Fatigue Crack Propagation Rate Research Articles

Influence of plastic prestraining on the fatigue crack propagation rate of S355MC and S460MC structural steels

AbstractCold forming induces significant plastic deformation at corner locations of structural steel members, resulting in both material strain hardening and the development of residual stresses, which influence the fatigue resistance of the steel members. Reliable fatigue assessment at such critical locations requires knowledge of the influence of pre‐existing plastic deformation on material behavior. The effect of plastic prestraining on the fatigue crack growth resistance of S355MC and S460MC structural steels has been evaluated experimentally. A comparative study, including fatigue crack growth and fracture resistance tests for both reference (non‐prestrained) and prestrained materials, has been performed. The experimental results, which are correlated to examined microstructural features and fracture characteristics of the materials revealed that fatigue crack growth rates of the prestrained material are higher than the reference material in the low Δκ region. Also, the S460MC steel exhibits higher crack growth rates compared to S355MC steel within the examined Δκ range.

Fatigue crack propagation behavior of a GH 4169 argon–arc welded joint at high temperature

ABSTRACT Argon–arc welds of GH 4169 were produced. Microstructural observations and microhardness tests were performed to distinguish the different regions of the weld. The fatigue crack propagation (FCP) behaviours of the different locations were investigated at 650°C compared with that at 25°C. Besides, the fracture surfaces were observed by scanning electron microscopy (SEM) and the influences of temperatures and regions on FCP behaviour of GH 4169 argon–arc welded joint were discussed. The welds showed excellent high-temperature resistance to FCP, which related to the columnar crystals formed in the fusion zone. Besides, the FCP rates of welded samples were accelerated at 650°C and the failure mode changed from mixed transgranular and intergranular fracture to wholly intergranular fracture.

Zoning study on the fatigue crack propagation behaviors of a double-sided electron beam welded joint of TC4 titanium alloy with the thickness of 140 mm

This study investigated fatigue crack propagation rates in the base metal (BM), heat affected zone (HAZ) and weld metal (WM) of a double-sided electron beam welded (EBW) joint of Ti alloy with a thickness of 140 mm in different layers. Firstly, this research measured fatigue crack propagation rates at different zones in each layer of the joint and drew curves thereof. The results demonstrate that the WM in the second layer of the double-sided EBW joint shows poor resistance to fatigue crack propagation. Secondly, the metallographic microstructures of different zones in different layers of the joint were observed. It is found that there are a lot of needle-shaped martensites growing in a specific direction at the HAZs in different layers of the joint. Finally, precipitates in different zones in each layer of the joint were studied by using a high-resolution scanning electron microscope (SEM). The results show that needle-shaped martensites in the WM of the joint are interlaced and present basketweave microstructures, while there are needle-shaped, lamellar martensites at the HAZs in each layer of the joint.

Effect of Heat Treatment Procedure on Mechanical Properties of Ti-15V-3Al-3Sn-3Cr Metastable β Titanium Alloy

In this study, the effects of various heat treatment procedures on the mechanical properties of the Ti-15V-3Al-3Sn-3Cr metastable β titanium alloy were investigated. Single-step and duplex aging treatments were applied to the alloy at low and high aging temperatures. Duplex aging treatments were carried out by selecting two different pre-aging times and temperatures: 10 h at 300 °C and 24 h at 250 °C. Thus, the effects of pre-aging on the mechanical properties of the alloy were examined. Optimum tensile properties were obtained in the duplex-aged samples, which was aged at 550 °C for 20 h after being pre-aged at 250 °C for 24 h. While fine, high volume fraction and uniformly dispersed α phases increased the tensile strength and % elongation of the alloy in the aging treatment, the fatigue crack propagation rate also increased. Also, it was observed that the threshold stress intensity factor range (ΔKth) increased with the aging treatment. While the lowest ΔKth value was obtained at the solution-treated sample, the highest ΔKth value was obtained in the sample to which duplex aging treatment was applied depending on precipitation of fine and high volume fraction of α phases.

Fatigue Crack Propagation Analysis of Orthotropic Steel Bridge with Crack Tip Elastoplastic Consideration

Abstract: Due to the complex structure of the orthogonal steel bridge deck and dense weld, fatigue cracks are prone to occur in the typical welding details. Welding residual stress will cause a plastic zone at the crack tip. In this paper, an elastoplastic constitutive model of Chaboche material was introduced, and the extended finite element method (XFEM) was used to study the influence of material elastoplasticity and crack tip plastic zone on the law of fatigue crack propagation. By judging the stress state of the residual stress field at the crack tip and selecting different crack propagation rate models to investigate the crack propagation law when plastic deformation was considered, the propagation path and propagation rate of fatigue crack of the orthotropic bridge deck were obtained. The results show that, whether the residual stress field is considered or not, the plastic deformation at the crack tip will not cause the obvious closure of the fatigue crack at the U-rib toe during the crack propagation process, but will significantly affect the crack propagation path. When material plasticity is considered, the propagation angle of fatigue crack at the U-rib toe basically remains unchanged along the short-axis direction of the initial crack, but increases along the long-axis direction, and the crack tip plastic zone inhibits the propagation of the crack tip on one side. Compared with linear elastic materials, the crack propagation law considering material plasticity is more consistent with the actual fatigue crack propagation law in bridge engineering. In terms of the propagation rate, if the residual stress field is not considered, the fatigue crack propagation rate at U-rib toe with plasticity considered is slightly higher than that without plasticity considered, because plastic deformation will affect the amplitude of energy release rate. When considering the welding residual stress field, the fatigue crack propagation rate at U-rib toe is increased due to the combined actions of plastic deformation and stress ratio R.

Residual stress effects on fatigue crack propagation in Butt–Welded joints for 304 stainless steel sheets

Welded joints are sensitive to fatigue failure due to cyclic loading, as well as fatigue crack propagation influenced by the distribution of welding residual stress. In this study, the fatigue crack propagation rates in butt-welded joints for 304 stainless steel sheets were evaluated in the presence of welding residual stresses. The analysis consisted of two separate models: first, a 3D-finite element (FE) model was used to predict the residual stresses due to welding; second, a numerical study was undertaken to predict fatigue crack propagation in the presence and absence of residual stress using the extended finite element method (XFEM). The crack growth model (NASGRO) and available experimental data were applied to verify the simulation results. The XFEM without residual stress effects shows good agreement with the experimental data and the NASGRO model. However, in the presence of residual stress, the simulation results show less agreement with the NASGRO model. The level and the nature of residual stress have significant effects on crack growth. A faster crack propagation rate is recognized due to the effect of tensile residual stress at the crack tip, while a higher resistance to crack growth is developed due to a compressive residual stress field.

Fatigue crack density of asphalt binders under controlled-stress rotational shear load testing

Fatigue crack is one of the most common distresses in asphalt pavement. In this study, the fatigue crack behaviors of asphalt binders were evaluated under repeated cyclic controlled-stress loadings. Fatigue crack density, defined by the initial sample radius and effective radius of the undamaged part during fatigue testing, was used as an indicator. Also, an effective radius function was developed using the torque equilibrium principle and dissipated strain energy (DSE) equilibrium principle. The data revealed that fatigue crack density of asphalt binder manifested three changing tendencies, in which the secant method was employed to partition their profiles based on concavity and convexity characteristics. Specimens made of #50, #70 unmodified asphalt, SBS modified asphalt, and #50 asphalt mastic, respectively, were tested by time sweep tests on a Dynamic Shear Rheometer (DSR) at different stress levels and rest time. The variations in fatigue crack density at the three stages, as well as the changes in fatigue crack density growth rate in the second stage, were all analyzed. The asphalt binder showed fatigue crack density profiles composed of three stages: crack initiation, steady growth, and unstable growth. Stage II had the smallest increase in fatigue crack density among the three stages but underwent more than 50% load cycles throughout the whole fatigue process. Hence, fatigue crack density growth rate (RD) could be used to evaluate the fatigue resistance of asphalt binders. A positive power-law relationship between RD and stress level was recorded since higher stress levels meant more serious damage. By contrast, RD displayed a negative power-law relationship with resting time, revealing that the rest time, that is, self-healing, effectively postponed the fatigue crack propagation rate.

Influence of microstructural evolution and localized delta ferrite number on high-cycle fatigue crack opening and propagation rate

In this research, percentages of nickel as austenizer and chromium as ferritizer, in the molten metal pool of SS304L weldments are varied to analyze the effect on the produced delta ferrites morphology and volume. The resulting localized precipitation of delta ferrite and its effect on the fatigue crack propagation rate of the material are examined in this work. The estimation of localized delta ferrite number for a particular weld zone is assessed through the development of a MATLAB code for image processing. It is observed that a reduction in the percentage of nickel in filler alloy yields randomly scattered granular ferrites in the fusion zone and equiaxed grain growing in the heat-affected zone with the development of blocky ferrites on the grain boundaries, whereas increased percentage of chromium in the filler causes the precipitation of dendritic clusters and vermicular ferrites in fusion zone and heat-affected zone of weldments, respectively. The effect of ferrite numbers and their morphology on the crack opening displacements and the corresponding fatigue crack propagation rate in different weld zones are analyzed. It is observed that the ferrite number ranging between 15 and 19 provides optimum fatigue strength for SS 304L. Paris curves are plotted from the test data, which shows that the weldment attained with filler alloy having Cr 26.60% and Ni 10.92% depicts the highest fatigue life compared to other studied weldments.

Crack propagation analysis of 3D printed functionally graded titanium alloy components

This paper presents an approach with a modified M-integral for determining the stress intensity factors of cracked functionally graded titanium alloy (FGTA) components. A derivative term of the strain energy density with respect to the spatial coordinate is embedded in the modified M-integral to account for the effect of the varying elastic moduli of functionally graded material. The validity of the modified M-Integral is validated by comparison with the experimental results of the crack propagation of FGTA specimens that are manufactured by using three-dimensional (3D) printing. Resistance tests of the cracked FGTA specimens are simulated to investigate the effects of the controlled changes in the material properties on the residual strength and crack-tip stress intensity factor. Further simulations of the fatigue crack propagation of the cracked FGTA specimens are performed to examine the influence of the changes in the material properties on the fatigue crack propagation life and crack propagation rate. The analysis results indicate that the numerical method in this study can accurately simulate the crack propagation of FGTA specimens. At present, due to the complexity and limitations of additive manufacturing of materials with gradient variations, it is not available to manufacture fully graded components through 3D printing for experiments. Therefore, this study assumes an FGTA beam model that is a fully graded material model to examine the effect of material gradient on crack propagation.

Three-dimensional fatigue crack propagation simulation using extended finite element methods for steel grades S355 and S690 considering mean stress effects

The assessment of fatigue crack propagation of steel structures is essential and important especially to improve the application of high strength steel in construction. The load ratio R, reflecting mean stress effects, will be changed with crack extension in the steel structures with complicated geometry. In this paper, the Walker equation is employed to fit the fatigue crack propagation rate of steel grades S355 and S690 based on experimental data in the literature to incorporate the mean stress effects. The material fatigue crack propagation parameters with 95%, 97.7%, and 99% guarantee of Walker equation were obtained by a stochastic analysis using the Monte Carlo method. The fatigue life was firstly predicted by the analytical method and was used as a baseline for numerical fatigue crack propagation simulation. A user-defined fatigue crack propagation subroutine based on the Walker equation was developed using phantom nodes-based extended finite element method (PN-XFEM) and Virtual Crack Closure Technique (VCCT) to consider the mean stress effects. The proposed three-dimensional fatigue crack propagation simulation subroutine is successfully validated of both steel grades, S355 and S690.

Effects of residual stresses on fatigue crack propagation of an orthotropic steel bridge deck

Orthotropic steel decks (OSD's) are susceptible to fatigue failure due to cyclic loading. Often fatigue cracks are found in the joint between the deck plate and the trough. Due to the welding process, residual stresses are present in and around the joint. In this paper, the effect of residual stresses on the fatigue crack propagation rate has been evaluated. First, a FE model has been made to predict and validate the residual stress field of the OSD due to welding. The validation of residual stresses is made comparing measured data at the surface of the OSD and over the thickness of the deck flange. The residual stresses are used to subsequently model for a crack propagation analysis based on extended finite element method (XFEM). The fatigue crack simulation including residual stress field shows good correlation compared to the experimental data, while the simulation without residual stress field shows less correlation. The effects of the residual stresses are relatively large as the tensile transversal residual stresses increase the crack propagation, while the tensile longitudinal residual stresses decrease the crack propagation rate. The optimal modelling of the component of residual stresses is investigated.

Microstructure and fatigue crack growth behavior in welding joint of Al-Mg alloy

In this paper, the microstructure and fatigue crack growth behavior of Al-Mg alloy welding joint by metal inert-gas welding (MIG) were studied by means of metallographic microscope, scanning electron microscopy (SEM) and electron backscattered diffraction (EBSD). The results show that the welded joint is characterized by obvious heterogeneous microstructure. The weld zone is dominated by equiaxed cellular dendritic crystals, with a small amount of welding bubbles and microcracks. The position of welding joint with bubble, grain boundary, interface between hard phase particles and matrix, and hole of coarse impurity particles debonded tend to generate fatigue crack origins, and further develop into the main crack. Due to the influence of stress state, grain orientation, welding bubble and coarse phase particle, the fatigue crack propagation in the weld zone will appear deflection, bifurcation, convergence, bridging connection and other path choices, which will affect the direction and rate of fatigue crack propagation.

Irreversible cyclic cohesive zone model for prediction of mode I fatigue crack growth in CFRP-strengthened steel plates

Experimental studies on various strengthening systems for steel elements under fatigue loading showed that the use of carbon fiber reinforced polymer (CFRP) strengthening system could significantly enhance the fatigue lifetime. Besides, more recently it was shown that the use of prestressed unbonded CFRP strengthening system results in an additional reduction of the fatigue crack propagation rate and promotes crack arrest. Different models have been proposed to evaluate the fatigue lifetime of CFRP-strengthened steel members (e.g. S-N curves and fracture mechanics-based models making use of Paris’ law or similar). As an alternative approach in this study, the numerical assessment of mode I (tensile mode) fatigue crack growth of an existing macrocrack in unstrengthened and CFRP-strengthened (both nonprestressed bonded and prestressed unbonded) tensile steel members is investigated by using a cyclic cohesive zone model (CZM). The key advantage, compared to the above-mentioned methods, is that it introduces a constitutive relationship of the material, capable of being calibrated for different materials and being used for any geometry and loading condition. In this way, the crack initiation, crack propagation, crack retardation as well as crack arrest are the natural outcomes of the model. It is shown that the finite element (FE) model can be readily coupled with an interface traction-separation law (TSL), to predict the damage evolution in the steel-CFRP interface. The comparison between the numerical and experimental results validated the proposed FE modelling, which has also been used to perform a parametric study with respect to the main design parameters.

The Mechanism of Creep during Crack Propagation of a Superalloy under Fatigue-Creep-Environment Interactions.

In this study, we examine the mechanism of fatigue-crack propagation in 718Plus superalloy at 704 °C under fatigue–creep–environment interactions, in this case, a new turbine disc material used in aero-engines at high temperatures. The effect of creep on the fatigue-crack propagation of the superalloy at high temperature was also researched. There was an unusual inhibitory effect on the propagation of fatigue cracks in 718Plus alloy, in which the propagation rate of fatigue cracks decreased with the increase of creep time through exploration of dwell-fatigue-crack growth (DFCG) test with different creep times. In particular, under lower stress intensity factor range (ΔK) conditions, the fatigue-crack growth rate with a 90 s hold-time was one order of magnitude lower than that with a 5 s hold-time. Conversely, the gap between the two DFCGs gradually decreased with the increase of ΔK and the creep effect became less apparent. The mechanism of crack propagation in 718Plus alloy under two creep conditions was investigated from a viewpoint of the microstructure, oxidation rate at high temperature and crack path morphology under different conditions.

Crack Propagation in the Spur Gear of Tubler Gear Mechanism of Lathe Machine

An estimation of the spur gear service life and crack path analysisis proposed in this paper. Ip theory using numerical solution is used to find the fatigue crack propagation rate, service life, and crack path due to bending fatigue. A spur gear of tumbler gear mechanism of lathe machine, is implemented for this analysis. An experimental test on gear specimens was carried out to analyze the crack propagation due to fatigue. Besides, service life estimated by the theory was compared with the most common Paris law. Hence further, extended finite element approach (Ansys)is implemented for the simulation of gear crack propagation trajectory. The simulation results are verified with experimental test. The service life and crack trajectory of the gear is obtained with ‘Ip’ theory and found in good agreement with experiment.In overall, this study signifies the gear failure prediction methodology during crack propagation in respect of life cycles and trajectory path.

Effect of Oxygen Variation on High Cycle Fatigue Behavior of Ti-6Al-4V Titanium Alloy.

The element oxygen is expected to be a low-cost, strengthening element of titanium alloys due to its strong solid solution strengthening effect. High cycle fatigue behaviors of Ti-6Al-4V alloys with different oxygen contents (0.17%, 0.20%, 0.23% wt.%) were investigated in this paper. The results illustrated that Ti-6Al-4V-0.20O alloy possesses the highest fatigue strength and the lowest fatigue crack propagation rate. The fatigue fracture morphology verified that the fatigue cracks propagated transgranularly in both Ti-6Al-4V-0.17O and Ti-6Al-4V-0.20O alloys, and the fatigue cracks tended to extend intergranularly in the Ti-6Al-4V-0.23O alloy. The maximum nano-hardness varied from the <0001> direction to the and directions with the increasing oxygen content, which suggested that the dominant slip system varied from prismatic slip to pyramidal slip. The number of the type dislocations increased with the oxygen content, which indicated that the number of the first-order pyramidal and the second-order pyramidal slip systems increased. The oxygen can significantly change the fatigue fracture mechanism of Ti-6Al-4V alloy: From transgranular fracture to intergranular fracture. These results are expected to provide valuable reference for the optimization of the composition and mechanical properties of titanium alloys.

Fatigue crack propagation behavior of fuel cell membranes after chemical degradation

The durability of fuel cells is significantly impaired by chemical and mechanical degradations of perfluorosulfonic-acid membranes. However, how the mechanical degradation, especially the fatigue crack propagation behavior, is impacted by chemical degradation is not clear. In this paper, the fatigue crack propagation behavior of Nafion 212 and Nafion XL composite membranes after chemical degradation are investigated. The fluoride release rates of Nafion 212 and Nafion XL membrane are 0.335 and 0.095 μmol cm−2 h−1, respectively. Likewise, the fatigue crack propagation rate of Nafion 212 membrane is also larger, which is attributed to two fatigue crack propagation mechanisms induced by chemical degradation, including bubble collapsing and pore interconnecting. By contrast, the fatigue crack propagation mechanism of Nafion XL membrane is not significantly changed where chemical degradation only accelerates crack growth in exterior surface layers. These findings provide new insights into the failure mechanisms under combined chemical and mechanical degradations.

Uncertainty Modeling of Fatigue Crack Growth and Probabilistic Life Prediction for Welded Joints of Nuclear Stainless Steel.

Welded joints are widely used in the pipeline connection of nuclear power plants. Defects in these joints are an important factor leading to the failure of welded joints. It is critical to study the fatigue crack growth and life prediction methods for the welded joints with defects, to reduce their likelihood. In this paper, we present our study of the uncertainty of fatigue crack propagation and probabilistic life prediction for welded joints of nuclear stainless steel. The standard compact tension (CT) specimens were fabricated according to the American Society for Testing and Materials (ASTM) standard. Fatigue crack propagation tests with different stress ratios were performed on CT specimens, using the Mei Te Si (MTS) fatigue test system. A fatigue crack propagation rate model considering the uncertainty of material parameters, and based on the Paris formula and crack propagation experimental data, was established. A probabilistic life prediction method based on Monte Carlo simulation was developed. The fatigue crack propagation prediction result of a CT specimen was compared with the actual tested result, to verify the effectiveness of the proposed method. Finally, the method was applied to an embedded elliptical crack in welded joints of nuclear stainless steel, to predict the fatigue crack growth life and evaluate the reliability.

Room and elevated temperature fatigue crack propagation behavior of Inconel 718 alloy fabricated by laser powder bed fusion

The fatigue crack propagation (FCP) behavior of Inconel 718 alloy manufactured by laser powder bed fusion (L-PBF) was studied at 25 and 650 °C, and compared to those of conventionally processed counterpart. The FCP rates of L-PBF Inconel 718 alloy were found to be significantly higher than those of conventionally processed counterpart at 25 °C in low and intermediate ΔK regimes. The FCP rates decreased in near-threshold ΔK regime at 650 °C, while the reversal occurred above ΔK of 7 MPa√m, as compared to those at 25 °C. The FCP behavior of L-PBF Inconel 718 alloy was discussed based on micrographic and fractographic analyses.

Experimental study on fracture and fatigue crack propagation processes in concrete based on DIC technology

A method to determine crack tip position and fracture process zone (FPZ) in concrete was proposed based on Bažant Crack Band Model (CBM) and digital image correlation (DIC) technique, which was described as: firstly, the critical crack opening displacement of concrete (wcr) was determined based on CBM and the basic mechanical properties; secondly, DIC method was used to obtain the displacement field on the surface of specimens; thirdly, the position where the crack opening displacement equaled to wcr was defined as the crack tip position. In the fatigue crack propagation tests, the whole fatigue crack was FPZ at the beginning of fatigue loading until the non-cohesive crack grew and the length of FPZ decreased. The length of FPZ tended to be stable when the number of loading cycles was close to half of the fatigue life. Fatigue crack propagation curves (a-N curves) were obtained and fitted by logistic function to reduce the influence of the data scattering. Fatigue crack propagation rate da/dN was separated into descending and ascending crack propagation functions based on the increment of crack Δa and stress intensity factor ΔK, respectively. The da/dN-Δa curves for the descending segment showed obvious scatter. Paris’ law was applied to describe fatigue crack propagation rate well for the ascending segment.