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The 7075 aluminum alloy is a promising material for the aerospace industry due to its combination of light weight and high strength. This study proposed a method for predicting fatigue crack initiation of the 7075 aluminum alloy by crystal plasticity finite element analysis considering microstructures. In order to accurately predict the total fatigue life, it is necessary to calculate the number of cycles for fatigue crack initiation, small crack growth, and long crack growth. The long crack growth life can be estimated by the Paris law, but fatigue crack initiation and small crack growth are sensitive to the microstructures and have been difficult to predict. In this work, the microstructure of 7075 aluminum alloy was reconstructed based on experimental observations in the literature and crystal plasticity simulations were performed to calculate the elasto-plastic deformation behavior in the reconstructed polycrystalline model under cyclic deformation. The calculated local plastic strain was introduced into the crack initiation criterion (Tanaka and Mura, 1981) to predict fatigue crack initiation life. The predicted crack initiation life and crack morphology were in good agreement with the experimental results, indicating that the proposed method is effective in predicting fatigue crack initiation in aluminum alloys. From the obtained results, future issues regarding the prediction of fatigue crack initiation were discussed.

The Geotechnical Monitoring System (GTMS) based on Distributed Temperature and Strain Sensing has been used for more than 15 years to mitigate geohazards along pipeline-crossing regions as distinct as tropical mountain ranges and the far north. The gained experience can be extended to similar infrastructures such as highways, railways, power cables or waterways. The present work reviews the optical fiber technology, the challenges and outcomes of the monitoring. It also draws a general applicable methodology. Different cases are reviewed, demonstrating the variety of geohazards that can be detected at an early stage.

The fatigue behavior of four extruded Mg-Y-Zn alloys containing different volume fractions of long-period stacking ordered (LPSO) grains was investigated through a comparative study combining experiments and crystal plasticity finite element simulations. Strain controlled low-cycle fatigue experiments were conducted at different strain amplitudes and revealed a limited cyclic hardening in Mg89Zn4Y7 alloy or softening in Mg99.2Zn0.2Y0.6 and Mg97Zn1Y2 alloys. A decrease in the fatigue life against the plastic strain with the increase in LPSO phase volume fraction was observed and was related the limited ductility of extruded LPSO grains. Stress-strain hysteresis curves were used to calibrate and validate a crystal plasticity model taking into account twinning and detwinning. The interaction of the different phases on the distribution of local micro-mechanical fields at the grain scale was then analyzed on synthetic microstructures under strain-controlled conditions. Deformation twinning activity was predicted in coarse unrecrystallized grains and tended to disappear with the increase in the LPSO phase volume fraction. Cleavage-like facets observed in LPSO grains were related to high tensile stress, especially at the Mg/LPSO interface, due to the limited number of deformation mechanisms in LPSO crystal to accommodate out-of-basal plane strain. The increase of the fatigue limit with the increase in LPSO phase volume fraction was finally associated with the decreasing presence of coarse unrecrystallized α-Mg grains due to a higher dynamic recrystallization activity during the extrusion process.

Fatigue crack initiation and short crack propagation was systematically investigated in a rolled α / β Ti-6Al-4V alloy through a combination of in situ optical microscopy observations during fully-reversed uniaxial fatigue experiments and post-mortem electron backscattering diffraction analyses. The effect of microstructural attributes (phase, crystal orientation, grain boundary) on fatigue crack formation and early growth was statistically quantified based on the analysis of a large dataset of grains. Crack initiation was mainly observed on prismatic slip systems which was related to the effect of the rolling texture. Crack propagation was found to be mostly transgranular and crystallographic on both basal and prismatic slip systems. An analysis of the effect of the β phase was proposed to explain the preferential slip systems for propagation in lamellar colonies. Crack deflection was quantified for a large set of grain boundaries. A combination of elastic, plastic and geometric compatibility criteria was identified to consistently predict the preferential slip systems for crack propagation across grain boundary. Finally, a discussion on the transition from microstructure-sensitive to microstructure-insensitive crack propagation was proposed.

An analysis of the fatigue behavior of rolled AZ31 magnesium alloy through numerical simulations of polycrystalline aggregates using the crystal plasticity finite element method is presented. A phenomenological pseudo-slip model including twinning, detwinning and subsequent slip in the twinned region was considered in this study. It was calibrated against uniaxial monotonic and cyclic experiments on rolled AZ31 Mg alloy. Multiple periodic two-dimensional synthetic microstructures were generated based on morphological and crystallographic data obtained from electron backscattering diffraction (EBSD) measurements. Fatigue simulations were conducted at different stress amplitudes and mesoscopic non-local fatigue criteria for slip-induced and twin-induced crack initiation were investigated. It was observed that basal slip and twinning-detwinning are the main deformation mechanisms taking place during cyclic loading but that these deformation modes generally occur in different grains. The comparison of the investigated criteria with experimental data also revealed that the twin-induced criterion exhibit better trends in terms of mean behavior, relative dispersion and stress amplitude sensitivity compared with the slip-induced criterion. Finally, the predictive capabilities and limitations of the current modeling approach are discussed.

A layered α/β Ti alloy was developed by cold rolling and subsequent aging heat treatment of a metastable β Ti-12Mo alloy. The microstructure consisted of coarse equiaxed β grains containing a large number of deformation twins. After aging, α phase was found to preferentially precipitate at twin boundaries following a fixed crystallographic orientation relationship with respect to both the twin and the matrix. Finely dispersed α precipitate were also observed in the matrix and inside some thick twins and were inferred to be due to the presence of parallel nano-sized α” martensite inside thick twins and ω phase formed during cold rolling. The deformation behavior of aged sample was finally characterized by a combination of digital image correlation (DIC) and electron backscatter diffraction (EBSD) analyses. An increase in yield strength was noted when compared with the solution treated sample. It was found that plastic deformation was highly heterogeneous and mostly located in the α layer at the interface between the matrix and thick twins. This heterogeneity was finally discussed based on an elastic stiffness analysis and the specific crystallographic configuration at this interface.

A new fatigue prediction method for wide range of welded structures has been developed by integrating several advanced computational techniques such as thermo-mechanical finite element method, crystal plasticity model and extended finite element method. The method consists of the following procedures: (i) computation of materials properties; (ii) analysis of temperature field, microstructure and residual stress generated during a welding process; (iii) stress distribution analysis under cyclic loading; (iv) fatigue crack initiation analysis by the Tanaka-Mura model; (v) fatigue crack propagation analysis. Using the proposed methodology, the fatigue life of butt joint was evaluated and compared with the experimental data.

The study aims to investigate the influence of the grain morphology and rolling texture on the fatigue crack initiation in BCC polycrystals. Several 2D polycrystalline aggregates having different grain size and shape distributions are generated based on an anisotropic tessellation and an ellipse packing algorithm. Uniform and typical BCC rolled textures containing both the α and γ fibers are considered for the crystallographic texture of the material. Finite element simulations using a crystal plasticity model are performed under cyclic uniaxial tension-compression condition. Twenty simulations are conducted for each type of microstructure in order to obtain significant statistical data. A critical plane fatigue indicator parameter (FIP) following the Tanaka-Mura model is applied to quantify the sensitivity of the microstructure against fatigue crack initiation. The distributions of relevant geometrical, crystallographic and mechanical quantities are analyzed. The effect of the grain size distribution, morphology and texture on the fatigue performance and scattering are discussed.

The effect of the crystal orientation on the local stress–strain path and ductile damage initiation in dual-phase steels was characterized through an integrated experimental–numerical study. Interrupted tensile tests were carried out on two different steels with varying microstructure to clarify the damage initiation mechanisms, their locations, sequences and proportions. It was found that damage initiated early and mainly from ferrite/martensite decohesion and martensite/martensite cracking with different ratio in the two materials. Finite element simulations were carried out on semi-synthetic microstructures generated from optical micrographs of specimen surfaces, using either an isotropic elasto-plastic model or a phenomenological crystal plasticity model calibrated from macroscopic stress–strain curves. The accuracy of the simulations was assessed through a comparison of the equivalent plastic strain fields estimated experimentally by digital image correlation and predicted numerically. It was found that both models predicted similar plastic strain distribution but slightly different stress triaxiality and Lode angle fields which highlighted the effect of the crystal orientation. However, in the vicinity of strain concentration regions where voids nucleated, the two fields were relatively close suggesting that the phase distribution and strength contrast had a predominant effect on these fields. The local stress–strain paths at void sites were extracted and were observed to significantly differ from that predicted by macroscopic simulations, especially in the case of ferrite damage which highlighted the importance of considering microstructural effects. A general ductile damage model dependent on the equivalent plastic strain, the stress triaxiality and the Lode angle was calibrated based on the stress–strain path history of void sites, taking into account experimental uncertainties. Similar fracture loci were predicted by the isotropic and the crystal plasticity model. Estimated damage initiation parameters suggested that martensite fracture was insensitive to the stress triaxiality and the Lode angle, while a certain sensitivity against the stress triaxiality was observed for ferrite/martensite decohesion.

In this study, fatigue crack initiation in pure α-iron is investigated through a microstructure-sensitive framework. At first, synthetic microstructures are modeled based on an anisotropic tessellation that accounts for the information of the grains morphology extracted from electron backscatter diffraction (EBSD) analysis. Low-cycle fatigue experiments under strain-controlled conditions are conducted in order to calibrate a crystal plasticity model and a J2 model including isotropic and kinematic hardening. A critical plane fatigue indicator parameter (FIP) based on the Tanaka-Mura model is then presented to evaluate the location and quantify the driving force for the formation of a crack. The FIP is averaged over several potential crack paths within each grain defined by the intersection between a given slip plane and the plane of the model thus accounting for both the lattice orientation and morphology of the grain. Several fatigue simulations at various stress amplitudes are conducted using a sub-modeling technique for the attribution of boundary conditions on the polycrystalline aggregate models including an elliptic defect. The influence of the microstructure attributes and stress level on the location and amplitude of the FIP are then quantified and discussed.

In this study, a method for the prediction of cyclic stress&ndash, strain properties of ferrite-pearlite steels was proposed. At first, synthetic microstructures were generated based on an anisotropic tessellation from the results of electron backscatter diffraction (EBSD) analyses. Low-cycle fatigue experiments under strain-controlled conditions were conducted in order to calibrate material property parameters for both an anisotropic crystal plasticity and an isotropic J2 model. Numerical finite element simulations were conducted using these synthetic microstructures and material properties based on experimental results, and cyclic stress-strain properties were calculated. Then, two-point correlations of synthetic microstructures were calculated to quantify the microstructures. The microstructure-property dataset was obtained by associating a two-point correlation and calculated cyclic stress-strain property. Machine learning, such as a linear regression model and neural network, was conducted using the dataset. Finally, cyclic stress-strain properties were predicted from the result of EBSD analysis using the obtained machine learning model and were compared with the results of the low-cycle fatigue experiments.

Geotechnical monitoring based on optical fiber sensor technology has been used over more than a decade to detect hazards than can affect the integrity of pipelines. In particular when these sensors are implemented in the form of distributed temperature and strain sensors, respectively known as DTS and DSS, they provide information about hazard location and spatial extension. In addition, these sensors can capture the speed at which the event developing in particular when implemented as a permanent monitoring solution. So far these sensors were implemented as part of an alarming system detecting events such as landslides, erosion and subsidence. The current work aims at presenting simple method to extract additional information about the hazard such as the amplitude of the soil displacement in the case of landslides and subsidence or dirt cover for erosion. Estimation of stress in soil is also discussed based on the cable strain-stress relation obtained from the sensing cable qualification. The approach is validated by academic works conducted in parallel of the technology development. The method use is then illustrated by its application to field data collected from several events occurred over the past 10 years.

A numerical procedure centered around the crystal plasticity finite element method was developed for the simulation of fatigue crack initiation and propagation in metallic materials. Damage indicators were derived from theoretical dislocation-based models for the prediction of the crack initiation, crack path and crack growth rate, taking into account slip transfer across grain boundaries. They were incorporated into a numerical framework in which an explicitly growing crack was considered via remeshing technique to account for the redistribution of the micromechanical fields ahead of the crack tip. In this regard, a non-local averaging scheme was applied to account for both the spatial heterogeneity of the stress–strain fields and the grains morphology. The proposed framework was tested on a rolled bi-modal Ti-6Al-4V alloy subjected to a fully reversed uniaxial loading condition. Statistically representative synthetic microstructures were generated by a multi-scale anisotropic tessellation procedure considering the presence of multiple lamellar variants within a prior β -phase grain sharing the same orientation relationship. The predictions of crack initiation, crack path and growth rate were statistically compared with experimental data from prior study to emphasize the predictive capabilities and limitations of the proposed approach.

A crystal plasticity finite element (CPFE) simulation framework has been proposed in this study for the prediction of combined detrimental effect of mean stress and defects on the fatigue behavior of aluminum alloy. Experimental data for mean-stress effect on fatigue life and crack growth behavior was obtained on metal inter gas (MIG) welded joints of Al-5083/Al-5.8%Mg alloy plates and has been detailed in authors’ previous work, referred later. The present study focuses on its prediction using computational framework only. A 2D representative model for material’s microstructure was used for the simulations, generated using an anisotropic tessellation algorithm using the EBSD measurements data. A total of 10 different microstructure models were generated for each loading condition using six-node plane strain type quadratic triangular (CPE6) elements of mesh size 6 μ m. Two different types of cases were investigated: one without defect and other with a semi-circular surface defect. The simulated loadings at different stress ranges and stress ratios (R-ratio) were similar to the experimental conditions for the better comparison of the results. Significant heterogeneity in the distribution of R-ratios and the far-field applied R-ratio was observed. When defect was not considered, a clear deviation in the predicted fatigue lives from the experimental data was observed at different R-ratios: the predicted fatigue lives were higher than the experimentally observed fatigue lives. This was probably because of not considering the detrimental effect of defects on fatigue lives. But, when the defects were considered, the predicted results for different R-ratios were consistent with the experimental fatigue lives. The proposed CPFE simulation framework not only predicted well the effect of defects and mean stress on the fatigue lives, but also the scatter induced in them due to the defects.

Many pipelines are built in regions affected by harsh environmental conditions where changes in soil texture between winter and summer increase the likelihood of hazards. Pipeline routes also cross mountains that are characterized by steep slopes and unstable soils as in the Andes and along the coastal range of Brazil. In other cases, these pipelines are laid in remote areas with significant seismic activity or exposure to permafrost. Depending on weather conditions and location, visual inspection is difficult or even impossible and therefore remote sensing solutions for pipes offer significant advantages over conventional inspection techniques. Optical fibers can help solve these challenges. Optical fiber based geotechnical and structural monitoring use distributed measurement of strain and temperature thanks to the sensitivity of Brillouin scattering to mechanical and thermal effects. The analysis of scattering combined with a time domain technique allows the measurement of strain and temperature profiles. Temperature measurement is carried out to monitor soil erosion or dune migration through event quantification and spatial location. Direct measurement of strain in the soil also improves the detection of environmental hazards. As an example, the technology can pinpoint the early signs of landslides. In some cases, actual pipe deformation must be monitored such as in the case of an active tectonic fault crossing. Pipe deformation monitoring operation is achieved by the measurement of distributed strain along fiber sensors attached to the structure. This paper comprehensively reviews over 15 years of continuous development of pipeline geohazard risk monitoring with optical fiber distributed sensors from technology qualification and validation to its implementation in real cases as well as its successful continuous operation. Case studies presented include pipeline monitoring in Arctic and Siberian environment as well as in the Andes which illustrate how the technology is used and demonstrate proof of early detection and location of geohazard events such as erosion, landslide, settlement and pipe deformation.

Yield and fatigue strength of pure titanium periodic plates were investigated for three different mesh designs using acoustic emission (AE) method. Simulations of tensile test were also conducted to estimate yield strength numerically and compare with experiments. By analyzing AE during deformation, it was found that the origin of multiple AE count rate peaks is due to various yielding locations inside the specimens. For fatigue experiments, AE was used to detect the time of first fracture as it corresponds to the failure of a single cell. However, the difference in time with the final fracture of the plate was not significant, especially for low stress amplitude. [doi:10.2320/matertrans.M2014404]

A framework for uncertainty quantification of fatigue life prediction in welded structures has been developed based on multi-scale finite element analysis (FEA) considering microstructure. The multi-scale FEA consists of the following procedures: (i) mechanical and thermal properties are estimated by using commercially available software and database; (ii) temperature field, residual stress and distortion generated during a welding process is calculated on the global model by thermo-mechanical FEA; (iii) macroscopic stress field under cyclic loading condition is calculated with a hardening constitutive model; (iv) the mesoscopic stress field is derived by crystal plasticity finite element analysis with sub-model technique and the cycles for a fatigue crack initiation is analyzed by strain energy accumulation on the slip plane; (v) the cycles for fatigue crack propagation is analyzed by extended finite element method and the total number of cycles to the failure is obtained. The possible uncertainties in the current model include physical variability (weld shape, residual stress, loading and microstructure), data uncertainty due to measurement error and modeling uncertainty in numerical approximations and finite element discretization. A sensitivity analysis was performed by changing the input parameters in the proposed simulation. Additionally, fatigue database of weld joint were aggregated from public databases, published papers and academic resources, and compared with simulation results to quantify the contribution of each source of uncertainty. The effects of residual stress, toe radius and microstructure on the fatigue life of welded joints were evaluated by the proposed methodology. It was shown that the fatigue life increased with the toe radius, and the scattering in high cycle region appeared due to considering microstructure. The features and limitations of our methods will be discussed.

Mechanical anisotropies of an extruded Mg–Al–Ca–Mn alloy plate were investigated under both monotonic and cyclic conditions through a combination of experiments and crystal plasticity finite element simulations. To this end, monotonic tensile, compression as well as fully-reversed strain-controlled and load-controlled fatigue tests were conducted on specimens aligned either in the extrusion or transverse direction. Plastic activities of the deformation mechanisms for the different loading conditions were quantitatively analyzed based on the results of numerical simulations. The asymmetrical mechanical response between tension and compression along the two loading directions was found to be associated with the lack of axi-symmetry of the relatively strong basal texture. In addition, the peculiar and lower tensile flow stress in the transverse direction was explained by the activation of deformation twinning in grains with their c-axis aligned along the loading direction. Under low-cycle fatigue conditions, a lower asymmetry of stress–strain hysteresis curves was observed in the transverse direction due to the occurrence of twinning/detwinning in both the tensile and compressive portions. High-cycle fatigue experiments revealed lower fatigue lives in the transverse direction than in the extrusion direction but similar fatigue limits. Crystallographic analyses of multiple secondary cracks revealed crack initiation from basal slip, twin bands, grain boundaries and non-metallic inclusions. The evaluation of slip-induced and twin-induced fatigue criteria from the results of fatigue simulations of explicitly modeled polycrystals suggested that the most detrimental crack initiation mechanism was crack initiation from basal slip as it predicted a significantly lower initiation life in the transverse direction than in the extrusion direction, as observed experimentally.

The fatigue properties of two Mg-Y-Zn alloys with different amount of long-period stacking order (LPSO) phase, Mg 97 Zn 1 Y 2 and Mg 89 Zn 4 Y 7 , were investigated. Low-cycle fatigue experiments were conducted under strain-controlled conditions. Both the monotonic and cyclic yield strength increased with the LPSO volume fraction. Cleavage-like facets were found to be at the origin of crack initiation in both alloys. High-cycle fatigue experiments were conducted under microscopy monitoring. Increasing the LPSO-phase volume fraction decreased the fatigue life but improved the endurance limit. Cracks initiated from α -Mg grains in both materials. Crack growth rates were found to be relatively similar.

Structural materials having higher performance in strength, toughness, and fatigue resistance are strongly required. In the conventional materials development, many fatigue tests need to be conducted to validate statistical behavior of fatigue failure. Accordingly the evaluation of fatigue properties with shorter time becomes quite essential. Based on such background, we are developing fatigue prediction methods for wide range of structural materials by multi-scale finite element analysis (FEA) and machine learning in the Materials Integration (MI) system. The multi-scale FEA consists of the following procedures: (i) mechanical and thermal properties are estimated by using commercially available software and database; (ii) temperature field, residual stress and distortion generated during a manufacturing process is calculated on the macroscopic model by thermo-mechanical FEA; (iii) macroscopic stress field under cyclic loading condition is calculated with a hardening constitutive model; (iv) the microscopic stress field is derived by finite element model with the polycrystalline structures and the cycles for a fatigue crack initiation is analyzed by strain energy accumulation on the slip plane; (v) the cycles for fatigue crack propagation is analyzed by extended finite element method (X-FEM) and the total number of cycles to the failure is obtained. The second approach is to use machine learning techniques to obtain empirical prediction formula. The database was prepared from published resources and experiments. Deterministic machine learning techniques such as multivariate linear regression and artificial neural network provided accurate equations to predict fatigue strength from materials and process parameters. Additionally, the concept of model-based machine learning was adopted to incorporate prior knowledge of microstructures and properties, and to account for uncertainty on fatigue life. The results showed that model-based machine learning was a promising tool for predicting fatigue performance in structural materials. The features and limitations of our prediction methods will be discussed.

This paper proposes a numerical framework to predict fatigue life on welded joints by integrating several computational techniques. The framework consists of five steps: i) materials properties estimation; ii) welding simulation using thermo-mechanical finite element method; iii) macroscopic stress field analysis under cyclic loading; iv) mesoscopic stress field analysis using crystal plasticity finite element method (CPFEM); v) analysis of fatigue crack growth. The total number of cycles to failure is eventually obtained by the sum of initiation life calculated by CPFEM and propagation life calculated by X-FEM. A fatigue life of butt joint is evaluated by the proposed method. The results demonstrated the possibility of evaluating the fatigue life and its scattering by the proposed framework.

Submillimeter crack is detected with a dedicated fiber-optic strain cable, a 1-m-spatial-resolution (w) distributed Brillouin sensor and an advanced signal processing technique. The signal processing approach consists in spectrum shape analysis and multiple peaks detection.