Initiation Of Short Cracks Research Articles

Ex-situ observation of the short crack initiation and propagation behaviour of nickel-base wrought alloy Inconel 718 for critical rotating components under fatigue conditions at elevated temperatures

In this paper, the initiation and propagation behaviour of short fatigue cracks in nickel-base alloy Inconel 718 is studied extensively in the fatigue loading regime by ex-situ scanning electron microscopy on component-near notched specimens in the temperature range between 400°C and 600°C. To investigate the influence of different surface treatments, both specimens with polished and shot-peened surfaces were tested. For polished specimens, most fatigue cracks initiated at oxidised non-metallic inclusions, whereas for shot-peened specimens, the process-induced surface roughness also served as crack starters. It was found, that the observed scatter in lifetime for repetitive tests is mainly caused within the short crack fatigue regime below 50μm, while cracks larger than 100μm showed similar growth characteristics.

The behavior of anisotropic fatigue short crack initiation and propagation for a directionally solidified superalloy CM247LC at room temperature

AbstractThe effects of mechanical and microstructural anisotropy on short fatigue crack initiation and propagation behaviors of a directionally solidified superalloy have been studied. An unusual result was found where the fatigue lives of specimens with grains longitudinally aligned along the loading direction fail at lower lifetimes than specimens with transversely loaded grains when the applied stress is close to the yield stress. This is mainly attributed to the lower Young's modulus of the longitudinal specimen, which induces more local plastic strain (at stress concentration features) leading to earlier crack initiation and faster crack propagation under the applied test stress.

A framework to simulate the crack initiation and propagation in very-high-cycle fatigue of an additively manufactured AlSi10Mg alloy

Currently, no systematic approach exists to simulate the crack initiation and propagation process in very-high-cycle fatigue including the microstructure sensitivity. A combined crystal plasticity and cohesive zone model based computational framework is developed to simulate the defect-induced short crack growth of an additively manufactured AlSi10Mg alloy. The crystal plasticity formulation is used to model the anisotropic deformation in the grains. The accumulated plastic shear strain obtained by the crystal plasticity is introduced into the cohesive zone model as the crack initiation damage evolution criteria. The total damage is suggested to be divided into static damage, fatigue initiation damage and fatigue propagation damage. The framework can control the crack growth rate and the proportion of crack initiation damage in the total damage. Besides, an acceleration strategy is proposed to accelerate the computational efficiency for the very-high-cycle fatigue (VHCF). It provides an equivalence of loading cycles and the computational efficiency and accuracy are controlled by the acceleration factor. The internal defect induced short crack initiation and propagation of VHCF are investigated experimentally and simulated by the proposed framework. Experimental observations of tortuous crack paths and fine grain regions are reasonably well captured by the simulation.

Low‐cycle fatigue characteristics and fracture behavior of the die‐forged 2014 aircraft wheel

AbstractIn order to provide a sufficient theoretical basis for the fatigue‐resistant design of the aircraft wheels, strain‐controlled low‐cycle fatigue (LCF) tests were carried out on specimens machined in the extrusion direction (ED) and transverse direction (TD) of die‐forged 2014 aluminum alloy wheels. Although the TD specimens have lower tensile strength and yield strength, the fatigue test results show that the TD specimens have superior fatigue life compared with the ED specimens at total strain amplitudes of 0.5–0.8%. This is predominantly caused by the coarse Al12(MnSi)2(FeCu) intermetallic particles close to the surface layer, which results in a relatively short crack initiation stage for the ED specimens. In contrast, TD specimens with finer and more uniform recrystallized grains exhibit more excellent resistance to fatigue crack initiation (FCI) and propagation (FCP). Moreover, the fatigue life of alloys could be accurately predicted via a Manson–Coffin–Basquin (MCB) model based on total strain.

Study on the short fatigue crack initiation and propagation behavior of 42CrMo

A micro-tensile fatigue test of 42CrMo alloy steel was carried out. The result shows that the crack initiation and propagation of the material could be characterized with randomness and localization. There existed two sorts of short fatigue crack propagation behaviors for the tested material, that is, self-propagation and joint propagation with other short cracks. A short fatigue crack growth rate function was built considering the propagation characteristics. Moreover, a simulation model of short fatigue crack evolution was built. Total three samples were tested and simulated. The experimental result was in good consistent with that from simulation, both showing that the crack growth rate accelerated first and then decelerated during propagation.

Research on evolution behavior of 42CrMo microscopic fatigue short crack based on microstructure effect of material

To investigate the causes of damage dispersion in fatigue life of alloy materials, fatigue tests on 42CrMo are carried out under microscope with the aid of a micro fatigue testing device. The fatigue crack initiation and propagation evolution process of 42CrMo are studied on the micromechanical scale. The fatigue short crack and short crack growth rates are measured respectively under loads of 1400, 1550, and 1650 N. The experimental result shows that the initiation of fatigue short cracks is related to the randomness of microstructure defects. The initiation and propagation of fatigue short cracks on the surface of samples are random and local, while the propagation is characterized by first deceleration and then acceleration.

In-situ SEM investigation on the fatigue behavior of Ti–6Al–4V ELI fabricated by the powder-blown underwater directed energy deposition technique

Underwater directed energy deposition (UDED) is a promising technology for on-site maintenance and repair of Ti–6Al–4V alloy. The microstructurally short fatigue crack initiation and growth behaviors of Ti–6Al–4V repaired by UDED were investigated through in-situ SEM fatigue testing. The results were compared with those of the Ti–6Al–4V repaired by in-air directed energy deposition (DED) and a substrate. The experimental results show that the microstructure of the Ti–6Al–4V repaired by UDED was dominated by α′ martensite. The α′ martensite in the Ti–6Al–4V repaired by in-air DED decomposed into α+β due to the intrinsic heat treatment. Both the strength and microhardness of the Ti–6Al–4V repaired by UDED were higher than those of the Ti–6Al–4V repaired by in-air DED. Compared with the Ti–6Al–4V repaired by in-air DED, the Ti–6Al–4V repaired by UDED presented a short micro crack initiation time and a poor fatigue crack propagation resistance. The short fatigue crack initiation and propagation behaviors of the as-deposited Ti–6Al–4V were primarily controlled by the structures and morphologies of the microstructures rather than the residual stresses. In addition, the prior-β boundaries in the Ti–6Al–4V repaired by UDED had a resistance effect on the fatigue crack growth rate. However, the resistant effect of prior-β boundaries in the Ti–6Al–4V repaired by in-air DED was not obvious and the local colony α+β played a dominant role in the resistance of short fatigue crack propagation.

Effect of Rod-like Structure on Fatigue Life, Short Surface Crack Initiation and Growth Characteristics of Extruded Aluminum Alloy A2024 (Analysis via Modified Linear Elastic Fracture Mechanics).

In the Al alloy A2024-T3 extruded material, a rod-like structure is generated parallel to the extrusion direction. In this study, the effects of rod-like structures on fatigue crack initiation and growth behavior were comprehensively investigated. Two types of specimens were used in a fatigue experiment, in which the direction of the load stress amplitude was parallel (specimen P) and perpendicular (specimen V) to the rod-like structure. Based on the experimental and analytical results, the following findings were obtained regarding the fatigue life, location of crack initiation, and fatigue crack growth behavior. Because the fatigue life of specimen P was longer than that of specimen V, it is inferred that the rod-like structure significantly affects the fatigue life. In specimen P, fatigue cracks were generated from the grain boundaries of the Al matrix. By contrast, in specimen V, cracks were generated from the Cu–Mg-based intermetallic compound in the Al matrix. In specimen P, fatigue cracks were more likely to propagate across the rod-like structure, which decreased the fatigue crack growth rate. In specimen V, fatigue cracks did not propagate across the rod-like structure; instead, they propagated through the Al matrix. Therefore, the fatigue crack growth resistance of specimen V was lower than that of specimen P. The relationship between the fatigue crack growth rate and the modified linear elastic fracture mechanics parameter could be used to predict the S–N curve (stress amplitude vs. fatigue life) and fatigue crack growth behavior. The predicted results agreed well with the experimental results.

The Role of Crystallographic Texture and Basal Plane Slip on Microstructurally Short Fatigue Crack Initiation and Propagation in Forged Billet and Rolled Bar Ti-6Al-4V Alloy

The Role of Crystallographic Texture and Basal Plane Slip on Microstructurally Short Fatigue Crack Initiation and Propagation in Forged Billet and Rolled Bar Ti-6Al-4V Alloy

Crack initiation and propagation from geometric microdefects: Experiment and transition fatigue behavior

AbstractThe present work aims to study short fatigue crack initiation and propagation induced by geometric microdefects. An empirical approach is used to evaluate the microstructural influences on crack growth behavior. Small fatigue crack test on titanium alloy Ti‐6Al‐4V is performed to investigate the crack induced from a geometric microdefect. Analyses on crack initiation and fracture are respectively performed to study the microstructural influences on the stages of initiation and fracture. Subsequently, an empirical multiscale model is employed to predict the short fatigue crack propagation of alloys underlining the transition features. Grain size variation is adopted to reflect the main microstructural contribution to the short crack propagation. An alpha factor is defined to demonstrate the transition interim from microstructurally short crack to physically short crack stages. Furthermore, a relationship between transition factor and grain size is correlated. The results indicate that the value of transition factor tends to decrease with the augment of average grain size. The empirical approach is validated through a variety of fatigue experimental data of alloys including Ti‐6Al‐4V, 2024‐T3, and GH4169.

Effects of Stress Amplitude Ratio and Phase Angle on Short Fatigue Crack Behavior and Fracture Mode of EA4T Steel

The loads on railway axles in actual service conditions are complex and variable, which may lead to local changes in the fatigue performance, which then affect the fatigue reliability and safety evaluation results of the components and structures. At present, studies on the short fatigue crack behavior and fracture failure mode of EA4T steel, a common material for high-speed trains, are limited. Therefore, fatigue tests on smooth funnel solid round bar specimens under different stress amplitude ratios and different phase angles were performed. Replica technology was used to record the initiation and propagation information of short fatigue cracks. Due to the homogenous and fine microstructure of all the specimens, the same short fatigue crack behavior was observed regardless of the loading condition. The fracture morphology observed by a scanning electron microscope shows that the cracks initiated on the surface of the specimens, and the specific fracture morphology was exceedingly distinct in each region. In addition, the short fatigue crack growth rate and fatigue life were compared and analyzed. The results show that the stress amplitude ratio and phase angle have a considerable influence on the fatigue life and short crack growth rate of the specimens. The results of this research can provide a reference for practical engineering applications.

Discrete crystal plasticity modelling of slip-controlled cyclic deformation and short crack growth under low cycle fatigue

For metals under fatigue, microplastic strain localisation leads to the formation of discrete slip bands, which contributes to the initiation and propagation of short cracks. In this paper, a discrete slip band model is introduced to investigate the slip-controlled cyclic deformation and short crack growth in a single crystal alloy. In conjunction with crystal plasticity and normal factor-based critical resolved shear stress, finite element simulations demonstrated the success of the discrete model in describing orientation-dependent cyclic stress-strain responses. The proposed approach is also capable of predicting slip-controlled short crack growth, based on element deletion technique and individual cumulative shear strain criterion.

Effects of laser shock peening on the mechanisms of fatigue short crack initiation and propagation of AA7075-T651

A laser shock peening (LSP) treatment was performed on AA7075-T651 for maximum fatigue improvement. Surface and microstructural characterisation techniques (micro-hardness, SEM-EBSD, contact-profilometry) showed LSP surface modification was limited, and LSP generated deep compressive residual stresses above −300 MPa. Fatigue testing showed a two-order magnitude increase in overall life, due to the mechanism of crack initiation changing from surface second-phase particles to subsurface crack initiation dependent on the local stress field. Modelling highlights the sensitive balance between surface roughness (including LSP-induced pits) and residual stress on the micro-mechanism of crack initiation, and how this can be used to maximise fatigue life extension.

Room-temperature low-cycle fatigue and fracture behaviour of asymmetrically rolled high-strength 7050 aluminium alloy plates

The asymmetrical rolling (ASR) process with shear deformation is considered as a promising technology to adjust/improve through-thickness microstructure homogeneity and integrated properties of high-strength aluminium alloy plates. But the advantages may come with caveats that are the subject of our research. In this paper, the room-temperature low cycle fatigue properties and fracture behaviour of the ASR-ed AA7050 aluminium alloy plates are compared with the symmetrical rolling (SR) one. It is shown that after either type of rolling the plates exhibit similar low-cycle fatigue lives but the SR-ed one displays a better cyclic deformation ability and slightly higher fatigue lives at high strain amplitudes. It is demonstrated that the severe surface-localized deformation contributes to the formation of slip relief on the surface and subsequently initiates micro-cracks that are propagated via transgranular and/or intergranular fracture modes along with obvious fatigue striations. Recrystallised grains with coarse grain boundary precipitates and wide precipitate-free zones near the upper/bottom layers as well as numerous and larger secondary particles in the ASR-ed plates may cause early crack initiation for a short crack initiation life. However, the SR-ed plate with more frequent subgrains near surface layers, numerous fine subgrains and less indissoluble particles could possess better crack initiation/propagation resistance and cyclic loading behaviour. Fine subgrains with higher microhardness/strength can facilitate passing of dislocations or slip bands into adjacent grains so as to delay crack propagation such as via energy-intensified transgranular fracture for extending fatigue life. Properly balancing the through-thickness strain/deformation distribution and the formation of recrystallization/indissoluble particles via implementing a feasible ASR process becomes a critical issue to achieve fracture-resistant microstructures for high-strength aluminium alloy plates. The underlying causes/mechanisms regarding the differences of microstructures and mechanical behaviour are revealed and discussed based on modelling through-thickness temperature/strain distribution and detailed microstructure characterization.

A dual-imaging framework for multi-scale measurements of fatigue crack evolution in metallic materials

This paper presents a dual-imaging methodology to investigate fatigue crack initiation & propagation along the lateral and transverse directions in Al7075 flat specimens having a one-sided V-notch. The framework consists of a confocal and a digital microscope. The confocal microscope can detect the short crack initiation at a length-scale of 3 µm while the digital microscope has the capability of reliably measuring cracks at a scale of 0.5 mm. The apparatus provides a unique advantage of characterizing the multi-scale fatigue crack evolution without any significant restrictions on maximum specimen size and maximum loading or frequency.

Crystal plasticity finite‐element modelling of cyclic deformation and crack initiation in a nickel‐based single‐crystal superalloy under low‐cycle fatigue

AbstractNickel‐based single‐crystal superalloys are predominantly used for turbine blades in aircraft engines and land‐based gas turbines. Understanding and predicting the fatigue failure of Ni‐based single‐crystal superalloys are critical to ensure the safety of these components during operation. In this paper, low‐cycle fatigue experiments were carried out to investigate cyclic deformation of a nickel‐based single‐crystal superalloy MD2, recently developed by GE Power, with different crystallographic orientations. Specialty in situ scanning electron microscope (SEM) tests were also conducted to study the slip‐controlled initiation of short cracks under low‐cycle fatigue. In particular, the stress–strain response for both [001] and [111] orientations was used to calibrate a crystal plasticity model, which allowed us to simulate the activation of crystallographic slip systems and predict the initiation of short fatigue crack. Using the accumulated shear strain as a criterion, the simulations confirmed that the slip system with the maximum accumulated shear strain appeared to control the crack initiation. The location and direction of slip traces and short cracks, captured by the crystal plasticity finite‐element simulations, agreed with the in situ SEM observations. The modelling tool will be valuable for assessing the structural integrity of critical gas turbine blades.

Fatigue strength modelling of high-performing welded joints

This paper investigates the fatigue strength modelling of high-performing welded steel joints. The work considers the strain-based, notch stress, averaged strain energy density approaches, and linear elastic fracture mechanics. A comparison of the methods with experimental data shows that the predictions vary significantly for different modelling assumptions. Only the microstructure-sensitive strain-based approach can predict the fatigue life of various weld geometries and plate thicknesses. The explicit modelling of the localised plasticity using the microstructure-dependent representative volume element is required for accurate prediction of the short crack initiation and growth periods, which dominate the fatigue life modelling of high-performing welds.

Fractographic Study on Naturally Initiated Short Fatigue Cracks in a Near-Lamellar TiAl Alloy at Room Temperature

Short crack phenomena are considered important for lamellar structures in γ-TiAl alloys and have been thoroughly investigated in the past. However, the short cracks in the previous studies were nearly all introduced artificially. No particular investigations have looked into the initiation of fatigue short cracks. Therefore, naturally initiated short fatigue cracks at room temperature under two different stress ratios (0.1 and 0.5) were investigated in a near-lamellar γ-TiAl alloy (Ti-45Al-2Mn-2Nb) in this study. The observations show that the fatigue crack initiation behaved differently at low and high stress ratios. At low stress ratio, the specimens failed at lower ultimate stress levels (σmax = 450 and 475 MPa), and the crack initiated from the cluster of interlamellar fracture near mode-I orientation or stress concentration areas. At the higher stress ratio, the specimens failed at higher but consistent stress levels (σmax = 560 and 570 MPa), and in the specimen crack initiation areas, the interlamellar fractures were still the primary fracture mode, whereas some were found at tilted angles due to shear deformation. The results suggest that short fatigue cracks can naturally initiate in lamellar γ-TiAl alloys, thus attention should be paid to their microstructure design, surface finishing and cleanliness.

Customized weight function for cracks initiated from semi-circular notches

A new weight function is analytically derived for calculating the stress intensity factor of a crack initiated from a semi-circular notch root. This function is dependent on crack size and notch radius and independent from loading condition. The employed method is based on the solution of the weight function for an edge crack. A proper modification factor is applied to considering the effect of the notch and crack geometry on the weight function of the edge crack. It is initially assumed that the notch is a part of the crack. This assumption is used in an analytical method to obtain accurate solution for the weight function. The modification factor is obtained in terms of dimensionless parameters for the ease of application. The provided weight function is examined for different geometries under different loading conditions, and the results are compared with finite element solutions. The weight function is found accurate and reliable. Accuracy is not altered by crack size, notch geometry, and loading conditions. Therefore, the weight function can be used to study the different stages of the initiation and growth of short and large cracks with suitable accuracy.

Influence of microstructures on microstructurally short fatigue crack initiation and propagation in the forged and rolled Ti-6Al-4V bars

Influence of microstructures on microstructurally short fatigue crack initiation and propagation in the forged and rolled Ti-6Al-4V bars