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.     

Fracture behavior of magnesia refractory materials under combined cyclic thermal shock and mechanical loading conditions

To investigate the effect of cyclic thermal shock and mechanical loading on the fracture behavior of magnesia refractories showing different brittleness, the as-received and cyclic thermally shocked specimens are subjected to monotonic and cyclic wedge splitting test. The whole duration of test is monitored by digital image correlation and acoustic emission. Both thermal and mechanical fatigue resistance increase with the reduction of brittleness. Repetitive thermal shock results in pronounced reduction of strength. However, the specific fracture energy and nonlinearity increase after exposure to thermal shock due to the expanded micro-crack network inducing the development of a significant fracture process zone. Periodic loading mainly leads to the decrease of strain bearing capacity, as the fatigue loads favor the extension of crack tip instead of fracture process zone expansion. The combined application of periodic thermal shock and mechanical loads gives a new insight into the progressive damage behavior of refractory under critical conditions.     

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.     

Deformation and pore formation mechanism under tensile loading in isotactic polypropylene

Four different polypropylene (PP) samples were prepared through isothermally crystallizing at 0 °C (PP‐Q), 80 °C (PP‐80), 100 °C (PP‐100) and 120 °C (PP‐120). The results of differential scanning calorimetry, wide‐angle X‐ray diffraction, polarized light microscopy and tensile testing indicate that the spherulite structure gradually improves with increasing isothermal crystallization temperature. Meanwhile, the interface between spherulites becomes more obvious due to the larger dimension and the higher strength of spherulites. Therefore, the trend of interfacial debonding during stretching is enhanced distinctly. In addition, based on the structural characterization of samples at different draw ratios, two completely distinct morphological changes are demonstrated. There are no defects generated after longitudinal stretching within PP‐Q, because intra‐spherulitic deformation predominates, which is caused by the imperfect spherulites of PP‐Q. As a result, no microporous structure is produced after sequential biaxial stretching. And the improvement of the crystalline structure makes interfacial debonding more likely to occur. Therefore, fully developed crazes and cracks disperse between microfibril structures after longitudinal stretching. Furthermore, numerous microporous structures are produced through debonding of fully developed crazes and cracks after sequential biaxial stretching. Meanwhile, the quantity, dimension and uniformity of the microporous structures and the porosity are gradually improved. © 2017 Society of Chemical Industry     

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.     

Hetero-blast from a structural reactive material cylinder under explosive loading

A structural reactive material (SRM) cylinder is considered here as a limiting case of a dense metallic energetic system in which a mixture of metal particles is consolidated to the theoretical maximum density excluding porosity, to possess both high energy density and mechanical strength. Dynamic fragmentation and free-field explosion of a 103 mm inner diameter SRM cylinder charge is experimentally studied, with a wall thickness varying in a range of metal-to-explosive mass ratio M/C=1.3 to 4.0. Under explosive loading, the SRM cylinder produces a designated fragment size distribution divided into two groups: fine fragments with sizes on the order of 10² μm and below, and coarse fragments with sizes on the order ranging between 10⁰-10¹ mm. Prompt detonation shock-induced reaction (DSIR) of the expanding cloud of high-concentration fine fragments supplements the energy to enhance the primary blast as it propagates, while the coarse fragments form a high-speed, high-concentration metal momentum flux crossing the fireball and blast front to contribute to the total impulse loading to a nearby structure. Rapid impact-induced reaction (IIR) of the secondary fragments from high-speed coarse SRM fragments further enhances the reflected blast loading or generates a high interior explosion pressure as fragments perforate into the structure. The above distinctive characteristics of a unique hetero-blast are coupled effectively in the near-field range.     

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.     

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.     

Creep mechanical properties and creep model of high water material under step loading

High water material, a kind of filling and supporting material, is widely used in underground engineering, whose creep mechanical properties are directly related to the underground engineering stability. In this paper, creep tests of different types of high water material are carried out. The results show that: for the complete specimen and the single and double crack specimens with crack inclination angles of 0°, 30°, and 60°, the curves present initial elastic, creep attenuation and creep stabilization in the first four stages, and the creep acceleration appears in the fifth stage, leading to the final failure of the specimen; for the single and double crack specimens with a crack inclination angle of 90°, no creep acceleration appears. According to the test results, a new nonlinear viscoplastic body is proposed, and connected with the traditional Nishihara model, forming the nonlinear Nishihara model. The new model can clearly describe the creep characteristics of complete and cracked specimens at different stages. Finally, according to the new creep model, crack parameters are discussed with the creep rate index and the starting time of accelerated failure.     

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.