Monotonic Deformation Research Articles

Microstructural evolution and mechanical behavior of pearlitic steel under multi-directional forging

The mechanical behavior and the microstructural evolution of a pearlitic SAE 1080 steel were analyzed along 12 Multi-Directional Forging (MDF) cycles at room temperature and strain amplitude Δε ≈ 0.30 (MDF0.30). The resulting cumulative strain flow curve is typical of metals undergoing extensive dynamic recovery, displaying appreciable work hardening up to a strain εt ≈ 0.9, followed by a very low work hardening, where flow stress rises from ≈1400 MPa to ≈1510 MPa. Similarly, the microhardness raises up to a total cumulative strain εt ≈ 1.8 and seems to stabilize for further straining, reaching ∼3626 MPa after εt ≈ 10.8. Conversely, yield strength initially rises and then falls remarkably after εt > 5.4. This behavior is related with directional cross effects that favor dynamic recovery as the total applied strain increases. These results differ remarkably from those connected to a monotonic deformation process such as axisymmetric drawing. In addition, directional cross effects decrease the material yield stress as the total applied strain increases. Microstructural results revealed that MDF0.30 leads to increasing shearing, bending and fragmentation of the cementite lamellae into particles <50 nm in diameter dispersed in the ferritic matrix as the total imposed strain rises. As a consequence, only limited alignment and changes in the cementite interlamellar spacing are observed, up to a total applied strain ≈10.8. It is considered that the work hardening of pearlitic steel deformed through MDF0.30 is dominated by the dynamic recovery of the interlamellar ferrite and a limited contribution from the cementite. Such results have never been reported in the literature and are of practical importance, since extensive cold working of eutectoid steel through MDF can be performed with standard presses and large specimens.

Probing phase transformation and dislocation evolution in high-entropy alloy under cyclic loadings

Stress-induced FCC-BCC phase transformation plays a crucial role in the mechanical behaviors of high-entropy alloys (HEAs). While there has been extensive research on this transformation during monotonic deformation, studies on fatigue behavior are extremely limited. Here, we use molecular dynamics simulations to investigate phase transformation and dislocation evolution in HEAs under strain-controlled symmetric tension–compression cycles. Our results show that cyclic deformation behavior is sensitive to strain amplitude, revealing three distinct cyclic responses. Notably, a progressive FCC-BCC phase transformation process occurs at a high strain amplitude of 4.8%. Grain boundaries and their triple junctions are identified as preferred sites for phase transformation under cyclic loading conditions. These findings provide valuable atomic-scale insights for understanding fatigue deformation in HEAs with transformation-induced plasticity.

Plastic deformation delocalization at cryogenic temperatures in a nickel-based superalloy

A nickel-based superalloy is examined during monotonic deformation from ambient to cryogenic temperatures, reaching as low as liquid helium temperature. A detailed multimodal analysis of the microstructure and plasticity is conducted to discern changes in deformation mechanisms and plastic deformation localization under cryogenic conditions. This study employs high-resolution digital image correlation and transmission electron microscopy to identify the deformation mechanisms and understand their influence on plastic deformation localization as the temperature varies. At cryogenic temperatures, unusual plastic deformation localization processes are observed, attributed to the competing activation of a range of deformation processes. Furthermore, a mechanism of slip delocalization, i.e., local plastic deformation homogenization through closely spaced slip, is noted at these extreme temperatures. Ultimately, the impact of the microstructure is identified across the temperature range, from room to cryogenic temperatures.

Degradation of molecular structure and residual strength of polycarbonate under cyclic loading: Insights from coarse-grained molecular dynamics simulation

Coarse-grained molecular dynamics simulations have been employed to provide insights into the molecular behavior and degradation of monodisperse polycarbonate subjected to cyclic loading with varying molecular structures characterized by two distinct parameters: radius of gyration and molecular entanglement. The fatigued polycarbonate structures were subjected to monotonic deformation in order to assess their residual strength. Our findings reveal that structures characterized by a higher degree of molecular entanglement exhibit a more significant loss of residual strength, attributable to the disentanglement induced by cyclic deformation. Furthermore, an increase in the radius of gyration contributes to greater resilience against cyclic loading, resulting in a higher level of residual strength compared to the impact of molecular entanglement.

Computer vision-based real-time deflection monitoring of complex and sizeable steel structures

Deflection, as an intuitive index, plays a pivotal role in assessing the load-bearing capability and structural integrity of complex and sizable steel structures. Notwithstanding, the conventional contact measurement is limited to static deflection and necessitates work stands and manual readings. Therefore, it hinders the attainment of dynamic deflection and fails to cater to the engineering demands of real-time and prolonged monitoring. By leveraging the advancements in computer vision technology, we propose an innovative system for real-time deflection monitoring of complex and sizable steel structures, particularly suitable for monotonic deformation induced by prolonged loading. Specifically, the off-axis-based displacement measurement method was adopted to surmount the constraint of requiring the optical axis of the camera to be perpendicular to the target. Additionally, the inverse compositional Gauss-Newton (IC-GN) algorithm and parallel computation based on seed point diffusion were exploited to boost the matching and computing efficiency for attaining real-time monitoring of multi-points. To validate the efficacy of the proposed system, we conducted static load monitoring tests on a Bailey beam, with a height of 29.5 m and length of 16.5 m per span, as part of the Alibaba Jiangsu headquarters project in Nanjing. The collected test data were compared with the results from a laser displacement sensor and the ABAQUS model. The outcomes substantiate that the system is capable of non-contact, expeditious, and straightforward installation, besides achieving high accuracy and real-time deflection monitoring. This system serves as a sophisticated solution for health monitoring in constructing complex and sizeable steel structures and further contributes to realizing construction intelligence.

Prediction on residual flexural fatigue life of rock-concrete interface based on a deformation-controlled fatigue crack propagation criterion

To assess the cracking resistance of rock-concrete interface, three-point bending tests were carried on composite rock-concrete specimens under the monotonic and high flexural fatigue loading. The crack opening displacements along the ligament of the beam were continuously recorded in the whole test process. According to the measured crack mouth opening displacements (CMODs), the correlation between the monotonic and fatigue deformations at different cracking statuses was discussed. It has been shown that the deformation development of composite specimens under flexural fatigue loading was mainly caused by the propagation of interfacial crack. The P-CMOD curve under monotonic loading was the envelope curve of those under the different maximum fatigue loads. There were three CMOD control points in the monotonic deformation response to indicate the fatigue crack propagation states, which corresponded to the initial cracking load, the ultimate load and the applied maximum fatigue load in the post-peak part of the monotonic response, respectively. Correspondingly, they manifested the fatigue crack initiation, the inflection point of the fatigue crack propagation rate from deceleration to acceleration and the fatigue failure of the rock-concrete interface. Based on the three CMOD control points, a deformation-controlled fatigue crack propagation criterion was proposed to assess the cracking state of the rock-concrete interface under fatigue loading. Accordingly, a theoretical prediction model, termed as fatigue deformation evolution law, was established to predict its residual flexural fatigue life. This method was verified by the reasonable agreements between experimental and predicted results in the residual fatigue life.

Cooperative deformation mechanisms in a fatigued CoCrNi multi-principal element alloy: A case of low stacking fault energy

Multi-principal element alloys (MPEAs) of low stacking fault energies (SFEs) often exhibit outstanding damage tolerance. In contrast to the well-understood monotonic deformation behaviors, fatigue deformation mechanisms of the low-SFE MPEAs remain fundamental questions. Using TEM, this work demonstrated two main dislocation configurations cooperating in an exemplary low-SFE CoCrNi MPEA, namely slip bands and veins/walls/cells, which are usually formed in materials with low- and high-SFEs, respectively. Under low strain amplitude of 0.3%, the deformation features are found to include primary and double slip bands (dominating in ≈71% and 29% grains, respectively). Upon increasing strain amplitude, apart from the slip bands, dislocation substructures formed (i.e., veins/walls/cells present in ≈ 33% and 60% grains under strain amplitude of 0.5% and 0.7%, respectively) due to cross slip, despite the alloy's low-SFE. Moreover, the slip bands and walls/cells are found to serve more likely as geometrically necessary dislocations and statistically stored dislocations, respectively. Besides, the constraints from neighboring grains and Copley-Kear effect (rather than grain orientation) were suggested to play more important role in determining these dislocation patterns formation. Lastly, this work evidenced unexpected partial dislocations and stacking faults (in addition to usually observed screw dislocations) shuttling between the walls/cells by planar slip. This predominant planar slip behavior explains well the exceptional damage tolerance of low-SFE MPEAs. These insights can also advance the understanding of fatigue deformation of conventional alloys with low-SFEs.

A study on micro‐mechanical deformation behaviour of heat‐treated 20MnMoNi55 steel

AbstractSeveral heat treatment procedures are designed considering critical temperatures of phase transformation evaluated through dilatometric testing of 20MnMoNi55 steel to transform low carbon bainitic as‐received material into ferrite‐martensite dual‐phase steels consisting of varied martensite fractions. A thorough metallographic study correlated with the micro‐hardness of constituent phases ensures morphological characteristics along with its fractional variations in as‐received and dual‐phase steels. The impact of fractional variation in constituent phases on the uniaxial monotonic deformation characteristics of dual‐phase steels has been observed with a correlation study between experimental tensile and finite element simulated results. Therefore, a physical‐based model with a 2‐dimensional representative volume element has been established, addressing actual morphological characteristics obtained from metallographic studies. Moreover, the constitutive flow behaviours of ferrite and martensite are also derived from the dislocation‐based hardening model to address the actual deformation phenomenon. Finally, an inhomogeneous deformation behaviour among constituent phases and localization of plastic strain in ferrite matrix has been observed with von‐Mises stress, and equivalent plastic strain distribution through finite element simulated results. This phenomenon is again confirmed with kernel average misorientation mapping and geometrically necessary dislocation density evaluation through electron backscattered diffraction of tensile samples subjected to different degrees of plastic strain.

Monotonic and cyclic load of pine wood under uniaxial compression: experiments and modeling

To analyze the behavior of wood under uniaxial monotonic deformation, we applied an analytical model to separately control of the pre-peak and post-peak branches of the load-displacement curve. Two criteria for the transition of wood into the plastic stage of deformation are considered. To analyze the behavior of wood under cyclic uniaxial deformation, we used the load-displacement curve obtained for monotonic deformation as an envelope. Equations for the plotted of load-displacement curves under cyclic loading with incomplete unloading are proposed. As an experimental and model illustration, the behavior of a pine specimen under cyclic uniaxial compression with incomplete unloading in the range from 8 to 15 kN is considered. Because the highest cyclic load exceeds the load at the point of transition of wood into the plastic stage, the deformation process has already after 54 cycles moved to the downward branch of the load-displacement curve for monotonic deformation. The simulation results are in agreement with experimental and literature data, which suggests the suitability of this method for the study of some wood-based composites.

Dislocation density and twinning-related constitutive modeling of substructural evolution in Sn-5Sb-0.5(Ag/Cu) alloys under monotonic deformation

The development of a numerical model and experimental evaluation of microstructural evolution during the deformation of such materials are of pronounced significance. In the current work, evolutions of the substructure in three Sn alloys, namely Sn-5Sb, Sn-5Sb-0.5Ag, and Sn-5Sb-0.5Cu, were investigated using a dislocation density constitutive equation. The mechanical behavior of such alloys has been assessed, taking advantage of Ag and Cu addition and substructure-based factors like dislocation density, size, and misorientation. A comprehensive constitutive equation was developed for modeling the flow behavior of Sn-based alloy through the substructure-based tensile response of the material. It has been perceived that Cu is more in effect to improve the strength of Sn than Ag and Sn-5Sb-0.5Cu alloy is with enhanced work hardening rate during the tensile state of deformation. Dislocation density formulation results have direct implications on the capability to explicitly model the superimposed effect of dislocation densities and subgrain boundaries on the macroscopic mechanical properties of polycrystals. A numerical description of twinning stress in studied BCT alloys is described by the dislocation production kinetic model considering dislocation pile-up as the dominant mechanism. The developed slip-twinning transition model is incorporated into the deformation maps to identify the twinning domain.

Heterogeneous slip localization in an additively manufactured 316L stainless steel

Additively manufactured 316L stainless steels display significantly higher yield strength than their as-cast or wrought counterparts. This is associated with the micro-scale cellular structure and complex grain and sub-grain structure, resulting from high cooling rates occurring during the additive manufacturing process. The consequences of these peculiar microstructural features on plastic localization early in the plastic regime at the sub-grain scale are investigated. The plastic localization involved during monotonic deformation of conventional and additive manufactured 316L stainless steels is investigated using high-resolution digital image correlation. Significant heterogeneous slip localization is observed in the additively manufactured 316L stainless steels compared to the wrought 316L stainless steels. The cellular structure and low-angle grain boundaries are observed to control the incipient plasticity. In addition, slip localization characteristics indicate that the additional strengthening in the AM material is mainly related to the cellular structure acting as a dislocation forest-type obstacle.

Origin of superior low-cycle fatigue resistance of an interstitial metastable high-entropy alloy

In this study, the deformation behaviors and related microstructural evolutions were investigated in either monotonic or cyclic deformation modes in an interstitial metastable high-entropy alloy. These investigations aimed to reveal the mechanisms underlying the superior low-cycle fatigue (LCF) life of this alloy. A thermomechanical process was applied to induce fine-grained (FG) and coarse-grained (CG) microstructures in Fe–30Mn–10Co–10Cr–0.4C (atomic percentage) alloy. Their superior combination of strength and ductility was attributed to the appearance of deformation-induced ε-martensite and the presence of carbon. The CG alloy showed a greater volume fraction of ε-martensite than the FG alloy in the monotonic deformation mode, and vice versa in the cyclic mode. Such a disparity was interpreted in light of the back-stress effect of the relaxed γ-grain boundaries in the latter mode. Meanwhile, the γ-to-ε phase transformation under cyclic loading at low strain amplitudes (0.4%) barely led to an improved fatigue life as compared with that at higher strain amplitudes (≥ 0.55%). The high reversibility of partial dislocation motions under cyclic loading and delaying the formation of dislocation cells through the martensitic transformation could explain why the alloys investigated in this study exhibited a superior LCF life compared with high-entropy alloys reported in previous studies.

A criterion for when an emulsion drop undergoing turbulent deformation has reached a critically deformed state

Turbulent breakup in emulsification devices is a dynamic process. Small viscous drops undergo a sequence of oscillations before entering the monotonic deformation phase leading to breakup. The turbulence-interface interactions prior to reaching critical deformation are therefore essential for understanding and modeling breakup. This contribution uses numerical experiments to characterize the critically deformed state (defined as a state from which breakup will follow deterministically, even if no further external stresses would act on the drop). Critical deformation does not coincide with a threshold maximum surface area, as previously suggested. A drop is critically deformed when a neck has formed locally with a curvature such that the Laplace pressure exceeds that of the smallest of the bulbs connected by the neck. This corresponds to a destabilizing internal flow, further thinning the neck. Assuming that the deformation leads to two spherical bulbs linked by a cylindrical neck, the critical deformation is achieved when the neck diameter becomes smaller than the radius of the smallest bulb. The role of emulsifiers is also discussed.

Micromechanism of Plastic Accumulation and Damage Initiation in Bearing Steels under Cyclic Shear Deformation: A Molecular Dynamics Study

Fatigue failure usually occurs on the subsurface in rolling bearings due to multiaxial and non-proportional fatigue loadings between rolling elements. One of the main stress components is the alternating shear stress. This paper focuses on the micromechanism of plastic accumulation and damage initiation in bearing steels under cyclic shear deformation. The distribution of subsurface shear stress in bearings was firstly investigated by finite element simulation. An atomic model containing bcc-Fe and cementite phases was built by molecular dynamics (MD). Shear stress–strain characteristics were discussed to explore the mechanical properties of the atomic model. Ten alternating shear cycles were designed to explore the mechanism of cyclic plastic accumulation and damage initiation. Shear stress responses and evolutions of dislocaitons, defect meshes and high-strain atoms were discussed. The results show that cyclic softening occurs when the model is in the plastic stage. Severe cyclic shear deformation can accelerate plastic accumulation and result in an earlier shear slip of the cementite phase than that under monotonic shear deformation, which might be the initiation of microscopic damage in bearing steels.

Experimental research on seismic damage of T-shaped CFST column to steel beam joints

To investigate the seismic damage of T-shaped concrete-filled steel tubular (CFST) column to steel beam joints, eight specimens with the scale of 1/2 were tested under various loadings. The failure modes, load–displacement curves, energy dissipation capacity, cyclic strength degradation, and strain distribution are analyzed. The test results indicate that the failure pattern of the specimens is mainly the shear failure of the joint panel zone and the fracture of the beam end. The hysteretic curves have no remarkable pinch effect, and the joints have excellent energy dissipation capacity. The shear bearing capacity and shear deformation capacity of specimens improve with the growth of depth to width ratio. The seismic damage of specimens with a bigger depth to width ratio develops more slowly. During the mixed loading, the monotonic deformation capacity of specimens decreases significantly with the increase of seismic damage of cyclic loading. Under cyclic loading with constant amplitude, the accumulative damage plays a major role in the failure of the specimens, and the specimens under a bigger constant amplitude are more prone to failure. The column web within the joint panel zone shows a larger damage degree than the column limb.

Dislocation structures in a Ti–6Al–4V alloy subjected to cyclic tensile deformation

The present work investigates dislocation structures developed within both the α and β phase of a Ti–6Al–4V alloy undergoing cyclic tensile straining. The cyclic loading-unloading was performed using the stress levels of 830 and 580 MPa (R∼0.7) at a strain rate of 2 × 10-3 s-1 to 15125 cycles, as part of a synchrotron in-situ experiment. The macroscopic yield stress was about 950 MPa. The dislocation arrangement, type and density were studied post-mortem employing transmission electron microscopy, together with the automatic determination of crystal lattice orientations via precession nanobeam diffraction. The deformation took place exclusively through dislocation slip and no presence of deformation twins was detected within either of the studied phases. The α phase dislocation structure displayed some similarity to that reported for monotonic deformation and no evidence of the formation of regular planar dislocation networks, proposed in the literature, has been found. The prevalent deformation modes were prismatic and basal glide, based on the experimentally determined Burgers vectors together with the global Schmid factor and reported critical resolved shear stress values. The <a>-type dislocations observed after deformation generally possessed a large screw component, with most of them being of a pure screw type. There were also some <c+a> dislocations present within the α phase and these typically displayed a mixed character. Dislocation Burgers vectors present after straining within the β phase frequently deviated from the Schmid factor rule and these dislocations were often wavy and largely possessed a significant screw component.

Damage behaviour of an Al2O3 particle-reinforced 6061 alloy induced by monotonic and cyclic deformation

Abstract The influence of damage and its correlation with the failure behaviour is discussed in the light of microstuctural aspects. The failure induced by microstructural mechanisms of the Al2O3-particle reinforced aluminium alloy 6061 under uniaxial tensile as well as under cyclic loading was investigated. The damage behaviour as a function of the strain ε can be correlated with stiffness E(ε). Under cyclic loading conditions the damage increases with the number of cycles and can be correlated with the strain amplitude εa. Furthermore, the damage is associated with crack formation and propagation and the damage evolution depends sensitively on the kind of deformation. The crack formation was detected by metallographic investigations and calculated by numerical methods (FEM), taking the size and shape of particles into account. Generally, damage under mechanical loading is initiated by broken particles and agglomerations of particles. In case of the particulate-reinforced specimens the evolution of damage is quite different for monotonic and cyclic loading conditions but, nevertheless, in both cases the damage parameter is given by inhomogeneities characteristic for the MMC which act as seeds for crack initiation.

Crack initiation and propagation behavior in γ’-Fe4N layer during monotonic and cyclic tensile deformation in the nitrided ultra-low carbon steel

Crack initiation and propagation behavior in γ’-Fe4N layer during monotonic and cyclic tensile deformation in the nitrided ultra-low carbon steel

Basic Creep-Fatigue Models Considering Cavitation

Cavitation plays a central role during creep-fatigue. During recent years, fundamental models for initiation and growth of creep cavities that do not involve any adjustable parameters have been developed. These models have successfully been used to predict creep rupture data for austenitic stainless steels again without using adjustable parameters. However, it appears that basic models have not yet been applied to creep-fatigue assessments. A summary of the fundamental cavitation models is given. A model for monotonous deformation is transferred to cyclic loading. The parameter values are kept except that the dynamic recovery constant is raised due to increased interactions between dislocations during cycling. This model is successfully compared with observed LCF and TMF hysteresis loops. A new model for cavity growth due to plastic deformation is presented. The model is formulated in such a way that the condition for constrained growth is automatically satisfied. In this way, it is avoided to overestimate the growth rate.

Cyclically loaded risers and pipelines in cohesive soil

Lateral resistance and lateral displacement are important design factors of risers/pipelines. This work presents the behavior of the riser/pipeline-soil interaction due to the “zig-zag” cyclic movement (combined of vertical and lateral movements). This study has been based on a series of monotonic, lateral cyclic, and T-bar tests using a single gravity model test. This study aims to evaluate (1) the fundamental mechanism of riser/pipeline-soil interaction due to the zig-zag cyclic loading, (2) large monotonic deformation at different embedment depths of 0.5 and 1.0 D (where D = pipe diameter), (3) zig-zag cyclic loading for cyclic displacement amplitudes of 0.15–0.5 D, (3) lateral resistance degradation, and (4) trench formation. The findings from this paper are that (1) the pipe tends to rise due to the behavior of light pipes during the first few cycles. Continuing towards more cycles, the pipe tends to move downward, (2) the riser/pipeline lateral resistance drops after trench formation, continuing towards more cycles, the resistance starts to lightly increase, (3) lateral soil resistance on a shallowly embedded pipe (w/D = 0.3) drops significantly after forming the trench, and it is less pronounced at increased embedment depths, and (4) the depth and width of the trench depend on the zig-zag displacement amplitude.