First, the purpose
1: Use the static strain gauge TST3826 to complete the bridge test of strain gauges;
2: Understand the principle of electrical stress measurement and master the addition and subtraction characteristics of DC measurement bridges;
3: Analyze the relationship between the strain gage bridge and the deformation of the beam, and deepen the understanding of the concept of equal strength beams.
Two: instruments and equipment used
1: One TST3826 static strain tester;
2: Weight (5N) * 4
3: a set of equal strength beams
4: One computer.
The equipment and instruments are shown in the following figure:
Three: Test principle
Figure 2-1 Experimental device diagram
â‘´ Measuring principle of resistance strain gauge
At present, the commonly used foil resistance strain gauge is made of 0.003 ~ 0.01mm high-impedance nickel-copper foil material through chemical corrosion and other processes to make a resistance foil grid, and then the lead wire is welded, coated with insulating glue and fixed to the plastic base film. When in use, it is only necessary to firmly adhere the base film surface to the measuring point of the component with special glue. In this way, when the component is deformed by force, the resistance strain gauge will also deform, and the resistance value of the resistance strain gauge will change. The characteristic relationship is: ΔR / R 0 ∕ ΔL / L 0 = K That is to say, the ratio of the change rate of the strain gauge resistance to the change rate of the length is a constant K, and the change rate of the length ΔL / L 0 = ε. The constant K is also called the sensitivity coefficient of the resistance strain gauge. When the resistance strain gauge is delivered as a product, it will give K, R 0 and L 0.
Therefore, as long as a special electronic instrument can measure the resistance change rate ΔR / R 0 of the strain gauge, the stress measurement σ = Eε can be completed
This specialized electronic instrument has been widely used as a static resistance strain gauge. ⑵ Measuring principle of static resistance strain gauge
The static resistance strain gauge is measured according to the principle of Wheatstone bridge.
The Wheatstone bridge is shown in Figure 2-2:
If a DC voltage V AC is given between nodes A and C, there is a voltage output V BD between B and D, and V BD = (R 1 R 3 -R 2 R 4) V AC / (R 1 + R 2) (R 3 + R 4), when R 1 R 3 = R 2 R 4, the bridge is said to satisfy the equilibrium condition, at this time V BD = 0, and the characteristics of the bridge know that when R 1 = R 2 = When R 3 = R 4 = R, the bridge is a full-arm bridge.
dV BD = (ΔR 1 / R-ΔR 2 / R + ΔR 3 / R-ΔR 4 / R)
Since the resistance strain gauge has ΔR / R = Kε, the above formula can be written as:
dV BD = K (ε 1 -ε 2 + ε 3 -ε 4) This is the measuring principle of static resistance strain gauge. At the same time, it also shows the addition and subtraction characteristics of the measurement bridge. Using the addition and subtraction characteristics of the bridge, single arm, half bridge, full bridge and other measurements can be achieved according to different measurement requirements. It should be remembered that the display value of the static resistance strain gauge is micro-strain (με), ε = 10 6 με. Four: Test Step 1: In this experiment, we input the full bridge method and connect the strain gauges D1-D4 to the first channel of the TST3828 dynamic static strain gauge as shown in the figure below. Such a solution can further cancel or method the strain output value. . 2: Connect the instrument to the power supply and turn on the power switch, connect the computer and the instrument with a cable, and then open the ST3828 dynamic and static test software. 3: Create a new project in the software, set various parameters, and change the sensitivity coefficient in the software from 2.00 to 2.07 (the sensitivity coefficient of the equal strength beam). 4: Balance and clear the system in the software without adding weight. 5: Start collecting signals without adding weights, add a weight (50g) to the test beam after 5S, add a weight after 5S, and so on, until all four weights are added And then stop collecting. The obtained signal acquisition picture is as follows:
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Five: The test data calculates the average strain of each segment of ε1-ε5 according to the function of the software itself, as follows: ε1 = 0.723με ε2 = 179.6με ε3 = 424.9με ε4 = 665.6με ε5 = 879.7με Value Added:
△ ε1 = ε2-ε1 = 178.9με
△ ε2 = ε3-ε2 = 245.3με △ ε3 = ε4-ε3 = 240.7με
△ ε4 = ε5-ε4 = 214.1με
From this, ε = (△ ε1 + △ ε2 + △ ε3 + △ ε4) / 4 = 219.7 με This is the experimental measured value of the strain at the measured point. The theoretical value is calculated as follows:
According to the bending theory, it can be known that D 1, D 3 and the measuring point are under tension; D 2 and D 4 are under compression and are in a unidirectional stress state. The stress σ = 6FX / bh 2 and Hooke's law σ = Eε, when E = 206Gpa, we know: ε = 6FX / Ebh 2; this is the theoretical calculated value of the measured point strain.
ε = 6FX / Ebh 2 = (6 * 19.6 * 0.09) /206000000000*0.0085*0.005*0.005= 241.1 με
Six: Test conclusion
It can be seen from the above data: the actual measured value is very close to the theoretical value data. Therefore, it can be concluded that the method of measuring strain using the strain gauge bridge is feasible
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