Micromechanical Behavior Research Articles

A Multiscale Finite Element Analysis of Balloon Kyphoplasty to Investigate the Risk of Bone-Cement Separation In Vivo.

During the past decade there has been a significant increase in the number of vertebral fractures being treated with the balloon kyphoplasty procedure. Although previous investigations have found kyphoplasty to be an effective treatment for reducing patient pain and lowering cement-leakage risk, there have been reports of vertebral recollapse following the procedure. These reports have indicated evidence of in vivo bone-cement separation leading to collapse of the treated vertebra. The following study documents a multiscale analysis capable of evaluating the risk of bone-cement interface separation during lying, standing, and walking activities following balloon kyphoplasty. Results from the analysis found that instances of reduced cement interlock could initiate both tensile and shear separation of the interface region at up to 7 times the failure threshold during walking or up to 1.9 times the threshold during some cases for standing. Lying prone offered the best protection from interface failure in all cases, with a minimum safety factor of 2.95. The results of the multiscale analysis show it is essential for kyphoplasty simulations to take account of the micromechanical behavior of the bone-cement interface to be truly representative of the in vivo situation after the treatment. The results further illustrate the importance of ensuring adequate cement infiltration into the compacted bone periphery during kyphoplasty through a combination of new techniques, tools, and biomaterials in a multifaceted approach to solve this complex challenge.

Inhibiting effect of superheated steam processing on milling characteristics deterioration induced by storage of common buckwheat

Superheated steam (SS) was used to improve the milling characteristics of common buckwheat grains during storage. The changes in milling characteristics, pasting properties, micro-mechanical behavior and protein-starch interactions of buckwheat during storage were investigated. Results showed that the average particle size, color and pasting viscosities of untreated buckwheat decreased during storage, while the damaged starch content increased significantly. SS slowed down these deteriorations of milling characteristics by alleviating intracellular cleavage of starch during the process of buckwheat milling. SS also kept pasting properties of starch stable during the continuous heating and shearing process. X-ray diffraction showed less variation in crystallinity and stable structure of starch induced by SS. The results from SDS-PAGE and confocal laser scanning microscopy showed that these stabilizations were associated with the disulfide bonds of protein and its interaction with starch granules. It meant SS processing retarded the quality deterioration during storage and protected buckwheat starch against mechanical damage by promoting protein aggregation and letting the starch to be wrapped tightly by the protein, so as to protect the swollen starch granules from disintegration. Therefore, SS processing was potentially applicable for improving milling characteristics of buckwheat.

In Situ SEM Study of the Micro-Mechanical Behaviour of 3D-Printed Aluminium Alloy

Currently, 3D-printed aluminium alloy fabrications made by selective laser melting (SLM) offer a promising route for the production of small series of custom-designed support brackets and heat exchangers with complex geometry and shape and miniature size. Alloy composition and printing parameters need to be optimised to mitigate fabrication defects (pores and microcracks) and enhance the parts’ performance. The deformation response needs to be studied with adequate characterisation techniques at relevant dimensional scale, capturing the peculiarities of micro-mechanical behaviour relevant to the particular article and specimen dimensions. Purposefully designed Al-Si-Mg 3D-printable RS-333 alloy was investigated with a number of microscopy techniques, including in situ mechanical testing with a Deben Microtest 1-kN stage integrated and synchronised with Tescan Vega3 SEM to acquire high-resolution image datasets for digital image correlation (DIC) analysis. Dog bone specimens were 3D-printed in different orientations of gauge zone cross-section with respect to the fast laser beam scanning and growth directions. This corresponded to the varying local conditions of metal solidification and cooling. Specimens showed variation in mechanical properties, namely Young’s modulus (65–78 GPa), yield stress (80–150 MPa), ultimate tensile strength (115–225 MPa) and elongation at break (0.75–1.4%). Furthermore, the failure localisation and character were altered with the change in gauge cross-section orientation. DIC analysis allowed correct strain evaluation that overcame the load frame compliance effect and helped to identify the unevenness of deformation distribution (plasticity waves), which ultimately resulted in exceptionally high strain localisation near the ultimate failure crack position.

Nanoindentation and electron backscatter diffraction mapping in laser powder bed fusion of stainless steel 316L

This study on laser powder bed fusion (L-PBF) of AISI 316 L addresses the anisotropy in microstructure and micro-mechanical properties, from the observation of electron backscatter diffraction (EBSD) and nanoindentation maps in parts made with different volumetric energy densities (VED) and in grains at different crystallographic orientations. Similar experimentation was done with conventionally made AISI 316 L, with the aim of comparing anisotropy and micro-mechanical response. It was observed that the variations of VED, calculated from different process parameters combinations, have an influence on crystallographic oriented grain growth evolution affecting the nanoindentation performance. This progression was responsible of the variation in grain size and average misorientation angles and played important role in determining the micro-mechanical behavior. The highest nanoindentation response was obtained at a VED = 56.67 J/mm3, with an average indentation modulus of 196.77 GPa and hardness of 3.68 GPa. From Texture Pole Figures (PFs) and Inverse Pole Figures (IPFs) of EBSD maps, it was revealed that the highest texture strength of 〈101〉 oriented grains along with the evolution and domain of 〈111〉 oriented grains enhanced the micro-mechanical properties at that VED value. The other causes of this behavior are the relatively lower grain size (11.62 μm) and a higher average misorientation angle (11.74°), when comparing it with rest of the L-PBF AISI 316 L parts. The additional but undesirable strong 〈001〉 texture somehow contributed to the slight mitigation on the micro-mechanical properties in conventional AISI 316 L stainless steels and these parts were found to be comparatively less anisotropic than the parts obtained from L-PBF process.

Design and manufacture of a micro-tensile testing machine for in situ optical observation and DIC analysis: application to 3D-printed and compression-molded ABS

In this paper, a novel micro-tensile testing apparatus is designed and built to characterize the micromechanical behavior of materials. Furthermore, a special geometry for micro specimens is introduced that could facilitate problems related to the handling, gripping, and aligning such specimens. The device can be integrated with an optical microscope to observe the microstructural evolution taking place during deformation. Using the developed system, the micromechanical response of additively-printed and compression-molded ABS was determined. 3D-printed specimens demonstrated higher mechanical properties over the compression-molded specimens. Digital image correlation (DIC) technique was also applied to the optical images taken during deformation to measure the strain field in a micron-sized area of interest. DIC results revealed a more intense strain localization for the 3D-printed specimen compared to the monolithic sample. Comparing stress–strain curves and DIC results indicate that the apparatus and technique were quite successful in the in situ characterization of mechanical response.

Quantifying the effects of elongation and flatness on the shear behavior of realistic 3D rock aggregates based on DEM modeling

It is significant for industrial production and engineering practice to study the macro and micromechanical behaviors of realistic particles in nature. Based on the rock aggregates database obtained by 3D scanning, this study investigated the effect of particle shape on the shear behaviors of particles by discrete element method (DEM) modeling. First, 1200 rock particle models were acquired by white-light scanning, and the elongation index (EI) and flatness index (FI) of the 1200 particles were calculated. After initial dense samples were created for particles with specific EI and FI values by the isotropic compression method, all the samples were sheared in drained triaxial compression tests under a quasi-static condition. Then, the mechanical behaviors of the samples at the peak and critical states were analyzed. Meanwhile, the evolution of internal mechanical behaviors during the shearing of samples with different EI and FI values was evaluated. Finally, through the analysis of the stress-force-fabric relationship, the underlying mechanism explaining why the macroscale mechanical behaviors of samples were dominated by particle shape was revealed from the perspective of fabric anisotropy.

Evolution of rapidly grown cellular microstructure during heat treatment of TiAl-based intermetallic and its effect on micromechanical properties

Continuous growth of rapidly grown cellular microstructure (i.e., cellular microstructure obtained by rapid solidification) could be realized by the method of laser powder bed fusion. However, there is still lack of research about how to adopt post heat treatment further optimizing this type of homogeneous and fine microstructure. Herein, the phase and microstructure evolution of rapidly grown cellular microstructure of TiAl-based intermetallic during heat treatment was specifically investigated; besides, both the micromechanical properties and behavior of the heat-treated microstructures were analyzed. The phase and microstructure evolution of Ti–48Al–2Cr–2Nb (in at.% unless otherwise specified) rapidly grown cellular microstructure during heating at 694–1180 °C was: the α2 rapidly grown cellular microstructure transformed into cellular microstructure with α2/γ nanolamellae substructures at 694 °C; at 900 °C, almost all of the rapidly grown cellular microstructure transformed into γ phase, and began to transform into equiaxed microstructures by recrystallization; when the temperature increased to 936 °C, α2 phase began to be precipitated in or between the equiaxed γ grains; after being held at 1000 °C for 30 min, the rapidly grown cellular microstructure completely decomposed into equiaxed near-γ microstructures. The nanohardness of the heat-treated microstructures first decreased then increased with increasing heat-treatment temperature, the maximum nanohardness (8.697 GPa on average) was obtained after heat treatment at 700 °C. The cellular microstructure with nanolamellae substructures can further improve the strength and toughness of the rapidly grown cellular microstructure, effectively realizing the strengthening and toughening of TiAl-based alloy.

Scale and surface morphology effects on the micromechanical contact behavior of granular materials

Scale effects and the influence of surface roughness were investigated on the contact behavior of glass specimens of the grain-grain, grain-block and block-block configurations, where the block corresponds to the limiting case of infinite contact radius. The tests were carried out using in-house micromechanical apparatuses and the results showed that both the normal and tangential stiffness increased with increasing grain diameter but also decreased with increasing equivalent surface (RMS) roughness. The interface friction increased with increasing equivalent surface roughness but did not depend heavily on grain size. Rough interfaces had extended slip displacements, thus slower degradation of stiffness.

Ultra High-Temperature Deformation in a Single Crystal Superalloy: Meso-Scale Process Simulation and Micro-Mechanisms

A mesoscale study of a single crystal nickel-base super-alloy subjected to an industrially relevant process simulation has revealed the complex interplay between micro-structural development and the micro-mechanical behavior. As sample gauge volumes were smaller than the length scale of the highly cored structure of the parent material from which they were produced, their subtle composition differences gave rise to differing work hardening rates, influenced by varying secondary dendrite arm spacings, γ′ phase solvus temperatures and a topologically inverted γ/γ′ micro-structure. The γ′ precipitates possessed a characteristic ‘X’ morphology, resulting from the simultaneously active solute transport mechanisms of thermally favored octodendritic growth and N-type rafting, indicating creep-type mechanisms were prevalent. high resolution-electron back-scatter diffraction (HR-EBSD) characterization reveals deformation patterning that follows the γ/γ′ micro-structure, with high geometrically necessary dislocation density fields localized to the γ/γ′ interfaces; Orowan looping is evidently the mechanism that mediated plasticity. Examination of the residual elastic stresses indicated the 'X' γ' precipitate morphology had significantly enhanced the deformation heterogeneity, resulting in stress states within the γ channels that favor slip, and that encourage further growth of γ' precipitate protrusions. The combination of such localized plasticity and residual stresses are considered to be critical in the formation of the re-crystallization defect in subsequent post-casting homogenization heat treatments.

Influence of partitioning parameters on the mechanical stability of austenite in a Q&P steel: A comparative in-situ study

The transformation-induced plasticity (TRIP)-effect is an efficient way to increase the formability in high performance steels. Hence, an optimal stability of the retained austenite is crucial to benefit the most from this effect. In the present work, in-situ high energy X-ray diffraction was used to study the austenite to martensite transformation upon uniaxial tensile loading of a TRIP-assisted steel produced by the quenching and partitioning (Q&P) process. A detailed analysis of the diffraction patterns recorded during deformation allowed to study the austenite stability with respect to the applied partitioning conditions. The austenite stability was found to strongly depend on the applied heat treatment, and could be mainly attributed to the carbon content and to the tempering degree of the surrounding martensitic matrix. Partitioning at 260 °C resulted in a poor austenite stability, while the austenite was almost too stable after partitioning at 360 °C. The optimal combination of strength and ductility was found for partitioning at 400 °C.The micromechanical behavior was analyzed by the evolution of individual lattice strains and the change of full width at half maximum (FWHM). Yielding of austenite could be clearly identified by an increase of FWHM. Martensite showed an unexpected peak narrowing upon yielding. In the case of 2-step Q&P, austenite started to yield after martensite, while yielding occurred almost simultaneously in the case of 1-step Q&P.

Micromechanical Behaviors and Plastic Deformation Mechanisms of Ceramics

Micromechanical Behaviors and Plastic Deformation Mechanisms of Ceramics

Numerical and experimental evaluation of the energetic release during static tensile tests on short fiber reinforced composite material

In this work, a simple numerical model is presented in order to evaluate the energetic release of short fiber composite PA66GF35 under static tensile tests. The micro-mechanical behavior of the fiber-matrix system was taken into account, as well as the fiber distribution and orientation. Numerical simulation at macro scale level were carried out to predict the temperature evolution of the material under monotonic load. Experimental validation was performed on dog-bone specimens monitoring the surface temperature with an infrared camera. A deviation from the linearity of the thermoelastic effect has been noticed for a stress level below the yielding strength of the material. The analysis of the temperature trend evolution may be a useful aid in order to identify damage initiation within the material that would lead to failure, especially under fatigue load, even adopting a simple and rapid static tensile test.

Deformation mechanism and in-situ TEM compression behavior of TB8 β titanium alloy with gradient structure

Severe plastic deformation is known to induce grain refinement and gradient structure on metals’ surfaces and improve their mechanical properties. However, the fundamental mechanisms behind the grain refinement and micromechanical properties of materials subjected to severe plastic deformation are not still well studied. Here, ultrasonic surface rolling process (USRP) was used to create a gradient microstructure, consisting of amorphous, equiaxed nano-grained, nano-laminated, ultrafine laminated and ultrafine grained structure on the surface of TB8 β titanium alloy. High energy and strain drove element co-segregation on sample surface leading to an amorphous structure during USRP processing. In situ transmission electron microscope compression tests were performed in the submicron sized pillar extracted from gradient structure and coarse grain, in order to reveal the micromechanics behavior of different grain morphologies. The ultrafine grained layer exhibited the lowest yield stress in comparison with single crystal and amorphous-nanocrystalline layers; the ultrafine grained layer and single crystal had an excellent strain hardening rate. The discrepancy among the grain sizes and activated dislocation sources led to the above mentioned different properties. Dislocation activities were observed in both compression test and microstructure evolution of USRP-treated TB8 alloy. An evolution of dislocation tangles and dislocation walls into low angle grain boundaries and subsequent high angle grain boundaries caused the grain refinement, where twinning could not be found and no phase transformation occurred.

Finite element and experimental structural analysis of endodontic rotary file made of Cu-based single crystal SMA considering a micromechanical behavior model

Shape Memory Alloys (SMAs) are widely used in endodontics as instruments for root canal preparation namely endodontic rotary files. Despite their high performances, compared to stainless steel instruments, it is still possible to enhance their cutting efficiency by acting on their shape and material properties. As an example, increasing the exhibited reversible martensitic transformation strain makes them more flexible without decreasing their mechanical strength. It becomes possible with single crystal SMAs. Cu-based (Cu-Al-Be, Cu-Zn-Al, Cu-Al-Mn) single crystal SMAs start emerging and can reach about 12% of martensitic transformation strain, in addition to their interesting antimicrobial properties. This study investigates, both numerically and experimentally, the development of endodontic files made of Cu-based single crystal SMAs. The numerical analysis is carried out by the finite element method. The geometry of the instrument is parameterized. Depending on the applied boundary conditions, bending, torsion, or combined bending-torsion loadings are represented. A micromechanical constitutive law is implemented in Abaqus via the UMAT subroutine to describe the thermomechanical behavior of the Cu-based single crystal SMA. Following the thus obtained numerical results, Coltene-Micromega company has manufactured endodontic file prototypes made of Cu-Al-Be single crystal SMA. A specific setup applying the same boundary conditions of torsion-bending loading is used to characterize these SMA file prototypes. The good agreement between experimental and numerical responses, for a combined bending-torsion loading, proves the relevance of the proposed approach and the pertinence of considering Cu-based SMAs for endodontic file applications.

Exploring micromechanical behaviors of soft rock joints through physical and DEM modeling

The joint surface of a rock mass is one of the most important factors that strongly affect its shear strength, which is critical in rock engineering. Specifically, the joint roughness coefficient (JRC) is a parameter that represents the profile characteristic of the joint. In the authors’ earlier investigation, any joint profile with a given JRC might be generated randomly for engineering purposes. In this study, a series of direct shear tests in both laboratory and numerical modeling (through discrete element method (DEM)) was conducted for soft rock joints to elucidate the mechanical properties of a randomly-generated JRC profile. A 2D-DEM model was adopted to simulate the joint specimen under the same direct shear conditions as in the laboratory tests. A reasonable agreement is found between the experimental direct shear tests performed on an artificial gypsum plaster model and the numerical modeling that was carried out, showing that the numerical model can be used in the interpretation of the direct shear tests of joint surfaces. Besides, the peak shear strength of the gypsum model also compares well with that predicted by Barton’s equation. Based on the lab test results and numerical simulation, the failure mechanism of the joint specimen is correlated with the normal stress applied. From a microscopic viewpoint, the distribution of contact forces is most concentrated at the early stage during shearing, especially at the time of the peak shear stress. The distribution of shear stresses along the shear plane is not uniform, depending on the degree of joint undulation. The peak shear strength of the soft rock joints mostly comes from the roughness along the joint surface. However, the residual strength is mobilized from reduced roughness and the shearing-off of the joints.

Micromechanical Simulation Approach of Dual Phase Steel Artificial Microstructure Using Random Field Model

In this study, a random field (RF) model with a Gaussian kernel was applied to generate an artificial microstructure of dual phase (DP) steels. Micrographs obtained from Scanning Electron Microscopy (SEM) were analyzed using image processing software to extract the grain size and the volume fraction of each phase. Based on watershed (Ws) segmentation and quantitative analysis, the real and artificial microstructures were compared by analyzing grain features related the solidity, grain size and aspect ratio (the proportional relationship between its width and its height). Consequently, this approach allows to simulate the overall stress-strain behavior of the analyzed microstructures. As a result, it was shown that the strain localization starts to develop at the ferrite/martensite interface and that the RF model could replicate the micromechanical behavior of DP steels.

Micromechanical evaluation of cortical bone using in situ XCT indentation and digital volume correlation

The overall mechanical behaviour of cortical bone is strongly dependant on its microstructure. X-ray computed tomography (XCT) has been widely used to identify the microstructural morphology of cortical tissue (i.e. pore network, Haversian and Volkmann's canals). However, the connection between microstructure and mechanics of cortical bone during plastic deformation is unclear. Hence, the purpose of this study is to provide an in-depth evaluation of the interplay of plastic strain building up in relation to changes in the canal network for cortical bone tissue. In situ step-wise XCT indentation was used to introduce a localised load on the surface of the tissue and digital volume correlation (DVC) was employed to assess the three-dimensional (3D) full-field plastic strain distribution in proximity of the indent. It was observed that regions adjacent to the imprint were under tensile strain, whereas the volume underneath experienced compressive strain. Canal loss and disruption was detected in regions of higher compressive strains exceeding −20000 με and crack formation occurred in specimens where Haversian canals were running parallel to the indentation tip. The results of this study outline the relationship between the micromechanical and structural behaviour of cortical bone during plastic deformation, providing information on cortical tissue fracture pathways.

Exploring the micromechanical behaviour of sand-rubber mixtures using X-ray micro-tomography

The micromechanical behaviour of sand-rubber mixtures (SRMs) under monotonic triaxial shear were investigated using X-ray micro-tomography. The localisation of sand particle rotations that occurred in a pure sand sample under shear was inhibited in the sand mixed with 30% rubber grains by mass. Meanwhile, the SRMs exhibited an evolution of sand-sand contact coordination number that is not negatively correlated with sample porosity, dramatically different from that was observed in pure sands. Substantially increasing anisotropy degree of sand-rubber contacts compared with minor changes of sandsand contact fabric was observed, implying the increasingly important role of sand-rubber contacts in the transmission of deviatoric loads as the shear of SRMs progressed.

Micromechanical modeling of glass fiber reinforced plastic material

The objective of this paper is to detail the steps taken in creating a material model capable of describing the micromechanical behavior of a composite material made up of a polyphthalamide matrix reinforced with glass fiber inclusions. The composite material model was created using the specialized software program Digimat, through its Mean-Field and Finite Element modules. In order to validate this material model, the true stress–strain multilinear curves were compared with the equivalent ones obtained from tensile testing done with the same composite material as the modeled one, following multiple angles of fiber orientation to also characterize the anisotropy of the material.

Characterization of metakaolinite micro-mechanical behaviors using MD simulation

The mechanical behavior of metakaolinite under nanoindentation and nanoscratching was investigated by means of molecular dynamic (MD) simulations in this study. A 6 nm radius spherical indenter was pressed on the (001) and 001¯ basal planes up to a 3.5 nm indentation depth to study the anisotropy of metakaolinite in hardness and Young’s modulus. The higher hardness was found in silica facet as it possessed a hexagonal-close-packed plane that is the most densely packed plane. During scratching, a 3 nm radius spherical indenter was used to scratch the substrate surfaces up to 8 nm length, the scratching behavior in different scratching depth and speed was studied. Both normal and scratch forces increased as the scratching processed, while scratch force showed a high dependence on scratching depth. At high scratching depth of 2.5 nm and 3.5 nm, pile-up of atoms towards the free surface and formation of chip via the shear zone were observed, the chip volume was found to increase with the increase of scratching depth. The metakaolinite appeared to soften slightly, as demonstrated by normal hardness decreased with depth. The depth dependence was more conspicuous on the scratch force than for the normal force, while the scratching speed had little effect on the scratching process.