COMMENTS FROM INCOMING PRESIDENT T. DIXON DUDDERAR 1994–1995

Last year we celebrated the 50th anniversary of the founding of the organization that we now call the Society for Experimental Mechanics or the SEM. As your incoming SEM President rather early in the next 50 years of our Society's history, I feel both excited and challenged by the opportunity at hand.     

Wide-band measurement of frequency-dependent viscoelastic properties using an inverse nyquist plane parameter extraction methodology

This paper presents a new method for dynamically characterizing viscoelastics materials. A multi-degree of freedom vibration model is employed to extract frequency-dependent storage moduli and loss factors from experimental frequency-response functions of the elastomer specimen. The frequency range over which reliable data are available spans from 300 to 6,000 Hz. Hundreds of estimates can be obtained from a single test, Compared with conventional methods, it exhibits the remarkable advantages of covering a wider frequency range, of being more testwise efficient, and of providing more estimates within the frequency range of interest. Paper was presented at the 1989 SEM Spring Conference on Experimental Mechanics held in Cambridge, MA on May 28–June 1.     

Bounds for interpolating complex effective moduli of viscoelastic materials from measured data

The complex effective moduli of viscoelastic materials can be experimentally measured by a number of techniques giving relaxation data for materials at discrete frequencies. In this paper we present a method which allows one to find bounds on the moduli of the material at the unmeasured frequencies. Our bounds can be very tight, and are, therefore, good approximations to the data. Using our bounds, experimentalists can concentrate their efforts on gathering highly accurate data at only a few frequencies and numerically generate data at other frequencies that may be of interest. Received: 10 August 1999 Accepted: 21 December 2000     

New approximate solution for N-point correlation functions for heterogeneous materials

Statistical N-point correlation functions are used for calculating properties of heterogeneous systems. The strength and the main advantage of the statistical continuum approach is the direct link to statistical information of microstructure. Two-point correlation functions are the lowest order of correlation functions that can describe the morphology and the microstructure-properties relationship. Experimentally, statistical pair correlation functions are obtained using SEM or small x-ray scattering techniques. Higher order correlation functions must be calculated or measured to increase the precision of the statistical continuum approach. To achieve this aim a new approximation methodology is utilized to obtain N-point correlation functions for non-FGM (functional graded materials) heterogeneous microstructures. Conditional probability functions are used to formulate the proposed theoretical approximation. In this approximation, weight functions are used to connect subsets of (N?1)-point correlation functions to estimate the full set of N-point correlation function. For the approximation of three and four point correlation functions, simple weight functions have been introduced. The results from this new approximation, for three-point probability functions, are compared to the real probability functions calculated from a computer generated three-phase reconstructed microstructure in three-dimensional space. This three-dimensional reconstruction was based on an experimental two-dimensional microstructure (SEM image) of a three-phase material. This comparison proves that our new comprehensive approximation is capable of describing higher order statistical correlation functions with the needed accuracy.     

Laser diffraction methods for high-temperature strain measurements

Optical methods, utilizing the interference fringes produced by diffraction of a laser light beam passing through a narrow slit, have been investigated for strain measurements at high temperatures above 1000°C. Two methods are reviewed, one suitable for dynamic loading conditions and the other for slow rate and static loading conditions. The basic principles of the methods, new instrumentation, and new measurement techniques are described. Experimental verification of the basic optical theory, system calibration, results of cross calibrations with foil strain gage, and sample applications at high temperatures are presented. Test results show that both methods are capable of performing accurate and reliable strain measurements at high temperatures.Paper was presented at the 1989 SEM Spring Conference on Experimental Mechanics held in Cambridge, MA on May 28–June 1.     

DIC Challenge: Developing Images and Guidelines for Evaluating Accuracy and Resolution of 2D Analyses

With the rapid spread in use of Digital Image Correlation (DIC) globally, it is important there be some standard methods of verifying and validating DIC codes. To this end, the DIC Challenge board was formed and is maintained under the auspices of the Society for Experimental Mechanics (SEM) and the international DIC society (iDICs). The goal of the DIC Board and the 2D–DIC Challenge is to supply a set of well-vetted sample images and a set of analysis guidelines for standardized reporting of 2D–DIC results from these sample images, as well as for comparing the inherent accuracy of different approaches and for providing users with a means of assessing their proper implementation. This document will outline the goals of the challenge, describe the image sets that are available, and give a comparison between 12 commercial and academic 2D–DIC codes using two of the challenge image sets.     

Pore Characterization Methodology by Means of Capillary Sorption Tests

In this study, a methodology is proposed for obtaining information about the porous structure of materials by analysing data supplied by a hydric absorption test. A model of vertical cylindrical pores is used for studying the variation of absorbed mass versus time. The proposed methodology requires an estimation of the numerical interval in which the radius distribution must be calculated and a minimum amount of experimental data. The experimental tests are developed with a system, designed and built in our laboratory, that allows capture the amount of data necessary to use the calculus methodology proposed. This methodology is based on the derivative of normalized mass in relation to the square root of time of the capillary absorption tests. The simplicity and low cost, in many cases, of the capillary absorption tests compared to other experimental techniques such as mercury porosimetry or SEM, make it feasible to design a simple methodology to obtain valuable information about the structure of a porous material, pore size distribution, and tortuosity factor.     

Accurate measurement of three-dimensional deformations in deformable and rigid bodies using computer vision

Recently, digital-image-correlation techniques have been used to accurately determine two-dimensional in-plane displacements and strains. An extension of the two-dimensional method to the acquisition of accurate, three-dimensional surfacedisplacement data from a stereo pair of CCD cameras is presented in this paper. A pin-hole camera model is used to express the transformation relating three-dimensional world coordinates to two-dimensional computer-image coordinates by the use of camera extrinsic and intrinsic parameters. Accurate camera model parameters are obtained for each camera independently by (a) using several points which have three-dimensional world coordinates that are accurate within 0.001 mm and (b) using two-dimensional image-correlation methods that are accurate to within 0.05 pixels to obtain the computer-image coordinates of various object positions. A nonlinear, least-squares method is used to select the optimal camera parameters such that the deviations between the measured and estimated image positions are minimized. Using multiple orientations of the cameras, the accuracy of the methodology is tested by performing translation tests. Using theoretical error estimates, error analyses are presented. To verify the methodology for actual tests both the displacement field for a cantilever beam and also the surface, three-dimensional displacement and strain fields for a 304L stainless-steel compact-tension specimen were experimentally obtained using stereo vision. Results indicate that the three-dimensional measurement methodology, when combined with two-dimensional digital correlation for subpixel accuracy, is a viable tool for the accurate measurement of surface displacements and strains. Paper was presented at the 1989 SEM Spring Conference on Experimental Mechanics held in Cambridge, MA on May 29–June 1.     

Reliability for design of planetary gear drive units

Gear drive units are important components of technical systems (TS) and need to be of high quality. Planetary gear units are very compact and efficient mechanical power transformers, but further increase of operating quality level requires the application and development of the new design methodology. The subject of this contribution is presentation of Reliability for design as the new approach of reliability modelling suitable for the new design methodology application, especially for planetary gear units using various kinds of experimental and exploitation data. The methodology follows V-model for TS design which is in this work adapted for gear units design and for presentation of the new methodology based on property based design, axiomatic design and robust design methodology. To this end, the procedure for total reliability of TS decomposition, and methodology for elementary reliability for design of structure components calculation is developed and presented. The reliability for design is established in reverse form of reliability for maintenance which presents common perception of the “reliability” term. This approach is intended to provide further increase of planetary gear unit’s quality and efficient usability of gear unit component resources. The design directions are oriented to providing equal level of elementary reliability of components.     

Digital filtering of speckle-photography data

Optical methods for nondestructive analysis of strain fields are continuing to generate a great deal of interest among experimentalists; the potential of these methods is considered to be great. One persistent obstacle to the accuracy of such methods is the contamination of the data by ‘optical noise’. This paper describes a digital method which allows extensive reduction of noise components from data records obtained in point-by-point interrogation of double-exposure speckle photographs. The computations prove to be inexpensive and effective. Sample results are given and further applications are suggested.     

A simple orthotropic elasticity-based constitutive model for reinforced concrete

Using strain-induced orthotropic material concepts, a new numerical constitutive model is demonstrated by predicting the ultimate capacity of a unique simple test specimen and two cylindrical shell structures. Only the uniaxial compressive strength of the concrete and the yield stress of the reinforcing steel are needed as input to predict both constitutive behavior and structural strength. The approach is simple, reliable and computationally efficient.Paper was presented at the 1988 SEM Spring Conference on Experimental Mechanics held in Portland, OR on June 5–10.     

Stability of plane poiseuille flow of a highly elastic liquid

The stability of fully developed pressure driven plane laminar flow of a Maxwell fluid has been studied using linear hydrodynamic stability theory. Elasticity is destabilizing in the inertial regime, but the flow is found to be stable to infinitesimal disturbances at low Reynolds numbers. This result contradicts previous calculations, which predicted a low Reynolds number flow instability at a critical recoverable shear of order unity. The previous calculations were carried out using less accurate numerical methods; the eigenvalue problem which must be solved is a delicate one, requiring sophisticated umerical techniques in order to avoid the calculation of spurious unstable modes.This work has direct bearing on the question of the mechanism of a low Reynolds number extrusion instability known as “melt fracture”. It is observed that the intensity of melt fracture increases with increasing die length for high density polyethylene, and it is therfore believed by some experimentalists that fully-developed die flow is unstable for this polymer above a critical recoverable shear. The analysis appears to be at variance with this interpretation of the experimental results.     

Changes in the experimentalist's role in the future of mechanics

At present there exist considerable opportunities for experimental studies. In large part this prognosis derives from the observation that analytical capability has, to some extent, outstripped our detailed understanding of many aspects of engineering concerns, so that there exists a need for providing input into the modeling of mechanics (of materials) problems. This favorable situation is supported by increasingly sophisticated experimental methods which, in turn, depend heavily on the computer, as well as on optical and other field methods. Examples are given that illustrate the gradual transition of macroscopic material related studies to those that require submicron resolution as demanded by ever increasing relevance in advanced materials related research. Paper was invited lecture at the 1995 SEM Spring Conference on Experimental Mechanics held in Grand Rapids, MI on June 12–14.     

复合材料板冲击损伤检测的二维间隔平滑法

本文发展了一种基于振动的复合材料板的损伤检测方法,将原有的一维间隔平滑法(1D GSM,one dimensional gapped smoothing method)发展至二维(2D GSM),并进一步提出二维间隔平滑法的标准化损伤指标.与其他多数基于振动分析的损伤检测方法不同,该方法只需分析含有损伤结构的检测数据,无需结构健康时的数据或理论、计算结果作为对比信号,即可判定缺陷的存在,并能准确定位.针对由冲击造成的准各向同性碳纤维增强复合材料板中的层裂损伤,本文采用压电片阵列组合激励的方式,得到了复合材料板多频率扭转振型的同时激励,可实现快速、高效的损伤检测.通过扫描式激光测振仪测得结构在不同固定频率下的结构响应ODS(operational deflection shapes),利用提出的二维间隔平滑法,分析得出损伤指标.实验结果表明,二维间隔平滑法可以准确地检测碳纤维增强复合材料板的冲击损伤,并具有较好的精度.     

Advances in scanning electron microscope moiré

The scanning electron microscope (SEM) moiré method for microscopic measurements based on electron beam lithography and an SEM has been well developed. Although it has been a reliable method, some drawbacks exist: reinforcement effects, complicated processing and low sensitivity. To improve the SEM moiré method, new grating-casting techniques and a fringe-viewing technique must be developed. In this study, a carbonaceous grating technique and a total imaging technique are introduced. Accordingly, there are two techniques available for grating-casting (i.e., the carbonaceous grating technique and the existing lithography grating technique) and three techniques available for fringe viewing (i.e., the total imaging technique, the existing, monitor viewing technique and the existing photographic viewing technique). A total of six new imaging techniques of SEM moiré methods are available for microscopic measurements by combining one technique from each of the two groups. This study demonstrates the feasibility of the individual techniques and discusses the characteristics and limitations of each. Based on the presented total imaging technique, the sensitivity of the moiré method is only dependent on the frequency of specimen grating. Because it can be made as high as 10,000 lines/mm, the SEM moiré method can achieve sensitivity as high as 0.01 percent.     

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在解读亚里士多德、伽利略等人相关著作的基础上,通过分析比较,探讨了人类认 识落体运动的深化过程,说明科学的发展不但需要知识的积累和创新,还需要先进 的思想和科学方法.     

DNS-DERIVED FORCE DISTRIBUTION ON FLEXIBLE CYLINDERS SUBJECT TO VORTEX-INDUCED VIBRATION

We use direct numerical simulation (DNS) based on spectral methods to simulate turbulent flow past rigid and flexible cylinders subject to vortex-induced vibrations (VIV). We present comparisons of amplitude, and lift and drag forces, at Reynolds number 1000 for a short and a long cylinder, and we examine differences between a traveling wave response and a standing wave response. The DNS data suggest that the often-used empirical formula proposed by Skop, Griffin & Ramberg in 1977 overpredicts the drag coefficient. We propose an appropriate modification and present preliminary results that indicate that low-dimensional modeling may be an accurate and efficient approach in predicting forces in VIV. Given the lack of any benchmark experiments in VIV currently, the DNS results presented here, both distributions as well as span- and time-averaged quantities, should be helpful to experimentalists and modelers.     

On the Convection in a Porous Medium with Inclined Temperature Gradient and Vertical Throughflow. Part II. Absolute and Convective Instabilities,and Spatially Amplifying Waves

This article presents a new methodology to estimate the effective permeability of random fractured media of any anisotropy containing both microfractures and a large number of long fractures crosscutting the representative volume element. The fractures are replaced by fictitious permeable materials for which the tangential permeability is deduced from a Poiseuille flow. A self-consistent scheme is proposed to derive the macroscopic permeability. On the one hand, the contribution of long fractures to the effective permeability writes by simple superposition of the fracture tangential permeabilities. On the other hand, the contribution of microfractures needs to resort to auxiliary problems requiring the computation of second-order Hill (or Eshelby) tensors related to ellipsoids embedded in an anisotropic matrix, for which a complete procedure is detailed. The effect of the microfracture normal permeability is put in evidence in the upscaling scheme and analyzed. In particular, it is shown that it must be chosen large enough to allow the connections between families. Examples are finally developed and compared to numerical simulations in the 2D case.     

Large-scale molecular dynamics simulations of dense plasmas: The Cimarron Project

We describe the status of a new time-dependent simulation capability for dense plasmas. The backbone of this multi-institutional effort – the Cimarron Project – is the massively parallel molecular dynamics (MD) code “ddcMD,” developed at Lawrence Livermore National Laboratory. The project’s focus is material conditions such as exist in inertial confinement fusion experiments, and in many stellar interiors: high temperatures, high densities, significant electromagnetic fields, mixtures of high- and low-Z elements, and non-Maxwellian particle distributions. Of particular importance is our ability to incorporate into this classical MD code key atomic, radiative, and nuclear processes, so that their interacting effects under non-ideal plasma conditions can be investigated. This paper summarizes progress in computational methodology, discusses strengths and weaknesses of quantum statistical potentials as effective interactions for MD, explains the model used for quantum events possibly occurring in a collision, describes two new experimental efforts that play a central role in our validation work, highlights some significant results obtained to date, outlines concepts now being explored to deal more efficiently with the very disparate dynamical timescales that arise in fusion plasmas, and provides a careful comparison of quantum effects on electron trajectories predicted by more elaborate dynamical methods.     

践行工程科学思想的体会

郑哲敏先生一生积极践行和倡导钱学森先生的工程科学思想, 传承和发扬工程科学思想对推动我国技术创新有十分重要的现实意义. 本文首先阐述了工程科学的定义、方法论和特点, 强调了提出新的解决方案是工程科学的核心内容以及工程科学家对技术创新的作用. 其次通过分析水下爆炸处理地基、三峡围堰相关技术等工程案例, 诠释了工程科学在技术创新中的作用; 讨论了工程计算与数值模拟的理论基础及工程科学属性, 选取广义计算变量、构建解的结构是数值模拟中创造性的工作; 连续非连续计算方法的核心在于构建了计算过程中时域内可增减的广义计算变量, 列举了几个体现工程科学方法论的计算案例; 介绍了滑坡灾害防治关键力学问题研究中践行工程科学思想的工作, 讨论了监测与数值模拟相结合的可行性及渐进破坏和破裂度概念. 最后基于开发高压卸荷矿石粉化技术的认知提出了岩石物理学发展的几个工程科学研究方向.