Study Of Fatigue Crack Growth Research Articles

Study of fatigue crack growth of al 6061-T6 welds obtained by gas metal arc welding along longitudinal direction

Study of fatigue crack growth of al 6061-T6 welds obtained by gas metal arc welding along longitudinal direction

An innovative approach to planar mixed-mode fatigue crack growth study

Determining fatigue crack growth parameters under mixed-mode loading is crucial for damage tolerance design, particularly in assessing a component's residual life with defects to prevent disasters. This study introduces a novel methodology for determining fatigue crack growth rate. A digital microscope is utilized to track the fatigue crack tip with respect to local coordinate systems established on the specimen surface. A grid is drawn and subdivided into units with proposed nomenclature. Compact Tension and Shear (CTS) specimens, extracted from AA-7085 plates, undergo fatigue loading in a servo-hydraulic universal testing machine. A fixture proposed by Richard et al. induces planar mixed-mode loading cases in the specimen. During experiments, the digital microscope tracks the crack tip using grid unit corners as local origins. The local coordinates of the crack tip are transformed into the crack tip coordinates with respect to the pre-crack tip. Crack evolution data are used to determine the fatigue crack growth rate by analyzing tangents of a three-parameter fitted curve. The equivalent SIF ranges are computed at different crack lengths in an XFEM script written in Ansys. The modified Paris parameters are found and experimentally observed crack deflection angles are compared with the Maximum Tangential stress, Strain energy density, and Richard’s criterion.

A fractographic study of fatigue crack growth from a cold expanded fastener hole at an engineering “crack initiation” scale

AbstractThe effects of cold expansion on fatigue crack growth from fastener holes in aircraft structure are a well‐studied problem, with many contributions focusing on both residual stresses and “long crack” experimental data. In this study, the authors have used quantitative fractography techniques to measure fatigue crack growth from naturally occurring discontinuities on an engineering “crack initiation” scale. Fatigue crack growth rates were measured through the full range of crack depths, allowing the effects of beneficial residual stresses to be directly observed. The almost identical early crack growth rates in cold expanded and non‐cold expanded specimens were demonstrated up to crack depths of approximately 0.1–0.2 mm. Beyond this depth, fatigue crack growth in cold expanded specimens was markedly slowed to the point at which failure ultimately arose from cracks away from, or growing into, the fastener hole. The available life improvement factor for cold expansion is therefore maximized through a crack growth‐based lifing approach.

Fatigue Crack Growth Evaluation of Pipeline Steels and Girth Welds

Abstract Fatigue test specimens were prepared and tested with an API 5 L X70 spiral welded pipe steel and girth weld. For a few selected specimens, two unloading compliance techniques (elastic compliance and back-face strain compliance) were applied simultaneously to a single specimen for direct comparisons of in situ crack size estimation. This paper also includes fatigue crack growth rate (FCGR) data of other pipe steels and welds available in the literature. It was observed that most FCGR curves of pipeline steels (X65–X100) remained within the BS 7910 mean and upper bound design curves in the Paris region. It was noted that the FCGR of austenitic stainless pipe steel and girth weld obtained from Arora et al. (2014), “Fatigue Crack Growth Studies on Narrow Gap Pipe Welds of Austenitic Stainless Steel Material,” Procedia Eng., 86, pp. 203–208) showed a very superior fatigue property.

A study on the effect of different Paris constants in mixed mode (I/II) fatigue life prediction in Al 7075-T6 alloy

Mixed-mode (I/II) fatigue crack growth experiments in Al 7075-T6 alloy have been performed using a newly proposed single edge cracked circular (SECC) specimen. An equivalent stress intensify factor (ΔKeq) model based on the generalized MTS criterion (GMTS) has been tested for the first time for mixed-mode fatigue crack growth rate correlation and prediction of the fatigue life. Along with this proposed model, other widely used ΔKeq models have also been considered to study the effect of Paris constants on the fatigue life prediction. Three different mode mixity configurations (30°, 45°, and 60°) have been considered for experiments with multiple specimens to study the reproducibility aspect. With the help of detailed fatigue crack growth finite element simulations and experimental results, the performance of the various ΔKeq models has been tested in terms of correlation with the fatigue crack growth rate and prediction of the fatigue life. The results of the present investigation show that using the mixed-mode Paris constants along with Tanaka’s and Richard’s ΔKeq models accurately and consistently predicts the fatigue life compared to other Paris constants. The GMTS criterion has been shown to exhibit excellent performance in the prediction of crack growth path and thus signifies the importance of T- stress under mixed-mode fatigue loading. Additionally, the results of the present investigation establish the potential application of the SECC specimen for mixed-mode fatigue crack growth studies of metallic materials. Furthermore, fractographic studies also confirm the experimental observations in Al 7075-T6 alloy.

Bridging macro to micro-scale fatigue crack growth by advanced fracture mechanical testing on the meso-scale

Fatigue and fracture mechanics testing is strictly regulated by standards that specify a minimum specimen size. However, when component sizes or material volume decrease below this limit, test methods need to be adapted for small scales. In fatigue crack growth studies, experiments become complex and challenging. Thus, there is lack of systematic scale-bridging studies from meso- to micro-scales on ductile materials. This study outlines a method to bridge this scale gap by performing meso-scale studies. Cantilver bending specimens with a cross section of 50x50µm2 and a lever arm length of 200µm of nanocrystalline nickel, as a model material, are milled by Xe plasma focused ion beam technique and tested following ASTM E647 standard. The gathered da/dN-curves are highly reproducible. The threshold stress intensity factor ΔKth is obtained by step-wise load reduction and discussed in context of crack closure. So, not only the gap between macro and micro-scaled testing was bridged, but also a pre-cracking step was shown to be essential for obtaining reliable data, and a new method to determine the intrinsic threshold stress intensity factor was introduced.

Numerical study on high-cycle fatigue crack growth of sinusoidal interface based on cyclic cohesive zone model

The numerical study of the high-cycle fatigue crack growth for the double cantilever beam (DCB) specimen with sinusoidal interface under remote mode-I loading is carried out based on the discrete interface element and the cyclic cohesive zone model (CCZM). Compared with the continuous cohesive element, this discrete interface element is more suitable for complex interface, easier to implement, and has better computational convergence. According to the summarized laws by the systematic researches, the general process of calibrating CCZM is proposed. The accuracy of proposed method is verified by the benchmark models representing different modes fatigue crack. In the simulation results of fatigue crack growth for sinusoidal DCB specimens, the as-N (as is the crack growth length and N is the number of load cycles) curves exhibit periodic oscillation behavior. The constructed rate model indicates that, the oscillation behavior intensifies with the increase of aspect ratio A/λ (A = amplitude, λ = wavelength). Moreover, the sinusoidal interface can delay fatigue crack growth, and this effect is more obvious with the increase of interface roughness. Due to the stress concentration at the wave crests and troughs of sinusoidal interface, the fatigue damage is easier to accumulate, which is the mechanism of fast and slow alternating propagation. In addition, the results of this paper also demonstrate that, the periodic oscillation behavior will be more intense when the materials on both sides of interface are inconsistent.

Equivalent Stress Intensity Factor: The Consequences of the Lack of a Unique Definition

The concept of an equivalent stress intensity factor Keq is used in the study of fatigue crack growth in mixed-mode situations. A problem seldom discussed in the research literature are the consequences of the coexistence of several alternative definitions of mixed mode Keq, leading to rather different results associated with the alternative Keq definitions. This note highlights the problem, considering several Keq definitions hitherto not analyzed simultaneously. Values of Keq calculated according to several criteria were compared through the determination of Keq/KI over a wide range of values of KI/KII or KII/KI. In earlier work on Al alloy AA6082 T6, the fatigue crack path and growth rate were measured in 4-point bend specimens subjected to asymmetrical loading and in compact tension specimens modified with holes. The presentation of the fatigue crack growth data was made using a Paris law based on Keq. Important differences are found in the Paris laws, corresponding to the alternative definitions of Keq considered, and the requirements for candidate Keq definitions are discussed. A perspective for overcoming the shortcomings may consist in developing a data-driven modelling methodology, supported by material characterization and structure monitoring during its life cycle.

Crack growth life calculation approach of surface cracked landing gear strut under eccentric compression load by bulging effect

The landing gear strut is mainly subjected to eccentric compression load during service, and the calculation of its stress intensity factor is often considered as a challenge to calculate the surface crack growth life. Based on the bulging effect, we modify the stress intensity factor of the surface cracked cube, and obtain the stress intensity factor formula of the surface cracked strut. It calculates the stress intensity factor by using geometric characteristics and load characteristics, and calculates the crack growth life without meshing finite element model. In the study of surface fatigue crack growth of steel strut under eccentric compression load, the proposed approach to calculate the crack growth life of surface cracked strut is proved to be feasible. The method is simple and conservative.

Dwell Fatigue Crack Growth Behavior of CP-Ti TA2 Welded Joint at 25 °C and 200 °C

We report on studies of the dwell fatigue crack growth (FCG) behavior of commercial pure titanium (CP-Ti) TA2 weld joints at 25 °C and 200 °C. Taking into account the effects of load ratio and dwell temperature, the impact of dwell on FCG behavior and the associated fracture mechanism are clearly demonstrated. Meanwhile, in order to illustrate creep–fatigue interaction, finite element simulations are also performed to analyze the evolution of stress and strain field near the crack tip. The results show that with the increase in dwell temperature, the FCG rate is increased. Furthermore, the effect of dwell on FCG behavior is more pronounced at higher load ratios. Finite element simulation results indicate that dwell induces creep stress relaxation and leads to an increase in the equivalent plastic strain near the crack tip. With the increase of dwell temperature and load ratio, the more pronounced creep deformation may lead to a creep-dominated FCG behavior. As consistent with the above analysis, the examination of the fracture surface reveals that more cavities and secondary cracks may be observed because of the increased creep deformation at the higher dwell temperature and load ratio.

Mixed mode (I/III) fracture studies using a new specimen setup

In the present work, mixed mode (I/III) fracture studies have been investigated using a new specimen and an out-of-plane loading fixture that can be used with the conventional uniaxial universal testing machines. The proposed specimen is a single edge cracked circular (SECC) specimen which can be employed on both metallic and non-metallic materials under static and fatigue loads. A large number of three-dimensional finite element analyses have been performed on the proposed specimen setup to demonstrate the efficacy of the proposed setup. Experimental fracture studies have also been performed under mixed mode (I/III) loading conditions. The present experimental results are in very good agreement with the available mixed mode (I/III) fracture criteria. Experimental results indicate that the pure mode III fracture toughness is nearly 1.46 times greater than that of the mode I. This has been found due to an increase in the inelastic region around the crack tip in pure mode III and is in accordance with the published results. Detailed analyses of the fractured surfaces have also been carried out. It has been demonstrated that the fractured surfaces become more rough as the mode III component increases. Preliminary results of mixed mode (I/III) fatigue crack growth studies on Al 7075-T6 using the metallic SECC specimen have also been presented. The results of the present study confirm that the proposed specimen setup can be used to study the mixed mode fracture behavior of linear elastic materials under static and fatigue loading.

Fatigue crack behaviour of composite- and mono-coated marine structural components

ABSTRACT In this study, experimental and numerical studies of fatigue crack growth (FCG) mechanisms in coated and uncoated structural components of EN8 steel (EN8) and aluminium alloy 6063 (AA6063) samples were compared. Electroplating (EP) (of nickel (Ni) and chromium (Cr)) and physical vapour deposition technique (PVD) (of aluminium nitride (Al-N) and nanocrystalline layers of titanium aluminium nitride (Ti-Al-N)) methods were used for mono and composite metallic coatings of EN8 and AA6063. The present study focuses on the implications of the coating-substrate interface during FCG initiation and propagation by considering the loading-bearing ability of the coating. Fatigue cracking mechanism depends on coating fracture toughness (FT) at the interface of mono-composite coating and its thickness. In comparison, composite-coated PVD samples proved better than mono-coated electroplated samples due to the delayed plastic deformation process on the surface of the substrate sample.

Experimental studies of fracture and fatigue crack growth in dissimilar metal welded joints of nickel-based superalloys: Cracks oriented along the weld

Nickel-based superalloys, Inconel 718 and Nimonic 80A, were welded without any interlayer for Blisk manufacturing in aerospace industries. Fracture toughness and fatigue crack growth rate of the dissimilar metal welded joints were experimentally studied to confirm their structural integrity. Cracks were oriented in and along the weld. Base metals, Inconel 718, and Nimonic 80A differ in yield strength but have the same modulus of elasticity. Electron beam welding was used to butt weld Inconel 718 to Nimonic 80A. J resistance curve ( J vs. crack extension) was experimentally found to determine the elastic–plastic fracture toughness. The welded specimens showed lesser fracture toughness and higher crack growth rates than base metals. An increase in load ratio resulted in an increase in fatigue crack growth rate in the high Δ K region and a decrease in Δ Kth. Fractography studies confirmed linear elastic and plane strain fracture all along the fatigue crack growth rate path except for the final fracture with plane stress.

Numerical study of surface fatigue crack growth in steel plates repaired with CFRP

This paper presents a numerical study of the fatigue behaviour of carbon fibre-reinforced polymer (CFRP) repaired steel plates with a surface crack under uniaxial cyclic tension. A simulation programme for surface fatigue crack propagation has been developed by means of Abaqus-Python scripting. Validation of models was first performed analytically and experimentally, through evaluation of the stress intensity factor (SIF) and fatigue life of unrepaired specimens, respectively. The developed programme was then used to simulate the fatigue behaviour of surface cracked specimens repaired with CFRP sheets. Different aspect ratios of surface cracks, numbers of CFRP layers, CFRP sheet widths and types and CFRP repair schemes (i.e. single- and double-sided reinforcement) were considered, to assess their influence on the CFRP repair efficiency. The results showed that applying CFRP sheets could effectively reduce the SIF of the surface cracks, thereby prolonging the fatigue life of the steel plates. Further, the repair performance improved with more and wider CFRP sheets and higher equivalent modulus but remained almost unchanged for different crack aspect ratios, while the single-sided CFRP repair outperformed the double-sided repair in efficiency. The present study is of instructive value for application of CFRP materials to the repair of surface cracked metallic structures.

Fatigue crack growth in CNT-reinforced polymer composite

Carbon nanotubes (CNT)-reinforced polymer composite has many engineering applications due to attractive properties, like high strength to weight ratio. These materials are subjected to cyclic fatigue loading during their service life. This work proposed a dual scale modeling approach to predict fatigue crack growth behavior of CNT-reinforced polymer composite under the thermo-mechanical loading environment. The effective orthotropic property of CNT-reinforced polymer composite is estimated using the mean-field homogenization technique in meso-scale modeling. Further, the equivalent composite property is used for macro-scale fatigue crack growth study using higher-order extended finite element method. To present the efficiency of the proposed modeling approach, various combinations of polymer composite (by volume fraction of reinforcement at 7.5%, 10%, 12.5%, 15%, 17.5%, and 20%) are taken for analysis. At macro-scale modeling, XFEM with higher-order enrichment terms is employed to improve the solution accuracy at the crack tip region. The proposed computational approach has been employed with the in-house developed MATLAB code and DIGIMAT software. Several numerical examples are considered with geometrical discontinuities like multiple holes and cracks.

Numerical simulation-aided particle filter-based damage prognosis using Lamb waves

Damage prognosis generally resorts to a damage evolution model and the current damage state to predict the future state and the remaining useful life (RUL). As a properly defined damage-sensitive statistical feature extracted from the Lamb waves can be used to online quantify the damage state, it is here exploited within a particle filter (PF) scheme to perform damage prognosis in structural health monitoring. An accurate mapping between the damage state and this feature would require a sufficient set of experimental Lamb waves collected in correspondence of different damage levels occurring during the run-to-failure process, which, however, is not usually available in real practice due to the high costs and the complex logistics involved in such experimental campaigns. In order to deal with this issue, this paper develops a new numerical simulation-aided particle filter-based damage prognosis framework, where the process equation is still built on the basis of available physical knowledge about the degradation process, whereas the measurement equation is built by means of a data-driven modeling approach using the features extracted from the numerically simulated Lamb waves. The PF framework serves as the state estimation tool which allows to identify the damage state and the parameters in the two equations. The future damage states and the RUL can finally be predicted by projecting the PF estimates in the future using the process equation. The proposed framework is demonstrated with reference to experimental studies of fatigue crack growth in aluminum lug structures with online Lamb wave monitoring.

Identification and prediction of fatigue crack growth under different stress ratios using acoustic emission data

On-line monitoring and quantification of fatigue cracks are essential for ensuring the reliability of engineering structures. The acoustic emission (AE) technique is one of the structural health monitoring (SHM) techniques and is capable of detecting the growth of defects in real time. In this study, the identification and prediction of fatigue crack growth (FCG) of 316LN stainless steel under different stress ratios were investigated by the AE technique. The purpose of this study is to extract multiple AE parameters from AE waves and relate AE signals with FCG under different load ratios for qualitatively identifying the fatigue damage and quantitatively predicting the crack size. The results show that three damage stages associated with crack initiation, stable crack growth, and rapid crack growth are accurately identified by using multiple AE parameters during FCG under various load ratios. The quantitative relationships between Fatigue crack growth rates (FCGRs) and AE parameters are linear in log–log scale and independent of the stress ratio. Moreover, AE energy is found to be the most superior parameter for qualitatively identifying the fatigue damage and quantitatively relating FCGR to AE data. Finally, a combined qualitative and quantitative approach for FCG assessment based on AE monitoring is proposed. The criterion for diagnosing the critical damage state and estimating the fatigue crack size is also clarified. Results from this study will provide a strategy for effectively assessing FCG with AE data and guide complementary studies for damage identification with AE monitoring.

A new simple specimen for mixed-mode (I/II) fracture and fatigue tests: Numerical and experimental studies

In the present investigation, a simple and efficient specimen geometry for conducting mixed mode (I/II) static fracture tests and fatigue crack growth studies has been proposed. No additional complicated fixtures are needed for loading the specimen and it can be used for both metallic and non-metallic materials. Large number of finite element analyses and mixed mode fracture experiments have been conducted. Present results of the experimental studies have been compared with widely used mixed mode (I/II) fracture criteria in terms of fracture toughness. The present experimental results are in very good agreement with the selected mixed mode (I/II) fracture criteria. Results of the present investigation clearly recommend the proposed specimen geometry as a useful mixed mode (I/II) specimen for fracture and fatigue studies.

A Polynomial Equation Model for Fatigue Crack Propagation in an Aeronautical Steel Material

The model widely used in the fatigue crack growth study in materials science and fracture mechanics is the Paris-Erdogan, which relates the stress intensity factor with the subcritical crack growth, under a fatigue regime. In this work, a seven degree polynomial is used as an alternative to model the crack propagation behavior and to be able to analyze the three regions charts of the cycles by load, in contrast to the common model that it performs at a slight approximation of the propagation phase of the trinca (region II). Finally, a comparison has been made between the proposed model and the secant method by the ASTM 647,2000 standard that adjusts the points obtained experimentally. This proposed equation is new as another alternative analytical model for which the adjustment parameter “R” are compared in relation to the classical approach in a aeronautical ABNT 4130 steel.

Study on fatigue crack growth behavior of selective laser‐melted Ti6Al4V under different build directions, stress ratios, and temperatures

AbstractThe experimental study of fatigue crack growth (FCG) behavior in Ti6Al4V alloy manufactured by selective laser melting (SLM) was carried out on an in situ fatigue testing machine. Two specimen orientations relative to the build direction (horizontal and vertical), two temperatures (room temperature, RT, and 400°C), and two stress ratios (0.1 and 0.5) were considered, and the FCG curves with the threshold values were determined. The results showed that the FCG properties were affected by the stress ratio due to different degree of crack closure and temperature with grain softening at 400°C in threshold and Paris region. The threshold value is highly dependent on the build direction of the alloy, which was caused by the anisotropic microstructure, while in the Paris region, the effect of material direction can be negligible and FCG rate tends to be consistent. The fracture mechanism at different stages was discussed and revealed by the fracture morphology observation using scanning electron microscopy.