Изменение теплопроводности многослойного массива углеродных нанотрубок при его поперечном сжатии

Numerical simulation of thermal conductivity across a multilayer array of single-walled carbon nanotubes is carried out. The effect of transverse compression of the array on thermal conductivity has been studied. It is shown that the compression of the array can occur uniformly when all the nanotubes of the array are compressed equally, and it can occur inhomogeneously when a part of the nanotubes is strongly compressed, and the other part is weakly compressed. With homogeneous compression, the thermal conductivity of the array increases, but with inhomogeneous compression, it does not change and may even decrease in case of a large number of layers. This effect is especially pronounced for arrays of nanotubes of small diameter (D < 2 nm)

Study of the Mechanical Properties of Wood under Transverse Compression Using Monto Carlo Simulation-Based Stochastic FE Analysis

Wood is an anisotropic material, the mechanical properties of which are strongly influenced by its microstructure. In wood, grain compression strength and modulus are the weakest perpendicular to the grain compared to other grain directions. FE (finite element) models have been developed to investigate the mechanical properties of wood under transverse compression. However, almost all existing models were deterministic. Thus, the variations of geometry of the cellular structure were not considered, and the statistical characteristic of the mechanical property was not involved. This study aimed to develop an approach to investigate the compression property of wood in a statistical sense by considering the irregular geometry of wood cells. First, the mechanical properties of wood under radial perpendicular to grain compression was experimentally investigated, then the statistical characteristic of cell geometry was extracted from test data. Finally, the mechanical property of wood was investigated using the finite element method in combination with the Monte Carlo Simulation (MCS) techniques using randomly generated FE models. By parameter sensitivity analysis, it was found that the occurrence of the yield points was caused by the bending or buckling of the earlywood axial tracheid cell wall in the tangential direction. The MCS-based stochastic FE analysis was revealed as an interesting approach for assessing the micro-mechanical performance of wood and in assisting in understanding the mechanical behavior of wood based on its hierarchical structure.

Transverse Vibration of Viscoelastic Sandwich Structures: Finite Element Modeling and Experimental Study

In the present work, the nonlinear vibration behavior of elastic-viscoelastic-elastic sandwich (EVES) beams is studied. A finite element (FE) equation taking intoaccount the transverse compression deformation of the viscoelastic core for the EVES beams is derived. In order toaccurately characterize the frequency-dependent feature of the viscoelastic materials layer, athird-order seven-parameter Biot model isused. A 2-node 8-DOF element is established to discretize the EVES beams. The experimental testing onEVES beams validates the numerical predication of the FE model. Numerical and analytical investigations are carried on a series of EVES beams with different thicknesses. The results indicate that the presented FE model has better accuracy in predicting the natural frequency of the sandwich beams, and in predicting damping, the accuracy is related to the thickness of each layer. The results of this paper have important reference values for the design and optimization of the viscoelastic sandwich structure.

Effect of Strain Rate on the Transverse Tension and Compression Behavior of a Unidirectional Non-Crimp Fabric Carbon Fiber/Snap-Cure Epoxy Composite

The strain rate-dependent behavior of a unidirectional non-crimp fabric (UD-NCF) carbon fiber/snap-cure epoxy composite loaded along the transverse direction under quasi-static and dynamic conditions was characterized. Transverse tension and compression tests at quasi-static and intermediate strain rates were performed using hydraulic testing machines, while a split Hopkinson pressure bar (SHPB) apparatus was used for transverse compression tests at high strain rates. A pulse shaper was used on the SHPB apparatus to ensure dynamic equilibrium was achieved and that the test specimens deformed homogenously with a nearly constant strain rate. The transverse tensile strength at a strain rate of 16 s−1 increased by 16% when compared to that at quasi-static strain rates, while distinct localized fracture surface morphology was observed for specimens tested at different strain rates. The transverse compressive yield stress and strength at a strain rate of 325 s−1 increased by 94% and 96%, respectively, when compared to those at quasi-static strain rates. The initial fracture plane orientation for the transverse compression tests was captured with high-speed cameras and found to increase with increasing strain rate. The study provides an important data set for the strain rate-dependent response of a UD-NCF composite material, while the qualitative fracture surface observations provide a deeper understanding of the failure characteristics.

STRENGTH SIZE EFFECT IN FIBER COMPOSITES FAILING UNDER LONGITUDINAL AND TRANSVERSE COMPRESSION

The size effect in the structural strength of fiber reinforced composites has been typically analyzed for tensile failures. However, this is not true for the equally important compressive failures, primarily due to the difficulties in conducting compression tests on specimens of multiple sizes. These size effects are analyzed here numerically for two important compressive failure mechanisms in composites, viz. (i) fiber kink bands forming under longitudinal compression (typically accompanied by axial splitting matrix cracks) and (ii) inclined shear cracks forming under transverse compression. The former mechanism is modeled by a semi-multiscale microplane model, while the latter by the fixed crack model. Both models are calibrated and verified using available test data on carbon fiber composites and then used to predict the failure and load bearing capacities of geometrically scaled pre-cracked specimens of different sizes. In all cases, the predicted failure is found to be of a propagating nature, accompanied by release of strain energy from the specimen causing a distinct size effect in the nominal strength. For the composite considered here, under longitudinal compression, the fracture process zone (FPZ) is found to be fairly small (<1 mm) and the strength size effect is seen to follow linear elastic fracture mechanics (LEFM). The size effect deviates from LEFM for smaller specimen sizes due to increased flaw size insensitivity but cannot be fitted by Bažant's size effect law since the geometric similarity of the failure mode is lost. On the other hand, under transverse compression the FPZ is found to be much larger (34 to 42 mm) and the size effect is found to obey Bažant's size effect law, deviating from LEFM. The failure is geometrically similar despite being inclined to the pre-crack. These findings provide evidence of the general applicability of fracture mechanics-based size effect laws to compressive failure in fiber composites, and prompt suitable experimental investigations.

Comparison of the Compressive and Tensile Strength Values of Rocks Obtained on the Basis of Various Standards and Recommendations

The objective of the current study was to compare results relating to the compressive and tensile strength of rocks obtained during research undertaken according to Polish Standards (as part of the European standards known as Eurocodes), American Society for Testing and Materials (ASTM) Standards, and the recommendations of the International Society for Rock Mechanics (ISRM). A total of 130 experiments for uniaxial compression on axisymmetric samples, point loads, and transverse compression (so-called Brazilian tests) were performed on rock samples comprising granite, limestone, and sandstone. Geometric properties of the samples were selected depending on the applied research method, and the relationship between the specimen’s slenderness and shape, and the obtained values of compressive and tensile strength, were analyzed. The results of the study showed that values of compressive and tensile strength obtained in a laboratory depend significantly on specimen slenderness, different values of which are imposed by various ISRM standards and recommendations, wherein this sensitivity was much higher in the case of compressive strength. The study also raised doubt about the usefulness of the so-called point load test as a method for determination of the compressive strength of rocks and potential estimation of the tensile strength.

Контроль одноосных напряжений в сталях с учетом их магнитоупругой чувствительности

The magnetoelastic demagnetization of a number of steels in the state of delivery under longitudinal elastic tension, transverse compression and impact has been investigated. The estimation of the magnetoelastic sensitivity of the studied steel grades in the linear approximation is given. The possibility of determining the tensile and compressive stress in the mode of magnetoelastic memory is considered, taking into account the magnetoelastic sensitivity of the selected section of steel, and a method for its determination using dosed loading (impact or compression) is proposed. It is shown that the regression equation in the form of a polynomial is in satisfactory agreement with the experimental results.