A reusable biaxial strain transducer

This paper describles a transducer which is, in effect, a reusable strain-gage rosette. The sensitive member of the instrument is a flat element shaped in the form of a hollow equilateral triangle. At the corners of the triangle are styli mounted perpendicular to the plane of the triangular element. On each side of each arm, an electric-resistance strain gage is centrally mounted parallel to the edges of the arm. The two gages on an arm form a temperature-compensated pair. In operation, the styli are pressed into the test surface, and subsequent straining of the surface induces bending and twisting strains in the arms of the element. The bending strain in an arm is detected by the strain gages mounted thereon and is proportional to the strain in the test surface in the direction parallel to the arm. The bending strain in a particular arm is unaffected by the forces in the other two arms. The theory of the transducer is discussed and experimental evidence which supports the theory is provided.     

A new reusable quadriaxial strain transducer

The reusable quadriaxial strain transducer consists of an eight-arm (at 45 degrees from one another) star-shaped plane-sensitive element. Two strain gages are mounted to the inner and outer sides of each arm in the neighborhood of the extremity where hard oblique pins are fixed. The pins are impressed into the surface of the material so the strains on the surface induce bending and torsional displacements of the arms. The paper describes the transducer design and the calibration procedure. Experimental results obtained on wood materials are analyzed.     

A stiff and compact impact-force transducer based on strain measurement

A stiff and compact transducer for both static and impact compressive forces has been developed. The principle of the transducer is based on measurement of tangential strain around the periphery of an axially loaded short and axixymmetric body. It is shown that for an appropriate choice of transducer geometry the output is nearly insensitive to the distribution of the force over the loaded surface. In an experimental investigation, a prototype of the transducer was subjected to both static and impact loads of different distributions. The results show only a few percent variation in transducer sensitivity for the load distributions used. The transducer consists essentially of a homogeneous body and is simple to male. The size and material can be chosen to fit special applications.     

Measurement of concrete internal strain using a three-dimensional transducer system

A new three-dimensional transducer system is introduced in this paper, which enables the measurement of six internal components of strain tensor in concrete members. Laboratory tests and strain analyses were carried out to evaluate this tranducer system. The results show that the system is effective in measuring internal strains of concrete members. It was then embedded in a reinforced concrete bridge deck in Macomb County, Michigan. Field tests were conducted to measure the strains in the deck under truck wheel load. The results were used to understand the deck's strain and stress behavior under truck wheel load.     

Measuring soil compaction and soil behavior under the tractor tire using strain transducer

Soil compaction can occur due to machine traffic and is an indicator of soil physical structure degradation. For this study 3 strain transducers with a maximum displacement of 5 cm were used to measure soil compaction under the rear tire of MF285 tractor. In first series of experiments, the effect of tractor traffic was investigated using displacement transducers and cylindrical cores. For the second series, only strain transducers were used to evaluate the effect of moisture levels of 11%, 16% and 22%, tractor velocities of 1, 3 and 5 km/h, and three depths of 20, 30 and 40 cm on soil compaction, and soil behavior during the compaction process was investigated. Results showed that no significant difference was found between the two methods of measuring the bulk density. The three main factors were significant on soil compaction at a probability level of 1%. The mutual binary effect of moisture and depth was significant at 1%, and the interaction of moisture, velocity, and depth were significant at 5%. The soil was compressed in the vertical direction and elongated in the lateral direction. In the longitudinal direction, the soil was initially compressed by the approaching tractor, then elongated, and ultimately compressed again.     

Dynamic micro-torque transducer

The desing and characteristics of a torque transducer developed to measure sinusoidally oscillating torques from 10^?6 lb-in. to 1.0 lb-in. and from near static to 500 Hertz are discussed. The transducer was used to transmit, as well as measure, the driving-point torque to a 0.5-in. diam circular cylinder of low-modulus test material whose viscoelastic, deviatoric moduli were to be determined. The more salient features of the transducer are its ability to cancel up to 98 percent of the signal due to its own inertial load and its insensitivity to transverse vibrations as a result of internal network compensation. Piezo-electric crystals were used as transducing elements.     

The interferometric strain gage

The interferometric strain gage consists of two very shallow grooves ruled on a highly polished surface. The grooves are cut with a diamond and are 4×10^?5 in. deep and 5×10^?3 in. apart. Coherent, monochromatic light from a He?Ne gas laser incident upon these grooves will produce fringe patterns. A fringe pattern with the fringes parallel to the grooves is formed on each side of the impinging beam. The position of these patterns in space is related to the distance between the two grooves. As this distance changes, the fringes shift. Measurement of these fringe shifts enables one to determine the local strain of the specimen. In this paper, the theory of the measurement is developed first. The strain, ∈, is given by ∈=ΔFλ/d _o sin α _o where ΔF is the average fringe shift of the two patterns, λ is the wavelength of light,d _o is the initial distance between grooves, and α _o is the angle between the incident light beam and the fringe patterns. A procedure for making static measurements with the interferometric strain gage is presented. The sensitivity for these measurements is 0.5 percent strain per fringe shift, and the maximum strain is 4 percent. The method is evaluated by comparing its results with other accepted means of measuring large plastic strain. These other techniques are: post-yield foil gages, a 2-in. clip gage, and an Instron testing machine. The average percent difference among these techniques is less than 0.4 percent based on a full-scale measurement of 4-percent strain. The interferometric strain gage has the following features: a gage integral with the specimen surface, a very short gage length, relatively easy application, and the ability to measure large strains.     

The diffractographic strain gage

A method for measuring strain using diffraction of light from a single aperture is described, and results of a comparison tensile test with an electrical-resistance strain gage are presented. The “diffractographic strain gage” is shown to have high sensitivity, linearity, accuracy and temperature compensation and the ability to operate in a variety of hostile environments. It is furthermore simple, inexpensive, and the data can be collected by eye without assistance from further instrumentation.     

The response of strain gages to longitudinally sweeping strain pulses

The current state-of-the-art for estimating the maximum rise time of a strain gage which is subjected to an axially sweeping strain pulse ist _rg≤0.8L/c+0.5μsec wheret _rg is the 10/90 rise time of the strain gage,^* L is the gage length andc is the strain-pulse velocity. This paper shows that the effect of the 0.8L/c term can be significantly reduced by utilizing an analytical compensation technique. In addition, it is shown that the 0.5 μsec additive constant can be reduced to approximately 0.1 μsec by reinterpreting data published by a previous author.     

Designing an all-purpose force transducer

Many design and analysis situations require the determination of loads being transmitted to a structure. Since it is not always possible to insert a load cell, it is proposed that the structure itself and a set of strain gages be used to determine each of the applied loads (forces and moments) responsible for the measured strains. In essence, the structure becomes the transducer.Although a set of loads will uniquely deform a structure, the measurements can only be made at a finite number of points. However, in most situations, these data are sufficient to reveal the combination of loads that caused the deformation.The development of a technique for determining these loads is described. Limitations of the technique are discussed along with the associated error estimates. A worked example is given at the end.Paper was presented at the 1990 SEM Spring Conference on Experimental Mechanics held in Albuquerque, NM on June 3–6.     

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.     

The interferometric strain rosette technique

An interferometric strain rosette can be used to measure three in-plane strains. The strain rosette consists of three microindentations produced on an object surface. Upon illuminating the indentations with laser light, each pair of indentations acts as a two-point source generating a pair of Young's interference fringe patterns. When the object is deformed, the distance between the indentations is altered. By measuring the change of spacing of the Young's fringes, the strain in the direction of the separation of the indentations is measured. Three indentations are at the vertexes of an equilateral triangle to constitute a strain rosette. Each pair of the microindentations enables measurement of an axial strain in the indentation separation. The rosette measures simultaneously three in-plane strains in the directions of the triangular sides. As three in-plane strains are measured, the in-plane shear and two normal strains can be found. Compared with a resistance strain rosette, the interferometric strain rosette has great features such as non-contacting and a short gage length. In addition, the interferometric strain rosette can measure large elastoplastic strains and is applicable to measurements at elevated temperatures. The theory with experimental evaluation is presented. Measurement sensitivity of the technique is discussed. Potential applications and limitations of the technique are to be described as well.     

A biaxial stress transducer for fabrics

A transducer^* is developed for measurement of biaxial stresses in fabrics and other flexible sheet materials. The stress transducer responds directly to the far-field stresses by means of strains induced in an elastic inclusion which are monitored by strain gages. Design of the transducer is based on the analytical solution for such inclusions in linear-anisotropic materials, but the concept is applied and tested in nonlinear-orthotropic structural fabrics. Calibration and verification tests are conducted for principal far-field stresses in the yarn directions in vinyl-coated polyester and tefloncoated fiberglass.     

Tests of a fiber-optic velocity transducer

A novel transducer is developed and tested. The transducer utilizes optical fiber to measure mean and instantaneous flow rates in turbulent flows, and is capable of detecting flow reversal. Calibration of the transducer is conducted in both air and water. The dynamic response of the transducer is tested against hot-wire anemometery in the wake flow of a circular cylinder over a wide range of Reynolds number.List of symbols C _D drag coefficient - D diameter of cylinder - d diameter of fiber - E modulus of elasticity of the fiber - e output voltage - F drag force per unit length of a cylinder - f frequency (Hz) - L length of the fiber cantilever - M magnification factor - m mass per unit length of the fiber - Re Reynolds number - q dynamic pressure (= 1/2 U ²) - U free stream velocity - density - v kinematic viscosity     

Increased torque transducer sensitivity via oversampling

The sensitivity of a commercial torque transducer under oscillatory shear conditions is extended by about a factor 3–5 into the small torque region. This increased sensitivity is a result of a straightforward data treatment in the time domain termed “on the fly oversampling”. This method is enabled by the emergence of modern ADC-cards. The underlying ideas of oversampling together with a first experimental verification of this method are described in detail. Received: 22 August 2000 Accepted: 14 November 2000