B Pre Lab Quiz Ans1

MAE244

Electrical Resistance Strain Gages

PreLab Quiz - 1

PreLab Quiz:  Strain Gage for Beam Bending Application  Section #### : Name(Print) : Student ID : With Answer 1. Strain is dimensionless (1 point). [ True, False ] True 2. The device used to directly measure the change in gage length is called Extensometer. (1 point) 3. What is the principle of operation of the electrical resistance strain gage? (1 point) The electrical resistance of a conducting wire changes when its length changes.

4. The strain gage uses a straight wire (1 point). [ True, False ] False 5. Strain gages measure only normal component of strain directly . [ True, False ] True (1 point) 6. The resistance of a strain gage mounted on a structure was measured to be 120 ohms with no load. After loading, the resistance changed to 121.5 ohms. Assuming gage factor Sg = 2.10, what is the strain? (1.5 point) 1 ∆R 1 121.5 − 120 = 0.005952381 Solution: ε = , so ε = Sg R 2.10 120 7.

For the Wheatstone bridge shown in the figure, determine the strain measured by gage R1 if voltage drop E is measured to be 0.0050 volts. (Gage factor Sg = 2.10, and R1 = R2 = R3 = R4 at zero load.) (2 point) Solution: 3.5V Vr  ∆R1 ∆R2 ∆R3 ∆R4  ∆E = − + − (1 + r)2  R1 R2 R3 R4  For single active gage, r = 1 and only R1 Changed, V ∆R1 V R2 = S gε R1 so ∆E = 4 R1 4 E 4∆E 4 × 0.0050 ε= = = 0.002721088 R4 R3 VS g 3.5 × 2.10

8. For a cantilever beam of 10" long with a load at the free end, calculate the theoretical strain at 7" from the free end when the deflection at loading point is 0.3" (Assume a beam thickness of 0.1"). (1.5 point) 3 δ ⋅t ⋅d ε= Use the equation (17) from the theory handout, 2 L3 , in this problem, δ =0.3, t=0.1, d=7 and L=10, so ε = 0.000315