Photoelastic and strain-gage investigation of penetration of thin plates

An experimental investigation of the central normal impact and penetration of plates of Columbia Resin No. 39, simply supported at the corners, was conducted using both photoelastic and strain-gage techniques. Both conically-pointed and blunt-nosed steel projectiles of 1/4-in. diam were employed at initial velocities ranging from 191 to 272 fps. Sequences of fringe photographs were obtained by repeating shots on different plates, using various predetermined time intervals between impact and photograph. A dynamic value of Young's modulus has been calculated from the velocity of propagation of tension waves, and a strain-optic coefficient of 398 microinches per fringe for the material was determined from a quasi-static test. Both fringe order and strain histories for various impact conditions have been ascertained and are compared.Paper was presented at 1961 SESA Spring Meeting held in Philadelphia, Pa., on May 10–12.     

Vibration and dynamic buckling of shear beam-columns on elastic foundation under moving harmonic loads

The vibration and buckling of an infinite shear beam-column, which considers the effects of shear and the axial compressive force, resting on an elastic foundation have been investigated when the system is subjected to moving loads of either constant amplitude or harmonic amplitude variation with a constant advance velocity. Damping of a linear hysteretic nature for the foundation was considered. Formulations in the transformed field domains of time and moving space were developed, and the response to moving loads of constant amplitude and the steady-state response to moving harmonic loads were obtained using a Fourier transform. Analyses were performed to examine how the shear deformation of the beam and the axial compression affect the stability and vibration of the system, and to investigate the effects of various parameters, such as the load velocity, load frequency, shear rigidity, and damping, on the deflected shape, maximum displacement, and critical values of the velocity, frequency, and axial compression. Expressions to predict the critical (resonance) velocity, critical frequency, and axial buckling force were proposed.     

Critical needs of fringe-order accuracies in two-color holographic interferometry

Two-color holographic interferometry is a promising technique for separating simultaneous concentration and temperature variations in solution for crystal-growth experiments on earth and in microgravity conditions in space. The ultimate success depends on two linearly independent fringe patterns due to the different wavelengths. With available practical lasers and typical crystal-growth experimental conditions the two interference fringe patterns (other than scale effects) may not be too different. The slight error in the measured fringe order can then yield large uncertainty in temperature and concentration determination. This aspect is analytically considered for the first time in this article. A simple cell (rectangular parallelpiped) is considered containing the fluid. For simplicyt, we assume a constant field along the object-wave-propagation direction in the cell. The two reconstructed fringe patterns are then represented in terms of temperature and concentration variations. Solving two equations theoretically yields the desired temperature and concentration. However, once the fringe-postition measurement error is introduced, error in the temperature and concentration results. These errors are analytically determined. A particular case of TGS (triglycine sulfate) aqueous solution with HeNe (λ=632.8 nm) and HeCd (λ=441.6 nm) lasers for holography is critically discussed. It is found that a high degree of accuracy in the fringe order is required in this particular case. To improve fringe-position measurements, special techniques such as electronic phase measurement or heterodyne detection may be necessary. The study provides the analytical guidelines for designing the experiments and critical needs of desired physical parameters. Paper was presented at the 1990 SEM Fall Conference on Experimental Mechanics held in Baltimore, MD on November 5–8.     

Photomechanical properties of some birefringent polymers around their glass transition temperatures

This paper describes an investigation and comparison of the optical and mechanical properties of the three polymers—PMMA (polymethyl methacrylate), CT200 and MY750 (both based on bisphenol-A epichlorohydrin). The work was undertaken with the specific purpose of assessing the suitability of these polymers for use in contact studies involving three-dimensional photoelasticity and the stress-freezing technique. The optical property investigated was the variation of photoelastic fringe constant with maximum stress-freezing temperature. The mechanical properties sought were the variation of Young's modulus with temperature and the stress-strain behavior at the stress-freezing temperature. The effects of the magnitude of stress and the soak time in the thermal cycle were also investigated for the MY750 resin. The results provide the optimum peak temperature for the thermal cycle to achieve repeatable values of Young's modulus and fringe constant during stress freezing. The nonlinear stress-strain behavior is quantified and should be a useful reference. The dependence of strain on load and soak time is also shown and is useful in specifying the optimum cycle time for stress freezing and the appropriate stress level. It was concluded that the resin MY750 was the most suitable for photoelastic applications which involved high localized stress, such as contact problems or fracture mechanics studies.     

Rewetting analysis of hot surfaces with internal heat source by the heat balance integral method

A two region conduction-controlled rewetting model of hot vertical surfaces with internal heat generation and boundary heat flux subjected to constant but different heat transfer coefficient in both wet and dry region is solved by the Heat Balance Integral Method (HBIM). The HBIM yields the temperature field and quench front temperature as a function of various model parameters such as Peclet number, Biot number and internal heat source parameter of the hot surface. Further, the critical (dry out) internal heat source parameter is obtained by setting Peclet number equal to zero, which yields the minimum internal heat source parameter to prevent the hot surface from being rewetted. Using this method, it has been possible to derive a unified relationship for a two-dimensional slab and tube with both internal heat generation and boundary heat flux. The solutions are found to be in good agreement with other analytical results reported in literature.     

冲击荷载与火灾联合作用下SFRC梁的力学行为

为了探究冲击荷载与火灾联合作用下钢纤维混凝土(steel fiber reinforced concrete, SFRC)梁的力学性能,联合应用高性能落锤试验系统、四点弯曲实验装置与装配式电炉开展了4根SFRC梁的冲击实验与高温恒载实验,观察了其破坏模式并记录了跨中位移和钢筋应变的时程曲线,探讨了冲击损伤SFRC梁的抗火性能。此外,在实验研究的基础上,考虑材料的应变率强化效应及温度软化效应,建立数值模型,首先对梁进行冲击加载模拟,并以冲击模拟结果为初始状态,采用热-力“顺序”耦合方法,对冲击加载与高温恒载联合作用下SFRC梁的力学行为进行了三维宏观有限元数值模拟。同时,考虑混凝土内部结构非均质性的影响,采用类似步骤,开展了细观模拟。宏/细观模拟结果与实验结果的良好吻合验证了本文数值方法的合理性与有效性,并体现了细观方法的优越性。研究发现,冲击能量较小时,SFRC梁在冲击荷载作用下,尽管局部混凝土开裂,梁整体残余变形较小,抗火性能有一定程度的下降;随着钢纤维掺量增大,混凝土基体抗剪强度增大,SFRC梁在冲击荷载作用下的开裂形态由弯剪裂缝并存向以弯曲裂缝为主转变;冲击损伤SFRC梁在高温恒载作用下裂缝分布较为集中,且发生脆性破坏。     

Characterization of 10% Ballistic Gelatin to Evaluate Temperature, Aging and Strain Rate Effects

Ballistic gelatin is widely used as a soft tissue simulant in physical surrogates for the human body to evaluate penetrating impacts and, more recently to evaluate blunt impact and blast loading effects on soft tissues. It is known that the properties of gelatin are sensitive to temperature and aging time, but this has not previously been quantified. The mechanical properties of 10% ballistic gelatin were measured using a compression test apparatus with temperature controlled platens to maintain the sample temperature at a fixed level. Penetration testing was undertaken using a standard BB impact test to assess the effect of aging. The gelatin was found to be within calibration after 3?days (72?h of aging), based on the standard penetration test. The material properties were evaluated using the stress at failure, strain at failure and material stiffness as characterized by the Neo-Hookean constitutive model. The stress at failure and material stiffness increased with decreasing temperature and increasing strain rate, as expected, while the strain at failure remained relatively constant for the test conditions considered (1 to 23??C, strain rate from 0.01 to 1.0?s^?1). The study showed that the penetration resistance was consistent after 72?h of aging, while the mechanical study demonstrated increasing failure stress and stiffness with decreasing failure strain at longer aging times, suggesting that these effects offset one another so that the penetration resistance remains relatively constant. The primary contribution of this study was to show the importance of temperature and aging time, through mechanical and penetration testing, to achieve appropriate and consistent response from ballistic gelatin.     

An Evaluation of the Double Torsion Technique

Double Torsion (DT) is a powerful testing technique for fracture mechanics characterisation of brittle materials as, in principle, it provides a crack length independent test configuration. However, several corrections have been proposed to address variations of experimental results reported from various laboratories. These correction factors address the validity of the DT configuration and its crack length independent stress intensity. Never the less, there seems to be no consensus in literature on the various corrections and the reason of reported variations. This paper presents firstly a critical review of the DT technique, followed by proposed corrections through an experimental analysis using the proposed corrections, a Finite Element model of the geometry and the use of Digital Image Correlation to measure out-of-plane surface deformations. It focuses on the validity of the constant stress intensity regime and the independence of crack length in a critical evaluation using Polymethylmethacrylate test specimens. Assessment of three un-grooved specimen geometry configurations demonstrated the apparent regime of approximately constant stress intensity, although a small but clear dependence of the stress intensity on crack length was observed in all specimen configurations. This dependence is attributable to significant load-point deflections and out-of plane deformations that are not accounted for in the DT analysis. Revisions of the proposed analysis methodologies show that a crack length independent specimen geometry can be achieved, however at the cost of less accurate data. Reliable and accurate data can be achieved with a DT testing configuration using an optimum specimen configuration.     

A bonded polariscope for three-dimensional photoviscoelastic studies

A method of incorporating an internally bonded polariscope into a transparent model has been developed for use with viscoelastic materials. The newly developed technique serves to isolate one or more planes in a three-dimensional model so that a threedimensional photoviscoelastic analysis can be performed. The model may be subjected to a variety of loadings at room temperature, and only the photomechanical effects in the isolated plane will be observable. In the experimental program, a viscoelastic cement was developed whose mechanical properties matched those of a chosen model material. The cement was used to bond the polarizing sheet into the model. The restraining effect of the stiff elastic Polaroid sheet was reduced by cutting a gridwork of slits into this material. Tests performed on simple disk models and on star-grain rocket models containing bonded-polariscope elements showed structural continuity through the cemented joint. The fringe pattern observed through the bonded polariscope in star-grain models was the same as that observed using a conventional polariscope. On the basis of the favorable test results, the newly developed technique was judged acceptable.     

On the integral transform solution of low-magnetic MHD flow and heat transfer in the entrance region of a channel

The main goal of the present study is to show the procedure, application and main features of the hybrid numerical/analytical approach known as GITT (Generalized Integral Transform Technique) by solving a magnetohydrodynamic channel flow with heat transfer, sustained by a pressure gradient and subjected to a uniformly applied and undisturbed transversal magnetic field. Although not the primary objective, application of this novel method provides a critical review of previously published numerical results on developing channel flows with uniform or non-uniform velocity and temperature profiles at the channel inlet. This is bounded by non-conducting horizontal walls at constant temperature and lateral walls that are electrically perfect conductors (open or short-circuited). Transport properties, namely, fluid viscosity, thermal and electrical conductivities, are taken as either constant or thermally-dependent, exponential-type, functions. Due to the hybrid numerical-analytical nature of the integral transform approach, benchmark results for velocity and temperature fields and the main correlated potentials are produced as a function of the primary dimensionless governing parameters, such as Hartmann, Prandtl and Eckert numbers, as well as the electrical parameter, for typical situations.     

大电流快速加热条件下LY12铝板动态变形的云纹干涉法实验研究

本文研究了中间带孔受拉铝板在大电流热冲击条件下的动态变形测试．在试件表面制作高温高频光栅，预加机械载荷后放入双光束云纹干涉光路中，用大电流加热器对试件进行快速加热，利用高速摄影机拍摄记录试件表面圆孔附近区域干涉条纹的变化情况，同时利用测温系统对试件的温度变化情况进行了测试记录．实验结果表明，用高速拍摄方法摄影热冲击条件下的云纹干涉条纹变化是可行的．     

Model for Gas–Condensate Phase Equilibria for Estimating C7+ Critical Properties Using Genetic Algorithm

A reliable prediction of the phase behavior is necessary in determining the compositions of the gas and liquid phases at various pressures. These calculations require stepwise computational procedure using a cubic equation of state (EOS). Since the heavy components in the petroleum mixtures have the strongest effect on the characteristics of the fluids, critical properties must be estimated for the petroleum fractions making up heptanes-plus. A phase equilibria calculation of a gas–condensate system with Peng–Robinson equation of state was done by satisfying the condition of chemical equilibrium. A genetic algorithm was used to determine the optimum critical properties of heptanes-plus (C₇₊) fraction. The predictions of the model are compared with the experimental results of the constant volume depletion (CVD) test.     

C80机车救援抬轮器受力的实验研究

采用DH5923动态信号测试分析系统对C80机车救援抬轮器受力进行了实验测试.整个测试利用计算机1394接口进行长时间,无间断记录多通道动态信号.记录了机车按照不同速度前行、倒行时相应各点应变随时间的变化情况;给出了六个测试点不同测试时段峰值应变列表;并实时记录了整个测试阶段轴承温度的变化曲线;发现了在道岔处测试点应变和温度变化剧烈;同时得到了整个运行过程中抬轮器轴承的最高温度值.实验表明抬轮器性能指标达到设计要求,能够安全运行,也为以后抬轮器进一步改进提供了一定的实验数据.     

Rheology and gelation kinetics of PVC plastisols

Oscillatory rheological experiments at different temperatures and over a wide range of frequencies have been used to investigate the gelation process and, more particularly, the sol–gel transition of various poly(vinyl chloride) (PVC) plastisols. The sol–gel transition process was found to be universal with respect to the temperature and solid volume fraction according to the similarity of the fractal structure in PVC plastisols. The variation of the gel time (t _gel) with temperature for any composition of PVC plastisols was predicted from the Dickinson’s model (E. Dickinson, J Chem Soc Faraday Trans, 93:111–114, 1997). Dynamic viscoelastic properties of PVC plastisols have also been studied as a function of temperature that allowed us to follow the gelation process of various plastisols. Thus, the influence of the type and concentration of PVC resins in gelation process was investigated. The variation of the complex shear modulus at a constant frequency was depicted by a master curve regarding the dependence of the moduli on PVC concentrations.     

Shrinkage Measurement in Concrete Materials using Cure Reference Method

This paper describes the details of using Cure Reference Method (CRM) to determine the shrinkage that develops in concrete materials. The technique involves the replication of diffraction grating on the concrete specimen during curing. After demolded, the specimen is stored in a chamber where a specific drying condition is maintained for 6 days. Every day in this period, the specimen is removed from the chamber and a set of the consecutive moiré fringe patterns are recorded with the help of a specially-designed stage. An automated fringe analysis program is developed to obtain the full-field displacement and strain information. Shrinkage as a function of location, time, and drying conditions is measured. A numerical method is developed in order to obtain material properties from the complex geometry used in the tests. The test in different drying condition and the ring test are performed, and their results are compared with FEA to validate the constructed model.     

Photoviscoelasticity on a microsecond time scale

A previous paper¹² has described the optical calibration of photoviscoelastic materials on a microsecond time scale. In this paper, attention is given to the experimental determination of the two-dimensional plane-stress field in a body subjected to arbitrary loads. Symmetry of load and geometry is not assumed. Since the stress state depends on the history of fringe order and isoclinic angle, these basic data must be recorded. Due to nonlinearity in the material studied, the photoviscoelastic data were limited to fractional fringe orders. This introduced problems of initial birefringence and polariscope imperfections. Techniques are described which correct for these imperfections and allow for acquiring and analyzing the basic photoviscoelastic data.     

Photoelasto-plastic analysis of notched-bar configuration subjected to bending

Knowledge of the three-dimensional stress distribution near a notch for a strain-hardening material in an elasto-plastic state is limited, to say the least. This experimental investigation is concerned with obtaining some insight concerning the three0dimensional elastoplastic stress distributions and the associated plastic-zone sizes. A notched-bar configuration (scaled-up Charpy specimen) subjected to flexure was selected for this purpose. The three-dimensional elasto-plastic stress distributions were determined along the plane of symmetry adjacent to the notch for two levels of applied bending moment. The experimental-stress-analysis method utilizes the creep and frozen-stress characteristics of an epoxy resin when subjected to a thermal cycle whose maximum temperature is significantly less than the critical temperature of the model material. The resulting frozenstress-strain behavior is characterized by the generation of a nonlinear effective stress-strain curve. This effective stress-strain curve was generated by subjecting uniaxial tensile specimens to constant stress and the appropriate thermal cycle. Also, an effective birefringence-stress curve was obtained from these tensile specimens (calibration). Then the notched-bar configuration was subjected to the thermal cycle and an applied bending moment which would develop a plastic zone (determined by using a distortion-energy-yield criteria). The stress distributions were determined from photomechanical analysis of the slices removed from the model.     

Photomechanical behavior of cellulose acetate under viscoelastoplastic deformation

To confirm the possibilities of cellulose acetate as a material for a model analysis during viscoelastoplastic deformation, the time-dependent photomechanical properties of the material were examined by means of creep tests under constant stress and recovery tests after removal of stress. Consequently, though the strain and the fringe order of cellulose acetate during creep and recovery are greatly influenced by stress and room temperature, both of them can be described simply by a power function of time, and the coefficient of each of these formulas can be represented by a function of the ratio of active stress to yield stress only. The effect of temperature is included in the formulation of the yield stress. In addition, the strain and the fringe order can be represented by the viscous-viscoelastic model proposed by Findleyet al.,^1,2 in which both of them are divided into four components: elastic, plastic, time-dependent irrecoverable viscous and time-dependent recoverable viscoelastic. The relation between viscoelastic strain and viscoelastic fringe order, and the relation between viscous strain and viscous fringe order were verified to be equivalent to that between plastic strain and plastic fringe order, all of which do not depend on stress, temperature or time. Therefore, the strain distribution of cellulose acetate under viscoelastoplastic deformation can be determined directly from the value of the fringe order measured.     

Transient free convection flow between long vertical parallel plates with ramped wall temperature at one boundary in the presence of thermal radiation and constant mass diffusion

The unsteady laminar free convection flow between two long vertical parallel plates with ramped wall temperature at one boundary has been investigated in the presence of thermal radiation and chemical species concentration. The exact solutions of the momentum, energy and concentration equations have been obtained using the Laplace transform technique. The velocity and temperature profiles, skin-friction and Nusselt number variations are shown graphically and the numerical values of the volume flow rate, the total heat rate and species rate added to the fluid are presented in a table. The influence of different system parameters such as the radiation parameter (R), buoyancy ratio parameter (N), Schmidt number (Sc) and time (t) has been analyzed carefully. A critical analysis of the coupled heat and mass transfer phenomena is provided. The free convective flow due to ramped wall temperature has also been compared with the baseline case of flow due to constant wall temperature.     

Determination of thermal strains by moiré interferometry

An existing method is extended to measure thermal strain distributions on an absolute basis. Free thermal expansion and stress-induced deformations are separated, allowing the determination of coefficients of expansion, stress-induced strains, normal stresses and shear stresses. The method is applicable to many steady-state and transient thermal-strain problems. A crossed-line grating is replicated on the specimen at elevated temperature. A zero-expansion mold is used for the replication, so that the grating frequency at the replication temperature can be retained to null the moiré interferometer at room temperature. When the specimen is viewed in the moiré interferometer, the fringe patterns reveal theU andV displacements induced by any change from the replication temperature. In addition, carrier fringe techniques are introduced for steady-state problems to subtract off the free thermal expansion and produce fringe patterns of the stress-induced deformations themselves. The method is demonstrated by analysis of a bimaterial plate subjected to a uniform change of temperature. Paper was presented at the 1988 SEM Fall Conference on Experimental Mechanics held in Indianapolis, IN on November 6–8.