Predict Fatigue Crack Growth Research Articles

Fatigue crack growth resistance of the austenitic stainless steel Alloy 709 at elevated temperatures

Fatigue crack growth resistance of an austenitic stainless steel Alloy 709 has been evaluated at temperatures of 550, 650 and 750 °C in air and vacuum. Tests were conducted at a frequency of 0.25 Hz and a stress ratio of 0.1. The linear elastic stress intensity factor range (ΔK) has been used to characterise fatigue crack growth resistance. A modest detrimental effect of air at elevated temperatures on fatigue crack growth is identified and discussed. Striated transgranular fatigue is found to be the failure mechanism for all test conditions. The formation of striations and the interaction of crack growth with slip traces are further investigated using in-situ testing (within a scanning electron microscope), together with transmission electron microscopy carried out on samples extracted by focused ion-beam milling perpendicular to fracture surfaces. Finally, an analytical model is proposed to predict fatigue crack growth in air for Alloy 709.

Machine learning approach to predict fatigue crack growth

Prediction of fatigue crack growth is an important requirement during estimation of residual life of machine components or during failure analysis. The existing theoretical models are specialized to predict the crack growth for one of the three stage of classical crack growth diagram. In this work, a novel and unified machine learning based approach has been developed to cover both stage-II and stage-III regions of crack growth rate. Three alternative machine learning algorithms are investigated to identify the most suitable algorithm for prediction of fatigue crack growth rate. The models are trained using experimental data conducted on CT specimens of carbon steel subjected to different types of cyclic loading. The comparison of mean squared error and R2 score in terms of accuracy in percentage obtained from the three models is presented. The guidelines for training and tuning of machine learning models are discussed.

Experimental investigation of overload effects on fatigue crack growth behaviour of 7050‐T7451 aluminium alloy

AbstractFatigue damage and fracture of metals are the major failure modes of mechanical structures. Studying the fatigue crack growth (FCG) behaviour under overload effects can better explain the loading interactions and develop more accurate FCG prediction models. This research innovatively designed and established the experimental system that is suitable for FCG test under variable amplitude loading (VAL). Systematically, FCG experiments were carried out by introducing single tensile overload, single compressive overload and tensile/compressive sequential overload in constant amplitude loading (CAL). The effects of overload ratio, stress intensity factor (SIF) range ∆K and tensile/compressive overload sequence on the FCG behaviour of 7050‐T7451 aluminium alloy were comparatively analysed. The retardation or acceleration effects on the macro FCG and the micro crack deflection, crack bifurcation and particle falling effects of different overloads as well as their interactions were revealed.

Curved fatigue crack growth prediction under variable amplitude loading by artificial neural network

The focus of this study is to predict curved crack FCG failure under variable amplitude load effectively and accurately. Based on the artificial neural network (ANN) and FCG path/life prediction models, a numerical calculation method is designed. The proposed method considers the underlying physical mechanism of cracked structure, and only a relatively small amount of finite element calculations are required to predict FCG problems with different initial conditions. Furthermore, it can be found that the geometric parameters of hole and crack have an effect on FCG. Finally, compared with the experimental and simulation results of different examples, the effectiveness of the new method is verified.

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.

Fatigue crack growth prediction for small-scale yielding (SSY) and non-SSY conditions

A model to predict fatigue crack growth (FCG) for small-scale yielding (SSY) and non-SSY conditions where J-integral applies is established. The ΔJ-characterized elastic-plastic fracture mechanics (EPFM) analysis is performed based on a rearranged FCG process to achieve the crack-tip geometry and stress state. The root stress of blunted crack and the FCG rate are calculated from the cycle-wise local and global energy balance. To consider effects of geometry, load level and strain hardening, a correction factor is further developed in the governing equation of global energy balance. The model is validated using the test data of several types of metals.

Influence of crack driving force on correlating stress ratio effects in fatigue crack growth rate of a nickel base super alloy IN720

Damage tolerance evaluation under service loads requires a unified constant amplitude fatigue crack growth law for the material with nullified stress ratio effects. Several types of crack driving force parameters have been proposed in the past to correlate stress ratio effects on fatigue crack growth rates in various materials. In the present investigation, an effort was made to use four different crack driving force parameters to eliminate stress ratio effects in a nickel base super alloy IN720. Constant amplitude fatigue crack growth rate of Inconel 720 (IN720) was experimentally determined at various stress ratios, R ranging from R = 0.1 to 0.7. All the tests were performed at room temperature and in laboratory air condition in a 100-KN servo-hydraulic test machine using compact tension specimen and following ASTM test standard procedures. As expected, increasing the stress ratio increased crack growth rates and decreased threshold stress intensity factor range, ΔKth. The conventional crack growth rate, da/dN versus stress intensity factor range, ΔK data was modified and re-plotted as a function of four different crack driving force parameters, viz., (a) Kujawski’s K*, (b) two parameter ΔK*, (c) Walker’s ΔKw and (d) Foreman’s ΔKf. It was observed that Kujawski’s crack driving force parameter, K*, was correlating stress ratio effects better than all other models in all the three regimes of crack growth rates in this material. Further, all these models were employed to predict fatigue crack growth behaviour under a truncated FALSTAFF spectrum load sequence. It was observed that the total fatigue crack growth life estimated varied from about 25 blocks to about 75 blocks. Thus, the use of appropriate crack growth model appears to influence accuracy of life prediction under spectrum loads.

Uncertainty Quantification in Small-Timescale Model-Based Fatigue Crack Growth Analysis Using a Stochastic Collocation Method

Due to the uncertainties originating from the underlying physical model, material properties and the measurement data in fatigue crack growth (FCG) processing, the prediction of fatigue crack growth lifetime is still challenging. The objective of this paper was to investigate a methodology for uncertainty quantification in FCG analysis and probabilistic remaining useful life prediction. A small-timescale growth model for the fracture mechanics-based analysis and predicting crack-growth lifetime is studied. A stochastic collocation method is used to alleviate the computational difficulties in the uncertainty quantification in the small-timescale model-based FCG analysis, which is derived from tensor products based on the solution of deterministic FCG problems on sparse grids of collocation point sets in random space. The proposed method is applied to the prediction of fatigue crack growth lifetime of Al7075-T6 alloy plates and verified by fatigue crack-growth experiments. The results show that the proposed method has the advantage of computational efficiency in uncertainty quantification of remaining life prediction of FCG.

Analysis of fatigue crack growth in cylinder head bolts of gasoline engine based on experimental data

Engines cylinder head bolts failure from the thread location in cylinder block causes the problems during repairing time. Exact prediction of fatigue crack growth in bolts by experimental analysis and finite element method modeling can affect the Maintenance Planning and Scheduling Implementation. In this study, the fatigue analysis of cylinder head bolts of a four-cylinder gasoline engine imposed by premature failure is investigated. The bolts suffer a mechanical seizure on the threads about 2 × 108 cycles in service. For such aim, standard specimens are cut off from the bolts and examined to evaluate the mechanical properties and material chemical composition. Optical microscopes and scanning electron microscopy are employed to consider the microstructures, defects, fracture surface and failure cause. The morphology of the fracture surface shows the fatigue crack growth marks, including the beachmarks, the ratchet marks and the river cracks. The finite element analysis model is presented applying the elastic–plastic finite element analysis for the bolts under variable combustion pressure. The stress history is then used to calculate stress intensity factors and fatigue life of bolts. Numerical results show that crack existence to depth of 0.35 mm is the source of failure of premature fracture.

Analysis of Surface Crack using Various Crack Growth Models

This research focusing on fatigue crack growth experiment and assessments of fatigue crack growth model. Fatigue has been considered as the most important phenomena in the engineering problems. It has been found to occur in every engineering field. Many methods were introduced to overcome fatigue problem. One of it is fatigue crack growth prediction models. Fatigue crack growth model prediction can determine the residual life of the mechanical component. Fatigue crack growth is an important parameter in engineering. It may begin from undamaged area and propagate afterwards. In this case, a mechanical component condition before an unstable crack still can be used until fatigue. With the study of fatigue crack growth, it can predict the crack life of the component and can reduce cost to repair the component. In this thesis, primary objective is to develop, test and provide computational fracture mechanics model that emphasized an uncertainties quantification for surface crack. Fracture and failure study that has been promoted by S-version finite element method is focus on the analysis of the surface-crack problem. In this project, a few limitations of the study have been implemented as to present a specific scope of the investigation. Both fatigue crack growth model is considered; Paris law model and modified Forman model. The specimen in this analysis is aluminium al 2024 T3. The effect of load on the fatigue crack growth rate is investigated on aluminium 2024 T3. It is concluded that modified Forman has a low exponent compared to the Paris law. Further the crack growth prediction based on modified Forman is lower than Paris law. Paris law only can predict the crack in the region II while modified Forman can predict the crack in region II and III. After finish setup the project based on the instruction, the project is proceeded and the data is collected. The result is analysed based on the objective by using data collected from the project. After that, conclusion is made based on the result of the project.

Accounting for initial plastic deformation for fatigue crack growth predictions under TMF loading condition

Crack propagation rates from isothermal and in-phase high temperature thermomechanical fatigue crack propagation tests for Inconel 792 resulted in a rather large scatter band when plotted against the linear-elastic stress intensity function ΔK. Previous observations indicated that the residual strains generated during the first few cycles in the uncracked specimen can explain, to a large extent, the differences between the tests. An effective ΔKeff was calculated for each test based on non-linear FE simulations. When the crack propagation rates for each test were plotted against the calculated ΔKeff, the curves collapsed to a smaller scatter band.

Equivalent thickness-based three dimensional stress fields and fatigue growth of part-through cracks emanating from a circular hole

Accurate characterization of stress states is essential to assess structural integrity and predict fatigue crack growth life. Strong three dimensional constraints always raise at cracks and holes, but have never been investigated in the frame of three dimensional fracture theory for part-through cracks emanating from a hole. Here the three dimensional constraint and its effect on crack tip fields of corner and surface cracks emanating from a circular hole in tensile plates are comprehensively analyzed using finite element method. Distributions of the out-of-plane constraint factor Tz are strongly dependent on relative circular hole size and relative thickness parameters, but the K-Tz description of the stress field remains efficient with the present of circular hole. The equivalent thickness conception based on Tz distributions is applied to crack tip stress field description. Based on the equivalent thickness conception and plasticity-induced fatigue crack closure model, evaluation of fatigue growth of corner crack emanating from a circular hole is demonstrated and validated against test data in the literature.

A Numerical Study of the Effect of Isotropic Hardening Parameters on Mode I Fatigue Crack Growth

The consideration of plastic crack tip opening displacement (CTOD, δp), as a crack driving force has given us the opportunity to predict fatigue crack growth (FCG) rate numerically, and, therefore, to develop parametric studies focused on the effect of loading, geometrical, and material parameters. The objective here is to study the effect of the isotropic hardening parameters of the Voce law on FCG, which are the isotropic saturation stress, YSat, and the isotropic saturation rate, CY. The increase of these hardening parameters causes δp to decrease. However, this effect is much more pronounced for YSat than CY. The variation is non-linear, and the rate of variation decreases with the increase of isotropic parameters. The increase of YSat increases the crack closure phenomenon. Finally, the influence of the isotropic parameters is more relevant for pure isotropic hardening than for mixed hardening.

Model for fatigue crack growth analysis

The application of damage tolerance to the design of components is based on the ability to predict fatigue crack growth (FCG) rate precisely. A literature review about analytical models showed a great number of models developed for specific materials and loading conditions. A numerical analysis of a CT specimen made of 304L stainless steel showed the complex influence of material parameters on FCG, which also depends on loading parameters, geometry and environmental conditions. Therefore an alternative to analytical models is proposed here, based on plastic CTOD, assuming that this is the crack driving force. A material law must be first obtained relating da/dN with plastic CTOD range, δp, obtained numerically using the finite element method or experimentally using DIC. This law changes with material and includes all material parameters and also environmental conditions (temperature and atmosphere). The design of a specific cracked component is made using numerical tools in order to obtain δp for different crack lengths. This second analysis includes the effect of geometrical and loading parameters.

Fatigue crack growth lessons from thirty-five years of the Royal Australian Air Force F/A-18 A/B Hornet Aircraft Structural Integrity Program

As the Royal Australian Air Force (RAAF) retires its fleet of F/A-18 A and B model Hornets, it is opportune to reflect upon some of the in-service fatigue crack growth lessons accrued from 35 years of the aircrafts Aircraft Structural Integrity Program (ASIP). In this paper, the authors have selected a small number of the key lessons and observations arising from real-world fatigue problems in the RAAF Hornet fleet that have contributed to advances in the state of the art of fatigue crack growth measurement and prediction. The importance of the lead crack concept, quantitative fractography, fatigue testing, nucleating discontinuities, the physically short crack regime, as well as individual aircraft tracking are given prominence. It is hoped that this review of the metal fatigue challenges for a highly stressed lightweight structure that have driven research into fatigue crack growth will be valuable to those aspiring to make impactful contributions to the field and the sustainment of aircraft fleets worldwide.

Structural fatigue investigation of transverse surface crack growth in rail steels and thermite welds subjected to in-plane and out-of-plane loading

This study investigates the structural fatigue crack growth behavior of observed transverse surface cracks in rail steel and thermite weld subjected to in-plane and out-of-plane loading. Stress Intensity Factor (SIFs) solutions were derived from finite element model analysis for transverse surface cracks located at five different crack initiation locations in the cross-section of rail steel and thermite weld. The SIFs were presented by the conventional geometry factor Y and weld magnification factor Mk solutions for rail steel and thermite weld. Fracture toughness and fatigue crack growth rate tests were conducted to determine the mechanical properties of thermite weld materials and are compared with reported data for rail steels (R260 and R350HT). Fatigue crack growth prediction algorithm was developed using the coupled root mean square integration approach and was compared with the conventional uncoupled two-degree of freedom approach for semi-elliptical surface crack growth in the depth and surface directions respectively. The fatigue crack growth algorithm and propagation life prediction provide valuable observations on the crack shape evolution. It proves that out-of-plane bending loading is a more significant failure mode as compared to in-plane bending loading for thermite weld joints at curve tracks. Fatigue crack propagation results provide fatigue stress range-life curves for weld-toe surface cracks initiated at the head-to-web region of thermite welded joint structure.

Airframe metal fatigue revisited

Good fatigue design and prognostics are essential for safe and sustainable aircraft. A common feature in all metallic fatigue failures is the presence of structural cracks that propagate during the life of the structure. There are fatigue crack growth prediction tools (and certification processes) that have largely ensured flight safety by utilizing crack growth rate data generated from physically large cracks. However, unexpected fatigue cracking and non-critical failures still occur, which increases the cost of aircraft sustainment. This paper attempts to define some of the reasons why these sustainment-related cracking problems can occur and some means of redress. The discussions are limited to production quality metals and fabrication processes.

Hybrid Approach of Data-Driven and Physics-based Methods for Estimation and Prediction of Fatigue Crack Growth

This paper aims to develop a hybrid method to estimate the fatigue crack growth of an aluminum lap joint specimen with and without Lamb wave signals. The proposed method is validated on the two validation specimens (T7 and T8), using the training data sets of six different specimens (T1-T6). Each validation data set includes crack length estimation of few loading cycles with the given Lamb wave signals, followed by crack estimation without the signals. First, the crack length estimation using the signals for T7 and T8 sets was performed by the data-driven based method. A set of features was extracted from the preprocessed signals. Then, a random forest model was used to estimate crack lengths with grid search-based feature selection and hyper-parameter optimization. Next, different approaches were used to estimate the crack length without the signals, since T7 and T8 were tested under different loading conditions. Assuming that the homogeneous constant loading condition leads to a similar fatigue crack growth patterns, an ensemble prognostics approach with simplified particle filter-based weight update was used to predict the crack lengths of T7 specimen. In contrast, Walker’s equation model-based approach was chosen for T8 specimen as it was tested under a different loading condition. Considering the uncertainties of the model parameters, Walker’s equation models were generated by Monte Carlo methods. The average of generated models were used to predict the remaining crack lengths of T8 specimen. The proposed method led to Top 3 in 2019 PHM Conference Data Challenge.

The prediction of fatigue crack growth of rotary kiln shell welded under cyclic loading

In this paper, fatigue crack propagation in the kiln shell welded under cyclic loading has been estimated using computerized finite element analysis (S-Version FEM). By using the S-FEM technique, it makes the local mesh which is re-meshed automatically, then it becomes easier to make a numerical study of crack growth on the welded kiln shell. Virtual crack closure integral method (VCCM) uses the stress intensity factor evaluated. From the results, it was observed that the crack depth grows faster as the number of cycles increased. It was found that the initial crack of welded shell propagates slowly, and beyond 8 × 105 cycles increase quickly. In addition, the effect of stress distribution and stress intensity factor on cracking have been discussed and evaluated. The life of the component depends on the initial crack depth, stress intensity factors and crack inclination angle.

Crack growth rate and lifetime prediction for aviation gas turbine engine compressor disk based on nonlinear fracture mechanics parameters

In this study, the fatigue crack growth and life-time prediction of aviation gas turbine engine compressor disks under operating conditions are analyzed. Different combinations of rotational speed, temperature, surface flaw forms and sizes, as well as elastic–plastic titanium alloy BT3-1 properties are considered. A crack growth rate equation, which included the fracture process zone size and nonlinear stress intensity factor, is derived. The assessments of the structural integrity of the rotating disk are compared for elastic and elastic-plastic solutions. It is indicated that the traditional elastic crack growth models overestimate the residual fatigue lifetime with respect to the nonlinear fracture mechanics approach.