaries. Their form and height are found using scanning electron microscope observations and focused ion beam sectioning. A mechanism similar to intergranular grain boundary crack initiation is considered to explain the formation of grain boundary facets. A novel mechanism of intergranular fatigue crack growth is proposed based on the damaging effect of the extrusions and intrusions produced in the cyclic plastic zone of the growing fatigue crack.

Fracture toughness and fatigue crack growth resistance of precipitate-free and precipitation hardened NiTiHf shape memory alloys

The present work investigates fracture toughness, and actuation and mechanical fatigue crack growth responses of Ni50.3Ti29.7Hf20 HTSMAs across martensitic transformation with two different microstructures, one with H-phase nanoprecipitates and one without. H-phase precipitation is known to stabilize the actuation cycling response of NiTiHf HTSMAs and notably impacts transformation-induced plasticity. The fracture toughness tests performed reveal that precipitate-free NiTiHf has a higher fracture toughness and undergoes significantly more inelastic deformation than the one with the precipitates resulting in toughness enhancement, i.e., stable crack advance during fracture toughness experiments, which is not observed in the precipitated NiTiHf for the crack configuration and loading conditions tested. Furthermore, the precipitate free NiTiHf has higher actuation and mechanical fatigue crack growth resistance than the precipitation-hardened microstructure. This is attributed to plasticity buildup, which exacerbates the manifestation of retained martensite upon repeated transformations. The fatigue crack growth rates obtained from both actuation and mechanical fatigue experiments align to a single Paris Law Curve for the precipitation-hardened NiTiHf. This work aims to determine if unified Paris Law curves can be generated from mechanical and actuation fatigue experiments, irrespective of composition and microstructure, to estimate actuation fatigue crack growth rates, laborious and challenging to measure, from easier to detect mechanical fatigue crack growth rates.

Generalization of crack growth mechanisms under isothermal and thermomechanical fatigue by COD and ERR parameters

An experimental-computational study of the crack opening displacement (COD) was performed to evaluate the ability of this parameter through the hysteresis loop area and energy release rate (ERR) to characterise fatigue crack growth under isothermal and thermomechanical conditions. To this end, a range of experimental crack growth tests were carried out including isothermal fatigue (IF), creep-fatigue interaction (CFI), in-phase (IP), and out-of-phase (OOP) thermomechanical fatigue (TMF). The crack growth experimental results in terms of COD and ERR were interpreted based on multi-physics finite element analyses of the stress–strain and displacement fields performed by incorporating Maxwell 3D, Fluent, and the transient structural modules of ANSYS. As a result of measurements and computations, the COD on the outer surface of the SENT specimen at the location of the original notch was found to be directly related to the crack tip opening displacement (CTOD). To gain insight into the fatigue crack growth mechanism operating under isothermal and TMF conditions, the fracture surfaces of all the tested specimens were examined using a scanning electron microscope to compare the morphology and changes in the dominant failure mechanisms. It was found that propagating cracks under isothermal and thermomechanical conditions interact with the microstructure of the material, and supporting fractography evidenced subtle differences in fracture mechanisms in terms of COD and ERR parameters. The relationship between the experimentally determined energy release rate and degradation function in the fatigue phase-field fracture methodology is discussed.

Low cycle fatigue behavior and life prediction of a directionally solidified alloy

Low cycle fatigue behavior and life prediction of a directionally solidified alloy

An analysis of fatigue crack features at various crack lengths through mating fracture surface pairs

AbstractThis paper presents a novel high‐resolution direct analysis of mating AA7050‐T7451 fracture surfaces at crack lengths from 0.1 to 1.3 mm (Kmax of 4.51 to 16.39 MPa√m) for the purpose of analyzing the fatigue growth mechanisms. The various fractographic features observed at these crack lengths were then compared to the known accelerative effects of an applied loading sequence that contained underloads at regular intervals. A correlation was found between some of the observed features and the local fatigue crack growth rate. This study provides new insight into the prominent mechanisms that dictate crack growth rate and ultimately should assist with developing crack growth prediction methods that more accurately represent the physics of the fatigue crack growth mechanisms.

Mechanism of intergranular fatigue crack growth in copper polycrystal

Polycrystalline copper specimens (notched and smooth) were subjected to constant strain amplitude loading and the fatigue fracture surface was inspected and analyzed. The majority of facets on the fracture surfaces indicated intercrystalline fracture. Facets with well-developed relief were inspected. The relief was formed by persistent slip markings consisting of extrusions and intrusions similar to that on the specimen surface in the cyclic plastic zone of the growing crack. The mechanism of grain boundary cracking in the cyclic plastic zone of the growing crack similar to that recently suggested for intergranular crack initiation was drawn up. The crack growth mechanism based on the interconnection of cracked grain boundaries and the tear of the islands between them was proposed. The effect of grain size, orientation, and crack growth rate on the grain boundary cracking was discussed. Preliminary results on other FCC materials indicate a more general validity of the proposed intergranular fatigue crack growth mechanism.

RELATIONSHIP BETWEEN STRESSES FOR THE BOUNDARIES OF SCALE LEVELS OF FATIGUE DIAGRAM AND THE DIFFERENCE OF THE MESOAND MACROSCALE FRACTURE MECHANISMS

Statistical data analysis was performed based on the results of standard tests with metallic materials. The relationship was found between the so-called fatigue limit, which is considered as the transition boundary between microand mesoscale fatigue fracture processes, and mechanical characteristics under monotonic tension. Using heat-resistant alloy EI698 and titanium alloy VT22 as examples, it was shown that the mechanisms of fatigue crack initiation and growth in materials with different ratios of fatigue strength to yield strength are different. The mechanism of slip band formation determines the fatigue crack initiation and its early growth stage for materials with a σ-1/σ0.2 ratio close to 1 and higher, as was shown for EI698. For materials with a σ-1/σ0.2 ratio below 1, the fatigue crack initiation is associated with mechanisms other than slipping, as demonstrated for VT22.

Extrusions and intrusions in the cyclic plastic zone of the growing fatigue crack in polycrystalline copper

Surface relief in the tip and along the path of a growing fatigue crack in polycrystalline copper has been observed and analyzed. A crack started from a semicircular notch in a prismatic specimen cycled with constant strain amplitude. In the notch tip and along the crack path on both sides of the specimen the surface relief of the grains was characterized by a high density of the PSMs belonging to two, up to three slip systems. Extrusions and intrusions of these PSMs intersect forming specific surface relief. Similar surface relief, corresponding to the grain boundary surface, has arisen on the fracture surface. It means that in the persistent slip bands inside the crystal, the material is pushed towards the neighboring grains or removed from the grain boundary (analogically to extrusions and intrusions on the surface). This mass transfer locally increases stresses on the grain boundaries creates discontinuities and weakens grain boundaries. Such novel mechanism of intergranular fatigue crack growth is discussed. The suggestion of an intergranular crack growth mechanism agrees with the predominantly intergranular crack path of the growing fatigue crack.

Conventional shot peening treatment effects on thermal fatigue crack growth and failure mechanisms of a solid solution alloy

This study presents the effects of conventional shot peening as a surface post-treatment method on the fatigue crack growth behavior and failure mechanism of a solid solution alloy used in the flat seal component under thermal fatigue conditions. The study employs a combination of experimental tests and finite element simulations to evaluate the effect of shot peening on thermal fatigue crack growth behavior. The corrected J-integral and Paris model are used to calculate the stress intensity factor and predict the thermal fatigue crack growth behavior, respectively. Additionally, the ZENCRACK method is employed to predict thermal fatigue life. The results indicate that shot peening induces compressive residual stress, leading to a decreased distribution of thermal stress, particularly along the crack propagation path. The simulated fatigue life curve obtained by ZENCRACK demonstrated a good agreement between the simulated and experimental crack growth results. The incorporation of shot peening caused a significant reduction in the rate of thermal fatigue crack growth, and the simulated stress intensity factor analysis revealed an increase during the heating stage. Consequently, the thermal fatigue crack propagated during the heating-up process. SEM evaluation reveals that the shot peening post-treatment impacts both thermal fatigue crack initiation and propagation. Furthermore, the failure mode of shot-peened specimens involves inter- and trans-granular cracking, while non-shot-peened specimens exhibit trans-granular cracking.

Short fatigue crack growth mechanism in Ni-Co based superalloy at elevated temperatures and in oxidative atmospheres

In this study, to elucidate the oxidation effect on the short fatigue crack growth (SFCG) characteristics of Ni-Co based TMW-4M3 superalloy, fatigue tests were conducted at room/elevated temperature in air/vacuum and three-dimensional microscopic observation of the SFCs using a plasma focused ion beam – scanning electron microscope (PFIB – SEM) system. Fatigue lives tested under vacuum at elevated temperature were comparable to those at room temperature while those tested at elevated temperatures in air showed shorter fatigue life in higher stress regions and longer fatigue life in lower stress regions than the others. In situ observation of SFCs at elevated temperatures in air revealed SFCG deceleration in the small ΔK regions and acceleration in large ΔK regions. SFCs opening/closing behaviours at elevated temperatures measured by digital image correlation (DIC) showed the crack opening stress to be higher at elevated temperature in air, possibly due to oxide-induced crack closure. However, the crack closure effect did not fully explain the difference in FCG rate between room and elevated temperatures in air. Three-dimensional investigations revealed SFCs to form at elevated temperatures in air, showing straight transgranular FCG to be insensitive to microstructure in slow growth regions, and intergranular FCG to precede that in the surrounding material in fast growth regions, in contrast to microstructural SFCs features at room temperature. It appears that slow and straight unique SFCG at elevated temperatures might occur due to intermittent brittle fracture of oxides formed at the crack tip. This suggests a possible role of the oxide layer at the crack tip in broadening the plastic deformation distribution, thus contributing to FCGR deceleration.

High-cycle fatigue behaviour and crack growth mechanism of Ni-base single crystal superalloy under intermediate temperature

This study investigates the high cycle fatigue behaviour of a Ni-base single crystal superalloy under 760 °C and 850 °C. At 760 °C, the crack initially grows in the crystallographic shearing mode and the crack tip propagation mechanism is non-crystallographic mode. Furthermore, near the crack tip propagated at 760 °C, the oxide film distributes along the γ/γʹ interface and the Ni-rich γʹ phase dissolution channels pass through the matrix in different ways, resulting in the wavy-like and dendritic cracks. When the temperature rises to 850 °C, recrystallization grains are observed in the fatigue initiation region. Moreover, the stress reduction leads to the increment of accumulative strain and the aggravation of the recrystallization. This will enhance the crack growth driving factor, promoting the crack to grow in crystallographic shearing mode. Finally, the mechanisms of high cycle fatigue crack growth influenced by temperatures and stresses are obtained and enhancement conditions of crack growth driving factor are revealed.

Heat treatment effects on near threshold region for AISI 4340 steels

Abstract The microstructure effect is critical in the near-threshold region in terms of fatigue crack propagation. Despite numerous studies on the crack growth phenomenon in the literature, there is still no comprehensive understanding of the mechanism behind it. The fatigue crack growth mechanism occurs in the plastic zone region, which is quite small in size; the order is regarded as microstructural units, particularly at low stress intensities. Microstructural differences caused by heat treatment methods are frequently attributed to changes in monotonic and yield strength, resulting in differences in plastic zone size. The driving force required for crack growth under alternating loading is proportional to the plastic zone size ahead of the crack tip. When the microstructure is modified using isothermal transformations, the stress intensity near the threshold and corresponding crack propagation rates were found to be affected by stress ratio, material yield strength, particle size distribution, and impurity segregation. The crack growth threshold ΔK 0 is discovered to be inversely related to steel strength, and a relationship between ΔK 0 and cyclic yield stress is established. In the scope of this paper, annealed and tempered conditions were investigated to assess near threshold behavior for AISI 4340 steel. Effect of microstructure will be detailed around low stress intensities via performed crack growth tests.

A Review on the Corrosion and Fatigue Failure of Gas Turbines

Since gas turbines are used in airplanes, ship engines and power plants, they play a significant role in providing sustainable energy. Turbines are designed for a certain lifetime according to their operating conditions and the failure mechanisms they deal with. However, most of them experience unexpected and catastrophic failure as a result of synergistic effects of more than one damage mechanism. One of the main causes of failure in turbines is corrosion fatigue, which results from the combination of cyclic loads and corrosive environments. In the current review paper, an attempt has been made to investigate the damages related to corrosion and fatigue in turbines such as fatigue corrosion, hot corrosion and oxidation, thermomechanical fatigue, emphasizing their synergistic effect. In this regard, the mechanism of fatigue crack initiation and growth in a corrosive environment is also taken into consideration. Moreover, a summary of the results reported in the literature regarding the influence of the loading conditions, characteristics of the corrosive environment and properties of the turbine materials on this failure is presented. Finally, common methods of dealing with corrosion fatigue damage, including surface treatment and cathodic protection, are briefly reviewed.

Effects of the exposure to moist air on crack-tip deformation and striation marking in a 7175 T7351 aluminum alloy

This paper investigates the effects of the exposure to moist air on the mechanisms of fatigue crack growth in 7175 T7351 aluminum alloy. A special attention is paid to the crack-tip deformation mechanisms, controlling the fatigue crack growth rate as well as the fatigue striation markings, firstly under moist air and at low loading frequency, corresponding to a high exposure rate, and then as compared to a low exposure rate condition obtained in an inert atmosphere. It is shown that a high exposure rate induces the development at the crack-tip of a 800 nm-thick surface layer, divided into subdomains characterized with a pronounced and highly localized lattice rotation. In contrast, under very low exposure rate, a significantly lower lattice rotation associated with a smoother gradient is observed. This suggests that the formation of subdomains at high exposure rate is actually induced by some environmental effect, presumably in relation with an assistance of hydrogen highly accumulated in a thin subsurface layer in the intense localization of crack-tip deformation resulting into lattice rotation. Finally, it is clearly established that fatigue striation spacing in moist air exactly coincides with the distance separating two successive subdomains exhibiting the highest lattice disorientation.

Vibration fatigue behavior and life prediction of directionally solidified superalloy based on the phase transformation theory

Directionally solidified nickel base superalloy DZ125L is the main material that has been widely used to manufacture aviation gas turbine blades and hot-end structural parts. Aeroengine is under vibration load for a long time in the service process, which makes vibration fatigue become one of the main failure modes of aero-engine structures. In this study, vibration fatigue behavior and excitation frequency response curves of DZ125L alloy under different stress levels were investigated through vibration fatigue tests. The fatigue crack initiation and growth mechanism of DZ125L alloy under vibration load is studied based on the results of the vibration fatigue test and finite element simulation. A vibration fatigue life prediction model based on the phase transformation theory is developed. Each parameter has clear physical significance in the model. The comparison between the model prediction results and the test results shows that the theoretical prediction results have reasonable accuracy.

Mechanical properties of laser powder directed energy deposited NASA HR-1 superalloy: Effects of powder reuse and part orientation

The feasibility of NASA HR-1 powder reuse for laser powder directed energy deposition (LP-DED) of thin-walled structures in the as-built surface condition was investigated in this study. Although continuously reusing the powder five times slightly reduced the number of fine particles in the powder and increased the mean particle size, changes in powder rheological properties were negligible. The similarities in powder flow between all reused conditions resulted in no variation in surface roughness, defects’ size and spatial distributions, and microstructure of the as-deposited parts and, thus no differences on tensile and fatigue properties. The effects of different specimen orientations, loading direction being perpendicular or parallel to the building direction, on mechanical properties were also investigated. While no change in tensile strength was noted, tensile ductility was lower for the parallel specimens because of surface micro-notches being perpendicular to the loading direction, which led to an early onset of fracture. Due to accelerated crack initiation from these micro-notches, parallel specimens were also less resistant to fatigue failures. The fatigue failures always initiated from the specimen surfaces, and all specimens experienced a ductile fatigue crack growth mechanism characterized by clear striations on the fracture surface.

Experimental investigation on anisotropic fatigue crack growth characteristics of Q420C steel

The hot-rolled steel sheets often exhibit anisotropy in their mechanical characteristics due to the grain’s elongation along the rolling orientation. The anisotropy characteristic also appears in the fatigue crack growth behaviour. However, it has not been thoroughly studied for some types of steel materials although they have been widely used for wind turbine towers. In this study, the longitudinal fatigue crack growth behaviour of the hot-rolled steel sheet (Q420C) is investigated by an experimental programme. A total of six specimens were tested under constant amplitude loading cases with different load ratios. It was found that longitudinal cracks are approximately 15% faster than transverse cracks at the load ratio of 0.1; at the load ratio of 0.3, they are approximately 22% faster. The anisotropy of crack growth rates is attributed to differences in the microstructure of the material. A comparison of the crack growth rate is carried out for Q420C steel and some widely used American-made steel materials. Furthermore, the applicability of design curves in the British Standard BS 7910 is explored, the crack closure phenomenon is analysed and the fatigue crack growth mechanism is discussed.

Effect of crack flank holes on fatigue crack growth

The effect of drilling two symmetric holes along the crack flanks on the fatigue crack growth (FCG) rate was evaluated numerically. The FCG increases (decreases) when the crack tip is behind (ahead) the holes. This behaviour is enlarged both by increasing the diameter of the holes and by reducing the distance between them. This is consequence of the geometrical effect, which modifies the plastic zone size. Experimental work validated the numerical model, indicating that that cumulative plastic strain at the crack tip is an adequate crack driving force and that cyclic plastic deformation is the main damage mechanism of FCG.

Micromechanics of fatigue, creep-fatigue interaction and thermo-mechanical crack growth of XH73M nickel alloy

This study presents interpretation and evaluation of a range of isothermal experimental crack growth data on polycrystalline XH73M nickel-based alloy generated by two type tests carried out by stress-controlled pure fatigue and creep-fatigue interaction conditions. The tests have been carried out using cycles with a triangular and trapezoidal waveform and a temperature range of 23-750°C. It was found that from the crack growth rate point of view, the following order of arrangement of fatigue fracture diagrams is formed: isothermal creep-fatigue interaction, isothermal pure fast (f=10 Hz) and slow (f=1 Hz) fatigue. The ordering of the crack growth rate curves is supported by detailed fractographic analysis which shows transgranular or intergranular mechanism of fatigue crack growth depending on thermo-mechanical test conditions. The regularities of fatigue crack propagation were established by fractographic analysis. It was found for example that the intergranular crack growth mechanism become dominant at 750°C for the pure fatigue and at 550°C for creep-fatigue interaction conditions. The secondary crystals on the grain boundary of primary grain at intergranular fracture of specimen tested at 650°C under creep-fatigue interaction conditions appeared.

Influence of build orientation on high temperature fatigue crack growth mechanisms in Inconel 718 fabricated by laser powder bed fusion: Effects of temperature and hold time

Inconel 718 manufactured by Laser Powder Bed Fusionhas been characterized in terms of the microstructure, effect of post-processing heat treatment, and hence dependence of fatigue crack growth (FCG) on build orientation. Long crack fatigue tests were conductedat different temperatures and frequencies. It was found that at 350 ⁰C (transgranular crack propagation) build orientation had no effect on FCG rates. However,at higher temperatures (time-dependent crack propagation)build orientation had a significant effect on FCG rates.This was primarily related to microstructural differences caused by the printing strategy in grain size distribution, grain shape and grain boundary character.