A technique for measuring the dynamic behavior of materials at high temperatures

A new experimental technique has been developed for the performance of high temperature, high-strain-rate experiments in the compression Kolsky bar (split-Hopkinson pressure bar or SHPB). The new technique (referred to as the High-Temperature Compression Kolsky Bar or HTCKB) uses an infra-red spot-heater to rapidly heat the specimen to the desired temperature, a!nd an electropneumatic actuation system to minimize the development of temperature gradients in the sample. The technique is cheap and relatively easy to implement and yet provides accurate, repeatable results. As an illustration of the application of the technique, we have examined the high-temperature response of the BCC metal vanadium at high-strain rates. Stress–strain curves are obtained for the material at strain rates of 4 × 10³ s⁻¹ and at temperatures ranging from 300 to 1100 K (27–800°C). Quasistatic (10⁻³ s⁻¹) experiments have also been performed on vanadium over a slightly smaller range of temperatures, and the results are compared with the new high-temperature, high-strain-rate data. It is observed that the rate of thermal softening is a function of the strain rate. These results illustrate the importance of including the coupling between temperature and strain r!ate in thermoviscoplastic constitutive models.     

A laser extensometer for measuring strain at incandescent temperatures

A laser-extensometer system has been developed and installed in an electron-beam tensile vacuum furnace to measure the strain of tensile-test specimens at incandescent temperatures. The laser extensometer operates by measuring the length of time during which the laser light can be “seen” by a photosensitive device while the light beam is rotated at a constant angular velocity. The light can be “seen” by the photosensitive device only when the light passes through a gap between two opaque flags which are affixed to the specimen so that the change in the gap width is proportional to the strain in the specimen. Tests indicate that the laser extensometer is capable of measuring strain with a maximum error of less than 100 μin./in. based on a 1-in. specimen gage length. The sensitivity and accuracy of the first model of the laser extensometer increase with a decreasing strain rate and are well within the range of requirements for metallurgical evaluation of modern metals and alloys for use at very high temperatures. Operation of the extensometer has indicated that greatly improved sensitivity and reduction of electronic noise and drift can be achieved by simple modifications yet to be made.     

A High-frequency High-temperature Optical Strain/Displacement Gage

This paper describes an optical method for measuring strain or crack-opening displacement at high frequencies (20 kHz) and high temperatures (590°C) on a near-real-time basis. Two small reflective markers are placed on a smooth specimen or across a crack. When illuminated with a laser, interference fringes are generated; their motion can be monitored with photomultiplier tubes. The data acquisition system acquires 200 points per 50 microsecond cycle. These are processed, displayed, and stored at a rate of 25 Hz. Applications are in the general area of very high cycle (10⁹ cycles or more) fatigue. Demonstration tests at 20 kHz at room temperature with a strain range of 0.45 percent and at 590°C with a range of 0.2 percent are presented along with room temperature displacements up to 0.7 μm across the center of a 1.4 mm long crack.     

确定材料在高温高应变率下动态性能的Hopkinson杆系统

描述了一种利用Hopkinson杆装置确定在高温（温度可高达1 173 K）、高应变率下材料动态性能的试验方法。在试样加温过程中,试样不与入射杆及透射杆接触。当试样加热到预定温度时,气压驱动同步组装系统,推动透射杆及试样,使得应力波到达入射杆与试样接触面时,入射杆、试样及透射杆紧密接触。利用以上系统,完成了连铸单晶铜及上引法连铸多晶铜从室温到1 085 K范围内的应力应变曲线。测试结果表明,不论是上引法连铸多晶铜还是连铸单晶铜,流动应力随温度的升高而下降,在温度低于585 K时,材料的应变硬化率明显大于在温度高于585 K时的应变硬化率。     

A new experimental method in determining crack propagation transition temperature in steel

This study concentrates on a new experimental method for determining propagation transition temperature (PTT) in carbon steel (0.45% C). The method is based on the Hopkinson bar concept, by which the nature of pressure-time relationship can be studied when a pulse is applied at the active end of the bar. This concept enables one to determine terminal crack velocity using the round tensile specimen of a proper geometry with the V-notch at the half of its length; the total speimen length is 900 mm. When such a specimen is pulled in a testing machine, the fracture occurs at the notch cross section. The fracture event produces elastic waves which can be analyzed, and thus, the crack velocity can be estimated. This technique has been applied in several temperatures to monitor crack velocity changes. The analysis of oscillograms indicated a sharp change in the average crack velocity as well as in the average acceleration within a very narrow range of temperatures. It has been observed that the propagation transition temperature occurs within the limits ΔT ≈ 10 K, and for steel 0.45%C PPT ≈ 252 K.     

High-temperature tensile and creep tester for wire

To provide test facilities for determining the tension and short-time creep properties of small-diameter tungsten wire at high temperatures, special equipment has been designed and built, employing rf (radio frequency) heating as the means of attaining temperatures up to 2600° C. This paper describes the problems which had to be solved in designing and building the equipment, and gives results to tests made after the equipment was assembled. The equipment had to meet these requirements: it had to be capable of providing tension and short-time creep data on tungsten wire in sizes from 0.001 in. diam to 0.009 in. at temperatures up to 2600° C, it had to provide an autographic stress-strain curve for the tension tests, the loading rate during tension tests had to be constant, and all of this had to be done in good vacuum. Basically the equipment consists of a loading frame which supports a calibrated beam-type load dynamometer, a synchronous electric-clock motor for applying the load, rf equipment for attaining the desired temperature, an X-Y recorder for recording stress-strain curves, and a two-color automatic optical pyrometer for measuring the temperature. The test arrangement is mounted on a vacuum base plate under a bell jar. For creep testing, the flexible beam is replaced by a rigid beam, and load is applied by means of dead weights. Creep strain is measured with a cathetometer or Optron.     

Elevated temperature testing with the torsional split hopkinson bar

The torsional split-Hopkinson-bar technique is modified for high-strain-rate testing at elevated temperatures by heating the specimen rapidly and keeping the rest of the apparatus at room temperature. Tests have been conducted with specimens made of several materials (Haynes-188, 1020 steel, and 1151 steel) at temperatures ranging from 650°C to 1060°C and strain rates on the order of 1000 s^–1.     

Noncontacting strain measurements during tensile tests

The application of an innovative noncontracting Doppler laser extensometer is presented. True axial strain has been measured during tensile tests conducted on stainless-steel metal sheets over a range of strain rates (from 10^–4 to 10² 1/s) and temperatures (from –40°C to 400°C). The laser radiation scattered at the surface of the specimen is recorded during the duration of the experiment. The signals are then used to determine the evolution of the axial strain, which is subsequently combined with the load signal to construct the stress-strain curve for the material. Excellent agreement has been obtained between the total elongation predicted by the laser measurements and the actual values measured from the specimens. This technique offers several advantages over traditional strain-measuring technologies.     

高温SHPB实验技术及其应用

介绍了高温分离式Hopkinson压杆(SHPB)实验方法,建立了一套高温SHPB实验系统,利用该系统研究了温度对某种抗氢钢动态压缩力学性能的影响,实验温度最高达到1000 ℃,应变率为500～1000 s-1。仅对试件进行加温,并利用一套气动装置在加载前快速完成系统的组装,以尽量减小试件中温度分布的不均匀性。研究结果表明：该气动装置可以将加载前杆端与试件的完全接触时间控制在500 ms内;该抗氢钢的温度软化效应很明显,且温度敏感性随温度升高而下降。     

Synchronous Full-Field Strain and Temperature Measurement in Tensile Tests at Low,Intermediate and High Strain Rates

Tensile tests with simultaneous full-field strain and temperature measurements at the nominal strain rates of 0.01, 0.1, 1, 200 and 3000 s^?1 are presented. Three different testing methods with specimens of the same thin and flat gage-section geometry are utilized. The full-field deformation is measured on one side of the specimen, using the DIC technique with low and high speed visible cameras, and the full-field temperature is measured on the opposite side using an IR camera. Austenitic stainless steel is used as the test material. The results show that a similar deformation pattern evolves at all strain rates with an initial uniform deformation up to the strain of 0.25–0.35, followed by necking with localized deformation with a maximum strain of 0.7–0.95. The strain rate in the necking regions can exceed three times the nominal strain rate. The duration of the tests vary from 57 s at the lowest strain rate to 197 μs at the highest strain rate. The results show temperature rise at all strain rates. The temperature rise increases with strain rate as the test duration shortens and there is less time for the heat to dissipate. At a strain rate of 0.01 s^?1 the temperature rise is small (up to 48 °C) but noticeable. At a strain rate of 0.1 the temperature rises up to 140 °C and at a strain rate of 1 s^?1 up to 260 °C. The temperature increase in the tests at strain rates of 200 s^?1 and 3000 s^?1 is nearly the same with the maximum temperature reaching 375 °C.     

The recrystallization technique for local plastic-zone observation in type 304 stainless steel in the temperature range of −196° to 950° C

The recrystallization technique has been extended for direct observation of plastic zone in Type 304 stainless steel in the temperature range of ?196° to 950°C. It can reveal plastic deformation with plastic strain above 0.02 in the range of ?196° to 850° C and that with plastic strain above 0.06 at 950°C. Results of plastic-zone observation in notched specimen in the range of ?196° to 950° C are presented to illustrate the technique's capability.     

Determination of thermal-expansion characteristics of metals using strain gages

A method has been developed to measure thermal-expansion characteristics of metals using bonded resistance strain gages. The method allows rapid and accurate determination of expansion properties at similar or lower cost (depending on the particular application) than conventional dilatometric techniques. Other advantages include elimination of a need for perfectly flat sample material, elimination of specimen machining, and applicability to structures and components. To utilize this technique, the ‘apparent strain’ of the gage is determined by attaching it to a ‘standard’ material for which the thermal-expansion characteristics are accurately known and subtracting the known thermal response of the material from the total gage output. ‘Apparent strain’ is therefore the temperature-induced output of the gage when bonded to a material having a thermal-expansion coefficient of zero. When the gage is then attached to a test material and cycled through the same temperature range, this ‘apparent strain’ is subtracted from the total gage output to obtain the actual unit-length change of the test material. Using this technique, mean-expansion coefficients of experimental alloys were determined over the temperature range ?320°F (?196°C) to room temperature.     

Balanced Biaxial Testing of Advanced High Strength Steels in Warm Conditions

The main purpose of the present work is to measure the stress–strain behavior under warm conditions (about 100 °C) of advanced high strength steel (AHSS) sheets up to large strains compared to uniaxial tension. The test equipment consists of two main parts, i.e., a hydraulic bulge tester and a heating device. A mechanical system is attached to the test equipment for measuring the membrane stress and thickness strain at the bulge pole. The stress–strain curves were measured for three kinds of AHSS sheets with the proposed test method for various initial temperatures (10, 50 and 100 °C). The proposed method does not provide isothermal stress–strain curves because the specimen temperature increases during the test due to the effect of deformation-induced heating. A numerical scheme using thermo-mechanical finite element (FE) simulations is suggested to deconvolute the isothermal stress–strain curves.     

The imbedded disk retraction method for measurement of interfacial tension between polymer melts

A method for measuring interfacial tension of high viscosity polymer melts at elevated temperatures is described. The method involves the tracking of the shape evolution of a disk of one material imbedded in a second one. This makes it possible to determine the interfacial tension over a relatively short time period. The technique of preparing the samples makes it possible to measure on practically any combination of polymer melts without restrictions on viscosities and melting temperature, as long as one of the materials is transparent in the molten state.The retraction of the disk is observed by using a microscope with a high resolution video camera. The camera is connected to a video frame grabber in a personal computer which is programmed to collect images with preselected time intervals. Data of the retraction is acquired by using an image analysis software, measuring the average radius of the disk.The driving force for the shape evolution is interfacial tension and it is balanced by viscous forces. The analysis of the retraction process is done analytically with a simplified one-dimensional model. The model has been compared to experiments with the system PS/PMMA at 210 °C, covering viscosity ratios over a range of six decades and five different molecular weight values of PS. It is shown that interfacial tension can be determined over the whole range and a value of 1.1±0.2 mN/m was obtained for all samples.     

极端环境下连续碳纤维增韧的陶瓷基复合材料的力学行为

连续纤维增韧的碳化硅复合材料(以下简称C/SiC),作为超高速飞行器热结构使用时,有可能在高温环境下受到高速撞击的作用,因此,掌握其在极端环境(高温、高应变率)下的力学性能是进行结构安全设计的基础。本文采用具有高温实验能力的分离式Hopkinson杆,在293~1273K温度范围内进行了动态压缩力学性能测试,研究了环境温度和加载速率对材料力学性能的影响。结果表明:C/SiC复合材料的高温压缩力学性能主要受应力氧化损伤和残余应力的共同影响。实验温度低于873K时,应力氧化损伤的影响很小,而由于增强纤维和基体界面残余应力的释放使界面结合强度增大,复合材料的压缩强度随温度的升高而增大;当实验温度高于873K时,应力氧化损伤加剧,其对压缩强度的削弱超过残余应力释放对强度的贡献,材料的压缩强度随温度的升高逐渐降低。由于应力氧化损伤受应变率的影响很大,当温度由873K升高至1273K时,高应变率下压缩强度降低的程度要比应变率为0.0001/s时低得多。     

The Effect of Temperature on Anisotropy Properties of an Aluminium Alloy

The temperature influence on the mechanical behaviour during plastic deformation of an AA5754-O aluminium alloy has been investigated by several experimental tests. First, monotonous tensile tests were carried out from room temperature up to 200°C with a classical tensile machine and with a less conventional testing apparatus involving the heating of the sample by Joule effect. With this second testing apparatus, the strain fields and tensile curves were obtained in function of temperature by means of a non-contacting optical 3D deformation measuring system. Moreover, shear tests were performed in the same temperature range. It is shown that the anisotropy coefficients are rather constant within this temperature range, with a relative variation less than 8%. For both tensile and shear tests, the stress levels are similar at the beginning of straining at room temperature and 150°C, except that the Portevin?CLe Chatelier (PLC) phenomenon disappears at elevated temperature, and then evolves differently. At 200°C, the stress level is clearly below whatever the deformation. In the framework of drawing process, the formability of this alloy at temperatures higher than 150°C seems to be improved.     

Temperature-Dependent Mechanical Response of an UFG Aluminum Alloy at High Rates

High temperature (298 K–573 K) and high strain rate (4000 s⁻¹) compression experiments were performed on a cryomilled ultra-fine grained (UFG) Al-5083 using a modified Kolsky bar with a heating system designed to reduce “cold contact” time. The resulting stress strain curves show a reduction in strength of approximately 300 MPa at the highest temperature tested. This softening has been related to a thermally activated deformation mechanism. In addition, an experimental procedure was developed to investigate the microstructure evolution during the preheating, prior to mechanical loading, so as to identify the intrinsic mechanical response of the material at high temperatures. The results of this procedure are in good agreement with a TEM study on material that has been heated but not loaded.     

YB-2航空有机玻璃的应变率和温度敏感性及其本构模型

为了理解和评价YB-2航空有机玻璃在极端环境下的动态力学性能,采用电子万能试验机和分离式Hopkinson压杆对YB-2航空有机玻璃在218~373 K温度范围、10^-3~3 000 s^-1应变率范围内的压缩力学行为进行了研究,得到了材料的应力应变曲线。结果表明：随着温度的升高,材料的流动应力逐渐减小而破坏应变呈现增大的趋势;温度相同时,材料的流动应力随应变率的增加而增大,破坏应变随应变率的增加而减小。随着应变率的提高,材料的应变软化效应更加剧烈。基于朱-王-唐(ZWT)本构模型,得到了考虑温度效应的本构参数。结果显示,在8%应变范围内,改进的考虑温度效应的本构模型可以较为理想地表征该材料的应力应变响应。

     

Analysis of biaxial-fatigue damage at elevated temperatures

Two-level cumulative-damage fatigue tests were conducted on tubular 304 stainless-steel specimen under biaxial-strain conditions at elevated temperatures. Effects of temperature, biaxiality and sequence of straining were investigated. The experimental results are forwarted with a new approach that utilizes the Miner cumulative-damage rule. This approach has shown that fatigue damage at elevated temperatures of 538°C (1000°F) and 649°C (1200°F) accelerates and decelerates as a result of time of exposure to a given loading sequence. The effect of biaxiality is shown through the behavior of the material under axial and shear-strain components. The axial (tensile) strain component has shown to be the severest detrimental damaging component when compared to a shear-strain component. A damage mechanism emerges from the interaction of temperature and loading sequence. Its significance can be observed only when a certain life ratio has been exhausted.     

Bestimmung der Temperaturleitfähigkeit von Toluol und Methanol mittels laserinduzierter thermischer Gitter

To determine the thermal diffusivity of liquids, the laser-induced thermal grating technique has been developed. This experimental technique features (a) a measurement technique applicable to all liquids, (b) a contact-free measurement, (c) an absolute technique without calibration, (d) a very short measuring time, (e) a quite small temperature rise, and (f) a very small sample volume. Using this technique, the thermal diffusivity of liquid toluene and methanol was measured in the temperature range 295 to 375 K, near the saturation line. The measuring results are presented and compared with those from other sources.