New method for testing composites at very high strain rates

A method was developed for testing and characterizing composite materials at strain rates in the 100 to 500 s^?1 regime. The method utilizes a thin ring specimen, 10.16 cm (4 in.) in diameter, 2.54 cm (1 in.) wide and 6–8 plies thick. This specimen is loaded by an internal pressure pulse applied explosively through a liquid. Pressure is measured by means of a calibrated steel ring instrumented with strain gages. Strains in the composite specimen are measured with strain gages. Strains in the calibration and specimen rings are recorded with a digital processing oscilloscope. The equation of motion is solved numerically and the data processed by the mini-computer attached to the oscilloscope. Results are obtained, and plotted by an X-Y plotter in the form of a dynamic stress-strain curve. Unidirectional 0-deg, 90-deg and 10-deg off-axis graphite/epoxy rings were tested at strain rates up to 690 s^?1. Times to failure ranged between 30 and 60 μs. The 0-deg properties which are governed by the fibers do not vary much from the static ones with only small increases in modulus. The 90-deg properties show much higher than static modulus and strength. The dynamic in-plane shear properties, obtained from the 10-deg off-axis specimens, are noticeably higher than static ones. In all cases the dynamic ultimate strains do not vary much from the static values.     

Compression-impact testing of aluminum at elevated temperatures

This paper presents and experimental technique for determining compressive stress-strain curves well into the plastic range of relatively soft metals at strain rates from 300 to 2000 sec^?1 at six temperatures from 30 to 550° C. More than 100 curves were obtained on annealed 1100° F aluminum. The strain-rate dependence in these tests could be fitted quite well either by a power function (log-log plot) or by a semilogarithmic plot, but the power function gave a better correlation of the present data with that obtained at lower strain rates by Alder and Phillips.¹     

Burnout at very high heat fluxes

The paper provides a brief overview of the present state-of-the-art in the field of the critical heat flux (CHF) in subcooled flow conditions, with particular regard to high liquid velocity and highly subcooled conditions. These thermofluid-dynamic conditions to reach very high values of the CHF (up to 80 MW/m²) as requested by fusion technology requirements. After reporting the general features of the problems, some details are given about the main parameters affecting CHF in subcooled flow boiling (subcooling, pressure, duct diameter and length, fluid velocity, duct orientation and flow direction, heat flux distribution), and the respective knowledge and possible enhancement techniques for CHF (extended surfaces, hypervapotron, surface roughness, swirl flow, helically coiled wires) are reviewed. As fundamental aspects of highly subcooled flow boiling are not known and, on the other hand, such knowledge is necessary for any possible enhancement technique evaluation and for a theoretical modelling of the phenomenon, recent experiments carried out over the last five years on CHF in subcooled flow boiling are reviewed together with the main correlations and models currently used for data prediction, and suggestions for particular and general research guidelines are given.Der Aufsatz gibt einen kurzen Überblick über den gegenwärtigen Stand der Technik im Bereich der kritischen Wärmestromdichte bei unterkühltem Strömungssieden, mit besonderem Schwerpunkt auf hohe Flüssigkeitsgeschwindigkeiten und hohen Unterkühlungsbedingungen. Diese thermo- und fluiddynamischen Bedingungen ermöglichen es, sehr hohe Werte der kritischen Wärmestromdichte (bis zu 80 MW/m²) zu erreichen, wie sie in den Anforderungen der Fusions-Technik verlangt werden. Nach einem Abschnitt über allgemeine Gesichtspunkte des Problems werden ein paar Einzelheiten der Hauptparameter bezüglich der kritischen Wärmestromdichte bei unterkühltem Strömungssieden (Unterkühlung, Druck, Rohrleitungsdurchmesser und Länge, Fluidgeschwindigkeit, Rohr-Orientierung und Strömungsrichtung, Wärmestromverteilung) genannt. Es wird ein Überblick gegeben über den jeweiligen Wissensstand und mögliche Verbesserungstechniken für die kritische Wärmestromdichte (vergrößerte Oberflächen, Hypervapotron, Oberflächenrauheit, Drallströmung, spiralförmig gewickelte Drähte). Da grundlegende Aspekte von extrem unterkühlten Strömungssieden unbekannt sind, eine Kenntnis dieser Aspekte andererseits für jede mögliche Entwicklung von Verbesserungstechniken sowie für eine theoretische Modellbildung über das Phänomen notwendig ist, wird eine Übersicht über die neuesten Experimente gegeben, die in den letzten fünf Jahren über die kritische Wärmestromdichte bei unterkühltem Strömungssieden durchgeführt wurden. Gleichzeitig werden die wichtigsten Korrelations-Gleichungen und Modelle besprochen, die heutzutage für Daten-Vorausberechnungen verwendet werden sowie Vorschläge für besondere und allgemeine Forschungsrichtlinien unterbreitet.Dedicated to Prof. Dr.-Ing. U. Grigull's 80th birthday     

Measuring static strains at high temperatures

A recently introduced strain gage is described. It depends on the variation in electrical capacitance brought about by the deflection of an arched strip when the separation of its feet is altered. The gage has a wide range (1-percent strain) and good stability even in the 1100–1200°F range. With the design adopted, the output when a gage is heated but not stressed can be controlled and reduced. Using the techniques described, problems of connections for field work are not severe and remote readings can be made of the small capacitances involved, which are about 0.5 pF. Several trials are in progress. In one installation, automatic logging from scores of gages has proved successful. The gage has particular attractions for monitoring the formation of cracks in structures working at high temperatures.     

高温作用下钢管混凝土构件侧向撞击性能

通过耦合ABAQUS有限元软件中的隐式静态分析和显示动态分析,提出钢管混凝土构件在火灾与撞击联合作用下的数值计算方法,分别对已有钢管混凝土构件的温度场试验、火灾下轴向撞击试验和常温下侧向撞击试验进行数值模拟,以验证本文方法的合理性。在此基础上建立了钢管混凝土构件在不同温度下的侧向撞击有限元模型,分别对不同温度下的挠度和撞击力时程曲线进行对比,采用极值后平均撞击力和吸能系数对高温作用下构件的抗侧向撞击性能进行量化分析,并分析了600 ℃下构件撞击全过程。结果表明：温度对钢管混凝土构件的侧向撞击性能影响明显,随着温度升高,构件跨中挠度大幅增加,撞击时程变长;高温下构件的撞击力时程曲线与常温下差异明显,高温下曲线可分为震荡阶段、下降阶段和卸载阶段;构件主要通过整体弯曲变形吸收落锤的动能,随着温度升高,极值后平均撞击力和吸能系数逐渐降低,表明构件的抗撞击性能逐渐降低,当温度超过400 ℃后,构件抗撞击性能损失严重。

     

The ionization-rate constant at high temperatures high electron concentrations

The electron distribution function and the rate constant for ionization of atoms by electron impacts have been calculated as they apply to the conditions that are characteristic of a shock wave — namely, the energy distribution of the electrons and the ionization-rate constants are determined as functions of the temperature of the heavy particles. The energy dependence of the effective cross section for the excitation of an atom by electron impact is assumed to be linear. Equations of the Fokker-Planck type are used in the solution of the problem, and the range of temperatures and concentrations in which the deviation of the distribution from Maxwellian leads to a substantial change of the ionization-rate constant is determined.Translated from Zhurnal Prikladnoi Mekhaniki i Tecknicheskoi Fiziki, No. 2, pp. 32–40, March–April, 1971.     

Structural testing at high temperature

This paper describes the equipment and procedures used in testing a large multicell box beam under various combinations of applied loading and transient heating. Data are presented on the performance of radiant-heat lamps when enclosed by aluminum reflectors and powered by automobile-type batteries. In addition, solutions are given to the following special problems introduced by simulating fuel conditions: (1) sealing the specimen without disturbing the normal paths for heat flow; (2) protecting strain gages and thermocouples from the simulated fuel.     

Optical properties of teflon at high temperatures

Journal of Applied Mechanics and Technical Physics - When considering thermal decomposition of thermoplastics by a-thermal flux, the case wherein radiation is the dominant heat-transfer mechanism...     

Dynamics of aerospace vehicles at very high altitudes

Some basic questions of the flight mechanics of aerospace vehicles at very high altitudes are investigated and in some cases reviewed. In particular those of the spacecraft trajectories are considered along which the aerodynamic actions take place in a range of the Knudsen number between transition and free molecular flow. A recent physico-mathematical model is adopted in the evaluation of the aerodynamic and heat transfer coefficients. The stability characteristics are discussed of two reference vehicle configurations for which the time histories are also calculated relative to significant equilibrium conditions. In the final section of the paper the initial step of an aeroassisted orbit transfer is considered in order to evaluate the constraints due to the aerodynamic heating on this manoeuvre.Si studiano alcune questioni fondamentali della meccanica del volo di veicoli aerospaziali a quote molto elevate. Si considerano, in particolare, quei tratti delle traiettorie lungo i quali le azioni aerodinamiche si esplicano in un campo di numeri di Knudsen tra la transizione ed i flussi di molecole libere. Si adotta un recente modello fisico-matematico per calcolare i coefficienti aerodinamici e di scambio termico. Dopo aver discusso le caratteristiche di stabilit`a per due configurazioni di velivoli di riferimento, se ne calcolano le leggi orarie a partire da condizioni di equilibrio significative. Il lavoro termina con lo studio di un traferimento orbitale aeroassistito al fine di valutare i limiti a questa manovra dovuti al riscaldamento aerodinamico.     

Experimental procedures for low-cycle-fatigue research at high temperatures

Testing procedures are reported for lowcycle-fatigue experiments on hydraulic fatigue machines operated with closed-loop servocontrol of the strain in inductively heated test specimens of the hourglass type. Typical equipment performance data and fatigue-test results are presented.     

Determination of stress-strain characteristics at very high strain rates

A modified version of the Kolsky thin-wafer technique is described. The method permits one to obtain the dynamic plastic properties of materials at strain rates as high as 10⁵ sec^?1. Data obtained from compression tests on high-purity aluminum are presented for strain rates ranging from 4000 to 120,000 sec^?1 at room temperature. Specimen-size effects and the effect of lateral inertia are taken into account in analyzing the data. The results plotted as stress vs. strain rate at constant strains (5 to 20 percent) show that, at the highest strain rates, the stress rises very rapidly with strain rate suggesting that a limiting strain rate is being reached. At the lower strain rates (10³ to 10⁴ sec^?1), the stress is linearly proportional to the strain rate indicating that the material is deforming in a viscous manner.     

Local shrinkage and stress induced by thermo-oxidation in composite materials at high temperatures

The paper is concerned with the modelling, simulation and experimental characterisation of local shrinkage strains and stresses induced by thermo-oxidation phenomena in the IM7/977-2 carbon/epoxy composite material at elevated temperatures. The oxygen concentration and mechanical fields were established through a coupled model constructed from a unified multiphysical approach and the thermodynamics of irreversible processes. The model was implemented in the ABAQUS^® finite element commercial code. Simulations of thermo-oxidation-induced matrix shrinkage were run at a local scale, i.e., the scale of the elementary constituents of the composite, the fibre and the matrix. The experimental assessment was done at the same scale, and the local matrix shrinkage profiles were measured by confocal interferometric microscopy.A good agreement was found between the simulated and measured profiles, validating the unified model. The thermo-oxidation induced stress field was analysed to understand the influence of the environment on the onset of damage in composite materials at elevated temperature.     

Thermal stresses at high temperatures in stainless-steel rings by the moiré method

The research presented in this paper is a continuation of previous work published by one of the authors. Improvements in the techniques of recording and processing moiré data in thermal-strain fields have extended the maximum temperature at which useful information can be obtained from 400 to 1600° F. It has been found that the strains obtained experimentally do not depart very much from the values predicted by the theory of elasticity, although the stresses at some points of the model exceed the yield limit at the corresponding temperature. These results prove that the moiré method can be used with advantage to obtain experimental information in an area where experimental data are scarce if not nonexistent.     

A non-linear viscoelastic model for ceramics at high temperatures

High-temperature mechanical behavior of ceramics is characterized by non-linear rate dependent responses, asymmetric behavior in tension and compression, and nucleation and coalescence of voids leading to rupture. Moreover, rupture experiments show considerable scatter or randomness in fatigue lives of nominally equal specimens. To capture the non-linear, asymmetric time-dependent behavior, a new non-linear viscoelastic model is proposed. Non-linearity and asymmetry are introduced in the volumetric component. To model the random formation and coalescence of voids, each element is assigned a failure strain sampled from a lognormal distribution. An element is deleted when its volumetric strain exceeds its failure strain. Temporal increases in strains produce a sequential loss of elements (a model for void nucleation and growth), which in turn leads to failure. Non-linear viscoelastic model parameters are determined from uniaxial tensile and compressive creep experiments on silicon nitride. The model is then used to predict the deformation of four-point bending and ball-on-ring specimens. Simulation is used to predict statistical moments of rupture lives. Numerical simulation results compare well with results of four-point bending experiments.