Linear Elastic Behavior Research Articles

Mechanical behavior analysis of sandwich composite structures with different core geometries under three-point bending

This study presents a comprehensive investigation into the mechanical behavior of sandwich composite structures featuring six distinct core geometries under three-point bending conditions. Through detailed finite element analysis (FEA), the research examines the performance characteristics of I-shaped, traditional honeycomb (NIDA), O-shaped, X-shaped, semi-circular, and modified core geometries. The analysis focused on stress distribution patterns, displacement responses, and overall structural integrity. Results demonstrate that the semi-circular core geometry exhibits superior mechanical properties, achieving an 84.66% improvement in positive stress resistance and 74.79% enhancement in negative stress resistance compared to the traditional honeycomb reference model. The study revealed consistent linear elastic behavior across all models within the tested displacement range, with the semi-circular configuration showing optimal stress distribution and minimal core deformation. Using aluminum for facings and core materials, the research evaluated mechanical responses through displacement-controlled loading conditions, measuring stress concentrations at critical points and analyzing deformation patterns. This research provides valuable insights into the relationship between core geometry and mechanical performance, offering practical implications for engineering applications requiring high strength-to-weight ratios and enhanced flexural resistance in modern structural design.

Post-irradiation examination of UN-Mo-W fuels for space nuclear propulsion

The National Aeronautics and Space Administration's return to space nuclear propulsion stems from the need for a more efficient method of space travel. Nuclear thermal propulsion systems have been shown to be two times more efficient than chemical propulsion. NASA's Sirius program was created to fabricate and test fuels for space nuclear propulsion, specifically to determine their performance under prototypical startup conditions. The Sirius project featured 4 test capsules, Sirius-1 featured uranium nitride fuel dispersed in a matrix of tungsten and rhenium, while Sirius-2A, -2B, and -3 featured uranium nitride-molybdenum-tungsten fuel (UN-Mo-W). This study discusses the Sirius-2A and -2B irradiation experiments at the Idaho National Laboratory, specifically their performance under irradiation at the Transient Reactor Test Facility. It was found that the fuel samples overall did not exhibit significant cracking, though the Sirius-2A fuel did have one large crack on the surface of the fuel. There was minimal hydrogen absorption in the samples, though it is unknown if the absorption occurred during irradiation or during fabrication. Mechanical testing indicated that the UN fuel demonstrated ceramic behavior as expected, and the Mo/W matrix demonstrated linear elastic behavior to failure.

Flexural Behaviour and Environmental Impact of African Fan Palm Reinforced Concrete Beams under Symmetrical Loading Conditions

The paper presents the results of investigation on reinforced beams adopting eco-friendly materials for sustainable concrete. The use of African fan palm is explored to mitigate the environmental impact aimed at reducing carbon emissions associated with the production and use of steel reinforcement. The beams were tested under a four -point loading system with the ends simply supported. The test comprised twenty-three beams that were reinforced with African fan palm bars whilst two beams were reinforced with mild steel bars to serve as control beams. The study investigated the flexural strength and deformation characteristics of the beams under monotonic loading. Two concrete strengths of 15.34N/mm2 or 22.37N/mm2 were adopted and proposed optimal tensile ratios for an under reinforced fan palm section. The parameters considered included the tension bars ratio (ρ), concrete compressive strength, span-to-effective-depth ratio and loading conditions. A linear elastic behaviour of both fan palm and steel reinforced concrete beams was observed to the point of first crack load beyond which the beam stiffness continued to reduce until failure. A comparison of the experimental results with results of theoretical analysis of the beams revealed that a 1.22% increase in the tensile ratio of steel in the control beams increased the cracking moment (Mcr) to 36%, while the fan palm beam of the same size and same concrete strength with a higher tensile ratio of 9.69% exhibited a lower Mcr of 11.86%. This shows a significant effect of reinforcing steel bars on concrete cracking compared to the fan palm. The averages of experimental to theoretical failure loads of fan palm to steel were compared with failure loads of 36.52kN to 20.19kN and 32kN to 19.78kN respectively. These results of the study based on the failure mode, and failure loads, highlights the suitability of using African fan palm as substitute reinforcing material for low rise buildings. Furthermore, the study also proposed minimum and maximum tensile ratio of 0.76% and 5.60% respectively for fan palm reinforced concrete beams based on the Indian Standard and modulus of elasticity of steel.

The impact of using prestressed CFRP bars on the development of flexural strength

Abstract The load-carrying capacity and ductility of conventional steel reinforcement may be greatly increased by using ordinary and prestressed carbon fiber-reinforced polymer (CFRP) bars, resulting in reinforced concrete structures with greater sturdiness. This research investigates the flexural behavior of CFRP-prestressed beams with bonded tendons as opposed to conventional steel-prestressed beams. This article examines a less explored topic that often fails because post-tensioned beams with unbonded CFRP tendons are crushed by concrete. The linear-elastic behavior of CFRP accounts for much of the experimentally observed considerable variations in flexural characteristics between CFRP and steel-prestressed beams. Compared to unbonded CFRP specimens, bonded CFRP specimens are much stronger. Our knowledge of CFRP reinforcement in concrete infrastructure is advanced by this work, which highlights the need for better design techniques to increase strength and ductility. It is recommended that CFRP bars be bonded with epoxy resin to restrict fast energy dissipation and avoid concrete failure. As a result, CFRP-reinforced concrete beams will react less linearly and more ductilely. CFRP may improve prestressed concrete beam performance and lifespan in a number of scenarios, as shown by comparative research with traditional steel reinforcement. Six 1,800–mm-long RC beams with a 200 × 300 mm cross-section were examined and divided into three groups, analyzing important loading phases (i.e., cracked, ultimate, and mid-span deflection) using CFRP bars as prestressed tendons to replace the bottom steel rebars. The load-carrying capabilities and ductility of CFRP-reinforced beams were much greater than those of the reference beam. A single beam using normal CFRP bars for reinforcement showed a 21.2 mm mid-span deflection and an ultimate load of 157.2 kN, with a 57.7-kN fractured load. Another beam with normal CFRP bars also showed enhanced performance, with a mid-span deflection of 21.2 mm, an ultimate load of 160.6 kN, and a cracking load of 57.5 kN.

The structural-mechanical damage behavior of rice bran layer under tensile processing for moderate milling

Bran layer moderate removal during rice milling is a crucial process for reducing rice broken rate and nutrient loss. The bran layer structural-cracking behavior is analyzed using a self-developed in-situ observation tensile tester with high-speed image acquisition. The rice bran tissue microstructure, chemical composition, cracking morphology are in-depth investigated. The results show that rice bran pericarp cells are of reticulate network, cell shape with rectangular or elliptical.significant differences in cell distribution and density across rice varieties, with Japonica rice (NJ5055) with a denser and more uniform structure, exhibiting higher ductility and tensile strength. The rice bran exhibited linear elastic rheological behavior. The bran tensile property is different under tensing directions, the tensile strength shows longitudinal (Y) > radial (X). Cracking along the cell long axis is Y direction, showing orderly line cracking path. Cracking along cell short axis is X direction, showing zigzag cracking path. Pericarp cells geometrical structure determines the mechanical strength and cracking-removal resistance ability of rice bran. The pericarp layer significantly enhances the ductility of the bran layer, allowing it to stretch and deform under stress, thereby improving overall mechanical performance. Rice bran structural-mechanical properties are affected by micro-structure and components.

Numerical Analysis of the Cylindrical Shell Pipe with Preformed Holes Subjected to a Compressive Load Using Non-Uniform Rational B-Splines and T-Splines for an Isogeometric Analysis Approach

In this paper, we implement the finite detail technique primarily based on T-Splines for approximating solutions to the linear elasticity equations in the connected and bounded Lipschitz domain. Both theoretical and numerical analyses of the Dirichlet and Neumann boundary problems are presented. The Reissner–Mindlin (RM) hypothesis is considered for the investigation of the mechanical performance of a 3D cylindrical shell pipe without and with preformed hole problems under concentrated and compression loading in the linear elastic behavior for trimmed and untrimmed surfaces in structural engineering problems. Bézier extraction from T-Splines is integrated for an isogeometric analysis (IGA) approach. The numerical results obtained, particularly for the displacement and von Mises stress, are compared with and validated against the literature results, particularly with those for Non-Uniform Rational B-Spline (NURBS) IGA and the finite element method (FEM) Abaqus methods. The obtained results show that the computation time of the IGA based on the T-Spline method is shorter than that of the IGA NURBS and FEM Abaqus/CAE (computer-aided engineering) methods. Furthermore, the highlighted results confirm that the IGA approach based on the T-Spline method shows more success than numerical reference methods. We observed that the NURBS IGA method is very limited for studying trimmed surfaces. The T-Spline method shows its power and capability in computing trimmed and untrimmed surfaces.

Non-linear critical speed in high-speed ballasted railways with ground reinforcement

When constructing or upgrading high-speed railway lines on soft soils, reinforcing the soil foundation becomes crucial. This not only involves small track displacements but also increases the critical speed, thereby ensuring safe and efficient regular operations. Nevertheless, existing studies on the critical speed phenomenon in high-speed railway tracks with ground reinforcement are limited, and they all assume a linear elastic behavior for both the soils and reinforcement materials. The main novelty of this research is that it presents the first study on the effect of non-linear soil behavior on the critical speed of high-speed railway ballasted tracks with soil reinforcement and introduces a novel simplified approach based on an equivalent homogeneous soil layer by replacing the soil layer containing inclusions/columns. To accomplish this, a full 3D non-linear numerical model was developed and experimentally validated. The findings from a detailed and comprehensive parametric analysis demonstrate a significant influence of non-linear behavior on the critical speed, resulting in reductions of up to 30% compared to the linear elastic scenario. Furthermore, it is found that the soil-reinforcement stiffness contrast, the area replacement ratio, and the plasticity index of the soil foundation play a crucial role in the critical speed. Other aspects such as the installation pattern and the thickness of soft soil have a relatively lower impact on the critical speed.

Size effect analysis of mode I fracture performance of hot mix asphalt

The diameters of asphalt concrete cylindrical specimens typically measure either 100 mm or 150 mm, as these are the standard sizes for almost all conducted tests. However, the impact of specimen size on the fracture performance of asphalt mixtures has not been extensively explored. This study aimed to address this gap by performing laboratory tests on semi-circular bending (SCB) specimens. These tests were designed to apply the Size Effect Law (SEL) in evaluating the fracture performance of two asphalt mixture types (AC10 and AC16) at −10 ℃. The experiment selected four different diameters (76 mm, 100 mm, 125 mm, and 150 mm) and two notch-radius ratios (α0 = 0.2 and 0.4) to investigate the relationship between the specimen dimensions and both the nominal stress and fracture toughness (KIC). Results indicated that the nominal stress and KIC adhered to the SEL. It was observed that both aggregate gradation and notch-radius ratio (α0) significantly influenced the SEL’s effect on nominal stress. An increase in α0 from 0.2 to 0.4 shifted the fracture failure towards a more linear elastic fracture behavior. Specifically, the coarser gradation in AC16, compared to AC10, predisposed the fractures towards linear elastic behavior. Moreover, an increase in α0 notably augmented the brittleness number. For specimens with identical dimensions and α0, AC16 exhibited higher D/D0 values, indicating a stronger alignment with Linear Elastic Fracture Mechanics (LEFM) principles for the tested specimen sizes compared to AC10. As the size of the specimen increased, there was a gradual rise in KIC, with the enlargement from 76 mm to 150 mm resulting in KIC increases of 25.2 % and 12 % for α0 values of 0.2 and 0.4, respectively, in the AC10 mixture. Conversely, the AC16 mixture saw respective enhancements of 15.8 % and 7.6 % for the same α0 values. The influence of α0 on the apparent fracture toughness became less pronounced with its increase; KIC gradually decreased for AC10, yet increased for AC16 with rising α0 levels. For the same asphalt mixture type, KIC was found to escalate with both D/D’0 and α0. At an α0 of 0.4, the disparities between KIc and KIcm, – expressed as (KIcm- KIc)/ KIcm, – stood at 13.9 % and 9.2 % for AC10 and AC16, respectively, with a diameter of 150 mm. Based on these findings, it is recommended to utilize an SCB specimen with a diameter of 150 mm and an α0 of 0.4 for fracture performance testing, as these parameters appear to provide the reliable assessment of the materials’ fracture behavior.

Sputter-deposited Ni-rich NiTi thin films: Mechanical behavior and composition sensitivity

In this study, we investigated the mechanical behavior of sputter-deposited Ni-rich NiTi thin films with various chemical compositions. Freestanding thin films were successfully produced by employing magnetron sputtering and microfabrication techniques. Notably, films containing Ni within the range of 52.6–57.6 at. % exhibited distinct stress-induced martensitic transformations. Among these, 53.3 at. % and 54.2 at. % Ni–Ti films demonstrated a remarkable combination of strength exceeding 1.5 GPa and elastic strain approaching 5%. Conversely, films with the highest Ni content displayed nearly linear elastic behavior, showcasing an exceptional tensile strength of 2.2 GPa. As the element composition deviated from the equiatomic ratio, the grain size of the deposited films underwent a transition from several micrometers to nano-scale. In addition, the critical stresses showed lower sensitivity to chemical composition compared to bulk alloys, indicating that superelastic behavior persists over a wider composition range in Ni-rich NiTi thin films. By comparing the NiTi matrix and precipitate morphology between bulk alloys and thin films, we discussed the variations in the mechanical behavior of thin films.

Discovering 3D hidden elasticity in isotropic and transversely isotropic materials with physics-informed UNets

Three-dimensional variation in structural components or fiber alignments results in complex mechanical property distribution in tissues and biomaterials. In this paper, we use a physics-informed UNet-based neural network model (El-UNet) to discover the three-dimensional (3D) internal composition and space-dependent material properties of heterogeneous isotropic and transversely isotropic materials without a priori knowledge of the composition. We then show the capabilities of El-UNet by validating against data obtained from finite-element simulations of two soft tissues, namely, brain tissue and articular cartilage, under various loading conditions. We first simulated compressive loading of 3D brain tissue comprising of distinct white matter and gray matter mechanical properties undergoing small strains with isotropic linear elastic behavior, where El-UNet reached mean absolute relative errors under 1.5 % for elastic modulus and Poisson's ratio estimations across the 3D volume. We showed that the 3D solution achieved by El-UNet was superior to relative stiffness mapping by inverse of axial strain and two-dimensional plane stress/plane strain approximations. Additionally, we simulated a transversely isotropic articular cartilage with known fiber orientations undergoing compressive loading, and accurately estimated the spatial distribution of all five material parameters, with mean absolute relative errors under 5 %. Our work demonstrates the application of the computationally efficient physics-informed El-UNet in 3D elasticity imaging and provides methods for translation to experimental 3D characterization of soft tissues and other materials. The proposed El-UNet offers a powerful tool for both in vitro and ex vivo tissue analysis, with potential extensions to in vivo diagnostics. Statement of significanceElasticity imaging is a technique that reconstructs mechanical properties of tissue using deformation and force measurements. Given the complexity of this reconstruction, most existing methods have mostly focused on 2D problems. Our work is the first implementation of physics-informed UNets to reconstruct three-dimensional material parameter distributions for isotropic and transversely isotropic linear elastic materials by having deformation and force measurements. We comprehensively validate our model using synthetic data generated using finite element models of biological tissues with high bio-fidelity–the brain and articular cartilage. Our method can be implemented in elasticity imaging scenarios for in vitro and ex vivo mechanical characterization of biomaterials and biological tissues, with potential extensions to in vivo diagnostics.

Comparative Study on Flexural Behavior of Reinforced Concrete Beams Cast by Using Hydraulic Cement and Ordinary Portland Cement

This paper is a comparative study of the flexural behavior of reinforced concrete beams cast by using hydraulic cement and ordinary Portland cement for construction applications. The main variables used in this research comprise of type of cement, ultimate compressive strength of concretes, and curing time. Two different types of cement, including hydraulic cement and ordinary Portland cement, were used at three different ultimate compressive strengths of 18 MPa, 25 MPa, and 32 MPa with the curing times of 28 and 90 days. The beams' nominal span-to-depth (L/h) ratio is 8.0. The results showed that the reinforced concrete beams cast by using hydraulic cement and ordinary Portland cement had similar flexural behavior and failure patterns. Both types of the reinforced concrete beams exhibit linear elastic behavior up to approximately 80–90% of their maximum applied load. Then, the behavior of the beams is nonlinear in that the deflection increased rapidly with a slight increase in the applied load until reaching their failure. In addition, based on the experimental results obtained from this study, it was found that the ACI design equations are acceptable for predicting the design strength of the reinforced concrete beams cast by using hydraulic cement and the hydraulic cement can be an equivalent substitute for ordinary Portland cement for the reinforced concrete beams used in this study. Therefore, this application of the hydraulic cement is not only the environmentally friendly practices, but also promoting low-carbon society as well.

Does VIMC-FMC work well on highly ductile bimetal joints? An investigation on mixed mode I/II fracture of AA1050/Cu bimetal weakened by U-notches

In this study, it is attempted to check if the virtual isotropic material concept (VIMC), originally suggested for fiber-reinforced composite laminates, can be used as an appropriate simplifying tool in fracture estimation of notched bimetals. First, the aluminum alloy AA1050 and the copper are cladded together using the roll-bonding technique. Then, several U-notched bimetal specimens are manufactured to perform fracture tests under mixed mode I/II loading conditions. Due to the highly ductile behavior of the AA1050/Cu bimetal, the fictitious material concept (FMC) is adopted to equate the bimetal with a fictitious material having perfectly linear elastic behavior with the aim to avoid the elastoplastic fracture analysis. Finally, the VIMC and FMC are coupled with three famous brittle fracture models, i.e., the U-notch maximum tangential stress (UMTS), U-notch mean stress (UMS), and U-notch strain energy density (USED) criteria, for predicting the experimentally measured strength of the notched bimetal specimens. The results indicate that the maximum discrepancy between the theoretical predictions and the experimental results in all the combined failure criteria is less than 8%. Therefore, the VIMC-FMC combination is found to be quite effective as a converter to solve the complex problem of highly ductile fracture of notched inhomogeneous anisotropic bimetals under mixed mode I/II loading using only the linear elastic analysis of the fictitious homogeneous isotropic plates. Also, found in this research is that not only VIMC works well on fiber-reinforced composite laminates, but also on bimetal joints.

Prediction of single-layer antimony oxyselenide (Sb2O2Se2): metal-to-semiconductor transition via hydrogenation

In this study, the structural, electronic, vibrational, and mechanical properties of single-layer Antimony Oxyselenide (Sb2O2Se2) and its hydrogenated structure (Sb2O2Se2H2) are investigated by performing density functional theory-based first principles calculations. Geometry optimizations reveal that single-layer Sb2O2Se2 crystallizes in tetragonal structure which is shown to possess dynamical stability by means of phonon band dispersions. In addition, the mechanical stability of the predicted single layer is satisfied via the linear-elastic parameters. Electronically, it is revealed that single-layer Sb2O2Se2 exhibits metallic behavior whose highest occupied states are found to arise from the surface Se atoms, may be an indication for tuning the electronic features via surface functionalization. For the surface modification of Sb2O2Se2, top of each Se atom is saturated with a H atom and fully hydrogenated single-layer Sb2O2Se2H2 is shown to be an in-plane anisotropic structure. Phonon band dispersion calculations indicate the dynamical stability of Sb2O2Se2H2. Mechanically stable Sb2O2Se2H2 is found to possess anisotropic linear-elastic behavior, which is much softer than its pristine structure. Moreover, electronically a metallic-to-semiconducting transition is shown to occur as the unoccupied Se-orbitals are saturated via H atoms. Our work offers insights into prediction of a novel single-layer material, namely Sb2O2Se2, and reports the chemically-driven semiconducting behavior via hydrogenation, which may lead to the use of hydrogenated structure in solar cell, photoelectrode, or photocatalyst applications owing to its suitable band gap.

Atomic simulation of phosphorus segregation in nickel grain boundaries and its impact on deformation mechanisms

The segregation behavior of phosphorus on the grain boundary (GB) structure and mechanical response has been studied in the Ni–P system. Fourteen twist and tilt bi-crystals have been selected from the Olmsted dataset. Then, hybrid Monte Carlo/Molecular dynamics simulations (MC/MD) are used to obtain the equilibrium GB at 300 K and 1000 K, respectively. The present study indicates that solute phosphorus dissolved in the nickel matrix is inhomogeneous segregated to all of the GBs. GB characters including GB free volume and energy are found to correlate with segregation. The participated phase with Ni3P composition was observed only at 1000 K and for the lower energy GBs. Furthermore, we performed simulations of tensile deformation for the GBs with and without segregation. The twist and tilt GBs initially exhibit linear and non-linear elasticity behavior, respectively. The twist GBs demonstrate an increased yield strength with the help of phosphorus segregation. Besides, the thermal annealing process at 1000 K promotes uniform segregation at GB, leading to further strength enhancement. Conversely, the strength is weakened in segregated tilt GBs. This work illustrates the relationships among composition, GB structure, and mechanical properties.

Efficient and accurate uncertainty quantification in engineering simulations using time-separated stochastic mechanics

A robust method for uncertainty quantification is undeniably leading to a greater certainty in simulation results and more sustainable designs. The inherent uncertainties of the world around us render everything stochastic, from material parameters, over geometries, up to forces. Consequently, the results of engineering simulations should reflect this randomness. Many methods have been developed for uncertainty quantification for linear elastic material behavior. However, real-life structure often exhibit inelastic material behavior such as visco-plasticity. Inelastic material behavior is described by additional internal variables with accompanying differential equations. This increases the complexity for the computation of stochastic quantities, e.g., expectation and standard deviation, drastically. The time-separated stochastic mechanics is a novel method for the uncertainty quantification of inelastic materials. It is based on a separation of all fields into a sum of products of time-dependent but deterministic and stochastic but time-independent terms. Only a low number of deterministic finite element simulations are then required to track the effect of (in)homogeneous material fluctuations on stress and internal variables. Despite the low computational effort the results are often indistinguishable from reference Monte Carlo simulations for a variety of boundary conditions and loading scenarios.

Experimental Investigation of Ductility in GFRP RC Beams by Confining the Compression Zone

Nowadays, building structures in corrosive environments requires some considerations. Being lightweight, high tensile strength, and corrosion resistance are the features that make fiber-reinforced plastic (FRP) bars an alternative component for longitudinal steel reinforcement of concrete. On the other hand, the linear elastic behavior of FRP bars, alongside the brittle behavior of concrete, makes brittle members without considerable ductility. In this paper, the effect of compression region confinement with CFRP sheets on the FRP-reinforced concrete beams was experimentally investigated. Eight GFRP reinforced beams with 2 m length, including one reference beam and seven confined beams, were constructed and tested under a four-point bending test. Based on the type of confinement, specimens are categorized into four groups. Flexural behavior improvements, including load carry capacity, energy dissipation capacity, and ductility, were observed in at least one specimen of each confined group. According to the results, the specimen that was spirally confined with a 30 mm ribbon width and angle of 10° had the best total energy absorption up to about 110% improvement in comparison to the unconfined specimen. On the other hand, vertically confined specimens with 50 mm ribbon width showed the highest improvement in ductility indices and load carrying capacity up to 60% and 11% in comparison to unconfined specimens, respectively. Due to concrete compression zone fractures in flexural failure mode, the over-reinforce method is considered the design philosophy. Results indicate that regardless of the confinement type (discrete vertical, discrete spiral, or continuous spiral confinement), there is an optimal amount for width, blank space between ribbons, and depth of confinement to achieve the best flexural behavior.

Power law dependence of unstable slip velocity on decreasing shear loading stiffness

Shear loading stiffness plays a critical role in conditioning the stability of slip on reactivated faults. However, a relationship linking peak slip velocities and shear loading stiffness is lacking. To explore this, we shear granite faults in double direct shear with shear loading stiffnesses spanning two orders to define the effects of shear loading stiffness in conditioning the transition from stable to unstable slip. Our results show that peak slip velocity and acceleration decrease as power law relationships with respect to the shear loading stiffness ratio, with identical exponent, and with a linear relationship between the peak slip velocity and acceleration. The maximum acceleration occurs during the velocity weakening process that limits the peak slip velocity. The power law exponent increases linearly with increasing normal stress. Experimental results also indicate that slip magnitude, stress drop and recurrence time of unstable slip events increase, and slip durations decrease with decreasing stiffness ratios. The spans of limit cycles of velocity versus shear stress decrease, and their shapes evolve from triangular to semicircular with increasing loading stiffness ratio. Stress drops mostly occur during deceleration. The deceleration phase dominates the unstable slip duration that decreases as peak slip velocity increases. Our results indicate that the average stress drop rates over a slip duration increase as a power law relationship with reducing shear loading stiffness, which also contributes to a lower shear loading stiffness producing increased slip velocities and accelerations. Our findings highlight that loading stiffness ratio is an underlying mechanism defining unstable slip behaviors with the normal stress merely conditioning the exponent. The present relationship of unstable slip velocity with shear loading stiffness suggests a way to evaluate hazard of an impending instability event based on the initial shear loading stiffness ratio that can be calculated through the linear-elastic behaviors at the early quasi-static phase.

A new method for determining the ogden parameters of soft materials using indentation experiments

A full understanding of the material properties of skin tissue is crucial for exploring its tribo-mechanical behaviour. It has been widely accepted that the mechanical behaviour of skin tissue for both small and large deformations can be accurately described using a hyperelastic model, such as the one developed by Ogden. However, obtaining these Ogden parameters for in-vivo skin by in-vivo experiments no matter the indentation or suction tests is a significant challenge. The mathematical model used to describe the material behaviour during the test should consider not only the material nonlinearity but also the geometrical confinement of the tissue, the large deformations induced, and the fact that the specimens are relatively thin.A range of contact models is available to describe the contact behaviour during the indentation test. However, none of them can be used for hyperelastic materials with small thickness under large deformations. Simultaneously explaining material nonlinearity and geometric nonlinearity, either through theoretical equations or numerical calculations, poses a significant challenge.In this research, we propose a pragmatic method to obtain Ogden parameters for in-vivo skin tissue by combining experimental indentation results and numerical simulations. The indentation tests were used to obtain the force-indentation depth curves, while the numerical simulations were used to obtain the strain fields. The method assumes the material behaviour of specimens can be linearized in each small deformation increment, and the contact model developed by Hayes can be applied to accommodate each increment. Then, the linear elastic behaviour in each increment can be described by the elastic modulus E which were obtained using Hayes model, and the principal stresses in each increment were subsequently obtained using Hooke's law. By combining all stress fields, overall stress-strain curves can be constructed, from which the hyperelastic Ogden parameters can be obtained. A second numerical simulation of the hyperelastic indentation was then performed using the obtained Ogden parameters, allowing a comparison of the experimental and simulated relationships between force and indentation.

Fracture Toughness, Radiation Hardness, and Processibility of Polymers for Superconducting Magnets.

High fracture toughness at cryogenic temperature and radiation hardness can be conflicting requirements for the resins for the impregnation of superconducting magnet coils. The fracture toughness of different epoxy-resin systems at room temperature (RT) and at 77 K was measured, and their toughness was compared with that determined for a polyurethane, polycarbonate (PC) and poly(methyl methacrylate) (PMMA). Among the epoxy resins tested in this study, the MY750 system has the highest 77 K fracture toughness of KIC = 4.6 MPa√m, which is comparable to the KIC of PMMA, which also exhibits linear elastic behaviour and unstable crack propagation. The polyurethane system tested has a much higher 77 K toughness than the epoxy resins, approaching the toughness of PC, which is known as one of the toughest polymer materials. CTD101K is the least performing in terms of fracture toughness. Despite this, it is used for the impregnation of large Nb3Sn coils for its good processing capabilities and relatively high radiation resistance. In this study, the fracture toughness of CTD101K was improved by adding the polyglycol flexibiliser Araldite DY040 as a fourth component. The different epoxy-resin systems were exposed to proton and gamma doses up to 38 MGy, and it was found that adding the DY040 flexibiliser to the CTD101K system did not significantly change the irradiation-induced ageing behaviour. The viscosity evolution of the uncured resin mix is not significantly changed when adding the DY040 flexibiliser, and at the processing temperature of 60 °C, the viscosity remains below 200 cP for more than 24 h. Therefore, the new resin referred to as POLAB Mix is now used for the impregnation of superconducting magnet coils.

Notch strength evaluation for highly ductile Al/Cu bi-metal fabricated by the roll-bonding technique under pure mode I and pure mode II loading conditions

In this work, the aluminum alloy AA1050 and copper sheets are cladded together using the roll-bonding technique to make rectangular bi-metallic joints. To examine the notch strength of this bi-metal under quasi-static loading, central slits with two U-shaped ends are introduced into the bi-metal. The slit orientation angle is adopted such that for horizontal slit, the notched rectangular specimen experiences pure mode I loading. However, when the slit orientation angle changes to about 67 degrees, which is obtained by the finite element (FE) analysis, the U-ends of the slit are subjected to pure mode II loading. Considering three different notch tip radii, two loading conditions, and three replicates for each test configuration, totally 18 U-notched specimens are produced and tested with the goal to measure the notch strength of bi-metal experimentally. According to the experimental observations, the bi-metal joint is perfect such that crack initiates from the notch in the whole thickness, proposing that the whole bi-metal joint can be considered as a single bulk material. Based on these observations and thanks to the virtual isotropic material concept (VIMC), the AA1050/Cu bi-metal is firstly equated with a virtual bulk material having homogeneous and isotropic behavior. Secondly, due to highly ductile behavior of the bi-metal joint proven by the associated tensile stress–strain curve, the fictitious material concept (FMC) is used to equate the virtual isotropic material obtained from the previous step having elastoplastic behavior with a fictitious material having perfectly linear elastic behavior. Finally, the VIMC and FMC are linked to the U-notch maximum tangential stress (UMTS), U-notch mean stress (UMS), and U-notch strain energy density (USED) criteria to predict the notch strengths of the bi-metal joint experimentally measured. With always higher than 92% accuracy for all VIMC-FMC-UMTS, VIMC-FMC-UMS, and VIMC-FMC-USED combined criteria, it is shown that the two-level strategy of material simplification, i.e., VIMC-FMC, is quite successful to be combined with different brittle fracture criteria for predicting the notch strength of AA1050-Cu bimetals under both pure tensile and pure in-plane shear modes of loading.