Subsequent Mechanical Response Research Articles

An integrated mechanical degradation model to explore the mechanical response of a bioresorbable polymeric scaffold

Simulation of bioresorbable medical devices is hindered by the limitations of current material models. Useful simulations require that both the short- and long-term response must be considered; existing models are not physically-based and provide limited insight to guide performance improvements. This study presents an integrated degradation framework which couples a physically-based degradation model, which predicts changes in both crystallinity (Xc) and molecular weight (Mn), with the results of a micromechanical model, which predicts the effective properties of the semicrystalline polymer. This degradation framework is used to simulate the deployment of a bioresorbable PLLA (Poly (L-lactide) stent into a mock vessel and the subsequent mechanical response during degradation under different diffusion boundary conditions representing neointimal growth. A workflow is established in a commercial finite element code that couples both the immediate and long-term responses. Clinically relevant lumen loss is reported and used to compare different responses and the effect of neo-intimal tissue regrowth post-implantation on degradation and on the mechanical response is assessed. In addition, the effects of possible changes in Xc, which could occur during processing and stent deployment, are explored.

Non-dimensionalization and scaling of fracture processes in concrete and rock

Many types of engineering structures may fracture, due to extreme external loads or due to inherent damages arising while in service or during their manufacturing/creation. The fractures may form at various scales, particularly within geological or cementitious materials like rock and concrete. Understanding and simulating accurately the fracturing processes at different scales, from laboratory to field, assists with the optimization of structural design and enhances reliability. This paper proposes novel non-dimensional scaling laws for fracture mechanics problems. They improve the prediction and interpretation of crack initiation and evolution and the subsequent mechanical response of structures. They have direct applicability in advanced numerical models like phase field models, where the governing parameters and mesh sizes can be appropriately scaled prior to conducting the numerical simulation. This leads to a unique dimensionless model output for any given loading configuration. Rescaling the unique non-dimensional output, which may incorporate varying pre-peak and post-peak fracture resistances, allows for the fracture behavior of a structure at any scale to be determined. The non-dimensionalization and rescaling techniques are demonstrated and validated against independent analytical and experimental benchmark problems. The scaling laws enable the adoption of coarse meshes in numerical models, significantly reducing computational cost while maintaining accuracy and capturing the fundamental mechanics. It is shown that the challenges associated with modeling fracture processes across scales can be mitigated as long as the problem is appropriately scaled.

Predicting moisture penetration dynamics in paper with machine learning approach

In this work, we predicted the gradient of the deformational moisture dynamics in a sized commercial paper by observing the curl deformation in response to the one-sided water application. The deformational moisture is a part of the applied liquid which ends up in the fibers causing swelling and subsequent mechanical response of the entire fiber network structure. The adapted approach combines traditional experimental procedures, advanced machine learning techniques and continuum modeling to provide insights into the complex phenomenon relevant to ink-jet digital printing in which the sized and coated paper is often used, meaning that not all the applied moisture will reach the fibers. Key material properties including elasticity, plastic parameters, viscoelasticity, creep, moisture dependent behavior, along with hygroexpansion coefficients are identified through extensive testing, providing vital data for subsequent simulation using a continuum model. Two machine learning models, a Feedforward Neural Network (FNN) and a Recurrent Neural Network (RNN), are probed in this study. Both models are trained using exclusively numerically generated moisture profile histories, showcasing the value of such data in contexts where experimental data acquisition is challenging. These two models are subsequently utilized to predict moisture profile history based on curl experimental measurements, with the RNN demonstrating superior accuracy due to its ability to account for temporal dependencies. The predicted moisture profiles are used as inputs for the continuum model to simulate the associated curl response comparing it to the experiment representing “never seen” data. The result of comparison shows highly predictive capability of the RNN. This study melds traditional experimental methods and innovative machine learning techniques, providing a robust technique for predicting moisture gradient dynamics that can be used for both optimizing the ink solution and paper structure to achieve desirable printing quality with lowest curl propensities during printing.

A Multi-Phase Modeling Framework Suitable for Dynamic Applications

Under dynamic loading conditions and the associated extreme conditions many metals will undergo phase transformations. The change in crystal structure associated with solid–solid phase transformations can significantly alter the subsequent mechanical response of the material. For the interpretation of experiments involving dynamic loading it is beneficial to have a modeling framework that captures key features of the material response while remaining relatively simple. We introduce a candidate framework and apply it to the metal tin to highlight a range of behaviors that are captured by the model. We also discuss potential extensions to capture additional behaviors that could be important for certain materials and loading scenarios. The model is useful for analysis of results from dynamic experiments and offers a point of departure for more complex model formulations.

Interaction Between Edge Dislocation and Single-Layered Graphene in Aluminum Matrix Investigated by Molecular Dynamics Simulation

The influence of graphene on dislocation movement and subsequent mechanical response of aluminum is investigated by the computational method of molecular dynamics simulation. A Lennard–Jones potential describing Al-C interaction was obtained through ab initio calculation. It was observed that the 2D graphene could reinforce Al matrix similar to the traditional Orowan mechanism. The Al/graphene interface first attract the gliding dislocation to reduce the system energy, which is unlike the grain boundary to repel gliding dislocations through pile-up mechanism. With the increase of stress, dislocation attracted and trapped at the front of graphene could glide along the interface and finally bypass it through climbing when graphene is orientated out of the shear plane. In addition, the strengthening ability of graphene is size dependent, showing a linear relationship between strength increment and graphene size.

Effect of Static Rotor Eccentricity on End Winding Forces and Vibration Wearing

In order to study the vibration wearing regularity and the strength failure point of stator end windings before and after static rotor eccentricity, the three-dimensional electromagnetic forces and the subsequent mechanical responses are studied in this paper. The electromagnetic force, stress, and deformation on the end winding of the QFSN-600-2YHG turbo-generator are calculated by the finite element method (FEM) through an electromagnetic-structure coupling. The radial vibration characteristics of the winding are verified by experiments. It shows that the vibration wearing in the same layer is more serious than that between two neighboring layers. For different layers, the interphase coils endure a larger wearing risk than the innerphase coils. Inside the same phase, the last coil along the rotating direction has the highest risk of insulation wearing. The occurrence of static rotor eccentricity will significantly increase the electromagnetic forces and the vibration amplitudes on some coils. The end-phase coil which is close to the minimum air-gap point is the most dangerous one due to the lasting overstresses and the intensified deformations.

Step-by-step texture modification through strain path change toward improvement of the hardening capacity in a twinning-induced-plasticity steel

A novel texture analysis approach is aimed to elucidate the role of strain path change in extending the superior strain hardening capacity of a twinning-induced-plasticity steel up to the large compressive imposed strains at room temperature, in lights of step-by-step texture modification. In this context, activation of different deformation mechanisms during multi-axial forging (MAF) process is fully studied using macrotexture and microtexture analyses by means of calculating Schmid factors of slip and twinning. Synergetic development of deformation twin and dislocation substructure throughout the 1st MAF pass leads to exponential strain hardening rate. This is correlated with the formation of texture components with twin-favoring (near to <001>) and slip-favoring (near to <111> and <110>) orientations in each compression of one MAF pass. Such cooperation can also be reached for larger imposed strains during the 2nd MAF pass at ∑Ԑ = 2.4. However, it results in a steady-state flow behavior through continuous dynamic recrystallization stemming from turning dislocation cells of slip-favoring grains into the new ultrafine grains. This is phenomenal as can be understandable by recognizing the orientation dependency of dislocation substructures including Taylor lattice and dislocation cells as representatives for deformation twins and cross-slip, respectively. Having grains with dislocation cell and Taylor lattice respectively is the origin of facilitating restoration-based process and twinning activation, which modifies the texture by developing a twofold configuration in the course of imposing strain path change. As the second phase of this study, a detailed analyzation of subsequent mechanical responses of MAFed materials is focused through the room-temperature tensile and compression testing methods with the emphasis on the effect of pre-test microtexture on asymmetrical behavior. The shear-punch test is also used to prove the positive effect of recrystallized grains to enhance the mechanical properties.

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.

Collagen peptide simulated bending after applied axial deformation

Structural proteins in the extracellular matrix are subjected to a range of mechanical loading conditions, including varied directions of force application. Molecular modeling suggests that these mechanical forces directly affect collagen's conformation and the subsequent mechanical response at the molecular level is complex. For example, tensile forces in the axial direction result in collagen triple helix elongation and unwinding, while perpendicular forces can cause local triple helix disruption. However, the effects of more complicated mechanical loading, such as the effect of axial pretension on collagen bending and triple helix microunfolding are unknown. In this study we used steered molecular dynamics to first model a collagen peptide under axial tension and then apply a perpendicular bending force. Axial tension causes molecular elongation and increased the subsequent perpendicular bending stiffness, but surprisingly did not increase the predicted collagen triple helix microunfolding threshold. We believe these results elucidate a key potential mechanism by which microscale mechanical loads translate from cellular and micro scales down to the nano and atomistic. Further, these data predict that cryptic force-induced collagen triple helix unwinding is axial-deformation independent, supporting the possibility that cell traction forces could be a key molecular mechanism to alter the cellular matrix microenvironment to facilitate collagen enzymatic degradation and subsequent cellular migration, such as in tumor extravasation.

Effects of surface laser treatments on microstructure, tension, and fatigue behavior of AISI 316LVM biomedical wires

Recent developments in minimally invasive surgical techniques have led to a surge in advancing alternative manufacturing routes. In order to meet the functional needs of such small devices as cardiovascular stents, and guide wires, the use of new materials and delicate geometries has increased creating a new challenge for manufacturing and machining. In this work, two sets of laser parameters leading to different laser power output were employed to study the effects of systematic changes in microstructure and mechanical characteristics of AISI 316LVM biomedical grade wires. Uniaxial tension and cyclic strain-controlled fatigue with the use of flex bending machines were used in order to evaluate the mechanical properties. Surface quality, heat affected zone (HAZ), and subsequent mechanical response were also investigated. Improvements in the fatigue behavior of laser-treated wires were obtained but depended on the choice of laser parameters. Low power Nd:YAG laser-treated AISI 316LVM annealed wires exhibited better fatigue performance in both high cycle fatigue and low cycle fatigue compared to the annealed AISI 316LVM wires. In contrast, high power Nd:YAG laser-treated annealed wires exhibited poorer fatigue performance in both high cycle and low cycle fatigue. Nd:YAG laser-treated AISI 316LVM hard wires exhibited poorer fatigue performance in both high cycle fatigue and low cycle fatigue compared to the AISI 316LVM hard wires. The effects of laser processing conditions on the microstructural features responsible for these changes in fatigue behavior are discussed.

Effect of pre-fatigue on bending behavior of 2.5D C/C composites

This study investigates the influence of pre-fatigue on the bending behavior of needle-punctured 2.5D C/C composites with density between 1.75 and 1.80g/cm3 prepared by chemical vapor deposition (CVD). Three-point bending pre-fatigue tests are carried out under applied stress/static flexural strength ratio of 60%, 70%, 80% and 90% up to one million cycles. The results show that except for the 60% sample, flexural bending strength of most samples subjected to pre-fatigue is improved significantly compared with that without pre-fatigue. Detailed investigation indicates that the subsequent bending behavior of sample is closely related to both stress level in pre-fatigue and the resultant microstructure. Under low stress level (60–70%) during pre-fatigue, the composite is characterized by a plastic deformation and a slight enhancement of bending strength ascribed to the weakened fiber/matrix interface after fatigue. After pre-fatigue with high stress level (80–90%), the composite demonstrates brittle failure mode but much higher residual strengths during subsequent bending test. The reason for this is ascribed to the variation of the microstructure in fiber/matrix interface during the pre-fatigue, which strongly affects the subsequent mechanical responses.

Chromatic Mechanical Response in 2-D Layered Transition Metal Dichalcogenide (TMDs) based Nanocomposites.

The ability to convert photons of different wavelengths directly into mechanical motion is of significant interest in many energy conversion and reconfigurable technologies. Here, using few layer 2H-MoS2 nanosheets, layer by layer process of nanocomposite fabrication, and strain engineering, we demonstrate a reversible and chromatic mechanical response in MoS2-nanocomposites between 405 nm to 808 nm with large stress release. The chromatic mechanical response originates from the d orbitals and is related to the strength of the direct exciton resonance A and B of the few layer 2H-MoS2 affecting optical absorption and subsequent mechanical response of the nanocomposite. Applying uniaxial tensile strains to the semiconducting few-layer 2H-MoS2 crystals in the nanocomposite resulted in spatially varying energy levels inside the nanocomposite that enhanced the broadband optical absorption up to 2.3 eV and subsequent mechanical response. The unique photomechanical response in 2H-MoS2 based nanocomposites is a result of the rich d electron physics not available to nanocomposites based on sp bonded graphene and carbon nanotubes, as well as nanocomposite based on metallic nanoparticles. The reversible strain dependent optical absorption suggest applications in broad range of energy conversion technologies that is not achievable using conventional thin film semiconductors.

Apical and Basal Stress Fibers have Different Roles in Mechanical Regulation of the Nucleus in Smooth Muscle Cells Cultured on a Substrate

Actin stress fibers (SFs) play important roles in cellular mechanotransduction and in regulation of various cellular functions. Stress fibers generate internal tension and contribute to physical interactions between cells and extracellular matrices. We recently found that SFs in vascular smooth muscle cells (SMCs) cultured on a two-dimensional substrate mechanically interact with cell nucleus via nuclear membrane proteins, and that the internal tension of SFs is transmitted directly to the nucleus. However, SFs exist on both the apical side and the basal side of adherent cells on a substrate, and it remains unclear whether these two types of SFs play different roles on the mechanical environment around the nucleus. Here, we investigated differences between the apical and basal stress fibers (BSFs) in SMCs by using a laser nano-scissor technique. We microdissected apical SFs running across the top surface of nucleus (actin cap fibers: ACFs) or BSFs underneath the nucleus by using a laboratory-built laser nano-scissor and observed the subsequent mechanical responses of the SFs and the nucleus. Shortening of the dissected fibers was significantly greater in the ACFs than in the BSFs. Nuclei also moved in the direction of retraction of the dissected fibers, and displacement and local deformation of the nucleus was more remarkable after the dissection of the ACFs than after that of the BSFs. ACFs mostly aligned in the major axis of the nucleus, whereas BSFs showed a weak alignment with asymmetry: the direction of BSFs was rotated clockwise by ~10° from the major axis of the nucleus. These results indicate that ACFs and BSFs play different roles in mechanical regulation of the nucleus, and that intracellular tension is transmitted to the nucleus more efficiently by ACFs. ACFs may play significant roles in controlling the intranuclear distribution of DNA through intracellular orientation and positioning of the nucleus.

The effect of grain size on the high-strain, high-strain-rate behavior of copper

Copper with four widely differing grain sizes was subjected to high-strain-rate plastic deformation in a special experimental arrangement in which high shear strains of approximately 2 to 7 were generated. The adiabatic plastic deformation produced temperature rises in excess of 300 K, creating conditions favorable for dynamic recrystallization, with an attendant change in the mechanical response. Preshocking of the specimens to an amplitude of 50 GPa generated a high dislocation density; twinning was highly dependent on grain size, being profuse for the 117- and 315-μm grain-size specimens and virtually absent for the 9.5-μm grain-size specimens. This has a profound effect on the subsequent mechanical response of the specimens, with the smaller grain-size material undergoing considerably more hardening than the larger grain-size material. A rationale is proposed which leads to a prediction of the shock threshold stress for twinning as a function of grain size. The strain required for localization of plastic deformation was dependent on the combined grain size/shockinduced microstructure, with the large grain-size specimens localizing more readily. The experimental results obtained are rationalized in terms of dynamic recrystallization, and a constitutive equation is applied to the experimental results; it correctly predicts the earlier onset of localization for the large grain-size specimens. It is suggested that the grain-size dependence of shock response can significantly affect the performance of shaped charges.

Response of Metal Matrix Laminates with Temperature-Dependent Properties

An analytical, inelastic micromechanical model, with temperature-dependent matrix properties, is employed to study metal matrix composite (MMC) laminates subjected to thermomechanical loading. The predictions are based on knowledge of the thermomechanical response of transversely isotropic, elastic graphite or silicon carbide fibers and elastic-viscoplastic, work-hardening, temperature-dependent titanium or aluminum matrix. The model is applied to predict initial yielding and thermomechanical response of silicon carbide/titanium and graphite/aluminum laminates. The results demonstrate the effect of cooling from a stress-free temperature and the mismatch of thermal and mechanical properties of the constituent phases on the laminate's subsequent mechanical response. Typical results are presented for [±45], laminates subjected to monotonic tension, cyclic tension/compression, biaxial tension, and thermal loadings. It is shown that inclusion of temperature-dependent properties has a significant influence on both the initial yield surface and the inelastic response of metal matrix composites. It is also shown that the degree of applied biaxial loading has a significant effect on the response of laminates.

Thermo-Mechanical Response Predictions for Metal Matrix Composite Laminates

An analytical micromechanical model is employed for prediction of the stress-strain response of metal matrix composite laminates subjected to thermomechanical loading. The predicted behavior of laminates is based upon knowledge of the thermomechanical response of the transversely isotropic, elastic fibers and the elastic-viscoplastic, work-hardening matrix. The method is applied to study the behavior of silicon carbide/titanium metal matrix composite laminates. The response of laminates is compared with that of unidirectional lamina. The results demonstrate the effect of cooling from a stress-free temperature and the mismatch of thermal and mechanical properties of the constituent phases on the laminate's subsequent mechanical response. Typical results are presented for a variety of laminates subjected to monotonic tension, monotonic shear and cyclic tensile/compressive loadings.

Characteristics of the contractile response of rabbit aorta produced by cromakalim in calcium-free solution.

1 The effect of potassium channel opening compounds has been investigated in the smooth muscle of rabbit aorta under Ca-free conditions. Examination of the characteristics of the response has been performed using cromakalim as the prototype compound. 2 In order of potency, Ro 31-6930, cromakalim, minoxidil sulphate and pinacidil each produced a contraction in rabbit aortic strips bathed in Ca-free MOPS-buffered physiological salt solution (PSS). In contrast, forskolin, glyceryl trinitrate and nifedipine each failed to increase tension under identical conditions. Cromakalim also evoked contraction of bovine trachealis muscle bathed in Ca-free PSS. 3. The contractile response to cromakalim in rabbit aortic strips was of delayed onset (15-20 min) and reached a plateau after approximately 120 min (1.8 g maximum with 1 microM cromakalim). No cromakalim-induced tension changes were observed in either 1 mM or 2.5 mM Ca-containing PSS. 4. Raising the [KCl] of the Ca-free PSS to 65.9 mM fully inhibited the cromakalim-induced contraction in rabbit aortic strips. In addition, pretreatment of aortic strips with the sulphonylurea glibenclamide antagonized the subsequent mechanical response to cromakalim. 5. In Ca-free PSS, cromakalim (1 microM) stimulated 42K-efflux with a time-course corresponding to the contractile event. Glibenclamide (1 microM) inhibited this cromakalim-induced 42K-efflux. 6. In sharp microelectrode studies in bovine trachealis, cromakalim (10 microM) produced a sustained membrane hyperpolarization in normal PSS. In contrast, the cromakalim-induced hyperpolarization in Ca-free PSS was not sustained. The fading of the hyperpolarization was temporally correlated with the increase in tension under these experimental conditions. 7. It is concluded that the K-channel opener-induced smooth muscle contractile response revealed in Ca-free PSS is the consequence of K-channel opening. The nature of the detailed mechanism which underlies this contractile phenomenon remains to be determined.

The effects of pre-deformation and temperature of the transformation-deformation behavior of Fe-31%Ni-0.1%C

The influence of prior deformation of austenite at room temperature upon the subsequent mechanical response of the alloy at increasing degrees of metastability is described. The experimental results indicate that the tensile strength, the mean work-hardening rate and the volume fraction of martensite transformed up to necking are primarily controlled by the temperature of deformation. However, the overall extent of uniform deformation is improved if the pre-deformed material is reloaded below its M s σ temperature.

Thermomechanical treatment of high purity 6061 aluminum

The effects of combined deformation and aging on the subsequent mechanical response of high purity 6061 aluminum have been examined. It has been shown that the yield strength of this material can be increased by an appropriate choice of thermomechanical treatment. Furthermore, this increase need not be accompanied by a reduction in the notch toughness below that typically observed in a commercially available alloy. The procedure requires that the alloy be processed to obtain a uniformly tangled dislocation structure. Subsequent aging results in enhanced nucleation and growth of the primary strengthening phase, an intermediate needle-shaped precipitate. This practice involves a four-step age-deform cycle with a total aging time of 4 h at 448 K and a total reduction of 50 pct.