Dynamic fracture behavior of ceramics at elevated temperatures by caustics

The fracture behavior of partially stabilized zirconia (PSZ) and silicon-nitride ceramics (Si₃N₄) is investigated under dynamic loading at elevated temperatures up to 1200°C using the caustic method combined with an ultra high-speed camera. The values of the dynamic fracture toughnessK _Id and the crack-propagation fracture toughnessK _ID are obtained, and it is shown that a dynamic effect on these values is observed in PSZ but not in Si₃N₄. The dynamic crack arrest toughnessK _Ia is found to exist for PSZ.     

Cyclic behavior of a type 304 stainless steel in biaxial stress states at elevated temperatures

The results of companion, incremental/decremental, and stepup fatigue experiments on austenitic stainless steel tube (type 304) are presented. The experiments include proportional and nonproportional loading conditions at ambient as well as elevated temperatures. Empirical relations are developed between von Mises effective stress and strain, and these relations are shown to describe the cyclic behavior during proportional companion as well as incremental/decremental tests. In case of nonproportional incremental/decremental experiments, the material behavior is not accurately modeled either by using the von Mises effective stress and strain, or by relating Tresca's maximum shear stress and strain.     

High-flux magnetorheology at elevated temperatures

Commercial applications of magnetorheological (MR) fluids often require operation at elevated temperatures as a result of surrounding environmental conditions or intense localized viscous heating. Previous experimental investigations of thermal effects on MR fluids have reported significant reductions in the magnetorheological stress with increasing temperature, exceeding the predictions made by considering the thermal variations in the individual physical properties of the fluid and solid constituents of a typical MR fluid. In the low-flux regime, designers of MR fluid actuators can alleviate this thermal reduction in stress by increasing the applied magnetic field strength. However, this is not possible in the high-flux regime because of magnetic saturation, and it becomes necessary to explore and understand the intrinsic limitations of the fluid at elevated temperature. We describe a new magnetorheological fixture, which was designed as an accessory to a commercial torsional shear rheometer, capable of applying magnetic flux densities up to 1 T and controlling the sample temperature up to 150°C. During the design of the instrument, close attention was given to the uniformity of the magnetic field applied to the sample by using numerical simulations. Incorporation of a custom-built magnetic flux sensor which matches the environmental capabilities of the fixture enables in situ measurement of the local magnetic field at each temperature. The numerical results are also validated by spatially resolved measurements of the local magnetic field. Finally, we explore the ability of a shift factor between fluid magnetization and yield strength to describe the measured variation in the MR fluid response at elevated temperatures.     

Constitutive modeling of the material behavior at elevated temperatures and related material testing

Constant strain rate tests, both in tension and in torsion, were performed on 2618-T61 aluminum alloy at 200°C. The results were compared with those from the creep tests. It was demonstrated that the macroscopically observed material behavior was essentially a consequence of the inherent time dependence of the inelastic strain. Based on the experimental results, the traditional approach and the unified approach for modeling the material behavior at elevated temperatures were compared. The material tests required for the development of the constitutive model and their limitations were also discussed.     

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.     

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     

Mechanical behavior of Gamma-Met PX under uniaxial loading at elevated temperatures and high strain rates

Gamma titanium aluminides have received considerable attention over the last decade. These alloys are known to have low density, good high temperature strength retention and good oxidation and corrosion resistance. However, poor ductility and low fracture toughness have been the key limiting factors in the full utilization of these alloys. More recently, a new generation of gamma titanium aluminide alloys, commonly referred to as Gamma-Met PX, has been developed by GKSS, Germany. These alloys have been observed to have superior strength and better oxidation resistance at elevated temperatures when compared with conventional gamma titanium aluminides.The present paper discusses results of a study to understand the uniaxial mechanical behavior in both compression and tension of Gamma-Met PX at elevated temperatures and high strain rates. The compression and tensile tests are conducted using a modified Split-Hopkinson Bar apparatus at test temperatures ranging from room temperature to 900 °C and strain rates of up to 3500 s⁻¹. Under uniaxial compression, in the temperature range from room to 600 °C, the flow stress is observed to be nearly independent of test temperature. However, at temperatures higher than 600 °C thermal softening is observed at all strain rates with the rate of thermal softening increasing dramatically between 800 and 900 °C. The room temperature tensile tests show negligible strain-rate dependence on both yield stress and flow stress. With an increase in test temperature from room to 900 °C, the material shows a drop in both yield and flow stress at all levels of plastic strain. However, the measured flow stress is still higher when compared to nickel based super-alloys and other gamma titanium aluminides under similar test conditions. Also, no anomaly in yield stress is observed up to 900 °C.     

Fracture toughness of CIP-HIP Beryllium at elevated temperatures

The fracture toughness of CIP-HIP Beryllium was determined using the short bar fracture toughness (K_IcSB) method. The K_IcSB value measured was 10.96 MPa√m at room temperature. This falls well within the expected range of 9 to 12 MPa√m as observed from previous fracture toughness measurements of beryllium. Toughness increased rapidly between 400°F and 500°F reaching a value of 16.7 MPa√m at 500°F.     

Experimental study of dynamic plasticity at elevated temperatures

In this paper it is shown that the diffraction-grating technique and the optical-displacement technique used by the writer for the study of plastic wave propagation at room temperature, may both be extended to within 100° F of the melting point of aluminum. In addition to the measurement of stress history at the impact face obtained by the extension of the load-bar technique to elevated temperatures, strain-time, surface angle-time, time of contact, coefficient of restitution, and displacement-time behavior at the free end of the struck specimen may all be determined at elevated temperatures. Typical strain-time behavior is shown at 800, 1000, and 1100° F, for three types of impact situations.     

Failure analysis of stainless steel at elevated temperatures

In this paper, particular emphasis has been put on gathering information on the phenomena that take place at the crack tip of a crack propagating at 1100°F. Since the experimental program was directed toward studying crack propagation in tubing, the tests were conducted on rings.From the experimentally obtained data and from the correlation with the theoretically predicted values, the following picture emerges for the fracture behavior with full plasticity present. There is a region surrounding the crack tip where very large plastic deformations take place. This region is surrounded by a much larger region where the loading is nearly proportional and the behavior can be predicted well by the results of the deformation theory of plasticity and the theory of singularity fields. As the crack propagation initiates, there is a drastic change in the crack-tip configuration. The crack tip does not blunt and a fairly sharp crack-tip region is observed. The crack tip carries a large deformation field of a far more localized nature than that observed at the initiation of the crack growth.Paper was presented at 1978 SESA Spring Meeting held in Wichita, KS on May 14–19.     

Dynamic stress-strain characteristics of metals at elevated temperatures

A method for measuring the load, strain and strain rate inside a high-temperature environment is described. Results of tests on specimens of high-purity copper are reported. Results indicate that copper is strain-rate dependent within the temperature range 78°–1000° F up to strains of 0.6 percent and strain rates of 10³ sec^–1.Paper was presented at 1969 SESA Spring Meeting held in Philadelphia, Pa., on May 13–16.This work was sponsored by the Army Research Office (Durham) under Contract No. DA 31-124-ARO-D229.     

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。     

闭孔泡沫铝的高温局部压入力学响应

通过不同形状(平头和半球头)的压头在不同温度下对闭孔泡沫铝材料进行塑性压入实验,研究不同温度下闭孔泡沫铝的压入变形模式及载荷响应特性。并基于闭孔泡沫铝在高温下的准静态塑性压入载荷响应的实验结果,结合多种分析方法,(如量纲分析和有限元计算等),探索既考虑温度影响也包含压入深度影响的预测闭孔泡沫铝平头和半球头压入力学响应的经验公式。结果表明,本文得到的两种压头情况下的经验公式都能够较好地预测闭孔泡沫铝在不同温度下的压入力学响应。

     

A multi-scale algorithm for dislocation creep at elevated temperatures

Dislocation creep at elevated temperatures plays an important role for plastic deformation in crystalline metals. When using traditional discrete dislocation dynamics(DDD) to capture this process, we often need to update the forces on N dislocations involving ～N~2 interactions. In this letter, we introduce a multi-scale algorithm to speed up the calculations by dividing a sample of interest into sub-domain grids:dislocations within a characteristic area interact following the conventional way, but their interaction with dislocations in other grids are simplified by lumping all dislocations in another grid as a super one. Such a multi-scale algorithm lowers the computational load to ～N 1.5. We employed this algorithm to model dislocation creep in Al-Mg alloy. The simulation leads to a power-law creep rate in consistent with experimental observations. The stress exponent of the power-law creep is a resultant of dislocations climb for ～5 and viscous dislocations glide for ～3.     

Dynamic yield-strength determination at elevated temperatures after nanosecond pulse heating

An experimental method is described that has been used to determine the yield strength of 6061-T6 aluminum after extremely short times at elevated temperature. The method combines electron-beam pulse heating and onedimensional stress-wave loading. A 3.5-MeV pulsed electron-beam source (pulse width of 70 nanoseconds) is used to deposit energy uniformly through the thickness and along a limited region of a slender aluminum rod. An axial compressive stress wave, produced by projectile impact on one end of the rod, propagates into the heated region a few microseconds after energy deposition. The nanosecond electron-beam pulse increases the internal energy of the material before it can expand to equilibrium dimensions at the elevated temperature. Additional time is therefore required for the specimen to equilibrate mechanically through the propagation of radial release waves which originate at the stress-free boundary of the sample. The deformation produced by these radial relief waves is coupled with microstructural changes that also contribute to a reduction in the yield strength of the material at elevated temperature, as well as at room temperature following electron-beam heating.     

Apparatus for studying cyclic stress-strain and fatigue at elevated temperatures

An apparatus is described which permits tubular specimens to be heated to a uniform temperature, while being cyclically strained with a constant amplitude of alternating strain about a fixed mean strain. Under these conditions, tests were performed in which the mean stress was measured as a function of the number of cycles of repeated strain for the alloys Udimet 700 and Rene 41 at 1300°F. These data along with fatigue fracture data are reported.