Low Cycle Fatigue Crack Growth Research Articles

Low-cycle fatigue crack growth in brittle materials: Adaptive phase-field modeling with variable-node elements

This work presents an adaptive phase field framework for the simulation of crack nucleation and propagation in brittle materials subject to low-cycle loading. We introduce a fatigue history strain parameter within the phase-field framework to capture the fatigue effect. The resulting coupled differential equations are solved using a staggered iteration scheme. In order to improve the computational efficiency of the phase-field model, an adaptive refinement scheme is introduced. This scheme utilizes an artificial threshold that takes into consideration of both phase-field variables and cumulative fatigue history field variables to determine the elements to be refined. The handling of the hanging nodes resulting from mesh refinement is accomplished through the application of the variable-node element technique, offering a flexible and efficient mesh integration technique. We validate our proposed numerical scheme through five representative numerical examples: single-edge cracked specimen, compact tension specimen, double-edge cracked panel, plate with a hole and plate with multi-cracks and hole. The results demonstrate that the adaptive mesh refinement scheme significantly reduces computational costs without compromising the accuracy of the numerical predictions.

Fatigue Life and Crack Growth Rate Prediction of Additively Manufactured 17-4 PH Stainless Steel using Machine Learning

The present work is focused on machine learning-assisted predictions of the low cycle fatigue behaviour and fatigue crack growth rate (FCGR) of 17- 4 PH SS processed through L-PBF and post-processing. Various machine learning techniques reported in the literature provided a flexible approach for explaining the complex mathematical interrelationship among processing-structure-property of the materials. In the present work, four machine learning (ML) algorithms, such as K- Nearest Neighbor (KNN), Decision Trees (DT), Random Forests (RF), and Extreme Gradient Boosting (XGB) algorithms, are implemented to analyze the Fatigue Crack growth rate (FCGR) of 17-4 PH SS alloy. After optimizing the hyper parameters for these algorithms, the trained models were found to estimate the unseen data as equally well as the trained data. The four tested ML models are compared among each other over the training as well as the testing phase based on their mean squared error and R2 scores. Extreme Gradient Boosting model has performed better for the FCGR predictions providing the least mean squared errors and higher R2 scores compared to other models.

Effect of surface crack dimension on very low cycle fatigue crack growth behavior of circumferential cracked pipes

In this study, the effects of surface crack depth and length on the fracture behavior of circumferentially cracked pipes under very low cyclic loading were investigated via finite element (FE) damage analysis using an energy-based damage model. The FE analysis method was first validated by comparison with the SA508 Gr.1a surface-cracked pipe test results. Using FE damage analysis, a parametric study was conducted for a surface-cracked pipe with cracks of different depths and lengths, giving the same limit load. The variations in crack penetration and unstable fracture cycles depending on crack depth and length are discussed.

ProCrackPlast: a finite element tool to simulate 3D fatigue crack growth under large plastic deformations

Many structural components and devices in combustion and automotive engineering undergo highly intensive cyclic thermal and mechanical loading during their operation, which leads to low cycle (LCF) or thermomechanical (TMF) fatigue crack growth. This behavior is often characterized by large scale plastic deformations and creep around the crack, so that concepts of linear-elastic fracture mechanics fail. The finite element software ProCrackPlast has been developed at TU Bergakademie Freiberg for the automated simulation of fatigue crack growth in arbitrarily loaded three-dimensional components with large scale plastic deformations, in particular under cyclic thermomechanical loading. ProCrackPlast consists of a bundle of Python routines, which manage finite element pre-processing, crack analysis, and post-processing in combination with the commercial software Abaqus . ProCrackPlast is based on a crack growth procedure which adaptively updates the crack size in finite increments. Crack growth is controlled by the cyclic crack tip opening displacement varDelta CTOD, which is considered as the appropriate fracture-mechanical parameter in case of large scale yielding. The three-dimensional varDelta CTOD concept and its effective numerical calculation by means of special crack-tip elements are introduced at first. Next, the program structure, the underlying numerical algorithms and calculation schemes of ProCrackPlast are outlined in detail, which capture the plastic deformation history along with the moving crack. In all simulations, a viscoplastic cyclic material law is used within a large strain setting. The numerical performance of this software is studied for a single edge notch tension (SENT) specimen under isothermal cyclic loading and compared to common finite element techniques for fatigue crack simulation. The capability of this software is featured in two application examples showing crack growth under mixed-mode LCF and TMF in a typical austenite cast steel, Ni-Resist. In combination with a crack growth law identified in terms of varDelta CTOD for a specific material, the tool ProCrackPlast is able to predict the crack evolution in a 3D component for a given thermomechanical loading scenario.

Fatigue behavior of low-temperature hot isostatic pressed electron beam powder bed fusion manufactured Ti-6Al-4 V

Ti-6Al-4 V finds application in the fan and compressor modules of gas turbine engines due to its high specific strength. Ti-6Al-4 V components manufactured using one of the additive manufacturing (AM) techniques, the electron beam powder bed fusion (PBF-EB) process, has been an active area of research in the past decade. The fatigue life of such PBF-EB built Ti-6Al-4 V components is improved by hot isostatic pressing (HIP) treatment typically performed at about 920 ˚C. The HIP treatment at 920 ˚C results in coarsening of α laths and reduced static strength and therefore a low-temperature HIP treatment is performed at about 800 ˚C to limit the impact on static mechanical properties. In the present work, the low cycle fatigue and fatigue crack growth behavior of such a modified HIP (low-temperature HIP) treated material is assessed and compared with the respective data for the standard HIP-treated material. The modified HIP-treated material has fatigue performance comparable to the standard HIP-treated material. This work suggests that the modified HIP treatment improves the static mechanical properties without significantly impacting the fatigue performance. Also, fatigue life predictions were made from the measured defect size at the crack initiation site using a linear elastic fracture mechanics tool. The life predictions show good agreement with the experimental values for defects greater than the intrinsic crack length, where life is well predicted by large-crack growth methodology.

The crack growth rate and crack opening displacement of EH-36 steel under low-cycle fatigue loading

The aim of this work is to investigate the low-cycle fatigue (LCF) crack growth behavior of EH-36 steel from the perspective of crack opening displacement, proposing a simple method to calculate the crack growth rate. The crack tip opening displacement (CTOD) under cyclic loads is described by a theoretical formula that takes accumulative strain and crack closure into account. And, a series of quantitative load tests on LCF crack growth of EH-36 steel are carried out to investigate the variation of accumulative strain, crack opening displacement and crack growth life. Through fitting and verification analysis with the test data, the relationship between the crack opening displacement and crack growth rate is obtained. The findings show that the proposed calculate formula can effectively characterize and predict the crack growth behavior of EH-36 steel under the low-cycle fatigue regimes.

Crack Growth Conditions under Extreme Low Cycle Fatigue Crack Growth in Consideration of Stress Condition

Crack Growth Conditions under Extreme Low Cycle Fatigue Crack Growth in Consideration of Stress Condition

Aspects of simulating stable low-cycle fatigue crack growth in the main parts of aircraft gas turbine engines

The article presents theoretical basis for the industry-based approach for finite element modeling of stable crack growth in the main parts of an aviation gas turbine engine. An axial compressor disc is used as an example. Parameters of typical FE-models applied are provided. In addition, some effective practices of FE-modeling representing the novelty of this work are described: crack evolution increment under-relaxation and automation of the process of constructing a new crack front. Some simulation results are presented demonstrating implementation of the approach steps and benefits gained from the application of the listed features. Under-relaxation ensures maintaining the stability of a numerical solution for a significantly larger crack increment size. This leads to essential effort decrease as a result of reducing the total number of simulation cycles required. Automatic construction of a new crack front allows significant improvement in crack representation accuracy during the simulation process due to the greater number of points for which crack front evolution is determined.

Fatigue-resistant design in low-cycle regime by regulating the micro-structural gradient in a TWIP steel: Modelling and experiment

It remains an intricate problem how to enhance low-cycle fatigue (LCF) performance, in the premise of guaranteeing a relatively high strength (or cyclic stress) which is vital to long-time service, for engineering components. The underlying cause of this puzzle is the lack of practical theory on cracking evolution/life prediction for LCF. In this study, a two-stage constitutive LCF crack growth model was proposed for a homogeneous solid and later developed for graded structures. Based on the model, a material design strategy emphasizing the strength gradient architecture (informally, hard core and soft shell structure) is proposed and analyzed to improve the LCF properties of engineering structural materials, e.g., steels. Experimentally, sharp and gradual strength gradients were carefully introduced into a TWIP steel by a series of well-designed process combinations regarding various (heterogeneous) deformation and annealing treatments. The influences of strength/microstructure and their spatial distributions on the LCF behaviors were systematically investigated. The excellent LCF properties of the TWIP steel with a negative strength gradient are originated from the repeated strengthening and weakening of strain gradient during cyclic loading, which suppresses the fatigue crack initiation and growth. The crack initiation/propagation model developed in this study is conducive and applicable to the LCF life prediction. The theory and process of constructing microstructure gradient, and therefore mechanical property gradient, may provide new and important implications for engineering metals on anti-LCF design.

Experimental and Numerical Study on LCF Crack Propagation of Coiled Tubing Steel

Coiled tubing (CT) is a joint-less long oil pipe that is wound around a reel and can be run and pulled continuously. Due to the particularity of the operating process, low-cycle fatigue (LCF) failure of the CT constitutes the main production cost. Aiming at the characteristics of small diameter and thin wall of CT, a single-edge-notched (SEN) arc specimen was designed and machined. LCF tests were conducted with force-controlled mode. Cyclic softening of the CT steel was presented and crack growing rates were measured. Meanwhile, finite element simulation was carried out to obtain the relationships among J-integral, crack size and load. Based on the experimental and numerical results, the speed of the LCF crack growth of the CT steel is expressed as an explicit function of the J-integral. It provides a basis for predicting the LCF life of the CT under working conditions from the perspective of crack propagation.

Study of Crack Closure Effect of Hull Plate under Low Cycle Fatigue

The crack closure phenomenon significantly influences low cycle fatigue (LCF) crack growth. The crack closure theory deems that a crack can grow only when the applied load is greater than the fatigue crack opening and closing loads. The revised crack closure theory proposed in this paper provides a new understanding of crack growth: It is no longer the range of stress intensity factor ΔK that controls the crack growth rate, but the effective stress intensity factor ΔKeff. Therefore, it is of great importance to study the crack closure phenomenon of LCF. A combination of experiments and the finite element method (FEM) was used to study the effect of overload on the crack closure effect, and the study was carried out using compact tensile (CT) specimens made of AH32 steel. The FEM was used to obtain the stress changes near the crack tip and the opening displacement changes in the crack trailing area after a single tensile overload, to study the intrinsic mechanism of overload on crack closure, and to obtain the LCF crack opening and closing loads by the nodal displacement method. The effect of overload on crack morphology was observed by using high-magnification electron microscopy in combination with testing.

Fatigue dataset of high-entropy alloys

Fatigue failure of metallic structures is of great concern to industrial applications. A material will not be practically useful if it is prone to fatigue failures. To take the advantage of lately emerged high-entropy alloys (HEAs) for designing novel fatigue-resistant alloys, we compiled a fatigue database of HEAs from the literature reported until the beginning of 2022. The database is subdivided into three categories, i.e., low-cycle fatigue (LCF), high-cycle fatigue (HCF), and fatigue crack growth rate (FCGR), which contain 15, 23, and 28 distinct data records, respectively. Each data record in any of three categories is characteristic of a summary, which is comprised of alloy compositions, key fatigue properties, and additional information influential to, or interrelated with, fatigue (e.g., material processing history, phase constitution, grain size, uniaxial tensile properties, and fatigue testing conditions), and an individual dataset, which makes up the original fatigue testing curve. Some representative individual datasets in each category are graphically visualized. The dataset is hosted in an open data repository, Materials Cloud.

Insights on low cycle fatigue crack formation and propagation mechanism: A microstructurally-sensitive modeling

A novel model for revealing low cycle fatigue (LCF) fracture mechanism was established based on molecular dynamics, extended finite element method, and representative volume element technology. Stored strain energy criterion provided the basic relationship for the proposed model. The proposed model was the 3D model for comprehensively simulating LCF crack formation at outer surface and propagation along slip system. In addition, micro-scale stored strain energy damage equations were proposed. The capability of the proposed model was confirmed by the microstructural fractography and 3D X-ray computed tomography. The complicated LCF crack growth direction and the secondary cracks accompanying the main crack could be simulated successfully. Particularly, the LCF crack length, the LCF crack formation/fracture life and morphology, the active slip system proportion, and the fracture angle were compared with experiment quantitatively and qualitatively. Two formation mechanisms of branch-shaped and main crack-independent secondary crack were determined. In addition, the microstructure had a strong influence on the LCF growth rate changing from short crack to long crack and the fluctuating crack growth rate during the short crack period. The proposed model had great potential to reveal the correlation between microstructure and LCF fracture mechanism.

Energy-based damage model incorporating failure cycle and load ratio effects for very low cycle fatigue crack growth simulation

An energy-based damage model for simulating very low cycle fatigue (VLCF) crack growth is proposed to improve the prediction accuracy of our previous model. Modifications are made to explicitly include the terms related to failure cycle and load ratio. The parameters related to these terms are determined from mean fatigue life model of the material. The proposed damage model is validated by comparing with through-wall cracked pipe test data for Type 316 stainless steel. Predicted failure cycles are very close to experimental data, improving the prediction accuracy of our previous model. Comparison of fracture surfaces also shows good agreement.

Effects of strain rate on low-cycle fatigue crack growth behavior of 316LN weld metal in high-temperature pressurized water

Corrosion fatigue (CF) tests of 316LN weld metal were investigated in high-temperature water at different strain rates (0.0004–0.4%/s) under strain amplitude of 0.6%. Relationship between CF life and strain rate was proposed and environmental fatigue correction factor Fen was also calculated. It was found that the γ/δ phase boundary was more susceptible to cracking at low strain rates and δ-ferrite worked as a barrier to retard the crack growth at high strain rates. Mechanisms of CF damage involving the effects of strain rate, hydrogen, δ-ferrite and residual strain on crack growth are also discussed.

Analysis of biaxial proportional low-cycle fatigue crack propagation for hull inclined-crack plate based on accumulative plasticity

Fracture failures of ship plates subjected to in-plane biaxial low-cycle fatigue loading are generally the coupling result of accumulative plasticity and biaxial low-cycle fatigue damage. A biaxial low-cycle fatigue crack growth analysis of hull structure that accounts for the accumulative plasticity effect can be more suitable for the actual evaluation of the overall fracture performance of the hull structure in severe sea conditions. An analytical model of biaxial low-cycle fatigue crack propagation with a control parameter for ∆CTOD is presented for hull inclined-crack plate. A test was conducted for cruciform specimens made of Q235 steel with an inclined crack to validate the presented analysis. The biaxial accumulative plasticity behavior and the effects of biaxiality and stress ratios were investigated. The results of this study reveal a strong dependence of biaxial low-cycle fatigue crack propagation on biaxial accumulated plasticity.

Analysis of biaxial proportional low-cycle fatigue and biaxial accumulative plasticity of hull inclined-crack plate

Fracture induced failure of plates in hull structures subjected to in-plane biaxial low-cycle fatigue loading are generally a combination of accumulative plasticity and biaxial low-cycle fatigue damage. A biaxial low-cycle fatigue crack growth analysis of a hull structure that accounts for the accumulative plasticity effect can be suitable for the actual evaluation of the overall fracture induced failure of the hull structure in severe sea conditions. An analytical model of biaxial low-cycle fatigue crack propagation with a control parameter for the J integral was presented for hull inclined-crack plate. The test was conducted for cruciform specimens made of Q235 steel with an inclined crack to validate the presented analysis. The biaxial accumulative plasticity behaviour and the effects of the biaxiality and stress ratios were investigated by numerical analysis. The results of this study reveal a strong dependence of biaxial low-cycle fatigue crack propagation on biaxial accumulated plasticity.

Crystallographic evaluation of low cycle fatigue crack growth in a polycrystalline Ni based superalloy

We present the microscale fracture mechanics aspects of LCF cracks non-propagating till failure in the cylindrical specimens of Ni based superalloy Haynes 282 and attempt to unravel the underlying crystallographic factors. Two key parameters, Crack Tip Opening Angle (CTOA) and maximum tangential stress (θMTS) have been opted for characterization. CTOA variations along with a propagating crack, exhibit a non-linear decay followed by a stabilized regime and the fraction of the stabilized regime increases in lower strain amplitude where the fatigue life is more. Mixity of local KI and KII fields is directly proportional to θMTS and that has been assessed by measuring local deflections. The modal mixity has been found to be microstructure sensitive and an entirely statistical parameter. Different growth modes of these non-propagating cracks have been identified from the microstructural observations. The role of elasto-plastic incompatibility of neighboring grains has been addressed by conducting crystallographic analyses. There is a critical bound for Elastic Modulus (EM) and Schmid factor (SF) for the grains favoring subsurface crack propagation below which grains (mostly with high elastic compliance and low SF) are not participating in crack propagation. The favourable twin-matrix incompatibility of the microstructure has also been identified about the fatigue crack growth and twins in (211) plane is abundant in the cracked region. From the results of a detailed slip transfer analysis based on the Luster-Morris parameter (LMP), it is observed that there exists microstructurally controlled lower bounds that inhibits slip transfer (LMP value <0.1). Dislocation debris analysis using TEM has also been carried out to understand micromechanisms clearly. Both non coplanar and planar mode of dislocation activity has been observed and twin boundaries has been see to act as barrier to the planar dislocation motion.

Study of low-cycle fatigue crack growth behavior of central-cracked stiffened plates

This work is aims to study the low-cycle fatigue (LCF) crack growth behavior of central-cracked stiffened plates through experimental and numerical analysis methods. A central-cracked stiffened plate with two stiffeners is manufactured to test the influence of the stiffeners on the crack propagation in the base plate. The effects of loading amplitude, the stiffeners’ size and location on the crack growth life are investigated in the test. In numerical analysis, an elastic-plastic numerical model is developed to simulate the crack propagation of stiffened plates considering the influence of the plasticity-induced crack closure (PICC). The results show that PICC and the crack opening load can clearly reflect the crack arrest performance of the stiffener on the crack propagation in the plate. Increasing the stiffener stiffness or decreasing the space between the two stiffeners is beneficial to reduce the driving force acting on the crack tip, prolonging the crack growth life. This work is intended to contribute to a better understanding of the crack arrest performance of stiffened plates through the crack closure mechanism.

The fatigue properties, microstructural evolution and crack behaviors of low-carbon carbide-free bainitic steel during low-cycle fatigue

The low-cycle fatigue properties, microstructural evolution and crack growth behaviors of a low-carbon carbide-free bainitic steel subjected to austempering at 300, 320 and 350 °C have been investigated. The results indicated that the fatigue lifetimes of the specimens increase with decreasing the austempering temperature at lower total strain amplitudes, but a comparable fatigue lifetime or even an opposite result is observed at higher total strain amplitudes. During fatigue loading, the blocky retained austenite with inhomogeneous carbon distribution partially transforms into martensite, and the transformation amount increases with the total strain amplitude, especially for the specimens austempered at high temperatures. The transformed martensite and the untransformed austenite satisfy a Kurdjumov-Sachs (K–S) orientation relationship, which can increase the cooperative deformation capacity between the both phases. However, with continuous cycling, more martensite is formed, and these newly formed martensite grains are more likely to form micro-voids/cracks due to their brittleness, which reduces the crack initiation lifetime, and become the fast path for crack propagation, which deteriorates the crack propagation lifetime. Alternatively, more crack branching, crack deflection and severely twisted bainitic ferrite laths are observed for the specimens austempered at high temperatures, which helps to delay/hinder crack growth and increase the crack propagation lifetime, especially at high total strain amplitudes. This suggests that the fatigue lifetime can be closely associated with the interactions of the crack initiation and propagation caused by mechanically-induced martensite from retained austenite.