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     

Ultrasonic thickness gage

An ultrasonic method was developed for making approximate measurements of the thickness of a web moving between parallel guides. Since practical considerations ruled out mechanical contact, a phasesensitive detection scheme for measuring the thickness of the air spaces between the fixed guides and the web was chosen so that: (1) the apparatus was insensitive to variations in surface properties of the web, and (2) a linear response was closely approximated. Unwanted reflections and their influence were reduced through the design of the acoustical geometry and a dummy arrangement. Another set of dummies compensated for changes in sound velocity with temperature changes. Performance curves of the ultrasonic measuring device show that the device performs satisfactorily if all electrical-to-sonic transducers have identical temperature vs. phase-lag characteristics at the operating frequency.     

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.     

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.     

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.     

Virtual strain gage size study

Experimental Techniques -     

Lateral-deformation gage for rock-mechanics testing

Lateral-deformation measurements are important in rock-mechanics testing, both for providing material-behavior information and for acting as a control variable in servo-controlled experiments designed to obtain post-failure stress-strain curves. A gage for making these measurements must be compact, have a large linear range, and for tests on anisotropic rock, be able to monitor a discrete sample diameter. A gage which employs a strain-gaged sensing disk is described which meets all of these requirements.     

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.     

Concepts of the photoelastic stress gage

With the photoelastic stress gage birefringence readings are made with light that traverses a path parallel to the surface of the workpiece. Individual stresses are determined in the elastic range of deformation, rather than stress or strain differences. The theory of a circular and linear stress gage is developed, including the influence of Poisson's ratio, and stress gradients. Stresses in the surface of the workpiece are expressed in terms of measured birefringence. Instrumentation is extremely simple. High sensitivity is derived from the relatively long optical-path length through the transducer. Applications should include stress analysis, load analysis and transducer design.     

Strain gage behavior on sandwich glasses

Experimental Techniques - The tests demonstrated that when the strain gages are applied in the manner described in this paper, satisfactory results can be expected. The thermal coefficients of...     

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.     

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.     

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.     

An electromagnetic shear-stress gage for large-amplitude shear waves

In many materials, especially plastics, ceramics and rocks, large-amplitude shear-wave propagation studies could provide valuable information for the development of constitutive equations. A newly developed electromagnetic-gage configuration provides an output voltage which is directly related to the dynamic shear stress in the material. The electromagnetic shear-stress gage has been used to make direct measurements of shear-wave stresses in PMMA and Solenhofen limestone. Large-amplitude shear waves were obtained with a new plate-impact technique which generates shear waves by a controlled-reflection process. The configuration of the stress gage permits it to be used simultaneously with more conventional electromagnetic velocity gages, thus providing both types of data in one experiment.