Fatigue Crack Initiation Life Research Articles

Fatigue of X65 steel in the sour corrosive environment—A novel experimentation and analysis method for predicting fatigue crack initiation life from corrosion pits

AbstractOil and gas pipelines manufactured from API‐5L Grade X65 steel are generally subjected to cyclic loading and their internal surfaces are frequently exposed to corrosive sour fluids. Exposure of pipelines to these environments often leads to localized corrosion (pitting) and decreased fatigue life. Corrosion pits are geometrical discontinuities that may promote fatigue cracking by acting as stress raisers. In order to optimize asset inspection and repair scheduling, it is important to understand the fatigue behavior of X65 steel and in particular, the ability to predict the crack initiation from corrosion pit. To achieve this level of understanding, conducting fatigue tests in an environmental condition replicating the field environment is important. This paper presents the test protocol and results of environmental fatigue testing using bespoke laboratory apparatus to undertake in situ corrosion fatigue tests in a sour corrosive environment under uniaxial loading. The environment selected represent processes that are likely to occur at internal surfaces of oil and gas pipelines exposed to production fluids. The tests were carried out on smooth samples to obtain S‐N curve in this specific environment as well as on pre‐pitted samples. An electrochemical method is used to create corrosion pits on the samples. Also, a model is proposed to predict the crack initiation life from corrosion pit, using a local stress‐based technique, which has been validated by experimental test results. Post‐test fractography was carried out by scanning electron microscopy (SEM). The performance of our approach is demonstrated. The innovation is anticipated to encourage other workers to employ similar small‐scale tests requiring toxic and challenging test environments.

Paper Review of “Numerical investigation on fatigue crack initiation and propagation lives for non-load carrying fillet welded joint considering cyclic elasto-plasticity response of steel”

Paper Review of “Numerical investigation on fatigue crack initiation and propagation lives for non-load carrying fillet welded joint considering cyclic elasto-plasticity response of steel”

Relationship between non-inclusion induced crack initiation and microstructure on fatigue behavior of bainite/martensite steel in high cycle fatigue/very high cycle (HCF/VHCF) regime

High cycle and very high cycle fatigue (HCF/VHCF) in a bainite/martensite (B/M) multiphase steel with varying inclusion size and microstructural features was studied using ultrasonic axial cycling test. The fatigue crack initiation was predominantly induced by inclusions in specimens with large inclusion size, whereas fatigue crack initiated from the sub-surface microstructure in specimens with coarse microstructure. The fatigue life from granular bright facet (GBF) to fish-eye and from fish-eye to the critical crack size was calculated to obtain an estimate of the contribution to fatigue life by GBF, for the two modes of crack initiation within the HCF/VHCF regime. The results demonstrated that the majority of fatigue life was consumed by the crack initiation process along with the formation of GBF irrespective of whether the crack initiated from inclusions or from sub-surface microstructure. In the case of crack initiation from sub-surface microstructure, the ratio of fatigue crack initiation life to total fatigue life (Ni/Nf) had a wide scatter, which is attributed to varying B/M hierarchical structure in individual prior austenite grains.

Fatigue analysis of cruciform welded joint with weld penetration defects

For the fabrication of steel structures such as steel bridges, offshore structure etc, welding is nowadays considered as an efficient metal joining process. The type of fillet welded cruciform joint are widely used in construction of long spanned bridges with improved design and increased weld quality. However, the presence of geometrical discontinuities and metallurgical nonuniformities lead to crack initiations from different positions which are difficult to detect. Moreover, when these structures are subjected to repeated loading these cracks start propagating in different directions and lead to fatigue failure of welded structures. Also, the presence of locked in stresses and other factors affect the fatigue life and fatigue crack initiation. In the present work, the cruciform fillet welded joint with full and incomplete penetration were analyzed for both load types i.e load carrying and non-load carrying by using 3D fatigue FEM analysis. The experimental study of non-load carrying cruciform joint was also carried out. The effect of incomplete penetration depth, welding deformation, residual stresses and stress magnitude on fatigue crack initiation and fatigue life of fillet welded cruciform joints was investigated. Also, the S-N curves plotted for comparing with the results of hotspot stress and experiments. The results of 3D fatigue FEM analysis were compared with S-N curves recommended by IIW and DNV GL.

Fatigue life prediction of rubber-sleeved stud shear connectors under shear load based on finite element simulation

Rubber-sleeved studs, which are headed studs wrapped with rubber sleeves, have been found to be prospective to overcome some challenges in the design of steel–concrete composite structures with ordinary stud shear connectors. A finite element-based approach was provided for the fatigue life prediction of rubber-sleeved stud shear connectors. After calculating the stress and strain state of the connector by finite element models, the fatigue crack initiation life was predicted based on the critical surface method, and fracture mechanics was adopted to predict crack propagation life. The prediction method was validated by fatigue push-out test results. The subsequent finite element parametric study revealed the effect of shear stress range, rubber sleeve height, and elastic moduli of rubber and concrete on the fatigue life of shear connectors. Further, fatigue life prediction formulas for ordinary stud and rubber-sleeved stud shear connectors were proposed for facilitating the engineering application. The reliability of these formulas was verified by comparing with S-N curves in the design codes and those obtained from fatigue push-out tests.

Strain-based fatigue reliability assessment of welded joints in ship structures

A strain-based fatigue reliability assessment of welded ship structural joints is presented. To obtain a complete description of the fatigue stress acting on a butt-welded joint in a ship, a probabilistic distribution for the low cycle fatigue stress range is developed considering the uncertainty in the static and dynamic stresses. A model considering the effect of a secondary notch embedded into the primary notch is developed considering the low and high cycle fatigue regimes. The misalignment, secondary notch, residual stress and fatigue limit effects are incorporated into the notch strain approach on which the fatigue reliability assessment is based in the present study. The efficient Advanced Mean Value method is used to derive the distribution of the fatigue crack initiation life where a detectable crack is formed. The influence of different modelling options is analysed, and the importance of accounting for the misalignment, residual stress and especially the proposed secondary notch effect is discussed. The sensitivity of the reliability estimates to the variables’ uncertainty is investigated based on the First Order Reliability Method, and the dominant ones are identified.

Numerical investigation on the rolling contact wear and fatigue of laser dispersed quenched U71Mn rail

The influences of laser dispersed quenching (LDQ) technology on the rolling contact wear and fatigue of U71Mn rail were investigated numerically. Firstly, based on the improved Abdel Karim-Ohno’s material model, a three-dimensional elasto-plastic model with of LDQ rail was established, and the moving load method was used to apply the equivalent contact load. Subsequently, combined with the ductility exhaustion theory and Smith-Watson-Topper fatigue assessment method, the rolling contact wear and fatigue performances of U71Mn rail were investigated, respectively. The results show that the LDQ treatment has little effect on fatigue crack initiation life, but can significantly improve wear resistance of rails.

Internal Versus External Cracking—Their Impact on the Fatigue Life of Modern Smoothbore Autofrettaged Tank Gun Barrels

Abstract An extensive analysis of the fatigue life of a typical modern autofrettaged smoothbore tank barrel, cracked either internally or externally, in terms of the initial crack depth and shape, type and level of autofrettage, was conducted. Five overstraining cases were considered: no-autofrettage, 70% and 100% hydraulic autofrettage, and 70% and 100% swage autofrettage. KINmax, the maximum combined stress intensity factor (SIF) KINmax = (KIP + KIA) max, due to both internal pressure and autofrettage, as a function of crack depth for a large number of internal and external crack configurations was determined by the finite element method (FEM). A novel realistic experimentally based autofrettage model, incorporating the Bauschinger effect, was integrated into the finite element model, replicating both the hydraulic and swage autofrettage residual stress fields (RSFs) accurately. Fatigue lives were evaluated by integrating Paris' Law using the above values of KINmax. The following conclusions can be drawn from the results: hydraulic and swage autofrettage have a dramatic beneficial effect in extending the fatigue life of an overstrained barrel 4–11 times as compared to an identical nonautofrettaged tube. The fatigue life of overstrained barrels is controlled by internal cracking, for barrels overstrained by up to ε = 100% hydraulic autofrettage, by up to ε = 70% in the case of swage autofrettage, and by external cracking for ε = 100% swage autofrettaged. Eliminating or carefully designing stress concentrators on the tube's external face and keeping away from corrosive agents thus, extending the fatigue-crack initiation life of an external crack, enables the increase of the level of swage autofrettage to up to ε = 100%. Swage autofrettage is much more superior to hydraulic autofrettage. The fatigue life of a 70% swaged autofrettaged barrel is 1.5 times higher than that of a 100% hydraulically autofrettaged tube. If full swage autofrettage is permissible, the fatigue life of such a barrel is twofold that of a fully hydraulically autofrettaged tube. Unlike the commonly accepted concept, the level of hydraulic autofrettage should not be limited to 70%, and full hydraulic autofrettage should be used. Similarly, in the case of swage autofrettage, if the detrimental effect of external cracking is removed by proper design and maintenance of the tube's outer surface, the level of autofrettage can be increased to up to ε = 100%, thus, gaining an increase of 33% in the fatigue life as compared to overstraining the barrel to only ε = 70%. Initial crack depth and shape are major factors affecting the fatigue life of the barrel. The deeper the initial crack depth, a0, and the slenderer its shape, a/c→ 0, the shorter the fatigue life of the barrel.

Combined effect of rolling contact fatigue and corrosion on structural performance of rails

ABSTRACT An accurate structural assessment of rails under combined effect of fatigue and corrosion prevents a phenomenal disaster, also saves millions of dollars and people’s lives. Recent studies which are proposed to simulate a precise model fail to take into account the different underlying mechanism for rail corrosion. The current study presents a numerical procedure to investigate crack initiation under combined effect of fatigue and corrosion. A three-dimensional (3D) finite element model (FEM) which accounts for rolling contact stresses due to wheel-rail contact loads is developed. Then, stress distribution is used in a MATLAB code to estimate the fatigue crack initiation life. The advantage of the proposed model is considering the combined effect of rolling contact fatigue and corrosion on structural integrity of rails. This model is applied to a case study of rail assessment in Melbourne, Australia. The cracks orientations and locations which were based on the FEM results were in good agreement with the field observations.

Effects of hole reaming on fatigue performance of thin sheets for fuselage: DIC and FEM analysis

The effects of hole reaming (HR) after pre-fatigue damage on fatigue performance of AA2198-T8 were studied by using the digital image correlation (DIC) technique. The influences of stress distribution of the rivet hole on fatigue life were studied. When maximum axial strain is equal to 0.0005, the number of fatigue cycles is suggested as the fatigue crack initiation life. The prediction of fatigue lives by the calculation method combining Paris law and ABAQUS, and the experimental results are within a factor of 1.2-scatter band. This work provides a reference for the maintenance of fatigue damaged rivet holes in aircraft structure.

Corrosion kinetics study of 316 N weldment under cyclic loading in acidified chloride environment

Corrosion fatigue crack initiation (CFCI) life of AISI Type 316 N weldment in as-welded (AW) and heat treated (Solution annealed (SA), 550 °C/4h and 750 °C/1h) conditions was studied in high cycle fatigue regime (above 104 cycles) with a frequency (η) of 0.1 Hz and and R-ratio (σmin/σmaxwhere σmin and σmax signify minimum and maximum tensile stress in MPa, respectively) of 0.5 for different stress amplitude (σa) values in boiling acidified 5 M NaCl + 0.15 M Na2SO4 + 2.5 ml/l HCl solution under axial loading in load controlled mode. In-situ electrochemical measurements were employed during corrosion fatigue tests to study the passive film characteristics. SA weldment showed better corrosion fatigue (CF) resistance as compared to AW and heat treated conditions. Homogenization of microstructural and microchemical heterogeneities facilitates highest resistance to CF cracking process for SA weldment condition. Number of cycles to failure (Nf) increased with decrease in stress amplitude for all material conditions. CFCI mechanism was identified based on variation in the strain rate (ε°) due to combined effect of both emergence of persistant slip bands (PSBs) casued by rupture of passive film and initiation of microcrack caused by dissolution around depassivation site. The failure of weld metal occurred by a combination of transgranular (TG) and stress-assisted dissolution (SAD) of delta-ferrite. Crack growth zone was minimum for AW and heat treated (550 °C/4h and 750 °C/1h) conditions as compared to SA weldment.

New Crack Arresting Methodology Using Structural Optimizing Technique. (Dept. M. ( Production ) )

The stop-hole method is a simple and economic repair technique widely used to retard or even to stop the propagation of a fatigue crack in structural components that cannot be replaced immediately after the detection of the crack. Its principle is to drill a circular hole at or close to the crack tip to transform the crack into a notch, reducing in this way its stress concentration effect. In the present study, the stop-hole method was investigated with creating a non circular hole. The aim of the present work is to obtain an optimum stop hole shape that gives maximum fatigue crack initiation life. Fatigue crack initiation life depends at least on the crack size and on the hole diameter. An optimization technique had been used and a finite element program had been built to find this optimum shape of stop hole. It was found that the fatigue life obtained by using the optimum hole shape ranges from 2 to 9 times the fatigue life obtained by using the circular holes depending on the initial hole size and initial crack length. The effect of this optimum hole shape on crack reinitiating life for different specimen geometries has been studied. It was found that the optimum hole shape increased the initial fatigue life for all specimens used whatever its geometry was. A global optimum hole shape (which is practically suitable for all geometries ) is defined. Global optimum hole shape helps engineers to use this global hole shape directly in practice without carrying out any calculation much like the use of the stop hole size found in the literature. Opposite to the traditional stress concentration factor minimization problem where the nominal area remains constant during optimization, here it is allowed to vary nominal area using design variables resulting in decreasing of the nominal stress in addition to decreasing of the stress concentration factor. This leads to higher fatigue life compared to previous studies.

Numerical method for fatigue life of plane bolted joints under thermal load

In fatigue test, the fatigue life of metal components is affected by many factors, such as test temperature, stress ratio and loading frequency. In order to study the influence of temperature on fatigue life of bolted joints, thermal stress and fast coefficient are introduced. A numerical method of fatigue crack initiation life is proposed based on Manson-Coffin strain fatigue formula. The crack initiation life of 2024 aluminum alloy at different temperatures can be obtained by this method, which provides a theoretical basis for the fatigue life prediction of metals. Then, the stress severity factor SSF is introduced to calculate fatigue life of plane bolted joints. The data obtained from the model show that the crack initiation life of aluminum alloy specimen decreases significantly with test temperature rises, the same as the fatigue life of bolted joints.

Low cycle fatigue damage model and sensitivity analysis of fatigue crack initiation by finite element approach

To meet the design requirements, different types of defects are often machined on the surface of fatigue components. Local stress concentration formed at the notch accelerate the initiation of fatigue crack, therefore greatly shorten the service lives of such components. Based on the theory of continuous damage mechanics and the principle of irreversible thermodynamics, the damage evolution model of low cycle fatigue is investigated. By programming the damage evolution model as UMAT subroutine and coupling it to ABAQUS, the fatigue damage and crack initiation life of notched P92 steel samples under specific loads are simulated, and the crack initiation location is determined. Furthermore, the damage evolution and crack initiation sensitivity of notch morphology are considered. The results show that the crack initiation occurs easily in the notch root where the damage is greatest and the plastic strain accumulates fastest under cyclic loading. The fatigue damage accumulates slowly at the initial stage, but the damage accumulates rapidly after the cumulative damage reaches a critical value. The fatigue damage evolution and fatigue initiation life are very sensitive to the notch morphology parameters. The notch morphology need to be carefully analyzed, to improve the fatigue life of the notched samples.

A Methodology for Incorporating the Effect of Grain Size on the Energy Efficiency Coefficient for Fatigue Crack Initiation Estimation in Polycrystalline Metal

Estimating fatigue crack initiation of applied loading is challenging due to the large number of individual entities within a microstructure that could affect the accumulation of dislocations. In order to improve the prediction accuracy of fatigue crack initiation models, it is essential to accurately compute the energy dissipated into the microstructure per fatigue loading cycle. The extent of the energy dissipated within the microstructure as a fraction of the overall energy imparted by loading has previously been defined as the ‘energy efficiency coefficient’. This work studied the energy efficiency coefficient as a factor in the measurement of accumulated plastic strain energy stored at the crack initiation site during cyclic loading. In particular, the crystal plasticity constitutive formulation was known as ’length scale independent’ previously. As a result, a semi-empirical approach was presented whereby the potential effect of grain size can be accounted for without the use of a strain gradient plasticity approach. The randomized representative volume elements were created based on the experimental analysis of grain size distribution. The work was aimed at capturing some of the effects of grain size and utilizing them to complete a semi-empirical estimation of crack initiation in polycrystalline materials. The computational methodology ensured the representative of microstructural properties, including the elastic constant and critical resolved shear stress via appreciable fit achieved with the empirical tensile test results. Crystal plasticity finite element modeling was incorporated into a finite element code to estimate the potential for crack initiation. The energy efficiency coefficient was computed for a class of material with grain size to C11000 electrolytic tough pitch (ETP) copper. This methodology can improve fatigue crack initiation life estimation and advance the fundamental study of energy efficiency coefficient during fatigue crack initiation.

Investigations of micro-notch effect on small fatigue crack initiation behaviour in nickel-based alloy GH4169: Experiments and simulations

Small fatigue crack initiation behaviour in a series of specimens containing different sizes of micro-notch has been observed through RepliSet system during interrupted low cycle fatigue tests. From the replicating observations, very initial single crack or multiple cracks near the micro-notch area can be captured. Since then, the crack initiation life for each specimen can be determined. Computationally, a numerical procedure with a combination of crystal plasticity theory and finite element implementation has been constructed for predicting small fatigue crack initiation life as well as identifying the crack initiation sites near the micro-notches. With the help of fatigue indicator parameter represented by accumulated plastic slip, the predicted numbers of cycles for the submodels with various micro-notch sizes agreed with the experimental ones. In addition, the simulation results can successfully reveal the mechanism in crack initiation sites. The stress concentration effect was found to be responsible for corner crack initiation, while the combined effects of stress concentration and grain orientation around micro-notch area showed great influence on edge crack initiation.

Experimental and numerical study of mooring chain residual stresses and implications for fatigue life

Residual stresses in large mooring chains were measured for the first time in this study. Two measurement techniques were employed (neutron diffraction and hole drilling). Elastic-plastic finite element simulation of the proof loading was conducted, and the computed residual stresses were compared to the experimental measurements. Further, the cyclic plasticity of the material was taken into account to investigate residual stress redistribution caused by introduction of corrosion pits and cyclic service loads. A critical damage parameter was employed to estimate fatigue crack initiation life of corroded mooring chains subjected to various service load levels considering the effect of residual stresses.

Fatigue crack initiation and propagation life assessment of butt joint considering the effect of corrosion

Fatigue crack initiation and propagation life assessment of butt joint considering the effect of corrosion

Fatigue damage evolution and finite element simulation of the samples under different notch morphology

Based on the theory of continuous damage mechanics and the principle of irreversible thermodynamics, the damage evolution model of low cycle fatigue is investigated. By programming the damage evolution model as UMAT subroutine in ABAQUS, the fatigue damage and crack initiation life of notch materials under specific loads are simulated, and the crack initiation location is determined. Furthermore, the effects of root radius, depth and opening angle on crack initiation life are investigated. The results show that the crack initiation occurs easily in the notch root where the damage is the most serious and the plastic strain accumulates fastest under cyclic loading. With the increase of notch depth and root radius, the crack initiation life decreases. For V-notched specimens, when the notch depth remains unchanged, the crack initiation life will decrease with the increase of the opening angle.

Evaluation of different strain-based damage criteria for predicting the fatigue life of friction stir spot-welded joints under multi-axial loading conditions

In this article, fatigue life of dissimilar friction stir spot welds in Cross-Tension and Lap-Shear specimens is predicted using a number of fatigue damage criteria. The results revealed a relatively good correlation between predicted fatigue crack initiation lives and experimental data. Also, it has been shown that in the whole range of applied load levels, the least discrepancy between predicted fatigue lives and experimental data is related to the Smith–Watson–Topper criterion. Finally, an excellent agreement was found in predicting the location of the crack tips using the Fatemi–Socie criterion in both types of specimens.