Microscopic Strain Research Articles

Microstructure and tensile behaviors for a medium Mn steel with δ-ferrite phase under different annealing temperatures

In this experiment, we systematically investigated the microstructure evolution, mechanical properties, and deformation behaviors as functions of annealing temperature using cold-rolled Fe-0.05C–6Mn–1Al-1.5Si steel. Almost all annealed specimens are composed of the equiaxed and granular α-ferrite (α) grains due to recrystallization, reversed austenite (γ) grains because of reversed transformation from deformed martensite, and fibrous δ-ferrite (δ) grains only undergoing recovery during intercritical annealing except for 760 °C. It is interesting that several annealing twins are formed within γ grains. The outstanding combination of strength and ductility is obtained for annealing at 740 °C. The tensile strength and total elongation are 980 MPa and 32.1%, which are attributed to the ultrafine grains and optimal TRIP effect associated with rational γ content and stability. A phenomenon that yield point elongation (YPE) gradually shortens with increased γ content and decreased γ stability is observed. What's more, the mechanism that active α′ formation can promote the improvement of work hardening rate and further curtail YPE is verified using deep-cryogenic treatment. Based on SEM observation, microscopic strain of δ grains appears nearly consistent with matrix (α and γ grains) during tensile process, although δ phase presents as fibrous morphology with coarse size.

Microscopic characterisation of local strain field in healing tissue in the central third defect of mouse patellar tendon at early-phase of healing

Tendons exhibit a hierarchical collagen structure, wherein higher-level components, such as collagen fibres and fascicles, are elongated, slid, and rotated during macroscopic stretching. These mechanical behaviours of collagen fibres play important roles in stimulating tenocytes, imposing stretching, compression, and shear deformation. It was hypothesised that a lack of local fibre behaviours in healing tendon tissue may result in a limited application of mechanical stimuli to cells within the tissue, leading to incomplete recovery of tissue structure and functions in regenerated tendons. Therefore, the present study aimed to measure the microscopic strain field in the healing tendon tissue. A central third defect was created in the patellar tendon of mice, and the regenerated tissue in the defect was examined by tensile testing, collagen fibre analysis, and local strain measurement using confocal microscopy at 3 and 6 weeks after surgery. Healing tissue at 3 weeks exhibited a significantly lower strength and disorganised collagen fibre structure compared with the normal tendon. These characteristics at 6 weeks remained significantly different from those of the normal tendon. Moreover, the magnitude of local shear strain in the healing tissue under 4% tissue strain was significantly smaller than that in the normal tendon. Differences in the local strain field may be reflected in the cell nuclear shape and possibly the amount of mechanical stimuli applied to the cells during tendon deformation. Accordingly, restoration of a normal local mechanical environment in the healing tissue may be key to a better healing outcome of tendon injury.

Payne effect of carbon black filled natural rubber nanocomposites: Influences of extraction, crosslinking, and swelling

Rubber nanocomposites experiencing dynamic shears at large strain amplitudes (γ) exhibit the nonlinear Payne effect featured by decays of storage and loss moduli (G′ and G″) or by G′ decay accompanied with G″ overshoot near a critical strain amplitude. The occurrence of the Payne effect has been assigned to damages of “filler network” and rubber-filler interfacial interactions for a long time and to Rouse dynamics of rubber chains recently. To solve the dispute, influences of extraction, crosslinking, and paraffin swelling on the Payne effect of carbon black filled natural rubber nanocomposites are investigated systematically. Master curves of G′ as a function of γ could be always created, and overshoot of G″ in the filled vulcanizates weakens with increasing filler content and intensifies by dilution via paraffin swelling, suggesting that the Payne effect is not mainly rooted in the “filler network” and rubber-filler interfacial interactions. The filler reduces the onset strain amplitude of the Payne effect by amplifying microscopic strain amplitude of the rubber phase, irrespective of whether the matrix is crosslinked or not and whether the crosslinked matrix is swollen or not. Partial removal of bound rubber by compounding the paraffin swollen compounds could lower modulus and eliminate G″ overshoot of the deswollen vulcanizates without influence on the mechanism of G′ decay accompanying Payne effect. The overshoot is found to be closely related to the overall viscous characteristic of the vulcanizates in the linear viscoelastic regime. Provided herein are new insights for recognizing the important roles of the viscoelastic rubber phase on the Payne effect of the nanocomposites.

Piezoresistive bond lines for timber construction monitoring\u2014experimental scale-up

Several laboratory studies and experiments have demonstrated the usability of polymer films filled with electrically conductive filler as piezoresistive material. Applied to adhesives, the glue lines of wood products can achieve multifunctional—thus bonding and piezoresistive/strain sensing—properties. Based on critical load areas in timber constructions, upscaled test setups for simplified load situations were designed, especially with regard to a stress-free electrical contact. In a second step, another upscaling was done to small glulam beams. Based on an experimental test sequence, the piezoresistive reactions as well as the behaviour until failure were analysed. The results show in all cases that a piezoresistive reaction of the multifunctionally bonded specimens was measurable, giving a difference in the extent of relative change. Additionally, measured phenomena like inverse piezoresistive reactions, electrical resistance drift and the absence of a piezoresistive reaction were discussed, based on additional strain analysis by digital image correlation. A model of macroscopic and microscopic strains influencing the piezoresistive reaction of the electrically conductive bond line in wood was used to explain all experimental results. Finally, a first scale-up of piezoresistive bond lines from laboratory samples to glulam beams was possible and successful.

Correlating Prior Austenite Grain Microstructure, Microscale Deformation and Fracture of Ultra-High Strength Martensitic Steels

Herein, we correlate the prior austenite grain (PAG) microstructure to deformation and fracture mechanisms of an ultra-high strength martensitic steel. To this end, a low-carbon martensitic steel is subjected to five heat-treatments and the PAG microstructure in the material is reconstructed from the EBSD inverse pole figure maps of the martensitic microstructure. The deformation and fracture response of all heat-treated materials are characterized by in situ tension tests of dog-bone and single-edge notch specimens that allow us to capture both the macroscopic mechanical response and the evolution of microscopic strains via microscale digital image correlation. The experimental results, together with microstructure-based finite element analysis, are then used to elucidate the effect of the PAG microstructure on the mechanical response of the material. Our results show that the interaction between the heterogeneous deformation fields induced by the notch and the bimodal PAG size distribution leads to an increase in the propensity of shear deformation and degradation in the fracture response of the material with increasing heat-treatment temperature and time. Our results also suggest that achieving a unform distribution of fine grains is an effective way to enhance both the strength and fracture properties of this class of materials.

Microscopic deformation mechanism and main influencing factors of carbon nanotube coated graphene foams under uniaxial compression

Many experiments have shown that carbon nanotube-coated (CNT-coated) graphene foam (CCGF) has specific mechanical properties, which further expand the application of graphene foam materials in many advanced fields. To reveal the microscopic deformation mechanism of CCGF under uniaxial compression and the main factors affecting their mechanical properties, numerical experiments based on the coarse-grained molecular dynamics method are systematically carried out in this paper. It is found that the relative stiffness of CNTs and graphene flakes seriously affects the microscopic deformation mechanism and strain distribution in CCGFs. The bar reinforcing mechanism will dominate the microstructural deformation in CCGFs composed of relatively soft graphene flakes, while the microstructural deformation in those composed of stiff graphene flakes will be dominated by the mechanical locking mechanism. The effects of CNT fraction, distribution of CNTs on graphene flakes, the thickness of graphene flakes, and the adhesion strength between CNTs and graphene flakes on the initial and intermediate moduli of foam materials are further studied in detail. The results of this paper should be helpful for a deep understanding of the mechanical properties of CCGF materials and the optimization design of microstructures in advanced graphene-based composites.

In-situ investigation of strain partitioning and microstructural strain path development up to and beyond necking

Multi-phase metallic materials exhibit significant levels of strain partitioning and localization when plastically deformed. Connecting these microstructural processes to macroscopic limits of uniform deformation, e.g., plastic instability and fracture, can reveal guidelines for damage-resistant microstructure design. This connection, however, is hindered due to the absence of strain partitioning and localization data from high strain levels, near and beyond necking. A parallel gap in our current understanding of the plasticity of multi-phase materials is regarding the spatial variations in strain path development at the micro-scale. Microscopic digital image correlation, the most powerful experimental method to provide such insights, is typically limited to low strain levels (< ~15%) due to crystal-plasticity- or damage-induced surface topography evolution. Here, we address these challenges by developing a practical method that relies on serial in-situ scanning electron microscope (SEM) mechanical tests of samples pre-strained to different levels. Applying this method to a dual-phase steel, we observe that ferrite and martensite on average exhibit a linear strain partitioning trend throughout the deformation. However, the highly-deformed ferrite regions (i.e., the top 4% most strained ferritic subsets) exhibit an exponential increase in the level of strain they accommodate, as well as sequential activation of the strain localization processes in different microscopic strain paths. Martensitic constituents play an important role in the strain localization processes and the resulting microscopic strain paths.

A nonuniform transformation field analysis for composites with strength difference effects in elastoplasticity

Many composites and their constituents exhibit strength difference (SD) effects. To reduce the computational cost associated to twoscale simulations, the theory of nonuniform transformation field analysis (NTFA, originally proposed by Michel and Suquet, 2003) is extended to consider a general case of SD effects in elastoplasticity, containing several well-known plasticity models like von Mises or Drucker and Prager as special cases. A space-time decomposition is done separately for volumetric and deviatoric inelastic strain fields, thus leading to two sets of plastic modes with different charateristics. Localization rules are deduced from superposition principles for the microscopic strain and stress fields, which are then homogenized to obtain the effective response. A coupled model governing the evolution of the resulting reduced variables is proposed based on some dissipative considerations. A convenient matrix formulation for an FE-based implementation is presented in detail. The proposed coupled model is shown to be exact for homogeneous cases and sufficiently accurate for three-dimensional heterogeneous microstructures. Finally, an efficient structural analysis is performed by using corresponding algorithmic consistent tangent operators.

Modulation on the magnetic and electrical properties of Fe3O4 thin films through strain relaxation

A series of Fe3O4 thin films with growth temperature ranging from 400 to 600 °C are prepared by magnetron sputtering on α-Al2O3 (0001). XRD and XPS measurements indicate that all the samples are pure Fe3O4 (111) epitaxial thin films. Microscopic strain is highly sensitive to the growth temperature. As a result, the magnetic and electrical measurements show some interesting phenomena related to the strain relaxation. The saturation magnetization demonstrates a monotonous increase in relative change as large as 85.9% originating from the strain relaxation effect. The improvement of magnetic anisotropy in Fe3O4 thin films is attributed to the reduction of strain-induced anisotropy. However, the resistivity changes suddenly at the growth temperature of 450 °C, which is dominated by mesoscopic grain size rather than microscopic strain.

Effect of Deformation Rate on the Elastic-Plastic Deformation Behavior of GH3625 Alloy

GH3625 alloy is a typical polycrystalline material. The mechanical properties of a crystal within the alloy depend on the single crystal properties, lattice orientation, and orientations of neighboring crystals. However, accurate determination of single crystal properties is critical in developing a quantitative understanding of the micromechanical behavior of GH3625. In this study, the effect of deformation rate on the elastoplastic deformation behavior of GH3625 was investigated using in situ neutron diffraction room-temperature compression experiments, EBSD, and TEM. The results showed that the microscopic stress–strain curve included elastic deformation (applied stress, σ ≤ 300 MPa), elastoplastic transition (300 MPa 350 MPa) stages, which agreed with the mesoscopic lattice strain behavior. Meanwhile, the deformation rate was closely related to the crystal elastic and plastic anisotropy. The results of the lattice strain, peak width, and intensity reflected by the specific hkl showed that the deformation rate had little effect on the elastic anisotropy of the crystal, but had a significant effect on the plastic anisotropy of the crystal. With the increase in the deformation rate, the high angle grain boundaries gradually changed to the low angle grain boundaries, and the proportion of twin boundaries gradually reduced. Also, the grains transformed from uniform deformation to nonuniform deformation. Moreover, with the increase in deformation rate, the total dislocation density (ρ) of the alloy first decreased and then increased, whereas the geometrically necessary dislocation density (ρGND) monotonically increased, and the statistically stored dislocation (SSD) density (ρSSD) monotonically decreased. Meanwhile, the abnormal work hardening behavior of the sample at a deformation rate of 0.2 mm/min was mainly related to the SSD generated by uniform deformation. Additionally, the contribution of dislocation strengthening and TEM observation confirmed that the dominant deformation of GH3625 was dislocation slip, and its work hardening mechanism was dislocation strengthening.

Unraveling the Charge Transport Mechanism in Mechanochemically Processed Hybrid Perovskite Solar Cell.

The long-term operation of organic-inorganic hybrid perovskite solar cells is hampered by the microscopic strain introduced by the multiple thermal cycles during the synthesis of the material via a solution process route. This setback can be eliminated by a room temperature synthesis scheme. In this work, a mechanochemical synthesis technique at room temperature is employed to process CH3NH3PbI2Br films for fabricating perovskite solar cell devices. The solar cell device has produced a 957 mV Voc, a 16.92 mA/cm2 short circuit current density, and a 10.5% efficiency. These values are higher than the published values on mechanochemically synthesized CH3NH3PbI3. The charge transport properties of the devices are studied using DC conductivity and AC impedance spectroscopy, which show a multichannel transport mechanism having both ionic and electronic contributions. A much smaller defect density in the mechanochemically synthesized hybrid perovskite material is confirmed. A polarization assisted recombination mechanism is observed to have a dominant effect on the overall charge transport mechanism. However, no obvious grain boundary and intralayer lattice defect related responses are found in the perovskite layer. Interfacial charge transport and recombination are found to show major effects on both the temperature dependent and illumination dependent impedance spectra.

Multiscale numerical analyses of arterial tissue with embedded elements in the finite strain regime

Multiscale models based on representative volume elements (RVEs) might help unraveling the ways in which macroscopic loadings affect the microstructure of tissues reinforced by collagen fibers, and vice versa. Tissues such as arteries, however, are characterized by a significant collagen dispersion. Therefore, many fibers have to be included in the RVE to achieve a representative geometrical model of the microstructure. For this reason, when the finite element method is employed in the numerical homogenization, fibers are commonly modeled as 1D elements, either by considering a network of trusses or the embedded elements technique. With regard to the latter, there has been little attention in previous works concerning the influence of the chosen multiscale boundary conditions and RVE size. In order to address this issue, the present work combines a sound multiscale framework with the classical embedded elements technique to simulate four increasingly larger RVEs, which resemble the microstructure of the medial layer of the arterial wall. Each RVE is modeled as a ground substance with embedded collagen fibers and subjected to a macroscopic isochoric equibiaxial stretch up to 10%, according to four classical multiscale boundary conditions: Taylor-Voigt, linear boundary displacements, periodic boundary fluctuations and minimally constrained model. Results are evaluated both at the macroscopic level (homogenized response) and the microscopic level (strains in the ground substance and fiber stretches). At the macroscopic level, the homogenized response for the periodic boundary condition seems to converge faster than the other three with increasing RVE size. At the microscopic level, the periodic model is also less prone to concentrated effects at the boundaries of the RVE. Therefore, among the four classical multiscale boundary conditions, the periodic model seems to be better suited to simulate the microstructure of fibrous tissues employing the embedded elements technique. Importantly, the resulting microscopic strain fields are characterized by a considerable degree of inhomogeneity and some values are significantly larger than the macroscopic (imposed) strain. That could help to shed more light on relevant mechanotransduction mechanisms, e.g., cell signaling, that are known to happen in the arterial media and are linked to biological processes such as growth and remodeling. Therefore, the framework herein proposed may serve as a valuable tool for the investigation of microstructural phenomena that happen in arteries, or even other fibrous tissues.

Effects of random laminate misalignment on macroscopic and microscopic elastic/viscoplastic behaviors of ultrafine plate–fin structures

In this study, the effects of random laminate misalignment on the macroscopic and microscopic elastic/viscoplastic behaviors of ultrafine plate–fin structures are statistically investigated adopting Monte Carlo simulation. To this end, a multiscale method that analyzes the elastic–viscoplastic behavior of ultrafine plate–fin structures with random laminate misalignment both macroscopically and microscopically is proposed based on the homogenization theory for nonlinear time-dependent materials combined with the substructure method. Using the method, Monte Carlo simulations for elastic and elastic–viscoplastic behaviors of ultrafine plate–fin structures subjected to uniaxial tension or simple shear are performed for many cases of random laminate misalignment. The obtained results reveal that the random laminate misalignment affects the microscopic behavior of ultrafine plate–fin structures more than the macroscopic behavior; microscopic stress and viscoplastic strain exhibit considerable variability depending on the laminate misalignment, and their maximum values can become about 30–50% higher than those in the case without laminate misalignment. This suggests the importance of considering random laminate misalignment in the design and strength/life prediction of ultrafine plate–fin structures.

Enhanced strength and ductility AZ91 alloy with heterogeneous lamella structure prepared by pre-aging and low-temperature extrusion

In this study, AZ91 alloy with heterogeneous structure comprised coarse, fine grained layers and the heterogeneous precipitates was obtained via pre-aging and low-temperature extrusion. The aging prior to extrusion (APE) sample with heterogeneous lamella structure presented an optimum combination of ultimate tensile strength (405.3 MPa), yield strength (216.5 MPa) and elongation (18.5%), which are much superior than extrusion (E) and extrusion prior to aging (EPA) samples with homogeneous structure. The observed excellent mechanical properties stemmed from the synergy effect of more stable precipitations at low temperature extrusion, grain refinement and texture. Meanwhile, heterogeneous deformation-induced (HDI) strengthening and work hardening also play a significant role on improving mechanical properties. The macroscopic and microscopic strain distributions demonstrate that APE sample is conducive to strain transfer and alleviate local stress concentration during collaborative deformation of coarse and fine grained layers, resulting in the improvement of strength and ductility. The research of this work will enrich the preparation methods of heterogeneous materials with intense precipitation strengthening potential.

Review on Strain Localization Phenomena Studied by High‐Resolution Digital Image Correlation

Herein, a comprehensive review on the application of digital image correlation (DIC) for investigations of strain localization phenomena in the field of materials science and engineering (MSE) is provided. The Review is divided into three parts. In Part 1, the method of DIC is reviewed in terms of basic principles, correlation algorithms, and sources of errors. Part 2 is focused on the application of DIC in the field of MSE. This part is subdivided into the application of DIC in 1) combination with optical microscopy and 2) in combination with scanning electron microscopy (SEM). In Part 3, results of high‐resolution DIC obtained in combination with SEM are presented for high‐strength austenitic stainless steels—transformation‐induced plasticity steels and twinning‐induced plasticity steels—exhibiting microscopic strain localization phenomena. These investigations are supplemented by electron backscattered diffraction for interpretation of strain fields. Although DIC allows for both 2D and 3D calculations of displacement and strain fields, herein, 2D digital correlation is referred to only, as in particular with SEM only in‐plane displacements are calculated so far.

Evolution of microscopic strains, stresses, and dislocation density during in-situ tensile loading of additively manufactured AlSi10Mg alloy

The AlSi10Mg alloy produced by laser powder bed fusion (LPBF) possesses a novel microstructure and higher mechanical properties compared with its casting counterpart. So far, the crystallographic orientation-dependent lattice strains, average phase stresses, and dislocation density during the tensile loading of the LPBF AlSi10Mg are not well understood. This fact is impeding further optimization of microstructure and mechanical properties. High energy synchrotron X-ray diffraction providing deep penetration capability and phase-specific measurements of various bulk properties of crystal materials is applied to investigate the LPBF AlSi10Mg under loading. The crystallographic orientation-dependent lattice strains and elastoplastic properties of the Al matrix are assessed. The average phase stresses are calculated to quantify load partitioning between the Al and Si phases. The nano-sized Si particles that bear high-stress are efficient strengthening particles. The maximum value of the average phase stress of Si reaches up to ~2 GPa. Based on the modified Williamson-Hall and the modified Warren-Averbach methods, the dislocation density and its evolution during the plastic deformation are determined. A multistage strain hardening behavior is detected in the Al matrix, which is associated with the interactions between the dislocations and the cell boundary network.

Effect of germanium auto-diffusion on the bond lengths of Ga and P atoms in GaP/Ge(111) investigated by using X-ray absorption spectroscopy.

The germanium auto-diffusion effects on the inter-atomic distance between the nearest neighbors of the Ga atom in GaP epilayers are investigated using high-resolution X-ray diffraction (HRXRD) and X-ray absorption spectroscopy. The GaP layers grown on Ge (111) are structurally coherent and relaxed but they show the presence of residual strain which is attributed to the auto-diffusion of Ge from the results of secondary ion mass spectrometry and electrochemical capacitance voltage measurements. Subsequently, the inter-atomic distances between the nearest neighbors of Ga atom in GaP are determined from X-ray absorption fine-structure spectra performed at the Ga K-edge. The estimated local bond lengths of Ga with its first and second nearest neighbors show asymmetric variation for the in-plane and out-of-plane direction of GaP/Ge(111). The magnitude and direction of in-plane and out-of-plane microscopic residual strain present in the GaP/Ge are calculated from the difference in bond lengths which explains the presence of macroscopic residual tensile strain estimated from HRXRD. Modified nearest neighbor configurations of Ga in the auto-diffused GaP epilayer are proposed for new possibilities within the GaP/Ge hetero-structure, such as the conversion from indirect to direct band structures and engineering the tensile strain quantum dot structures on (111) surfaces.

Strain evolution of L-shaped precast columns spliced by laminated metal plates, part I

ABSTRACT Mechanical joints for both irregular steel-concrete composite precast frames and reinforced concrete precast frames were developed to replace conventional cast-in-place concretes in the previous study of the authors. Novel connections modified from conventional steel joints having fully restrained moment connections offer rapid and facile erections. This study aims to numerically identify microscopic strain evolution and failure modes of the laminated metal plates splicing L-shaped precast composite columns. Numerical parameters including material properties of structural components of mechanical joint were implemented to determine damages and strength degradations with associated fractures. Comprehensive numerical results agreed well with test data, which explored structural behavior and contribution of each structural element of mechanical joint to flexural capacity. Strain evolution showed that degradation of the mechanical joints was retarded by interior bolts to a level similar to monolithic columns, resulting in contribution of mechanical joint to flexural capacity of columns. Influence of axial loads on the behavior of columns with mechanical joints was also evaluated to provide design recommendations.

Microscopic full field strain measurement of unidirectionally fiber reinforced plastics with the Kriging-digital image correlation and region splitting method

Microscopic strain measurements of unidirectional glass fiber reinforced plastic under transverse tensile loading were conducted. The microscopic full field strain measurement incorporated with digital image correlation (DIC) accompanied with an empirical semivariogram-based Kriging method. Since there are many factors causing inaccurate measurement results, a noise-resistant and region splitting approach was employed to improve the accuracy and validity of the strain measurement of composite material at the micro-level. Specimens with a few unidirectional fibers were fabricated, and the microscopic strain fields under tensile load were investigated using K-DIC. A detailed finite element model based on the specimen geometry was generated and its strain fields were numerically analyzed. The experimental and numerical results obtained by a standard approach like the moving least square method-based method were compared to discuss both the accuracy and effectiveness of the proposed approach.

Measuring coarse grain deformation by digital image correlation

AbstractThis work presents results from oedometric compression of coarse granular material. Coarse granular media exhibit significant deformations making it complicated to predict the settlement of structures. In this paper, a measurement technique was developed for the analysis of two‐dimensional (2‐D) images of a deforming coarse granular medium to investigate its deformation. This was achieved by realising grain‐based image correlation to measure the grain transformation in gravel with the use of a digital image correlation technique. The 2‐D displacement fields enable us to explore the behaviour of granular media at different scales: microscopic, mesoscopic and macroscopic scales. The mesoscopic scale is defined from branches that connect the centres of three neighbouring grains, using a Delaunay triangulation to account for an equivalent continuum media. Whereas the consistency of the macroscopic strain and the average mesoscopic strain is assessed, it is shown that a deviation from the normalised microscopic vertical displacement is an indicator of the heterogeneity of the mesoscopic strain field. The proposed mesoscopic analysis allows us to investigate these heterogeneities. Another important result is that even if the amplitude of the microscopic strain is small (approximately 100 times smaller) compared with the other strain measures, it confirms that the grains are not rigid and that their ultimate strain can be estimated using the proposed approach.