A Pinned-end Fixture for Off-axis Testing

Experimental Techniques -     

A New Apparatus for Large Scale Dynamic Tests on Materials

The development of an innovative apparatus, based on Hopkinson bar techniques, for performing large scale dynamic tests is presented and discussed. The activity is centered at the recently upgraded HOPLAB facility, which is basically a split Hopkinson bar with a total length of approximately 200 m, with bar diameters of 72 mm and where force pulses up to 2 MN and 40 ms duration can be generated and strain rates up to 50 s^?1 can be achieved. Several modifications in the basic configuration have been introduced: twin incident and transmitter bars have been installed with strong steel plates at their ends where large specimens can be placed. A series of calibration and quantification tests has been conducted in order to prove the reliability of the experimental technique proposed. Moreover, real tests on concrete cylindrical samples of 200 mm diameter and of up to 400 mm length have been performed. Analyses of recorded signals indicate proper Hopkinson bar testing conditions and reliable functioning of the facility.     

A Novel Method for Strain Controlled Tests

The present paper aims at providing a contribution to the testing strategies in the field of mechanics of materials, with particular reference to low cycle fatigue in the strain control mode. After a detailed analysis of the state of the art on possible techniques for strain controlling, the paper points out the difficulties that could be encountered when a conventional longitudinal contact extensometer cannot be used. This methodology, based on controlling the strain at a particular specimen location, by controlling the relative displacement between its ends, was developed to provide an alternative solution in such occurrences. The paper introduces its analytical fundamentals for its most general application in the execution of fatigue or even static tests. Particular attention was devoted to the validation of the proposed methodology: this task was conducted by applying the suggested technique to both static and fatigue testing of hourglass specimens, by analyzing results also in comparison to other experimentations or numerical simulations, always observing a good agreement. The methodology proved to be efficient and reliable on a wide range of strain amplitudes.     

多模块集成软件的通用操作管理系统GOMS

GOMS系统采用菜单作业方式,具有自动加载、咨询、批作业等功能,用户可把应用软件模块加载到GOMS系统中,形成特定的应用软件操作管理系统。用户作业时只需选择相应的菜单。GOMS系统为用户提供了一个友好的人机界面操作环境,它的通用性很好地解决了一类“多模块集成软件”的人机界面性能的改造问题。     

A New Technique for Curved Rod Bending Tests Based on Digital Image Correlation

Background

The study of the deformation of curved rods subjected to bending and its associated stress state is a complex task that has not been treated in depth in the literature, which makes difficult to obtain constitutive models or Finite Element Models (FEM) in which it is necessary to know all the components of the stress and strain tensors.

Objectives

This study focuses on a new calculation methodology to obtain stress and strain tensors of curved rods under bending.

Methods

The stress and strain tensors have been determined based on the theory of continuum mechanics and differential geometry of curves (moving bases), in a general methodology and valid for large strains, curved geometries and variable cross-sections along the specimen. This has been applied to the human rib and, in addition, a new experimental method for bending of curved specimens based on Digital Image Correlation (DIC) is presented.

Results

Both the test method and the proposed calculations applied to the human rib show results according to expectations, allowing to know the rib curvature changes along the test, the stresses and strains along the rib and the components of both stress and strain in all directions, in order to build the stress and strain tensors. In addition, the results of stress, strain and young’s modulus correspond to those of previous literature in tensile testing of human rib cortical bone.

Conclusions

The proposed calculations allow the construction of the strain and stress tensors of a curved specimen subjected to bending, which is of great importance for the development of constitutive models. Moreover, since with this method it is possible to calculate both tensors along the entire length of the specimen and in all directions, it is possible to apply this method in finite element models. Finally, the new test methodology allows to know the stress and strain in curved specimens such as the human rib, from bending tests.

     

A Braking System for Tests with a Prescribed Deceleration Pulse

Tests with a prescribed deceleration pulse are fundamental to the development and certification of crashworthy structures. At the Politecnico di Milano Laboratory for the Safety of Transports these tests are carried out using a horizontal impact-sled facility. Test articles are mounted on a trolley which is launched down a rail and then arrested by means of a braking system that allows prescribed deceleration pulses to be obtained. The oleo-pneumatic braking system customarily used in these tests is difficult to use and user-defined time-histories of the deceleration pulse cannot be obtained. In an effort to overcome these limits, a new braking system was developed. The idea was to arrest the trolley by means of a number of beams that varied in length and that were positioned at variable distances from each other. Experimental tests and numerical simulations were carried out in parallel to verify the feasibility of the new system and improve on the initial design. A mathematical model and genetic algorithm were also developed and used as part of a method to find the test set-up that allowed user-defined deceleration pulses to be obtained. A full-scale test with the deceleration pulse prescribed for small airplane seat certification was carried out to assess the performance of the braking system. The test revealed a weakness of the braking system that was then further developed before being put into full operational service.     

A New Analytical Formulation for Interwell Finite-Step Tracer Injection Tests in Reservoirs

Analytical models to describe tracer transport in reservoirs commonly set conditions either on the tracer concentration or the tracer flow at the injection border. Here a different formulation based on tracer sources is presented. This approach avoids some of the physical inconsistencies that can be found when setting conditions on the tracer concentration. The case of a tracer injected as a finite-step in an infinite one-dimensional homogeneous reservoir with a uniform flow is considered. The solution is analytically obtained. The results are confronted against the two common boundary cases. The new approach predicts slightly delayed and broader pulses. The tracer breakthrough curves differences can be large for small Peclet numbers. These differences weakly reduce by increasing the injection period. The new approach contains tracer injection elements that can make it suitable to describe real conditions found in reservoir tracer tests.     

复合型裂纹断裂和扩展速率试验夹具及裂纹长度测量方法

本文设计了复合型裂纹断裂和扩展试验夹具及试样,借助此夹具可以在拉压疲劳试验机上进行多种复合型裂纹断裂及扩展速率试验,针对此夹具和试样提出了复合型裂纹长度测量的显微镜直读比拟法,并应用设计的夹具和提出的裂纹长度测量方法的MTSnew810上进行了多种复合型裂纹扩展速率试验,获得了复合型裂纹扩展的a-N曲线及一些新的结果。     

A Priori Tests of RANS Models for Turbulent Channel Flows of a Dense Gas

Dense gas effects, encountered in many engineering applications, lead to unconventional variations of the thermodynamic and transport properties in the supersonic flow regime, which in turn are responsible for considerable modifications of turbulent flow behavior with respect to perfect gases. The most striking differences for wall-bounded turbulence are the decoupling of dynamic and thermal effects for gases with high specific heats, the liquid-like behavior of the viscosity and thermal conductivity, which tend to decrease away from the wall, and the increase of density fluctuations in the near wall region. The present work represents a first attempt of quantifying the influence of such dense gas effects on modeling assumptions employed for the closure of the Reynolds-averaged Navier–Stokes equations, with focus on the eddy viscosity and turbulent Prandtl number models. For that purpose, we use recent direct numerical simulation results for supersonic turbulent channel flows of PP11 (a heavy fluorocarbon representative of dense gases) at various bulk Mach and Reynolds numbers to carry out a priori tests of the validity of some currently-used models for the turbulent stresses and heat flux. More specifically, we examine the behavior of the modeled eddy viscosity for some low-Reynolds variants of the \(k-\varepsilon \) model and compare the results with those found for a perfect gas at similar conditions. We also investigate the behavior of the turbulent Prandtl number in dense gas flow and compare the results with the predictions of two well-established turbulent Prandtl number models.     

A Simple Closed-Loop Active Control of Electrodynamic Shakers by Acceleration Power Spectral Density for Environmental Vibration Tests

This work presents the main results of a simple closed-loop active control for an electrodynamic shaker in order to generate acceleration Power Spectral Densities (PSD) according to prescribed Standards used in environmental vibration tests. The main idea is to start generating acceleration pseudo-signals obeying the prescribed Power Spectral Density and then to acquire acceleration data from the electrodynamic shaker’s table behaviour. So the Power Spectral Density of the acquired acceleration is computed and compared with the required PSD and then the time-varying pseudo-acceleration is updated to reflect this corrected PSD. It was noticed that for piecewise narrow bands frequencies, the electrodynamic shaker acceleration behaves near linearly, both in frequency and voltage, for the input signals. A code in AgilentVee 7.5 software to acquire, send and process signals for the active control in a closed-loop scheme was developed. The used A/D D/A hardware was a single PC sound card with specific characteristics. The control could be accomplished sending and acquiring at the same time with a range of input/output of ±1.5 V with 16 bits of resolution, at 48 kHz and assistance of an external sound amplifier.     

基于实验的数值反演的滚动轮胎稳态温度场的有限元分析

根据轮胎温度场的单向解耦分析思想,形成了一个基于ABAQUS程序的轮胎稳态温度场的分析方法,单向解耦过程分为变形、损耗、热传导三个分析过程。变形分析中,采用了平衡态的超弹性材料模型;损耗分析中,依据变形分析获得的应力应变场,结合材料粘性损耗特性来获得损耗能量;热传导分析中,依据实测的轮胎胎侧温度场,提出了一种基于实验的数值反演方法来确定胎侧的对流热边界条件。由于轮胎胎侧的形状和结构细节,其对流热边界不同于旋转平圆盘的对流热边界,本文的数值反演方法避免了实测胎侧对流热交换系数的困难。     

A Finite Element Method for the Determination of Optimal Viscoelastic Material Properties from Indentation Tests of Polymer Film and Wire with Polymer Insulation

The time-dependent behavior of bulk polymer film and wire with polymer insulation is studied using indentation. The indenter is displaced into the material at a constant rate and then held at a fixed indentation depth to monitor load relaxation. A finite element simulation of the experiment is performed; this analysis is parameterized in terms of the unknown shear compliance modeled as a Prony series. An optimization method is then presented to determine the unknown material parameters by minimizing the RMS error between the model and the experimental data. The method is demonstrated with poly (vinyl chloride) (PVC) films after thermal aging and pristine polyethylene sheet; excellent agreement between the model and the data is demonstrated. The method is also demonstrated to successfully characterize the material properties for the compression of a wire with PVC insulation; the resulting properties are then shown to adequately predict the crossed-cylinder indentation behavior of the same wire using a 3D finite element model. The chief benefit of the method is that an analytical solution method is not required for its implementation; as such, the optimization approach can be readily applied to the determination of material properties from arbitrarily complex experimental geometries.

Tests of a new numerical scheme on a “non-reflection and free-transmission” open-sea boundary for longwaves

A new numerical scheme of a “non-reflection and free-transmission” boundary for longwave equations proposed by Hino (1987) has been tested for a variety of cases. The test results verify the effectiveness of the method for (a) a single progressive wave train on a horizontal bottom, (b) two wave trains each propagating in opposite directions on a horizontal bottom, (c) a single wave train propagating on a sloping bottom with friction, (d) oscillatory flood waves in an open channel flow, (e) two-dimensional waves travelling obliquely to open boundaries and (f) water surface oscillation in a harbor by waves incident through an opening.     

过阻尼地震检波器阻尼系数测量的研究

对动圈式恒速度型过阻尼地震检波器相对阻尼系数的测量方法进行了研究。首先引入了动圈式地震检波器的运动方程和输出方程,随同参数定义给出了这种检波器对直流电流的响应函数;然后讨论了当前两种过阻尼系数的测量方法———经典的参数辨识法及两点法,明确了它们的适用范围;最后提出了一种线性方法。根据该方法,相对过阻尼系数能够表示成检波器响应曲线的半峰高宽度和检波器自然频率乘积的线性函数。研究表明,利用本方法可在较宽阻尼范围取得更好性能,且该方法便于硬件实现。     

A note on faxen laws for nonspherical particles

The advantages of new forms of Faxen laws with derivatives of finite order are discussed and specifically illustrated for prolate spheroids. Earlier ideas on the connection between the Faxen laws and certain “associated solutions” (single-particle solutions of the Stokes flow where the ambient velocity is a uniform stream or linear field) have been refined for the case where the associated solution has a relatively simple singularity solution.     

A note on thin-layer theory for Bingham plastics

The combination of Bingham-type rheological models with lubrication or thin-layer theory can lead to paradoxical results in that material which is below the yield point, and therefore, rigid, is nevertheless found to be deforming. The purpose of the present note is to show how these difficulties can be removed, and a consistent theory constructed, by the use of a suitably modified rheological model.     

A note on failure conditions for layered materials

Summary Some failure conditions for heterogeneous layered media, formed by two alternating homogeneous materials, are presented. The failure criteria have been obtained by a homogenization procedure, on the basis of the criteria of the materials forming the layered medium. The paper deals with layers formed by Mohr-Coulomb's type materials (the cases of cohesionless materials or materials provided by the same friction angle are discussed), as well as with layers undergoing the maximum tensile stress criterion. Theoretical predictions are in good agreement with the experimental results obtained by other authors on layered sands and laminated rocks.

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Sommario Vengono presentati alcuni criteri di rottura per materiali stratificati eterogenei, caratterizzati da regolare altenanza di due materiali omogenei. Tali criteri sono stati ottenuti mediante una procedura di omogeneizzazione, sulla base delle condizioni di rottura dei materiali che compongono il continuo stratiflcato. Sono stati considerati strati formati da materiali del tipo di Mohr-Coulomb (per i casi in cui i materiali sono o privi di coesione, oppure sono dotati dello stesso angolo d'attrito), o da materiali che obbediscono al criterio della massima trazione. Le pre-dizioni teoriche mostrano un buon accordo con i risultati di prove sperimentali eseguite da altri autori su sabbie e rocce stratificate.

     

A review on design criteria for vortex tubes

In this study, the past investigations of the design criteria of vortex tubes were overviewed and the detailed information was presented on the design of them. Vortex tubes were classified and the type of them was described. All criteria on the design of vortex tubes were given in detail using experimental and theoretical results from the past until now. Finally, the criteria on the design of them are summarized.