Mismatch In Mechanical Properties Research Articles

Mesoscale modeling of epoxy polymer concrete under tension or bending

Epoxy polymer concrete (EPC) has high volume fraction of aggregate and extraordinary mismatch of mechanical properties between the granite aggregate and the epoxy matrix, which make the mesoscale modeling a challenging issue. This paper demonstrates how the mesoscale model of EPC is generated and employed to investigate the mechanical responses of EPC under tension and bending. By introducing rotation operation into take-and-place procedure, high aggregate volume fraction (over 75%) of irregular shaped aggregate can be achieved efficiently following various practical gradations. Based on the obtained geometrical model, a three-phase model consisting of aggregate, matrix, and interfacial transition zone (ITZ) is constructed, in which the ITZ is defined as stiff as the granite aggregate and as strong as the epoxy matrix to coordinate the deformation of the other two phases in EPC. The proposed model demonstrates good performance in finite element analysis to explore the stress–strain relation and damage development of EPC during tensile deformation.

Homogeneity Permitted Robust Connection for Additive Manufacturing Stretchable Electronics.

Interface is the Achilles' heel in flexible electronics, where slipping, delamination, or cracking resulting from the intrinsic mechanical property mismatch of different materials might fail the device or cause degradation. Robust connections with strong interfacial strength and low interfacial impedance are highly desired in all kinds of flexible devices, especially when they require stretchability. Here, a new strategy of employing homogeneity is proposed to facilely achieve robust connection without introducing a third-party interlayer. The key to this strategy is implementing different functions in the same component materials, and in this work, this strategy is demonstrated by exploring the uniformity of carbon nanotube (CNT) in thermoplastic polyurethane (TPU) elastomer. Tuning the uniformity of CNT in TPU elastomer, two kinds of CNT-TPU composites (CTCs) are fabricated. They present contrasting properties of conductivity (281 folds), Young's modulus (7.1 folds), and strain sensitivity (15.7 folds) with the same weight fraction of CNT (15 wt %), and thus they can respectively act as strain sensor and interconnector in all-CNT-based stretchable strain monitoring electronics. The interlocking effect caused by spontaneous entanglement of surface polymer chains and the reconstruction of percolation network at the interface during drying confer the connection with robustness. The practical value of this strategy has been exemplified by the fabrication of a robust epidermis device via additive manufacturing. This epidermis device is patterned in a fractal motif for conformal contact and can sensitively monitor body motions and radial artery pulse.

Flexible, diamond-based microelectrodes fabricated using the diamond growth side for neural sensing

Diamond possesses many favorable properties for biochemical sensors, including biocompatibility, chemical inertness, resistance to biofouling, an extremely wide potential window, and low double-layer capacitance. The hardness of diamond, however, has hindered its applications in neural implants due to the mechanical property mismatch between diamond and soft nervous tissues. Here, we present a flexible, diamond-based microelectrode probe consisting of multichannel boron-doped polycrystalline diamond (BDD) microelectrodes on a soft Parylene C substrate. We developed and optimized a wafer-scale fabrication approach that allows the use of the growth side of the BDD thin film as the sensing surface. Compared to the nucleation surface, the BDD growth side exhibited a rougher morphology, a higher sp3 content, a wider water potential window, and a lower background current. The dopamine (DA) sensing capability of the BDD growth surface electrodes was validated in a 1.0 mM DA solution, which shows better sensitivity and stability than the BDD nucleation surface electrodes. The results of these comparative studies suggest that using the BDD growth surface for making implantable microelectrodes has significant advantages in terms of the sensitivity, selectivity, and stability of a neural implant. Furthermore, we validated the functionality of the BDD growth side electrodes for neural recordings both in vitro and in vivo. The biocompatibility of the microcrystalline diamond film was also assessed in vitro using rat cortical neuron cultures.

3D printing of high-strength, porous, elastomeric structures to promote tissue integration of implants.

Despite advances in biomaterials research, there is no ideal device for replacing weight-bearing soft tissues like menisci or intervertebral discs due to poor integration with tissues and mechanical property mismatch. Designing an implant with a soft and porous tissue-contacting structure using a material conducive to cell attachment and growth could potentially address these limitations. Polycarbonate urethane (PCU) is a soft and tough biocompatible material that can be 3D printed into porous structures with controlled pore sizes. Porous biomaterials of appropriate chemistries can support cell proliferation and tissue ingrowth, but their optimal design parameters remain unclear. To investigate this, porous PCU structures were 3D-printed in a crosshatch pattern with a range of in-plane pore sizes (0 to 800 μm) forming fully interconnected porous networks. Printed porous structures had ultimate tensile strengths ranging from 1.9 to 11.6 MPa, strains to failure ranging from 300 to 486%, Young's moduli ranging from 0.85 to 12.42 MPa, and porosity ranging from 13 to 71%. These porous networks can be loaded with hydrogels, such as collagen gels, to provide additional biological support for cells. Bare PCU structures and collagen-hydrogel-filled porous PCU support robust NIH/3T3 fibroblast cell line proliferation over 14 days for all pore sizes. Results highlight PCU's potential in the development of tissue-integrating medical implants.

Effect of mismatch in mechanical properties on interfacial strength of aluminum alloy/steel dissimilar joints

The strength of a dissimilar joint between aluminum and steel is generally dependent on the thickness of the intermetallic compound (IMC) layer. Mechanical factors such as the residual stress and stress field under loading also likely contribute to the fracture behavior of the joint. However, a clear understanding of the effects of the interfacial structure (i.e., the IMC layer thickness) and mechanical factors on joint strength is lacking in the existing literature. The purpose of this study was to elucidate the influence of both the IMC layer thickness and the difference in the strengths of dissimilar metals on the interfacial strength of aluminum/steel joints through experimental evaluations and finite element (FE) analyses. The relationship between the strength of a friction-stir-welded joint of 6061 aluminum alloy (A6061) and 780-MPa grade high-tensile-strength steel (HT780) and the IMC layer thickness was investigated by controlling the IMC growth promoted by the annealing process. The results showed that the joint strength decreased with increasing IMC layer thickness. Fracture mainly occurred at the joint interface for a thin IMC layer of approximately 100 nm, whereas the joint between A6061 and 304 stainless steel, with a lower proof stress than HT780, showed a higher joint strength that resulted in fracture in the A6061 alloy. The FE analysis revealed that a high maximum principal stress is applied near the interface due to the difference in the degree of deformation between steel and aluminum, a behavior that is likely to appear in any joint consisting of materials with considerably different strengths.

Internal short circuit and failure mechanisms of lithium-ion pouch cells under mechanical indentation abuse conditions：An experimental study

Electromechanical structural integrity and thermal stability dictate the safety performance of lithium-ion batteries. Progressive deformation and failure across microscopic and macroscopic lengths scales that are responsible for internal short circuit (ISC) in lithium-ion cells under mechanical abuse conditions remains elusive. In this study, a series of indentation tests were conducted on lithium-ion cells with different capacities up to the occurrence of ISC. The external response and internal configuration of these cells were investigated. It is discovered that cells with different capacities and state of charges exhibited different behaviors. Maximum temperature, which is often regarded as the most important parameter related to thermal runaway (TR), varied considerably due to the complicated contact configurations. X-ray computed tomography (XCT) showed that ISC was a collective result of shear band or other strain-localization modes in the electrode assembly, shear offsets in the granular coatings of electrodes, and the accompanying ductile fracture in the metal foils. We believe that the irregular strain-localization modes (kinks, cusps, and buckles), radical mismatches in mechanical properties of different layers, and geometric features of the indenter eventually lead to the tearing/puncture of cell separator at various locations. The results could provide useful guidance for the micromechanical modeling of lithium-ion cells.

Two stress rupture modes observed in Alloy 617–9%Cr dissimilar welded joint

The fracture transition behavior during the stress rupture test for Alloy 617–9%Cr dissimilar welded joint was investigated in the present study. The results showed that the specimen with applied stress of 240 MPa at 566 °C ruptured at the heat-affected zone of 9%Cr steel (9%Cr-HAZ), while the rupture location transferred to the interface between the weld metal (WM) and 9%Cr-HAZ with the decreased applied stress. The rupture behavior occurred in the 9%Cr-HAZ was caused by type IV crack due to the matrix softening. The notch was primarily formed in the interface due to the difference of oxidation resistance and mismatch of mechanical properties between the WM and 9%Cr-HAZ, and therefore the cracks initiated and propagated along the interface with the continuous load, eventually leading to the interface failure.

Biomimetic surgical mesh to replace fascia with tunable force-displacement

Here we mimic the mechanical properties of native fascia to design surgical mesh for fascia replacement. Despite the widespread acceptance of synthetic materials as tissue scaffolds for pelvic floor disorders, mechanical property mismatch between mesh and adjacent native tissue drives fibrosis and erosion, leading the FDA to remove several surgical meshes from the market. However, autologous tissue does not induce either fibrosis or adjacent tissue erosion, suggesting the potential for biomimetic surgical mesh. In this study, we determined the design rules for mesh that mimics native fascia by mathematically modeling multi-component polymer networks, composed of elastin-like and collagen-like fibers, using a spring-network model. To validate the model, we measured the stress-strain curves of native bovine and nonhuman primate (Macaca mulatta) abdominal fascia in both toe and linear regions. We find that the stiffer collagen-like fibers must remain limp until the elastin-like fibers extend to the initial length of spanning collagen-like fibers under uniaxial tension. Comparing model results to experiment determines the product of fiber volume fraction and elastic modulus, a critical design parameter. Dual fiber mesh with mechanical properties that mimic fascia are feasible. These results have broad application to a wide range of soft tissue replacements including ~200,000 surgeries/year for pelvic floor disorders, because standard-of-care mesh contain only stiffer polymers that behave more like collagen than native tissue.

Fabrication and Characterization of Pectin Hydrogel Nanofiber Scaffolds for Differentiation of Mesenchymal Stem Cells into Vascular Cells.

Despite significant progress over the past few decades, creating a tissue-engineered vascular graft with replicated functions of native blood vessels remains a challenge due to the mismatch in mechanical properties, low biological function, and rapid occlusion caused by restenosis of small diameter vessel grafts (<6 mm diameter). A scaffold with similar mechanical properties and biocompatibility to the host tissue is ideally needed for the attachment and proliferation of cells to support the building of engineered tissue. In this study, pectin hydrogel nanofiber scaffolds with two different oxidation degrees (25 and 50%) were prepared by a multistep methodology including periodate oxidation, electrospinning, and adipic acid dihydrazide crosslinking. Scanning electron microscopy (SEM) images showed that the obtained pectin nanofiber mats have a nano-sized fibrous structure with 300-400 nm fiber diameter. Physicochemical property testing using Fourier transform infrared (FTIR) spectra, atomic force microscopy (AFM) nanoindentations, and contact angle measurements demonstrated that the stiffness and hydrophobicity of the fiber mat could be manipulated by adjusting the oxidation and crosslinking levels of the pectin hydrogels. Live/Dead staining showed high viability of the mesenchymal stem cells (MSCs) cultured on the pectin hydrogel fiber scaffold for 14 days. In addition, the potential application of pectin hydrogel nanofiber scaffolds of different stiffness in stem cell differentiation into vascular cells was assessed by gene expression analysis. Real-time polymerase chain reaction (RT-PCR) results showed that the stiffer scaffold facilitated the differentiation of MSCs into vascular smooth muscle cells, while the softer fiber mat promoted MSC differentiation into endothelial cells. Altogether, our results indicate that the pectin hydrogel nanofibers have the capability of providing mechanical cues that induce MSC differentiation into vascular cells and can be potentially applied in stem cell-based tissue engineering.

Micromechanical investigation of the role of percolation on ductility enhancement in metallic glass composites

In contrast to the catastrophic failure arising from the unconstrained shear band propagation in monolithic bulk metallic glasses (BMGs), the introduction of second phases in BMG composites (BMGCs) provides an extrinsic way of reducing the magnitude of strain localization, and thus may significantly improve the ductility. For this mechanism to be effective, a multitude of experiments have found that the volume fraction should reach a rather high threshold of about 30–40%, corresponding to the percolation limit with variations depending on the second phase morphology. However, we demonstrate here that an enhanced ductility can be realized in a BMGC made from crystalline Ni foam (completely percolated even with a volume fraction of about 11%), suggesting the resistance in the second phase to the impinging shear-force dipole from neighboring shear bands be the dominant factor for such a ductility improvement. The shortcomings of percolation approach, as well as the synergistic effects of microstructure and mismatches in mechanical properties, are investigated here in a thermo-mechanical finite element method based on the free volume model and plasticity-induced heating. The effectiveness of ductility improvement will be demonstrated by calculating the probability distributions of the shear-band effective strain.

Conductive Silk-Based Composites Using Biobased Carbon Materials.

There is great interest in developing conductive biomaterials for the manufacturing of sensors or flexible electronics with applications in healthcare, tracking human motion, or in situ strain measurements. These biomaterials aim to overcome the mismatch in mechanical properties at the interface between typical rigid semiconductor sensors and soft, often uneven biological surfaces or tissues for in vivo and ex vivo applications. Here, the use of biobased carbons to fabricate conductive, highly stretchable, flexible, and biocompatible silk-based composite biomaterials is demonstrated. Biobased carbons are synthesized via hydrothermal processing, an aqueous thermochemical method that converts biomass into a carbonaceous material that can be applied upon activation as conductive filler in composite biomaterials. Experimental synthesis and full-atomistic molecular dynamics modeling are combined to synthesize and characterize these conductive composite biomaterials, made entirely from renewable sources and with promising applications in fields like biomedicine, energy, and electronics.

Skin Aging &amp; Modern Edge Anti-Aging Strategies

Skin Aging &amp; Modern Edge Anti-Aging Strategies

Small-caliber vascular grafts based on a piezoelectric nanocomposite elastomer: Mechanical properties and biocompatibility

The development of small-caliber grafts still represents a challenge in the field of vascular prostheses. Among other factors, the mechanical properties mismatch between natural vessels and artificial devices limits the efficacy of state-of-the-art materials. In this paper, a novel nanocomposite graft with an internal diameter of 6 mm is proposed. The device is obtained through spray deposition using a semi-interpenetrating polymeric network combining poly(ether)urethane and polydimethilsyloxane. The inclusion of BaTiO3 nanoparticles endows the scaffold with piezoelectric properties, which may be exploited in the future to trigger beneficial biological effects. Graft characterization demonstrated a good nanoparticle dispersion and an overall porosity that was not influenced by the presence of nanoparticles. Graft mechanical properties resembled (or even ameliorated) the ones of natural vessels: both doped and non-doped samples showed a Young's modulus of ∼700 kPa in the radial direction and ∼900 kPa in the longitudinal direction, an ultimate tensile strength of ∼1 MPa, a strain to failure of ∼700%, a suture retention force of ∼1.7 N and a flexural rigidity of ∼2.5 × 10−5 N m2. The two grafts differed in terms of burst strength that resulted ∼800 kPa for the control non-doped samples and ∼1100 kPa for the doped ones. The graft doped with BaTiO3 nanoparticles showed a d33 coefficient of 1.91 pm/V, almost double than the non-doped control. The device resulted highly stable, with a mass loss smaller than 2% over 3 months and an excellent biocompatibility.

Effect of Post-weld Heat Treatment on Microstructure and Mechanical Properties of Dissimilar Metal Weld Used in Power Plants

High base metal dilution (50 pct) dissimilar metal weld between A508 low-alloy steel and 309L clad layer was fabricated to investigate the effect of extended post-weld heat treatment (PWHT) on microstructure and mechanical properties. Extended PWHT at 607 °C caused significant carbon migration and microstructural change across the fusion boundary. Following PWHT at 607 °C for 20 hours, the carbon-enriched zone exhibited little to no reduction in hardness although 20 pct hardness reduction occurred in the remainder of the first butter layer. This is because tempering of the martensite in the carbon-enriched zone was entirely offset by the formation of a high density of chromium carbides. In contrast, following PWHT at 607 °C for 20 hours the average hardness of the carbon-depleted zone decreased from 241 HV to 169 HV due to the elimination of martensite, ferrite grain coarsening and carbon depletion. In addition, the extended PWHT significantly enhanced the mismatch in mechanical properties between the carbon-enriched zone (351 HV) and the adjacent carbon-depleted zone (169 HV). The formation of chromium carbides (less than 200 nm in size) in the carbon-enriched zone reduced the carbon concentration in the matrix, generating a continued driving force for carbon migration from A508 steel to the weld metal during PWHT.

Effect of interface mechanical discontinuities on scaffold-cartilage integration.

A consistent lack of lateral integration between scaffolds and adjacent articular cartilage has been exhibited in vitro and in vivo. Given the mismatch in mechanical properties between scaffolds and articular cartilage, the mechanical discontinuity that occurs at the interface has been implicated as a key factor, but remains inadequately studied. Our objective was to investigate how the mechanical environment within a mechanically loaded scaffold-cartilage construct might affect integration. We hypothesized that the magnitude of the mechanical discontinuity at the scaffold-cartilage interface would be related to decreased integration. To test this hypothesis, chondrocyte seeded scaffolds were embedded into cartilage explants, pre-cultured for 14 days, and then mechanically loaded for 28 days at either 1N or 6N of applied load. Constructs were kept either peripherally confined or unconfined throughout the duration of the experiment. Stress, strain, fluid flow, and relative displacements at the cartilage-scaffold interface and within the scaffold were quantified using biphasic, inhomogeneous finite element models (bFEMs). The bFEMs indicated compressive and shear stress discontinuities occurred at the scaffold-cartilage interface for the confined and unconfined groups. The mechanical strength of the scaffold-cartilage interface and scaffold GAG content were higher in the radially confined 1N loaded groups. Multivariate regression analyses identified the strength of the interface prior to the commencement of loading and fluid flow within the scaffold as the main factors associated with scaffold-cartilage integration. Our study suggests a minimum level of scaffold-cartilage integration is needed prior to the commencement of loading, although the exact threshold has yet to be identified. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res.

Characterization of the Interfacial Toughness in a Novel “GaN-on-Diamond” Material for High-Power RF Devices

GaN thin film integrated to polycrystalline diamond substrates is a novel microwave transistor material with significantly improved heat dissipation capability. Due to the thermal and mechanical properties mismatch between GaN and diamond, a natural concern arises in terms of its interfacial stability as currently there is no established method to evaluate the interfacial toughness in GaN-on-diamond material. Using three generations of “GaN-on-Diamond” materials with varying process parameters, a comprehensive study has been carried out to identify the most appropriate fracture mechanics-based methods for reliable evaluation of the interfacial toughness in this novel material system. Several techniques were assessed, and the results are cross-compared; these include an ex situ nanoindentation induced buckling method and two-step indentation approach together with several analytical models. Additionally, a microcantilever bending method was adopted to measure an upper bound for the interfacial fracture toughness. For the three generations of materials, the interfacial toughness, GIc, was determined to be 0.7, 0.9, and 0.6 J·m–2, respectively. Postmortem analysis of the micro- and nanostructure of fractured interfaces indicated that the systems with better heat spreading capability displayed smoother fracture surfaces, i.e., were more brittle due to the lack of active toughening mechanisms. Potential modifications to the interface for improved mechanical stability were proposed based on the experimental results.

Fabrication and modification of wavy multicomponent vascular grafts with biomimetic mechanical properties, antithrombogenicity, and enhanced endothelial cell affinity.

A mismatch of mechanical properties and a high rate of thromboses are two critical challenges of creating viable artificial small-diameter vascular grafts (SDVGs). Herein, we propose a method to fabricate wavy multicomponent vascular grafts (WMVGs) via electrospinning using an assembled rotating collector. The WMVGs consisted of a wavy silk/poly(lactic acid) (PLA) inner layer and a thermoplastic polyurethane (TPU) outer layer, which mimic the structures and properties of collagen and elastin in native blood vessels, respectively. Attributed to the wavy structure and the combination of rigid silk/PLA and elastic TPU biomaterials, WMVGs are capable of mimicking the nonlinear tensile stress-strain relationship and "toe region" of native blood vessels. In addition, they have sufficient mechanical strength to meet implantation requirements in terms of tensile strength, suture retention, and burst pressure. Further modification of silk/PLA fibers with dopamine and heparin gave the grafts antithrombogenic properties and greatly enhanced endothelial cell affinities. Human umbilical vein endothelial cells (HUVECs) cultured on modified silk/PLA showed high viability, high proliferation rate, and favorable cell-substrate interactions. Moreover, HUVECs were able to fully cover and freely migrate upward on the lumen of the modified WMVGs without needing a special circulation bioreactor. Therefore, the modified WMVGs possessed biomimetic properties, antithrombogenicity, and enhanced endothelialization, making them a promising candidate for SDVGs. © 2019 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater 107B: 2397-2408, 2019.

Magnetomechanical model for coating/substrate interface and its application in interfacial crack propagation length characterization

During the service process of remanufactured components, the stress and crack may occur at the interface between coating and substrate due to their mechanical property mismatch. Based on the spontaneous magnetization phenomenon in ferromagnetic materials, the stress distribution and crack propagation length along the interface can be characterized. In this paper, the magnetomechanical constitutive model for interface is established according to Timoshenko beam theory and Jiles' stress-magnetization model. The distributions of stress and residual magnetization along the interface are analyzed under the effect of typical three-point bending (TPB) load. The results show that the interfacial residual magnetization Mr can reflect the stress distribution very well and its peak value at supporting seat Mr3 is closely related to interfacial crack propagation length. In order to verify the theoretical model, the metal magnetic memory (MMM) technique is used to collect the residual magnetic field along the interface in the TPB testing. The experimental results are consistent with the theoretical predictions. They indicate that the magnetomechanical model for coating/substrate interface established in this paper can provide the theoretical support for magnetic non-destructive testing and the calculation results can be applied to the interfacial crack propagation length characterization in MMM.

Skin aging as a mechanical phenomenon: The main weak links.

From a mechanical point of view, human skin appears as a layered composite containing the stiff thin cover layer presented by the stratum corneum, below which are the more compliant layers of viable epidermis and dermis and further below the much more compliant adjacent layer of subcutaneous white adipose tissue (sWAT). Upon exposure to a strain, such a multi-layer system demonstrates structural instabilities in its stiffer layers, which in its simplest form is the wrinkling. These instabilities appear hierarchically when the mechanical strain in the skin exceeds some critical values. Their appearance is mainly dependent on the mismatch in mechanical properties between adjacent skin layers or between the skin and sWAT, on the adhesive strength and thickness ratios between the layers, on their bending and tensile stiffness as well as on the value of the stress existing in single layers. Gradual reduction of elastic fibers in aging significantly reduces the skin’s ability to bend, prompting an up to 4-fold reduction of its stability against wrinkling, thereby explaining the role of these fibers in skin aging. While chronological and extrinsic aging differently modify these parameters, they lead to the same end result, reducing the critical strain required for the onset of instabilities. Comparing of mechanical properties of the skin presented as a bi-, tri- or tetra-layer structure demonstrates the particular importance of the papillary dermis in skin aging and provides the arguments to consider the undulations on the dermal-epidermal and dermal-sWAT interfaces as the result of mechanical bifurcation, leading to structural instabilities inside of the skin. According to this model, anti-aging strategies should focus not as much on the reinforcement of the dermis, but rather aim to treat the elastic mismatch between different adjacent layers in the skin and sWAT as well as the adhesion between these layers.

Global Sensitivity Analysis for the Elastic Properties of Unidirectional Carbon Fibre Reinforced Composites Based on Metamodels

A fast and effective numerical method to predict mechanical properties of carbon fibre reinforced polymer (CFRP) composites, even elastic properties, is complicated due to the mismatch of mechanical properties among the constituents. Furthermore, it is not possible to completely characterise the influence of multiple parameters including mechanical and structural parameters on the bulk properties of CFRP by experiments. In this study, a three-phase finite-element model consisting of matrix, carbon fibre and interface was developed to predict the elastic mechanical behaviour of unidirectional CFRP. The elastic properties in terms of two Young's moduli, two Poisson's ratios and a shear modulus were calculated by means of a homogenisation method. High-accuracy Kriging surrogate models were constructed to fast-calculate the elastic responses for a large number of samples. Combining Kriging and high-dimensional model representation (HDMR) methods, a global sensitivity analysis was performed to study how the microscopic parameters influence the elastic responses to get a deeper understanding of elastic property-structure relationship. Eleven parameters, including mechanical and geometry properties of constituent phases, were chosen as inputs. Independent and cooperative effects of input parameters on the elastic properties of the studied composites were surveyed via first- and second-order sensitivity indices, respectively. An importance ranking of these parameters for each elastic response was derived directly by these indices. The procedure proposed in this work could serve as a theoretical guide for further design optimisation of CFRP.