Stress. Strain is the ratio of the change in length of a material to the original, unaffected length. The most widely used strain gage is the bonded metallic strain gage. It is named after Siméon Poisson and denoted by the Greek letter ‘nu’, It is the ratio of the amount of transversal expansion to the amount of axial compression for small values of these changes. Axial strain measures how a material stretches or pulls apart. A valid service agreement may be required.
the x-y plane) and different properties in the direction normal to this plane (e.g. Semiconductors, medical equipment, lasers, optics and aviation and aerospace. The number of active strain gages and arrangement of the grid (longitudinal or transverse strain with the Poisson's number ν) can be summarized in the bridge factor "B": Bridge type Bridge factor"B" Number active strain gauges; full bridge: 4: 4: Strain gages are configured in Wheatstone bridge circuits to detect small changes in resistance. The three types of strain gage configurations, quarter-, half-, and full-bridge, are determined by the number of active elements in the Wheatstone bridge, the orientation of the strain gages, and the type of strain being measured. Calculators for Stress, Strain and Young's Modulus (Modulus of Elasticity, Elastic Modulus) Stress, Strain and Young's Modulus are all factors linked to the performance of a material in a particular setting. Embedded Control and Monitoring Software Suite, Engineer's Guide to Accurate Sensor Measurements, Download the Engineer's Guide to Accurate Sensor Measurements, Learn more about NI strain measurement hardware. Any smooth figure of revolution if R 2 is less than infinity Uniform internal or external pressure, q force/unit area; tangential edge support Stress and Deflection Equation and Calculator. To ensure accurate strain measurements, consider the following: To learn how to compensate for these errors and review other hardware considerations for strain measurements, download the Engineer's Guide to Accurate Sensor Measurements. Poisson’s ratio (v), is the measure of this effect and is defined as the negative ratio of strain in the transverse direction to the strain in the axial direction. The grid pattern maximizes the amount of metallic wire or foil subject to strain in the parallel direction. Therefore the strain has little effect on this dummy gage, but any temperature changes affect both gages in the same way. This document provides information to help you understand basic strain concepts, how strain gages work, and how to select the right configuration type. Installing strain gages can take a significant amount of time and resources, and the amount varies greatly depending on the bridge configuration. Ideally, strain gage resistance should change in response to strain only. Given the strains at a space point in the body, this calculator computes the strains of the same space point in a rotated coordinate system.
For example, the full-bridge type I configuration is four times more sensitive than the quarter-bridge type I configuration. Figure 6. Ideally, the resistance of the strain gage should change only in response to applied strain. Therefore the strain has little effect on this dummy gage, but any temperature changes affect both gages in the same way. The grid is bonded to a thin backing called the carrier, which is attached directly to the test specimen. When a material is compressed in one direction, the tendency to expand in the other two directions perpendicular to this force is known as the Poisson effect. For a summary of the various types of strain gages, refer to the following table. Poisson's ratio is which is the negative ratio of transverse (x and y) and longitudinal (z) strains. The number of bonded gages, number of wires, and mounting location all can affect the level of effort required for installation. Although dimensionless, strain is sometimes expressed in units such as in./in. Online calculator for the conversion of the bridge output into strain. Note: The strain measure exy is used in this calculation. Bending strain measures a stretch on one side and a contraction on the other side. STRAIN GAUGE The level of transverse sensitivity of a strain gauge is usually related to the grid geometry and in particular its size, especially the length/width ratio. Any change in resistance in any arm of the bridge results in a nonzero output voltage. Additionally, full-bridge strain gages are significantly more expensive than half-bridge and quarter-bridge gages. Because the temperature changes are identical in the two strain gages, the ratio of their resistance does not change, the output voltage (Vo) does not change, and the effects of temperature are minimized. Similarly, long lead wires can add resistance to the arm of the bridge, which adds an offset error and desensitizes the output of the bridge. Strain is the geometrical measure of deformation representing the relative displacement between particles in the material body. Strain can be positive (tensile), due to elongation, or negative (compressive), due to contraction.
Under these conditions, the bridge is said to be balanced. Certain bridge configurations even require gage installation on opposite sides of a structure, which can be difficult or even impossible. Towers, turbines, gearboxes; processes for shaping and finishing component parts. Strain is defined as the ratio of the change in length of a material to the original, unaffected length, as shown in Figure 1.
Figure 1. Strain gage manufacturers attempt to minimize sensitivity to temperature by processing the gage material to compensate for the thermal expansion of the specimen material for which the gage is intended. Shear strain measures the amount of deformation that occurs from a linear force with components in both the horizontal and vertical directions. the z-axis).Such materials are called transverse isotropic, and they are described by 5 independent elastic constants, instead of 9 for fully orthotropic. This configuration is commonly confused with the quarter-bridge type II configuration, but type I has an active R3 element that is bonded to the strain specimen. Input: displayed bridge unbalance on the measuring amplifier in mV/V, Parameter: k-factor, Poisson's number, bridge type. To measure such small changes in resistance, strain gage configurations are based on the concept of a Wheatstone bridge.
A strain gage with a GF of 2 exhibits a change in electrical resistance of only 2 (500 x 10-6) = 0.1%.
Requires a passive quarter-bridge completion resistor known as a dummy resistor, Requires half-bridge completion resistors to complete the Wheatstone bridge, R4 is an active strain gage measuring the tensile strain (+ε), R3 is an active strain gage compensating for Poisson’s effect (-νε), R3 is an active strain gage measuring the compressive strain (-ε), R1 and R3 are active strain gages measuring compressive strain (–e), R2 and R4 are active strain gages measuring tensile strain (+e), R1 is an active strain gage measuring the compressive Poisson effect (–νe), R2 is an active strain gage measuring the tensile Poisson effect (+νe), R3 is an active strain gage measuring the compressive strain (–e), R4 is an active strain gage measuring the tensile strain (+e), R1 and R3 are active strain gages measuring the compressive Poisson effect (–νe), R2 and R4 are active strain gages measuring the tensile strain (+e), Bridge completion to complete the required circuitry for quarter- and half-bridge strain gages, Excitation to power the Wheatstone bridge circuitry, Remote sensing to compensate for errors in excitation voltage from long lead wires, Amplification to increase measurement resolution and improve signal-to-noise ratio, Filtering to remove external, high-frequency noise, Offset nulling to balance the bridge to output 0 V when no strain is applied, Shunt calibration to verify the output of the bridge to a known, expected value. Torsional strain measures a circular force with components in both the vertical and horizontal directions. Figure 2. Please adjust accordingly when using the engineering shear strain .
Roarks Formulas for Stress and Strain for membrane stresses and deformations in thin-walled pressure vessels.
Therefore, the strain experienced by the test specimen is transferred directly to the strain gage, which responds with a linear change in electrical resistance.
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