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In this paper, nanoindentation test of zirconia ceramics was carried out under different strain rates by continuous stiffness measurement (CSM) technique at room temperature. This work aimed at studying the influence of indentation size effect (ISE) on strain rate sensitivity (SRS). The results showed that the contact stiffness increased almost linearly with the depth of the indentation. Under different strain rates, the contact stiffness and elastic modulus remain basically unchanged, but the hardness increases with the increase of the strain rate. The SRS exponent m is strongly dependent on the indentation depth. A calculation method for nano-hardness without ISE was proposed by detecting the relationship between hardness and indentation depth.

Temperatures of −25 °C, +5 °C, and +35 °C were selected to study the creep behavior of high-density polyethylene (HDPE). The ultimate tensile strength of HDPE materials was obtained through uniaxial tensile experiments and the time–strain curves were obtained through creep experiments. When the loaded stress levels were lower than 60% of the ultimate strength, the specimens could maintain a longer time in the stable creep stage and were not prone to necking. In contrast, the specimens necked in a short time. Then, the time hardening form model was applied to simulate the time–strain curve and the parameter values were solved. The parameter values changed exponentially with the stresses, thereby expanding and transforming the time hardening model. The expanded model can easily and accurately predict creep behaviors of the initial and stable creep stages as well as the long-term deformations of HDPE materials. This study would provide a theoretical basis and reference value for engineering applications of HDPE.

In this investigation, an asymmetrical yield criterion is proposed, and applies it to a hardness model which considers the interaction between statistically stored dislocations (SSDs) and geometrically necessary dislocations (GNDs). Based on this modified model, the relationship between pre-compression strain and hardness is analyzed by nanoindentation tests. In order to obtain the yield behavior of the CoCrFeNiMn high entropy alloy (HEA), the combined loading and uniaxial loading tests are conducted. Then different compression plastic strain is applied to specimens under various levels of temperature to investigate the interaction between SSDs and GNDs of CoCrFeNiMn HEA. Finally, the result shows that the effect of pre-compression depends on the competition between the pre-compression induced dislocations and nanoindentation nucleated dislocations because pre-compression causes the change of grain size.

Dynamic uniaxial and multiaxial tests were conducted to characterize the mechanical behavior of poly(methyl methacrylate) (PMMA). In this case, the multiaxial yield behavior of materials can be captured by the shear-compression samples (SCS). The experimental results were compared with different yield criteria, and which does not have a good agreement between theoretical prediction and experimental yield loci of PMMA undergoing combined shear-compression. Therefore, a phenomenological yield criterion is developed, and it notably involves hydrostatic stress sensitivity, lode angle, and one parameter. The parameter can be expressed by the asymmetry of yield strength between the tension and compression. The proposed criterion was verified by the utilization of differently combined shear-compression tests. The investigative results reveal that the proposed criterion can be successfully applied to describe the complex yield behavior of PMMA under dynamic loading conditions. The present study shows an efficient method to develop yield criterion of the reviewed materials.

A series of uniaxial and triaxial experiments are performed to analyze the mechanical behavior of foamed concrete (FC). The experimental results indicate that for uniaxial loading, foamed concrete demonstrates brittle failure under a uniaxial tension and small compression deformation. A 1D damage constitutive model is then established and expected to predict the nonlinear responses of foamed concrete under a large compression deformation. For triaxial loading, the level of confining pressure significantly influences the multiaxial mechanical behavior of foamed concrete due to its hydrostatically compressible characteristic. An elliptic phenomenological yield criterion based on three loading paths is further proposed and extended to consider the density-dependent effect by modifying the corresponding material constants as a function of material density. The comparisons between the theoretical predictions and experimental results reveal that the present yield criterion can give a good description of the tension/compression asymmetry, pressure dependence, and density sensitivity of the yield loci under the effective stress-mean stress space.

Quasi-static compression, tension and combined shear–compression tests were conducted to characterize the mechanical behavior of PMMA. The cylinder samples and dog-bone samples were used for uniaxial testing. The shear–compression samples of four different angles (15°, 30°, 45°, and 50°) were used to obtain the multiaxial stress state instead of complex loading equipment. The yield behaviors of PMMA cannot be described by the Tresca or Mises criterion due to the effects of both the first invariant of the stress and the third invariant of deviatoric stress. A criterion is proposed that includes the first invariant of the stress and the third invariant of deviatoric stress in this paper. In other word, the effects of hydrostatic pressure and Lode angle are considered. The proposed yield criterion has the capability to precisely capture the experimental yield loci of PMMA.

Two kinds of innovative re-entrant and hexagonal cell honeycomb sandwich structures filled with reactive powder concrete were proposed, and the ballistic resistance and energy absorption of the sandwich structures were investigated by numerical simulations. The deformation and failure modes of the different structures were analyzed and evaluated in detail. The honeycomb sandwich structures filled with reactive powder concrete prisms improved the capacity of ballistic resistance and energy absorption significantly, compared to the normal reactive powder concrete plates and sandwich structures without reactive powder concrete prisms. The analysis shows that the auxetic re-entrant cell honeycomb sandwich structures have a better ballistic performance than the hexagonal cell honeycomb sandwich structures. The sandwich structures were subjected to impact by three kinds of projectiles: flat, hemispherical and conical nosed. The ballistic limit of the flat nosed projectile is the highest, while the impact performance of the conical and hemispherical nosed projectiles is obviously different from the flat nosed projectile, especially in a relative high velocity range. The sharper nose leads to a higher value of exit velocity and mass loss. In addition, effects of different design parameters on ballistic resistance were also studied by changing the thickness of honeycomb cell and face plates. Results indicate that the thickness of honeycomb walls and face plates have significant effect on the ballistic resistance and energy absorption in a relative low velocity range, while there are no big differences when the initial impact velocity exceeds 400 m/s.

Based on the elongated Voronoi models, the yield behaviors of transversely isotropic foams have been explored by using finite element method. A pressure-dependent phenomenological constitutive model (stress potential function) has been established for transversely isotropic foams through the characteristic stress and strain. The stress potential function and its evolution can be determined in terms of the uniaxial and equi-triaxial tensile/compressive characteristic stress-strain curves. The stress potential function is expected to predict the elastoplastic mechanical responses of transversely isotropic foams. The material properties (i.e., relative density, transversely isotropic degree, matrix material and cell morphology) exhibit significant influence on the profile of initial yield surfaces. The pressure dependence of yield surfaces decreases as the relative density and transversely isotropic degree increase. The predictions of pressure-dependent constitutive model match excellently with the simulated yield data under different stress states and characteristic strain stages.

In this work, a modified reaction device was used to deposit metallised Ti films on Al2O3 substrate through the chemical vapour deposition method under different temperatures. Then, the mechanical characteristics of the films were investigated through scratch to determine the plastic behaviours and wear tests to determine the wear mechanism of the thin films. The scratch morphology, wear tracks and wear debris of the films were subjected to microstructural characterisation using a scanning electron microscope equipped with an energy-dispersive spectrometer. Results showed that the scratch and wear resistance of the films are systematically correlated with the plasticity and toughness of the films. The combined film mechanical parameters, such as H3/E2 and 1/HE2, obtained through the nanoindentation test correspond well with the highly loaded sliding contact behaviours of the films and can be used to describe the deformation mechanisms of the films. Furthermore, the film deposited at 1050 °C has potential applications in ceramic pre-treatments given its combined properties of plasticity and toughness. This combination confers durability to the film by providing an effective energy dissipation mechanism.

Based on the method of the geometric stretch, the elongated tetrakaidecahedron and Voronoi models in the rise direction have been established to investigate the multiaxial creep behavior of approximately transversely isotropic open-cell foams. A stretch factor R (the ratio of the cell length in the rise direction to that in the transverse direction) is introduced to quantificationally characterize the degree of transverse isotropy. Uniaxial simulation results indicate that, for the open-cell foams, the relative density has significant influence on the secondary creep rate and the effects of the microstructure cannot be ignored. Additionally, a modified phenomenological model is proposed to predict the multiaxial secondary creep rate of transversely isotropic foams, and the corresponding material constants are determined as a function of the stretch factor R by modifying the theoretical equations or fitting the FEA results. Then, the capability of the modified model is examined by simulating the creep behavior under axisymmetric loading. The simulation results indicate that the modified phenomenological model can give a good description of the uniaxial/multiaxial creep response of the transversely isotropic foams.

Finite element models, i.e. 2D honeycombs and 3D tetrakaidecahedron models, have been established to study defect-effect on the creep behavior of the cellular materials. Simulation results indicated that relative density have dominant effects on the steady state creep rate of the cellular materials. The creep behavior of the cellular materials is significantly correlated with the fraction of missing struts. A theoretical equation is proposed to predict the creep rate of cellular materials as a function of the ratio of different types of missing struts. The capability of the theoretical model was examined by simulating the creep behavior of two types of numerical models. The simulation results revealed that the theoretical model gives a good description of the creep response.

The thermal property of closed-cell aluminum foam is studied numerically and the effects of the distribution of the circular pore on the thermal property are studied theoretically. When the convection and radiation are ignored, the effects of porosity, cell size, and distribution forms of pore on the apparent thermal conductivity are investigated. Moreover, the effects of air in the pore on the thermal property are analyzed as well. Simulation results show that apparent thermal conductivity linearly increases with the increase of porosity, while the cell size and the distribution have negligible effects on the thermal property. By comparison, thermal conductivity of air has slight effect on thermal property of foamed aluminum in the context of small size pore.

An experiment was conducted to study the strain rate sensitivity of yield behavior in an amorphous polymer, poly(methyl methacrylate) (PMMA). Compression, tension, shear, and combined shear-compression tests were conducted at four different loading rates. Experimental results indicate that strain rates had a pronounced effect on the yield behaviors of PMMA. Therefore, a strain rate–dependent yield function was developed, and the influences of hydrostatic pressure and strain rate on yield were analyzed. Based on the detailed discussion of the three material constants of the yield criterion, the influence of hydrostatic pressure on the tension and compression asymmetry was analyzed. A modified Johnson-Cook model was introduced to determine the effect of strain rate hardening on flow stress. The results of a compression test were described well by this mode. The change in the trend of parameter C of the mode (which characterizes the effect of strain rate) was also analyzed under the conditions of quasi-static loading.

FINITE ELEMENT ANALYSIS WAS USED TO INVESTIGATE THE IMPACT RESISTANCE ABILITY OF THE FOAM-CORED SANDWICH SHELL STRUCTURES UNDER THE BALLISTIC IMPACT. THE BALLISTIC LIMIT, FAILURE MODES OF THE FACES AND ENERGY ABSORPTION OF THE UNIFORM/GRADED CORE WERE DISCUSSED IN THIS PAPER. AND BASED ON THE IMPACT PHASE DIAGRAMS, THE EFFECTS OF SEVERAL FACTORS, I.E. PROJECTILE SHAPE AND DIAMETER, FACE-SHEETS CURVATURE AND THICKNESS, ON THE NORMAL/OBLIQUE IMPACT BEHAVIOUR WERE ANALYZED IN DETAIL. RESULTS INDICATE THAT BLUNTER, LARGER DIAMETER OF PROJECTILE, HIGHER IMPACT VELOCITY AND THICKER SHEETS CAN SIGNIFICANTLY RAISE THE ABILITY OF IMPACT RESISTANCE FOR THE TARGETS. IN ADDITION, THE BEHAVIOUR OF BALLISTIC IMPACT BECOME MORE COMPLEX AS THE RESULT OF INCREASING THE OBLIQUE ANGLE.

This paper proposes a new concept of loadability for interconnected distribution systems, in which feeders are connected with normally-open tie-switches and the N-1 security principle is applied. Firstly, this paper presents a nonlinear model to formulate the N-1 loadability, in which network topology, thermal rating of lines and substation transformers, voltage limits and losses are considered. Secondly, Generalized Reduced Gradient (GRG) method is then used to solve the nonlinear programming problem. Meanwhile, a backward/forward sweep algorithm is also employed to solve the load flow calculation in the model. Finally, both an IEEE RBTS case and a real system are given to illustrate the proposed model and calculation method; and the accuracy is verified by the traditional N-1 contingency simulation.

Three different types of tests, i.e. uniaxial compression, shear and combined shear-compression have been conducted on PMMA (polymethyl methacrylate) at different loading rates in order to investigate its failure behavior under different loading conditions. The experiment results show that the material exhibits the rate-dependent failure and post-failure behavior. Macroscopic failure loci of PMMA were was determined experimentally in the shear-normal stress space. Further, a macroscopic failure criterion was proposed to predict the material's failure locus as a function of the loading rate. The capability of the failure criterion was examined by simulating the failure behavior of the material at different load rates. The simulation results revealed very good agreement between the experimental data and the responses determined from the proposed failure criterion.

A series of dynamic tests were conducted on a closed-cell aluminum alloy foams in order to determine experimental failure surface under impact loading conditions. Quasi-static tests have also been performed to investigate failure mechanism under different stress paths. Three typical types of deformation modes can be observed, which corresponds to the different failure mechanism. The failure loci of the foam in principal stress plane are explored from quasi-static to dynamic loading conditions. A significant strength enhancement is identified experimentally. The expansion of the failure locus from the quasi-static to the dynamic test is almost isotropic. A modified failure criterion for the metallic foam is proposed to predict failure locus as a function of strain rate. This rate-dependence failure criterion is capable of giving a good description of the biaxial failure stresses over a wide range of the strain rates.