Change in the structural state of grain boundaries under high-rate mechanical loading

The molecular-dynamic modeling of the behavior of a three-dimensional crystallite that contains a specific-type grain boundary under shear loading is performed. It is found that the accommodation of displacements of the material grains can be realized owing to structural changes in the intergrain boundaries. The crystal-like structure of the grains can be restored after the external action terminates. The results obtained give deeper insight into the nature of the structural response of the material under mechanical loading at the atomic level. Translated fromFizika Goreniya i Vzryva, Vol. 35, No. 6, pp. 112–114, November–December 1999.     

Effect of filler and void content on mechanical properties of pultruded composite materials under shear loading

This paper presents the results of an analytical investigation aimed at examining the mechanical behavior of fiber‐reinforced polymer matrix materials containing nonhomogeneous constituents such as fine clay fillers under shear loading. Analytical formulae that account for the presence of inhomogeneities in the matrix materials were developed for estimating the extensional and in‐plane shear moduli and Poisson's ratio. The analytical results were compared with the properties obtained from experiments. For the experiments, the pultruded composites containing clay fillers and voids as well as glass fiber and vinylester resin were selected, analyzed, and tested under shear loading. The effects of presence of constituents on the mechanical properties of matrix materials were also investigated based on the obtained volume fraction of their constituents. POLYM. COMPOS., 26:181–192, 2005. © 2005 Society of Plastics Engineers     

Nucleation of structural defects in materials with a perfect crystal lattice by thermal fluctuations under dynamic loading

A molecular-dynamic study of nucleation of structural defects in materials with an initially perfect crystal lattice by thermal fluctuations under high strain rates is performed. It is shown that thermal fluctuations can generate structural defects. There exists some threshold value of strain at which abrupt growth of regions with local structural changes is observed. __________ Translated from Fizika Goreniya i Vzryva, Vol. 42, No. 4, pp. 135–138, July–August, 2006.     

Propagation of solitary compression waves in heterogeneous materials under high-speed loading

Based on the method of molecular dynamics, we investigated the special features of the passage of soliton-like waves, initiated by high-speed loading, through regions with a low atomic density. It is shown that the shape of these waves is mainly determined by the structure of a material region in which they propagate. The decrease in the amplitude of solitary waves is shown to be determined by the defect concentration. This effect is of interest from the viewpoint of developing the methods of monitoring the microdamage build-up in materials. Translated fromFizika Goreniya i Vzryva, Vol. 34, No. 3, pp. 123–125, May–June 1998.     

Behaviour of cordierite materials under mechanical and thermal biaxial stress

Abstract

A commercial cordierite powder (< 0.17 wt-% impurities) was selected for a study of material behaviour under mechanical and thermal stresses. Disks were slip cast, sintered for 2 h at 1450°C, and indented (Vickers, 44.1 N) at the centre of the surface to be subjected to mechanical and thermal shock tests. The sintered bodies (84 wt-% cordierite, 10 wt-% mullite, 6 wt-% glass) reached 95% of theoretical density. The microstructure consisted of homogeneous, mainly equiaxed grains (mean size ≈0.5 μm) and a few elongated grains (aspect ratio ≈1.9). A glass phase was identified at triple points, and intergranular pores (< 10 μm) and a few isolated larger pores (up to 40 μm) were observed. The fracture strength σ_F was measured by biaxial flexure, employing a ball on discontinuous ring configuration with displacement con1 trol (0.05 mm min ^-1). In each thermal shock test, the indented specimen was heated to a selected temperature and the disk centre was then suddenly cooled using a high velocity air jet at room temperature. The initial temperature was increased by increments of 10°C until crack propagation was detected and the value of the thermal shock resistance Δ T_C was evaluated. The values obtained were compared with cordierite disks without indents and with alumina materials. The fracture features of the specimens broken in both mechanical and thermal shock tests (crack patterns and fracture surfaces) were characterised, taking into account the developed microstructures (grains, phases, pores) and the fracture origin at the controlled size defect introduced by indentation.     

Predicting the behavior of nonlinear viscoelastic materials under spring loading

Structural parts made of plastics are usually tested under creep loading conditions, i.e., the stress is applied almost suddenly and then kept constant, while defomation is measured. In practice, however it happens that such a structural part is loaded by an elastic member (for example, by a spring). In this case, the acting force is no longer constant; it decreases in the course of time, while the deformation of the specimen increases (and, obviously, that of the spring decreases). The present paper describes a numerical approach for the solution of this problem, based on the assumption that the creep behavior of the material is known. An example is presented.     

The mechanical response characteristics of sapphire under dynamic and quasi-static indentation loading

Sapphire is widely used as optical materials and substrate materials due to its excellent physical and chemical properties. The mechanism of crack propagation and fracture damage evolution has important significance for improving the manufacturing quality and application performance of sapphire parts. In this study, dynamic and quasi-static indentation tests have been performed on the c-plane and a-plane of sapphires by Hopkinson pressure bar tester and continuous indentation tester, respectively. The crack propagation path in sapphire has been captured by High-speed camera and the crack velocity has been calculated. The crack propagation and fracture damage evolution has been analyzed based on the fracture morphology of specimen. It was found that the bearing capacity of sapphire is related to the loading velocity, while the crack propagation is affected by the crystal orientation. Under the indentation loading, the cracks in sapphire first propagate steadily, and then the cracks begin to propagate uncontrollably after reaching the critical conditions, where the crack propagation velocity obviously increases, typically from 204?m/s to 1006?m/s (dynamic indentation) or from 0.0032?m/s to 820?m/s (quasi-static indentation). And the crack propagation velocity depends on the loading speed at stable stage. The r-planes of sapphire are weaker than other crystal planes and are prone to crack propagation.     

Mechanical behaviour of thick structural adhesives in wind turbine blades under multi-axial loading

Wind turbine blades are made of integrated composite parts bonded together using structural adhesives. The blades are among the most severely multi-axial fatigue loaded structures and the bonded joints play an important role in their structural integrity. For better understanding of the mechanical performance of the bonded joints, thorough knowledge is required on the multi-axial behaviour of the bulk adhesive. In this study, tubular specimens consisting of glass/epoxy bonding paste were subjected to uniaxial (tension, compression and torsion) and biaxial (tension–torsion and compression–torsion) static tests. Different biaxial ratios were used and the stress–strain responses were recorded using strain-gauges. The imposed biaxial stress ratios influenced the stress–strain behaviour of the material system, especially the compression and the shear stress–strain. A material model was developed based on the experimental observations taking into account the non-linear behaviour and the effects of the biaxial ratios and it was implemented together with a progressive damage scenario into a finite-element model. The experimental failure patterns were compared with the numerical simulations and a good match was found.     

Stationary and traveling hot spots in the catalytic combustion of hydrogen in monoliths

In the course of catalytic combustion of hydrogen (1-5% H₂ in air) in monolith reactors, strongly localized stationary and traveling hot spots arise in response to a sudden and persistent rise of gas flow velocity. Such hot spots may occur, e.g. in a catalytic converter following the acceleration of a car or in a catalytic combustor as a result of a load increase. This phenomenon is illustrated by simulations using a two-phase reactor model. The temperature overshoot of the adiabatic limit is typically of the order of the adiabatic temperature rise itself.The following mechanism underlies this behavior. Light fuel is supplied to the catalytic wall by fast diffusion (in the direction perpendicular to flow), while the heat released by reaction is removed from the wall by the slower, mixture-averaged heat conduction. This leads to accumulation of heat at the catalytic surface that eventually saturates at high temperatures. The hot spots may exhibit intricate dynamics, propagating downstream or upstream, or they may remain stationary. The direction of propagation depends on the relative strength of convective downstream and conductive upstream contributions to the overall displacement of reaction fronts. Generally, the hot spot tends to drift downstream at low flow velocities, remain stationary at intermediate flow velocities, and drift upstream at high flow velocities.     

Zirconia-based structural materials under conditions of flame-initiated pyrolysis of hydrocarbons

Problems arising in processing lower hydrocarbons into valuable final products are considered. It is shown that the role of heads of different configurations on the initiating burner is very important for economic use of the jet heat in pyrolysis of propane, methane, and domestic gas. The use of highly dispersed powders of stabilized zirconia obtained from alcoholates of metals made it possible to solve the problem of preparing high-temperature ceramic heads with maximum requisite properties. It is shown that it is possible to increase considerably the selectivity of formation of ethylene and propylene in flame-initiated pyrolysis compared to thermal pyrolysis. The use of special ceramic heads decreases substantially the temperature gradient between the reactor wall and the flame and the contact time of the raw material in the jet zone.Translated from Ogneupory, No. 10, pp. 8–11, October, 1995.     

Long-range linear elasticity and mechanical instability of self-scrolling binormal nanohelices under a uniaxial load

Mechanical properties of self-scrolling binormal nanohelices with a rectangular cross-section are investigated under uniaxial tensile and compressive loads using nanorobotic manipulation and Cosserat curve theory. Stretching experiments demonstrate that small-pitch nanohelices have an exceptionally large linear elasticity region and excellent mechanical stability, which are attributed to their structural flexibility based on an analytical model. In comparison between helices with a circular, square and rectangular cross-section, modeling results indicate that, while the binormal helical structure is stretched with a large strain, the stress on the material remains low. This is of particular significance for such applications as elastic components in micro-/nanoelectromechanical systems (MEMS/NEMS). The mechanical instability of a self-scrolling nanohelix under compressive load is also investigated, and the low critical load for buckling suggests that the self-scrolling nanohelices are more suitable for extension springs in MEMS/NEMS.     

Indentation crack profiles of cordierite materials under mechanical and thermal biaxial stresses

The indentation crack profiles under mechanical and thermal stresses of cordierite materials obtained from a commercial powder were studied. Disks were prepared by slip casting and sintering, and they were indented (Vickers; 44.1 N; 15 s) at the center of one of their surfaces to be subjected to flexion in the mechanical test or to a sudden temperature change in the thermal shock test. Indented specimens were fractured in biaxial flexure (BF) or thermal shock (TS) tests. Some indented disks were thermally treated following two different schedules that involve exposures at high temperatures without (T₁) and with a sudden cooling (T₂). These specimens were fractured in biaxial flexure to observe the fracture surfaces. In all specimens (BF, TS, T₁ and T₂), the evolution of the indentation crack profiles was fractographically analyzed by SEM. In order to study the crack shape and the profile evolution, both the length and the depth of the indentation cracks were measured.     

压缩条件下发泡PS黏弹塑性力学模型研究

为提高发泡聚苯乙烯(EPS)工程应用的长期稳定性,分析了压缩条件下EPS的力学性能。实验中EPS块体的受压变形呈现出黏弹塑性特征,并可分为3个阶段。随着加载速率的增大,屈服应变略微增大,但对切线弹性模量影响较小,在实验的基础上建立了表观密度与3个EPS压缩实验参数的关系。构造EPS块体的压缩黏弹塑性模型与实验结果的误差为0.37%。比较模型预测值与现场实测的压缩变形结果,结果表明两者图形趋势较为一致,用该模型可预测EPS块体的压缩变形。     

Compounding of glass fiber-reinforced polypropylene and investigation of its mechanical properties under simple and complex loading

Polypropylene is one of the important thermoplastics that has been reinforced with glass fibers to give a reasonably good engineering plastic. Because of its inert nature, it does not easily adhere to the glass surface and hence some improvement in mechanical properties. In the present study a vinyltriethoxysilane coupling agent in polymeric form has been tried and seen to improve the properties. Tests have been carried out at (a) constant strain rate (in the Instron testing machine), and (b)constant stress rate (water loading on a biaxial testing machine). Under constant strain rate only tensile properties have been studied while under constant stress rate, the tensile, torsional and combined tensile-torsional tests have been carried out. In each of the above cases, a considerable improvement in elastic modulus has been observed. The Tensile strength is improved to the extent of about 25 percent. There is only a slight improvement in torsional strength. Different fiber volume contents were used and their effect studied on modulus and strength. The mechanical data for reinforced-polypropylene samples have been discussed in terms of the fiber-length distribution in the composite. Detailed analysis of tensile data suggests that at low strains, when the critical length is relatively low, the fibers contribute to a high modulus. With increasing strain the critical length increases and the load carrying capacity of the fibers is reduced. Consequently the gain in strength is relatively less. The effect of fiber length distribution in torsion and combined tension-torsion tests would be expected to be similar and the results seem to confirm this.