To deeply understand the failure characteristics of defective rock under actual stress condition, impact tests were conducted on prismatic granite containing two rectangular holes with different ...axial static pre-stresses by a modified split Hopkinson pressure bar (SHPB), and uniaxial compression tests were also carried out for comparison. Combined with digital image correlation (DIC), the dynamic damage and fracture process of specimens were observed by low-speed and high-speed cameras. Moreover, the energy evolution characteristics of specimens were analyzed to further understand the failure mechanism. The results indicate that the pre-stress has dual effects on the dynamic mechanical behavior of rock specimens, and the transition mechanism of the effect of pre-stress can be revealed by the elastic deformation limit. Observations show that the failure of specimens under different loads is caused by the growth of secondary cracks at hole corners. However, with the increase in pre-stress, the crack mode tends to shear and the strain localization tends to concentrate on sidewalls, resulting in severe rock bursting and extensive fracturing. Four coalescence modes around two rectangular holes were summarized: diagonal shear coalescence under static load, no coalescence under dynamic load, shear coalescence inside the middle rock bridge area under the pre-stress of 25–55% UCS, and indirect coalescence outside the rock bridge area under the pre-stress of 75% UCS. The specimen with the pre-stress of 75% UCS releases the internal strain energy during dynamic failure process, while the specimen with lower pre-stress absorbs the external impact energy. Finally, some insights are provided for deep rock engineering based on the test results.
An austenitic stainless steel with large number density of nanosized NbC precipitates distributed in matrix was developed with the addition of small amount of Mn and application of proper ...thermo-mechanical process. Micro-pillar compression tests were performed to evaluate the strengthening effect of nanosized NbC. Despite much higher number density of NbC precipitates in the Mn-added alloy, the strengthening effect was measured to be significantly less than expected. Then, the compressed micro-pillars were then subjected to post-mortem microstructure analyses focusing on the dynamic evolution of NbC precipitates and austenite matrix. Extensive dissolution and re-precipitation of NbC precipitates were observed in the elastically and plastically deformed micro-pillars. In addition, stacking faults and deformation twin were also observed near the precipitates. From the relationship between the number density of the NbC precipitates, microstructure evolution, and dissolution of NbC, it was found that higher number density of nanosized NbC precipitates associated with dislocations in austenitic stainless steel enhanced dissolution of them by nano-scale yielding during elastic deformation, which limited the strengthening effect of nanosized NbC precipitates.
Abstract
A method of numerical solution of one-dimensional problem of cylindrical shear wave propagation in an elastic and elastic-plastic soil is developed in the article using the finite difference ...method. The numerical results obtained are presented in the form of graphs. From the results obtained, the attenuation of the parameters (shear stress, shear strain, and angular velocity) of cylindrical wave propagation with distance in an elastic and elastic-plastic soil was determined. The attenuation of waves with distance is justified by the dissipation of strain energy on the expanding cylindrical soil layer. In the case of load exceeding the elastic limit, plastic strains occur in soil near the point of load application. The boundaries of elastic-plastic strain of soil are determined.
Polycaprolactone (PCL) has been one of the most popular biomaterials in tissue engineering due to its relatively low melting temperature, excellent thermal stability, and cost-effectiveness. However, ...its low cell attraction, low elastic modulus, and long-term degradation time have limited its application in a wide range of scaffold studies. Dimethyl sulfone (DMSO2) is a stable and non-hazardous organosulfur compound with low viscosity and high surface tension. PCL and DMSO2 composites may overcome the limitations of PCL as a biomaterial and tailor the properties of biocomposites. In this study, PCL and DMSO2 composites were investigated as a new bio-scaffold material to increase hydrophilicity and mechanical properties and tailor degradation properties in vitro. PCL and DMSO2 were physically mixed with 10, 20, and 30 wt% of DMSO2 to evaluate thermal, hydrophilicity, mechanical, and degradation properties of the composites. The water contact angle of the composites for hydrophilicity decreased by 15.5% compared to pure PCL. The experimental results showed that the mechanical and degradation properties of PCL and DMSO2 were better than those of pure PCL, and the properties can be tuned by regulating DMSO2 concentration in the PCL matrix. The elastic modulus of the composite with 30 wt% of DMSO2 showed 532 MPa, and its degradation time was 18 times faster than that of PCL.
Coherent precipitates have long been known to alter the anisotropy in mechanical properties of rolled products with respect to rolling direction, which is manifested in terms of yield strength ...anisotropy (YSA) of age hardenable wrought aluminium alloys. The effect of anisotropy of matrix due to texture and of precipitate due to its morphology has been captured previously using elastic and plastic inclusion approaches with limited success. In the present investigation, yield strength of polycrystalline sheet specimen of Al-Mg-Si alloy in different ageing conditions was calculated by considering solid solution, grain boundary and shearable precipitate strengthening. Effect of coherent directional precipitates on yield strength anisotropy was analyzed using both elastic as well as plastic inclusion models. Additional elastic or plastic work required to deform the precipitates is taken into account along with work done in deforming the matrix to predict YSA. The concept of effective Taylor factor that accounts for anisotropic contribution from precipitates and crystallographic texture of matrix is proposed and incorporated in the existing strengthening models to predict YSA in aged samples. It is observed that modified plastic inclusion model that incorporates anisotropy in flow stress of precipitates is able to capture the experimental YSA in differently aged samples. Universal nature of modified plastic inclusion model is demonstrated by extending the concept to Al-Zn-Mg-Cu alloy.
The laser energy directly affects the strengthening of materials during laser shock peening (LSP). In this paper, the effect of propagation and attenuation shock wave on LSP was studied. Combined ...with the strengthening effect and defect degree, the best impact parameters and the effects of different impact pressures were obtained. The simulation and experimental results demonstrated when the unloading wave chased the shock wave and caught up at 100-200µm in the depth direction, the possibility of strengthening defects increased and the occurrence position deepened with the increase of laser energy. There was no strengthening defect on the surface of alloy when the impact pressure 4σ
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> (P) > 3.5σ
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. Strengthening defects appeared in the form of cracks 184.61µm from the surface of the alloy. When P > 4σ
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, the strengthening defects appeared on both the surface and subsurface, where the surface defects in the form of holes. The subsurface defects in the form of cracks that appeared 200µm away from the surface resulted from the unloading wave meeting the shock wave. Based on the research and the Hugoniot elastic limit, the optimal impact pressure range to demonstrate the best strengthening effects and to mitigate the formation of strengthening defects is 2σ
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< P < 3.5σ
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.
To simulate the complex human walking motion accurately, a suitable biped model has to be proposed that can significantly translate the compliance of biological structures. In this way, the simplest ...passive walking model is often used as a standard benchmark for making the bipedal locomotion so natural and energy efficient. This paper is devoted to a presentation of the application of internal damping mechanism to the mathematical description of the simplest passive walking model with flexible legs. This feature can be taken into account by using the viscoelastic legs, which are constituted by the Kelvin–Voigt rheological model. Then, the update of the impulsive hybrid nonlinear dynamics of the simplest passive walker is obtained based on the Euler–Bernoulli’s beam theory and using a combination of Lagrange mechanics and the assumed mode method, along with the precise boundary conditions. The main goal of this study is to develop a numerical procedure based on the new definition of the step function for enforcing the biped start walking from stable condition and walking continuously. In our previous work, it was shown that the period-one gait cycle can be produced by adding the proportional damping model with high damping ratio to the elastic legs. The present paper overcomes the practical limitations of this damping model and similarly demonstrates the stable period-one gait cycles for the viscoelastic legs. The study of the influence of various system parameters is carried out through bifurcation diagrams, highlighting the region of stable period-one gait cycles. Numerical simulations clearly prove that the overall effect of viscoelastic leg on the passive walking is efficient enough from the viewpoint of stability and energy dissipation.
The primary purpose of this study is to develop an asymptotic formulation for boundary value problems in a non-local elastic half-space. For the sake of simplicity, the non-locality is limited to the ...vertical direction, which is represented by a one-dimensional exponential kernel, and the problem is formulated within the framework of Eringen’s theory. The proposed asymptotic approach is based on the assumption that the internal characteristic length is significantly smaller than a typical wavelength. This assumption allows for the development of an asymptotic formulation that expresses the considered boundary value problem in terms of local stresses. Additionally, the formulation includes explicit correction terms to the classical boundary conditions, which arise from the non-local effects. As an example application of the derived formulation, the Rayleigh surface waves in a plane strain problem are considered. Finally, numerical results are presented for certain specific values of elastic parameters to illustrate the effects of non-locality on the analyzed system.
Using large-scale nonequilibrium molecular dynamics simulations, we study the roles of interfaces and layer thickness in the response of experimentally observed Cu/Nb nanolayered composites to shock ...compression. We observe a critical layer thickness (<20nm) below which lattice dislocations nucleate preferentially from the Cu/Nb interfaces. Within this regime of interface dominance, samples with a layer thickness of 5nm have the largest Hugoniot elastic limit (the critical shock pressure required for dislocation production), which then decreases for finer layer thicknesses, where dislocation transmission across the interfaces becomes more frequent. The dislocation slip systems emitted and transmitted across the interfaces are strongly linked to the interface structure and crystallography. The strong layer thickness and interface structure effects found here can provide insight for the design of shock-resistant nanolayered composites.
•Development of a computationally-efficient approach for seismic fragility evaluation.•Provides enhancements to an existing equivalent linearization techniques.•Introduces the concept of “Equivalent ...Linear Limit State” for fragility estimation.•Adopts fragility model based on concept of Equivalent Linear Limit State as a prior.•Obtain posterior estimates of fragility in a Bayesian updating framework.
Conventionally, the seismic response of primary structures such as buildings and secondary systems such as piping is evaluated through uncoupled analyses. Many studies have illustrated that the two systems interact in many different ways (mass interaction, non-classical damping, phasing, etc.). An analysis of the coupled system is not only rational but also eliminates the excessive conservatism that exists in an uncoupled analysis. Consequently, fragility assessments based on uncoupled analysis are also incorrect and a coupled analysis must be conducted in such evaluations. However, nonlinear analyses of such complex systems particularly in the context of fragility assessment, which requires a large number of nonlinear analyses, becomes computationally prohibitive. Tadinada and Gupta (2017) presented an equivalent elastic limit state concept with an intent to reduce the computational effort needed in these assessments and yet evaluate the seismic fragility with sufficient accuracy. This paper outlines some of the limitations that have been experienced in the use of originally proposed equivalent limit-state formulation and presents valuable enhancements. The novel contribution of this study is focused on accounting for the effect of uncertainty in nonlinear characteristics and the effect of non-classical damping. Unlike the originally proposed formulation, the proposed formulation also considers the asymmetric variation of the equivalent limit state with respect to tuning ratio. Furthermore, a Bayesian approach is incorporated into the proposed methodology for increasing the accuracy of seismic fragilities in the case of tuned or nearly tuned primary-secondary systems. Numerical examples are used to illustrate that the modified form improves the accuracy for both the tuned and the detuned systems. In summary, the proposed approach provides an efficient framework of seismic fragility assessment and risk evaluation for coupled systems.
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