应变片技术在动态力学测量中的应用

本文详细讨论了应变片技术,特别是半导体应变片技术在动态力学测量中的若干技术问题,主要有应变片及起动态应变仪的频率响应问题,应变片灵敏系数的动态标定问题。文章指出,随着数据处理微机化的日益普及,半导体材料所存在的非线性特性及拉压不对称性将不再影响到半导体应变片技术的推广使用。相反地,灵敏系数极高这一优良特性将使该项技术获得日益广泛的应用。     

Application of carbon-powder-polymer composite as a continuous crack-length sensor

This paper explores the application of carbon-powder-impregnated polymeric composites for measuring crack extensions. The strain sensitivity of the gage material is shown to be very small. In the first series of tests, the gage material is characterized by measuring the change in electrical resistance due to machined slits for various gage lengths. The measured response is compared with the response predicted from a very simple electrical model. On the basis of good correlation and repeatability, the usefulness of such gages to measure crack extensions is assessed by a second series of tests. Further work to improve the gage response by optimizing the shape of the gage and making the gage and the adhesive layer thinner is proposed. The presented concept, with improvements, can result in a reliable, inexpensive crack gage requiring inexpensive instrumentation. Paper was presented at the 1990 SEM Spring Conference on Experimental Mechanics held in Albuquerque, NM on June 3–6.     

Errors in high-temperature strain measurements

The results of a technical evaluation of the accuracy of two types of commercially available high-temperature electric-resistance strain gages, and a special high-temperature gage under development at the Liquid Metal Engineering Center (LMEC) are presented. These gages, the BLH type HT 1212-5A, Microdot type SG420, and the LMEC gage, were selected for evaluation because of the need for reliable electric-resistance strain gages for high-temperature stress or strain measurements and process instrumentation in the temperature range of 900 to 1200° F. The BLH gage is rated by the manufacturer as a 1000 to 1200° F gage; the Microdot gage is rated as a 900° F gage. The special LMEC gage was made for use up to 1200° F. Instrumentation of this type is needed to determine or ensure component structural integrity and over-all system reliability of fast breeder reactors.     

Small attachable interferometric strain gages

Laser-based interferometry from two tiny reflective indentations can be used to measure in-plane strain/displacement over a very short gage length (on the order of 100 μm). If the specimen material is not reflective, then some other means of generating the interference patterns must be found. This paper describes two kinds of attachable gages: plated acetate replicas of indentations and reflective foils that are indented after application. In either case, the gage is applied with the techniques used for foil-resistance gages and the gage itself is very small. The manufacturing procedures are described. The results of experiments comparing the strain to that measured with foil-resistance gages are presented. Finally, the small interferometric gage is used to measure strain on one of the metal strips in a foil-resistance gage.     

Dynamic strain measurement with the interferometric strain gage

The interferometric strain gage has a short gage length, high-frequency response and the capability of measuring large plastic strains. Furthermore, the gage is easily ruled directly onto the specimen, and no mechanical or electrical contact needs to be made during the measurement. These features make the interferometric strain gage particularly suitable for dynamic plastic-strain measurement. In this paper, the details of an experimental setup for generating and measuring dynamic plastic strain are given. The photometric techniques of measuring the fringe motion of the interference patterns are describle as well as the data-reduction procedure. A typical result is presented, and the validity of the method is established.     

Measurement of static strain at 2,000° F

This paper describes the work being done at Hughes Aircraft Company using capacitance strain gages to measure static strains at temperatures up to and including 2,000° F. A new configuration of the capacitance gage is described with a gage length of 0.50 in. Methods of attaching the gage have been developed for welding, bonding and flame-spray installations. Instrumentation procedures and methods are defined. Statistical accuracy and test results are discussed on the results obtained from a lot of 60 gages. Results discussed were obtained on Rene 41, titanium and L605 stainless steel. Data analysis on the gage includes gage factor, gage factor vs. temperature, apparent strain, linearity, hysteresis, temperature effects, drift, and zero shift.     

Pressure effects on the response of foil strain gages

The effect of pressure on the gage factor (the strain coefficient of resistance) for a bonded constantan gage was investigated by measuring the compliance of single-crystal quartz in the direction of itsc-crystallographic axis as a function of pressure and by comparing these results with those derived from ocoustic-velocity data. The gage factor is indicated as increasing with pressure at a rate of approximately 0.4 percent per kilobar. This increase may be due to one or both of two factors: an increase in the resistivity-strain tensor or a thinning and stiffening of the gage cement.     

Gage-length errors in the resolution of dispersive stress waves

The effect of measuring the time history of a passing dispersive wave with a finite-length gage is to introduce time and amplitude errors between the actual values and the measured values of the wave function. In this paper, mathematical expressions relating the size of these errors to the gage length, properties of the material, and spectral content of the incident wave are derived. These expressions are used to predict the response of a long, resistance strain gage, attached in the longitudinal direction on the lateral surface of a long slender rod of a linear viscoelastic material (Lexan), to a short pulse. The shape of the pulse just before and just after the region of the long gage was measured by means of very short gages. The dispersion and attenuation properties of the material were obtained from the short-gage data. This information and the previously derived mathematical-correction equations were used to predict the response of the long gage. Good agreement was obtained between the predicted and measured values of the response of the long gage. In addition, the correction equations and the response of the long gage were used to predict the incident pulse. Good agreement with the known experimental values was obtained.     

准静态压缩应力-应变曲线测量方法的探索

本文介绍了用应变片直接测量材料的准静态应力-应变曲线的试验研究。在MTS810材料试验机上分别对93W、G50、砂浆等几种材料进行了准静态压缩试验。由于仪器的系统误差不能由MTSCod规准确的得到材料的真实的弹性变形。为此在试件的中部贴应变片得到材料的弹性变形,塑性变形仍旧由MTSCod规记录,从而得到试样的真实的应变,准确获得准静态压缩应力-应变曲线。试验结果表明:在试件中部贴应变片的方法能够准确得到该材料的杨氏模量;在试件两端垫块的刀口上安置Cod规可得到材料的应力应变曲线。两者的合理组合即可得到准确而完整的准静态压缩应力-应变曲线。试验中还发现准静态实验中试件的断面加工不平,偏心压缩等都会影响E的准确测量。     

Two types of high-temperature weldable strain gages: Ni−Cr half-bridge filaments and Pt−W half-bridge filaments

The historical development of the single active and “true” (zero gage factor) dummy weldable strain gage is reviewed. The combination of these devices into a half-bridge gage results in transducers capable of static-strain measurements at temperatures of 650 and 950° F for Ni?Cr alloy elements and Pt?W alloy elements, respectively. General application notes concerning these instruments are also included.     

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.     

The—S/N—Fatigue-life gage: A direct means of measuring cumulative fatigue damage

This paper describes the present state of development of a new type of sensor, a fatigue-life gage which generates an irreversible resistance change that is a continuous function of the fatigue experience of the structure to which it is attached. The gage accumulates fatigue-damage information whether or not it is connected to excitation or readout devices. Measurements of cumulative fatigue damage can therefore be made intermittently with simple instruments which are connected to the gage only long enough to measure its resistance. Results to date give promise of a powerful method of predicting many forms of structure failure.     

The use of a high-modulus-inclusion gage in nonlinear viscoelastic materials

The behavior of an inclusion in a host material subjected to a stress system depends primarily on the ratio of the tangent moduli,E inclusion/E host. An inclusion of suitable material used in the form of a gage will give an identifiable photoelastic-fringe pattern. This pattern is related to the applied biaxial stresses in the diametral plane of the gage, and is independent of the actual modulus and strains in the host material provided that the moduli ratio is more than 300. A program of work has been carried out to verify the use of such an inclusion gage in low-modulus nonlinear viscoelastic materials. The gage geometry used in this work consisted of a hollow cylinder of birefringent material with a ratio of outside diameter to inside diameter of 5 to 1. The host materials were either unfilled or highly filled carboxyl-terminated polybutadiene rubbers. The moduli ratios for both host materials were such that the gages act as rigid inclusions. A theoretical study has also been conducted to find the optimum measuring points within the gage and the fringe patterns created by selected biaxial-stress ratios. The study also showed that the gage sensitivity is virtually independent of Poisson's ratio but depends on the biaxial ratio of the stresses. The values of the sensitivity factor obtained experimentally were close to those derived theoretically. The stressfringe order at the optimum measuring points was obtained by Tardy compensation, and the biaxial-stress ratio determined either from fringe-pattern recognition or by measuring points. Future applications and uses of such a stress-measuring technique will be described.     

Development of New Strain Gage for High-Pressure Hydrogen Gas Use

The output changes of two conventional strain gages (Cu–Ni and Ni–Cr) and a newly-selected strain gage for high-pressure hydrogen gas use (Fe–Cr–Al) in 90 MPa hydrogen and nitrogen gases were measured under unloading conditions to find a high-performance strain gage for high-pressure hydrogen gas use. The changes in the outputs of the Cu–Ni and Ni–Cr gages in hydrogen gas were much larger than those in nitrogen gas, and the Fe–Cr–Al gage showed almost the same output changes in both gases. These results imply that the Fe–Cr–Al gage is superior to the others as a strain gage for high-pressure hydrogen gas use. A large amount of hydrogen entered the Cu–Ni and Ni–Cr foils, and the electrical resistances of these foils were significantly changed by hydrogen exposure, whereas almost no hydrogen entered the Fe–Cr–Al foil, and its electrical resistance was not changed. These resistance changes of the foils as a result of hydrogen entry were consistent with the gage output changes in hydrogen gas.     

High-temperature strain gages for use in sodium environments

High-temperature long-term static strain measurements have, for many years, been a problem to the experimental stress analyst. Strain gages operating in environments from 600–1000° F over extended periods of time have varying characteristics as to temperature compensation, gage-factor shifts and, also, drift. The weldable-type high-temperature gage offers a great deal of promise, considering the ease of application to nonlaboratory-type structures. In addition, the Microdot-type weldable gage offers additional protection against most environmental conditions. In the high-temperature range, the Microdot half bridge containing a platinum-tungsten alloy encased in magnesium oxide and protected by an outer shell of Inconel-X was evaluated. One of two major difficulties encountered with this gage is the silver-gold alloy used for brazing all of the connections that provide a sealed gage element. This alloy was a difficulty because the gages were to be used in a sodium environment which easily dissolves the silver-gold elements. The other difficulty is the highly individual characteristics of the gages. This paper describes two significant developments made in adapting this gage for use in sodium environments at high temperatures. These are:

1. 1.
   A calibrating technique whereby the bonded-gage characteristics of each gage can be obtained prior to actually bonding the gage to the structure under test     
   
   

Performance of miniature resistance strain gages in low-cycle fatigue

Experimental results describing the behavior of five types of miniature resistance strain gages subjected to cyclic strains of high amplitude are presented. Test procedure and instrumentation are described. Changes in zero drift and changes in gage sensitivity are discussed with respect to various strain gage and test variables. Mechanisms of gage failure, effect of variation of imposed strain and hysteresis in gage response are discussed.     

Monitoring short fatigue cracks with miniature strain gages

An experimental technique to monitor the length and the opening level of a short fatigue crack is presented. It is based on the progressive decrease with crack length of the response of miniature strain gages installed on the surface near the crack plane. A first gage installed close to the crack plane can monitor cracks from 10 μm in depth to half a millimeter where the response saturates. Other gages at larger distances from the crack plane are less sensitive but can monitor longer cracks. The response is measured so that it is independent of strain-gage calibration, Young's modulus and Poisson's ratio. The paper first presents the basic principles and possibilities of the technique as well as a finite-element analysis performed on automatic welded joints with straight-fronted cracks for which the technique has been developed. The results give a correlation between gage response, crack length and gage location and the conditions of replacement of a gage reaching saturation. The practical exploitation of the technique has required further work to derive a continuous calibration of the gage response that includes corrections to account for the gage finite dimensions and the crack-plane inclination. This calibration is shown to give crack lengths that compare well with fractographic marks and typical results that have been obtained on short crack growth at the weld toe are presented. In particular, the resolution of the technique is put into evidence with results on the initial growth of a 0.1 mm nonpropagating crack. The paper finally points out the distinctive features that appear in current works to adapt the technique to the growth of semi-elliptical cracks of low and high aspect ratio.     

A ten-inch extensometer measuring small low-frequency strains

A strain gage is described for the measurement of small low-frequency dynamic strains in concrete structures. It is in the form of a demountable extensometer, and its sensitivity is attained by a combination of mechanical amplification, electronic amplification, the use of semiconductor gages and the adoption of suitable filters. The gage has been successfully used for measuring strain in the range 0.1–5.0 microstrain at about 3 Hz on a 10-in, gage length.     

M-shaped Specimen for the High-strain Rate Tensile Testing Using a Split Hopkinson Pressure Bar Apparatus

An experimental technique is proposed to determine the tensile stress–strain curve of metals at high strain rates. An M-shaped specimen is designed which transforms a compressive loading at its boundaries into tensile loading of its gage section. The specimen can be used in a conventional split Hopkinson pressure bar apparatus, thereby circumventing experimental problems associated with the gripping of tensile specimens under dynamic loading. The M-specimen geometry provides plane strain conditions within its gage section. This feature retards necking and allows for very short gage sections. This new technique is validated both experimentally and numerically for true equivalent plastic strain rates of up to 4,250/s.