Strain-controlled Tests Research Articles

Investigations of Metallurgical Differences in AISI 347 and Their Influence on Deformation and Transformation Behaviour and Resulting Fatigue Life.

Due to variations in chemical composition and production processes, homonymous austenitic stainless steels can differ significantly regarding their initial microstructure, metastability, and thus, their fatigue behavior. Microstructural investigations and fatigue tests have been performed in order to evaluate this aspect. Three different batches and production forms of nominally one type of steel AISI 347 were investigated under monotonic tensile tests and cyclic loading under total strain and stress control in low and high cycle fatigue regimes, respectively. The deformation induced α'-martensite formation was investigated globally by means of in situ magnetic measurements and locally using optical light microscopy of color etching of micrographs. The investigation showed that the chemical composition and the different production processes influence the material behavior. In fatigue tests, a higher metastability and thus a higher level of deformation induced α'-martensite pronounced cyclic hardening, resulting in significantly greater endurable stresses in total strain-controlled tests and an increase in fatigue life in stress-controlled tests. For applications of non-destructive-testing, detailed knowledge of a component's metastability is required. In less metastable batches and for lower stress levels, α'-martensite primarily formed at the plasticization zone of a crack. Furthermore, the formation and nucleation points of α'-martensite were highly dependent on grain size and the presence of δ-ferrite. This study provides valuable insights into the different material behavior of three different batches with the same designation, i.e., AISI 347, due to different manufacturing processes and differences in the chemical composition, metastability, and microstructure.

Cyclic response of unsaturated weathered red mudstone: Experimental investigation and a cyclic model

The mechanical properties and constitutive model of unsaturated soils under cyclic loading are crucial for understanding the behavior of foundations and slopes subjected to dynamic motions such as earthquakes and traffic loading. In this study, multilevel strain-controlled cyclic simple shear tests of unsaturated weathered red mudstone (WRM) were conducted. The detailed investigation focused on cyclic responses, including shear stress-strain behavior and volume change, strain-dependent secant shear modulus and damping ratio, and stress–dilatancy behavior. This study revealed the significant influences of the degree of saturation and vertical stress on these responses, with the initial static shear stress mainly affecting the shear stress-strain behavior and volume changes at the initial loading stage. Based on the experimental observations, a cyclic constitutive model was proposed for unsaturated WRM. The model incorporates a slightly revised Davidenkov model and Masing criterion to generate shear stress-strain hysteresis loops with or without initial static shear stress. Additionally, a stress-dilatancy equation was included to capture the volume changes during cyclic loading. The proposed model was verified by comparing representative test data and calculation results, demonstrating the excellent performance of the proposed model in modeling the main features of unsaturated WRM under cyclic loading.

Characterization of Portevin–Le Chatelier effect in aluminum alloy AA5052 under strain-controlled tensile test via digital image correlation and acoustic emission

Characterization of Portevin–Le Chatelier effect in aluminum alloy AA5052 under strain-controlled tensile test via digital image correlation and acoustic emission

Physics-informed transfer learning model for fatigue life prediction of IN718 alloy

To address the challenges posed by inadequate data and data utilization in multiple scenarios of fatigue loading, a Physics-informed Transfer Learning (PITL) model has been developed to predict the fatigue life of IN718 superalloy. Strain-controlled low-cycle fatigue tests were carried out at 400 °C with three distinct strain ratios, which were subsequently segmented for individual transfer learning tests. PITL models with significant engineering value were built by integrating transfer learning methodologies rooted in TrAdaBoost with a physics-based model that hinges on the principles of equivalent strain theory. The findings suggest that PITL models exhibit improved accuracy and greater robustness compared to both transfer learning and physics models.

Non-linear mechanical behaviour of thermoplastic elastomeric materials and its vulcanizate under tension/tension fatigue deformation by fourier transform rheological studies

Fourier transform (FT) rheology opens up novel frontiers in understanding non-linear mechanical behaviours of polymeric materials under sustained long-term dynamic load. The prediction of the exact point of appearance of a crack in a sample under dynamic mechanical strains of large amplitude and the fatigue analysis has been made possible to such degrees of precision, previously not possible through conventional rheological and mechanical analysis. In this work, the fatigue behaviour of a thermoplastic elastomeric material (TPE) and a thermoplastic vulcanizate (TPV) from thermoplastic polyurethane (TPU) and epichlorohydrin− ethylene oxide−allyl glycidyl ether (GECO) rubber is investigated by Fourier transform studies under oscillatory strain-controlled tensional test to understand the linear and non-linear mechanical behaviour. Fatigue analysis is performed through the fingerprint harmonics of the material’s stress response, i.e., the second and third harmonics, ( I 2/1 and I 3/1) to establish the stress nonlinearity, asymmetry, and the formation of macrocracks. Furthermore, these higher harmonics are fitted with the Neo-Hooke and Mooney-Rivlin model to fundamentally understand the mode of fatigue failure, and a close agreement between theoretical fitting and experimental outcomes is established. Strain-life curves were utilized for the thorough investigation of the fatigue behaviour of the specimens and more than 9-fold enhancement in the strain life of the TPV is observed over TPU at a strain amplitude of ∼1.05 indicating an increasing ductility. Finally, the necessity of FT rheology was further emphasised as it is observed that I 2/1 and I 3/1 harmonics are more sensitive towards fatigue response as compared to the storage modulus and the complex modulus. Henceforth, the FT rheology analysis has enabled the current study to efficiently establish the ductility of each individual specimens and for prediction of the dynamic service lifetime of the developed materials.

Microcrack characteristics of granite under circumferential strain-controlled uniaxial compression test using UDEC-GBM

Rockburst is one of the significant challenges for underground excavation stability. The post-peak behaviour of rock controls the way it fails, and the released energy is potentially involved in the rockburst. However, understanding of both microscopic and macroscopic behaviours of brittle rock in the post-peak stage is limited due to the lack of proper loading methods and quantitative microscopic analysis methods. This study proposes a circumferential strain-controlled loading algorithm in UDEC to control the failure process, which has been verified against previous literature. By employing the improved geometry generation method, the microcrack evolution is quantitatively assessed. Energy analysis reveals that up to 80% of the input energy is stored in the granite prior to complete failure, which could be released if the rock fails at this moment. Conversely, after the complete failure, intragranular cracks account for the majority of energy dissipation, indicating its importance in post-peak stability. A small portion of the peak stored strain energy needs to be extracted to achieve a controlled failure process. Furthermore, it is found that the significant increases in the formation of intergranular shear and low-angled microcracks potentially feature the complete failure of the specimen. The proposed model offers a complementary method to systematically investigate the post-peak behaviour of rock, including the evolution of microcracks and energy.

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.

Fatigue life prediction considering conversion of mean stress for titanium alloy under multiaxial thermo-mechanical random loading

This paper presents a thermo-mechanical fatigue (TMF) life prediction method for titanium alloy TA15 under multiaxial random amplitude loading. Through strain-controlled TMF tests, it was found that under full-reversed cyclic loading, there was a significant presence of mean stress in the stress response. The induced mean stress led to the anomalous phenomenon where the fatigue life of thermal phase angle out-of-phase (TOP) was shorter than that of thermal phase angle in-phase (TIP). Therefore, a new damage calculation model was proposed, which can take into account the pure fatigue damage considering mean stress conversion and the cumulative environmental damage. Uniaxial TMF tests, constant amplitude multiaxial TMF tests, and random amplitude multiaxial TMF tests are conducted to verify the proposed method. The predicted results for all types of tests are in a good agreement with the experimental data.

Quasi-static tensile and fatigue behaviour of a metastable austenitic stainless Cr-Ni-Cu-N steel

The tensile as well as the fatigue behaviour of an experimental copper-alloyed metastable austenitic stainless steel has been investigated. Three batches were studied: (i) cast material as reference state and two states were recrystallised after (ii) rotary swaging and (iii) forward impact extrusion. Deformation-induced α’-martensite was formed during both rotary swaging and forward impact extrusion with volume fractions of 48 vol% and 4 vol%, respectively. The material was then heat treated to form a fine-grained, fully austenitic microstructure. These fine-grained material states exhibited a higher strength compared to the cast state. However, strain-hardening was highest in the cast state. The effect of the initial microstructure on the cyclic deformation behaviour and fatigue life was investigated using strain-controlled tests. During cyclic deformation all three states exhibited a martensitic phase transformation and concurrently a pronounced cyclic hardening with the formation of up to approximately 70 vol% α’-martensite. The fine-grained state produced by rotary swaging and reversion annealing showed a significantly higher fatigue life of about factor 4 compared to the coarse-grained cast state. Relative to forward impact extrusion, rotary swaging provided a higher fatigue life and fatigue strength.

Effect of loading modes on uniaxial creep-fatigue deformation: A dislocation based viscoplastic constitutive model

A comprehensive investigation was conducted on the stress-strain responses and microstructural evolutions of 9Cr3Co3WCu martensitic steel at 650 ℃ subjected to low cycle fatigue tests, strain-controlled creep-fatigue tests (SNCFTs), and hybrid stress-strain-controlled creep-fatigue tests (HSSCFTs). The creep strain accumulation per cycle in HSSCFTs exhibited three stages: an initial decrease in the early cycles, followed by a prolonged period of steady increase, culminating in a rapid increase prior to fracture. In contrast, the creep strain accumulation in SNCFTs showed a consistent decreasing trend throughout the test. Through the employment of advanced characterization techniques, the evolution of dislocation density exhibited a decreasing rate in all cases, and a fracture morphology characterized by a large size of creep voids was observed in HSSCFTs. Consequently, a dislocation-based viscoplastic constitutive model was developed, incorporating a relaxation parameter, a slip deformation resistance model and a dislocation density evolution model interacting with the grain boundary. The proposed model demonstrated excellent congruences under fatigue, creep, creep-fatigue, and multi-step creep- fatigue tests between experimental and simulated results, thereby confirming its capability to comprehensively capture cyclic deformation under various loading modes.

Application of Instrumented Indentation Procedure in Assessing the Low-Cycle Fatigue Properties of Selected Heat-Treated Steels.

The paper presents an analysis of the low-cycle fatigue (LCF) properties of C45, X20Cr13, and 34CrNiMo6 steels subjected to various heat treatment processes. Strain-controlled LCF tests were carried out with a total cyclic strain amplitude equal to 0.5, 1 and 1.5%. Fatigue life, cyclic stress-strain behavior and hardness were analyzed. Qualitative and quantitative relationships between material LCF properties resulting from the heat treatment processes, were related to the indentation force P*, which was derived experimentally by applying an instrumented indentation procedure with the use of the Vickers indenter. The proposed parameter P* and its changes ΔP* seem to be promising for the identification of the structural stress parameter σ* that is necessary for deriving values of the fatigue strength coefficients σf' corresponding to different tempering temperatures. The common feature of all steels analyzed in this paper is that the elastic parts of the strain-life characteristics remain parallel after being subjected to different tempering temperatures.

Mechanistic model for hydrogen accelerated fatigue crack growth in a low carbon steel

Fracture by hydrogen accelerated fatigue crack growth is a severe type of environmental failure. Although fatigue crack growth has been the subject of intense investigation over several decades, a complete mechanistic and predictive model is still lacking. Such a crack growth model is even more rare in the case of hydrogen in view of the lack also of constitutive models for material deformation under cycling loading that account for the hydrogen effect. In this study, we present a model for fatigue crack propagation induced by alternating crack tip plastic blunting and re-sharpening, which in the presence of hydrogen can be accelerated by hydrogen enhanced dislocation motion and generation. The Chaboche constitutive model, which is a nonlinear kinematic hardening model capable of capturing many features of material behavior under cyclic loading, is used for the calculation of the stress and strain fields at the propagating crack tip. The Chaboche model is calibrated using a sequence of experimental data from uniaxial strain-controlled cyclic loading tests and uniaxial stress-controlled ratcheting tests with a low carbon steel, JIS SM490YB, in the absence and presence of hydrogen. The numerical simulation results indicate that the proposed crack propagation model can predict Paris law behavior and can successfully demonstrate acceleration of fatigue crack growth in the presence of hydrogen. Significantly, the profiles of the steady-state opening stress and strain ahead of the fatigue crack tip in a compact tension (C(T)) specimen were found to have sections over which they vary as ln(1/r) with distance r from the crack tip, consistent with the crack-tip strain field for a non-stationary crack.

A new perspective on Lüders band formation in medium-Mn steels based on Lüders-strain-rate and dislocation evolution

Propagating Lüders bands with pronounced strain localization are often observed in medium-Mn transformation-induced plasticity (TRIP) steels, which adversely limits the application of such alloys in engineering structures. The present work aims to understand this phenomenon from a dislocation-based perspective, with a focus on the specific role of dislocation multiplication. The material studied is a typical dual-phase medium-Mn steel with a ferrite matrix and a second phase of retained austenite. A warm-rolling process was used as a pre-straining approach to adjust the microstructure of the material, such that materials with two different microstructural states can be obtained: as-received material and pre-strained material. They show distinct differences in initial dislocation density. Stress-controlled rather than conventional strain-controlled tensile testing was adopted, which allowed the Lüders band to develop in an unconstrained manner. In this way, the intrinsic Lüders-strain-rate can be obtained. The results show that the Lüders band-related mechanical parameters, including Lüders strain, Lüders-strain-rate and yield stress drop, all depend on the initial microstructure state and the dislocation evolution process during Lüders band formation. In particular, Lüders-strain-rate is a critical microstructure-sensitive factor. It is closely related to the dislocation multiplication in the formation of Lüders band, which can be characterized and quantified using scanning transmission electron microscope and synchrotron X-ray diffraction. Finally, a quantitative relationship between Lüders-strain-rate and dislocation multiplication rate is established, which allows providing a better understanding of the exceptional Lüders band phenomenon in medium-Mn steels.

Local ratcheting and stress relaxation at notch root of CP-Ti specimen under cyclic loading: Experiment and simulation

Damage and crack always origin from notch, and it is necessary to understand the local ratcheting and stress relaxation at notch root for commercially pure titanium (CP-Ti). Symmetric strain-controlled cyclic test for smooth CP-Ti specimen is conducted to determine the cyclic constitutive relationship based on the combined hardening model. Asymmetric force-controlled cyclic test combined with Digital Image Correlation (DIC) technology is conducted to reveal local ratcheting strain distribution at notch root of CP-Ti specimen, while the distributions of stress, local stress ratio, and local hysteresis curve at notch root are obtained by finite element (FE) simulation. It is found that plastic zone, local ratcheting and stress relaxation at notch root area change significantly with cyclic load levels. In the low stress amplitude region with the mean nominal stress lower than 87 MPa, the small-scale yield occurs, and the notch root is in a state of plastic shakedown. With the mean nominal stress increasing to 96 MPa in the transition region, the notch root is in a state that exceeds the plastic shakedown limit but the continuous ratcheting progress is not found. In the high stress amplitude region with the mean nominal stress higher than 104 MPa, the plastic zone significantly expends, and the notch root is in a continuous ratcheting progress state, accompanied by stress relaxation. For a specific structural configuration, the local ratcheting at notch root is affected by both the load level and the cyclic deformation characteristics of the material. In the high stress amplitude region, the reverse yield occurs at the notch root during unloading, which promotes continuous ratcheting progress and stress redistribution, resulting in stress relaxation. The higher the load level, the larger the ratcheting strain rate, and the lower the stress relaxation rate.

Low cycle fatigue behavior and life prediction of a directionally solidified alloy

Low cycle fatigue behavior and life prediction of a directionally solidified alloy

Evaluations and enhancements of fatigue crack initiation criteria for steels subjected to large shear deformations

While large accumulated plastic deformations occur in the rail surface layer where rolling contact fatigue cracks initiate, many available Low Cycle Fatigue (LCF) crack initiation criteria focus on small plastic strains. Accordingly, this paper evaluates available fatigue crack initiation criteria for highly shear-deformed R260 steels, reflecting the conditions in the surface layer of rails. Furthermore, modified crack initiation criteria are suggested. The evaluation is based on three different experiments: Large shear strain increments under varying axial loading (predeformation), strain-controlled LCF tests after some predeformation, and axial High Cycle Fatigue (HCF) experiments. For the predeformation, Finite Element (FE) simulations, with a large-strain plasticity model for cyclic and distortional hardening, provide predictions of the local stress and strain histories. A cross-validation procedure is used to assess the accuracy and reliability of both established and modified fatigue crack initiation criteria. The proposed modifications to one of the criteria show an improved fit to the experimental data. However, there is a tendency to overfitting, which can be improved by including more experimental data.

Effect of Non-Plastic Fines Content on the Pore Pressure Generation of Sand-Silt Mixture Under Strain-Controlled CDSS Test

Effect of Non-Plastic Fines Content on the Pore Pressure Generation of Sand-Silt Mixture Under Strain-Controlled CDSS Test

Creep-Fatigue Life Evaluation for Grade 91 Steels with Various Origins and Service Histories

Grade 91 steel is widely used in the boilers and piping of thermal power plants. There has been significant research interest in understanding the variations in creep characteristics among different heats of this steel for effective plant management. In recent years, thermal power plants have been subjected to frequent load changes and startup/shutdown to adjust power supply and demand and stabilize frequencies. These operational shifts have raised concerns regarding the potential for creep-fatigue damage in high-temperature components. Therefore, this research focuses on creep-fatigue properties of Grade 91 steel and their predictability. Tensile, creep, strain-controlled fatigue, and strain-controlled creep-fatigue tests were performed on six Grade 91 steels with different heats and/or histories, and the characteristics in each test were compared. As a result, the variations in creep-fatigue life among the materials were found to be correlated with the difference in creep characteristics and stress level during stress relaxation. Furthermore, the study involved a comparative assessment of the predictive performance of creep-fatigue life using five different approaches: time fraction, classical ductility exhaustion, modified ductility exhaustion, energy-based, and hybrid approaches. Among these approaches, the hybrid approach, based on inelastic strain energy density at fracture formulated as a function of inelastic strain rate, exhibited the most accurate predictive performance.

Effect of lateral confinement method on excess pore pressure generation under strain-controlled cyclic direct simple shear test

Effect of lateral confinement method on excess pore pressure generation under strain-controlled cyclic direct simple shear test

The Effect of Buckling Restraint on Axial Strain Life Fatigue Data

Abstract Focused on the common problem of easy specimen buckling in the axial strain–controlled fatigue testing of thin automobile steel sheets, a strain fatigue test method for thicknesses less than 2.5 mm is proposed. Because of the unrestrained grip end of specimens between the anti-buckling device and the testing machine fixture, specimens were prone to buckling in this area. In addition, if the designed specimen sizes were unreasonable, buckling may have occurred in the width and thickness directions of the gauge length of the specimen. Both situations can lead to invalid tests. This paper adopted a testing machine fixture and an anti-buckling plate as a mortise-and-tenon structure for the anti-buckling method; the upper and lower fixtures of the testing machine were cut in straight grooves, and the left and right anti-buckling plates were designed to be connected with a mortise and tenon. This is equivalent to narrowing the width of the unrestrained part of the specimen’s grip end to avoid buckling failure due to plane strain. The factors affecting the stiffness of fatigue specimens are also discussed herein. In order to avoid common instability in the thickness and width directions of the gauge length and unconstrained part of the specimen’s grip end at a strain ratio of Re = −1 during testing, six recommended specimen sizes are given on the premise of ensuring maximum specimen stiffness. When using the anti-buckling device, the recommended specimen sizes and test points of this study (i.e., the lower limits for yield strength and thickness of strain control testing) are 110 MPa and 0.5 mm, respectively, with which continuous and smooth stress–strain curves that result in accurate and reliable experimental data can be obtained.