Bridge Testing

Purpose of the Testing

The purpose of the bridge testing is to test out the tensile strength of the material used in our bridge. The cardstock is generally considered as a brittle material, which means it would not undergo plastic deformation prior to failure. The bridge testing would tell us the maximum weight it can take compression and tension in order to warn the engineers of the limits the bridge can do.

Spreadsheet for BOM, Graphs , and Data Tables

Bridge Testing

Bill of Materials For Tension

Bill of Materials For Compression

Process of Making Tubes and Bars

Becasue my partner Ryan was sick, I did all of the design by myself. For bars I just created rectangles and I did the tubes by putting 5 rectangles together. Below is my design for the bridge testing.

Testing Machine Used

Epilog Laser Cutter
A Testing Lever

Tension Testing Analysis

Tension Test Procedures

First clamp the tesing machine onto the table and then put the pucket on one end of the testing machine. Put a chair under the bucket
Place one of the bars into position, centered on the T-Line.
Next, clamp both ends of the bars onto the tesing machine. The clamp must be very tight.
Then allow the bucket to pull the specimen tight.
Fill the bucket with sand gradually with scoops
Wait for 5 seconds and then pour another scoop of sand
The specimen would break without warning
Measure the weight of the bucket and the sand
Input data into the spreadsheet

Testing Pictures

I glued the bars on the squares I cut and then we were ready to perform the tension test.

We set up the tesing lever following the instruction by clamping it to the table and then measured the L1 ans L2. We clamped

After gradually pouring weight into the bucket, the bar broke in a sudden. Then we weighed the bucket to see what the tension force is.

Tension Tesing Data Table and Graph

Questions:

q. Is cardboard a ductile or brittle material? How do you know?

The cardboard is a brittle material because it do not go through plastic deformation before it snaps, which means it fails suddenly without a warning.

2. Describe the load-deformation curve for the material. Did the cardboard show elastic behavior?

The load-deformation curve for the material is going to be linear. The carboard did show elastic behavior but its elasticity is really low which means the elastic behhavior is really hard to tell.

Compression Analysis

Testing Procedures for Compression Test

First clamp the tesing machine onto the table and then put the pucket on one end of the testing machine. Put a chair under the bucket
Place one of the tubes into position, centered on the T-Line.
Notes: The L1 and L2 should be on the same side
Next, clamp both ends of the bars onto the tesing machine. The clamp must be very tight.
Then allow the bucket to pull the specimen tight.
Fill the bucket with sand gradually with scoops
Wait for 5 seconds and then pour another scoop of sand
The specimen would break without warning
Measure the weight of the bucket and the sand
Input data into the spreadsheet

Process of Testing

Since some of the tubes are really short, we had to put books under the testing machine to make the lever parallel to the table.

Here's a picture of how the testing machine would look like in a compression test.

Then we gradually pour sand into the bucket until the tube is crashed.

IMG_3730.MOV

Here's a video that shows the whole process of the compression test.

Here's a picture of how the tube looks like after being crashed by the weight

Tension Tesing Data Table and Graph

Question:

How did the length of the member affect its compressive strength?

The longer the tubes are, the lower its compressive strength is.

Conclusion & Sources of Error

Through this bridge testing activity, I find out that the shorter the bar is, the higher its tensile strength is, which is opposite than what I expected. I also figure out that every material has its elasticity to some extent, but it is really hard to tell sometimes because its elasticity can be really low, like the cardstock. I also learned what a lever is and how to use it to claculate forces. The compression strength is higher whne the tube is shorter as well.

There are maybe some errors caused during the experiments since the data we got for compression does not match with the result the example gives us. Factors below may be the reason that causes the error:

Did not wait after 5 seconds to pour sand again: this may cause the actual data to be higher than what the specimens can actually bear
The error caused by measurement of L1 and L2: Because we are using naked eyes to estimate the length of L1 and L2, the lengths of the lever can be measured wrong
The tightness of the clamp: The performance of tension test specimens can be affected by how tight we tighten the clamp to fix the position of the specimen
The change of testing machine: different testing machines can have different L1 and L2, which can cause variation in the calculation
The difference of strength of specimens: some of the specimens maybe folded before the testing which can greatly affect that their performances

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