In practical application, the selection of strain gauge should be considered from the test environment, the properties of strain, the gradient of strain change, the paste space, the radius of curvature, the measurement accuracy and the characteristics of strain gauge itself. The test environment mainly considers temperature, temperature and electromagnetic field. The properties of strain are divided into static strain and dynamic strain. The strain gauge with small lateral effect is selected for static strain measurement. Strain gauges with strong fatigue life are selected for dynamic strain measurement. For the measured object with uniform change of strain field, there is no special requirement on the length of the strain gauge grid, and the strain gauge with long grid can be selected, which is easy to paste; For measuring points with large variation of strain gradient, the strain gauge with small grid length can be selected. The available paste space also affects the choice of strain gauge, especially the narrow space should choose a small grid length strain gauge. The selected strain gauge should have no defects such as bubbles, mildew and rust spots, and the resistance value should be within 120±2Ω, with self-compensating strain gauges. The coefficient of thermal expansion of the strain gauge is shown in the table on page 6, from which a self-compensating strain gauge suitable for the measured object can be selected.
Select different strain gauges according to different test objects. The strain gauge used for concrete strain testing requires a long sensitive grid length, such as the linear plate sensitive grid length should be 60, 70, 80, 120mm; The sensitive grid length of the foil plate should be 10, 20, 30mm. The length of the sensitive grid of the foil strain gauge used for the strength test of composite materials should be 2 or 5mm. The grating length of the foil strain gauge used for printed circuit board testing should be 0.2 and 1mm. The strain gauge with 5mm grid length should be selected for the strain measurement of wood and glass. Generally, the strain gauge with grid length of 1~6mm should be selected for the strain measurement of metal and propylene. A single or two-axis 5-piece strain gauge with a grid length of 0.15~2mm should be used for stress concentration testing. The strain gauge with a relatively narrow space and the strain gauge under rapid conditions such as collision stress should be measured with a grid length of 0.12~1mm. There are also special strain gauges for measuring residual stress, large deformation and measuring bolt axial force.
A single strain gauge is generally used to measure uniaxial strain, and strain flowers are used to measure the state of plane stress. The orthogonal biaxial strain flower should be used to measure Poisson's ratio of materials. The triaxial strain flower of 0/90/45 degrees should be selected for stress analysis. Two-axis 90-strain flowers are used in situations where the principal stress direction is known, and three-axis and four-axis strain flowers are used in situations where the principal stress direction is unknown. The 60Ω strain gauge is often used for bending correction (both gauges are located on the same bridge arm), the 120Ω strain gauge is used for general stress measurement, and the 350 to 1000Ω strain gauge is used to make a strain sensor.
