Modulus Gradient Research Articles

Prediction of Field Aging Gradient in Asphalt Pavements

The aging of asphalt pavements is a key factor that influences pavement performance. Aging can be characterized by laboratory tests and prediction models. Common aging prediction models use the change of physical or chemical properties of asphalt binders based on regression techniques or aging reaction kinetics. The objective of this study was to develop a kinetics-based aging prediction model for the mixture modulus gradient in asphalt pavements to study long-term in-service aging. The proposed model was composed of three submodels for baseline modulus, surface modulus, and aging exponent to define the change of the mixture modulus with pavement depth. The model used kinetic parameters (aging activation energy and preexponential factor) of asphalt mixtures and combined the two reaction rate periods (fast-rate and constant-rate). Laboratory-measured modulus gradients of 29 field cores at different ages were used to determine the model parameters. The laboratory testing condition was converted to the field condition at a given age and corresponding temperature by introducing the rheological activation energy to quantify the temperature dependence of field cores at each age. The end of the fast-rate period or the beginning of the constant-rate period was accurately identified to model these two periods and to determine the associated parameters separately. The results showed that the predictions matched well with the measurements and the calculated model parameters were verified. The proposed aging prediction model took into account the major factors that affect field aging speed of an asphalt pavement, such as the binder type, aggregate type, air void content, pavement depth, aging temperature, and aging time.

Nano-Mechanical Studies on Polyglactin Sutures Subjected to In Vitro Hydrolytic and Enzymatic Degradation.

An experimental investigation on the effects of in vitro hydrolytic and enzymatic degradation on mechanical properties of polyglactin 910 monofilament sutures was performed by conducting nanoindentation studies using an atomic force microscope (AFM). For hydrolytic degradation, the sutures were incubated in phosphate buffered saline (PBS) solution at three different pH conditions, 5, 7.4, and 10. For enzymatic degradation, esterase was employed at pH condition of 7.4. The property of the sutures changed with time at different conditions were investigated by nanoindentation, tensile test experiments, image analysis using both of scanning electron microscopy (SEM) and AFM, and also Fourier transform infrared spectroscopy (FTIR). The effects of degradation on gradation of Young's modulus values across the cross section of the sutures were studied by doing progressive nanoindentation from center to surface. FTIR studies revealed the formation of new hydroxyl bonds due to both hydrolytic and enzymatic degradations. Nanoindentation results indicated that the degradation does not cause a gradient of Young's modulus of the polyglactin 910 monofilament sutures across the cross section from center to surface at different degradation times for both hydrolytic and enzymatic degradations. However, in general, the Young's modulus of all samples was decreased over 4 weeks of degradation. The microscopic evaluation of the samples also showed both qualitative changes in surface morphology and quantitative changes in surface roughness on the surface of degraded sutures. This study provided a deep understanding of the polyglactin sutures subjected to in vitro hydrolytic and enzymatic degradation, and also opened a new avenue to study the biomaterials at nano-scale.

Characterization of strength and damage of hard rock pillars using a synthetic rock mass method

In this research a 3D Synthetic Rock Mass (SRM) method is used to numerically characterize the strength and damage of hard rock pillars. The SRM is an integrated model incorporating a Discrete Fracture Network (DFN) within a Particle Flow Code 3D (PFC3D) particle assembly. Based on the numerical results of a joint-free pillar model, laterally fixed loading platens are suggested to simulate uniaxial compression tests on rock pillars. An internal-strain loading method is meanwhile used to ensure more realistic model behaviour. The peak strength, post-peak strain-softening gradient and deformation modulus of a series of jointed pillar models are then quantified, in order to investigate the effects of the inserted joint sets. The simulated peak strengths demonstrate a U-shape relationship when the joint sets are rotated; the peak strength also decreases with increasing joint size. A brittle post-peak behaviour is observed for pillar models with vertical joint sets of low persistence, the post-peak behaviour becoming more ductile when the joint sets are inclined and of higher persistence. A correlation is identified between the post-peak pillar behaviour and the simulated tensile cracking events, where a brittle post-peak corresponds to a high cracking rate. The effects of the joint sets on the pillar deformation modulus are observed to be similar to the effects on the pillar peak strength. Particular attention is given to the characterization of the crack initiation stress (σci) and crack damage stress (σcd) thresholds of each pillar model, where the ratio of the crack initiation stress/peak strength is between 0.3 and 0.45, and the ratio of the crack damage stress/peak strength is between 0.75 and 0.98. The simulated cracks are compared between the jointed pillars and detailed cracking modes are plotted as 3D views and as 2D thin layers for selected pillar models.

Fiber bias effect on characterization of carbon fiber-reinforced polymer composites by nanoindentation testing and modeling

Fiber bias effect on characterization of carbon fiber-reinforced polymer composites by nanoindentation testing has been investigated by computer modeling and validated by experiments. IM7/PEEK composite was selected as a case study material. Nanoindentation tests using Berkovich indenter were carried out on fiber longitudinal direction and in the near fiber region of the matrix to attempt to determine the nanomechanical properties of the fibers and the interphase. A 3D finite element analysis model for simulating nanoindentation was developed, taking into account the carbon fibers’ transversely isotropic properties. A fiber bias effect inducing a gradient in modulus in fiber–matrix interphase region was observed from tests and confirmed by modeling, and this property transition thickness induced by the fiber bias is about 1–2 µm depending on the indentation depth, which suggests an inherent difficulty when attempting to determine the true interphase properties experimentally using the nanoindentation testing.

An assessment of the effective density fluid model for backscattering from rough poroelastic interfaces

The effective density fluid model (EDFM) was developed to approximate the behavior of sediments governed by Biot’s theory of poroelasticity. Previously, it has been shown that the EDFM predicts reflection coefficients and backscattering strengths that are in close agreement with those of the full Biot model for the case of a homogeneous poroelastic half-space. However, it has not yet been determined to what extent the EDFM can be used in place of the full Biot model for other cases. In this work, the finite element method is used to compare the backscattering strengths predicted using the EDFM with the predictions of the full Biot model for three cases: a homogeneous poroelastic half-space with a rough interface, a poroelastic layer overlying an elastic half-space with both interfaces rough, and an inhomogeneous poroelastic half-space consisting of a shear modulus gradient with a rough interface. [Work supported by ONR, Ocean Acoustics.]

Modulus evolution of asphalt pavement based on full-scale accelerated pavement testing with Mobile Load Simulator 66

A full-scale accelerated pavement testing was conducted with Mobile Load Simulator 66 in Chongming Island, Shanghai, to study the performance response of a semi-rigid base asphalt pavement with fine-sand subgrade designed for the G40 construction. As an index sensitive to temperature and load, three different moduli including the seismic modulus, field falling weight deflectometer back-calculation modulus and interior material testing system resilient modulus were all measured during and after the test. The modulus evolution of asphalt pavement with the loading applications, rutting, strain and temperature, and the comparison of three methods for measuring modulus were analysed. There exists a modulus gradient from top to bottom, which can be considered in the pavement design or fatigue life prediction. Because of the strain level differences, the seismic modulus was the largest, followed by back-calculation and resilient modulus; the development of seismic and back-calculation modulus better reflected the anti-deformation changes.

Electromechanical study of polyurethane films with carbon black nanoparticles for MEMS actuators

Pure polyurethane and nanocomposite carbon black (CB) polyurethane solutions were deposited by spin-coating on a silicon substrate using gold as the adhesion layer and electrode. Different test structures were achieved for electrical and mechanical characterizations. The incorporation of CB nanoparticles in the polyurethane matrix has a significant influence on the dielectric permittivity of the material with an increase of about one third of its value. The Young's modulus of PU and nanocomposite PU films was determined by different characterization methods. Nanoindentation experiments have pointed out a Young's modulus gradient through the film thickness. By performing mechanical tests (tensile, bulge, point deflection) on freestanding films, an average Young's modulus value of about 30 MPa was found as well as a residual stress value of about 0.4 MPa. However, no influence of the presence of the nanoparticles was found. Finally, several MEMS actuators were realized and characterized. At their fundamental resonance frequency, the actuation of the nanocomposite membranes is more efficient than that of pure polyurethane. However, the time constant of the material seems to provide a major barrier for the development of high-frequency PU-based micro-actuators.

Distribution of Young's Modulus in Porcine Corneas after Riboflavin/UVA-Induced Collagen Cross-Linking as Measured by Atomic Force Microscopy

Riboflavin/UVA-induced corneal collagen cross-linking has become an effective clinical application to treat keratoconus and other ectatic disorders of the cornea. Its beneficial effects are attributed to a marked stiffening of the unphysiologically weak stroma. Previous studies located the stiffening effect predominantly within the anterior cornea. In this study, we present an atomic force microscopy-derived analysis of the depth-dependent distribution of the Young's modulus with a depth resolution of 5 µm in 8 cross-linked porcine corneas and 8 contralateral controls. Sagittal cryosections were fabricated from every specimen and subjected to force mapping. The mean stromal depth of the zone with effective cross-linking was found to be 219±67 µm. Within this cross-linked zone, the mean Young's modulus declined from 49±18 kPa at the corneal surface to 46±17 kPa, 33±11 kPa, 17±5 kPa, 10±4 kPa and 10±4 kPa at stromal depth intervals of 0–50 µm, 50–100 µm, 100–150 µm, 150–200 µm and 200–250 µm, respectively. This corresponded to a stiffening by a factor of 8.1 (corneal surface), 7.6 (0–50 µm), 5.4 (50–100 µm), 3.0 (100–150 µm), 1.6 (150–200 µm), and 1.5 (200–250 µm), when compared to the Young's modulus of the posterior 100 µm. The mean Young's modulus within the cross-linked zone was 20±8 kPa (2.9-fold stiffening), while it was 11±4 kPa (1.7-fold stiffening) for the entire stroma. Both values were significantly distinct from the mean Young's modulus obtained from the posterior 100 µm of the cross-linked corneas and from the contralateral controls. In conclusion, we were able to specify the depth-dependent distribution of the stiffening effect elicited by standard collagen cross-linking in porcine corneas. Apart from determining the depth of the zone with effective corneal cross-linking, we also developed a method that allows for atomic force microscopy-based measurements of gradients of Young's modulus in soft tissues in general.

Gradient composite materials for artificial intervertebral discs.

Composites with the gradient of Young's modulus constitute a new group of biomimetic materials which affect the proper distribution of stresses between the implant and the bone. The aim of this article was to examine the mechanical properties of gradient materials based on carbon fibre-polysulfone composite, and to compare them to the properties of a natural intervertebral disc. Gradient properties were provided by different orientation or volume fraction of carbon fibres in particular layers of composites. The results obtained during in vitro tests displayed a good durability of the gradient materials put under long-term static load. However, the configuration based on a change in the volume fraction of the fibres seems more advantageous than the one based on a change of the fibres' orientation. The materials under study were designed to replace the intervertebral disc. The effect of Young's modulus of the material layers on the stress distribution between the tissue and the implant was analyzed and the biomimetic character of the gradient composites was stated. Unlike gradient materials, the pure polysulfone and the non-gradient composite resulted in the stress concentration in the region of nucleus pulposus, which is highly disadvantageous and does not occur in the stress distribution of natural intervertebral discs.

Optimizing Stem Cell Engineering for Orthopaedic Applications

Optimizing Stem Cell Engineering for Orthopaedic Applications

Dynamics of Bio-Inspired Pressure Generation

The capability of plant cells to establish osmotic gradients allows them to actuate structures such as guard cells and the pulvini responsible for the swift nastic motion of mimosa pudica. The work presented here investigated the dynamics of osmotically-driven pressure generation in membrane-bound compartments, taking into account volume expansion, solute dilution, surface area to volume ratio, membrane hydraulic permeability, and changes in osmotic gradient and bulk modulus. We developed and experimentally validated a numerical simulation tool for characterizing and predicting the dynamics of pressure generators as a function of parameters that will be encountered in most systems that generate pressure osmotically. Finally, we demonstrate two applications of an osmosis-based pressure generator: actuation of a soft robot and continuous volume delivery over long periods of time.

THE EFFECTS OF ELASTIC STIFFENING ON THE EVOLUTION OF THE STRESS FIELD WITHIN A SPHERICAL ELECTRODE PARTICLE OF LITHIUM-ION BATTERIES

Poor cyclic performance of lithium-ion batteries is calling for efforts to study its capacity attenuation mechanism. The internal stress field produced in the lithium-ion battery during its charging and discharging process is a major factor for its capacity attenuation, research on it appears especially important. We established an electrochemical –mechanical coupling model with the consideration of the influence of elastic stiffening on diffusion for graphite anode materials. The results show that the inner stress field strongly depends on the lithium-ion concentration field, greater concentration gradients lead to greater stresses. The evolution of the stress field is similar to that of the concentration gradient but lags behind it, which shows hysteresis phenomenon. Elastic stiffening can lower the concentration gradient and increase elastic modulus, which are two major factors influencing the inner stress field. We conclude that the latter is more dominant compared to the former, and elastic stiffening acts to increasing the internal stress.

Effects of particle/matrix interfaces on the mechanical properties for SiCp or YAl2p reinforced Mg–Li composites

YAl2p or SiCP reinforced Mg–14Li–3Al (LA143) matrix composites were prepared by stir-casting. The composites were subjected to fracture toughness and tensile tests. The particle/matrix interfaces were investigated by nanoindentation combined with scanning electron microscopy (SEM). The effects of the particle/matrix interfaces on the mechanical properties of the composites were discussed through a unit cell model with a transition interface layer. The results show that a transition interface layer with smoother hardness and modulus gradient is developed in the YAl2/LA143 composite. Both the fracture toughness and ductility for the YAl2/LA143 composite are higher than those for the SiC/LA143 composite. The failure behavior is determined by particle breakage with little interfacial breakage for the YAl2/LA143 composite, while being due to interfacial breakage for the SiC/LA143 composite. The superiority of the mechanical properties for the YAl2/LA143 composite may result from the failure behavior of particle breakage, which are correlated to the better physical compatibility between the YAl2 intermetallics and LA143 matrix.

Study of the moisture/stress effects on glass fibre/epoxy composite and the impact of the interphase area

This paper shows the influence of interphase areas in a UD glass fibre/epoxy composite on water diffusion process and mechanical properties thanks to experimental, analytical and numerical analysis. The interphase is first characterized in terms of molecular mobility by micro-thermal analysis and in terms of mechanical properties by force measurements with atomic force microscopy. The measurements show an increase in elastic modulus and in glass transition temperature around each fibre. Optical microscopy reveals an important heterogeneity of fibre distribution inside the matrix with many contacts between fibres. A realistic microstructure including the interphase size (defined by μTA) and fibre distribution has then been considered to model the composite accurately. Diffusion parameters resulting from water absorption measurements on both the resin and the composite were also used in the finite element analysis. The comparison between experimental data, analytical and finite element models shows that the water diffusion coefficient is about five times higher in the interphase area than in the matrix whereas the gradient in modulus around the fibres barely modify the composite mechanical response.

Viscoelastic Properties of Field-Aged Cores

Nowadays, engineers begin to pay more attention on field-aged asphalt concrete cores because they have more reality than the lab-made cores and the results can tell what happened to the pavement. However, it is not accurate to estimate the full asphalt mixture because as usual the pavement engineers can only extract the binder from it, not the mixture. Now there is a newly developed way to analysis the data more accurately using the Direct Tension Test. Because the field aged asphalt mixtures are not aged uniformly with the pavement depth because of various reasons, there is a modulus gradient going through deeply through the asphalt layer and usually the stiffest layer is the surface because normally the surface is more aged. A method has been developed to analysis this behavior. Tests are both taken on field cores which are Hot Mix Asphalt (HMA) and Warm Mix Asphalt (WMA). During the research, the authors found that it is both applicable for all the field cores. More information about this method and tests are detailed in the following paragraphs.

Analysis of Performance Decay Behavior for Asphalt Pavement Based on Aging

Aging is a main factor affecting the durability of asphalt pavement. To study decay behavior of asphalt pavement with aging, aged asphalt was extracted from stratified pavement mixtures for different service-life. The changes of asphalt properties with service time and depth variations of the pavement were discussed. And numerical simulation analysis of pavement structure was conducted with pavement gradient modulus changes caused by aging. The results indicate that asphalt stiffness increases and low-temperature performance decays sharply with the extension of pavement service life, especially in the first several years. The vertical aging differences from top to bottom of pavement were significant, the aging extents decrease continuously from the surface, which cause the gradient changes of pavement modulus. The maximum tensile stress and maximum shear stress all increase with surface modulus increasing, so more serious aging can induce greater gradient modulus, shear stress and tensile stress are larger under the same loads, which have more serious damage to the pavement structure.

Evaporated Thick Polysilicon Film With Low Stress and Low Thermal Budget

In this letter, we report on the use of e-beam evaporation under ultrahigh vacuum conditions to deposit smooth, low stress, and thick polysilicon films for low thermal budget applications. The film stress, surface morphology, and crystallographic characteristics have been investigated at various deposition temperatures, deposition rates, and annealing temperatures. Cantilever beams formed from such films have been released and characterized for the stress gradient and Young's modulus. The results show that crystallized grains start forming in the evaporated films below 400°C substrate temperatures and the stress levels in the film can be easily controlled and minimized. Residual stress of 20 MPa and a stress gradient of only 1.6 MPa/μm have been obtained for a 10 μm film deposited at 500°C and annealed at 600°C.

Experimental and Numerical Modeling of Surface Indentation Response of Plastically Graded Materials

Materials with customized spatial gradients in mechanical properties are increasingly used in high performance applications requiring enhanced resistance to contact loads, wear, and fatigue. In many engineering materials, multiple property and microstructural gradients may occur simultaneously with depth. In this manuscript, two case carburized steels are analyzed for their gradient in hardness with depth, emphasizing the resulting variation in surface hardness under increasing indentation loads. A parametric study using finite element analysis is then conducted in order to characterize the influence of individual property gradients on the surface indentation response of graded materials. It is shown that the measured surface hardness value decreases rapidly under increasing surface indentation loads in materials with sharp negative hardness gradients. It is also shown that this trend is independent of the magnitude of the strain hardening exponent of the material, as well as the gradient in strain hardening exponent. Gradients in elastic properties were also shown to have negligible influence on surface hardness trends for a fixed gradient in hardness. Finally, it is revealed that the depth of subsurface plastic deformation increases with sharper gradients in hardness, while being insensitive to changes in strain hardening exponent. For elastically graded materials, a decreasing gradient in elastic modulus limits the depth of plastic deformation.

Lithological and structural prerequisites for localization of ore deposits in the Mesoproterozoic-Neoproterozoic Uchur-Maya depression (Siberian Platform)

A set of sketch maps has been compiled including a structural map for the Uchur-Maya depression, maps of the fractures based on the data derived from the digital elevation model processed using the modulus of the topographic gradient for discriminating the fault boundaries, and maps of the lineament distribution and the density of the elementary linear structures. On these sketch maps, the location of the ore deposits and the occurrences of various metals are plotted within the bounds of the Uchur-Maya depression with the defining structural and lithological factors that control the ore occurrences. The locations of the ore objects relative to the differently oriented faults and the lineament network based on the data of the digital elevation model’s processing and the density of the linear structures have been analyzed. The most important lithological factors that control the ore deposits are the pre-Mesoproterozoic structural-stratigraphic unconformity zones, the contact zones between the Mesoproterozoic formations with their contrasting physical and chemical properties, and the high-porosity (cavernous) dolomite member in the Yudomian Group of the Vendian.

Approximations for modulus of gradients and their applications to neighborhood filters

We define an integral approximation for the modulus of the gradient |∇u(x)| for functions f: Ω ⊂ ℝn → ℝ by modifying a classical result due to Calderon and Zygmund. Our integral approximations are more stable than the pointwise defined derivatives when applied to numerical differentiation for discrete data. We apply our results to design and analyse neighborhood filters. These filters correspond to well-behaved nonlinear heat equations with the conductivity decreasing with respect to the modulus of gradient |∇u(x)|. We also show some numerical experiments and evaluate the effectiveness of our filters.