Prediction Of Crack Growth Rate Research Articles

Prediction of primary water stress corrosion crack growth rates in Alloy 600 using artificial neural networks

After reviewing of the data for primary water stress corrosion cracking (PWSCC) for Alloy 600 in the literature, a crack growth rate (CGR) database was assembled, and an ANN model was developed and trained upon the data, in order to model PWSCC in Alloy 600. The dependence of PWSCC CGR on each of the principal independent variables of the system has been predicted. Sensitivity analyses were conducted via “fuzzy logic” and the importance of each variable was analyzed and show that IGSCC in Alloy 600 is primarily mechanical in character with the electrochemistry being a significant contributor.

The influence of stress-regime dependent creep model and ductility in the prediction of creep crack growth rate in Cr–Mo–V steel

In this paper, the stress-regime dependent creep model and ductility have been implemented in a ductility exhaustion based damage model, and their influence on creep crack growth (CCG) behavior of materials have been analyzed. By using the stress-regime dependent creep model and ductility, the CCG rate in a Cr–Mo–V steel over a wide range of C* has been predicted by finite element analyses. The predicted CCG rates agree with the available experimental data in the literature. The analysis results show that with increasing C*, the creep model and ductility for determining crack-tip creep damage accumulation change from the low-stress regime model and ductility through a combination of low- and high-stress regime model and ductility to high-stress regime model and ductility. These changes lead to the line segments of the da/dt−C* curves. In CCG life analyses and designs of high temperature components, the stress-regime dependent creep model and ductility need to be used.

Evaluation of three current methods for including the mean stress effect in fatigue crack growth rate prediction

AbstractThe fatigue crack growth rates curves of engineering materials depend on two parameters. In addition to the dependence on the classical stress intensity factor (SIF) range ΔK, there is a dependence on the mean load (or mean SIF), mainly in the near‐threshold region. The present paper provides some useful suggestions and good practices for using three of the current available methods to reduce this second dependence through the use of tuning constants. The methods considered here are the Elber, Walker and Vasudevan (or unified approach). For each approach, multiple regression analyses are performed on experimental data from the literature, and the correlations in two and three dimensions are graphically analyzed. Numerical examples of crack growth analysis for cracks growing under nominal stresses of constant amplitude in single‐edge and notch/hole geometries are performed, assuming an identical material component to that of the available experimental data. The resulting curves of crack size versus number of cycles (a versus N) are then compared. All three models gave approximately the same (a versus N) curves in both geometries. Differences between the behaviors of the (a versus N) curves in both geometries are highlighted, and the reasons for these particular behaviors are discussed.

Prediction of crack growth rate in Type 304 stainless steel using artificial neural networks and the coupled environment fracture model

A crack growth rate (CGR) database has been developed to train an artificial neural network (ANN). The trained ANN and the extended coupled environment fracture model (CEFM) were used to predict the CGR in Type 304SS as a function of each of the principal variables of the system. The ANN revealed the underlying relationships that map the dependencies of the CGR on the various input independent variables. A sensitivity analysis revealed that IGSCC in sensitized Type 304SS in high temperature aqueous environments is primarily electrochemical in character. Comparison between the ANN-predicted CGR and CEFM-predicted CGR reveal good agreement.

Calibration and validation of an HMA fracture mechanics based enhanced cracking performance model

An HMA fracture mechanics based enhanced cracking performance model (HMA-FM-E) was calibrated and validated using selected Florida field sections. The model calibration was undertaken by matching as closely as possible predicted top-down cracking performance with field observations. Also, predictions using the calibrated model showed that the predicted crack growth rates and crack initiation times were inversely-related, which is reasonable and agrees with our field experience. The model validation effort using the prediction sum of squares approach demonstrated the strong predictability of the calibrated model. Further calibration of the model is recommended when more field sections with high quality data are available. Also, further development and integration of submodels such as the predictive relationship for initial fracture energy based on gradation characterization and volumetric properties can be employed to make the HMA-FM-E a Level-3 design tool suitable for use in the mechanistic-empirical pavement design.

Fatigue Response of Hybrid Ti/APC-2 Nanocomposite Laminates with Single-Edged Cracks

The aims of this study are to fabricate Ti/APC-2 hybrid composite laminates with and without (W/WO) nanoparticles and investigate the mechanical properties of laminates with single-edged cracks due to both tensile and cyclic tests. The mechanical properties such as ultimate tensile strength and longitudinal stiffness of original composite laminates W/WO nanoparticles were first obtained from the static tensile tests. However, the load-displacement diagrams were plotted for the crack laminates. The constant stress amplitude tension-tension cyclic tests were conducted to receive the S-N curves and fatigue data. The ultimate strengths for both Ti/APC-2 composite laminates W/WO nanoparticles were very close at varied crack length. Ti/APC-2 cross-ply nanocomposite laminates had better fatigue resistance than that of laminates without nanoparticles. The longer the crack length is, the more their properties are reduced. Also, the values of fracture toughness of both hybrid cracked laminates W/WO nanoparticles were obtained by rule of mixtures and found acceptable. Finally, in predicting crack growth rate and residual life, instead of commonly used Paris Law for metals, the modified method was adopted for hybrid cracked laminates. The prediction is not satisfactorily acceptable, even if most results are in good agreement with empirical data.

Fatigue Crack Prediction of LC9 with Corrosion Damage Considering Effect of Load Frequency

Considering the degradation effect of corrosion damage on fatigue behavior of aero aluminum alloy, the present thesis made a research on corrosion fatigue crack growth rate. Taking into account the effect of load frequency on fatigue crack growth, a concept of corrosion fatigue frequency factor is proposed. Based on the fact that low frequency will lead to high corrosion fatigue crack growth rate, and frequency higher enough will make little difference between corrosion fatigue and pure mechanical fatigue behavior, an exponential expression of corrosion fatigue frequency factor is proposed. The crack growth rate prediction from proposed formula is proved to be in good agreement with experimental results for steadily extended corrosion fatigue crack.

Stress Corrosion Cracking in Pipeline Steels

Clusters of fine stress corrosion cracks on the external surface of buried steel natural gas pipelines in contact with groundwater have been examined and studied. The growth rates of transgranular stress corrosion cracks have been modeled and determined by conducting laboratory tests under similar conditions to those recorded in practice. The steel samples were immersed in an anaerobic dilute, near neutral solution with an open circuit potential for various times under stress. Metallographic examination of the resulting stress corrosion cracks was then conducted. Transgranular fracture, similar to that observed in the field, was observed following tests carried out under low frequency cycling in combination with a high stress ratio (R= minimum load/maximum load). A quantitative relationship between the frequency and stress ratio was developed giving crack growth rates similar to those observed in practice. Also, a superposition model was developed and applied to the experimental data which gave very good agreement between the actual and predicted crack growth rates. Applying the superposition model to the operating natural gas pipeline data showed that realistic predictions of crack growth result when taking interaction of the cracks into account.

Customization of the CEFM for predicting stress corrosion cracking in lightly sensitized Al–Mg alloys in marine applications

The Coupled Environment Fracture Model (CEFM) has been modified and calibrated to predict crack growth rate (CGR) in aluminum marine alloys. The customized CEFM provided quantitative predictions of the effects of O2, electrochemical potential, stress intensity factor, and conductivity on CGR in lightly sensitized AA5083-H321 in 3.5 wt.% NaCl solution, as well as explaining the development of a semi-elliptical surface cracks. The importance of the properties of the external environment, such as conductivity, oxidant/reductant concentration, and the kinetics of the cathodic reactions on the surfaces external to the crack has been confirmed. Crack growth is attributed to a sequence of microfracture events at the crack front, the frequency of which is determined by the mechanical conditions that exist at the crack tip, as governed by the stress intensity while the microfracture dimension is determined by hydrogen-induced fracture, with the CGR being the product of these two quantities. The success in explaining the intergranular stress corrosion cracking (IGSCC) of aluminum alloys, argues that the basic concept of the CEFM, that the internal and external environments are strongly coupled, is sound and that the CEFM, which was originally developed to describe IGSCC in sensitized stainless steels is equally applicable for describing IGSCC in lightly sensitized aluminum alloys.

An assessment of a strain‐life approach for fatigue crack growth

PurposeFatigue crack growth models based on elastic‐plastic stress‐strain histories at the crack tip region and strain‐life damage models have been proposed. The UniGrow model fits this particular class of fatigue crack propagation models. The residual stresses developed at the crack tip play a central role in these models, since they are applied to assess the actual crack driving force. This paper aims to assess the performance of the UniGrow model based on available experimental constant amplitude crack propagation data, derived for several metallic materials from representative Portuguese bridges. It also aims to discuss key issues in fatigue crack growth prediction, using the UniGrow model, in particular the residual stress computation and the suitability of fatigue damage rules.Design/methodology/approachThe UniGrow model is assessed using data derived by the authors for materials from Portuguese riveted metallic bridges. Strain‐life data, from fatigue tests on smooth specimens, are used to propose a convenient fatigue damage model. Predicted crack growth rates are compared with experimental crack propagation data obtained by authors using fatigue tests on compact tension specimens. Since the UniGrow model is a residual stress‐based propagation model, elastoplastic finite element analysis is proposed for comparison with the analytical approach implemented in the original UniGrow model.FindingsThe use of the Smith‐Watson‐Topper damage parameter overestimates the stress R‐ratio effects on crack propagation rates, mainly if the material shows crack propagation rates with small to moderate sensitivity to stress R‐ratio, which is the case of the materials under investigation in this paper. Alternatively, the application of the Coffin‐Manson damage law leads to consistent fatigue crack growth predictions for the investigated range of positive stress R‐ratios. The stress R‐ratios effects may be solely attributed to the residual stresses. Their estimation, using an analytical approach, may lead to inconsistent results, which is demonstrated by an alternative elastoplastic finite element analysis.Originality/valueContributions for more accurate predictions of fatigue crack propagation rates, for several stress ratios, using a strain‐based approach is proposed. This approach is valuable since it may be used to reduce the time consuming and costly fatigue crack propagation tests. Furthermore, the proposed approach shows potential for an unified crack initiation and propagation approach.

Effect of KIII on Fatigue Crack Growth Behavior

In this study, mixed-mode fatigue tests are conducted using surface-cracked specimens. Slant surface-cracked specimens are prepared with crack angles of 15 deg, 30 deg, 45 deg, and 60 deg. It is shown that a “factory roof” fracture is formed at the deepest point of the surface crack due to ΔKIII and causes the crack growth rate to decrease. Additionally, fatigue crack growth is simulated using the superposition finite element method (FEM) with crack growth criteria. It is shown that conventional crack growth criteria are not applicable to factory roof fractures. Finally, a modified criterion for the prediction of crack growth rate is proposed, fatigue crack growth simulation is conducted using this criterion, and the results are compared with experimental results.

Continuum approach to fatigue crack initiation and propagation in welded steel joints

In this paper, a continuum-based discrete crack growth approach is introduced for the fatigue life analysis of welded steel joints. The proposed approach applies cumulative damage models utilising the material hardness and grain size statistics. The approach is validated with fatigue strength tests, and the predicted crack growth rates and fatigue lives have been observed to be in good agreement with the experiments.

Fatigue crack growth in fibre metal laminates with multiple open holes

ABSTRACTThe fatigue crack growth behaviour of hybrid S2‐glass reinforced aluminium laminates (Glare) with multiple open holes was investigated experimentally and analytically. It was observed that the presence of multiple‐site fatigue damage would increase crack growth rates in the metal layers as two propagating cracks converged. An analytical crack growth model was established for predicting crack growth rates based on empirical Paris equation. The effective stress intensity factor at crack tips is a function of mode I far‐field stress intensity factor, crack opening stress intensity factor and effective non‐dimensional stress intensity factor that incorporated the crack‐bridging effect in fibre metal laminates. The predicted results under different applied stress can capture the trend of averaged crack growth rates in experiments, although deviation exists in the predictions.

Size effects on residual stress and fatigue crack growth in friction stir welded 2195-T8 aluminium – Part II: Modelling

The use of residual K ( K res ) approaches for prediction of fatigue crack growth rates in residual stress fields was studied. Finite element models of the samples were built and the measured residual stress data put into the model. The virtual crack closure technique (VCCT) was used to calculate K res (stress intensity factor from residual stress) together with its changes with crack length using data from the part I paper. Local K res values were used to calculate effective R values. K op and Δ K eff values throughout the crack path in the weld. The master curve approach was used to relate these to corresponding values for crack growth rates. Predicted crack growth rates were compared with experimental results. Changes in crack growth rate found as the crack grows through the weld can successfully be predicted via application of this closure based model. Agreement between predictions and experimental data was best for tensile residual stress fields and was not as exact in compression. Possible reasons for this discrepancy are discussed.

Visualization and Modelling to Understand Rail Rolling Contact Fatigue Cracks in Three Dimensions

The article presents an extensive survey of experimental data on rolling contact fatigue (RCF) crack shape and propagation characteristics in rails removed from service, where such cracks are angled to the rail axis. The data include re-analysis of previously published experimental data to extract crack shape information and new experimental work on crack shapes at different stages in the early RCF life. Periods from initiation (ratcheted ‘flake cracks’) have been considered through very early growth to the limit of one prior austenite (PA) grain and on to rail-head visual cracks. Techniques included multi-sectioning through single cracks and crack zones, on used rail and test discs, to build up real three-dimensional (3D) data on crack shapes and propagation characteristics. This data have been compared with the UK rail system guidance charts relating to visual crack length and respective vertical depth; all data fell within the indicated guidance zones. The configuration of such angled cracks, typically found in curves, so aligned due to the vector of both lateral and longitudinal traction, rather than just axially, was identified as an important case for modelling. A fracture mechanics-based model has been developed to predict modes I and II stress intensity factors for such cracks covering multiple PA grains. An important geometry effect is revealed by which a contact approaching a crack angled to the rail axis is effectively ‘offset’ from the approach direction considered in 2D models, thereby resulting in lower predicted peak stress intensity factor values, compared with 2D, for the prediction of crack growth rates.

Effect of KIII on Fatigue Crack Growth Behavior (2nd Report, Verification of Crack Growth Criterion)

Mixed mode fatigue tests are conducted using through cracked specimen. Slant through cracked specimens are made where crack angle is 15°, 30° and 45°. It is shown that factory roof is made in all crack tips due to ΔKIII, and crack growth rate decreases by the factory roof. Fatigue crack growth is simulated using S-version FEM( Finite Element Method) using crack growth criteria. It is shown that conventional crack growth criteria are not available to predict fatigue crack growth with factory roof. In authors' previous study, modified new criterion for the prediction of crack growth rate is proposed. In this study, by using new criterion, fatigue crack growth simulation is conducted, and results are compared with those of experiments and discussed.

Deterministic Analysis of Crack Growth Rates in Carburized Steels

In view of the need to understand low cycle fatigue (LCF)-microstructure relationships in carburized steels, bending fatigue tests and fatigue modeling were conducted on carburized 4320 steel. The starting case microstructure of the specimens, directly quenched and tempered after carburizing, consisted of plate martensite and retained austenite. This paper compares linear elastic fracture mechanics (LEFM) based models that predict crack growth rates in the transgranular crack growth region with an effective stress intensity reduction due to stress/strain induced transformation of austenite to martensite in front of the crack tip. These models more accurately predict fatigue lives than a simple linear elastic fracture mechanics model without a reduction in the stress intensity. A sensitivity analysis on the model results as a function of input parameters was also performed.

High temperature fatigue crack growth in Alloy 718 – Effect of tensile hold times

The present investigation aims to clarify the mechanisms behind hold-time fatigue crack growth in Alloy 718 by using well designed tests where the crack length is carefully monitored. The results indicate that there is a significant embrittlement in a zone ahead of the crack tip during the hold-time, which is cracked open on the next load reversal. This leads to a very large cyclic contribution to the total crack length increment during a cycle, orders of magnitude larger than expected from purely cyclic tests at higher frequencies. During the hold-time follows the growth determined from pure sustained load tests, with the exception of an initial transient after the opening of the embrittled zone. An attempt to model the crack growth rate was made using a superposition model where the crack growth increments from high frequency da/dN testing and sustained load tests were added. The predictions of the total crack growth rate are generally adequate, but when the predictions of the individual contributions are scrutinized, it is obvious that the simple model does not correlate with the physical reality. Therefore both inter- and extrapolations from such a model are uncertain. Further, the test results show a decreased sustained load crack growth immediately after unloading/reloading of the crack. This transient behavior can potentially explain the reduction in crack growth rates previously explained by overload effects.

Analysis of Material Properties for the Numerical Simulation of Fatigue Crack Growth under Variable Amplitude Loading

The numerical simulation of crack closure is employed to assist on the prediction of crack growth rate. Under fatigue load, the stress-strain response of metals is altered due to cyclic loading. For this reason, the material properties characterization is of prime concern as an input parameter to obtain reliable results. From numerical simulations, it was observed that simple material models do not provide accurate data for long crack lengths. In this paper, the effect that different hardening models have on the opening response of a cracked component when it is subject to variable amplitude loading is analyzed. The interaction effects (crack arrest/acceleration) for long crack length simulation are specially highlighted. For this purpose, a 6082-T6 aluminium alloy was analyzed experimentally and numerically in order to measure crack closure, and then, those data were used to predict fatigue crack growth rate under different patterns of overload. The Paris equation and the Elber crack closure concept were employed. The results showed that small variations in the opening stresses obtained from different material models produce high overestimated simulations of crack growth rate. Also, it was proved that the crack closure mechanism is able to take into account interaction effects due to variable amplitude loading.

A CRACK GROWTH MODEL BASED ON FATIGUE DAMAGE ACCUMULATION

A novel model for predicting the fatigue crack growth had been developed based on the concept of the fatigue damage accumulation. Fatigue crack growth was considered as a process of continuous crack nucleation. The crack tip failed to form a fresh crack while the accumulative fatigue damage reached the critical damage. A simplified model of the general crack growth model was proposed with the assumption that the damage zone can be divided into many different zones and each zone had the same crack growth rate. The model was applied to predict the crack growth of the compact specimen made of 16MnR steel under the constant amplitude loading. The predicted crack growth rate was in excellent agreement with the experimental observations.