Uniaxial Straining Research Articles

Why Do Grains Remain Roughly Equiaxed during Steady State Discontinuous Dynamic Recrystallization?

The combination of advection and migration of grain boundaries is analyzed on the basis of a simple mesoscale model, where parallelepipedic grains are considered under uniaxial compression straining. Strain hardening and dynamic recovery are described by the classical Yoshie-Laasraoui-Jonas equation. Grain-boundary migration is driven by the difference in dislocation densities between one representative grain and the average over the material. Finally, nucleation is assumed to occur at grain boundaries. Special attention is paid to the aspect ratio, which starts from unity (infinitely small cubic nucleus) and tends to zero when the grain disappears. In spite of the role of migration, the average shape of the grains is determined as a first approximation by their lifetimes.

Investigation of ductile damage in dual phase steel during tensile deformation by in situ X-ray computed tomography

In situ synchrotron radiation X-ray computed tomography (CT) is used to understand the void nucleation and growth behaviour of dual phase (DP) steel, DP800, during uniaxial tensile deformation. The voids were identified post mortem as having nucleated primarily in the (41 vol%) martensite phase. The behaviour of individual voids was found to be broadly in line with a Rice and Tracey style growth model (their volume increasing by a factor of 10), while the mean void size was broadly in line with a model that included the continuous nucleation of new voids, as well as the growth of existing voids, throughout straining. Voids tended to elongate during uniaxial (low stress triaxiality) straining, but then to dilate more isotropically as necking led to increased triaxiality. One large pre-existing void was found to elongate quicky at first, but then for growth to slow, presumably because it was not located in the highest triaxial stress (necked) region. Perhaps surprisingly the average size of the void population grows only slowly (by a factor of 2) during straining; this is because while the size of each void grows, new small voids are always being nucleated. Finally, the changes of stress triaxiality and shear stress state during tensile deformation of the square cross-section smooth tensile specimen are investigated via finite element modelling, to qualitatively assess the impact of sample geometry on void nucleation behaviour and fracture strain in the current study.

Hydrogen-enhanced grain boundary vacancy stockpiling causes transgranular to intergranular fracture transition

The attention to hydrogen embrittlement (HE) has been intensified recently in the light of hydrogen as a carbon-free energy carrier. Despite worldwide research, the multifaceted HE mechanism remains a matter of debate. Here we report an atomistic study of the coupled effect of hydrogen and deformation temperature on the pathway to intergranular fracture of nickel. Uniaxial straining is applied to nickel Σ5(210)[001] and Σ9(1-10)[22-1] grain boundaries with or without pre-charged hydrogen at various temperatures. Without hydrogen, vacancy generation at grain boundary is limited and transgranular fracture mode dominates. When charged, hydrogen as a booster can enhance strain-induced vacancy generation by up to ten times. This leads to the superabundant vacancy stockpiling at the grain boundary, which agglomerates and nucleates intergranular nanovoids eventually causing intergranular fracture. While hydrogen tends to persistently enhance vacancy concentration, temperature plays an intriguing dual role as either an enhancer or an inhibitor for vacancy stockpiling. These results show good agreement with recent positron annihilation spectroscopy experiments. An S-shaped quantitative correlation between the proportion of intergranular fracture and vacancy concentration was for the first time derived, highlighting the existence of a critical vacancy concentration, beyond which fracture mode will be completely intergranular.

In situ synchrotron tensile investigations on ultrasonic additive manufactured (UAM) zirconium

The microstructure evolution during room temperature uniaxial tensile straining of ultrasonic additive manufactured (UAM) zirconium was evaluated using the Advanced Photon Source (APS) facility. Miniature dog-bone tensile specimens of two orientations were cut from a UAM-fabricated zirconium bar for in situ synchrotron tensile tests. Wide-angle X-ray scattering (WAXS) scanning was used to unveil the changes in microstructure of the entire gauge regions throughout the straining. A series of WAXS data analysis methods were utilized to quantify both elastic and plastic deformation mechanisms within the strained specimens. Stress concentrations were identified during early stages of plastic deformation, which become candidate necking positions and eventually lead to failure. Fracture surface analysis implied that these stress concentration locations may be correlated to the fabrication defects, providing insightful guidance for future improvements of the UAM zirconium fabrication process.

Tensile Response of As-Cast CoCrFeNi and CoCrFeMnNi High-Entropy Alloys

In this research, we systematically investigated equiatomic CoCrFeNi and CoCrFeMnNi high-entropy alloys (HEAs). Both of these HEA systems are single-phase, face-centered-cubic (FCC) structures. Specifically, we examined the tensile response in as-cast quaternary CoCrFeNi and quinary CoCrFeMnNi HEAs at room temperature. Compared to CoCrFeNi HEA, the elongation of CoCrFeMnNi HEA was 14% lower, but the yield strength and ultimate tensile strength were increased by 17% and 6%, respectively. The direct real-time evolution of structural defects during uniaxial straining was acquired via in situ neutron-diffraction measurements. The dominant microstructures underlying plastic deformation mechanisms at each deformation stage in as-cast CoCrFeNi and CoCrFeMnNi HEAs were revealed using the Convolutional Multiple Whole Profile (CMWP) software for peak-profile fitting. The possible mechanisms are reported.

Hydrogen-induced transgranular to intergranular fracture transition in bi-crystalline nickel

It is known that hydrogen can influence the dislocation plasticity and fracture mode transition of metallic materials, however, the nanoscale interaction mechanism between hydrogen and grain boundary largely remains illusive. By uniaxial straining of bi-crystalline Ni with a Σ5(210)[001] grain boundary, a transgranular to intergranular fracture transition facilitated by hydrogen is elucidated by atomistic modeling, and a specific hydrogen-controlled plasticity mechanism is revealed. Hydrogen is found to form a local atmosphere in the vicinity of grain boundary, which induces a local stress concentration and inhibits the subsequent stress relaxation at the grain boundary during deformation. It is this local stress concentration that promotes earlier dislocation emission, twinning evolution, and generation of more vacancies that facilitate nanovoiding. The nucleation and growth of nanovoids finally leads to intergranular fracture at the grain boundary, in contrast to the transgranular fracture of hydrogen-free sample.

Improved fracture resistance of Cu/Mo bilayers with thickness tailoring

The fracture toughness of Mo in Cu/Mo bilayers on polyimide was assessed with in situ X-ray diffraction during uniaxial tensile straining. The fracture resistance of Mo acting as an adhesion layer greatly depends on the thickness of the Cu layer exhibiting a toughening effect with increasing Cu layer thickness. In contrast, the presence of the Mo interlayer greatly decreases the apparent KIc of the Cu layers. The quantification of KIc for Mo with a Cu top layer provides further evidence that when brittle layers are used, a thicker ductile layer is advantageous to create fracture resistant stretchable systems.

Investigation of Magnetic Anisotropy and Barkhausen Noise Asymmetry Resulting from Uniaxial Plastic Deformation of Steel S235

This study investigates alterations in magnetic anisotropy and the marked asymmetry in Barkhausen noise (MBN) signals after the uniaxial plastic straining of steel S235 obtained from a shipyard and used as standard structural steel in shipbuilding. It was found that the initial easy axis of magnetisation in the direction of previous rolling, and also in the direction of loading, becomes the hard axis of magnetisation as soon as the plastic strain attains the critical threshold. This behaviour is due to the preferential matrix orientation and the corresponding realignment of the magneto-crystalline anisotropy. Apart from the angular dependence of MBN, the asymmetry in the consecutive MBN bursts at the lower plastic strains is also analysed and explained as a result of magnetic coupling between the grains plastically strained and those unaffected by the tensile test. It was found that, by increasing the degree of plastic strain, the marked asymmetry in MBN tends to vanish. Moreover, the asymmetry in MBN bursts occurs in the direction of uniaxial tension and disappears in the perpendicular direction. Besides the MBN technique, XRD and EBSD techniques were also employed in order to provide a deeper insight into the investigated aspects.

A computationally tractable framework for nonlinear dynamic multiscale modeling of membrane woven fabrics

AbstractA general‐purpose computational homogenization framework is proposed for the nonlinear dynamic analysis of membranes exhibiting complex microscale and/or mesoscale heterogeneity characterized by in‐plane periodicity that cannot be effectively treated by a conventional method, such as woven fabrics. The framework is a generalization of the “finite element squared” (or FE2) method in which a localized portion of the periodic subscale structure is modeled using finite elements. The numerical solution of displacement driven problems involving this model can be adapted to the context of membranes by a variant of the Klinkel–Govindjee method1 originally proposed for using finite strain, three‐dimensional material models in beam and shell elements. This approach relies on numerical enforcement of the plane stress constraint and is enabled by the principle of frame invariance. Computational tractability is achieved by introducing a regression‐based surrogate model informed by a physics‐inspired training regimen in which FE2 is utilized to simulate a variety of numerical experiments including uniaxial, biaxial and shear straining of a material coupon. Several alternative surrogate models are evaluated including an artificial neural network. The framework is demonstrated and validated for a realistic Mars landing application involving supersonic inflation of a parachute canopy made of woven fabric.

Evaluation of Strain Transition Properties between Cast-In Fibre Bragg Gratings and Cast Aluminium during Uniaxial Straining.

Current testing methods are capable of measuring strain near the surface on structural parts, for example by using strain gauges. However, stress peaks often occur within the material and can only be approximated. An alternative strain measurement incorporates fibre-optical strain sensors (Fiber Bragg Gratings, FBG) which are able to determine strains within the material. The principle has already been verified by using embedded FBGs in tensile specimens. The transition area between fibre and aluminium, however, is not yet properly investigated. Therefore, strains in tensile specimens containing FBGs were measured by neutron diffraction in gauge volumes of two different sizes around the Bragg grating. As a result, it is possible to identify and decouple elastic and plastic strains affecting the FBGs and to transfer the findings into a fully descriptive FE-model of the strain transition area.We thus accomplished closing the gap between the external load and internal straining obtained from cast-in FBG and generating valuable information about the mechanisms within the strain transition area.It was found that the porosity within the casting has a significant impact on the stiffness of the tensile specimen, the generation of excess microscopic tensions and thus the formation of permanent plastic strains, which are well recognized by the FBG. The knowledge that FBG as internal strain sensors function just as well as common external strain sensors will now allow for the application of FBG in actual structural parts and measurements under real load conditions. In the future, applications for long-term monitoring of cast parts will also be enabled and are currently under development.

Surrogate Modeling of Viscoplasticity in Steels: Application to Thermal, Irradiation Creep and Transient Loading in HT-9 Cladding

The use of structural metals in extreme environments relies both on the characterization of the mechanical response and microstructure changes in service and on modeling predictions. Data scarcity creates a need for predictive constitutive models that can be used in regimes outside calibration domains. While crystal plasticity models can be applied to non-monotonic loads and complex environments, their computational cost typically prohibits use at the level of an engineering structure. As an alternative, the present study introduces a surrogate constitutive model derived from crystal-plasticity predictions of the mechanical response of HT9 subjected to irradiation, stresses and temperatures. The surrogate law is then tested in the cases of uniaxial straining, stress cycling, thermal cycling and thermal ramping. Finally, using this constitutive relationship, finite element simulations of a pressurized tube subjected to a stress and thermal transients are performed and analyzed.

Atomistic insights into metal hardening.

For millennia, humans have exploited the natural property of metals to get stronger or harden when mechanically deformed. Ultimately rooted in the motion of dislocations, mechanisms of metal hardening have remained in the cross-hairs of physical metallurgists for over a century. Here, we performed atomistic simulations at the limits of supercomputing that are sufficiently large to be statistically representative of macroscopic crystal plasticity yet fully resolved to examine the origins of metal hardening at its most fundamental level of atomic motion. We demonstrate that the notorious staged (inflection) hardening of metals is a direct consequence of crystal rotation under uniaxial straining. At odds with widely divergent and contradictory views in the literature, we observe that basic mechanisms of dislocation behaviour are the same across all stages of metal hardening.

Effective Mechanical Responses of a Class of 2D Chiral Materials

Chiral materials may exhibit negative Poisson's ratio and deformation‐mode coupling phenomena. The finite element numerical method is adopted to analyze a class of 2D chiral and nonchiral materials and to show the effects of microstructural geometry on their effective elastic properties and coupling between tension/compression and bending. With the same area fraction (AF), nonchiral samples show larger effective moduli than chiral ones. The number of unit cells may reduce negativity in effective Poisson's ratio of the chiral samples due to nonuniform lateral deformation under uniaxial straining. Increasing AF in a hierarchical pattern in the chiral samples makes their Poisson's ratio more negative. Bending occurs in the chiral samples when they are under uniform, uniaxial, tensile, or compressive straining due to the coupling of deformation modes. The sensibility of tension–bending coupling may be controlled by the chiral microstructure. Optimization of the coupling sensitivity may help develop novel mechanical sensors.

Modelling Hydrogen Embrittlement using Density Functional Theory: A theoretical approach to understanding environmentally assisted cracking in 7xxx series aluminium alloys

The effects of H segregation to a Σ11 symmetric tilt Al grain boundary are investigated using atomistic simulations, as part of a wider study on cracking in 7xxx series alloys. Density functional theory based simulations of uniaxial straining of grain boundaries containing 11 different concentrations of H were performed under the cohesive zone fracture mechanics framework. The theoretical strength of grain boundaries is shown to be supressed by H segregation, and the cause of this is attributed to the prevention of the formation of Al ligaments across grain boundaries. Segregated concentrations of relevant alloying elements (Zn, Mg, and Cu) show minimal impact on the H embrittlement process investigated, namely H enhanced decohesion (HEDE). Further modelling, of H transport and grain boundary precipitates, is required to confirm the validity of the HEDE mechanism in the case of 7xxx alloys.

Effects of pore morphology and pore edge termination on the mechanical behavior of graphene nanomeshes

We report results of a systematic computational study on the mechanical response of graphene nanomeshes (GNMs) to uniaxial tensile straining based on molecular-dynamics simulations of dynamic deformation tests according to a reliable bond-order interatomic potential. We examine the effects on the GNM mechanical behavior under straining along different directions of the nanomesh pore morphology and pore edge passivation by testing GNMs with elliptical pores of various aspect ratios and different extents of edge passivation through termination with H atoms of under-coordinated edge C atoms. We establish the dependences of the ultimate tensile strength, fracture strain, and toughness of the GNMs on the nanomesh porosity, derive scaling laws for GNM strength-density relations, and find the GNMs' mechanical response to uniaxial straining to be anisotropic for pore morphologies deviating from circular pores. We also find that the GNM tensile strength decays exponentially with increasing GNM porosity and that pore edge termination with H atoms causes a reduction in the GNMs' elastic stiffening, strength, deformability, and toughness; this hydrogen embrittlement effect is more pronounced at a high level of pore edge passivation that renders the edge C atoms sp3-hybridized. The underlying mechanisms of crack initiation and propagation and nanomesh failure for the various types of GNMs examined also are characterized in atomistic detail. Overall, even highly porous GNMs remain particularly strong and deformable and, therefore, constitute very promising 2D mechanical metamaterials.

Numerical Study of Stress Relaxation in Nanostructures in the Course of Uniaxial Straining

Stress relaxation in a nano-sized rod containing structural defects in the course of constant-rate uniaxial straining is studied, and the reasons for the onset of this phenomenon are determined. Under the assumption that structural defects can serve as carriers of irreversible strain of a higher level than dislocations, the problem is solved by the molecular dynamics method. It is found that stress relaxation is accompanied by the transition of the entire system to a steady state with a deeper potential minimum as compared to the system energy before the stress relaxation process, resulting in a temperature increase and reduction of the strain tensor components.

Structure-properties relations in graphene derivatives and metamaterials obtained by atomic-scale modeling

ABSTRACTRecent findings of atomic-scale modelling studies are reviewed on graphene derivatives and metamaterials fabricated through chemical functionalization and/or defect engineering of graphene sheets. Results of molecular-statics and molecular-dynamics simulations according to a reliable bond-order potential, as well as first-principles density functional theory calculations are reviewed that have established useful structure-properties relations in two-dimensional materials, such as graphene nanomeshes (GNMs), electron-irradiated graphene, and interlayer-bonded twisted bilayer graphene. Quantitative relationships are established for the elastic moduli, mechanical properties, and thermal conductivity of GNMs as a function of the nanomesh porosity and the mechanical response of GNMs to uniaxial tensile straining is explored over the range of nanomesh porosities. The dependence of structural, mechanical, and thermal transport properties of electron-irradiated graphene sheets on the density of irradiation-induced defects is reviewed, highlighting an amorphization transition accompanied by a brittle-to-ductile transition and a transition in thermal transport mechanism beyond a critical defect concentration. The tunability of the electronic band structure, mechanical properties, and structural response to mechanical loading of graphene-diamond nanocomposite superstructures consisting of nanodiamond superlattices in interlayer-bonded twisted bilayer graphene also is demonstrated by precise control of the density and distribution of covalent interlayer C–C bonds.

Ion current and action potential alterations in peripheral neurons subject to uniaxial strain.

Peripheral nerves, subject to continuous elongation and compression during everyday movement, contain neuron fibers vital for movement and sensation. At supraphysiological strains resulting from trauma, chronic conditions, aberrant limb positioning, or surgery, conduction blocks occur which may result in chronic or temporary loss of function. Previous in vitro stretch models, mainly focused on traumatic brain injury modelling, have demonstrated altered electrophysiological behavior during localized deformation applied by pipette suction. Our aim was to evaluate the changes in voltage‐activated ion channel function during uniaxial straining of neurons applied by whole‐cell deformation, more physiologically relevant model of peripheral nerve trauma. Here, we quantified experimentally the changes in inwards and outwards ion currents and action potential (AP) firing in dorsal root ganglion‐derived neurons subject to uniaxial strains, using a custom‐built device allowing simultaneous cell deformation and patch clamp recording. Peak inwards sodium currents and rectifying potassium current magnitudes were found to decrease in cells under stretch, channel reversal potentials were found to be left‐shifted, and half‐maximum activation potentials right‐shifted. The threshold for AP firing was increased in stretched cells, although neurons retained the ability to fire induced APs. Overall, these results point to ion channels being damaged directly and immediately by uniaxial strain, affecting cell electrophysiological activity, and can help develop prevention and treatment strategies for peripheral neuropathies caused by mechanical trauma.

Membrane Mechanical Properties Regulate the Effect of Strain on Spontaneous Electrophysiology in Human iPSC-Derived Neurons

Peripheral nerves contain neuron fibers vital for movement and sensation and are subject to continuous elongation and compression during everyday movement. At supraphysiological strains conduction blocks occur, resulting in permanent or temporary loss of function. The mechanisms underpinning these alterations in electrophysiological activity remain unclear; however, there is evidence that both ion channels and network synapses may be affected through cell membrane transmitted strain. The aim of this work was to quantify the changes in spontaneous activity resulting from application of uniaxial strain in a human iPS-derived motor neuron culture model, and to investigate the role of cell membrane mechanical properties during cell straining. Increasing strain in a custom-built cell-stretching device caused a linear decrease in spontaneous activity, and no immediate recovery of activity was observed after strain release. Imaging neuronal membranes with c-Laurdan showed changes to the lipid order in neural membranes during deformation with a decrease in lipid packing. Neural cell membrane stiffness can be modulated by increasing cholesterol content, resulting in reduced stretch-induced decrease of membrane lipid packing and in a reduced decrease in spontaneous activity caused by mechanical strain. Together these results indicate that the mechanism whereby cell injury causes impaired transmission of neural impulses may be governed by the mechanical state of the cell membrane, and contribute to establishing a direct relationship between neural uniaxial straining and loss of spontaneous neural activity.

On the correlation between multiscale mechanisms of deformation in uniaxial dynamic straining and high velocity penetration

On the correlation between multiscale mechanisms of deformation in uniaxial dynamic straining and high velocity penetration