Stress-Strain

Equipment

| Included: | | 1 | Stress-Strain Apparatus | AP-8214A | 1 | Force Sensor | PS-2104 | 1 | Rotary Motion Sensor | PS-2120 | 1 | Calipers | SF-8711 | | Required but Not Included: | | 1 | 850 Universal Interface | UI-5000 | 1 | PASCO Capstone Software | UI-5400 |

Introduction

The objective of this lab is to find the relationship between tensile stress and strain for various materials. The Stress-Strain Apparatus stretches (and in some cases breaks) a test coupon while it measures the amount of stretch and force experienced by the test coupon. Software is used to generate a plot of stress versus strain, which allows Young's Modulus, the elastic region, the plastic region, the yield point, and the break point to be ascertained.

Theory

The ratio of the force (F) applied to the cross-sectional area (A) of a material is called the stress: Stress=FA (1)

The ratio of the change in length (L) to the original length (Lo) of a material is called the strain: Strain=∆LLo (2)
Stress (Pa)
Strain
Elastic Region
Plastic Region
Yield Point
Stress (Pa)
Strain
Elastic Region
Plastic Region
Yield Point

In the elastic region, the stress is proportional to the strain and the proportionality constant is called Young’s Modulus, Y. Y=StressStrain (3)

Set-Up
Figure 1. Stress/Strain Apparatus Assembly 1. Open the PASCO Capstone software on the computer. 2. Connect the Stress/Strain apparatus to the computer using USB cable and the PASCO Capstone software will automatically recognize the device. 3. Make a calculation for the stretch. For every 360o turn of the Rotary Motion Sensor, the screw moves 1 mm, so the stretch, x, in mm is given by

x = [position in radian]/ 2*π (4)

Calibration (Not required)

During the experiment, as the crank turns, force will be applied to the test coupon, causing it to stretch. However, this force will also cause the apparatus platform and the Force Sensor to bend. The displacement registered by the Rotary Motion Sensor will be the combination of the coupon stretching and the rest of the apparatus bending. Regardless of how much the coupon stretches, the deformation of the rest of the apparatus is constant for a given force. After repetitive measurement with the calibration bar it was found that the movement of force sensor is negligible compared to the stretching of the sample, hence the calibration step is not required.

Procedure

1. When installing coupons, remove the nuts and the clamp and place it at secure place, it is not needed for composite material testing. The coupon should be slid completely over the coupon holding screws on each end.

Figure 3: Clamping a Coupon
Figure 3: Clamping a Coupon

Figure 2: Washer Arrangements
Figure 2: Washer Arrangements

2. Pre-loading Coupons: This is the procedure you will follow each time you test a coupon. You must pre-load the coupon so the initial slack is taken up and the force sensor is zeroed at position zero.

a. In PASCO Capstone, setup continuous mode measurement with recording condition of start and stop when force is greater than 1 and greater than 45 N, respectively (figure 4).

45
45
1
1

Figure 4: Setting up recoding conditions

b. In PASCO Capstone, set up a Digits display of the Actual Force. You can setup graphs and table for data recording and display on the screen. Select the time and force to be displayed on the graph and collect the radian and force data in the table.

c. Turn the crank slowly so the lever bar does not touch the force sensor. Then zero the force sensor.

d. Press the start recoding button.

e. Turn the crank at speed of 3 N/second. That means the complete run from 1N to 45N should take about 15 seconds. Be careful when the load is greater than 20 N, hold the Pasco stress-strain device firmly in order to avoid movement of the device during experiment. Stop recoding when the force reach to 45 N or the coupon breaks.

f. Rename the data run to identify the type of material that was tested.

g. Copy the table data (position (radian) vs force (N)) and paste them in in excel spreadsheet for further analysis.

h. Generate data columns for stress and strain from the force and position data for further analysis and plotting graphs.

3. Test total of 4 (n=4) coupons for each type of composite.

Data processing

1. In excel spreadsheet; filter out data representing negative force values (if any). 2. Make a new column for zeroing the position by subtracting the initial position from all other position values. Simply, zero should appear as the starting point of the position values. 3. Divide all position values of radian by 2*π and force values by 5. 4. Convert position and force values into strain and stress values, respectively by using appropriate equations. 5. Plot the graph of stress vs strain for each type of composite material and find the Young’s modulus.

Analysis

1. Select the initial part of the straight slope at the beginning of the stress vs. strain plot. Fit a straight line to it and find Young's Modulus from the slope. 2. Make a table of comparison for average values of Young's Modulus of two types of composites.

3. What is the difference between two composite materials?

4. Which type of composite if tough?

| inch | mm | Length of Coupon | 4.134 | 105.0036 | Width of Coupon | 0.5 | 12.7 |