Large Strain Accumulation Research Articles

A study on the evolution mechanism of small diameter thin-walled stainless steel bellows during a bending process

In order to accurately construct the evolution mechanism of the matrix during the bending process, this paper combines finite element simulation and material characterization methods to study the evolution process of the matrix in detail. Firstly, a repeated bending model of the bellows was constructed by ABAQUS finite element software. The deformation hardening, geometry evolution, stress and strain evolution of the matrix were analyzed in depth. At the same time, by characterizing the microstructure of the trough under different bending times, the evolution of the microstructure during the fracture process was obtained. Combined with the fracture characteristics, it can be seen that the deformation hardening of the trough is significant and the stress is large, so it is most likely to become the failure zone. The waveform distortion increases the deformation inhomogeneity of the trough at different positions, resulting in local large strain accumulation and further aggravating the fracture failure of the trough. During the bending process, a large number of martensite tissue appears in the central area of the bellow wall thickness, and the dislocation gradually moves from the inner and outer surfaces to the center. The crack initiates first at the bellow outer surface and propagates along the circumference and wall thickness direction. Bending four times is the key node of crack propagation. Moreover, the crack propagation models of the trough in the wall thickness and circumferential direction are established, and the fracture failure mechanism of the trough during repeated bending is revealed.

Fatigue crack growth behavior of marine steel under variable amplitude loading-combining DIC technique and SEM observation

This paper experimentally studies the fatigue crack growth (FCG) behavior of AH36 grade marine steel under variable amplitude loading. FCG testing is conducted using compact tension specimens. Digital image correlation technique (DIC) is employed to study the evolution of strain distribution and cyclic fatigue characteristics. Fractographic analysis is performed to explore the overload retardation mechanism. Results indicate that the FCG rate varies significantly under overload application, leading to the extension of fatigue life by 1.03, 1.18 and 2.42 times under the overload ratios (OLRs) of 1.5, 2.0 and 2.5. Reasonable evaluation of the fatigue performance depends on the loading history. Extension of the fatigue life for periodic overload (POL) cases is not a simple superposition of that under two single ones, the interaction effect between the adjacent tensile overloads cannot be neglected. Greater plastic dissipated energy corresponds to more severe plastic deformation at larger OLRs and crack lengths. The larger strain accumulation at the crack tip should account for the greater FCG resistance and more evident overload retardation effect. The fresh fracture surface is destroyed at a certain region after overload due to the premature contact of the crack flanks, confirming that overload induced crack closure exists.

Crystal plasticity simulation of in-grain microstructural evolution during large deformation of IF-steel

High-resolution three-dimensional crystal plasticity simulations are used to investigate deformation heterogeneity and microstructure evolution during cold rolling of interstitial free (IF-) steel. A Fast Fourier Transform (FFT)-based spectral solver is used to conduct crystal plasticity simulations using a dislocation-density-based crystal plasticity model. The in-grain texture evolution and misorientation spread are consistent with experimental results obtained using electron backscatter diffraction (EBSD) experiments. The crystal plasticity simulations show that two types of strain localization features develop during the large strain deformation of IF-steel. The first type forms band-like areas with large strain accumulation that appear as river patterns extending across the specimen. In addition to these river-like patterns, a second type of strain localization with rather sharp and highly localized in-grain shear bands is identified. These localized features are dependent on the crystallographic orientation of the grain and extend within a single grain. In addition to the strain localization, the evolution of in-grain orientation gradients, misorientation features, dislocation density, kernel average misorientation, and stress in major texture components are discussed.

Determining the static Young’s modulus and Poisson’s ratio, and compressive strength of the friable Erin Formation rocks using P-wave velocity

It is important to understand the mechanical properties (elastic properties and strength) of rocks as these properties are used to model the rock's deformation in civil and underground engineering projects. Determining these properties for friable rocks are very difficult and scarce due to the rock's poor cementation making it challenging to retrieve samples from outcrop and boreholes, and difficult to prepare specimens for laboratory testing. To improve on the lack of mechanical properties of friable rocks, this study aims to measure the mechanical properties and P- and S-wave velocities and determine if empirical relationships exist between them. Experiments were carried out at effective pressures up to 130 MPa under dry and saturated conditions on friable sandstone and thin-bed shale lithofacies from the Erin Formation. The static Young's modulus of the rocks is less than 8 MPa and the unconfined strength ranged from 1.0 to 2.2 MPa, which indicate that the rock is very weak. The confined compressive strength is relatively high due to large strain accumulation (average of 20% strain), which resulted in significant strain hardening. At low effective pressures, velocity measurements were highly attenuated, resulting in limited S-wave velocity measurements. Empirical relationships established between the mechanical properties and P-wave velocities were predominantly weak to strong. Moderate and strong linear relationships were established between the unconfined compressive strength and static elastic properties and between the tensile strength and static Poisson's ratio. The results of this study could be carefully applied to velocity data that are readily available from well logs to estimate the mechanical properties of friable rocks.

Interseismic Slip and Coupling along the Haiyuan Fault Zone Constrained by InSAR and GPS Measurements

The Haiyuan fault zone is an important tectonic boundary and strong seismic activity belt in northeastern Tibet, but no major earthquake has occurred in the past ∼100 years, since the Haiyuan M8.5 event in 1920. The current state of strain accumulation and seismic potential along the fault zone have attracted significant attention. In this study, we obtained the interseismic deformation field along the Haiyuan fault zone using Envisat/ASAR data in the period 2003–2010, and inverted fault kinematic parameters including the long-term slip rate, locking degree and slip deficit distribution based on InSAR and GPS individually and jointly. The results show that there is near-surface creep in the Laohushan segment of about 19 km. The locking degree changes significantly along the strike with the western part reaching 17 km and the eastern part of 3–7 km. The long-term slip rate gradually decreases from west 4.7 mm/yr to east 2.0 mm/yr. As such, there is large strain accumulation along the western part of the fault and shallow creep along the Laohushan segment; while in the eastern section, the degree of strain accumulation is low, which suggests the rupture segments of the 1920 earthquake may have been not completely relocked.

The effects of initial static deviatoric stress on liquefaction and pre-failure deformation characteristics of saturated sand under cyclic loading

Initial static deviatoric stress can affect the liquefaction behaviour and deformation characteristics of saturated sand under cyclic loading. A series of cyclic triaxial tests were performed on a saturated sand consolidated under a constant vertical stress but different lateral stresses. The dependence of failure mechanism, liquefaction resistance, and stiffness evolution of the sand on initial static deviatoric stress ratio was investigated on medium dense to dense sand samples (Dr = 0.44 to 0.82). The pre-failure deformation and the direction of strain accumulation were analysed under different static deviatoric stress ratios. The results indicate that under cyclic loading with stress reversal condition, the effective mean stress in the samples could reduce to zero, which leads to cyclic mobility failure and completely loss of stiffness due to liquefaction. The stress state after cyclic loading could be above critical state line. Under non reversal loading condition, however, the effective mean stress cannot reduce to zero and therefore the samples fail under shear due to large strain accumulation. In this condition, the stress state at the end of the cyclic loading is approximately at the critical state line and hardly affected by initial deviatoric stress. The failure resistance of medium dense to dense sand is not greatly affected by initial static deviatoric stress until it is large enough to meet non-reverse loading condition. The ratio of axial stain to excess pore water pressure accumulation (or Δϵacc a/Δϵacc v) increases with the average static deviatoric stress ratio ηav. The strain accumulation direction roughly follows the flow rule of Modified Cam Clay model, independent of relative density and failure mechanism.

An approach to calculating large strain accumulation for discrete element simulations of granular media

SummaryFor research on granular materials, establishing a method to calculate continuum strain from particle displacements is necessary for understanding the material behaviour at macro‐level and developing continuum constitutive models. Existing methods are generally based on constructing a mesh or background grid to calculate strain from particle motions. These methods offer rigorous ways to measure strain for granular materials; however, they suffer from several problems such as mesh distortion and lacking grid‐to‐particle strain mapping procedure, which hinders their capability of calculating strain accumulation during large deformation processes of granular media. To address this issue, this study proposes a new strain calculation method for discrete element simulations of granular materials. This method describes a particle assembly as an equivalent continuum system of material points, each of which corresponds to a particle centre and represents a continuous region with its initial volume/area presumably equal to the volume/area of Voronoi cells generated in accordance with the particle assembly configuration. Smooth Particle Hydrodynamics (SPH) interpolation functions are then employed to calculate strain for these material points. This SPH‐based method does not require any mesh or background grid for computation, leading to advantages in calculating strain accumulation under large deformation. Simulations of granular materials in both uniform and heterogeneous gradations were carried out, and strain results obtained by the proposed method indicate good agreements with analytical and numerical solutions. This demonstrates its potential for strain calculations in discrete element simulations of granular materials involving large deformations and/or large displacements.

Normal and dwell fatigue behavior of a near-alpha titanium alloy - IMI 834

Dwell sensitivity of titanium alloys at ambient temperature (~250 C) is a well-known phenomenon, although the question about the exact micromechanical reasons responsible for this still remains open. In this work, the normal and dwell fatigue response of a near-alpha titanium alloy, IMI 834, is studied. Samples with three different microstructures, namely, fully lamellar, fully equiaxed and bimodal, are evaluated for their dwell fatigue behaviors. A reduction in fatigue life by at least an order of magnitude is seen in all the three microstructures. Large plastic strain accumulation (almost equal to the monotonic ductility) was observed during the dwell fatigue loading condition and this is held responsible for this large debit in fatigue life. The normal fatigue lives decreased in the order, bimodal > fully equiaxed > fully lamellar, while the dwell fatigue lives decreased in the order, fully equiaxed > fully lamellar > bimodal. Bimodal microstructure showed a dwell fatigue debit of 17, while fully lamellar and fully equiaxed showed a debit of 9 and 10, respectively.

Seismic performance of steel beam-to-column moment connections with different structural forms

A novel scheme of steel moment connections shared by flange cover-plate strengthening and web opening weakening is proposed in this study. Four steel beam-to-column moment connections with different structural forms (standard connection, cover-plate reinforced connection, web-opening weakened connection, and equal strong (ES) connection) were tested under cyclic loading. The effects of the local structural forms at the beam end on the failure modes, bearing capacity, strength and stiffness degradation, and ductility and energy dissipation capacity of the connection were analyzed and discussed. The test results indicate that the connections with local structural forms achieve satisfactory ductile behavior by the plastic hinge moving outside. The strength and stiffness degrade gradually owing to a large plastic strain accumulation, and the plastic deformation capacity and energy dissipation capacity improve significantly. Specifically, compared to the standard connection, the displacement ductility coefficient and cumulative energy dissipation of the ES connection increased by 20.0% and 27.9%, respectively, under the premise that maximum strength and initial stiffness remain unchanged, indicating superior seismic performance. Moreover, finite element analysis considering a stress triaxiality dependent fracture criterion was performed, and demonstrated good agreement with the experimental results. Based on the experiment and numerical simulation, the feasibility of the ES connection was verified to provide practical design applications.

Postpeak Deformability Parameters of Localized and Nonlocalized Damage Zones of Rocks under Cyclic Loading

Previous studies on the effects of cyclic loading on the strength and deformability behavior of rocks under uniaxial conditions, i.e., strength damage together with the degradation of rocks’ mechanical properties, have solely taken into account the postpeak stress strain regime. Nevertheless, rock may experience cyclic loading history in the postpeak regime as well. In this study, the effects of cyclic loading on the deformational characteristics of both the nonlocalized damage zone (NLDZ) and the localized damage zone (LDZ) of rock in the postpeak regime are investigated. A series of postpeak damage-controlled tests were carried out on three different rock types, including sandstone, limestone, and granite specimens. Three-dimensional digital image correlation was implemented to measure the strain development in the surface of the specimens. Strain localization was found to be more significant in the lateral direction and less occurrent in the axial direction. In addition, it was observed that the material located in the NLDZ experiences much less damage in contrast to material located in the LDZ. Both large irreversible strain accumulation and Young’s modulus degradation were found to take place progressively in the LDZ, initially occurring at a lower rate and then faster as the number of loading-unloading cycles increased. In this regard, the extent of irreversible deformation and stiffness degradation in the LDZ is considered to be a major contributor to the global loss of strength of the entire specimen.

Macro‑meso scale modeling and simulation of surface roughening: Aluminum alloy tube bending

Surface roughening (SR) is one of the most common defects in deformation-based processing worsening surface quality and service performance of forming components, and in-depth understanding of SR is still a non-trivial and challenging issue for defect free processes since this phenomenon is interactively influenced by so many intrinsic and extrinsic factors. This paper aims to provide a panoramic discussion on the origination and evolution mechanisms of SR considering both intrinsic and extrinsic factors. A macro‑meso scale modeling framework, including Voronoi tessellation based geometrical model and crystal plasticity finite element (CPFE) based constitutive model, is established to explore non-uniform plastic deformation at grain-scale, and then the above framework is numerically implemented and applied into the 5052-O Al-alloy tube bending. The coupling effects of intrinsic factors (i.e., grain size, grain orientation and abnormal large grain) and extrinsic factors (i.e., deformation conditions, strain accumulation and strain rate) on SR are investigated. The results show that SR in plastic deformation stems from the accumulated intergranular non-uniform deformation through thickness direction, indicating that SR is an inherent characteristic and cannot be eradicated. SR is controlled by several factors: grain size, intergranular misorientation, the abnormal large grain, strain accumulation and deformation conditions. In a practical view, these are two effective ways to improve the surface quality by reducing grain size and removing abnormal large grains. From the extrinsic factor, it also makes sense to design the processing parameters reasonably and reduce friction in order to avoid local large deformation.

Earthquakes initiation and thermal shear instability in the Hindu Kush intermediate depth nest

AbstractIntermediate depth earthquakes often occur along subducting lithosphere, but despite their ubiquity the physical mechanism responsible for promoting brittle or brittle‐like failure is not well constrained. Large concentrations of intermediate depth earthquakes have been found to be related to slab break‐off, slab drip, and slab tears. The intermediate depth Hindu Kush nest is one of the most seismically active regions in the world and shows the correlation of a weak region associated with ongoing slab detachment process. Here we study relocated seismicity in the nest to constraint the geometry of the shear zone at the top of the detached slab. The analysis of the rupture process of the Mw 7.5 Afghanistan 2015 earthquake and other several well‐recorded events over the past 25 years shows an initially slow, highly dissipative rupture, followed by a dramatic dynamic frictional stress reduction and corresponding large energy radiation. These properties are typical of thermal driven rupture processes. We infer that thermal shear instabilities are a leading mechanism for the generation of intermediated‐depth earthquakes especially in presence of weak zone subjected to large strain accumulation, due to ongoing detachment process.

Tertiary creep behaviour of 9Cr–1Mo ferritic steel

The paper presents tertiary creep behaviour of thick section 9Cr–1Mo ferritic steel forging at 793 and 873K. The stress dependence of rupture life obeyed power law. The steel exhibited distinct low and high stress regimes characterised by separate values of power law exponent and constant in the creep rate–rupture life relationships of Monkman–Grant type. Extensive tertiary creep in terms of both the large time spent and the large creep strain accumulation during tertiary creep has been observed. The prolonged tertiary creep was associated with the high and distinct values of creep damage tolerance factors in the low and high stress regimes indicating dominance of microstructural degradation in the steel. Separate master creep curves have been obtained for low and high stress regimes in the coupled relationship for creep deformation and damage that depends only on creep damage tolerance factor envisaged in the continuum damage mechanics approach.

Mechanical properties of methacrylate‐based model dentin adhesives: Effect of loading rate and moisture exposure

The aim of this study is to investigate the mechanical behavior of model methacrylate-based dentin adhesives under conditions that simulate the wet oral environment. A series of monotonic and creep experiments were performed on rectangular beam samples of dentin adhesive in three-point bending configuration under different moisture conditions. The monotonic test results show a significant effect of loading rate on the failure strength and the linear limit (yield point) of the stress-strain response. In addition, these tests show that the failure strength is low, and the failure occurs at a smaller deformation when the test is performed under continuously changing moisture conditions. The creep test results show that under constant moisture conditions, the model dentin adhesives can have a viscoelastic response under certain low loading levels. However, when the moisture conditions vary under the same low loading levels, the dentin adhesives have an anomalous creep response accompanied by large secondary creep and high strain accumulation.

Stress relaxation and elastic follow-up using a stress range-dependent constitutive model

Despite the availability of detailed non-linear finite element analysis (FEA), some aspects of high-temperature design can still be best addressed through more simplified methods. One such simplified method relates to the problem of elastic follow-up where, typically in strain-controlled situations, elastic behaviour in one part of a structure can lead to large strain accumulation in another. Over the past 30 years, it has been shown that in regions with significant elastic follow-up, a plot of maximum stress against strain (a ‘stress-strain trajectory’) is virtually independent of the constitutive relation – a characteristic which can be used to estimate elastic follow-up for design purposes without detailed non-linear FEA. The majority of studies which have reported this independence on material behaviour have used simple constitutive models for creep strain, primarily based on power-law creep or variations. Recently, studies of the behaviour of high-temperature structures with a stress range-dependent constitutive law have begun to emerge. This article examines the problem of elastic follow-up using such a constitutive law for a classic two-bar structure and for a more complex structure using FEA. It is found that the independence of the stress–strain trajectory on constitutive equation is lost with a stress range-dependent relation.

Static strain and stress changes in eastern Japan due to the 2011 off the Pacific coast of Tohoku Earthquake, as derived from GPS data

The 2011 M 9.0 Tohoku earthquake induced regional crustal deformation not only in the Japanese Islands but also in north-eastern Asia. Strain release due to mainshock faulting should cause strain redistribution in the overriding plates. The dense GPS network in Japan enables us to calculate co-seismic strain and stress changes from observed data. Strain is a more objective indicator than displacement because no reference frame is required. The co-seismic strain field clearly indicates island-scale strain redistribution. Huge extensional strain changes were concentrated in the southern Iwate and northern Miyagi regions, with a maximum value of 45 × 10−6, which might correspond to approximately 225 to 450 years of strain accumulation. This implies relatively large strain accumulation and release in these regions. Small strain decay was observed in the northernmost NiigataKobe tectonic zone and a possible anomalous Coulomb failure stress change was observed in the Mt. Fuji region. Earthquakes triggered in the above regions might be associated with these anomalies, and/or these non-uniform crustal deformations may reflect crustal heterogeneity.

Effect of Nb Content on Hot Flow Stress, Dynamic Recrystallization and Strain Accumulation Behaviors in Low Carbon Bainitic Steel

Compressive deformation behaviors of low carbon steels with different Nb contents were investigated in the temperature range 900oC to 1100oC and strain rates from 0.05s-1 to 2s-1 by single pass deformation. Multi-pass compressive deformation processes were also carried out to examine strain accumulation under different Nb contents. In single pass deformations, dynamic recrystallization (DRX) can be observed in the case of low strain rate and high temperature, and the higher Nb steel exhibits higher deformation activation energy (Qdef) and critical strain value (εc) for the onset of DRX. However during multi-pass compression process (interval time of 3-4s), the higher Nb steel has larger strain accumulation between passes, so it is easier for high Nb steel that DRX happens during hot strip rolling process, which starts at relative high rolling temperature.

Optimum Pass Design of Bar Rolling for Producing Bulk Ultrafine-Grained Steel by Numerical Simulation

The groove design for creating ultrafine-grained low-carbon steel through a caliber rolling process was studied from the viewpoint of the large strain accumulation and cross-sectional shape variation in a bar. A three-dimensional finite element analysis was employed for this purpose. The caliber rolling process of foval (flat-like-oval)/square type was proposed as a method to efficiently introduce a large strain in material. The relationship among the foval configuration, strain, and cross-sectional shape was examined in the caliber rolling. The influence of the equivalent strain distribution by 1st pass (foval rolling) depends strongly on the strain distribution and a cross-sectional shape by 2nd pass, and the foval configuration to accumulate a large strain efficiently was shown. The optimum pass schedule to fabricate a 13mm square bar of ultrafine-grained steel from a 24 mm square bar by caliber rolling at warm working temperatures was proposed.

1310 数値シミュレーションを活用した超微細粒棒鋼創成における圧延パスの最適化(OS7-3 設計と最適化III-衝撃・加工-,OS7 設計と最適化)

The caliber configuration design for creating ultrafine-grained steel bars through a groove rolling process was studied from the viewpoint of a large strain accumulation and cross-sectional shape variation in a bar. A three-dimensional finite element analysis was employed for this purpose. The square/foval (flat-like-oval)/square rolling was proposed as a method to introduce a large strain efficiently in material. The relation among the foval configuration, a strain and cross-sectional shape was examined in the groove rolling. The influence of the equivalent strain distribution by 1st pass (foval rolling) was considered to clarify the strain distribution and a cross-sectional shape by 2nd pass, and then the foval configuration to accumulate a large strain efficiently was shown. The optimum pass schedule to fabricate an ultrafine-grained steel of 13mm square bar from a 24mm square bar was proposed in warm groove rolling.

実機溝ロール圧延機による超微細粒組織棒鋼の創成

A caliber-rolling process with new type groove characterized by multidirectional deformation was proposed to fabricate ultrafine-grained (UFG) steel bars efficiently. To impose a large strain into center of cross-section, the groove configuration of foval (flat-like-oval) type was designed. Three-dimensional finite element simulation (FE simulation) was utilized for this purpose, and the caliber-rolling process of square/foval pass was investigated from the viewpoints of a large strain accumulation and cross-sectional shape variation in a bar. The UFG steel bars were fabricated by using the commercial caliber-rolling with the grooves designed from FE simulation. The cross-sectional shape predicted from FE simulation was in excellent agreement with the shape of the rolled bars. The strain imposed in the rolled bars has a distribution with maximum at the center. The giant strain of 5.2 is accumulative in center regardless of reduction in area of 79%. The good correlation between the strain imposed and the hardness of the rolled bars was confirmed.