Tangent Elastic Modulus Research Articles

Cyclic cumulative strain of coarse-grained soil under large cyclic stress amplitude

The deformation behavior of coarse-grained soil under large cyclic stresses, such as those induced by strong earthquakes, has received limited attention. This study aims to investigate the cyclic accumulation behaviors and hysteresis loops of coarse-grained soil during drained cyclic triaxial tests, spanning a range from small to large cyclic stresses. The cyclic triaxial tests were primarily conducted under anisotropic consolidation conditions, with an axial-to-radial stress ratio of 2.0, confining pressures ranging from 100 to 500 kPa, and cyclic stresses varying from 0.01 to 1.85 times the confining pressure. Additionally, cyclic triaxial tests under isotropic consolidation conditions and constant mean stress conditions were performed for comparison and validation. The test results reveal that the properties of the hysteresis loops exhibit significant nonlinear behavior as cyclic stress increases, particularly concerning their shape, symmetry, degree of closure, and initial tangent modulus of elasticity. The cumulative axial strain displays three stages: strain increases slightly and gently at small cyclic stress, increases rapidly and substantially at medium cyclic stress, and decreases at large cyclic stress. The delineation of these phases is largely governed by the behaviors observed during the first cycle. Moreover, the cumulative volumetric strain increases monotonically with the increase of cyclic stress, with a more rapid increase at large cyclic stress. This study provides valuable insights into the cyclic deformation and constitutive modeling of coarse-grained soil under significant cyclic stress.

Evaluation of sustainable composites of sugarcane bagasse and combined algae-soy protein binders

The development of sustainably-sourced adhesives is essential to displace conventional particleboard adhesives made from formaldehyde and other petroleum derivatives. This work evaluates the mechanical properties of particleboards prepared from sugarcane bagasse, spirulina algae powder, soy flour, and sodium hydroxide at various algae to soy ratios and denaturation times. The rheology of the adhesive was characterized using the Hershel-Bulkley method to determine the yield stress (max. 288 Pa) and flow behavior index (0.21–0.54). The lap shear strength (max. 3.02 N/mm2) was also determined. For the particleboard, the secant modulus of elasticity (max. 1.12 GPa), tangent modulus of elasticity (max. 1.30 GPa), modulus of rupture (max. 11.62 N/mm2), and internal bond strength (max. 0.09 N/mm2) were determined. These particleboards met the minimum requirement set by ANSI/A208.1–1999 for general-use particleboards for the modulus of elasticity and modulus of rupture, but they did not meet the standard for internal bond strength.

Understanding the Mechanism of the Structure-Dependent Mechanical Performance of Carbon-Nanotube-Based Hierarchical Networks from a Deformation Mode Perspective.

Carbon nanotube (CNT)-based networks have wide applications, in which structural design and control are important to achieve the desired performance. This paper focuses on the mechanism behind the structure-dependent mechanical performance of a CNT-based hierarchical network, named a super carbon nanotube (SCNT), which can provide valuable guidance for the structural design of CNT-based networks. Through molecular dynamic (MD) simulations, the mechanical properties of the SCNTs were found to be affected by the arrangement, length and chirality of the CNTs. Different CNT arrangements cause variations of up to 15% in the ultimate tensile strains of the SCNTs. The CNT length determines the tangent elastic modulus of the SCNTs at the early stage. Changing the CNT chirality could transform the fracture modes of the SCNT from brittle to ductile. The underlying mechanisms were found to be associated with the deformation mode of the SCNTs. All the SCNTs undergo a top-down hierarchical deformation process from the network-level angle variations to the CNT-level elongations, but some vital details vary, such as the geometrical parameters. The CNT arrangement induces different deformation contributors of the SCNTs. The CNT length affects the beginning point of the CNT elongation deformation. The CNT chirality plays a crucial role in the stability of the junction's atomic topology, where the crack propagation commences.

Biomechanical properties of the aortic root are distinct from those of the ascending aorta in both normal and aneurysmal states

BackgroundAlthough aneurysms of the ascending aorta and the aortic root are treated similarly in clinical guidelines, how biomechanical properties differ between these 2 segments of aorta is poorly defined. MethodsBiomechanical testing was performed on tissue collected from the aortic root (normal = 11, aneurysm = 51) and the ascending aorta (normal = 21, aneurysm = 76). Energy loss, tangent modulus of elasticity, and delamination strength were evaluated. These biomechanical properties were then compared between (1) normal ascending and normal root tissue, (2) normal and aneurysmal root tissue, (3) normal and aneurysmal ascending tissue, and (4) aneurysmal root and aneurysmal ascending tissue. Propensity score matching was performed to further compare aneurysmal root and aneurysmal ascending aortic tissue. Clinical and biomechanical variables associated with decreased delamination strength in the aortic root were evaluated. ResultsThe normal aortic root demonstrated greater viscoelastic behavior (energy loss 0.08 [0.06, 0.10] vs 0.05 [0.04, 0.06], P = .008), and greater resistance against delamination (93 [58, 126] mN/mm vs 54 [40, 63] mN/mm, P = .05) compared with the ascending aorta. Delamination strength was significantly reduced in aneurysms in both the root and the ascending aorta compared with their normal states. Aneurysms of the aortic root matched to the ascending aortic aneurysms in terms of baseline characteristics including size, were characterized by a larger decrease in delamination strength from baseline (Δ −59 mN/mm vs Δ −24 mN/mm). Aging (P = .003) and the presence of hypertension (P = .02) were associated with weakening of the aortic root, while diameter did not have this association (P = .29). ConclusionsThe normal aortic root was found to have distinct biomechanical properties compared with the ascending aorta. When aneurysms form in the aortic root, there is less strength against delamination, without other biomechanical changes such as increased energy loss observed in aneurysmal ascending aortas. Age and hypertension were associated decreased aortic wall strength in the aortic root, whereas diameter had no such association.

A novel laboratory investigation to characterize the mechanical behaviors of polyurethane-ballast bonds under complex stress states

The polyurethane has been successfully applied to reinforce the railway ballasted track and the bonding properties of the polyurethane to ballast particles have significant effects on track performance. However, the mechanical behaviors of the polyurethane-ballast bonds(PBBs) haven’t been reported leaving the particle scale mechanism of the polyurethane reinforcement still unclear. A novel laboratory experimental method was developed in this paper to characterize the mechanical behaviors of the PBBs. Using standard test specimens with various specimen size and bond thickness, a series of laboratory tests were conducted to measure the stress–strain responses of the PBBs under tensile, compressive and shear loadings. The measured results show that the stress of a PBB increases nonlinearly with its strain when it subjects to uniaxial tensile or compressive loads. The test results also validate that the strength envelope of a PBB follows the linear Mohr-Coulomb criterion. A modified parallel bond model was proposed to predict the nonlinear stress–strain behaviors of the PBB with high accuracy. Furthermore, it is found that the bond thickness has distinct effects on the tensile and the cohesion strength of the PBB but negligible effect on its friction angle. The initial tangent elastic modulus of the PBB increases approximately linearly with the increase of the bond thickness. The experimental method, results and the modified bond model introduced in this paper are very helpful for the performance evaluation and the numerical simulations of polyurethane reinforced ballast.

Digitized design and mechanical property reprogrammability of multistable origami metamaterials

Origami-based multistable metamaterials have been recognized as a promising platform for diverse applications due to their exceptional mechanical properties. However, the current state of the art in designing origami metamaterials is mostly ad-hoc and narrowly focused on a particular mechanical property. In other words, there is a lack of basic research on deriving generic and systematic methodologies for the design, configuration tuning, and property programming of origami metamaterials, to explore their achievable ranges of mechanical properties comprehensively from the perspective of mechanics. In this work, a mathematically rigorous strategy for synthesizing and reconfiguring multistable origami metamaterials based on two basic modules is put forward, and a systematic approach to analyze and program the material properties is proposed. More specifically, by programming the binary design array of module arrangement and by mathematically expressing the connection constraints, a number of multistable origami cells with exceptional shape reconfigurability are generated. Building upon this, the fundamental mechanics properties, including the multistability, global stress-strain profiles, and tangent elastic moduli, are thoroughly investigated via examining the unit cell, i.e., the “infinitesimal element,” of the metamaterial. Considering the extreme cases of the unit cell architecture, boundaries of the material properties of the synthesized metamaterials at the stress-free configuration are derived, which are of vital importance for material selections and topological optimization. Moreover, due to the topologically different configurations, six categories of metamaterials with qualitatively different material densities and moduli programmability are revealed. In addition to the reprogrammable density and Young's modulus of conventional Miura origami metamaterial, more intriguing properties, such as reprogrammable shearing modulus, locking effect, and inner reconfiguration without modulus change, are discovered for the first time. A prototype made of dual-material 3D printing is created to experimentally evaluate and verify the effectiveness of the analysis method and the characterized material properties. Overall, this work provides a rigorous tool for designing multistable origami metamaterials and an effective strategy for characterizing their fundamental mechanical properties, which will greatly enhance the systematization of creating origami metamaterials.

On ellipticity of static equations of strain gradient elasticity and infinitesimal stability

Within the framework of strain gradient elasticity under finite deformations we formulate the strong ellipticity conditions of equilibrium equations. Within the model a strain energy density is a function of the first and second deformation gradients. Ellipticity involves certain constraints on the tangent elastic moduli. It is also closely related to infinitesimal stability which is defined as the positive definiteness of the second variation of the potential energy functional. Here we consider the first boundary-value problem, that is with Dirichlettype boundary conditions. For one-dimensional deformations we determine necessary and sufficient conditions of infinitesimal instability. The latter constitute two inequalities for elastic moduli.

Plasticity of cerebral microvascular structure and mechanics during hypertension and following recovery of arterial pressure.

Changes in vascular structure contribute to vascular events and loss of brain health. We examined changes in cerebral arterioles at the onset of hypertension and the hypothesis that alterations during hypertension would recover with the return of mean arterial pressure (MAP) to normal. MAP was measured with radiotelemetry in awake male C57BL/6J mice at baseline and during infusion of vehicle or angiotensin II (ANG II, 1.4 mg/kg/day using osmotic pumps) for 28 days, followed by a 28-day recovery. With ANG II treatment, MAP increased through day 28. On day 30, MAP began to recover, reaching levels not different from vehicle on day 37. We measured intravascular pressure, diameter, wall thickness (WT), wall:lumen ratio (W:L), cross-sectional area (CSA), and slope of the tangential elastic modulus (ET) in maximally dilated arterioles. Variables were similar in both groups at day 1, with no significant change with vehicle treatment. With ANG II treatment, CSA, WT, and W:L increased on days 7-28. Internal and external diameter was reduced at 14 and 28 days. ET versus wall stress was reduced on days 7-28. During recovery, the diameter remained at days 14 and 28 values, whereas other variables returned partly or completely to normal. Thus, CSA, WT, W:L, and ET versus wall stress changed rapidly during hypertension and recovered with MAP. In contrast, inward remodeling developed slowly and did not recover. This lack of recovery has mechanistic implications for the long-term impact of hypertension on vascular determinants of brain health.NEW & NOTEWORTHY Changes in vascular structure contribute to vascular events and loss of brain health. We examined the inherent structural plasticity of cerebral arterioles during and after a period of hypertension. Arteriolar wall thickness, diameter, wall-to-lumen ratio, and biological stiffness changed rapidly during hypertension and recovered with blood pressure. In contrast, inward remodeling developed slowly and did not recover. This lack of recovery of arteriolar diameter has implications for the long-term impact of hypertension on vascular determinants of brain health.

Carbon Nanostructures for Ocular Tissue Reinforcement.

PurposeThe purpose of this study was to improve the biomechanical properties of the cornea through the incorporation of carbon nanostructures.MethodsHealthy Japanese rabbits were used to evaluate the effect of carbon nanostructures’ incorporation in the cornea. Rabbits were divided in two groups A and B. In each of these groups, the corneas were divided in (i) corneas not submitted to any treatment (the control group), (ii) corneas modified either with carbon nanostructures (group A), or with the traditional cross-linking technology (group B). After modification, rabbits were euthanized at different time intervals. The biomechanical properties of the treated corneas were evaluated using the inflation method.ResultsBiomechanical tests based on the inflation method show that the incorporation of carbon nanostructures to the cornea and their proper distribution within it gives rise to a large improvement in the mechanical properties and tangential elastic modulus (up to 155%). These results anticipate that this novel and easy approach based on nanotechnology is able to compete with the actual cross-linking technology applied in clinical ophthalmology using a photosensitive molecule, such as riboflavin and unpleasant UV-A radiation.ConclusionsThe incorporation of carbon nanostructures (single-walled carbon nanotubes and graphene) in corneal stroma is proposed as a promising alternative to improve the mechanical properties in the treated eyes. The proper dispersion of the carbon nanostructures a few days after implementation (down to 60 micrometers depth) explains the successful results achieved.Translational RelevanceNanotechnology applied to the eye constitutes a promising approach for ocular tissue reinforcement.

Covalent and biodegradable chitosan-cellulose hydrogel dressing containing microspheres for drug delivery and wound healing

The purpose of this study is to prepare polysaccharide-based hydrogel dressing integrated with microspheres which has excellent self-healing ability and sustained antibacterial function for wound healing application. The hydrogel is derived from carboxymethyl chitosan (CMCS) and carboxymethyl cellulose (CMC) via the Schiff’s base reaction, without the addition of chemical catalyst. Also, the CMCS microspheres containing an antibacterial drug, e.g., tetracycline hydrochloride (TH), were prepared by an emulsion cross-linking. Parameters were optimized during the preparation experiment to improve various properties of the microspheres/hydrogel composite dressing. This gel dressing has good elasticity, when compressive strain was 0.7 mm/mm, the tangent elastic modulus was up to 1.7 MPa. Furthermore, its drug sustained release performance was significantly better than the drug directly embedded in the hydrogel. TH could sustain release stably within 12 h, without obvious burst release phenomenon. Antibacterial experiments showed that 20 mg/mL microspheres made composite dressing a better performance than control microspheres and hydrogel. Potential of the composite scaffolds was further demonstrated by culture of human adipose-derived stem cells (ASCs) in vitro. Cell culture showed that this composite hydrogel could support survival and proliferation of ASCs. These characteristics provide a potential opportunity to use this composite gel dressing in drug delivery and wound healing applications.

Time-course of the human thoracic aorta ageing process assessed using uniaxial mechanical testing and constitutive modelling

Age-related remodelling of the arterial wall shifts the load bearing from the compliant elastin network to the stiffer collagen fibres. While this phenomenon has been widely investigated in animal models, human studies are lacking due to shortage of donors’ arteries. This work aimed to characterise the effect of ageing on the mechanical properties of the human aortic wall in the circumferential direction.N = 127 thoracic aortic rings (age 18–81 years) were subjected to circumferential tensile testing. The tangential elastic modulus (Kθθθθ) was calculated at pressure-equivalent stresses ranging 60–100 mmHg. Further, the mechanical data were fitted using the Holzpafel-Gasser-Ogden hyperelastic strain energy function (HGO-SEF), modelling the superimposed response of an isotropic matrix (elastin) reinforced by collagen fibres.Kθθθθ increased with age across at all considered pressures (p < 0.001), although more strongly at higher pressures. Indeed, the slope of the linear Kθθθθ-pressure relationship increased by 300% from donors <30 to ≥70 years (4.72± 2.95 to 19.06± 6.82 kPa/mmHg, p < 0.001). The HGO-SEF elastin stiffness-like parameter dropped by 31% between 30 and 40 years (p < 0.05) with non-significant changes thereafter. Conversely, changes in HGO-SEF collagen parameters were observed later at age>60 years, with the exponential constant increasing by ∼20–50 times in the investigated age range (p < 0.001).The results provided evidence that the human thoracic aorta undergoes stiffening during its life-course. Constitutive modelling suggested that these changes in arterial mechanics are related to the different degeneration time-courses of elastin and collagen; likely due to considerable fragmentation of elastin first, with the load bearing shifting from the compliant elastin to the stiffer collagen fibres. This process leads to a gradual impairment of the aortic elastic function with age.

Study on shear strength and deformation characteristics of sandy silt improved by cement under hydrothermal action

At present, researches on the stability of sandy silt improved by cement (SS-C) are abundant, but there are few researches on the shear strength and deformation characteristics of it under the conditions of low temperature and different moisture content in cold regions. Firstly, sandy silt was selected as the research object, which the liquid limit (wL) is 28.6% and plasticity index (IP) is 8.9, the coefficient of nonuniformity (Cu) is 61.5, and the optimum cement incorporation ratio was determined as 6% according to the maximum direct shear strength at − 7 ℃. Secondly, the SS-C was tested by direct shear at different temperatures (−2 ℃, −5 ℃, −7 ℃, −14 ℃) and moisture contents (13.0%, 15.2%, 18.0%). Subsequently, the effects of temperature and moisture content on shear strength and shear modulus were calculated and analyzed. The results show that when the temperature drops from − 2 ℃ to − 7 ℃, the cohesion of the SS-C increases by 243.4%; when from − 7 ℃ to − 14 ℃, it continues to increase by 285.7% compared with that at − 2 ℃, and the internal friction angle of the SS-C also shows a similar variation law with the temperature. At − 7 ℃, when the initial moisture content increases from 13.0% to 15.2%, the increase of cohesion is about 8.6%; when from 15.2% to 18.0%, it is about 22.8%, while the change of internal friction angle is less than 1%. Finally, the impact of hydrothermal action on the failure ratio of the SS-C was analyzed, and the relation equation between shear strength and tangent elastic modulus of the SS-C was established.

Experimental and Simulation Studies of Temperature Effect on Thermophysical Properties of Graphene-Based Polylactic Acid.

Overheating effect is a crucial issue in different fields. Thermally conductive polymer-based heat sinks, with lightweight and moldability features as well as high-performance and reliability, are promising candidates in solving such inconvenience. The present work deals with the experimental evaluation of the temperature effect on the thermophysical properties of nanocomposites made with polylactic acid (PLA) reinforced with two different weight percentages (3 and 6 wt%) of graphene nanoplatelets (GNPs). Thermal conductivity and diffusivity, as well as specific heat capacity, are measured in the temperature range between 298.15 and 373.15 K. At the lowest temperature (298.15 K), an improvement of 171% is observed for the thermal conductivity compared to the unfilled matrix due to the addition of 6 wt% of GNPs, whereas at the highest temperature (372.15 K) such enhancement is about of 155%. Some of the most important mechanical properties, mainly hardness and Young’s modulus, maximum flexural stress, and tangent modulus of elasticity, are also evaluated as a function of the GNPs content. Moreover, thermal simulations based on the finite element method (FEM) have been carried out to predict the thermal performance of the investigated nanocomposites in view of their practical use in thermal applications. Results seem quite suitable in this regard.

Effects of temperature gradient on the granite’s mechanical properties

Temperature gradient significantly affects the production of surrounding rock stress in mining engineering. The mechanics and deformation characteristics of the rock will change under the temperature gradient, thereby increasing the probability of accidents in the roadway. This paper conducts uniaxial compression tests on granite at different temperatures from room temperature to 250∘C, and analyzes in detail the changes in the stress-strain curve, peak stress, peak strain and tangent modulus of granite under high temperature and different temperature gradient conditions. The results of this study are as follows: (1) Under high temperature conditions, the granite’s peak stress and tangent elastic modulus increased with temperature from 17 to 100∘C, then decreased from 100∘C to 250∘C, whereas the granite’s peak strain increased steadily with increasing temperature; (2) under temperature gradient, the granite’s peak stress and tangent modulus first decreased and then increased with increasing temperature gradient, while the granite’s peak strain first decreased and then increased at 100∘C, but first increased and then decreased from 150∘C to 250∘C.

Investigation of small- to large-strain moduli correlations of rockfill materials — application to Romaine-2 dam

The establishment of relationships between the shear wave velocity (Vs) and other geotechnical parameters of rockfills under large strains (e.g., oedometer-constrained modulus, Moedo, and tangent elastic modulus, Et) is a significant step toward precise modeling of earth structure stress–strain behavior. In this study, four specimens reconstituted from the rockfill used for Romaine-2 dam construction were investigated experimentally to correlate small-strain to large-strain moduli. The development of Moedoand Vswith consolidation was measured in a laboratory using the piezoelectric ring-actuator technique (P-RAT) incorporated in a large oedometer cell. A correlation between Moedoand the small-strain shear modulus (Go) was proposed. Moreover, numerical simulations were performed using the Duncan–Chang hyperbolic model to correlate Vsand the Duncan–Chang initial elastic modulus (Ei), which depends on the minor principal stress (σ3). Based on the experimental and numerical data, a relationship between the Eiand Vsof the rockfill specimens was established. Verification studies were also performed using in situ measurements obtained from the Romaine-2 dam construction, and the ability of the proposed relationships to predict Eifrom in situ Vsmeasurements was demonstrated. The proposed correlations can help geotechnical designers estimate the deformation characteristics of rockfill materials from in situ Vsmeasurements.

The influence of the reinforced concrete deformability in the design of slender columns

This work aims to evaluate the influence of the main parameters related to the deformability of reinforced concrete columns in the design of pin-ended rectangular slender columns, under combined bending and axial force. For this, the objective is to analyse the influence of the elasticity modulus of the concrete and the type of coarse aggregate on the deformability of slender columns and consequent dimensioning, and to propose an adaptation of the parabolic law, inserting the influences of the coarse aggregate and the initial tangent modulus of elasticity in this equation, being able to use it to evaluate the local second order effects. Concrete characteristic strengths in the range 20–50 MPa (Group I) and 55–90 MPa (Group II) and CA-50 steel (fyk=500MPa) and their respective constitutive laws are considered. Geometric non-linearity is defined from the exact solution of the second-order differential equation for two simultaneous loading cases: geometric imperfection of the column axis and equal moments applied at its extremities. Physical non-linearity is defined from the moment–curvature diagram of the cross-section under the action of the compression force. The constitutive laws used in this diagram are: the bilinear law with horizontal threshold for steel, and for concrete in compression, the parabola-rectangle diagram on the dimensioning of the cross-section at the ultimate limit state under combined bending and axial force, and the law given in MC-90, MC-2010 [13], EC2-2010 [17] and in the Brazilian Bridge Standard ABNT NBR 7187 [20]. It is also examined the parabola of degree n, corrected for the local physical non-linearity consideration (i.e. of the column’s length). These laws explicitly or implicitly include the modulus of elasticity of concrete and the type of aggregate in it, through the factor αe defined in ABNT NBR 6118 [14]. The concrete strength in tension is disregarded in this work, that is, the tension-stiffening effect for the cracked column is taken equal to zero, which is on the safe side in case of slender columns, as its cracking moment increases with increasing axial compression force. Besides, this strength depends also on the height of the cross-section, frequently considered in prestressed beams. Another important parameter, besides the concrete characteristic strength, is the geometric ratio of the total reinforcement. Thus, it is possible to determine more precisely and securely the second order effect of the slender column, and the consequent variation in reinforcement in relation to the basic case in which αe=1. The results showed that the influence of the type of aggregate can be considered irrelevant for the determination of reinforcement of columns with low slenderness. However, for columns with high slenderness and high strength concrete, the type of aggregate had a significant influence on the design of these columns, where the column with aggregate αe=0,7 required twice the reinforcement compared to the column with aggregate αe=1. In addition, a parabolic law adapted to analyse the second order effects of slender columns is proposed with the replacement of fckby∝0∙αe∙fcd0.

Shear performance of recycled asphalt mixture based on contact interface parameter analysis

The mechanical characteristics, quantity, and distribution of contact interfaces in the composite skeleton of recycled asphalt mixture affect the consistency of deformation of new and old aggregate composite skeleton, further affecting the macroscopic mechanical properties of material. To quantitatively analyze the effect of reclaimed asphalt pavement (RAP) content on the mechanical characteristics of contact interface and shear performance of recycled asphalt mixture, the tangential elastic stiffness of interface of new-new, new-old, and old-old contact types is defined as ksnew-new, ksnew-old, and ksold-old, respectively; interfacial shear tests were carried out for new-new, new-old, and old-old contact types. Digital image technology and statistical analysis were used to obtain the proportion distribution of αnew-new, αnew-old, αold-oldof three aggregate contact types under different RAP contents. Combined with the calculation results of interface tangential elastic stiffness and shear modulus with different RAP contents, an interface tangential elastic stiffness estimation model and a shear modulus calculation model were established, and the rationality of the model was verified. The results show that different interface contact types significantly affected the shear test results. The tangential elastic stiffness of old-old contact type is the highest, 0.3416 × 106 N/m. The tangential elastic stiffness of new-old and new-new contact types are 0.2254 × 106 N/m and 0.0992 × 106 N/m, respectively. The results of contact interface shear test of recycled asphalt mixture with different RAP contents show that the overall trend of shear curve is similar, and the peak value and mean value of shear force do not change consistently with the increase of RAP content. Using the least square mathematical analysis method for overdetermined equation, the goodness-of-fit ofks regression model with different RAP contents is 0.85. There is a good correlation between the shear modulus calculation results and uniaxial penetration test results, and the correlation coefficient is 0.80.

Contribution of Bowman layer to corneal biomechanics.

To compare the elastic modulus of thin corneal lamellas using 2D stress-strain extensometry in healthy ex vivo human corneal lamellas with or without the presence of Bowman layer. Center for Applied Biotechnology and Molecular Medicine, University of Zurich, Switzerland; ELZA Institute, Dietikon, Switzerland; Department of Ophthalmology, Philipps University of Marburg, Germany. Prospective experimental laboratory study. Healthy human corneas were stripped of Descemet membrane and the endothelium for Descemet membrane endothelial keratoplasty. After epithelium removal, corneas were divided into 2 groups. In Group 1, Bowman layer was ablated with an excimer laser (20 μm thick, 10 mm). In Group 2, Bowman layer was left intact. Then, a lamella was cut from the anterior cornea with an automated microkeratome. Elastic and viscoelastic material properties were analyzed by 2D stress-strain extensometry between 0.03 and 0.70 N. Twenty-six human corneas were analyzed. The mean lamella thickness was 160 ± 37 μm in corneas with Bowman layer and 155 ± 22 μm in corneas without. No statistically significant differences between flaps with and without Bowman layer were observed in the tangential elastic modulus between 5% and 20% strain (11.5 ± 2.9 kPa vs 10.8 ± 3.7 kPa, P > .278). The presence or absence of Bowman layer did not reveal a measurable difference in corneal stiffness. This may indicate that the removal of Bowman layer during photorefractive keratectomy does not represent a disadvantage to corneal biomechanics.

Towards the non-invasive determination of arterial wall distensible properties: New approach using old formulae

Arterial function and wall mechanical properties are important determinants of hemodynamics in the circulation. However, their non-invasive determination is not widely available. Therefore, the aim of this work is to present a novel approach for the non-invasive determination of vessel’s distensibility and elastic modulus.Simultaneous measurements of vessel’s Diameter (D) and flow velocity (U) were recorded to determine local wave speed (nC) in flexible tubes and calf aortas non-invasively using the lnDU-loop method, which was used to calculate the Distensibility (nDs) and Elastic Modulus (nE), also non-invasively. To validate the new approach, the non-invasive results were compared to traditionally invasive measurements of Dynamic Distensibility (Dsd) and Tangential Elastic Modulus (Em).In flexible tubes, the average nDs was higher and nE was lower than Dsd and Em by 1.6% and 6.9%, respectively. In calf aortas, the results of nDs and nE agreed well with those of Dsd and Em, as demonstrated by Bland-Altman technique.The results of nDs and nE are comparable to those determined using traditional techniques. Our results suggest that nDs and nE could be measured in-vivo non-invasively, given the possibility of measuring D and U to obtain nC. Further studies are warranted to establish the clinical usefulness of the new approach.

Nonlinear Inelastic Analysis of 2D Steel Frames

In this study, a new method for nonlinear analysis of 2D steel frames, by improving the conventional plastic hinge method, is presented. The beam-column element is established and formulated in detail using a fiber plastic hinge approach. Residual stresses of I-shape sections are declared at the two ends through fibers. Gradual yielding by residual stresses along the member length due to axial force is accounted for by the tangent elastic modulus concept. The P-δ effect is captured by stability functions, whereas the P-Δ effect is estimated by the geometric stiffness matrix. A nonlinear algorithm is established for solving nonlinear problems. The present study predicts the strength and behavior of 2D steel frames as efficiently and accurately as the plastic zone method did.