Miniaturized Compression Test at Very High Strain Rates by Direct Impact

A modified miniaturized version of the Direct Impact Compression Test (DICT) technique is described in this paper. The method permits determination of the rate-sensitive plastic properties of materials up to strain rate ∼10⁵ s⁻¹. Miniaturization of the experimental setup with specimen dimensions: diameter d _S = 2.0 mm and thickness l _S = 1.0 mm, Hopkinson bar diameter 5.2 mm, with application of a novel optical arrangement in measurement of specimen strain, makes possible compression tests at strain rates from ∼10³ s⁻¹ to ∼10⁵ s⁻¹. In order to estimate the rate sensitivity of a low-alloy construction steel, quasi-static, Split Hopkinson Pressure Bar (SHPB) and DICT tests have been performed at room temperature within the rate spectrum ranging from 5*10⁻⁴ s⁻¹ to 5*10⁴ s⁻¹. Adiabatic heating and friction effects are analyzed and the final true stress versus true strain curves at different strain rates are corrected to a constant temperature and zero friction. The results have been analyzed in the form of true stress versus the logarithm of strain rate and they show two regions of a constant rate sensitivity : relatively low up to the strain rate threshold ∼50 s⁻¹, and relatively high above the threshold, up to strain rate ∼4.5*10⁴ s⁻¹.     

Some computational aspects for analysis of low- and high-velocity impact of deformable bodies

In this paper, we present some numerical algorithms devoted to analysis of low- and high-velocity impact of deformable bodies. For modeling of contact phenomena involved in impact problems, an implicit method is used rather than the most popular penalty explicit method. For the appropriate time integration of the discretized equation of motion, a first order algorithm is suggested rather than the most common second order algorithms such as Newmark, Wilson, etc. Numerical results are presented for dynamic analysis such as low-velocity impact of an elastic plate on a rigid surface with rebounding and high-velocity impact of an elastic-plastic copper bar on a rigid wall.     

Theory of stimulated desorption of ions and neutrals

Summary To understand the electronic processes that occur during desorption, we must allow simultaneously for the intrinsic nonadiabatic character of electron states and for correlation-induced features like two-hole resonances. A recently proposed theory describes the Menzel-Gomer-Redhead and Knotek-Fiebelman mechanisms on equal footing and reveals a complex interplay of electronic and kinematic effects on the ion yield. Here, I wish to show how in the same scheme we can also obtain the yield of ions that have suffered (partial or total) re-neutralization in the course of desorption. Closed-form expressions for the relevant escape probabilities are given in terms of one-hole and two-hole Green’s functions. The surface molecule limit of the theory is worked out in detail assuming that the hopping parameters decay exponentially in time during desorption. It is shown that, in general, when a given species can be desorbed by different mechanisms, we can discriminate them experimentally by the energy and angular dependence of the yield.

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Riassunto Per comprendere i processi elettronici che avvengono nel desorbimento, dobbiamo tener conto che gli stati hanno un carattere intrinsecamente non adiabatico. Allo stesso tempo, occorre includere gli effetti di correlazione, come, ad esempio, le risonanze a due buche. Una teoria recente consente una descrizione unificata dei meccanismi di Menzel-Gomer-Redhead e di Knotek-Feibelman; essa mostra che effetti elettronici ed effetti cinematici concorrono in modo complesso a determinare la resa di ioni. Qui si vuol far vedere come tale teoria consenta di ottenere la resa anche per le specie che subiscono una rineutralizzazione parziale o totale nel corso del desorbimento. Le probabilità che lo ione desorba nei vari stati di carica sono espresse in forma chiusa in termini delle funzioni di Green a una e due buche. Il limite della molecola superficiale è risolto in dettaglio nel caso in cui i parametri di salto decadono esponenzialmente nel tempo durante il desorbimento. Si trova che, quando diversi meccanismi portano al desorbimento di una stessa specie, è generalmente possibile distinguerli per le diverse dipendenze dall’energia cinetica e dagli angoli.

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Резюме Чтобы понять электронные процессы, происходящие при десорбции, мы должны одновременно учитывать внутренний неадиабатический характер электронных состояний и особенности, подобные двухдырочным резонансам. Недавно предложенная теория описывает механизмы Менцеля-Гомера-Редхеда и Нотека-Фейбелмана и обнаруживает комплексное влияние электронного и кинематического эффектов на выход ионов. В рамках этой схемы мы хотим показать, как можно получить выход ионов, которые испытывают (частичную и полную) ренейтрализацию в процессе десорбции. Приводятся замкнутые выражения для соответствующих вероятностей вылета в терминах одно-дырочных и двух-дырочных функций Грина. Разрабатывается предел теории для молекулы на поверхности, предполагая, что параметры ?перепыгивания? экспоненциально затухают во времени в процессе десорбции. Показывается, что, когда заданный тип частиц может быть десорбирован за счет различных механизмов, мы можем дискриминировать их экспериментально, используя энергетическую и утловую зависимости выхода.

     

An equivalent method of jet impact loading from collapsing near-wall acoustic bubbles: A preliminary study

Cavitation damage is a micro, high-speed, multi-phase complex phenomenon caused by the near-wall bubble group collapse. The current numerical simulation method of cavitation mainly focuses on the collapse impact of a single cavitation bubble. The large-scale simulation of the cavitation bubble group collapse is difficult to perform and has not been studied, to the best of our knowledge. In this study, the equivalent model of impact loading of acoustic bubble collapse micro-jets is proposed to study the cavitation erosion damage of materials. Based on the theory of the micro-jet and the water hammer effect of the liquid–solid impact, an equivalent model of impact loading of a single acoustic bubble collapse micro-jet is established under the principle of deformation equivalence. Since the acoustic bubbles can be considered uniformly distributed in a small enough area, an equivalent model of impact loading of multiple acoustic bubble collapse micro-jets in a micro-segment can be derived based on the equivalent results of impact loading of a single acoustic bubble collapse micro-jet. In fact, the equivalent methods of cavitation damage loading for single and multiple near-wall acoustic bubble collapse micro-jets are formed. The verification results show the law of cavitation deformation of concrete using equivalent loading is consistent with that of a micro-jet simulation, and the average relative errors and the mean square errors are insignificant. The equivalent method of impact loading proposed in this paper has high accuracy and can greatly improve the calculation efficiency, which provides technical support for numerical simulation of concrete cavitation.     

Enhancing plasticity of high-strength titanium alloys VT 22 under impact-oscillatory loading

Abstract

The purpose of this paper is to provide new experimental data for high-strength VT 22 titanium α + β-type alloy under impact-oscillatory loading and dynamic non-equilibrium process. Based on the performed experimental studies, it was found that overall plastic deformation of this alloy can be increased by a factor 2.75 compared with its initial state without significant loss of strength. To achieve this goal, a new methodology to study the impact behaviour of materials under non-equilibrium process has been used. Physical research revealed that significant microstructural refinement of the alloy is observed after such type of loading, as the result of which the fine grains are formed with subgrain refinement which takes place within the basis of alloy.     

Non-linear analysis and calculation of the performance of a shelving protection system by FEM

The aim of this paper consists on the study, analysis and calculation of the efficiency of a shelving protection system by means of the finite element method (FEM). These shelving protection systems are intended to prevent the eventual damage due to the impacts of transport elements in motion, such as: forklifts, dumpers, hand pallet trucks, and so on. The impact loads may threaten the structural integrity of the shelving system. The present structural problem is highly non-linear, due to the simultaneous presence of the following nonlinearities: material non-linearity (plasticity in this case), geometrical non-linearity (large displacements) and contact-type boundary conditions (between the rigid body and the protection system). A total of 48 different FEM models are built varying the thickness of the steel plate (4, 5 and 6 mm), the impact height (0.1, 0.2, 0.3 and 0.4 m) and the impact direction (head-on collision and side impact). Once the models are solved, the stress distribution, the overall displacements and the absorbed impact energy were calculated. In order to determine the best shelving protection’s candidate, some constraints must be taken into account: the maximum allowable stress (235 MPa), the maximum displacement (0.05 m) and the absorbed impact energy (400 J according to the European Standard Rule PREN-15512). Finally, the most important results are shown and conclusions of this study are exposed.     

抗冲共聚聚丙烯中二甲苯可溶物的链结构与性能研究

抗冲共聚聚丙烯(IPC)是聚丙烯与乙烯-α-烯烃共聚产物在反应釜内形成的原位共混物,乙烯-α-烯烃无规共聚物(橡胶相)作为聚丙烯增韧剂,增韧能力受其组成、玻璃化转变温度(T_g)的影响很大。目前工业界主要采用二甲苯可溶物来表征IPC中橡胶相的含量。该研究采用红外光谱(FTIR)、核磁共振(NMR)和热分析(DSC)等方法对两种不同催化剂制备的IPC的二甲苯可溶物,进行了组成、链结构及热性能对比研究,结果显示二甲苯可溶物中除了乙烯-丙烯无规共聚物外,还含有少量的具有可结晶乙烯序列的乙烯-丙烯多嵌段共聚物,且两个样品中的乙烯-丙烯多嵌段共聚物中可结晶乙烯序列长度存在差别;两个样品中乙烯、丙烯单元在分子链上无规分布的程度比较接近;丙烯序列PPP的含量相对低且乙烯共聚单元含量高的样品具有更低的Tg,这将有利于其对聚丙烯抗冲击韧性的提高。     

A simplified model for nonlinear dynamic analysis of steel column subjected to impact

This paper presents a new simplified model of the nonlinear dynamic behavior of a steel column subjected to impact loading. In this model, the impacted column, which undergoes large displacement, consists of two rigid bars connected by generalized elastic–plastic hinges where the deformation of the entire steel column as well as the connections is concentrated. The effect of the rest of the structure on the column is modeled by an elastic spring and a point masse both attached to the top end of the column which is also loaded by a compressive force. The plastification of the hinges follows the normality rule with a yield surface that accounts for the interaction between M and N. The latter is described by a super-elliptic yield surface that allows ones to consider a wide range of convex yield criterion by simply varying the roundness factor that affects the shape of the limit surface. By including these features, the model captures both geometry and material nonlinearities. Both the flow rule and the equations of motion are integrated using the midpoint scheme that conserves energy. The non-smooth nature of impact is considered by writing the equations of motion of colliding masses using differential measures. Contact conditions are written in terms of velocity and combined with Newton's law to provide the constitutive law describing interactions between masses during impact. Numerical applications show that the model is able to capture the behavior of a column subjected to impact.     

Dynamic crushing characteristics of high strength steel cylinders with elliptical geometric discontinuities

Thin-walled members are commonly used as energy absorbers in engineering structures and often contain cutouts. This study performed numerical simulations of high strength steel cylindrical shells with elliptical cutouts subjected to dynamic axial impact. The LS-DYNA code was the primary analytical tool used to analyze the influence of cutout locations, cutout shapes and symmetry of cutout on the energy absorption capabilities and the crush characteristics of tubes with a cutout. For high strength steel tubes made from a rate sensitive material, the stress-strain curves of different strain rates were used to elucidate the effect of dynamic impact on the strain rate. Our results show that collapse crushing behavior is strongly influenced by the location and symmetry of cutouts and the variation of major axis influences the peak crush load.     

Quantitative Impact Testing of Energy Dissipation at Surfaces

Impact testing with nanoscale spatial, force, and temporal resolution has been developed to address quantitatively the response of surfaces to impingement of local contact at elevated velocities. Here, an impact is generated by imparting energy to a pendulum carrying an indenter, which then swings towards a specimen surface. The pendulum displacement as a function of time x(t) is recorded, from which one can extract the maximum material penetration x _max , residual deformation x _r , and indentation durations t _in and t _out. In an inverse application one can use the x(t) response to extract material constants characterizing the impact deformation and extent of energy absorption, including material specific resistance coefficient C_in, coefficient of restitution e, and dynamic hardness H _imp . This approach also enables direct access to the ratio H/E, or resilience of the deformed material volume, at impact velocities of interest. The impact response of aluminum was studied for different contact velocities, and the mechanical response was found to correlate well with our one-dimensional contact model. Further experiments on annealed and work hardened gold showed that dynamic hardness H _imp scales with contact velocity and highlighted the importance of rate-dependent energy absorption mechanisms that can be captured by the proposed experimental approach.