Macroscopic Mechanical Response Research Articles

The microstructural effects on the mechanical response of polycrystals: A comparative experimental-numerical study on conventionally and additively manufactured metallic materials

Metal additive manufacturing (AM) provides an unprecedented opportunity to explore extreme microstructural regimes and their associated effects on the mechanical response of polycrystals. In this work, a single alloy (composition) belonging to the high-Mn steels family was processed by a conventional thermo-mechanical treatment and two different AM processes. This resulted in three different materials with strikingly diverse microstructures with respect to the microstructural size and polarity. The influence of disparities in the statistical microstructural features arising from the different processing conditions on the (macroscopic) mechanical response was investigated through an experiment-simulation approach. The (as-processed/as-built) microstructures were analyzed on the meso-scale for derivation of a set of parameters, namely meso-structure descriptors, which adequately describe the meso-structural heterogeneity features. To link the microstructure and mechanical properties, we used a physics-based crystal plasticity modeling approach in the framework of full-field polycrystal homogenization. The meso-structure descriptors together with the submeso-scale/constitutive microstructural parameters were used to simulate the (anisotropic) mechanical response of the aforementioned materials. Then, for a reduced-order quantitative assessment of the effect of microstructural polarity on the anisotropic mechanical response, the plastic anisotropy indices were defined and calculated using the simulation results. This study provides new insights into the primary microstructural effects (the size and polarity effects) on the mechanical (stress and strain hardening) response of polycrystals as well as the propensity of micro-mechanisms accommodating polycrystal plasticity in highly different microstructural regimes, particularly those typically obtained by metal AM.

Origin of different thermal cycling effects in Fe80P20 and Ni60Nb40 metallic glasses

The effect of thermal cycling on structure and properties has been explored in many metallic glasses (MGs) within last 5 years. It is critical to establish the connection between macroscopic mechanical response and the underlying atomic level process during thermal cycling. The mechanisms underlying the complexity of thermal cycling effect are poorly understood. Here, we investigate the thermal cycling effect on structural evolution and property changes in brittle Fe80P20 and ductile Ni60Nb40 MGs. Our results demonstrate that thermal cycling treatment initially delays the cavitation behavior for Fe80P20 MG and induces local-to-non-localized deformation mode change with high probability for N60Nb40 MG, despite their opposite atomic structural evolution. Specifically, with initial thermal cycling, the Fe80P20 MG prefers to relax, while Ni60Nb40 MG prefers to rejuvenate. With further thermal cycling, the Fe80P20 MG rejuvenates while Ni60Nb40 relaxes back towards a relative stable state. To reveal the origin of such opposite thermal cycling effects in these MGs, we proposed ‘core-shell’ structural models of MGs. Our results shed light on the atomistic understanding of the relationship between the micro-structure and thermal cycling effect in MGs, which may offer useful insights for designing and processing MGs.

Anisotropic inertia rheology of ellipsoidal grains

Ellipsoidal grains show a complex macroscopic response associated with their microscopic ability to orient and align with respect to each other and the ambient flow. These additional microscopic degree of freedom is an evolving property that gives rise to a complex anisotropic macroscopic mechanical response. Available models for dense granular flow typically only consider grain size while ignoring grain shape, orientation and alignment and their effect on the macroscopic mechanical response. This paper discusses the construction of a generalized inertia rheology model for ellipsoidal grains. The construction of the continuum model incorporates the microstructure that accounts for the grains’ orientation and alignment and utilizes the representation theorem and dissipation inequality to yield a simple and physically meaningful phenomenological model. The model predictions show good agreement compared to available Discrete Element Method simulations of simple shear flow. The proposed model is complete in the sense that it consists of a constitutive law that relates the microstructure arrangement and the flow to the developed stresses supported by an evolution law for the microstructure arrangement.

Numerical simulation on dynamic damage evolution of high pure copper with different grain sizes

High-purity (HP) copper targets with grain sizes of 50, 130 and 200 μm are constructed by using the Voronoi method. Damage nucleation points are randomly prefabricated at the grain boundaries. A two-dimensional axisymmetric finite element model is established to simulate the spallation experiment of HP copper target. The effects of grain size and loading stress on the macro- mechanical response and meso-damage evolution of HP copper spallation are studied and compared with the relevant experimental results. Based on the analysis of free surface velocity profiles, the effects of grain size on the location of pull back velocity rebound point, velocity rebound slope and velocity rebound amplitude are revealed. It is demonstrated that the spalling strength corresponds to the peak value of tensile stress in the damage zone, which essentially represents the critical stress of micro damage nucleation or early growth. Based on the characteristic analysis of damage evolution nephogram, the evolution process of localized plastic strain field around the micro-voids in the growth and coalescence process is reproduced, and the strong dependence of micro-void coalescence behavior on grain size is clarified. The loading stress amplitude has little effect on the location of pull back velocity rebound point, but has a significant effect on the growth and coalescence behavior of micro-voids. The slope and amplitude of pull back velocity rebound increase with loading stress increasing, which is consistent with the relevant experimental result. With the increase of the loading stress, the micro-voids grow from independent growth to coalescence, thus forming spalling surface. The physical process of damage evolution determines the wave oscillation characteristics after the pull-back rebound point. The numerical simulation results reproduce the physical process of damage evolution and its influence on the macroscopic mechanical response, which is of great significance for further understanding spall damage evolution mechanism and theoretical model construction.

Computational analysis of the effects of geometric irregularities and post-processing steps on the mechanical behavior of additively manufactured 316L stainless steel stents

Advances in additive manufacturing enable the production of tailored lattice structures and thus, in principle, coronary stents. This study investigates the effects of process-related irregularities, heat and surface treatment on the morphology, mechanical response, and expansion behavior of 316L stainless steel stents produced by laser powder bed fusion and provides a methodological approach for their numerical evaluation. A combined experimental and computational framework is used, based on both actual and computationally reconstructed laser powder bed fused stents. Process-related morphological deviations between the as-designed and actual laser powder bed fused stents were observed, resulting in a diameter increase by a factor of 2-2.6 for the stents without surface treatment and 1.3-2 for the electropolished stent compared to the as-designed stent. Thus, due to the increased geometrically induced stiffness, the laser powder bed fused stents in the as-built (7.11 ± 0.63 N) or the heat treated condition (5.87 ± 0.49 N) showed increased radial forces when compressed between two plates. After electropolishing, the heat treated stents exhibited radial forces (2.38 ± 0.23 N) comparable to conventional metallic stents. The laser powder bed fused stents were further affected by the size effect, resulting in a reduced yield strength by 41% in the as-built and by 59% in the heat treated condition compared to the bulk material obtained from tensile tests. The presented numerical approach was successful in predicting the macroscopic mechanical response of the stents under compression. During deformation, increased stiffness and local stress concentration were observed within the laser powder bed fused stents. Subsequent numerical expansion analysis of the derived stent models within a previously verified numerical model of stent expansion showed that electropolished and heat treated laser powder bed fused stents can exhibit comparable expansion behavior to conventional stents. The findings from this work motivate future experimental/numerical studies to quantify threshold values of critical geometric irregularities, which could be used to establish design guidelines for laser powder bed fused stents/lattice structures.

Theoretical Study for Dynamic Strain Aging in Niobium: Effect of Temperature and Strain Rate on the Flow Stress

A constitutive model for niobium with the effect of dynamic strain aging is proposed. The crystal structure of metals hugely influences the dynamic strain aging phenomenon and causes considerable alterations in the material’s macroscopic mechanical responses. Dynamic strain aging needs to be accounted for in a constitutive model to obtain accurate predictions of material’s thermo-mechanical behaviors during deformation. The proposed constitutive model attempts to describe the material’s flow stress responses during deformation by separating the flow stress contributions into the athermal component, thermal component, and dynamic strain aging component. Two different mathematical equations are proposed to model the dynamic strain aging component. The proposed model attempts to describe the mechanical response of niobium for a wide range of strain rates: from quasi-static loading ( $$\dot{\varepsilon } = 0.001\,{\text{s}}^{ - 1}$$ ) to dynamic loading ( $$\dot{\varepsilon } = 3300\, {\text{s}}^{ - 1}$$ ) across the temperature ranges 77 K–800 K.

Depth-sensing ductile and brittle deformation in 3C-SiC under Berkovich nanoindentation

The interplay between ductile and brittle deformation modes in hard brittle materials exhibits a strong size effect. In the present work, indentation depth-dependent deformation mechanisms of single-crystal 3C-SiC under Berkovich nanoindentation are elucidated by finite element simulations and corresponding experiments. A novel finite element framework, that combines a crystal plasticity constitutive model for describing dislocation slip-based ductile deformation and a cohesive zone model for capturing crack initiation and propagation-induced brittle fracture, is established. The utilized parameters in the crystal plasticity model of 3C-SiC are calibrated according to the load-displacement curves obtained from corresponding Berkovich nanoindentation experiments. Subsequent finite element simulations and experiments of nanoindentation jointly reveal co-existing microscopic plastic deformation and brittle fracture of 3C-SiC at different indentation depths, which significantly affect the observed macroscopic mechanical response and surface pile-up topography. In particular, the predicted morphology of surface cracks at an indentation depth of 500 nm agrees well with experimental observation, and the correlation of crack initiation and propagation with surface pile-up topography is theoretically analyzed.

Experimental and numerical research on fracture behaviors of sandstone under different loading rates

The macroscopic deformation process of rock is the external performance of microscopic damage in essence, and research from the macro and micro perspectives can effectively reveal the failure mechanism of rock. To further investigate the fracture behaviors of yellow sandstone disc specimens under different loading rates, seven groups of Brazilian disc splitting tests with different loading rates from 1 × 10–6 to 1 × 10–4 s–1 were carried out. A systematical investigation of loading rate effect on macroscopic mechanical responses and microscopic damage characteristics has been conducted from the stress–strain relationship obtained from material testing system, from damage three-dimensional localization based on acoustic emission techniques, and from fracture morphology by scanning electron microscope. In addition to laboratory experiment, parallel bond models of different grain size distribution indexes based on particle flow code were established. The coupling effect of loading rate and grain size distribution index was further analyzed. The results interpret that the effect of loading rate on rock fracture behaviors is determined by the process of crack initiation, propagation and coalescence, and it is also affected by distribution of different size grains. The differences of rock fracture behaviors under different loading rates were discussed from the mechanism of energy input and dissipation. The research have actual significance for stability design and evaluation of rock excavation engineering.

Elastic-viscoplastic self-consistent modeling for finite deformation of polycrystalline materials

Anisotropic 1-site and 2-site self-consistent models are developed to describe the elastic-viscoplastic behavior of polycrystalline materials deformed to finite strains on the basis of rate-dependent crystallographic slip and a generalized Hill-Hutchinson self-consistent approach. The choice of rate-dependent constitutive law at single crystal level implemented in the models is discussed through fitting experimental data and calibrating viscous parameters. It is found that drag-stress type Norton law works well for the 1-site elastic-viscoplastic self-consistent (EVPSC) model while threshold stress type Norton law is suitable for the 2-site EVPSC model to assure that the viscoplastic inter-granular interaction is realistic. Both models have been verified by thoroughly fitting experimental data in literatures. For the 1-site EVPSC model, selected experimental data covers both macroscopic and microscopic mechanical responses of steels during deformation with a large range of strain rate from the quasi-static (10−4s−1) to the dynamic (~104s−1). For the 2-site EVPSC model, in situ neutron diffraction data of nickel-based superalloys with various microstructures was fitted. Both models generally fit the experimental data well. A comparison between the EVPSC and elastic-plastic self-consistent (EPSC) models on the prediction of lattice strains has also been made for both the 1-site and 2-site cases, which verifies the predictability on lattice strains of the newly developed EVPSC models. A validation of the homogenization approach for the EVPSC modeling has been performed, which confirms that the proposed EVPSC models are applicable for cubic structure materials with finite deformations. Our formulation of EVPSC modeling developed in this work shines a spotlight on the way of developing a multi-functional self-consistent model to predict both macroscopic and microscopic deformation behaviors of various polycrystalline materials under different loading rates of 10−4s−1~104s−1.

A continuum constitutive model for FDM 3D printed thermoplastics

Fused deposition modelling (FDM) is the most common additive manufacturing technology used for thermoplastic components. This layers-based manufacturing process results into direct links between printing parameters and the polymer mesostructure by means of porosity and structural anisotropy. These dependencies along with other features of thermoplastic polymers (i.e., nonlinearities, viscous and thermal responses) makes its constitutive modelling very challenging. This work distances from studies that model the 3D printing process. Instead, we aim at complementing such approaches with a continuum model to describe the macroscopic behaviour of FDM thermoplastics while preserving links with printing parameters. Prior to the modelling conceptualisation, experimental characterisation tests are conducted on ABS specimens to evaluate the influence of printing parameters on the macroscopic mechanical response. The physical fundamentals behind the deformation and failure mechanisms are identified and motivate the new constitutive model. This model is formulated for finite deformations within a thermodynamically consistent framework. The model accounts for: nonlinear response; anisotropic hyperelasticity related to a transversely isotropic distribution of porous; strain rate dependency; macroscopic stiffness dependent on 3D printing processing. Finally, the model is numerically implemented and calibrated for ABS with original experiments, demonstrating its suitability.

Frequency dependent inelastic response of collagen architecture of pig dermis under cyclic tensile loading: An experimental study

The evaluation of collagen architecture of the dermis in response to mechanical stimulation is important as it affects the macroscopic mechanical properties of the dermis. A detailed understanding of the processes involved in the alteration of the collagen structure is required to correlate the mechanical stimulation with tissue remodeling. This study investigated the effect of cyclic frequencies i.e. low (0.1 Hz), medium (2.0 Hz), and high (5.0 Hz) (physiological range) in the alteration of pig dermis collagen structure and its correlation with the macroscopic mechanical response of the dermis. The assessment of the collagen structure of virgin and mechanical tested specimens at tropocollagen, collagen fibril, and fiber level was performed using Fourier-transform infrared-attenuated total reflection (FTIR-ATR), atomic force microscopy (AFM), and scanning electron microscopy (SEM) respectively. After 103 cycles, a significantly higher alteration in collagen structure with discrete plastic-type damage was found for low frequency. This frequency dependent alteration of the collagen structure was found in correlation with the dermis macroscopic response. The value of inelastic strain, stress softening, damage parameter (reduction in elastic modulus), and reduction in energy dissipation were observed significantly large for slow frequency. A power-law based empirical relations, as a function of frequency and number of cycles, were proposed to predict the value of inelastic strain and damage parameter. This study also suggests that hierarchical structural response against the mechanical stimulation is time-dependent rather than cycle-dependent, may affect the tissue remodeling.

Self-assembled nanoparticle-coated interfaces: Capillary pressure, shell formation and buckling

HypothesisParticle accumulation at liquid-liquid or liquid-gas interfaces can significantly alter capillary behavior and give rise to unusual interfacial phenomena including the asymmetric macroscopic mechanical response of the interface. ExperimentsThis study explores the accumulation of cetyltrimethylammonium bromide-modified nanoparticles at fluid interfaces and the subsequent mechanical response of nanoparticle-coated droplets during contraction and expansion. Droplet tests involve the simultaneous recording of the droplet shape and the capillary pressure. Complementary single-pore experiments examine the response of particle-coated interfaces as they traverse a pore constriction. FindingsInterfaces promote order. The time-dependent nanoparticle accumulation at the interface is diffusion-controlled. The nanoparticle coated droplets can sustain negative capillary pressure before they buckle. Buckling patterns strongly depend on the boundary conditions: non-slip boundary conditions lead to crumples while slip boundary conditions result in just a few depressions. The particle-coated interface exhibits asymmetric behavior in response to particle-level capillary forces: an “oil droplet in a nanofluid bath” withstands a significantly higher capillary pressure difference than a “nanofluid droplet in an oil bath”. A first-order equilibrium analysis of interaction forces explains the asymmetric response. Single-constriction experiments show that the formation of particle-coated interfaces has a pronounced effect on fluid displacement in porous media.

Effect of particle morphology on mechanical behavior of rock mass

The effects on geotechnical properties of rock mass from the constituent particle morphology have been acknowledged since long. Particle morphology is among the most important parameters affecting the mechanical characteristics of rock, such as shear strength, cohesion, friction angle, porosity, groundwater seepage, and angle of repose. This review summarizes the current advancement in numerical simulation-related particle morphology and its impact on the mechanical behavior of rock mass. Several scholars assessed the influence of particle morphology on macroscopic mechanical response using experimental and numerical simulation techniques. The clump particle approach based on discrete elements methods (DEM) reproduces mechanisms between rock particles to illustrate the correlation of mechanical performance and particle morphology. However, this approach presents the statistical limits of particle shape and size distribution of clump particles to rock particles. Particle shape has distinct effect on geotechnical properties of rock; conversely, each descriptor is case sensitive to a specific attribute of shape. The diversity of the outcomes of the conducted study in the geotechnical field revealed an ambiguity associated with the correlation of particle size distribution with the mechanical behavior of rock and granular material. Nevertheless, further research is essential to address some of the issues that have not yet been tackled, few of which are identified throughout this review article.

Implementing a micromechanical model into a finite element code to simulate the mechanical and microstructural response of arteries

A computational strategy based on the finite element method for simulating the mechanical response of arterial tissues is herein proposed. The adopted constitutive formulation accounts for rotations of the adventitial collagen fibers and introduces parameters which are directly measurable or well established. Moreover, the refined constitutive model is readily utilized in finite element analyses, enabling the simulation of mechanical tests to reveal the influence of microstructural and histological features on macroscopic material behavior. Employing constitutive parameters supported by histological examinations, the results herein validate the model’s ability to predict the micro- and macroscopic mechanical behavior, closely matching previously observed experimental findings. Finally, the capabilities of the adopted constitutive description are shown investigating the influence of some collagen disorders on the macroscopic mechanical response of the arterial tissues.

Experimental and numerical investigation of microstructure effect on the mechanical behavior and failure process of brittle rocks

Abstract The microscopic characteristics greatly influence the nonlinear mechanical behavior and failure process of rock materials. In the present work, the experimental investigations are firstly performed including microstructural observations and macroscopic laboratory tests. Based on various experimental evidences, a micromechanical damage model is proposed according to the microscopic characteristics for the description of the local mechanical features such as mineralogical composition, plastic deformation and failure process caused by induced damage. The microstructure of the studied rocks is artificially reconstructed using Fast Fourier Transforms (FFT) based technique without meshing the complex geometry composed of clay matrix and two randomly distributed mineral inclusions. Comparisons between experimental data and numerical predictions are presented for uniaxial and triaxial compression tests. The characterization of damage evolution is visualized by the direct full field numerical simulations corresponding the macroscopic mechanical response. The effects of induced damage, the confining pressure and the overburden depth on the nonlinear mechanical behavior and failure process are schematically studied.

Glycosaminoglycan depletion increases energy dissipation in articular cartilage under high-frequency loading

High-frequency material behavior of cartilage at macroscopic lengths is not widely understood, despite a wide range of frequencies and contact lengths experienced in vivo. For example, cartilage at different stages of matrix integrity can experience high-frequency loading during traumatic impact, making high-frequency behavior relevant in the context of structural failure. Therefore, this study examined macroscopic dissipative and mechanical responses of intact and glycosaminoglycan (GAG)-depleted cartilage under previously unexplored high-frequency loading. These dynamic responses were complemented with the evaluation of quasi-static responses. A custom dynamic mechanical analyzer was used to obtain dynamic behavior, and stress relaxation testing was performed to obtain quasi-static behavior. Under high-frequency loading, cartilage energy dissipation increased with GAG depletion and decreased with strain; dynamic modulus exhibited opposite trends. Similarly, under quasi-static loading, equilibrium modulus and relaxation time of cartilage decreased with GAG depletion. The increased energy dissipation after GAG depletion under high-frequency loading was likely due to increased viscoelastic dissipation. These findings broaden our understanding of fundamental properties of cartilage as a function of solid matrix integrity in an unprecedented loading regime. They also provide a foundation for analyzing energy dissipation associated with cartilage failure induced by traumatic impact.

Microtensile properties and failure mechanisms of cortical bone at the lamellar level

Bone features a remarkable combination of toughness and strength which originates from its complex hierarchical structure and motivates its investigation on multiple length scales. Here, in situ microtensile experiments were performed on dry ovine osteonal bone for the first time at the length scale of a single lamella. The micromechanical response was brittle and revealed larger ultimate tensile strength compared to the macroscale (factor of 2.3). Ultimate tensile strength for axial and transverse specimens was 0.35 ± 0.05 GPa and 0.13 ± 0.02 GPa, respectively. A significantly greater strength anisotropy relative to compression was observed (axial to transverse strength ratio of 2.7:1 for tension, 1.3:1 for compression). Fracture surface and transmission electron microscopic analysis suggested that this may be rationalized by a change in failure mode from fibril-matrix interfacial shearing for axial specimens to fibril-matrix debonding in the transverse direction. An improved version of the classic Hashin's composite failure model was applied to describe lamellar bone strength as a function of fibril orientation. Together with our experimental observations, the model suggests that cortical bone strength at the lamellar level is remarkably tolerant to variations of fibrils orientation of about ±30°. This study highlights the importance of investigating bone's hierarchical organization at several length scales for gaining a deeper understanding of its macroscopic fracture behavior. Statement of SignificanceUnderstanding bone deformation and failure behavior at different length scales of its hierarchical structure is fundamental for the improvement of bone fracture prevention, as well as for the development of multifunctional bio-inspired materials combining toughness and strength. The experiments reported in this study shed light on the microtensile properties of dry primary osteonal bone and establish a baseline from which to start further investigations in more physiological conditions. Microtensile specimens were stronger than their macroscopic counterparts by a factor of 2.3. Lamellar bone strength seems remarkably tolerant to variations of the sub-lamellar fibril orientation with respect to the loading direction (±30°). This study underlines the importance of studying bone on all length scales for improving our understanding of bone's macroscopic mechanical response.

Influence of hierarchical porosity on the mechanical properties of porous woven composites under thermomechanical loading

The manufacturing process often results in hierarchical microstructures of porous woven composites, with pores and cracks in the matrix and fibers ranging from nanometers to hundreds of micrometers. This research is devoted to the mechanical behavior of porous woven composites with hierarchical pores of very different sizes under thermomechanical loading. A micromechanics-based damage model is developed within the continuum damage mechanics (CDM) considering the contribution of hierarchical pores for capturing the thermomechanical behaviour of porous woven composites. The predicted results of the proposed model are compared with available experimental results as well as other analytical methods to verify its correctness. The main advantages of the proposed model, comparing to the existing counterparts, are that the contributions of hierarchical pores can be examined on the material macroscopic mechanical responses, and the capability in characterizing the damage behaviour of matrix and fibers. This work can provide theoretical guidance for controlling the formation of hierarchical porosity during the manufacturing process.

Giant Single-Crystal Shape Transformation with Wide Thermal Hysteresis Actuated by Synergistic Motions of Molecular Cations and Anions.

Manipulating the collective molecular movements to implement macroscopic mechanical response of bulk material is attractive and challenging. Here, an organic-inorganic hybrid single crystal is synthesized, which exhibits a giant macroscopic shape transformation with a remarkable thermal hysteretic feature. The colossal anisotropic shape change, which manifests as an abrupt elongation of ca. 9 % along the crystallographic c-axis and a concomitant contraction of ca. 9 % in a perpendicular direction, is induced by a significant reorientation of imidazolium, accompanied with a substantial configurational variation in CuBr4 2- complex anions. The synergistic motions of both the molecular cations and anions engender a remarkable large thermal hysteresis (>30 K) in the shape transformation of the single crystal, implying that this material may play a role in alternating memory media. Furthermore, due to the stable crystal lattice, a single crystal that demonstrates naked-eye detectable large shape transformation was used as a thermal actuator to spontaneously control an electric circuit by temperature variation.

Experimental Investigation of Self-Loosening Behavior of Bolt Joints with Superelastic Shape-Memory Alloy by Macroscopic-Mechanical Response and Microscopic Evolution

AbstractThe self-loosening process of a bolted joint due to ratcheting consists of two distinct external loading stages. The first stage of self-loosening, with relatively small loading magnitude, ...