Fatigue Crack Propagation Data Research Articles

Numerical-experimental procedure for predicting fatigue life in SAE AMS 7475-T7351 aluminum alloy considering the effect of stress ratio.

This study addresses the prediction of fatigue life in SAE AMS 7475-T7351 aluminum alloys under variable loads, commonly used in the construction of aircraft fuselages. The main objective of the research was to develop a numerical-experimental procedure to analyze crack growth, using the Walker's approach which considers the effects of the stress ratio R on the fatigue crack growth rate d a / d N , combined with the Finite Element Method and Linear Regression of the Stress Intensity Factor. Observations showed that Walker's model effectively consolidated fatigue crack propagation data for various stress ratios when applied longitudinally to L-T rolling orientation, due to low dependence of exponent m on R -value in d a / d N equation. Simple averaging of m values effectively calculated Walker's exponent. The methodology employed experimental tests following ASTM standards for tension, fracture toughness, and fatigue, complemented by Finite Element Method (FEM) simulations. The Walker's model proved more effective, while the Paris-Erdogan model, which ignores the R effect, resulted in overly conservative service life estimates. The principle of similitude suggests that this methodology could be effective in predicting fatigue life in cases with complex geometries, where calculating the Stress Intensity Factor Fracture parameter is challenging and the Finite Element Method shows efficiency.

Scalable and portable computational framework enabling online probabilistic remaining useful life (RUL) estimation

This work demonstrates a framework that enables online prognostics in adhesive joints by estimating the real-time probabilistic remaining useful life (RUL) using ANNs based hybrid physics models and vectorized sequential Monte Carlo (SMC) simulations. The framework is developed by integrating the physics-based damage degradation model and uncertainty quantification (UQ) techniques to estimate both probabilistic fatigue failure life and RUL. The fatigue damage growth (FDG) simulator, a hybrid surrogate model that simulates real-time fatigue degradation in adhesive joints, is used. In the initial set of results, the generalized framework is validated by estimating the probabilistic fatigue failure life using two UQ methods: Markov Chain Monte Carlo (MCMC) and SMC method. The computational results are successfully compared against experimental data. The conventional MCMC sampling methods are inherently serial, which limits the exploitation of the computational speed-up provided by the FDG simulator and hinders the real-time life predictions. The SMC method quantifies the uncertainties by parallelizing the sampling process, significantly reducing computational time and enabling real-time prediction. Next, the generalized framework is used to estimate probabilistic RUL from the fatigue crack propagation data. The parallel SMC method showed very good speedup compared to the MCMC method. To further enhance the computational speed-up with SMC method, vectorized FDG simulations are introduced into the framework and good scalability is achieved. Finally, the portability of the framework is demonstrated by deploying it on the portable Raspberry Pi cluster.

Efficient Experimental and Data-Centered Workflow for Microstructure-Based Fatigue Data

BackgroundEarly fatigue mechanisms for various materials are yet to be unveiled for the (very) high-cycle fatigue (VHCF) regime. This can be ascribed to a lack of available data capturing initial fatigue damage evolution, which continues to adversely affect data scientists and computational modeling experts attempting to derive microstructural dependencies from small sample size data and incomplete feature representations.ObjectiveThe aim of this work is to address this lack and to drive the digital transformation of materials such that future virtual component design can be rendered more reliable and more efficient. Achieving this relies on fatigue models that comprehensively capture all relevant dependencies.MethodsTo this end, this work proposes a combined experimental and data post-processing workflow to establish multimodal fatigue crack initiation and propagation data sets efficiently. It evolves around fatigue testing of mesoscale specimens to increase damage detection sensitivity, data fusion through multimodal registration to address data heterogeneity, and image-based data-driven damage localization.ResultsA workflow with a high degree of automation is established, that links large distortion-corrected microstructure data with damage localization and evolution kinetics. The workflow enables cycling up to the VHCF regime in comparatively short time spans, while maintaining unprecedented time resolution of damage evolution. Resulting data sets capture the interaction of damage with microstructural features and hold the potential to unravel a mechanistic understanding.ConclusionsThe proposed workflow lays the foundation for future data mining and data-driven modeling of microstructural fatigue by providing statistically meaningful data sets extendable to a wide range of materials.

Experimental study and simulation analysis of crack propagation of Heavy-Duty milling cemented carbide tool material

The water chamber head is the core component of the nuclear island reaction evaporator. The material is 508III steel, and its main processing method is heavy-duty milling. During processing, cemented carbide tools always withstand high loads. The fatigue crack growth characteristics of them are quite different from those of traditional metal materials. The crack growth rate has a non-linear relationship with the load. The small change of load has a great influence on the fatigue crack growth. Firstly, the three-point bending test is carried out on the cemented carbide tool material, and the fracture morphology of Specimen is analyzed. Combined with the fatigue crack propagation data, the crack propagation rate is analyzed and the correlation of the data fitting can be tested. Secondly, the high temperature tensile test is performed on the hard. The high temperature damage characteristics of the cemented carbide tool materials were analyzed. Finally, based on the test, the three-point bending simulation analysis was carried out to investigate the fatigue damage characteristics, and the reliability was verified by comparison with the test results. Through the above research, it is expected to provide theoretical support for further research on tool failure mechanism and cutting parameter optimization.

Energy response of S355 and 41Cr4 steel during fatigue crack growth process

In this research, a novel approach of the fatigue crack growth rate description has been proposed. Based on theoretical and experimental approach, the mean stress effect expressed by R-ratio is present in classical da/dN–Δ K diagram. According to energy approach – based on the irrevocably dissipated energy accumulated in material (hysteresis loop) during fatigue process – the mean stress effect can be minimalized. Experimental validation of the proposed model was performed using results of fatigue crack propagation data for S355 and 41Cr4 steels in terms of strain energy density parameter Δ S or cyclic J-integral range –Δ J. In contrast to the force approach based on Kmax (or Δ K), the energy parameters Δ S or Δ J represent unambiguously the fatigue crack propagation rate, without influence of mean stress effect – R-ratio. However, in near threshold range of kinetic fatigue fracture diagram, the energy parameter displays a slight dispersion of the experimental data. According to the crack closure theory and its U-Elber parameter, the dispersion of experimental data is decreased. Therefore, the crack closure effects have a high significance in energy model – similar to the ‘force approach’ based on Δ K concept.

A recursive Bayesian approach to small fatigue crack propagation and detection modeling

Engineers have witnessed much advancement in the study of fatigue crack detection and propagation modeling. More recently, the use of certain damage precursors such as acoustic emission signals to assess the integrity of structures has been proposed for application to prognosis and health management of structures. However, due to uncertainties associated with small crack detection of damage precursors and crack size measurement errors of the detection technology used, applications of prognosis and health management assessments have been limited. In this article, a methodology is developed for the purpose of assessment of crack detection and propagation parameters and the minimization of uncertainties including detection and sizing errors associated with a series of known crack detection and propagation models that use acoustic emission as the precursor to fatigue cracking. The methodology is facilitated by the Bayesian inference of a joint-likelihood model which includes sizing and detection models. Examples where several dog-bone Al 7075T6 specimens are tested to produce fatigue crack initiation and propagation data and estimates based on remaining useful life support the effectiveness and usefulness of the proposed methodology.

Static mode fatigue crack propagation and generalized stress intensity correlation for fatigue–brittle polymers

Stable fatigue crack propagation is predominantly described by the Paris power law correlation of the crack growth rate with the amplitude cyclic stress intensity. The Paris relationship works well for most ductile materials but does not capture the response for fatigue–brittle materials lacking a cyclic damage mechanism, including ceramics and many polymers. Instead, crack growth rate of fatigue–brittle materials correlates to the peak cyclic stress intensity factor, $$\hbox {K}_{\mathrm{max}}$$ . This work shows that $$\hbox {K}_{\mathrm{max}}$$ correlation of fatigue crack growth is derived directly from static mode crack tip behavior with constant correlation coefficients, and that $$\Delta \hbox {K}$$ correlations are not generally applicable for static mode crack propagation in fatigue–brittle polymers. This derivation predicts load ratio, frequency, and waveform effects, which are included in a general static mode fatigue crack propagation law. Fatigue crack propagation data of a known fatigue–brittle polymer are presented to demonstrate static mode crack propagation behavior correlation with $$\hbox {K}_{\mathrm{max}}$$ with constant parameters.

MECHANICAL CHARACTERIZATION OF ANCIENT PORTUGUESE RIVETED BRIDGES STEELS

In repairing and retrofitting processes of ancient riveted steel bridges is crucial to assess the structural state of old metals to guaranty structural safety. Metals under long-time operations (mild, rimmed low carbon steels <0.1% C, puddle irons) and cyclic loading present a tendency for degradation processes. The case-studies of this work are five metallic bridges existing in Portugal (Luiz I, Eiffel, Fao, Pinhao and Trezoi). This work presents the study of some characteristics of materials extracted from the five case-studies, such as: monotonic tensile strength, chemical composition, microstructures, hardness, notch toughness and fatigue crack propagation. In terms of monotonic tensile tests, the materials from Luiz I, Eiffel and Fao bridges are similar to puddle steel and the materials from Pinhao and Trezoi bridges are similar to mild steel. In terms of toughness only the material from the Pinhao bridge exhibits acceptable toughness properties, considering current design requirements. The materials from the other bridges exhibit relatively low toughness properties. The fatigue crack propagation data from the old Portuguese riveted steel bridges were correlated using the Paris’s law and the possibility for a design crack growth rate was discussed.

Statistical analysis of fatigue crack propagation data of materials from ancient portuguese metallic bridges

In Portugal there is a number of old metallic riveted railway and highway bridges that were erected by the end of the 19th century and beginning of the 20th century, and are still in operation, requiring inspections and remediation measures to overcome fatigue damage. Residual fatigue life predictions should be based on actual fatigue data from bridge materials which is scarce due to the material specificities. Fatigue crack propagation data of materials from representative Portuguese riveted bridges, namely the Pinhão and Luiz I road bridges, the Viana road/railway bridge, the Fão road bridge and the Trezói railway bridge were considered in this study. The fatigue crack growth rates were correlated using the Paris’s law. Also, a statistical analysis of the pure mode I fatigue crack growth (FCG) data available for the materials from the ancient riveted metallic bridges is presented. Based on this analysis, design FCG curves are proposed and compared with BS7910 standard proposal, for the Paris region, which is one important fatigue regime concerning the application of the Fracture Mechanics approaches, to predict the remnant fatigue life of structural details.

Statistical evaluation of fatigue strength of double shear riveted connections and crack growth rates of materials from old bridges

The maintenance of old riveted bridges has become a major concern for railroad managers across Europe. These metallic structures, built on their majority on the second half of the 19th century, are indeed submitted to ever increasing traffic loads (augmentation in weight and speed of rolling stock).To assess the remaining life of these metallic bridges, some critical structural details have been identified and associated to S-N curves to be used in damage estimation (using Palmgren-Miner's rule for cumulative damage, for example). These constructional details are often described in construction standards, such as the EN 1993-1-9, the BS 5400 and the AASHTO standards. The particularity of older hot-riveted bridges is that their numerous geometric configurations are often under-represented, thus, limiting the accuracy of damage estimation.To compensate the absence of hot riveted assemblies in the standards, numerous research groups have performed extensive experimental campaigns. However, the heterogeneity and rarity of the material (no longer produced nowadays), as well as the very diverse geometrical configurations used at the time of the construction of the bridges (given the novelty proposed by the introduction of metal as a new material in monumental construction), makes difficult the comparison of experimental data.In order to facilitate interpretation and differentiation of fatigue data of a particular type of constructional detail, experimental data from double shear assemblies manufactured from three different metallic ancient bridges is considered (French and Portuguese puddled iron bridges). Then, through statistical analysis (linearized boundaries and the Fernández-Canteli model), the S-N curves corresponding to their structural configuration are identified. Comparisons to the EN 1993-1-9, the BS 5400 and the AASHTO standards are also proposed.The combination of the S-N curves and Fracture Mechanics approaches can be used to evaluate the residual life of the old riveted metallic bridges. Thus, a statistical analysis and discussion of experimental fatigue crack propagation data available for materials from the ancient riveted metallic bridges are made. This analysis is important for the application of the Fracture Mechanics approaches to predict the residual life of structural details.

Combined analytical-numerical methodologies for the evaluation of mixed-mode (I + II) fatigue crack growth rates in structural steels

This paper proposes an experimental study aiming to evaluate stress intensity factors (SIFs) for fatigue cracks propagating under pure mode I and mixed-mode I+II for a S235 structural steel. Compact tension (CT) specimens with a side hole were manufactured in order to generate a stress field, ahead of the crack tip, resulting in mixed-mode fatigue crack propagation. Specimens with distinct side hole locations were submitted to fatigue tests under stress controlled conditions for a stress ratio, R=0.01. Fatigue tests were coupled with digital image correlation (DIC) to assess mode I and mode II SIF from full-field displacement measurements. Besides, DIC data was used to directly estimate the crack tip location during the test. A validation study was carried out by comparing this approach with regard to conventional optical measurements. Finite element simulations were also performed to validate the crack branching models and direct identification of SIF values. The proposed DIC approach was demonstrated to be an efficient tool for the automation of crack path detection and stress intensity factors computation. Mixed-mode fatigue crack data correlated well with pure mode I fatigue crack propagation data using the Paris law and an effective stress intensity factor as proposed by Tanaka.

Modified CCS fatigue crack growth model for the AA2019-T851 based on plasticity-induced crack-closure

Several fatigue cracks growth laws have been suggested over the past 50years. Recently, a fatigue crack propagation law was proposed by Castillo-Canteli-Siegele (CCS model) based on the assumption that fatigue crack growth takes the form of a Gumbel cumulative distribution function. Besides many physical aspects, the fatigue crack propagation laws need to account for fatigue crack opening and closure effects. Thus, in this paper a modification of the CCS fatigue crack growth law is proposed to account for the crack opening and closure effects, as well as the stress R-ratio effect, Rσ. The fatigue crack opening and closure effects are taken into account using a plasticity-induced crack-closure model. Other fatigue crack closure models can also be used in the proposed modification of the CCS crack growth model through the quantitative parameter U=ΔKeff/ΔK. This modified CCS crack propagation model using the effective stress intensity factor range, ΔKeff, is a new version of an explicit fatigue crack propagation model, supported by mathematical and physical assumptions. In this paper, the proposed model is applied using the fatigue crack growth data and mechanical properties that were collected for the 2219-T851 aluminium alloy. Based on the plasticity-induced crack-closure model, which was first formulated by Newman, and in this paper modified with a boundary correction factor, Fw, the crack opening stress intensity factor, Kop, and quantitative parameter U are determined. The results showed a good agreement between the proposed modification of the CCS fatigue crack propagation model taking into account the plasticity-induced crack-closure model with the boundary correction factor and experimental results of the fatigue crack propagation data for the 2219-T851 aluminium alloy.

Experimental determination of accurate fatigue crack growth data in Tailored Welded Blanks

Joining of Tailored Welded Blanks (TWB) with Friction Stir Welding (FSW) is very attractive for future aircraft applications. This article provides the main findings of extensive studies of fatigue crack propagation in FSW-TWB made of AlMgSc alloy. The measurement of actual mechanical test conditions in combination with finite element simulations allows overcoming inherent drawbacks of conventional single edge notch tension (SENT) specimens. Residual stresses as result of the FSW process are measured via the cut-compliance-method. It is shown, that in case of FSW-TWB only the consideration of actual test conditions and residual stresses yields accurate fatigue crack propagation data.

Effect fracture resistance on retardation of crack growth

Purpose – The prediction of fatigue crack growth behaviour is an important part of damage tolerance analyses. Recently, the author’s work has focused on evaluating the FASTRAN retardation model. This model is implemented in the AFGROW code, which allows different retardation models to be compared. The primary advantage of the model is that all input parameters, including those for an initial plane-strain state and its transition to a plane-stress-state, are objectively measured using standard middle-crack-tension M(T) specimens. The purpose of this paper is to evaluate the ability of the FASTRAN model to predict correct retardation effects due to high loading peaks that occur during variable amplitude loading in sequences representative of an aircraft service. Design/methodology/approach – This paper addresses pre-setting of the fracture toughness K R (based on J-integral J Q according to ASTM1820) in the FASTRAN retardation model. A set of experiments were performed using specimens made from a 7475-T7351 aluminium alloy plate. Loading sequences with peaks ordered in ascending-descending blocks were used. The effect of truncating and clipping selected load levels on crack propagation behaviour was evaluated using both experimental data and numerical analyses. The findings were supported by the results of a fractographic analysis. Findings – Fatigue crack propagation data defined using M(T) specimens made from Al 7475-T7351 alloy indicate the difficulty of evaluating the following two events simultaneously: fatigue crack increments after application of loads with maximum amplitudes that exceeded J Q and subcritical crack increments caused by loads at high stress intensity factors. An effect of overloading peaks with a maximum that exceeds J Q should be assessed using a special analysis beyond the scope of the FASTRAN retardation model. Originality/value – Measurements of fatigue crack growth on specimens made from 7475 T7351 aluminium alloy were carried out. The results indicated that simultaneously evaluating fatigue crack increments after application of the load amplitude above J Q and subcritical increments caused by the loads at high stress intensity factors is difficult. Experiments demonstrated that if the fatigue crack reaches a specific length, the maximal amplitude load induces considerable crack growth retardation.

Near threshold fatigue crack propagation in railways’ steels: Comparison of two testing techniques

Mode I fatigue crack growth thresholds of steels for rail and wheel used in railways were determined according to two different procedures. Although fatigue crack propagation data in the Paris regime is usually straightforward to obtain, the same does not apply to the threshold regime, where crack growth rates are very small up to inexistent, and therefore tests are very time consuming. Furthermore, the very small fatigue crack growth rates involved in testing for threshold determination require very thorough and careful experimental procedures if extraneous effects – as for example interaction effects such as crack retardation or acceleration – are to be avoided. Two methods for evaluation of fatigue crack growth threshold value of the range of stress intensity factor were used: load shedding and constant maximum stress intensity factor with increasing load ratio R (R=minimum load/maximum load), taking into account the ASTM standard E647 prescriptions. The obtained results and lessons learned in conducting these tests are discussed, and the data generated is compared with data published.

Modelling probabilistic fatigue crack propagation rates for a mild structural steel

A class of fatigue crack growth models based on elastic–plastic stress–strain histories at the crack tip region and local strain-life damage models have been proposed in literature. The fatigue crack growth is regarded as a process of continuous crack initializations over successive elementary material blocks, which may be governed by smooth strain-life damage data. Some approaches account for the residual stresses developing at the crack tip in the actual crack driving force assessment, allowing mean stresses and loading sequential effects to be modelled. An extension of the fatigue crack propagation model originally proposed by Noroozi et al. (2005) to derive probabilistic fatigue crack propagation data is proposed, in particular concerning the derivation of probabilistic da/dN-?K-R fields. The elastic-plastic stresses at the vicinity of the crack tip, computed using simplified formulae, are compared with the stresses computed using an elasticplastic finite element analyses for specimens considered in the experimental program proposed to derive the fatigue crack propagation data. Using probabilistic strain-life data available for the S355 structural mild steel, probabilistic crack propagation fields are generated, for several stress ratios, and compared with experimental fatigue crack propagation data. A satisfactory agreement between the predicted probabilistic fields and experimental data is observed.

A three-parameter model for fatigue crack growth data analysis

A three-parameters model for the interpolation of fatigue crack propagation data is proposed. It has been validated by a Literature data set obtained by testing 180 M(T) specimens under three different loading levels. In details, it is highlighted that the results of the analysis carried out by means of the proposed model are more smooth and clear than those obtainable using other methods or models. Also, the parameters of the model have been computed and some peculiarities have been picked out.

A procedure to derive probabilistic fatigue crack propagation data

PurposeRecently, a new class of fatigue crack growth models based on elastoplastic stress‐strain histories at the crack tip region and strain‐life fatigue damage models have been proposed. The fatigue crack propagation is understood as a process of continuous crack initializations, over elementary material blocks, which may be governed by strain‐life data of the plain material. The residual stresses developed at the crack tip play a central role in these models, since they are used to assess the actual crack driving force, taking into account mean stresses and loading sequential effects. The UniGrow model fits this particular class of fatigue crack propagation models. The purpose of this paper is to propose an extension of the UniGrow model to derive probabilistic fatigue crack propagation data, in particular the derivation of the P–da/dN–ΔK–R fields.Design/methodology/approachAn existing deterministic fatigue crack propagation model, based on local strain‐life data is first assessed. In particular, an alternative methodology for residual stress computation is proposed, based on elastoplastic finite element analysis, in order to overcome inconsistencies found in the analytical approximate approaches often used in literature. Then, using probabilistic strain‐life fields, a probabilistic output for the fatigue crack propagation growth rates is generated. A new probabilistic fatigue field is also proposed to take mean stress effects into account, using the Smith‐Watson‐Topper (SWT) damage parameter. The proposed models are assessed using experimental data available for two materials representative from old Portuguese bridges.FindingsA new method to generate probabilistic fatigue crack propagation rates (P–da/dN–ΔK–R fields) is proposed and verified using puddle iron from old Portuguese bridges, usually characterized by significant scatter in fatigue properties. Also, a new probabilistic fatigue field for plain material is proposed to deal with mean stress effects.Originality/valueA relation between the P–ε–N and the P–da/dN–ΔK–R fields is firstly proposed in this research. Furthermore, a new P–SWT–N field is proposed to deal with mean stress effects.

Temperature and Load Interaction Effects on the Fatigue Crack Growth Rate and Fracture Surface Morphology of IN100 Superalloy

Abstract A study was conducted to explore some of the load and temperature interaction effects on the fatigue crack growth rate (FCGR) of polycrystalline superalloy IN100. Load interaction testing in the form of single overloads was performed at 316°C and 649°C. Temperature interaction testing was performed by cycling between 316°C and 649°C in blocks of 1, 10, and 100 cycles. After compiling a database of constant temperature, constant amplitude FCGR for IN100, fatigue crack growth predictions assuming no load or temperature interactions were made. Experimental fatigue crack propagation data were then compared with these predictions to assess interaction effects. The fracture mechanisms observed during interaction testing using a scanning electron microscope were compared with the mechanisms present during constant temperature, constant amplitude testing. Overload interaction testing led to full crack retardation at 2.0 × overloads for both 316°C and 649°C testing. Overloading by 1.6 × at both temperatures led to retarded crack growth, whereas 1.3 × overloads at 649°C created accelerated crack growth and at 316°C the crack growth was retarded. One block alternating temperature interaction testing grew significantly faster than the non-interaction prediction, while 10 block alternating temperature interaction testing also grew faster but not to the same extent. One hundred block alternating testing grew slower than non-interaction predictions. Possible explanations for the interaction effects responsible for the observed crack growth acceleration and retardation are discussed.

Utilising CODmax as an Indirect Fatigue Crack Length Measurement Parameter for M(T) Specimens of an Airframe Aluminium Alloy AA6056

Abstract Accuracy of indirect fatigue crack length measurement by potential drop method or by compliance technique may be affected at low load ratio due to fracture surface contact, crack closure or mixed mode fracture. As an alternative, the maximum value of crack opening displacement, CODmax, from a clip gauge was utilized. Middle crack tension M(T) specimens were used to obtain conservative data at a low load ratio (R = 0.1). Thin sheet specimens (B = 3.2 mm) with different widths (100 mm ≤ W ≤ 400 mm) of AA6056-T4 were investigated in the mid-regime (Paris regime), which is of interest for damage tolerance analysis. The use of CODmax is found to provide crack lengths equivalent to those measured optically. Hence, the method is very suitable for indirect crack length measurement. Furthermore, small width specimens provided data equivalent to large width specimens. Insofar, the size effect is found to be absent, and fatigue crack propagation data can be acquired on small width specimens when material availability is limited.