Fatigue Crack Propagation Rate Research Articles

Strategy of plasticity induced crack closure numerical evaluation

A propagating fatigue crack may be partly retarded thanks to a phenomenon called fatigue crack closure. The ability to accurately describe this phenomenon is of interest for the scientific and engineering community, because of its significant impact on the fatigue crack propagation rate. A strategy for numerical modelling of the most common closure mechanism – the plasticity induced crack closure – is presented in this paper. It was observed that the generally adopted suggestions for this type of simulations, such as the length of the crack growth or the number of substeps, are not necessarily valid in general, but require to be individually specified for particular conditions. The size of the elements in the vicinity of the crack front is also a widely discussed issue and it is shown here that even without convergence, the element size may be chosen as a fixed parameter leading to very reasonable closure values with low computational costs. A method of closure level determination based on change in specimen stiffness is described here and its performance is compared to the traditional first node displacement method with Load-Debond-Unload (LDU) and Load-Debond-Unload-Load-Unload (LDULU) loading schemes.

Declined Fatigue Crack Propagation Rate of a High‐Strength Steel by Electropulsing Treatment

The fatigue crack propagation behavior is a key issue for the service of engineering materials. The effect of electropulsing treatment (EPT) on the growth of fatigue crack in a high‐strength steel (AISI 4340 steel) is investigated. It is shown that the crack propagation rate could be significantly declined via high‐density EPT. The crack features of fracture surface are examined using laser scanning confocal microscopy (LSCM) and scanning electron microscopy (SEM). The crack deviation, local crack‐healing and bridging phenomenon are proposed to be beneficial to lower the crack propagation rate, which should be brought out by the current detour, selective Joule heating, compressive stress, and electrical breakdown phenomenon. All these EPT effects exhibit significant contributions to the improvement of fatigue properties in the widely used high‐strength steel.

The Effect of the Welding Direction on Fatigue Crack Propagation Rate of Welded Shell Kiln

In this paper, the fatigue crack growth rate (FCGR) properties of welded kiln shell at various positions relative to the weld has been proposed. This is achieved by combining an experimental and three-dimensional finite element model are used to demonstrate the results of fatigue crack growth rate curves in the kiln welded shell. Experimental marked crack fronts are used to determine the da/dN at local regions and provide the profile information for FE modeling. Stress Intensity Factor (SIF) at local regions is determined by FE analysis and the driving force ΔK of FCGR can be derived. Furthermore, from the mixture method, material constants of Paris law (c and m) on a welded-parts and non-welded part is measured. When crack grow across the weld-line, material constants of Paris law vary according to the effect of the welded part. In the result, the constant of Paris can be used to predict a fatigue crack growth of rotary kiln structure in finite element analysis.

Enhanced Fatigue Characteristics of Topologically Optimized Biomedical Titanium Porous Structure by Selective Laser Melting

Fatigue property is a critical consideration in the porous structures and most existing porous samples have unsatisfactory performance due to lack of structural optimization. This work finds that a topologically optimized structure of α-type commercial-purity titanium (CP-Ti) produced by selective laser melting presents excellent fatigue properties with an ultra-high normalized fatigue life of ~0.65 at 106 cycles at a low density of 1.3 g/cm3. All factors affecting fatigue have been studied, including material microstructure and porous structure stress analysis. The topologically optimized structure can effectively increase the fatigue life by reducing stress concentrations. The cyclic ratcheting effect plays a dominant role in fatigue crack initiation for the topologically optimized structure. As a results of twinning occurred during the fatigue process, the porous CP-Ti sample exhibits a higher ductility than the Ti-6Al-4V sample with the same structure, which delayed the fatigue crack initiation and therefore produced a higher fatigue life. In addition, the fatigue crack propagation rate was reduced significantly because of the large plastic zone ahead of the fatigue crack tip and the effect of fatigue crack deflection and bifurcation.

Influence of fillet end geometry on fatigue behaviour of welded joints

This paper presents a fatigue analysis of a type of fillet welded joint representative of one main joint of the steel box girder of the Alcácer do Sal railway bridge. From previous studies, it was found that the welded joint between the box girder diagonal and the central hanger gusset is one of the most stressed details of the bridge. This welded joint was not fully manufactured according to current construction procedures, as regards the fillet weld end configuration. In order to assess the fatigue behaviour of such welded joint, the present study combines an experimental campaign and numerical analysis. A total of four welded joint series were produced in order to allow the comparison of the fatigue performance of similar type of welded joint of the Alcácer do Sal bridge with welded joints produced according to existing recommendations, such as EC3. Since scale-down specimens were considered, two different thicknesses were included in this study for each joint configuration, to allow the verification of any thickness effect. Concerning the numerical analyses, two main numerical tools were used: the standard Finite Element Method (FEM) with ANSYS and the eXtended Finite Element Method (XFEM) with ABAQUS. Fatigue life predictions were performed including both fatigue crack initiation and fatigue crack propagation phases. The number of cycles to initiate a fatigue crack was computed using local notch strain-life approaches, and the number of cycles for fatigue crack propagation was computed by integrating the Paris fatigue crack growth law with stress intensity factors computed with ANSYS (virtual crack closure technique) and ABAQUS (contour integral method, 3D XFEM model). Experimental tests demonstrated little influence of fillet weld end geometry on fatigue behaviour of welded joints and plate thickness effects were also reduced as also confirmed by the similar fatigue crack propagation rates. Both numerical simulations provided very accurate predictions of the experimental S-N curves, however the XFEM modelling opens new possibilities for mix-mode fatigue crack propagation simulations.

Texture Evolution and Its Effect on Fatigue Crack Propagation in Two 2000 Series Alloys

The texture evolution and fatigue crack propagation (FCP) of 2000 series alloys were investigated, respectively, by using scanning electron microscopy and x-ray diffraction. Results showed that the intensity change in different textures (such as Goss, Copper, S and Brass) abides by a linear evolution law, and a new model for texture evolution is proposed to predict the texture intensity during different heat treatments. With increasing annealing temperature, the FCP rate increased, but the damage tolerance decreased. The intensity of P texture was the key to affecting FCP rate and damage tolerance of the alloys. A new model based on the average grain diameter and P texture volume fraction was proposed to predict FCP rate and damage tolerance, and the predicted FCP rates agree very well with the experimental FCP rates of the alloys after different T83 treatments.

Fatigue life prediction of z-fibre pinned composite laminate under mode I loading

A hybrid method is presented combining linear elastic fracture mechanics with nonlinear damage mechanics that can predict the fatigue crack growth rate in z-fibre pinned composites under mode I loading. The strain energy release rate is evaluated using the virtual crack closure technique via finite element analysis. Cohesive elements are used in the pinned region to represent the crack bridging force generated by the pins. The reduction of the pins' bridging force under the fatigue loading is accommodated by applying a degradation law, based on damage mechanics with empirical fitting parameters. A modified degradation law is proposed which is capable of accumulating fatigue damage under varying crack opening displacement ranges experienced by the pins during fatigue loading. Fatigue testing was performed with a z-pinned double cantilever beam at two different values of applied displacement amplitude. The predictions show reasonably good agreement with the test results in terms of the fatigue crack propagation rate and fatigue life.

Crystal plasticity assessment of crystallographic Stage I crack propagation in a Ni-based single crystal superalloy

Stage I fatigue crack propagation along crystallographic slip planes was experimentally and analytically investigated in a single crystal Ni-base superalloy, NKH-304. Fatigue crack propagation tests at room temperature were conducted using four types compact specimens with different combinations of primary and secondary orientations. It was revealed in the experiment that the fatigue crack propagated along crystallographic slip planes in a mixed mode with Mode I, II and III components. The mixture ratio and fatigue crack propagation rate were strongly influenced by the primary and secondary crystal orientations. To interpret the effect of crystal orientations on the Stage I cracking behavior, a crystal plasticity finite element analysis was conducted considering the actual geometry of the crystallographic crack planes. The slip activities of the individual octahedral slip systems were evaluated, and then a damage parameter was proposed based on the critical plane approach. As a result, a reasonable explanation was obtained for the effect of crystal orientations on the cracking path and propagation rate of the crystallographic Stage I cracking.

Щодо моделювання розповсюдження тріщин втоми в тонких ізотропнх пластинах в умовах двовісного асиметричного розтягу-стиску

The modelling of the fatigue fracture process of the thin isotropic infinite plates with cracks under external biaxial asymmetric cyclic loading is considered. The solution of the problem is based on the joint consideration of the fracture mechanics and continuous damage mechanics concepts and using two types of equivalent stress criteria’s. The first one reduces an asymmetrical cyclic load to the equivalent symmetric cyclic load in time of the rupture. The second one reduces a plane stress state in the vicinity of the top crack to a single-axial one. The obtained system of equations of the model a relatively equivalent stress intensity factor allows us to determine the duration of the incubation stage and the rate of fatigue crack propagation in plates with different stress concentrators. The calculated dependences of the crack length, which extends from the circular hole, from the number of load cycles in the infinite aluminum plate with a circular hole at the variation of the parameters of the asymmetrical cycle and the coefficient of the biaxiality loading are constructed.

An algorithm for fatigue crack growth applied to mixed and biaxial mode loadings

Fatigue is still one of the main concerns when dealing with mechanical components failure. While it is fundamental to experimentally determine the fatigue material behavior using standard specimens, testing large and complex component geometries can be complicated. In these cases, the Finite Element Method can be a cost-effective solution but developing fatigue crack growth models is still a complicated task. In order to solve this problem, an algorithm for automatic crack propagation was developed. Using three different modules, the algorithm can generate a complex Finite Element Method model including a fatigue crack; solve this model considering complex loading conditions, by applying the superposition method; and calculate the fatigue crack propagation rate, using it to update the original model. In order to benchmark this solution two different problems were analyzed, a modified compact tension specimen and a cruciform specimen. By modifying the compact tension specimen hole location and simulating an initial crack, it was possible to understand how mixed mode conditions influence the fatigue crack path. Different load ratios and initial crack directions on the cruciform specimen were analyzed. Increasing the load ratio will increase the crack deflecting angle. The obtain solutions were compared with experimental results, showing good agreement. Therefore the developed algorithm can be used to predict the fatigue crack growth behavior on complex geometries and when different types of loads are applied to the component.

Fatigue crack propagation simulation of orthotropic bridge deck based on extended finite element method

Orthotropic Steel Decks (OSDs) are widely used in various types of steel bridges due to its benefits of light weight, high load bearing capacity and speedy construction. However, fatigue remains as the predominant problem for OSDs. Many researchers have investigated fatigue issues of welded joints through experiments but is not a cost-effective solution. Therefore, it is necessary to combine experimental data with numerical approaches. Fracture mechanics approach has already shown its reliability and can be used to model and analyze fatigue crack propagation. In this paper, a numerical simulation is performed to predict the fatigue crack propagation using extended finite element method (XFEM). Two numerical models were considered namely CT-specimen and OSD, to evaluate the modelling efficiency. To verify the simulation, the results were compared with the experimental data. In predicting the fatigue crack propagation rate using two-dimensional CT-specimen, numerical results provided a good agreement with a maximum difference of 0.03% in the slope (m) and 1.48% in the intercept (C) of the power law equation. Furthermore, a simulation was performed on three-dimensional OSD structure to predict the fatigue crack growth.

Fatigue of thin periodic triangular lattice plates

A method for predicting the fatigue life of triangular lattices is proposed in this paper by considering fatigue properties of single lattice struts. Fatigue tests of different sizes of lattice plates of aluminium alloy, and tests of single struts with different maximum fluctuating loads, have been conducted to validate this method. It is found that the struts in a triangular lattice break near to strut intersections, where stress and strain concentrations occur. Similar crack propagation paths were observed in different lattice plate specimens: the cracks grew at a 30° angle to the initial edge crack in the upper half of lattice plate. The mixed-mode fatigue crack propagation rate was also studied and expressed using an effective stress intensity factor. A size effect on the crack growth rate of triangular lattice plates was also observed: a fatigue crack will propagate slightly quicker in larger triangular plates than in smaller ones.

Experimentally validated multi-step simulation strategy to predict the fatigue crack propagation rate in residual stress fields after laser shock peening

Laser shock peening (LSP) is a promising technology to retard the fatigue crack propagation (FCP) in metallic lightweight structures. A multi-step simulation strategy to predict FCP in LSP-induced residual stress fields is proposed and applied. The simulation strategy involves an LSP process simulation, a transfer approach to include the plastic strains in a C(T) specimen model to calculate the residual stresses and an FCP simulation to determine the stress intensity factors. The FCP rate is finally determined via FCP equations. The validity of the simulation strategy including the crack driving quantities prediction is experimentally demonstrated by a novel ‘simulation’ approach.

Precipitate Evolution and Fatigue Crack Growth in Creep and Artificially Aged Aluminum Alloy

The fatigue performance of high-strength Al-Cu-Mg alloys is generally influenced by the process of creep age formation when applied to acquire higher strength. The results show that creep aging accelerates the precipitation process, leading to a more uniform precipitation of strengthening phases in grains, as well as narrowed precipitation-free zones (PFZ). Compared with the artificially aged alloy, the yield strength and hardness of the creep aged alloy increased, but the fatigue resistance decreased. In the low stress intensity factor region (ΔK ≤ 7 MPa·m1/2), the fatigue crack propagation (FCP) rate was mainly affected by the characteristics of precipitates, and the fatigue resistance noticeably decreased with the increased creep time. In a 4 h creep aged alloy, the microstructure was dominated by Cu-Mg clusters and Guinier-Preston (GP) zones, while S″ phases began to precipitate in the matrix, showing better fatigue resistance. After aging for 24 h, the needle-shaped S’ phases were largely precipitated and coarsened, which changed the mode of dislocation slip, reduced the reversibility of slip, and accelerated the accumulation of fatigue damage. In stable and rapid crack propagation regions, the influence of precipitates on the FCP rate was negligible.

Multi-walled carbon nanotube reinforced polymer as a bonded repair for Al 2024-T3 fatigue crack growth

ABSTRACT The influence on multi walled carbon nanotubes (MWCNT, or simply CNT) as a reinforcement material in an epoxy resin in order to decrease the fatigue crack propagation rate in the 2024 T3 aeronautical Al alloy was studied. CT samples were pre-cracked in a resonant fatigue machine until a 4 mm pre-crack length. Four groups of samples were considered: a non-repaired reference group, two groups repaired with epoxy resin reinforced with two CNT proportions (0.5 and 1 vol %) and a group repaired by the conventional “stop drill” technique. The crack was propagated until a length of 16 mm, measuring the number of cycles to this crack propagation. Resin Hysol EA9320 NA was used, mixing it with the CNT and the hardener, by ultrasonic stirring. S-N curves (stress vs number of cycles) were plotted obtaining an increment of 104% for a 0.5 vol% of CNT, 128% for 1 vol% of CNT and 400% for “stop drill” repairing. These results were referred to the non-repaired samples at the lower load level. These results showed that in repaired samples with CNT reinforced resin, the initiation and propagation of cracks would be delayed, constituting this method a reasonable and convenient repairing procedure useful for aeronautical cracked structures.

P-Texture Effect on the Fatigue Crack Propagation Resistance in an Al-Cu-Mg Alloy Bearing a Small Amount of Silver.

P-texture effect on the fatigue crack propagation (FCP) resistance in an Al-Cu-Mg alloy containing a small amount of Ag, is investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electron back scattering diffraction (EBSD). Results shows that the high intensity P-texture sheet has lower , lower FCP rate and higher damage tolerance than random texture sheet. Fracture analysis indicates that the striations spacing of high intensity P-texture sheet is much smaller than that of random texture sheet and it has a rougher fatigue fracture surface, which causes a significant roughness induced crack closure (RICC) effect. The calculation results manifest that high intensity P-texture sheet possesses a higher crack closure level reaching 0.73 as compared to random texture sheet (only 0.25). The statistical analysis results reveal the P-grains have large twist angle of 105–170° and tilt angle of 5–60° with neighboring grains, which is similar to Goss-grains. This is the fundamental reason that P-texture sheet has the same FCP resistance and induces fatigue crack deflection as Goss-texture sheet. Additionally, the most {111} slipping planes of P-grains are distributed in the range of 30–50° deviating from transverse direction of the sheet. This results in more {111} slipping planes to participate in cyclic plastic deformation, which is beneficial to reduce fatigue damage accumulation and improve the damage tolerance of Al-Cu-Mg-Ag alloy.

Developing the ductility and thermal fatigue cracking property of laser-deposited Stellite 6 coatings by adding titanium and nickel

A design strategy is presented for the development of a laser-deposited Stellite 6 coating with high ductility and low thermal fatigue crack propagation rate. The strategy involved adding titanium to modify carbide morphology, accompanied by an increase in the nickel content to stabilize the γ-Co phase in the matrix. The molar ratios of the titanium to the carbon in coatings were 0.5, 1.0, 1.5, and 2.0. The amount of nickel added to the titanium-modified coatings was 10 and 20 wt%. The net-like M23C6 eutectic carbides in the Stellite 6 coating transformed to isolated TiC particles with increasing molar ratio of titanium to carbon. Adding nickel to the coatings resulted in an increase in the stacking fault energy and the stabilization of the γ-Co phase. Accordingly, the ductility increased with increasing nickel concentration. The modified coating, named 1.0Ti20Ni, in which the molar ratio of titanium to carbon was 1.0 and the nickel concentration was 20 wt%, exhibited the highest elongation to fracture (~12%), along with a high strength (1230 MPa) in all coatings, Besides, the 1.0Ti20Ni coating revealed the lowest thermal fatigue crack propagation rate.

Effect of laser surface remelting on the fatigue crack propagation rate of 40Cr steel

In the present work, quenched and tempered 40Cr steel was treated by laser surface remelting, followed by microstructure evolution, hardness, residual stress field and external morphology investigation to understand the effect of laser surface remelting on surface characteristics, as well as the grain refinement mechanism. Then, fatigue tests were carried out to obtain the fatigue crack growth rate and fracture morphology, thereby further discussing the fatigue crack propagation and fracture mechanism. Results indicate that the grain refinement mechanism of 40Cr steel treated by laser surface remelting can be attributed to the rapid melting-solidification process in surface, during which tempered sorbite is transformed into fine martensite. In addition, the residual compressive stress can be produced by the thermal effect. The fine microstructure and residual compressive stress lead to a significant decrease in crack propagation rate due to the high density of grain boundaries which hinder crack propagation.

Effect of hydrogen on the fatigue crack growth rate of quenched and tempered CrMo and CrMoV steels

In order to select the most appropriate steel to deal with pressurized hydrogen over long periods of time, the fatigue crack propagation rate of quenched and tempered CrMo and CrMoV steel grades was assessed by means of tests performed on thermally pre-charged specimens in a hydrogen reactor at 195 bar and 450 °C during 21 h. Cylindrical samples were used to measure the hydrogen content and their desorption kinetics at room temperature and compact tensile specimens to determine the fatigue crack growth rate. Under the aforementioned pre-charging conditions, significant amounts of hydrogen were introduced, being much larger in the CrMoV steel grades, which also have much lower apparent diffusion coefficients, as precipitation of fine vanadium carbides during tempering provides strong hydrogen traps. Moreover, the fatigue crack growth rate increased significantly due to the presence of internal hydrogen in the CrMo grades for test frequencies lower than 10 Hz in comparison with tests performed in air. Furthermore, the presence of vanadium carbides in the CrMoV steel significantly improved fatigue crack growth performance, the effective hydrogen diffusion distance per cycle and the hydrogen concentration in the process zone ahead of the advancing crack being considerably reduced.

The prediction models for fatigue crack propagation rates of mixed-mode I-II cracks

The Manson–Coffin law for the low-cycle fatigue of a representative volume element and the fatigue crack propagation rate of a cracked component are crucial for characterizing the fatigue resistance of a material. Using the average plastic-strain-energy density and the average linear damage accumulation in the plastic zone at the crack tip, two fatigue crack growth models for mixed-mode I-II cracks, without artificial parameters, are proposed, based on the low-cycle fatigue properties of a representative volume element under cyclic plastic strain. For 30Cr2Ni4MoV rotor steel, the proposed models are used for theoretically predicting the mixed-mode I–II fatigue crack propagation rates of compact tension shear specimens subjected to loadings at four different loading angles; and the theoretical prediction results are found to be consistent with the experimental ones.