Stress-deformation Curve Research Articles

Integrated waterproofing evaluation method for longitudinal joints of shield tunnel subjected to extreme surcharge: Numerical analysis and experimental validation

Sufficient waterproofing performance is critical to ensure a normal operation and safety of tunnels, and certain safety margin is essential to cope with uncertain external loads. This study proposes a novel methodology for evaluating the waterproofing performance of shield tunnels subjected to extreme surcharges, by integrating an analytic derivation, numerical analysis, and experimental validation. First, the analytical derivations of stresses at longitudinal joints and a finite element (FE) model for calculating the internal forces of tunnel linings under external loads are established; these are subsequently combined to compute the gasket deformation. Then, the stress–deformation curve of the gasket is fitted by a material-scale FE model to evaluate the contact stress against the underground water pressure. Finally, the methodology is validated via measurements from a full-scale experiment. The results help establish that the presumptions derived from the proposed method are consistent with the experimental results. It is also anticipated that the waterproofing performance would reach the actual failure point at the approximate 4–7 m depth of the equivalent dumped soil, which is remarkably consistent with the on-site observations in the Shanghai Metro tunnel. This study provides an efficient and economical method in terms of engineering applications, with emphasis on the importance of structural analysis in the waterproofing evaluation.

Determination of the mechanical properties of polymeric microneedles by micromanipulation

Precise characterization of the mechanical properties of polymeric microneedles is crucial for their successful penetration into skin and delivery of the loaded active ingredients. However, most available strategies for this purpose are based on compression of the whole patch, which only provide the average rupture force of the needles and can not give information on the variations across individual microneedles in the patch. In this study, we determined the mechanical strength of individual microneedles of two types of hyaluronic acid microneedles with or without loaded model drugs using a micromanipulation technique. The applied force as a function of displacement of the microneedles was recorded, which was used to determine the rupture displacement, rupture force, and then to derive and calculate normal stress-deformation curve, rupture stress and Young′s modulus of individual microneedles. The obtained data suggest that the molecular weight of the polymer and the loading of drug into the microneedles can significantly affect the rupture behavior and mechanical properties of the microneedles, which provides a foundation for preparing sufficiently strong microneedles for controlled drug delivery.

Experimental Study on the Complete Tensile Stress-Deformation Curve of Fully Graded Concrete

Concrete is a type of quasi-brittle material with high compressive strength and low tensile strength. To obtain the complete tensile stress-deformation curve of massive concrete specimens, a specially designed loading machine is proposed with a pair of hydraulic oil dampers parallel to the direction of the tensile loading. The dampers were designed with valves to modify the stiffness of the loading machine, enhancing the stiffness of the machine before the applied stress reaches 90% of the ultimate stress of the concrete. Several experiments were conducted with the proposed loading machine on prism-shaped specimens (size: 1350 mm× 450 mm × 450 mm), and the stress-deformation curves and mechanical parameters of the concrete were obtained. Finally, an empirical formula is presented and compared with the experimental results.

Torsion dissipated energy of hard rubbers as function of hyperelastic deformation energy of the Yeoh model

In this paper, we are proposing a new formulation of dissipated energy of hard rubbers as a function of the deformation energy expressed by the Yeoh hyperelastic model. Torsion deformation is considered as a planar deformation of a simple shear on the surface of a cylinder. Thus the deformation energy is dependent only on the first invariant of strain. Based on the experiment, a “hyperelastic proportional damping” (HPD) is proposed for hard rubbers under finite strains. Such damping is analogical to the model of proportional damping in the linear theory of viscoelasticity, i.e. the dissipated energy is proportional to the deformation energymultiplied by the frequency of dynamic harmonic loading. To obtain the experimental data, samples of hard EPDM rubbers of different harnesses were dynamically tested on a torsional test rig for different frequencies and amplitudes. The Yeoh model is chosen since the deformation function is dependent only on the first strain invariant for the description of the simple shear of a surface cylinder. The Yeoh constants are evaluated by curve fitting of the analytical stress function to the experimental torsion stress-deformation curve. The constants are used to express the deformation energy of the Yeoh model for specific cases of tested rubbers. The coefficients of hyperelastic proportional damping are evaluated on the basis of experimental results.

Torsion dissipated energy of hard rubbers as function of hyperelastic deformation energy of the Yeoh model

In this paper, we are proposing a new formulation of dissipated energy of hard rubbers as a function of the deformation energy expressed by the Yeoh hyperelastic model. Torsion deformation is considered as a planar deformation of a simple shear on the surface of a cylinder. Thus the deformation energy is dependent only on the first invariant of strain. Based on the experiment, a “hyperelastic proportional damping” (HPD) is proposed for hard rubbers under finite strains. Such damping is analogical to the model of proportional damping in the linear theory of viscoelasticity, i.e. the dissipated energy is proportional to the deformation energymultiplied by the frequency of dynamic harmonic loading. To obtain the experimental data, samples of hard EPDM rubbers of different harnesses were dynamically tested on a torsional test rig for different frequencies and amplitudes. The Yeoh model is chosen since the deformation function is dependent only on the first strain invariant for the description of the simple shear of a surface cylinder. The Yeoh constants are evaluated by curve fitting of the analytical stress function to the experimental torsion stress-deformation curve. The constants are used to express the deformation energy of the Yeoh model for specific cases of tested rubbers. The coefficients of hyperelastic proportional damping are evaluated on the basis of experimental results.

Experiment and analysis on mechanical properties of Artemisia selengensis stalk

The technology used for the storage and transportation of Artemisia selengensis is becoming increasingly important with its increasing consumption and demand; moreover, the amounts of artificial planting contribute to the challenges in Artemisia selengensis harvesting. Therefore, the mechanical property parameters of the Artemisia selengensis stalks were determined and researched to reduce the mechanical damage during harvesting, transportation, processing and storage. The Artemisia selengensis stalks were taken as the test object and a physical test method was adopted to study the impact of different positions, diameters, and directions on the mechanical properties by using the TMS-Pro texture analyzer; then, the relevant changing trends of the characteristic mechanical parameters were analyzed using statistical software. In the compression test, the compression load-deformation curve was observed and the breaking force and deformation were obtained; then, the compressive strength, elastic modulus and compression energy were computed. Next, the curve-fitting of the compressive strength and compression energy was carried out. In the shear test, the shear stress-deformation curve was obtained and the shear force, deformation, shear strength, and shear work were calculated. Then, the regression fitting of the section area, peak shearing stress and shearing work was conducted. Finally, in the last bending test, the bending stress-deformation curve, bending peak force, deformation, and bending work were obtained. Then the bending forces of other plants were tested, the results were compared and analyzed with the theoretical values, and finally the regression fittings were implemented. All the analysis results showed that Artemisia selengensis stalks can be considered to be anisotropic materials. The results also showed that the compressive strength and elastic modulus decreased with the height of stalk position, while the deformation increased. In addition, with the increase in the stalk diameter, the bending strength and fracture mechanical work increased, while the deformation decreased. The research results can provide a theoretical basis and reference for the design of the harvest equipment of Artemisia selengensis while minimizing its mechanical damage. Keywords: Artemisia selengensis, stalks, texture analyzer, mechanical properties DOI: 10.3965/j.ijabe.20171002.2660 Citation: Shi Y Y, Chen M, Wang X C, Zhang Y N, Odhiambo M O. Experiment and analysis on mechanical properties of Artemisia selengensis stalk. Int J Agric & Biol Eng, 2017; 10(2): 16–25.

Study of the last part of the stress-deformation curve of construction steels with distinct fracture patterns

The principal mechanical characteristics of construction steels are obtained by tensile testing. Nevertheless, the standards neglect the behaviour of steel beyond the maximum load point and do not define parameters related to the part of the stress-deformation curve that lies between the maximum load point and failure. The necking process that begins when the maximum load is reached makes it somewhat difficult to study the material behaviour beyond that point. However, the ductility of steel is highly affected by this last part of the load-deformation curve. For such a reason, and especially since structural safety is directly related to ductility, a deeper knowledge of this may help in designing safer structures. In this paper, this part of the load-deformation curve is analysed in two construction steels that exhibit distinct fracture patterns: one shows the typical cup-cone fracture surface, while the other shows a flat fracture surface with a dark region inside. An experimental campaign has been carried out with cylindrical specimens of contrasting diameters: 3mm, 6mm and 9mm for each material. The use of a digital image correlation system is shown to be extremely useful in studying the behaviour of steel beyond the maximum load point, with an innovative procedure for identifying the growth of the internal damage that leads to failure in a specimen being developed.

Manufacture of polycaprolactone and bovine bone filling Nukbone scaffold by 3D plotting system for bone tissue engineering

Event Abstract Back to Event Manufacture of polycaprolactone and bovine bone filling Nukbone scaffold by 3D plotting system for bone tissue engineering Karla Karina K. Gómez Lizárraga1, Carlos Escobedo2 and María Cristina Piña Barba1 1 National Autonomous University of Mexico (UNAM), Metallics and Ceramics, Mexico 2 Queen's University, Chemical Engineering, Canada Introduction: Bone tissue defects such as fractures, traumas, diseases and congenital problems are a devastating and costly problem in health care. One of the challenges is the development of new therapies or medical devices for improve the quality of life. Tissue Engineering[1] involves the use of biomaterials as scaffolds to provide structural support to cells, signaling molecules and cells to improve or restore tissue or organs that were damaged. The objective of this study is produces scaffolds from a blend of synthetic polymer polycaprolactone (PCL) and bovine bone filling (natural hydroxyapatite) using a rapid prototyping technology, 3-D Bioplotter® System and study their biocompatibility and mechanical properties to be use in bone tissue regeneration. Materials: PCL from Capa™ 6400, bovine bone filling Nukbone® (NKB) and synthetic hydroxyapatite (Sigma-Aldrich) were used. The blend was prepared by mixing PCL/NKB at different ratios. Method: All the scaffolds were built using a plotting system that consists in an extrusion process by the deposition of layer-by-layer until a 3D object was obtained. The blend was molten at 90°C in a high temperature stainless steel cartridge and plot by air pressure into a metal needle with a nozzle size of 300 µm. The compressive modulus was calculated from the stress-deformation curve. The chemical and physical characterization was development by FT-IR analysis, X-ray diffractometer and scanning electron microscopy. The biocompatibility of the scaffolds were tested with alamar blue reagent at 10% w/v and by seeding human mesenchymal stem cells from amniotic membrane (AM-hMSC) that were isolated according to methodology by Rodriguez-Fuentes et al[2]. and then differentiated into osteoblast-like cells. The visible absorbance was measured at 570nm. Finally the scaffolds were stained with Alizarin red staining 3 and Cristal violet dye. Results: The scaffolds showed biocompatibility, they encourage the function of cells demonstrated by the deposition of calcium salts over the 3-dimensional structure. The micrograph in SEM showed a porous structure with pores sizes about 620 µm an optimum value to allow cell attachment. There were slightly differences in compressive moduli between PCL and the blends of PCL/NKB samples. The XRD pattern exhibit low intensities in the peaks of the blend sample compared with the pure components and the FTIR spectra of the blend showed the chemical bands of each component of the mixture without chemical reaction. Discussion: The addition of hydroxyapatite into the blend increased the hydrophilicity of the PCL and as a consequence the biocompatibility. On the other hand, the use of an additive manufacture ensures the porosity of the 3D structure. Conclusion: The PCL/NKB scaffolds have been great potential to be used in tissue engineering due to mineralization of calcium salts. The porosity obtained and their interconnectivity played an important role in the biocompatibility with the cells. Dra. Nayeli Rodríguez Fuentes for her support in biological test; Prof. Brian Amsden and Dr. Leone Ploeg from Human Mobility Research Centre (HMRC) in Kingston, Ontario, Canada for the technical support provided.; PAPIIT for their financial support through IG100114 project, CONACyT through project 214128, to “The Association of Universities and Colleges of Canada (AUCC) by the support received by the scholarship Canada-Latin America and the Caribbean Research Exchanges Grants (LACREG)”; BECAS MIXTAS 2015 - MZO2016 Mobility abroad by the support received by the scholarship

Investigation of Hydraulic Fracture Propagation Using a Post-Peak Control System Coupled with Acoustic Emission

This study investigates the fracture mechanism of fluid coupled with a solid resulting from hydraulic fracture. A new loading machine was designed to improve upon conventional laboratory hydraulic fracture testing and to provide a means of better understanding fracture behavior of solid media. Test specimens were made of cement mortar. An extensometer and acoustic emission (AE) monitoring system recorded the circumferential deformation and crack growth location/number during the test. To control the crack growth at the post-peak stage the input fluid rate can be adjusted automatically according to feedback from the extensometer. The complete stress–deformation curve, including pre- and post-peak stages, was therefore obtained. The crack extension/growth developed intensively after the applied stress reached the breakdown pressure. The number of cracks recorded by the AE monitoring system was in good agreement with the amount of deformation (expansion) recorded by the extensometer. The results obtained in this paper provide a better understanding of the hydraulic fracture mechanism which is useful for underground injection projects.

A new constitutive law for the nonlinear normal deformation of rock joints under normal load

In view of the deviation of the fitting results of the classical exponential model and the hyperbolic model (the BB model) from several experiment data during intermediate stress period, a new constitutive model for the nonlinear normal deformation of rock joints under normal monotonous load is established with flexibility-deformation method. First of all, basic laws of the deformation of joints under normal monotonous load are discussed, based on which three basic conditions which the complete constitutive equation for rock joints under normal load should meet are put forward. The analysis of the modified normal constitutive model on stress-deformation curve shows that the general exponential model and the improved hyperbolic model are not complete in math theory. Flexibility-deformation monotone decreasing curve lying between flexibility-deformation curve of the classical exponential model and the BB model is chosen, which meets basic conditions of normal deformation mentioned before, then a new normal deformation constitutive model of rock joints containing three parameters is established. Two main forms of flexibility-deformation curve are analyzed and specific math formulas of the two forms are deduced. Then the range of the parameters in the g-δ model and the g-λ model and the correlative influence factor in geology are preliminarily discussed. Referring to different experiment data, the validating analysis of the g-δ model and the g-λ model shows that the g-λ model can be applied to both the mated joints and unmated joints. Besides, experiment data can be better fit with the g-λ model with respect to the BB model, the classical exponential model and the logarithm model.

Constitutive modeling of temperature and strain rate dependent elastoplastic hardening materials using a corotational rate associated with the plastic deformation

In this paper, a constitutive model with a temperature and strain rate dependent flow stress (Bergstrom hardening rule) and modified Armstrong–Frederick kinematic evolution equation for elastoplastic hardening materials is introduced. Based on the multiplicative decomposition of the deformation gradient,new kinematic relations for the elastic and plastic left stretch tensors as well as the plastic deformation-dependent spin tensor are proposed. Also, a closed-form solution has been obtained for the elastic and plastic left stretch tensors for the simple shear problem.To evaluate model validity, results are compared with known experimental data for SUS 304 stainless steel, which shows a good agreement with the results of the proposed theoretical model.Finally, the stress-deformation curve, as predicted by the model, is plotted for the simple shear problem at room and elevated temperatures using the same material properties for AA5754-O aluminium alloy.

EFFECTS OF CELL ANATOMY ON THE PLASTIC AND ELASTIC BEHAVIOUR OF DIFFERENT WOOD SPECIES LOADED PERPENDICULAR TO GRAIN

In radial compression, the shape of the stress-deformation curve varies for different wood species, particularly at the transition from elastic to plastic deformation and along the stress plateau. Due to differences in anatomy and cell wall microstructure, different responses to perpendicular loads were observed in spruce (ductile plastic deformation), oak (brittle failure), and beech (elastomeric yielding). Beginning plastic deformation was examined by SEM after the application of different compression levels and by dynamic observations during the loading process of small samples under a light microscope. It was demonstrated that radial compression of spruce is limited by the critical Euler buckling load of only a few cells closely behind the ring border. The compression behaviour of oak is determined by the buckling of the earlywood vessels and vasicentric tissue, whereas beech is characterised by the densification of the vessels at high plastic deformations.

Influence of Plasticizers on Tableting Properties of Polymers

This study relates tablet formation with relaxation properties of two polymers on the basis of the stress-deformation curve. The mechanical properties of the polymers were varied by changing tableting temperature, adding varying amounts of plasticizer, and incorporating a monomer with plasticizer effect on the polymer chain. The crucial parameter appeared to be the difference between the glass transition temperature and the tableting temperature. This temperature difference was found to determine the amount of energy stored during densification. The energy is manifested as the stress relaxation propensity of the material. Large stress relaxation yields porous and consequently weak tablets. At a low temperature difference (i. e., tableting temperature is much lower than the glass transition temperature), the amount of stored energy is large. An increase in tableting temperature, or a decrease in glass transition temperature, yields a decrease in stored energy as a result of a decrease in yield strength. Consequently, production of less porous and stronger tablet is possible. However, if the tableting temperature is higher than the glass transition temperature, the stress relaxation propensity of the deformed polymers is extremely high because the elastic modulus of the materials is low under these circumstances. This results in porous and even capped tablets. From the data it is concluded that, independent of the type of polymer and the method of plasticizing, compaction at a temperature of about 20 K under the glass transition temperature yields circumstances for which the amount of stored energy has a minimum. Consequently, tablet porosity has a minimum and tablet strength has a maximum. These circumstances are created by changing both the tableting temperature and the glass transition temperature of the powder.

Effect of water on deformation and bonding of pregelatinized starch compacts

This paper evaluates the tabletting process of pregelatinized starch with different moisture contents on the basis of the stress deformation curve. Simplification of the stress deformation curve enables the amount of elastically stored energy to be calculated. That stored energy, which is the driving force for relaxation of tablets, increases with compaction speed and decreases with increasing water activity of the material. This paper suggests a relation between absorbed water and stored energy. Interparticle bonding, however, also decreases with increasing amounts of adsorbed water. The decrease in stored energy with increasing water activity of the pregelatinized starch tends to produce stronger tablets at higher water activities, whereas the decrease of particle bonding with increasing water activity tends to produce weaker tablets at higher water activities. Given these two counteracting effects, the final tablet strength is a balance between viscoelasticity and bonding, resulting in a water activity where tablet strength has a maximum. In this case, the optimum water activity is about 0.70.

Porosity expansion of tablets as a result of bonding and deformation of particulate solids

This paper describes the tabletting process of y-sorbitol on the basis of the stress-deformation curve. This curve distinguishes between small, elastic deformation and large, viscous deformation. Small deformations can be quantified by the dynamic Young's modulus. The results demonstrated an effect of both rate of strain and initial particle size on the Young's modulus. The yield strength of compacts is a quantification of large deformations. There appeared to be an effect of strain rate on yield strength, but no clear relation between initial particle size and yield strength. The study relates elastic deformation with storage of elastic energy. The amount of stored energy was found to increase with compaction speed, and is postulated to be the driving force for changes of tablet porosities after compression. The attraction between particles causes resistance against porosity expansion, and is defined as expansion capacity. The expansion capacity showed no relation to compaction speed. It is therefore concluded that the effect of compaction speed on tablet properties is purely an effect of the amount of stored energy. The reciprocal value of expansion capacity demonstrated a direct relation with the constant that fits the relation between tablet strength and porosity. The expansion capacity is hence a quantification of bonding.

Fracture Energy and Tension Properties of High-Strength Concrete

A test setup and adequate instrumentation were developed to record the tension properties of high-strength concrete, including the postpeak softening response. The direct uniaxial tension tests were performed under a strain-controlled mode through a close-loop testing machine. The splitting tensile strength and modulus of rupture were also recorded conforming to standard ASTM test procedures. Test results revealed that high-strength concrete exhibits a more brittle and stiffer behavior with a large initial modulus of elasticity and a more sharply descending branch of the stress-deformation curve beyond the peak load. The unique softening behavior and the more brittle nature of high-strength concrete were expressed in terms of a stress-displacement (crack width) diagram and fracture energy. The fracture energy of high-strength concrete is estimated to be about five times the area under the ascending portion of the stress-deformation curve, compared to a corresponding value of 10 estimated for normal-strength concrete. Based on the test results, a constitutive relationship is recommended for the behavior of high-strength concrete in tension, including postpeak softening response.

時間依存型圧密履歴を受けた粉体層の力学的性質の変化

As was reported by the authors, it was recognized that the tensile strength of a powder bed prepared by compressive creep was increasing with time. This phenomenon is important in practical operations such as the storeage of powder.In the present work, to more quantitatively analyze this phenomenon, the transitional behavior of the tensile strength and stress-deformation curve are measured. The powder beds are prepared by three kinds of compressive histories, that is, creep, recovery of creep, and relaxation. In order to consider the mechanism of this phenomenon, the bottom stress distribution is measured during compressive histories. Materials used are Kanto loam, fused alumina, lactose and limestone powder.As results, it is recognized that the transitional processes of the tensile strength the depend on the preparations. In the case of creep and the recovery of creep and unlikely relaxation, the tensile strength is increasing with time. On the other hand, it is found that the bottom stress distribution tends slightly to become homogeneous for creep and recovery. From these results, the mechanism of increasing tensile strength with time is explained qualitatively by the homogenization of the bottom stress distribution.

Post-peak cyclic behaviour of concrete in uniaxial tensile and alternating tensile and compressive loading

Deformation controlled uniaxial tests have been carried out on cylindrical concrete specimens in order to establish the complete stress-deformation curve in tensile loading. Furthermore, cyclic tests have been performed under three types of loading: cycling from a tensile low stress, cycling from a compressive lower stress, and cycling between a lower and an upper stress to the envelope curve, respectively. The results seem to show that the tensile envelope curve is unique and the residual strength is the more affected the lower the stress at unloading is.

Some of the rheological characteristics of boron-siloxane polymer concentrates

The relaxation time and viscosity changes of 30–50% boron-siloxane polymer solutions as a function of the deformation rate γ yield curves of a complicated shape which have a central part indicating an anomalous increase. The part with the largest γ value on the stress-deformation curve shows in the pre-stationary stage a critical strength which characterizes the fracture of the intermolecular bonds formed during deformation. The exponential dependence of the viscosity on concentration has an index of 12, which contrasts with 6 for flexible, non-polar polymers; the activation energy of viscous flow is 6–17 kcal/mole instead of 3·5 kcal/mole applying to polydimethylsiloxane.

Dynamic strain aging in aluminum alloys

The athermal component of flow stress, as it is normally determined, is frequently in error for polycrystalline alloys of aluminum. The reason is that a transient occurs in the flow stress-deformation temperature curve for these alloys the character of which is a result of dynamic strain aging. The intensity of dynamic strain aging is a function of the initial structural state of the alloy as well as of its chemical composition. Dynamic strain aging tends to interfere with the normal tendency of an aluminum alloy when worked to form a distinct dislocation cell structure; the tendency to produce a non-cellular array of dislocations increases with an increase in the intensity of dynamic strain aging. Dynamic strain aging contributes significantly to the mechanical strengthening of aluminum alloys. Thermomechanical processing procedures which control this phenomenon allow unusual mechanical properties to be developed in aluminum alloys.