A piezoelectric field-effect strain gage

A new and highly sensitive strain transducer has been developed using a thin-film semiconductor deposited on a polished piezoelectric ceramic substrate. Field-effect coupling has been found to exist between the substrate and film in which the number of mobile carriers in the semiconductor is dependent on the electric-displacement vector of the substrate. Therefore, the conductivity of the semiconducting film can be altered by piezoelectric charge due to a strain applied to the substrate material. An effective gage constant has been calculated in terms of the piezoelectric and elastic constants of the substrate and electronic properties of the film. Experimental devices were constructed by depositingp type tellurium on polished lead-zirconate-titanate ceramic resulting in experimentally observed gage factors as high as 5800 compared to 100–200 for conventional semiconductor gages. The semiconductor film exhibits an electronic instability that limits its use, at present, to transient measurements with frequencies above 1 Hz. Data will also be presented to show that the gage constant is continuously variable between a positive and negative maximum value by altering the magnitude and direction of the substrate-polarization vector. It is believed that these gages will be useful in those cases where extremely small strains (～10^?7) are to be measured or when moderate strains (～10^?4) are to be determined in an electrically noisy background.     

A photoelastic fiber-optic strain gage

This paper reports on the development of a photoelastic fiber-optic strain gage sensitive to transverse strain. The sensing element is made from an epoxy resin which is stress frozen to passively achieve the quadrature condition. Light, emitted from an LED operating at 820 nm, is transmitted to and from the sensing element via multi-mode fibers and the signal is detected using a dual-channel operational photodiode/amplifier.This unique combination of optics and electronics produces a fiber-optic sensor having a high signal to noise ratio and a measurement system which is lead-in/out insensitive. Results show that strains on the order of 1 microstrain can be measured over an 800 microstrain range when a dummy gage is used for compensation.Paper was presented at the 1992 SEM Spring Conference on Experimental Mechanics held in Las Vegas, NV on June 8–11.     

Reversible strain gage

A reversible strain gage was developed for accurately measuring thermal strains, especially for use on large structures where strain gages cannot be welded. These strain gages can be peeled after taking required apparentstrain measurements in a furnace and can be attached reverse-side-up at the points of interest on a test structure. After many trials, a polyimide strain gage was developed that is the same on both the base side and the cover side. The thermal characteristics of the reversible strain gage—repeatability of apparent strain, gage-factor change, creep, drift and the output for a given mechanical strain—were investigated. The repeatability of apparent strains for 100 reversible gages was within 60 microstrain of difference at 250°C. The output of reversible gages for mechanical strain, after 2 to 3 heat cycles which were peeled and cemented in the reverse-side-up position, almost coincided with those of virgin reversible gages.Paper was presented at Fourth SESA International Congress on Experimental Mechanics held in Boston, MA on May 25–30, 1980     

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 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.     

Biaxial gage factors for piezoresistive strain gages

Theoretical considerations of piezoresistive strain gages show that the change in electrical resistivity depends on the biaxial state of strain at the surface of the specimen to which the gage is bonded. In particular, whenV is the initial voltage across the gage and ( \( \in _{11} , \in _{22} , \in _{12} \) ) is the surface-strain state at the point of attachment, the gage-voltage change ΔV is given by \(\frac{{\Delta V}}{V} = G_{11} \in _{11} + G_{22} \in _{22} + G_{12} \in _{12} \) whereG ₁₁,G ₂₂ andG ₁₂ are the biaxial gage factors. Experiments were conducted on a nominally one-dimensional gage. Kulite type DLP-120-500, bonded to a standard ASTM flat tensile specimen of CR 1018 steel. For this gage, typical values were found to beG ₁₁?26,G ₂₂??1.4 andG ₁₂??1.1. SinceG ₂₂ andG ₁₂ are less than 6 percent ofG ₁₁, it is concluded that contributions from these two factors (called transverse and shear sensitivities) will be significant only when the gage is oriented such that \( \in _{11}<< \left( { \in _{22} , \in _{22} } \right)\) . However, in the interest of completeness and accuracy, all biaxial gage factors should be reported.     

Virtual strain gage size study

Experimental Techniques -     

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.     

A simplified method for eliminating error of transverse sensitivity of strain gage

This paper presents a simplified method for eliminating error of transverse sensitivity of resistance strain gage. It is proved mathematically and mechanically that principal stress and directional angle are true values without any errors if calculated directly from the apparent values of Young's modulus and Poisson's ratio of specimen materials, and from all the apparent-strain readings.     

A basic study of a strain gage made of nickel foil

Strain gages made of nickel foil are devised for measuring the elastic surface stress of a specimen subjected to repeated loads. Sticking nickel foil on the surface of a specimen, the elastic stress is measured by observing slip-bands in the foil resulting from repeated strains. Calibration studies with rotating-bending tests are performed on round steel bars with nickel foil to determine the applicable test temperatures for the gage, and to establish the relation between the first appearance of slip-bands and the magnitude of cyclic stress. The peak stresses in grooved shafts under bending are measured with the nickel foil gages. The accuracy of the results is examined.Paper was presented at the 1988 SEM Spring Conference on Experimental Mechanics held in Portland, OR on June 5–10.     

Unified data reduction procedure for both standard and hole-drilling strain gage Rosettes

Experimental Techniques -     

Use of foil strain gage at high hydrostatic pressure

The sources of error inherent in the use of strain gages to measure strain, in various types of pressure or load transducers at high hydrostatic pressure have been pointed out. Two of these errors have generally been over-looked so far. A method to investigate the magnitude of these errors has been described. It is shown that up to 10,000 psi, the calibration of the gages is independent of the hydrostatic pressure.     

A displacement gage for the rock-mechanics laboratory

A gage for measuring displacements has been developed for use in the rock-mechanics laboratory and in the field. The gage consists of a support ring that holds a linear-variable-differential transformer (LVDT), a mounting screw, and a leaf spring. The gage is mounted to the test specimen at two points between which displacement is to be measured. At one point, contact with the specimen is by means of an adjusting screw. At the other point, contact is through a dimple in the leaf spring. The leaf spring in turn is rigidly connected to the support ring. An LVDT is mounted in the ring with its axis parallel to the line of measurement and its core rod attached in the dimple in the leaf spring. Any change in the length of the line between support points is directly communicated to the LVDT. Other gages using LVDTs have been constructed; but the technique for attaching the gage to the test specimen relied on the LVDT itself to support the ring. Because of the delicacy of the movement in precision-gage-head LVDTs, only small forces could be tolerated, leading to an unstable, nonrugged gage. For regular LVDTs, the free floating core is not well suited to supporting lateral forces. Using the leaf spring provides a secure mount, capable of bearing reasonable lateral loads with little flexure. The LVDT is left free of all load so that its precision is uncompromised. By its nature, a leaf spring is stiff in its plane which is the direction of the support forces in this case. In the normal direction, the leaf spring can be as compliant as desired. Accuracy is independent of the spring since the LVDT contacts the sample at the same point as the spring does.     

A delamination gage for carbon fiber composites

A graphite crack gage familiar to fracture testing of nonconductive polymeric materials has been adapted to measure delamination growth in carbon fiber composites. The gage consists of a continuous graphite film whose conductance changes linearly with respect to crack length. The development of an insulation technique so that the electrical film may be applied to carbon fiber composites is described. Further constraints on the gage design occur due to the narrow profiles of conventional delamination specimens. These limitations are reviewed in detail along with appropriate methods for manufacturing and calibration of the gage for delamination experiments. A simple shunt voltage measurement circuit is described along with a derivation of the relationship of crack length to voltage. Two example applications are provided: stable delamination growth in a conventional double cantilever beam (DCB) specimen and dynamic delamination growth in a single-edge-notched (SEN) strip. The electrical delamination length measurements from the DCB tests were found to compare well with the location of the delamination front determined by microscopy and radiography. These results give confidence in dynamic delamination results where growth rates exceeding 1000 m/s were measured. Sample evaluations of delamination toughness are made using the experimental data; compliance methods are used in the case of the DCB analysis, and dynamic finite element methods are used in the case of the SEN strip analysis.     

The evaluation of a strain gage having long-time stability characteristics

A standardizing strain gage that meets the requirements for long-time stability of strain measurements is described in this paper. Statistical analyses of the strain calibration results taken over a period of 13 months showed no significant differences between successive calibrations at a 5-percent significance level. From the results of the strain calibrations of 134 gages, the resistance-deformation characteristicdR/dW had a mean of 120.31 ohms/in. with a coefficient of variation of 1.43 percent. Evaluation tests using 6×12 in. concrete cylinders compared the strains measured by the standardizing gage to those measured by the Berry gage. Statistical analyses of these results showed equivalent accuracy of strain measured by the standardizing strain gage at comparable precision.     

Transient-velocity gage for structures

A compact gage has been developed to measure transient velocities up to about 600 cm/sec. The design is based on the principle of a highly overdamped seismic oscillator. Frequency response of the gage is flat from 1 to 500 cps. The gage can be used at any inclination and measures the velocity component along one axis only unaffected by crosswise motion.