Cyclic Material Behavior Research Articles

Productive automation of calibration processes for crystal plasticity model parameters via reinforcement learning

Crystal Plasticity Finite Element (CPFE), which merges crystal plasticity principles with finite element analysis, can simulate the anisotropic grain-level mechanical behaviour of polycrystalline materials. Due to the benefit of CPFE, it has been widely utilised to analyse processes such as manufacturing, damage, and deformation where the microstructure plays a prominent role. However, this method is computationally expensive and requires the robust calibration of its parameters, which can be many. In this work, we propose a framework to address difficulties in calibrating multi-parameter CPFE. The Deep Deterministic Policy Gradient (DDPG) algorithm, a Deep Reinforcement Learning (DRL) approach, is utilised to optimise the CPFE parameters. Additionally, a Python-based environment is developed to fully automate the calibration process. To allow comparison with the conventional optimisation method, the Particle Swarm Optimisation (PSO) algorithm is also used, which shows the DDPG framework yields more accurate calibration. The generalisation performance of the proposed framework is also demonstrated by calibrating each parameter set of two different CPFE models for monotonic loading of the stainless steel type, 316L. Moreover, the effectiveness of the framework in the more complex condition is also demonstrated by calibrating the CPFE parameters for a two-cycle cyclic behaviour of a 316H stainless steel material. The reliability of these calibrated parameters is also validated in the cyclic simulation after two cycles.

Predicting cyclic liquefaction behavior of saturated granular materials using an updated state evolution model

Liquefaction and dynamic response of granular materials under dynamic loading has been studied intensively in field and laboratory tests. However, theoretical modeling and analytical solutions on liquefaction are still lagging and investigations are mostly restricted to laboratory observations. To investigate undrained liquefaction shear deformation and fluidity of granular material, the updated state evolution model is proposed by introducing an excess pore water pressure ratio parameter. A series of undrained cyclic triaxial tests and DEM simulations are conducted to verify the proposed model. The result indicates that the liquefaction behavior of granular materials can be captured by the updated state evolution model both at constant and varying loading frequency. Furthermore, the state parameter based on the deviatoric strain and excess pore water pressure ratio is determined to quantify assess the fluidity of granular materials. It facilitates the refinement of the discriminative criteria for cyclic liquefaction of granular materials. This parameter increases slowly at the beginning of loading, followed by a rapid and fluctuating rise, and reaches the peak before the initial liquefaction. Another significant finding is that the turning point of the state parameter range from 0.89 to 0.95 in the θ−t/t0 plane and between 0.84 and 0.94 in the θ−ru plane, as affected by the cyclic loading conditions.

Analysis of the Anisotropic Cyclic Material Behavior of EN AW-1050A H24 Derived from Strain-Controlled Testing Using a Clip-On Extensometer and an Optical System

Due to its good conductive properties, unalloyed (pure) aluminum, such as EN AW-1050A H24, finds new fields of application in electromobility. To optimize components, the cyclic material behavior must be understood and described precisely as a foundation of a proper fatigue life estimation. Various cyclic tests were performed to not only derive the cyclic parameters to describe the material but also to find the most suitable procedure to deal with the challenges faced during the experiments. The main point of interest is the comparison between a surface-mounted clip-on extensometer and an optical system both used for strain control in cyclic tests. For the analysis of the anisotropic behavior of EN AW-1050A H24, un-notched flat specimens were extracted from sheet metal lengthways and crossways in respect to the rolling direction. While the cyclic material behavior for specimens of both directions of extraction is characterized by cyclic softening in general, the specimens extracted crossways show a strain-amplitude-dependent cyclic softening with strong strain localization especially at the contact points of the knives of the clip-on extensometer leading to an increased quantity of invalid experiments as well as sudden fractures. In the study, it was possible to show the benefits of a contactless optical strain control system when dealing with very soft metallic materials such as EN AW-1050A H24.

3D DEM simulations of cyclic loading-induced densification and critical state convergence in granular soils

Granular soils exhibit very complex responses when subjected to cyclic loading. Understanding the cyclic behavior of such materials is not only crucial for engineering applications but also the bottleneck of most of constitutive models. This study employs 3D Discrete Element Method (DEM) simulations to explore the accumulative plastic deformation and the internal fabric evolution within granular soils during cyclic loading. Two novel observations are identified: (1) A distinct and unique linear relationship between post-cyclic loading void ratio e and log (p*/p0) is found independent of the amplitude of cyclic load and the initial stress state prior to cyclic loading, where p* is the mean pressure incorporating cyclic loading stress and p0 is the mean pressure prior to cyclic loading; (2) When resuming drained triaxial loadings after cyclic loadings, we observe that both microstructural and macroscopic variables converge to the same values they would have reached for pure monotonic drained triaxial loadings. This intriguing behavior underscores and extends to more general loading paths the influential and attractive power of the critical state.

Fractional modeling of cyclic loading behavior of polymeric materials

Fractional modeling of cyclic loading behavior of polymeric materials

Microstructure and cyclic hardening/softening behavior of laser welded high strength steel joints under asymmetrical cyclic loading

The cyclic behavior of materials under load and its influence on the mechanical properties of materials have always been the focus of engineering research. In this study, high strength steel was successfully welded by laser welding. The microstructure of each region of the high strength steel DP590 post-welding was meticulously analyzed. Additionally, 500 cycles of low-cycle fatigue tests of laser welded high strength steel were carried out by using asymmetric stress control method. After fatigue test, tensile test shall be carried out on some samples. Under the specific combination of mean stress and stress amplitude, compare the specimens that have not been fatigue tested with those that have been fatigue tested, the change of properties of laser welded high strength steel joint under asymmetric cyclic load was analyzed. The results showed that, due to cyclic hardening, the tensile strength of the sample increased to 601 MPa and the yield strength to 325 MPa. Compared with the original sample, the tensile strength and yield strength of the sample after asymmetric cycling had increased by 8.65% and 31.3%, respectively.

Assessment of cyclic deformation behaviour of wire arc additively manufactured carbon steel

Wire Arc Additive Manufacturing (WAAM) has gained popularity due to its speed and cost-effectiveness. However, the current knowledge on the cyclic deformation behaviour of WAAM materials is limited. To address this issue, this study investigates the cyclic deformation behaviour of WAAM ER70S-6 carbon steel. Coupons were extracted from printed walls with horizontal and vertical orientations and from surface and interior locations. Low-cycle fatigue tests were performed under fully reversed strain-controlled conditions, spanning strain amplitudes from 0.20 % to 1.50 %. Regardless of the group, the material exhibited cyclic hardening for strain amplitudes above 0.60 % and cyclic softening for smaller strain amplitudes. Significant non-Masing hysteretic behaviour was also observed. Cyclic stress–strain curves and fatigue-life relationships written in terms of stress, strain and energy-based parameters were derived for all groups. Fatigue life was not significantly influenced by printing orientation or location across thickness. However, horizontal orientation resulted in a slightly superior fatigue response. A statistical analysis revealed no significant difference in the accuracy of fatigue-life relationships between the groups. Finally, a comparative study between WAAM materials and conventional steels showed promising results. Yet, conventional steels outperform WAAM carbon steel, at least doubling the fatigue life for the same value of the damage parameter.

Strain-life behavior of thick-walled nodular cast iron

Abstract For the design of thick-walled cast iron components used for wind energy turbines, large machines, presses, and engine parts made of nodular cast iron, the assessment of fatigue life of critical component areas is a crucial point to consider. Importance increases with the demand to safe resources by applying a lightweight design in combination with a certain resistance against extreme loads and misuse situations. Moreover, this gets more important when notches or deviations in the local microstructure are present. Nevertheless, due to accessibility, a lack in the description of the local strain-based material behavior still exists. Especially new cast iron grades such as ADI are not well investigated but offer an often-discussed potential for a lightweight design of the structure or to increase the highest admissible load on the structure. To determine the cyclic material behavior of nodular cast iron, different grades EN-GJS-400-18LT, EN-GJS-450-18, EN-GJS-500-14, EN-GJS-600-3, EN-GJS-700-2, and ADI in different wall thicknesses were investigated concerning their elastic–plastic material behavior under strain control. Moreover, new possibilities for the trilinear description of the strain-life and the stress–strain curve and the statistical and technological size effect are discussed in the following based on cyclic stress–strain and strain-life curves.

Fatigue Analysis of Steering Knuckles Using the Local Strain Approach

The study investigates the mechanical properties and fatigue behavior of steering knuckles used on commercial vehicles. The steering knuckle is made of hot forged bainitic steel (18MnCrMoV6-4-8), which is known to demonstrate high levels of fatigue strength, toughness, and hardness. The local strain concept was adopted to assess the durability of the steering knuckle based on the stabilized cyclic material behavior. For this purpose, experimental investigations have been conducted on both the steering knuckle as well as fatigue specimens under constant and variable amplitude loadings. The fatigue specimens were removed from the area next to the crack initiation location, to represent the microstructure in the critical area of the component. Fatigue life estimations were performed under different load ratios using the FKM guideline nonlinear, employing damage parameters PRAM and PRAJ. The assessment enables a fatigue strength assessment for the steering knuckle by considering the local non-linear material behavior. The estimations of the material's fatigue lifetime using the FKM guideline nonlinear approach were unsatisfactory.

Low cycle fatigue behaviour of engineering metallic materials: Review on cyclic deformation micro-mechanism

The micro-mechanism of low cycle fatigue deformation behaviour has been summarised, utilising the recent advances in the state-of-the-art facility for the characterisation of deformation microstructure, and micro-texture. Besides, recent development in the approach of numerical simulation of cyclic stress-strain behaviour of polycrystalline metallic materials at multi-scale has been discussed. It has been carried out by reviewing, and re-interpreting the evidences from the investigations carried out over the last few decades on several engineering metallic materials having critical applications in various industries. The understanding of the existing material behaviour will help us in the better characterisation of the low cycle fatigue deformation microstructure of the new generation metallic materials having complex microstructure.

Experimental investigation of the effects of fatigue on caprock materials in H2 storage projects

Experimental investigation of the effects of fatigue on caprock materials in H2 storage projects

A cyclic GTN model for ultra-low cycle fatigue analysis of structural steels

Ultra-low cycle fatigue (ULCF) is a critical concern in the seismic design of steel structures due to its adverse effects on the ductility and energy dissipation capacity of steel connections. The Gurson-Tvergaard-Needleman (GTN) model, a well-accepted micromechanical fracture model, cannot be applied directly to ULCF analysis, as it requires proper estimation of the effects of cyclic loadings on the ductile fracture of materials. In this paper, a cyclic GTN (C-GTN) model was proposed for the ULCF prediction of materials, in which the evolution of microvoids during ULCF loadings was addressed for tensile and compressive half load cycles, respectively, and the effect of the cumulative plastic strains on the material’s resistance to ductile fracture was considered by the reduction of the critical void volume fraction for void coalescence. Then, a VUMAT subroutine was programmed for the C-GTN model, in which the cyclic hardening behavior of materials was simulated with the Voce-Chaboche model. Finally, the C-GTN model was calibrated for the G20Mn5QT cast steel based on previous tests on notched round bar specimens of the material. The applicability of the C-GTN model and the VUMAT subroutine were validated in the ULCF analysis of double-hole plate specimens of the G20Mn5QT cast steel.

Air Cooling Martensites—The Future of Carbon Neutral Steel Forgings?

The air‐hardening ductile forging steels belong to the third generation of advanced high‐strength steels and achieve their final, martensitic microstructure simply by air cooling from the forging heat. The present conference paper reviews the 15 years of development, different alloying strategies, and metallurgical throwbacks like manganese embrittlement. Furthermore, the most recent results concerning microstructure, mechanical properties, and cyclic material behavior obtained from an industrial trial melt are discussed together with the implications of these results on the carbon footprint (CF) and their impact on the resource efficiency of forged steel components. It is demonstrated that the CF of a steel product needs to be discussed on different levels, as the material‐, process‐, and product‐level contributions need to be considered. Furthermore, a case is made for new high‐strength steels in the context of circular economy, as increase fatigue resistance enables lightweighting potentials.

Effects of surface roughness on fatigue behavior of additively manufactured stainless steel 316L

Surface roughness plays an important role which affects the fatigue performance of additively manufactured metal components. Surface roughness generally contains micro-notches which form stress concentration and significantly deteriorate the fatigue life. Based on nonlinear elastoplasticity finite element (FE) analyses, the roughness model of additively manufactured stainless steel 316L (SS316L) was performed, and the cyclic behavior of surface notch root material under different fatigue stress loads was captured. The stress concentration factor on the selected critical surface position is calculated and the notch root strain analysis are proved by depending on the applied load level. Finally two distinct deformation modes are found: elastic shakedown and ratcheting deformation.

Parametric study on lateral behaviour of composite shear walls with high-strength manufactured sand concrete

The traditional reinforced concrete (RC) shear wall showed limited seismic performance under extreme earthquakes when deployed at the bottom of high-rise buildings, due to the presence of high axial compression ratios. In this study, a novel composite shear wall was proposed to improve the seismic performance at high axial compression ratios, in accordance with recent demands for eco-friendly materials. The novel composite wall integrated the high-strength manufactured sand (MS) concrete, ring-stirrup and steel tube to fully maximize material efficiency and achieve inter-enhancement. A quasi-static test was at first carried out on the proposed composite shear wall, which validated the expected enhancement on its lateral mechanical behaviour preliminarily. On this basis, a refined finite element (FE) model was established for the test specimen by considering the nonlinearity and cyclic behaviour of materials, in order to provide further insights into its behaviour under lateral loads. The established FE model was verified against the test data, which showed a good agreement between the numerical prediction and the test result. Meanwhile, a list of key parameters was identified according to their influence on the lateral behaviour of the composite wall, including the axial compression ratio, yield strength of steel tubes, compressive strength of concretes, reinforcement ratio in concrete-filled steel tube (CFST) columns, and space between ring-stirrups. On this basis, a series of parametric investigations were carried out with the validated FE model, by varying the selected key parameters. In general, the research output offered a constructive guideline and data support for the expected engineering application of the proposed composite shear wall.

Low-Cycle Fatigue Behavior of Hot V-Bent Structural Components Made of AZ31B Wrought Magnesium Alloy

Recent studies have shown the change of microstructure during hot-bending in uniaxial specimens made of AZ31B alloy. They also investigated the influence of the changed microstructure on the quasi-static and cyclic material behavior under uniaxial stress states. However, studies on the fatigue behavior of hot-bent structural components in which a multiaxial inhomogeneous stress state occurs are still lacking. For this purpose, a novel hot-bent V-shaped specimen was developed, of which three different variants, each with a different bending radius, were fabricated and investigated. Microstructural analyses reveal that band-like accumulations of twinned grains are already formed in the compressively stressed area of the specimen during the bending process. Force-controlled low-cycle fatigue tests were performed to investigate the twinning evolution after cyclic loading. Subsequent microstructure analyses show that bands of twinned grains are no longer visible but also that the occurrence of twins is evenly distributed. Due to the specimen shape, the specimens are subjected to a multiaxial stress state. During LCF tests, the strain was measured using 3D digital image correlation and fatigue life was modeled successfully with the application of the concept of highly strained volume.

Viscoplastic constitutive model considering strain rate sensitivity under multiaxial thermomechanical fatigue loading

AbstractStrain rate sensitivity will change the cyclic mechanical response of materials under multiaxial thermomechanical fatigue loading; thus, it will lead to errors in fatigue life prediction if strain rate sensitivity is not considered in constitutive simulation. In order to more accurately express the time/rate dependence of viscoplastic behavior, a strain rate sensitivity factor is proposed to modify the viscoplastic function of Chaboche model first. On this basis, a viscoplastic constitutive model considering strain rate sensitivity under multiaxial thermomechanical fatigue loading is proposed, which can describe the varying influence of strain rate sensitivity on cyclic mechanical behavior. Meanwhile, the exponential isotropic hardening rule with the linear term is adopted to characterize the initial rapid softening and subsequent stable softening of the material under fatigue loading at elevated temperature. Moreover, the material‐dependent nonproportional hardening coefficient and the loading path‐dependent rotation factor are used in the kinematic hardening rule to consider the effect of nonproportional additional hardening on the cyclic mechanical behavior of materials. Finally, the proposed method is verified by the stress–strain data of titanium alloy TC4 and Ni‐based superalloy GH4169 under uniaxial and multiaxial thermomechanical fatigue loadings, and a good agreement is obtained.

Analyses of the model parameters of kinematic-isotropic hardening rule using genetic algorithm approach for predicting cyclic-plasticity of metallic structural materials

Modeling of engineering components under cyclic loading is complex because cyclic-plastic phenomena like the Bauschinger effect, ratcheting, shakedown, and mean stress relaxation are to be considered in the constitutive modeling. This has led to several investigations on modeling the cyclic-plastic behaviour of materials under stress-controlled and strain-controlled testing; but amongst these, Chaboche's isotropic-kinematic hardening (CIKH) model is quite popular and a frequently used one. A new methodology has been recently proposed by some of the present authors to use genetic algorithm towards optimization of the parameters of CIKH modeling with a demonstration of its applicability for a few materials. This investigation aims to elucidate that this approach is applicable to both cyclically stable materials as well as cyclic softening or hardening materials using several examples on structural materials. The considered metallic materials include several aluminum (like AA7075-T6 alloy), iron (like CS-1026and Sa333 C-Mn steel), titanium (like TA16 alloy), zirconium (like Zr-4 alloy)-base alloys or superalloys (like INCONEL718). The merit of the analysis of cyclic plastic deformation behaviour of materials with the current approach is inherent in its achieving higher accuracy of fitting to the experimental data with a single set of material parameters under both strain- and stress-controlled cycling. This has been established with a comparative analysis of the accuracy of fitting by the present approach with the ones available from the existing reports. The parameters obtained using the approach for the different materials are compared to get insights into the mechanism of plastic deformation.

Effects of initial static shear on undrained cyclic behavior of granular materials: energy evolution and micromechanical interpretation

Effects of initial static shear on undrained cyclic behavior of granular materials: energy evolution and micromechanical interpretation

Discrete-Element Simulation of Dense Sand under Uni- and Bidirectional Cyclic Simple Shear Considering Initial Static Shear Effect

Huge and critical structures, such as fuel storage tanks and nuclear power plants, are often built on dense sandy grounds, which may cause excessive plastic deformations under multidirectional cyclic shearing conditions caused by earthquakes. The lack of investigation of sand behavior under multidirectional loading may lead to unsafe designs, and thus significantly increase the maintenance cost for infrastructures during their long operational time, which might be longer than 100 years. This study presents a numerical investigation on the liquefaction behavior of dense sand under bidirectional cyclic simple shear using the discrete element method. The bidirectional cyclic simple shear tests with varying stress trajectories on the deviatoric stress plane, including figure-8, circular, and straight lines, were conducted. Additionally, a series of unidirectional loading tests was conducted in the simulation scheme for comparison purposes. The effect of the initial static shear on the cyclic behavior and liquefaction resistance of granular materials was examined. Four loading categories, including full reversal, partial reversal, intermediate reversal, and no reversal, were implemented in the numerical simulations. Although the presence of the initial static shear may enhance the cyclic resistance of granular samples under unidirectional loading, it mitigates the liquefaction resistance and promotes the failure of specimens under bidirectional shearing conditions. The microscopic analysis was performed to reveal the relationship between the fabric evolution and external loading, which can provide profound insights into the underlying mechanism of the cyclic behavior and liquefaction susceptibility of granular material when both the bidirectional shearing and initial static shear effects are considered.