Formation Moment Research Articles

A continuous phase-evolution model for cold and strain-induced crystallization in semi-crystalline polymers

The dynamic crystallization during extreme thermomechanical history under producing or service strongly influences the dimensional stability of the semi-crystalline thermoplastic polymers. To produce high-precision structural components, the proper thermomechanical history needs to be carefully designed to eliminate the crystallization-induced shrinkage. On the contrary, the rapidly developed four-dimensional (4D) printing technology tries to amplify the crystallization-induced shrinkage to directly produce the complex curvatures without supporting materials. However, the crystals formed at different deformation states might have different orientations, leading to an anisotropic residual deformation. Therefore, tracing of all crystals formed with different initial configurations is the key issue to ensure the geometric accuracy of both the high-precision and the deformable components. In this study, we developed a continuous phase-evolution model for both the cold and the strain-induced crystallization in semi-crystalline thermoplastics. The irreversible cold crystallization is analogized as the raindrop falling into the pool, and the reversible strain-induced crystallization is analogized as the nonequilibrium liquid-gas phase transformation. Using the continuous phase-evolution concept, the crystal growth is described by a series of continuously formed crystal phases sequentially added into the initial amorphous medium. Each newly formed crystal phase is in a stress-free state at the formation moment, and therefore the crystallization history coupled with the whole deformation history can be memorized. By introducing the oriented growth tensor representing the stress-free state of all formed crystals, the deformation-history dependent anisotropic crystallization and the corresponding residual deformation can be traced. The developed model is validated by comparing with the experimental data of the dynamic crystallization in amorphous poly l-lactide polymers under thermomechanical loading cycles. Finally, the predictive capability of the model is illustrated by several demonstrations, to show the influences of deformation-history dependent crystallization orientations and the corresponding anisotropic residual deformation.

A fractional dual-phase-lag generalized thermoelastic model of ultrashort pulse laser ablation with variable thermal material properties, vaporization and plasma shielding

The ultrashort pulse laser is widely used in micro scale non-contact machining of nonmetallic materials because of its high energy density, short pulse duration and small heat affected zone. Ablation is the main material removal mechanism in ultrashort pulse laser processing. This motivates us to establish the fractional dual-phase-lag (FDPL) generalized thermoelastic ablation model and investigate the transient responses of silicon ablated by the picosecond pulse laser. The temperature-dependent material properties, surface recession velocity and plasma shielding effect on the subsequent laser beam have been paid more attention. In calculation, the temperature distributions at the surface melting moment and plasma formation moment are taken as the initial conditions of the next Phases. The coupled governing equations containing fractional order parameter, lag times as well as spatial nonlocal parameter are formulated and solved by Laplace transform together with its numerical inversion. The temperature, displacement and stress with different laser intensity, pulse duration, fractional order parameter, lag time ratio as well as times are obtained and illustrated graphically. The accurate thermoelastic coupling description of silicon processed by the ultrashort pulse laser is obtained, which provides a reliable theoretical guide for high-quality laser processing of nonmetallic materials.

A numerical study on nested cavities during the water entry of deformable elastic spheres

Among the three typical cavities formed by elastic spheres, the nested cavities are investigated, including the formation moment, influencing factors, size and quantities. To achieve this, a fluid-solid interaction model is used to describe the water entry of elastic spheres. The results show that the formation moment is related to the sphere diameter, density and material shear modulus, but not to the impact velocity. The formation moment and number of nested cavities can be predicted from the material properties. Additionally, we have shown the change characteristics for the dimension parameters of the nested cavity with the dimensionless ratio of material shear modulus to impact hydrodynamic pressure and the sphere density. The higher the sphere density, the longer the nested cavity. The results also show that the cavity pinch-off time is modulated by the sphere size and density. With the increase of the dimensionless parameter for time D/(μ0/ρS)1/2, the pinch-off time grows rapidly first and then slowly.

Experimental study of cavity evolution and deformation during water entering into hyperelastic sphere

Hyperelastic materials, which have strong nonlinear mechanical properties, are commonly used in the engineering field. The application of hyperelastic materials to the water entry problem is a new interdisciplinary research topic. Unlike the water entering into a traditional rigid sphere, the hyperelastic sphere is very easy to deform during water entry. In order to explore the fluid-structure coupling problem with large deformations during water entry, a high-speed camera is used to study the problem of vertical water entering into hyperelastic sphere in this paper. Based on the experimental results, the effects of the material properties and impacting conditions on the cavity flow and sphere deformation behaviors during water entry are compared and analyzed. The experimental results show that the formation of the nested cavity after impacting a free surface of the hyperelastic sphere needs large enough impact conditions and small material shear modulus. The time for the nested cavity to be formed and retained during water entry is related to the material shear modulus and sphere diameter. The sphere displacement and length of cavity formed by the hyperelastic sphere increase with the increase of the impact velocity and material shear modulus, but decrease with the increase of the diameter of the sphere. The increase of the impacting velocity can only aggravate the deformation behaviors of the hyperelastic sphere, but does not affect the formation moment of the nested cavity. In addition, the characteristics for the deformation behaviors of the hyperelastic sphere to vary with the Froude number and the dimensionless ratio of material shear modulus to impacting hydrodynamic pressure are described and studied.

Dynamics of Mobile Concentration Fronts in Gas–Liquid Reaction Systems: Analysis and Numerical Experiment

A mathematical model has been developed for the process of chemosorption with the moving front of an instantaneous irreversible reaction at small Reynolds numbers in a gas–liquid system before and after the reaction front formation moment. An analytical solution of this model has been found for the time period to the formation of a mobile chemosorption front with consideration for the effect of a reaction product on the dynamics of chemosorption. In contrast to the earlier known results, the model takes into account the effect of the diffusion transfer of a reaction product on the kinetic characteristics of chemosorption and different stages of the formation and motion of the concentration front. Some results of the processing and analysis of numerical experiment data are presented. The characteristic depths and times of the penetration of a captured component are plotted as functions of all the process-control parameters. Sherwood numbers and absorption acceleration and mass transfer coefficients are calculated for the captured component in the liquid phase.

Comparison and analysis of shockwave characteristics between underwater pulsed discharge and metal wire explosion

In order to study the difference in energy conversion efficiency of the shockwaves induced by the expansion of the plasma channel generated by underwater metal wire explosion (MWE) and subsonic streamer breakdown discharge (SSBD), a test stand was constructed, and the experimental results were obtained based on the condition that the voltages of the two discharge modes were the same at the time of the formation of plasma channels. The results showed that the peak value of shockwaves induced by the expansion of the channel and the energy conversion efficiency of MWE were much higher than those of SSBD. The reason lies in the difference in the injected power at the formation moment of the plasma channel and the difference in the time distribution of energy deposited into channels.

Temperature Variations of Gold Nanoparticle and Dynamics of Plasmonic Bubble in Water Under Nanosecond Pulsed Laser

Suspended gold nanoparticle in water medium starts to warm up under nanosecond laser irradiation and creates a bubble around itself. The present study aims at evaluating the amount of nanoparticle size reduction at boiling temperature, the temperature variations of the nanoparticle, and its medium and finally the bubble formation moment. To this aim, Mie theory was used to calculate the absorption cross section of the nanoparticle in proximity of the bubble. Heat transfer equations were applied to determine the temperature of the nanoparticle and water. In addition, hydrodynamic equations were initiated to evaluate the expansion of the bubble. Then, these three groups of equations were coupled together and solved numerically. Based on the results, the bubble forms at the critical pressure and consequently due to the slow bubble velocity, temperature gradient in the medium is observed. Further, slight pulse width variations play a significant role on the nanoparticle temperature. The calculation of the nanoparticle heating associated with the creation of the bubble helps in controlling nanoparticle size and understanding the nanoscale heat transfer processes.

Experimental Studies of the Crack Formation Moment of Reinforced Concrete Beams

The main task of construction is providing buildings with the property to remain operable throughout the entire life cycle. The level of bearing capacity, both of individual structures and buildings as a whole, depends on many factors.

Estimation of the Limit State Criterion for Steels C1020 and A 516-55 with V-Notch under Quasistatic Loading Using Acoustic Emission Control

There is a need to determine the stresses value for responsible designs in the oil and gas refining, petrochemical and chemical industries, at which the defect reaches its maximum size. This stress value can be taken as the limit state criterion for strength calculations of vessels and apparatus. The article presents a technique for determining the limit state criterion for carbonaceous and low-alloy steels using the example of steels C1020 and A 516-55 in the state of delivery. As a limit state criterion it is proposed to use the stress value corresponding to the microcrack formation moment. The studies are based on the uniaxial tension tests results at room temperature of flat samples with V-shaped stress concentrators of 2 mm deep, accompanied by an acoustic emission nondestructive testing method. Based on the obtained pulse amplitude and pulse energy dependences on time in the process of testing and fractographic studies, a technique for estimating acoustic emission signals was developed to determine the microcrack formation moment for test steels. Stress values that can be taken as the limit state criterion for steels C1020 and A 516-55 in the delivery state for strength calculations vessels and apparatus are determined using the developed experimentally technique based on the high-amplitude acoustic emission to low-amplitude signals ratio. With the help of specimen fractures fractographic analysis the legitimacy of using the developed technique is justified. Thus, at room temperature for steel C1020, the stress at which a microcrack begins to form is 420 MPa, for steel A 516-55 it is 455 MPa.

Method of defining the crack formation moment in the bendable reinforced concrete components in the non-linear deformation model

The article proposes applying energy from law deformation mechanics of the rigid body to the development and improvement of the deformation method for calculating the strength of reinforced concrete constructions with the use of diagrams illustrating the deformation of concrete and reinforcement. In terms of the strength energy theory, concrete and rebar accumulate potential energy in the section of the construction component under stress; based on the contours of the diagram used in calculation it is possible to distinguish a stress diagram for concrete of the compressed zone. The value of the stress in the section is equivalent to the force used for the deformation of the concrete specimen under stress (prism- or cylinder-shaped); the force is equal to the area used in the calculation of the normable diagram. The calculations of the resolving equations are performed with the hypothesis of flat sections. The conditions of the stress balance in the section of the construction component are tested with the method of the successive approximation. The moment of cracking is calculated in reference to the neutral axis after defining the stress in concrete of the strained zone with the preconditioned accuracy.

Modeling of micrometeoric streams under the action of the high-power laser pulse on multicomponent polycrystal rocks

The paper presents the analysis of interaction of the high-power laser pulse with multicomponent polycrystal rocks. Experiments were completed on a laser facility “Saturn” with intensity of 1010-1013 W/cm2. Structural analysis of the materials from the spall crater and from the plasma flame show significant differences. The article demonstrates experimental results of the spall formation moment depending on the thickness of the radiated target. The scale of damage to aluminium 6 pm thick foil at the rare side of the target is illustrated when it is hit by the andesite fragments from a spall crater.

Electronic structure and vibrational spectrum of acetylthiocarbamide

An approximate quantum chemical optimization of the geometric parameters of the acetylthiocarbamide molecule CH3CONHCSNH2 was carried out using the MNDO/H approximation. Bond lengths, bond angles, enthalpy of formation, total energy, ionization potential, and dipole moment were estimated, and the effective charges on the atoms and the bond orders were calculated. An analysis of the normal vibrations of the acetylthiocarbamide molecule and its deuteroanalog CH3CONDCSND2 was carried out. The force fields have been estimated. The frequencies, potential energy distribution among the vibrational coordinates, and the frequencies for the partially and completely deuterated acetylthiocarbamide molecules have been calculated.