In practical application, the selection of the strain gauge should be considered from the aspects of the test environment, the nature of the strain, the gradient of the strain change, the paste space, the radius of curvature, the measurement accuracy, and the characteristics of the strain gauge itself. The test environment mainly considers temperature, temperature, and electromagnetic fields. The nature of strain is divided into static strain and dynamic strain. For static strain measurement, a strain gauge with a small lateral effect is selected; for dynamic strain measurement, a strain gauge with a strong fatigue life is selected. For the measured objects with uniform changes in the strain field, there is no special requirement for the grid length of the strain gage. A grid length can be selected for easy attachment. For measurement points with large strain gradient changes, a strain gage with a smaller grid length can be selected. The available paste space also affects the selection of strain gauges, especially for narrow spaces, it is appropriate to use strain gauges with small grid lengths. The selected strain gage should be free of defects such as bubbles, mildew spots, rust spots, etc., with a resistance value within 120 ± 2Ω, and a self-compensating strain gage. The coefficient of thermal expansion of the strain gauge is shown in the table on page 6, from which you can choose a self-compensating strain gauge suitable for the test object.
Choose different strain gauges according to the different measured objects. The strain gage used for the concrete strain test requires a longer sensitive grid length. For example, the length of the sensitive grid of the linear sheet should be 60, 70, 80, 120 mm; the length of the sensitive grid of the foil sheet should be 10, 20, 30 mm. The length of the foil-type strain gauge sensitive grid used for composite material strength test should be 2,5mm. The length of the foil strain gauge grid used for printed circuit board testing should be 0.2, 1mm. For strain measurement of wood and glass, a strain gauge with a grid length of 5mm should be used. Strain gauges with a grid length of 1 to 6 mm should be used for general metal and acrylic strain measurement. For stress concentration testing, a uniaxial or biaxial 5-piece strain gage with a grid length of 0.15 to 2 mm should be used. Strain gauges with a narrow space and fast measurement of impact stress should use strain gauges with a grid length of 0.12 ~ 1mm. There are also special strain gages for measuring residual stress, large deformation and measuring bolt axial force.
A single strain gauge is generally used to measure uniaxial strain, and a strain flower is used to measure the plane stress state. The measurement of Poisson's ratio of materials should be orthogonal biaxial strain flower. For stress analysis, a triaxial strain flower of 0/90/45 degrees should be used. The two-axis 90 strain flower is used when the principal stress direction is known, and the three-axis and four-axis strain flower is used when the principal stress direction is unknown. 60Ω strain gauges are often used for bending correction (two strain gauges are located on the same bridge arm), 120Ω strain gauges are used for general stress measurement, and 350 ~ 1000Ω strain gauges are used to make strain sensors.