Dynamic deformation of metals under high hydrostatic pressure

A method for investigating the static and dynamic deformation of materials subjected to hydrostatic confining pressures is described. Results of tests on high-purity (99.995 percent) copper in compression are given. These tests cover a range of hydrostatic pressures from one to 6,985 bars (100,000 psi) and strain rates from 0.001 to 10,000 sec^?1. The results show a definite strain-rate effect which is dependent on pressure.     

岩石拉压蠕变试验仪研制及应用

为了探索岩石在周期性恒定拉伸、压缩荷载作用下的蠕变行为,结合杠杆原理,设计研制了一种岩石杠杆式拉伸、压缩蠕变试验仪。该仪器具有挂重质量小、可方便切换拉、压荷载等特点。首先,通过标定好的数显式拉压荷重传感器对该试验仪拉伸、压缩荷载进行了校正,得出试验仪压缩、拉伸荷载杠杆扩力比分别为81.29倍与59.46倍,挂重质量与施加在岩样荷载呈线性关系(压缩作用下线性关系相关系数R2=0.99975,拉伸作用下R2=0.9991),荷载施加稳定。最后,采用该试验仪对红砂岩进行了单轴恒定拉、压循环荷载下的蠕变试验,探讨了受荷岩样拉压蠕变规律。上述成果丰富了岩石蠕变测试与研究内容,有助于岩石力学试验测试的发展。     

A traceable dynamic force transducer

A traceable dynamic force measurement system built at the National Physical Laboratory (NPL) for the frequency range of 0.1 to 100 Hz, is described. The strain on a load-cell element was measured by two independent methods, both calibrated in the static force standard at the NPL. All instrumentation and the measurement system for determination of the dynamic amplitudes were calibrated dynamically against reference sine waves. The load cell was then tested dynamically in a servohydraulic machine and the two force outputs compared. The two methods of strain measurement chosen were the strain gage and a capacitance gage, designed to measure the strain in as diverse a way as possible from the strain gages. Two load cells were built, with steel and aluminum elements. The strain-gage bridge was d-c excited, amplified, filtered and digitally sampled. The capacitance gage was a-c excited from a commercial capacitance bridge, which was calibrated by the injection of reference signals at the sideband frequencies. This technique is in principle applicable to any a-c excited instrumentation. The measurement system consisted of a digital storage oscilloscope, coupled to a computer. The force amplitudes were measured by cross correlation against sine waves generated in software at the frequency of the applied force. Comparison of the two methods of strain measurement and detection of systematic errors caused by the dynamic response of the capacitance electrodes lead to the determination of the uncertainty of dynamic force measurement. This was calculated to be ±0.4 percent over the frequency range of 0.001 to 100 Hz. The thermoelastic effect was also visible at frequencies below 0.1 Hz. Paper was presented at the 1989 SEM Spring Conference on Experimental Mechanics held in Cambridge, MA on May 28–June 1.     

Combined shear/compression structural testing of asymmetric sandwich structures

Asymmetric sandwich technology can be applied in the design of lightweight, non-pressurized aeronautical structures such as those of helicopters. A test rig of asymmetric sandwich structures subjected to compression/shear loads was designed, validated, and set up. It conforms to the standard certification procedure for composite aeronautical structures set out in the “test pyramid”, a multiscale approach. The static tests until failure showed asymmetric sandwich structures to be extremely resistant, which, in the case of the tested specimen shape, were characterized by the absence of buckling and failure compressive strains up to 10,000 μ strains. Specimens impacted with perforation damage were also tested, enabling the original phenomenon of crack propagation to be observed step-by-step. The results of the completed tests thus enable the concept to be validated, and justify the possibility of creating a much larger machine to overcome the drawbacks linked to the use of small specimens.     

Large Strain Shear Compression Test of Sheet Metal Specimens

A hybrid experimental-computational procedure to establish accurate true stress-plastic strain curve of sheet metal specimen covering the large plastic strain region using shear compression test data is described. A new shear compression jig assembly with a machined gage slot inclined at 35° to the horizontal plane of the assembly is designed and fabricated. The novel design of the shear compression jig assembly fulfills the requirement to maintain a uniform volume of yielded material with characteristic maximum plastic strain level across the gage region of the Shear Compression Metal Sheet (SCMS) specimen. The approach relies on a one-to-one correlation between measured global load–displacement response of the shear compression jig assembly with SCMS specimen to the local stress-plastic strain behavior of the material. Such correlations have been demonstrated using finite element (FE) simulation of the shear compression test. Coefficients of the proposed correlations and their dependency on relative plastic modulus were determined. The procedure has been established for materials with relative plastic modulus in the range 5?×?10^?4?<?(E _p /E)?<?0.01. It can be readily extended to materials with relative plastic modulus values beyond the range considered in this study. Nonlinear characteristic hardening of the material could be established through piecewise linear consideration of the measured load–displacement curve. Validity of the procedure is established by close comparison of measured and FE-predicted load–displacement curve when the provisional hardening curve is employed as input material data in the simulation. The procedure has successfully been demonstrated in establishing the true stress-plastic strain curve of a demonstrator 0.0627C steel SCMS specimen to a plastic strain level of 49.2 pct.     

Extending the stress-analysis range of brittle coatings

In this paper, an attempt is made to extend in both directions the range of the stresses that can be determined using brittle coatings: (1) by sensitizing the coating to determine small stresses, and (2) by the use of a new calibration device that allows the application of very large strains. The increase in sensitivity is obtained by bathing the coating with cold water under load. The coating used in this manner is shown to have a strain sensitivity as low as 100 μin./in., well below the 400 μin./in. limit imposed by crazing of the coating as it is normally used. The fixture to apply large strains is described and the method applied to several brittle coatings to calibrate them up to 10,000 μin./in. (1 percent) strain sensitivity. In conjunction with the above studies, a short test was conducted on the effects of repetitive loading on a brittle coating. Load history was shown to affect strain sensitivity in some cases.     

Tensile testing of materials at high rates of strain

A tension version of the split Hopkinson bar or Kolsky apparatus is developed for conducting tests in tension at high rates of strain up to 10³ s^?1. A number of aluminum, titanium, and steel alloys tested in tension show increasing degrees of rate sensitivity above 10 to 10² s^?1. Tests on 6061-T651 and 7075-T6 aluminum show measurable strain-rate sensitivity in tension at the highest strain rates, although similar tests in compression in the literature show essentially no strain-rate sensitivity. Details of the apparatus and instrumentation and guidelines for its use are presented.     

Further experimental studies on buckling of integrally ring-stiffened cylindrical shells under axial compression

An experimental study of the buckling of closely spaced integrally stiffened cylindrical shells under axial compression was carried out to determine the influence of shell and ring geometry on the applicability of linear theory. Twenty-nine specimens fabricated from 7075-T6 aluminum alloy with different geometries were tested. Test specimens were designed to fail in general instability and under low critical stresses to assure elastic buckling. Agreement of experimental results of the present study, and of those obtained in other studies with linear theory was found to be governed primarily by the ring area parameter, (A ₂ /ah). Values of “linearity,” ? (ratio of experimental buckling load to the predicted one), higher than 80 percent were obtained for (A ₂ /ah)>0.3 and a clear trend towards ?=1 was observed with increasing values of this parameter. Correlation with linear theory was also found to be influenced by ring spacing, (a/h), or rather the combination (a/h) [1+(A ₂ /ah]^?1/2. No significant effect of shell and other ring parameters on the correlation with linear theory could be discerned for the shells tested. By a conservative structural-efficiency criterion it was observed that only for low values of the area parameter, (A ₂ /ah)<≈0.5 ring-stiffened shells are more efficient than equivalent-weight isotropic ones. Highest efficiencies are obtained for (A ₂ /ah)≈0.2.     

High-strain-rate tests on titanium 6-6-2 utilizing a unique rate-testing machine

This paper presents the design of a unique materials-testing system capable of medium strain rates of from 10^?4 to 10²/s. The design incorporates both closed-loop hydraulic operation with that of open-loop pneumatic operation. A novel design permits accurate specimen alignment and a stiff frame which exceeds 17×10⁶ lb/in. (11.7×10⁴ MPa). The mechanine is able to perform conventional tension/compression tests, fatigue tests and, with slight modification, biaxial-stress-tube tests and triaxial-stress tests. The accurate alignment capability coupled with high frame stiffness and the pneumatic operation enables the testing of brittle materials with rigid grips. Titanium 6-6-2 was tested in both tension and compression at strain rates from 10^?4 to about 10/s at four selected temperatures. The material showed a slight strain-rate sensitivity. Yield stress was shown to increase with strain rate while ductility decreased at each test temperature.     

新型小载荷冲击磨损试验机的研制及其实验研究

设计并研制出新型小载荷冲击磨损试验机,介绍了该试验机的工作原理、结构及其主要技术参数,并采用所研制的试验机对TC4钛合金进行了8个不同周次的3组冲击磨损试验.结果表明:TC4钛合金的冲击坑形貌规整,具有明显的表面特征和累计损伤特征;在相同试验条件下试验结果的重现性较好,数据最大误差小于6%;试验机所加载荷稳定,长时运行无过流过温现象,能够满足小载荷冲击磨损试验的要求.     

Misalignment sensitivity test for load cells

A relatively simple test to measure the misalignment sensitivity of load cells is described. Eccentric compression loads are applied by the use of an angular block above the spherical loading button of the load cell. The results of replicate tests on six load cells, of five different types, produced by five different manufacturers, are presented.Richard A. Mitchell, formerly Mechanical Engineer, National Institute of Standards and Technology, Gaithersburg, MD, is now associated with Mitchell Associates, 2025 Lakebreeze Way, Reston, VA 22091.     

Applicability of normal and shear stress cells embedded in cohesionless materials

A normal stress cell and a shear stress cell have been designed and tested under very varied conditions including permanent strain. Results pooled from nine different tests with the cells embedded in cohesionless materials (sand and wheat) showed that the coefficient of variation of the normal stress-cell sensitivity was 0.04, while it was 0.10 for the shear cell. The agreement between predicted and measured sensitivity was considered to be good for the normal stress cell and reasonably good for the shear stress cell. The shear cell showed a systematic dependence, within acceptable limits, of the total stress state in the surrounding material. A qualitative explanation of this phenomenon is given. Paper was presented at the SEM VI International Congress on Experimental Mechanics held in Portland, OR in June 6–10, 1988.     

Changes in Intracellular Calcium during Compression of C2C12 Myotubes

In recent years, damage directly due to tissue deformation has gained interest in deep pressure ulcer aetiology research. It has been shown that deformation causes muscle cell damage, though the pathway is unclear. Mechanically induced skeletal muscle damage has often been associated with an increased intracellular Ca²⁺ concentration, e.g. in eccentric exercise (Allen et al., J Physiol 567(3):723–735, 2005). Therefore, the hypothesis was that compression leads to membrane disruptions, causing an increased Ca²⁺-influx, eventually leading to Ca²⁺ overload and cell death. Monolayers of differentiated C2C12 myocytes, stained with a calcium-sensitive probe (fluo-4), were individually subjected to compression while monitoring the fluo-4 intensity. Approximately 50% of the cells exhibited brief calcium transients in response to compression, while the rest did not react. However, all cells demonstrated a prolonged Ca²⁺ up-regulation upon necrosis, which induced similar up-regulations in some of the surrounding cells. Population heterogeneity is a possible explanation for the observed differences in response, and it might also become important in tissue damage development. It did not become clear however whether Ca²⁺-influxes were the initiators of damage.     

Measurements on scaled models of thrust pads

Reasons for use of scaled models in lubrication are put forward. It is shown how such models of thrust pads having a stepped surface can be used and how pressures, thrust and friction can be measured. In the family of pads chosen for experimentation, the lateral steps make an angle of cot^?12 with the free-stream direction, and the position of step which is at right angles to the oil flow and which joins the lateral steps is assumed to be at both optimum and non-optimum conditions with respect to the thrust/friction ratio. The mean experimental results for thrust load lie within +10 percent of the theory when obtained from pressure distribution and within +20 percent when obtained from direct-force measurements. For friction, the appropriate departures when obtained from pressure and direct-force measurements are ±2 percent and +8 percent.     

Enhancement of buckling characteristics for sandwich structure with fiber reinforced composite skins and core made of aluminum honeycomb and polyurethane foam

This experimental study is concerned with enhancing the buckling characteristics of sandwich structure when the 6061-T6 aluminum skins are replaced by carbon fiber reinforced composite for the same aluminum honeycomb and polyurethane core. Such an improvement can be attributed to the high strength to weight ratio of the composite skin while the softer core material acts on a relative base as a better energy absorbent and hence tends to stabilize the failure. This results in much higher post-buckling loads which corresponds to the remaining strength of the structure after the onset of buckling.Sandwich structures with core made of polyurethane foam with different densities were also tested in compression. The buckling load increased with the density of polyurethane up to 280 kg/m³ while deattachment of the core and skin occurred when the density is decreased below 100 kg/m³. Compatibility of the skin and core material is shown to play an important role in the buckling behavior of sandwich structure.     

Surface plasmon resonance reflectance imaging technique for near-field (~100 nm) fluidic characterization

Surface plasmon resonance (SPR) reflectance imaging technique is devised as a label-free visualization tools to characterize near-field (100 nm) fluidic transport properties. The key idea is that the SPR reflectance intensity varies with the near-field refractive index (RI) of the test fluid, which in turn depends on the micro/nano-fluidic scalar properties, such as concentrations, temperatures, and phases. The SPR sensor techniques have been widely used in many different areas, particularly in the biomedical and biophysical societies. While flow visualization techniques based on RI detection have been extensively well documented (Merzkirch 1987), the use of SPR imaging for fluidic applications has been introduced only recently since the author’s group presented a series of related studies in the past few years. The primary goal of this review article is two-fold: (1) Introduction of the working principles of the SPR imaging as a fluidic sensor, and (2) Presentation of example measurement applications for various fluidic scalar properties using the SPR imaging sensor technique. Section 1 summarizes the history and the basic principle of SPR by focusing on the Kretschmann’s theory and Sect. 2 describes the laboratory SPR imaging system specifically designed for fluidic applications. Section 3 presents the optical and material properties that affect the SPR measurement capabilities and sensitivity. Section 4 presents example applications of the implemented SPR for different near-field characterization problems, including (1) micromixing concentration field, (2) convective/diffusion of salinity distributions, (3) full-field thermometry, and (4) fingerprinting of crystallized nanofluidic self assembly. Sections 5 and 6 discuss the spatial measurement resolutions of the SPR imaging technique and the overall measurement sensitivities, respectively. Section 7 presents a few suggestions to further enhance the SPR measurement accuracy particularly for near-field fluidic characterization.     

基于原位观测的泡沫金属细观与宏观压缩实验研究

采用原位观测的方法研究了脆性泡沫铝材料在压缩载荷下细观与宏观断裂破坏规律和吸能机理。针对多孔泡沫金属材料提出一种细观原位加载实验方法,采用特别设计与制备的试件,在S570扫描电镜下研究了特定胞孔在压缩过程中孔壁的失效顺序和破坏规律,并揭示了能量吸收的细观机理。对块体材料的宏观压缩实验表明,脆性泡沫铝是以多个断裂带的形式破坏。研究发现,孔壁缺陷和胞孔形态缺陷是诱发断裂带形成与发展的重要因素。依据尺寸效应对细观与宏观实验下泡沫铝的性能进行了比较。     

Interaction of a wave with an obstacle in a multicomponent two-phase medium

The author's model [1] of a multicomponent liquid medium with nonlinear limiting compression diagrams and constant coefficient of viscosity is improved by the introduction of a coefficient of viscosity that varies during the deformation. The new model is used to obtain a numerical solution to the problem of the propagation of a plane wave produced by a shock load and the interaction of the wave with a fixed obstacle. Such a problem was solved earlier [2] in the case of a viscous medium for linear diagrams of static and dynamic compression and constant coefficient of viscosity. It is shown that the nonlinearity of the diagram of static compression leads with increasing pressure first to an increase in the reflection coefficient and then to a decrease of it. If the load has a sufficient duration, the initial section of the obstacle is subject to a succession of several waves, the number of which increases with increasing duration and amplitude of the load. The calculation was made for glycerine with air bubbles. It is assumed that at pressures up to 400·10⁵ N/m² glycerine is a linearly elastic medium In this case, the dynamic compression diagram of the two-component glycerine—gas-bubble medium is linear.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 3, pp. 183–187, May–June, 1981.I thank Yu. A. Sozonenko for discussion and valuable comments.     

Poisson's ratio variations in concrete

This paper presents the results of an investigation relating to the variations in the Poisson's ratio in compression of concrete. An embeddable strain-gage unit has been developed for use in this investigation, and the readings from this gage unit proved quite reliable. With embeddable gage units, and surface-bonded strain gages, concrete test specimens were prepared and tested under uniform load in compression. The test results from this investigation prove that the Poisson's ratio of concrete is minimum at the faces of the test specimens, and it increases as the point moves towards the specimens' axes, where it reaches a value about 56 percent larger than that at the faces of these specimens.     

High strain-rate behavior of ice under uniaxial compression

In the present study, a modified split Hopkinson pressure bar (SHPB) is employed to investigate the dynamic response of ice under uniaxial compression in the range of strain rates from 60 to 1400 s^?1 and at initial test temperatures of ?10 and ?30 °C. The compressive strength of ice shows positive strain-rate sensitivity over the range of strain rates employed; a slight influence of ice microstructure is observed, but it is much less than that reported previously for ice deformation under quasi-static loading conditions [Schulson, E.M., IIiescu, D., Frott, A., 2005. Characterization of ice for return-to-flight of the space shuttle. Part 1 – Hard ice. NASA CR-2005-213643-Part 1]. Specimen thickness, within the range studied, was found to have little or no effect on the peak (failure) strength of ice, while lowering the test temperature from ?10 to ?30 °C had a considerable effect, with ice behaving stronger at the lower test temperature. Moreover, unlike in the case of uniaxial quasi-static compression of ice, the effect of specimen end-constraint during the high rate compression was found to be negligible. One important result of these experiments, which may have important implications in modeling ice impacts, involves the post “peak-stress” behavior of the ice in that the ice samples do not catastrophically lose their load carrying capacity even after the attainment of peak stress during dynamic compression. This residual (tail) strength of the damaged/fragmented ice is sizable, and in some cases is larger than the quasi-static compression strength reported for ice. Moreover, this residual strength is observed to be dependent on sample thickness and the strain rate, being higher for thinner samples and at higher strain-rates during dynamic compression.