Search Results (8)

Freeze–thaw cycles lead to progressive damage in macro-defects within the rock mass, compromising its structural stability and ultimately resulting in frost-induced damage in rock mass engineering. Grouting plays a critical role in reinforcing fractured rock masses and enhancing their structural integrity. Investigating the physical and mechanical properties of grouted reinforcement bodies subjected to freeze–thaw cycles is of substantial theoretical and practical importance for ensuring the safe operation of rock engineering. This study focuses on fractured sandstone grouting reinforcement bodies to evaluate the impact of freeze–thaw cycles on their microscopic pore structure and macroscopic mechanical properties. Nuclear magnetic resonance (NMR) T₂ spectra demonstrate that freeze–thaw cycles progressively enlarge internal pores within the grouted reinforcement body, with pore characteristics evolving from micropores to mesopores and from mesopores to macropores. Triaxial compression test results indicate that as the number of freeze–thaw cycles increases, the peak strength, elastic modulus, cohesion, and internal friction angle of the grouted reinforcement body decrease, with both peak strength and elastic modulus following an exponential decline relative to the number of cycles. Furthermore, the crack dip angle and confining pressure exert significant influence on the failure mode of the grouted reinforcement body. Full article

The spectra of internal friction and temperature dependencies of the frequency of a free-damped oscillation process excited in the specimens of an amorphous–crystalline copolymer of polyoxymethylene with the co-monomer trioxane (POM-C) with a degree of crystallinity ~60% in the temperature range from −150 °C to +170 °C has been studied. It has been established that the spectra of internal friction show five local dissipative processes of varying intensity, manifested in different temperature ranges of the spectrum. An anomalous decrease in the frequency of the oscillatory process was detected in the temperature ranges where the most intense dissipative losses appear on the spectrum of internal friction. Based on phenomenological model representations of a standard linear solid, the physical–mechanical (shear modulus defect, temperature position of local regions of inelasticity) and physical–chemical (activation energy, discrete relaxation time, intensities of detected dissipative processes) characteristics of each local dissipative process were calculated. It was found that the intensities of dissipative processes remain virtually unchanged for both annealed and non-annealed samples. The maximum variation in the shear modulus defect is 0.06%. Additionally, according to computational data, small changes are also characteristic of the following parameters: the activation energy varies from 0.5 to 1.4 kJ/mol and the relaxation time changes from 0.002 to 0.007 s, depending on the presence or absence of annealing. As a result of annealing, there is a significant increase in the relaxation microinheterogenity of the polymer system across the entire temperature range (250% for the low-temperature region and 115% for the high-temperature region). Full article

A study was conducted on the internal friction spectra and temperature dependencies of the frequency of free damped oscillatory processes excited in the investigated samples of low-density polyethylene (LDPE) and high-density polyethylene (HDPE) over a temperature range from −150 °C to +150 °C. It was found that the internal friction spectra exhibit several local dissipative processes of varying intensity, which manifest in different temperature intervals. The structure of the internal friction spectra and the peaks of dissipative losses are complex, as evidenced by the occurrence of sharp, locally temperature-dependent jumps in the intensity of dissipative losses observed throughout the entire temperature range. A theoretical analysis was performed to explore the relationship between the anomalous change in the frequency of the oscillatory process and the defect in the shear modulus, as well as the mechanisms of internal friction for the most intense dissipative loss processes identified in the internal friction spectra. A significant difference was revealed in the structure of the internal friction spectra of LDPE and HDPE in the temperature range of −50 °C to +50 °C. A comparison of the LDPE and HDPE samples was conducted based on changes in their strength characteristics, taking into account the locally temperature-dependent changes in the shear modulus caused by local dissipative losses observed in the internal friction spectra. Full article

The article presents the chemical composition of vermiculite ores from the Kulantau deposits and the atomic structure of vermiculite during dehydration, in 1-, 2- and 3-layer hydrated states. It has been shown that vermiculites from deposits in the Turkestan region have significant differences in mineralogical composition. Rational ways of using Kulantau vermiculite as an innovative modifying additive in bitumen compositions are considered, which are intended to improve the asphalt concrete mixture during its preparation. A slight increase in structuring resins is a result of the elevated content of high-molecular-weight asphaltenes in the modified bitumen, as indicated by the analysis of the provided spectra. In turn, the systematization or structuring of bitumen leads to an increase in the mixture density, accompanied by a simultaneous rise in internal friction coefficients, and resistance to loads (shear and impact), as well as an enhancement in compressive strength. The influence of the chemical composition of Kulantau vermiculite on the structure of bitumen compositions is characterized by high adsorption capacity and effectively absorbs products resulting from the oxidation of hydrocarbons. Vermiculite is characterized by high efficiency in the activation phase, large pore volume, and selectivity. The alteration of the bitumen’s group composition due to the selective diffusion of oils, as well as resins in the material, occurs as a result of the interaction between bitumen and a material characterized by a fine-porous structure and high specific surface area. This process modifies the properties of bituminous layers on the surface of particles and leads to the formation of robust bitumen films appearing on the grain surfaces. Thus, enhancing the longevity of coatings, improving the quality of binding bitumen, and reducing asphalt concrete coverings necessitate the use of vermiculite in creating modified bituminous compositions. Full article

Velocity correlation spectra (VAS) in binary mixtures of water and glycerol (G/W), obtained by measurements using the modulated gradient spin echo (MGSE) NMR method, were explained by the interactions of water molecules with clusters formed around the hydrophilic glycerol molecule, which drastically change the molecular dynamics and rheology of the mixture. It indicates a thickening of the shear viscosity, which could affect the dynamics of submerged macromolecules. The calculation of the polymer dynamics with the Langevin equations according to the Rouse model, where the friction was replaced by the memory function of the retarded friction, gave the dependence of the dynamics of the polymer on the rate of shear viscous properties of the solvent. The obtained formula was used to calculate the segmental VAS of the polymer when immersed in pure water and in a G/W mixture with 33 vol% glycerol content, taking into account the inverse proportionality between the solvent VAS and friction. The spectrum shows that in the G/W mixture, the fast movements of the polymer segments are strongly inhibited, which creates the conditions for slow processes caused by the internal interaction between the polymer segments, such as interactions that cause disordered polypeptides to spontaneously fold into biologically active protein molecules when immersed in such a solvent. Full article

Cohesion is the main inter-controlled factor for the stability of fine-grained sediments in debris flow, and plays an important role in debris flow hazard early warning. At present, there is no cohesion rapid remote sensing detection model, which seriously affects the development of quantitative evaluation on debris flow stability. How to use remote sensing to quickly detect the cohesion of fine-grained debris has become an important scientific issue. Therefore, strengthening the research on the cohesion hyperspectral detection model, indicating its sensitive spectral bands, and establishing a quantitative model between cohesion and these bands are of great significance not only in discovering the stability mechanism, but also in quickly establishing the stability detection model for gully sediments. Taking the Beichuan debris flow as the study area, we carried out experiments on cohesion, cohesion influencing factors, and spectra. Firstly, six cohesion hyperspectral sensitive bands are indicated in red, near infrared portions of the electromagnetic spectrum, including 750, 1578, 1835, 2301, 2305, and 2309 nm; secondly, these bands discover the cohesion influencing factors. Band 750 nm indicates the characteristics of cohesion, effective internal friction angle, and permeability coefficient, while the other five bands indicate the characteristics of effective internal friction angle, density, and moisture; finally, a hyperspectral remote sensing detection model for the fine-grained sediments cohesion is established. With a correlation coefficient of 0.56, and p value less than 0.001, the model indicates that cohesion has a great significant correlation with the six bands. This not only provides sensitive bands for detecting cohesion of fine-grained sediments using remote sensing, but also provides a scientific basis for rapid detection of the fine-grained sediments’ stability in large areas. Full article

The study of spectra of internal friction $\lambda =f\left(T\right)$ and temperature dependencies of frequency of freely damped $\nu =f\left(T\right)$ oscillatory process excited in investigated samples of polyethylene with different degree of crystallinity in the temperature range from −150 ${}^{°}$C to +150 ${}^{°}$C. It is established that four local dissipative processes of different intensity shown in different temperature intervals are observed on the spectra $\lambda =f\left(T\right)$. These are $\mu$, $\beta$, $\alpha$, ${\beta }_k$ processes. The theoretical analysis of the relationship between the anomalous changes of the vibrational process frequency $\nu =f\left(T\right)$ and the shift modulus defect $\Delta G=f\left(T\right)$ and the internal friction mechanisms for each of the dissipative loss processes detected on the spectrum $\lambda =f\left(T\right)$ is carried out. The influence of supramolecular structures on local dissipative ${\beta }_k$ process in polyethylene is estimated. Full article

In stepped-sine testing of strongly nonlinear structures with the classical force-control strategy, corrective force perturbations of a standard controller used to capture the reference signal in the proximity of turning points of frequency response curves may often lead to a premature jump before reaching the actual resonance peak. Accordingly, a classical force-control approach is not suitable to identify backbone curves of strongly nonlinear structures. This paper shows that currently available commercial modal test equipment can accurately identify backbone curves of strongly nonlinear structures by using Response-Controlled stepped-sine Testing (RCT) and the Harmonic Force Surface (HFS) concept, both recently proposed by the authors. These methods can be applied to systems where there are many nonlinearities at several different (and even unknown) locations. However, these techniques are not applicable to systems where internal resonances occur. In RCT, the displacement amplitude of the driving point, rather than the amplitude of the applied force, is kept constant during the stepped-sine testing. Spectra of the harmonic excitation force measured at several different displacement amplitude levels are used to build up a smooth HFS. Isocurves of constant amplitude forcing on the HFS lead to constant-force frequency response curves with accurately measured turning points and unstable branches (if there are any), which makes it possible to identify backbone curves of strongly nonlinear structures experimentally. The validation of the proposed approach is demonstrated with numerical and experimental case studies. A five degree-of-freedom (DOF) lumped system with five cubic stiffness elements, which create strong conservative nonlinearity, is used in the numerical example. Experimental case studies consist of a cantilever beam and a control fin actuation mechanism of a real missile structure. The cantilever beam is supported at its free-end by two metal strips constrained at both ends to create strong stiffening nonlinearity. The control fin actuation mechanism exhibits very complex and strong nonlinearity due to backlash and friction. Full article