Macro-scale Mechanical Response Research Articles

Architected flexible syntactic foams: Additive manufacturing and reinforcing particle driven matrix segregation

Polymer syntactic foams are transforming materials that will shape the future of next-generation aerospace and marine structures. When manufactured using traditional processes, like compression molding, syntactic foams consist of a solid continuous polymer matrix reinforced with stiff hollow particles. However, polymer matrix segregation can be achieved during the selective laser sintering process with thermoplastic polyurethane (TPU). It is uncertain what role hollow particles play in forming this matrix segregation and its impact on the corresponding mechanical properties of syntactic foams. We show that the size of the hollow particles controls the internal microscale morphology of matrix segregation, leading to counter-intuitive macroscale mechanical responses. Particles with diameters greater than the gaps between the cell walls of the segregated matrix get lodged between and in the walls, bridging the gaps in the segregated matrix and increasing the stiffness of syntactic foams. In contrast, particles with smaller diameters with higher particle crushing strength get lodged only inside the cell walls of the segregated matrix, resulting in higher densification stresses (energy absorption). We show that stiffness and densification can be tuned while enabling lightweight syntactic foams. These novel discoveries will aid in facilitating functional and lightweight syntactic foams for cores in sandwich structures.

A microcracking-based model for the dynamic failure of carbon/carbon composites

The ‘pseudoplastic’ deformation and failure of the carbon/carbon composites (C/Cs) are strain-rate-sensitive while the mechanism has not yet been properly elaborated. To delve into the design and fabrication strategy of the C/Cs towards anti-impact applications, the influences of preform architectures and carbon nanotubes (CNT) doping on the dynamic mechanical behaviors of the C/Cs were studied, showing that the CNT-doped plain weave preform exhibits better anti-impact performances than the other structures. The experimental results indicate that the fragmentation of the C/Cs at high strain rate is due to the activation of all the possible cracking paths with excessive kinetic energy being deposited, which is significantly different with the growth of a main crack under the quasi-static loading. A two-scale modeling is established to backtrack from the macroscale mechanical response to the microscale cracking behavior, where the quasi-static and dynamic failure of the composite were elaborated within a unified framework of linear elastic fracture mechanism. The coupling of the experiments and modeling revealed that the CNT-doped layer would constrain the microcracking behavior at elevated strain rates. Further nanomechanical tests and molecular dynamics simulations verify that the buffering function and the enhanced energy absorption of the CNT-doped interphase are responsible for the improved anti-impact performances of the C/Cs.

Local electrical conductivity of carbon black/PDMS nanocomposites subjected to large deformations

The full-field microscale electrical behavior of polydimethylsiloxane (PDMS) matrix composites with 12 wt% (below the electrical percolation threshold) and 20 wt% (above the electrical percolation threshold) carbon black (CB) nanoparticles was obtained with the aid of Conductive Atomic Force Microscopy (C-AFM) as a function of uniaxial tensile strain. A comparison between the macroscale and the microscale 2D strain fields showed that 25 × 25 μm2 specimen domains can serve as a representative surface element (RSE) to describe the macroscale mechanical response under finite deformations. The same surface domain size was shown to serve as RSE for the local electrical behavior of CB/PDMS nanocomposites at small and large strains. The transverse specimen contraction due to the Poisson’s effect increased the through-thickness conductivity of 12 wt% CB specimens, with a rapid increase in the total electric current measured at the scale of the RSE taking place above 23% uniaxial true strain. The locally dominant tunneling conductance in the undeformed specimen state transitioned to a combination of Ohmic (linear local current-voltage (I–V) characteristics) and tunneling (parabolic local I–V characteristics) at 35% uniaxial true strain. In comparison, the local I–V characteristics of undeformed PDMS composites with 20 wt% CB were a combination of Ohmic (contributing the majority of the total current) and tunneling. Upon application of 35% uniaxial true strain the vast majority of local conduction sites became Ohmic, while the total electric current at the scale of the RSE increased relatively linearly with applied strain.

In-vivo high-speed biomechanical imaging of the cornea using Corvis ST and digital image correlation

In-vivo corneal biomechanical characterization has gained significant clinical relevance in ophthalmology, especially in the early diagnosis of eye disorders and diseases (e.g. keratoconus). In clinical medicine, the air-puff-based tonometers such as Ocular Response Analyzer (ORA) and Corvis ST have been used in the in-vivo biomechanical testing. In the test, the high-speed dynamic deformation of the cornea under air-puff excitation is analyzed to identify the abnormities in the morphological and biomechanical properties of the cornea. While most existing measurements reflect the overall corneal biomechanical properties, in-vivo high-speed strain and strain rate fields at the tissue level have not been assessed. In this study, 20 subjects were classified into two different groups: the normal (NORM, N = 10) group and the keratoconus (KC, N = 10) group. Image sequences of the horizontal cross-section of the human cornea under air puff were captured by the Corvis ST tonometer. The macroscale mechanical response of the cornea was determined through image analysis. The high-speed evolution of full-field corneal displacement, strain, velocity, and strain rate was reconstructed using the incremental digital image correlation (DIC) approach. Differences in the parameters between the NORM and KC groups were statistically analyzed and compared. Statistical results indicated that compared with the NORM group, the KC corneas absorbed more energy (KC: 8.98 ± 2.76 mN mm; NORM: 4.79 ± 0.62 mN mm; p-value <0.001) with smaller tangent stiffness (KC: 22.49 ± 2.62 mN/mm; NORM: 24.52 ± 3.20 mN/mm; p-value = 0.15) and larger maximum deflection (KC: 0.99 ± 0.07 mN/mm; NORM: 0.92 ± 0.06 mN/mm; p-value <0.05) on the macro scale. Further, we also observed that The maximum displacement (KC: 1.17 ± 0.06 mm; NORM: 1.06 ± 0.07 mm; p-value <0.005), velocity (KC: 236 ± 29 mm/s; NORM: 203 ± 17 mm/s; p-value <0.01), shear strain (KC: 24.43 ± 2.59%; NORM: 20.26 ± 1.54%; p-value <0.001), and shear strain rate (KC: 69.74 ± 11.99 s−1; NORM: 54.84 ± 3.03 s−1; p-value <0.005) in the KC group significantly increased at the tissue level. This is the first time that the incremental DIC method was applied to the in-vivo high-speed corneal deformation measurement in combination with the Corvis ST tonometer. Through the image registration using incremental DIC analysis, spatiotemporal dynamic strain/strain rate maps of the cornea can be estimated at the tissue level. This is constructive for the clinical recognition and diagnosis of keratoconus at a more underlying level.

Experiments and modeling of the macroscale mechanical responses of neutron-irradiated FeCrAl alloys

Iron chromium aluminum (FeCrAl) alloys have been regarded as candidate materials for accident-tolerant fuel cladding. In this study, the stress-strain curves of various neutron-irradiated and un-irradiated FeCrAl alloys at the test temperatures of 300 K and 653 K are obtained through the uniaxial tensile tests, with the fast neutron fluences of 2 × 1019 n/cm2 and 8 × 1019 n/cm2 (E ≥ 1 MeV). Simultaneously, improved shear strain rate model and damage evolution model are proposed in the crystal plasticity theoretical frame, with the irradiation effects involved. The developed models are numerically implemented, with the simulation results of the macroscale stress-strain curves agreeing well with the experimental data. The irradiation hardening and embrittlement phenomena are captured, together with the characteristics of strain hardening and softening. The influences of alloy compositions, temperatures, and fast neutron fluences on yield strength, ultimate tensile strength, yield ratio, and uniform elongation are discussed. Macro mechanical properties such as yield strength and yield ratio are found to be near-linearly correlated with crystal plasticity model parameters. A model for multi-objective optimization of the mechanical properties is established, with which the optimum design strategies of alloy composition for the materials in the in-reactor service environments are proposed. This study could provide a theoretical basis for the research and development of advanced FeCrAl alloys.

Evolution of Nanomechanical and Macroscale Mechanical Responses of Expansive Clay during Swelling

Evolution of Nanomechanical and Macroscale Mechanical Responses of Expansive Clay during Swelling

Crystal plasticity finite element modeling of twin band formation and evolution together with the macroscale mechanical responses of hexagonal metals

Deformation twinning is one of the major plastic deformation modes of hexagonal metals. In this study, a new system of crystal plasticity finite element models is developed, with the cooperative interactions of dislocation slip and deformation twinning comprehensively involved. The obtained stress-strain curves for several differently-orientated Mg single crystals match well with experimental results, and the formation and thickening features of twin bands are captured. Simulation results reveal that (1) some thin twin bands are firstly formed in the shearing direction of the twinning plane, because of the much lower twinning resistance than that in the normal direction; (2) the softening and hardening of the twinning resistance play a critical role in the anisotropic mechanical response. By optimizing the twinning activation behaviors, the favorable interactions of twinning and dislocation slip can occur and the plastic deformation ability can be improved.

Effects of tow arrangements on the homogenized response of carbon fiber woven composites

A set of micro-mechanical analyses is carried out to investigate the sensitivity of the homogenized response of two-ply carbon fiber woven composites on relative positioning of tow arrangements. In this paper, first, we present a convergence study to validate the applicability of the characteristic unit cell to capture the macroscale mechanical response of the composite. The study showcases the capacity of our models to capture the macroscale mechanical properties at different microscopic resolutions of the macroscale. The effects from phase different tow arrangements are then analyzed to quantify the mechanical response variability at the macroscale. Different tow arrangements considered in this work naturally stems from the relative inter-ply sliding at the manufacturing stage of woven composites. We consider several tow arrangements that can be interpreted as intermediate configurations between the two extremes; fiber in-phase and out-of-phase. Tows are assembled and constrained systematically to resemble different tow arrangements for the subsequent multiscale modeling. Corresponding macroscopic ABD stiffness matrices are extracted using the principle of virtual work, whereby the unit cell deformation is homogenized using periodic boundary conditions. The obtained results not only quantify the variability in A and D sub-matrices of the ABD stiffness matrix, but also explain possible reasons for the observed deviations between experimental and numerical results.

Microscopic and macroscopic numerical simulation of the progressive failure of granular materials

<p indent="0mm">The onset and propagation mechanisms of strain localization in granular materials have been analyzed from both discontinuous microscale discrete element modeling (DEM) and continuum mechanics-based finite element modeling standpoints. Through DEM, the mechanical response before and after the inception of strain localization is obtained. The evolution characteristics of the shear band are obtained by analyzing the relative displacement angle of particles. By analyzing the size effect of strain localization in granular materials, the relationship between the sample size-to-average particle size ratio and the thickness of the shear band is obtained. A generalized thermodynamics framework based on the micromorphic approach is adopted, where a nonlocal microvariable as an additional kinematics variable is introduced. An implicit gradient softening plasticity model considering pressure dependence is then proposed. The additional so-called Helmholtz partial differential equation is derived using the principle of virtual power and the second law of thermodynamics. A regularized finite element method coupling displacement and microvariable are proposed. Numerical simulations of plane strain compression tests show that the proposed numerical method can produce solutions, which are independent of the spatial discretization, verifying the applicability of the proposed model on regularizing ill-posed boundary value problems of strain-softening granular materials. Through analyses of the bearing capacity of the foundation and the stability of the tunneling surface, the effects of strain-softening behavior on the inelastic strain localization characteristics and macroscale mechanical responses are investigated. The results show that strain-softening behavior results in smaller active instability regions and more concentrated localized deformation.

A three-dimensional DEM modelling of triaxial test on gas hydrate-bearing sediments considering flexible boundary condition

The discrete element method (DEM) has been widely used to investigate the mechanical properties of gas hydrate-bearing sediments (GHBS). However, a flexible membrane boundary has not been incorporated into three-dimensional DEM modelling. Moreover, the roles of hydrates in stress transmission have not been comprehensively interpreted from a particulate scale point of view. This study adopted the finite difference method (FDM)-discrete element method (DEM) coupling method to establish the flexible boundary condition to capture the failure modes of GHBS and categorized the overall contact set into three contact subsets (i.e., soil-soil, soil-hydrate and hydrate-hydrate contact subsets) in a way that the stress transmission characteristics of hydrates can be identified. The simulated macroscale mechanical responses demonstrate a favourable agreement against the experimental results. The results show that the sample with a low hydrate saturation shows a single slant shear failure zone, while the sample with a high hydrate saturation shows an X-shape shear failure zone. The particulate scale analysis shows that the soil-hydrate contact subset not only plays a critical role in stabilizing the strong force chains but also has a significant contribution to the deviatoric stress. By contrast, the hydrate-hydrate contact subset accounts for a small contribution to the deviatoric stress.

Multiscale Geomechanical Behavior of Fiber-Reinforced Cementitious Composites Under Cyclic Loading Conditions—A Review

As construction materials, cementitious composites such as cemented paste backfill (CPB), cemented soil, and concrete may be subjected to extreme dynamic loadings including impact, blast, and/or seismic loads during their service life. To improve mechanical performance under dynamic loadings, fiber reinforcement technique has been considered a promising approach and extensively used in practice. In this manuscript, a new perspective on the multiscale geomechanical behavior of fiber-reinforced cementitious composites (FRCC) is provided through a comprehensive review on the macroscale constitutive behavior and the associated mechanical properties, and microscale failure processes under cyclic tensile, shear, and compressive loading conditions. For the macroscale mechanical response, this review includes a detailed analysis of the state-of-the-art research in stress-strain behaviors including pre- and post-peak response and hysteretic behaviors. Moreover, the effects of pore water pressure on the dynamic response of soft FRCCs such as CPB are discussed. Furthermore, the link between microscale crack propagation (including the formation of the interfacial transition zone and fracture process zone) and damage accumulation is established for each type of cyclic loading condition. In addition, a critical discussion on the future development of fiber reinforcement is conducted as well. Therefore, this review not only offers guidance and references to the experimental investigation on the multiscale behavior of FRCCs under cyclic loadings, but also promotes the further development of fiber reinforcement techniques.

CT image-based synthetic mesostructure generation for multiscale fracture analysis of concrete

Accurate prediction of multiscale fracture process of concrete relies on modeling of concrete mesostructures. Though high-resolution yet realistic mesostructures can be obtained by CT technique, the size limitation of reconstructed mesostructures is still an outstanding task. In this paper, a novel method of generating synthetic mesostructures of concrete based on generative adversarial networks is proposed. After training with CT images of a limited size, a large amount of mesostructures with arbitrary sizes are generated. Subsequently, a systematic scheme for investigating fracture process of concrete is established by incorporating the proposed method with cohesive zone model. The geometric features of generated mesostructures, including two-point correlation functions and aggregate size distributions, are compared with actual CT images to quantitatively verify performance of the proposed method. Simulated crack patterns and macroscale mechanical response of generated mesostructures demonstrate close agreements with those of actual mesostructures, thus validating that the proposed method is an accurate and effective tool in the study of multiscale fracture analysis of concrete.

Microplasticity at Room Temperature in α/β Titanium Alloys

The current understanding of room temperature microplasticity in α/β titanium alloys is reviewed with a special emphasis on dual-phase engineering alloys. As the interplay between microstructure and deformation mechanisms governs both the microscale and macroscale mechanical response, a brief description of the main features of α/β microstructures is first provided. Elastic and plastic deformation in individual phases is then described. The complex interactions that govern the effect of grain boundaries, phase interfaces and microtexture on deformation behaviour are reviewed. Crystal plasticity simulations have evolved over the past decade as a key technique to obtain a mechanistic understanding of the deformation of Ti alloys. Micromechanical aspects are emphasized with a discussion of input parameters required to achieve realistic constitutive modeling. As microplasticity is especially relevant in cyclic loading such as experienced in-service by components, the current understanding of the relation of this regime with fatigue and dwell-fatigue behavior is briefly summarized in the final section.

Mechanical properties of glassy polymers with controlled NP spatial organization

Industrially viable processes enabling effective control of nanoparticle (NP) spatial organization in polymers are vitally important for achieving potential of nano-composites as building blocks of dynamic composites. Here, we report on the small and large strain mechanical properties of glassy polymers with spatial organization of spherical NPs controlled by the balance of the (polymer-NP)/(NP-solvent) interfacial attraction and mixing conditions. While strong polymer-NP attraction yielded good NP dispersion regardless of the mixing rate, weak polymer-NP attraction and low shear rate mixing resulted in chain bound NP clusters. Polymers with individually dispersed NPs exhibited the largest elastic moduli and strength while polymers filled with deformable chain bridged NP clusters possessed the largest ductility, at the same NP content. Hybrid systems, consisting of chain bridged NP clusters embedded in a polymer with individually dispersed NPs, exhibited simultaneous enhancement of stiffness, strength and ductility with its extent tunable by the cluster content. Our results confirm the pivotal role of NP spatial organization in translating the nano- and micro-scale phenomena to macro-scale mechanical response of polymer nano-composites. The obtained knowledge enables the design of additive fabrication technologies capable of employing polymer nano-composites in manufacturing of lightweight, functional engineering structures.

High- vs. low-fidelity models for dynamic recrystallization in copper

We investigate the benefits and limitations of mesoscale models for discontinuous dynamic recrystallization (DDRX) in pure copper at elevated temperature with the two-fold aim of capturing microscale mechanisms and predicting the macroscale mechanical response during severe plastic deformation. Differing strongly in their computational expenses and the underlying constitutive assumptions, we introduce and compare an efficient Taylor model (which treats polycrystals as collections of spatially non-interacting grains) with a Field Monte-Carlo Potts (FMCP) model (which resolves spatially inhomogeneous deformation within grains by an FFT-based treatment). Both approaches are based on the same temperature-aware crystal plasticity model for pure copper and introduce only three model parameters for DDRX. The latter are fitted to stress–strain data from uniaxial compression experiments at elevated temperature levels where DDRX is prevalent. Both models capture grain refinement, texture evolution and the stress–strain history with convincing agreement with experiments. The fully-resolved model has highest accuracy, reveals pronounced texture formation, and captures the gradual formation of high-angle grain boundaries within grains as precursors to subgrain formation. The Taylor model, though being significantly more efficient, fails to capture spatially-correlated features including necklace formation and leads to comparatively high prediction errors. However, at temperatures where migration dominates the recrystallization behavior, we observe compelling agreement between the Taylor model and the FMCP model. Last, we demonstrate how reduced-order models facilitate identifying model parameters of the computationally more expensive models.

Direct numerical simulation of mechanical response in synthetic additively manufactured microstructures

Additive manufacturing (AM) processes for metals can yield as-built microstructures that vary significantly from their cast or wrought counterparts. These microstructural variations can in turn, have profound effects on the properties of a component. Here, a modeling methodology is presented to investigate microstructurally-influenced mechanical response in additively manufactured structures via direct numeral simulation. Three-dimensional, synthetic voxelized microstructures are generated by kinetic Monte Carlo (kMC) additive manufacturing process simulations performed at four scan speeds to create a thin-wall cylindrical geometry notionally constructed using a concentric-pathed directed energy deposition AM process. The kMC simulations utilize a steady-state molten pool geometry that is held constant throughout the study. Resultant microstructures are mapped onto a highly-refined conformal finite-element mesh of a part geometry. A grain-scale anisotropic crystal elasticity model is then used to represent the constitutive response of each grain. The response of the structure subjected to relatively simple load conditions is studied in order to provide understanding of both the influence of AM processing on microstructure as well as the microstructure’s influence on the macroscale mechanical response.

Tendon healing affects the multiscale mechanical, structural and compositional response of tendon to quasi-static tensile loading.

Tendon experiences a variety of multiscale changes to its extracellular matrix during mechanical loading at the fascicle, fibre and fibril levels. For example, tensile loading of tendon increases its stiffness, with organization of collagen fibres, and increases cell strain in the direction of loading. Although applied macroscale strains correlate to cell and nuclear strains in uninjured tendon, the multiscale response during tendon healing remains unknown and may affect cell mechanosensing and response. Therefore, this study evaluated multiscale structure-function mechanisms in response to quasi-static tensile loading in uninjured and healing tendons. We found that tendon healing affected the macroscale mechanical and structural response to mechanical loading, evidenced by decreases in strain stiffening and collagen fibre realignment. At the micro- and nanoscales, healing resulted in increased collagen fibre disorganization, nuclear disorganization, decreased change in nuclear aspect ratio with loading, and decreased indentation modulus compared to uninjured tendons. Taken together, this work supports a new concept of nuclear strain transfer attenuation during tendon healing and identifies several multiscale properties that may contribute. Our work also provides benchmarks for the biomechanical microenvironments that tendon cells may experience following cell delivery therapies.

Effects of weave architecture and mesoscale material properties on the macroscale mechanical response of advanced woven fabrics

This research explores the effects of mesoscale (yarn) material properties on the macroscale mechanical response of various woven fabric architectures. A unit cell finite element modeling for plain, basket, twill and satin weaves is adopted and coupled with a special design-of-experiments approach. To investigate the effects of the yarn material properties and weave architectures, two loading cases are studied: (1) Uniaxial tensile, and (2) Shear. Macroscale response outcomes such as woven fabric modulus, Poisson's ratio, frictional energy dissipation and shear resistance are defined for each loading case and the effects of the mesoscale material properties on these outcomes are studied. The method is capable of identifying significant parameters along with their relative effects and contributions to the outcomes. The results of the study revealed the importance of transverse shear modulus of the yarns. Moreover, the relative effects of the yarn material properties are determined using different weave architectures.

Development of physically based atomistic microstructures: The effect on the mechanical response of polycrystals

A method for the generation of atomistic realizations of polycrystalline aggregates from a phase field grain growth model is presented. Topologies of computational microstructures constructed from the proposed method as well as conventional Poisson Voronoi tessellation are quantitatively compared. While little difference is exhibited in the macroscale mechanical response, substantial differences in the resolved contribution to strain of deformation mechanisms of the structures under uniaxial tension are uncovered using post-processing kinematic metrics. These differences in the fundamental strain accommodation processes suggest that grain topology and grain boundary character significantly affect local responses of polycrystals in molecular dynamics simulations and that significant attention should be paid to the chosen starting microstructure.

Mechanical characterization of Ti-6Al-4V titanium alloy at multiple length scales using spherical indentation stress-strain measurements

Recent advances in spherical indentation stress-strain protocols and analyses have demonstrated the capability for measuring reliably the local mechanical responses in polycrystalline metal samples at different length scales, ranging from sub-micron (regions within individual grains) to several hundreds of microns (regions covering several grains). These recent advances have now made it possible to study systematically the mechanical behavior of a single material system at different length scales, with tremendous potential to obtain new insights into the role of individual phases, interfaces, and other microscale constituents on the macroscale mechanical response of the material. In this paper, we report spherical indentation stress-strain measurements with different indenter sizes (microns to millimeters) on Ti-6Al-4V (Ti-64) which capture the mechanical response of single phase alpha-Ti-64, single colony (alpha-beta), few colonies, and many colonies of Ti-64. The results show that the average mechanical response (indentation modulus and yield strength) from multiple indentations remains relatively unchanged from single phase alpha to many colonies of Ti-64, while the variance in the response decreases with indenter size. The work-hardening response in indentation tests follows a similar behavior up to indentation zones of many colonies, which shows significantly higher work hardening rates.