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Construction
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Your choices are...
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Electrical Resistance
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The resistance of an electrically conductive material changes with dimensional changes that take place when the conductor is deformed elastically. When such a material is stretched, the conductors become longer and narrower, which causes an increase in resistance. A Wheatstone bridge then converts this change in resistance to an absolute voltage. The resulting value is linearly related to strain by a constant called the gauge factor.
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Capacitance
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Capacitance devices, which depend on geometric features, can be used to measure strain. Changing the plate area, or the gap can vary the capacitance. The electrical properties of the materials used to form the capacitor are relatively unimportant, so capacitance strain gauge materials can be chosen to meet the mechanical requirements. This allows the gauges to be more rugged, providing a significant advantage over resistance strain gauges.
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Photoelectric
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A beam of light is passed through a variable slit, actuated by the extensometer, and directed to a photoelectric cell. As the gap opening changes, the amount of light reaching the cell varies, causing a varying intensity in the current generated by the cell.
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Semiconductor (Piezoresistive)
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In ferroelectric materials, such as crystalline quartz, a change in the electronic charge across the faces of the crystal occurs when the material is mechanically stressed. The piezoresistive effect is defined as the change in resistance of a material due to an applied stress and this term is used commonly in connection with semi conducting materials. The resistivity of a semiconductor is inversely proportional to the product of the electronic charge, the number of charge carriers, and their average mobility. The effect of applied stress is to change both the number and average mobility of the charge carriers. By choosing the correct crystallographic orientation and dopant type, both positive and negative gauge factors may be obtained. Silicon is now almost universally used for the manufacture of semiconductor strain gauges.
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Optical
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Photoelastic Strain Gauges When a photo elastic material is subjected to a load and illuminated with polarized light from the measurement instrumentation (called a reflection polariscope), patterns of color appear which are directly proportional to the stresses and strains within the material. The sequence of colors observed as stress increases is: black (zero stress), yellow, red, blue-green, yellow, red, blue-green, yellow, red, etc. The transition lines seen between the red and green bands are known as "fringes." The stresses in the material increase proportionally as the number of fringes increases. Closely spaced fringes means a steeper stress gradient, and uniform color represents a uniformly stressed area. Hence, the overall stress distribution can easily be studied by observing the numerical order and spacing of the fringes. Furthermore, a quantitative analysis of the direction and magnitude of the strain at any point on the coated surface can be performed with the reflection polariscope and a digital strain indicator.
Moire Interferometry Strain Gauges Moire interferometry is an optical technique that uses coherent laser light to produce a high contrast, two-beam optical interference pattern. Moire interferometry reveals planar displacement fields on a part's surface, which is caused by external loading or other source deformation. It responds only to geometric changes of the specimen, and is effective for diverse engineering materials. Contour maps of planar deformation fields can be generated from x and y components of displacements.
Holographic Interferometry Strain Gauges Holographic interferometry allows the evaluation of strain, rotation, bending, and torsion of an object in three dimensions. Since holography is sensitive to the surface effects of an opaque body, extrapolation into the interior of the body is possible in some circumstances. In one or more double-exposure holograms, changes in the object are recorded. From the fringe patterns in the reconstructed image of the object, the interference phase-shift for different sensitivity vectors are measured. A computer is then used to calculate the strain and other deformations.
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Fiber Optic
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The sensor measures the strain by shifting the light frequency of the light reflected down the fiber from the Bragg grating, which is embedded inside the fiber itself. Since it is possible to put several sensors on the same fiber, the amount of cabling required is reduced significantly compared to other types of strain gauges. Also, since the signal is optical rather than electronic, it is not affected by electromagnetic interference.
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Other
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Other unlisted, specialized, or proprietary strain gauge construction.
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Arrangement (If Applicable)
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The gage pattern refers cumulatively to the shape of the grid, the number and orientation of the grids in a multiple-grid (rosette) gage, the solder tab configuration, and various construction features that are standard for a particular pattern.
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Your choices are...
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Uniaxial
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Single grid strain gauge to measure strain in a single direction only.
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Dual Linear
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A double linear strain gauge is typically used for bending-beam type applications and has two gages that are parallel, so they sense strain in the same direction.
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Strip Gauges
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A strip gage consists of several strain-sensitive grids mounted on a common backing. This type of gage offers a number of advantages in the study of local strain distributions and strain gradients. In addition, the optical tooling employed in the manufacture of the strip gage ensures that all grids are precisely located. Grid spacing is also closer than can usually be achieved with individual gages, thus yielding better resolution of nonuniform strain fields.
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Diaphragm
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Strain gages are frequently used in pressure transducers incorporating a circular diaphragm as the spring element. They are typically designed around a full bridge pattern.
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Tee Rosette
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Two mutually perpendicular grids (0-90°). The tee rosette should be used only when the principal strain directions are known in advance from other considerations. Cylindrical pressure vessels and shafts in torsion are two classical examples of the latter condition.
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Rectangular Rosette
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Three grids, with the second and third grids angularly displaced from the first grid by 45° and 90°, respectively. Rectangular and delta rosettes may appear in any of several geometrically different, but functionally equivalent, forms.
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Delta Rosette
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Three grids, with the second and third grids 60° and 120° away, respectively, from the first grid. Rectangular and delta rosettes may appear in any of several geometrically different, but functionally equivalent, forms.
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Other
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Other unlisted, specialized, or proprietary arrangement of gauge pattern.
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Specialty Applications
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Your choices are...
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Crack Detection
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Convenient, economical method of detecting cracks or crack growth.
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Crack Propagation
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Multiple conducting grids on a single backing accurately indicate the rate of crack propagation. Crack gauges are designed to be bonded to structures or materials under testing to measure the propagation length and speed of an expected or existing crack.
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Extensometer
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An apparatus for indicating the deformation of metal while it is subjected to stress.
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Measures Temperature
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Strain gauge includes resistive sensors, constructed much like a strain gage but with nickel or nickel/manganin grids, used to measure the surface temperature of test specimens to which they are bonded.
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Residual Stress
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Residual stress rosettes for the practical, widely used hole-drilling method of residual stress determination.
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Shear Modulus Gauges
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Shear modulus gauges for accurately determining the shear modulus of composite materials.
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Transducer Gauge
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Transducer strain gauges are specifically manufactured for use in force, torque, pressure, and other transducers.
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Other
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Other unlisted, specialized, or proprietary specialty applications.
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