Big Mac's Blog

Preparation of Tensile Test

kilomac — Thu, 09 Jul 2009 17:33:05 +0000

To set up our composite for testing, we first had to cut it into strips. A band saw was used for this purpose. To ensure proper breakage of the samples (in the middle away from the clamps of the tensile machine) the samples had to be necked down in the middle. What we hadn’t accounted for was the amount of the sample eaten up by the saw blade. We had laid out the strings within the sample such that each test sample would be exactly one inch wide. Using the band saw, however, would take some of that one inch away which would make it very hard to neck the samples enough. Instead we decided to try to salvage some of our samples by sacrificing those immediately next to them. By cutting every other sample wider, we were able to salvage 4 samples with 4 strings, 3 samples with 2 strings, and 4 samples with no strings. A representation of this can be seen in the following photographs.

In all, we managed to retain labeled samples 1,3,5,7,9,11,13,14,15,16,and17.

More String Testing

kilomac — Tue, 07 Jul 2009 15:50:36 +0000

We found ourselves running the dead load test again, but this time on burlap. Burlap is simply jute twine in a plain weave (one warp to one weft) as can be seen below. To discover the strength of the burlap, individual strings were removed from the mesh and tested by pouring sand in a bucket until failure of the string.

Results showed that, like jute, the individual fibers withstood less stress than sisal fibers.

Composite Fabrication

kilomac — Sun, 28 Jun 2009 22:31:36 +0000

Our First VARTM:

After testing the constituent material properties, it was time to incorporate them in a composite part. To do this, we used vacuum assisted resin transfer molding (VARTM).

Materials:

The resin we used is called acrylated epoxidized soybean oil (AESO). This binding agent is also of natural origins, as it is derived from soybeans. The layering of this part is 5 sheets of recycled paper, 1 unidirectional layer of sisal twine, and then 5 more sheets of recycled paper. The dimensions of the plate are 14.5″ by 20″. The plate was set up to have three different sections; one section having 4 strings of twine per test strip, one having 2 strings of twine per test strip, and a control section without any twine.

Setup:

After, putting together the part, the VARTM process had to be set up. After cleaning the working surface and applying a non-stick chemical, a border of tacky tape was put down. The part was set within this area followed by a layer of peel-ply and a layer of distribution media. The distribution media is a multi-directional, ridged sheet plastic that allows for good infusion and faster flow across the surface of the part. Thee layer of peel-ply is what keeps the distribution media from adhering to the composite. Then the vacuum and infusion lines are put in and a plastic sheet is sealed to the tacky tape. The infusion process was set up as can be seen below and the part was infused from left to right.

Procedure:

A vacuum is applied within the tacky-taped area of the workbench with the assistance of an air pumping system connected to the orange vacuum bucket. This vacuum line is set up on the right side of the part across from the injection line. The injection line is fed by the resin mixture in the white container which is suspended inside of the blue bucket. To initiate the reaction of the resin, an initiator (Trigonox) is added to the resin:styrene mixture. To speed up the free-radical reaction, a catalyst (cobalt naphthenate) is also added. The water in the blue bucket acts as a coolant for the heat-evolving resin as the reaction takes place.

Results and Discussion:

The injection took place on Friday, June 19. After the part was allowed to cure for three days, we removed the vacuum and recovered our fabricated part. Here are some pictures of how it turned out.

From these pictures you can see a small area at the far end of the part that did not get saturated very well with the resin, but what isn’t seen is the bottom (or back side) of the part. The entire right half of the bottom of the composite is fairly dry as it did not get very well infused. A possible explanation for this is that the bottom sat flush with the workbench and the flow resistance of the surface prevented the entire part from wetting-out. Next time, perhaps using a layer of distribution media on the bottom as well would help to improve the fabrication of the part.

Twine Testing

kilomac — Tue, 16 Jun 2009 21:28:07 +0000

The first couple of days thus far I have found myself testing the structural properties of different types of twine. The two under analysis thus far are Sisal and Jute:

Sisal: Jute:

To test the tensile strength of these twines, a bucket was suspended from a beam by a length of twine. The diameter of the twine was taken at three locations (top, middle, and bottom) using digital calipers. The bucket was then filled with sand until the breaking point of the twine was reached. The combined weight of the bucket and sand is the strength at failure. By dividing the strength at failure by the approximately circular area of the twine, the material stiffness was determined.

Below is a link to a YouTube demonstration of this stiffness test. My research partner, Jonathan is adding sand to a bucket until brittle failure of the test specimen is observed:

http://www.youtube.com/watch?v=cj8cGFziQog

First Post on my First Blog

kilomac — Tue, 16 Jun 2009 18:45:55 +0000

Welcome to my blog.

Here is a starter pic from my suite:

Red sky at night...