Hardware

An important part of this project was for us to be able to characterize the material properties of the fungus. We had specialized equipment at our disposal, but not everyone has, so we decided to make an alternative for future teams to be able to follow in our footsteps without the headache of needing very expensive machinery. We built a hydraulic press for us to perform compressive and tensile strength tests. These kinds of tests are needed in order to determine the stiffness of the material, which is an important factor for evaluating the viability of a building material.
This page will give a brief introduction to mechanical testing and the physics behind it, building instructions of the hydraulic press, characterization and discussion of the press’s abilities.

Click herefor a fun video of the fabricating process, or click the image for a detailed description.

Material characterization

When wanting to determine material characteristics, one way to do it, is to perform destructive tests. These tests enable us to accurately predict how a material will act when under load. An important parameter is Young's Modulus. It predicts how much a material sample extends under tension or shortens under compression. The Young's modulus directly applies to cases of uniaxial stress, that is tensile or compressive stress in one direction and no stress in the other directions. It is a measure of how stiff/rigid a material is in its linear region. When a material is experiencing stresses, in its linear region, it will return to its original shape when the stress is removed (elastic deformation). Outside of the linear region, in the non-linear region, the material will undergo permanent deformation until catastrophic failure.

Young’s modulus is defined as: \begin{equation} E = \frac{\sigma(\varepsilon)}{\varepsilon} = \frac{F / A}{\Delta L / L_{0}} = \frac{FL_0}{A \Delta L} \end{equation} E is the Young's modulus (modulus of elasticity) [Pa or N/$\textrm{m}^{2}$]
F is the force exerted on an object under tension
A is the actual cross-sectional area, which equals the area of the cross-section perpendicular to the applied force;
$\Delta L$ is the amount by which the length of the object changes ( $\Delta L$ is positive if the material is stretched , and negative when the material is compressed)
$L_{0}$ is the original length of the object.

During our testing we measured compressive strength. To get a more complete picture of the material properties of our fungi, we’d need to also perform tensile strength tests.

Building Instructions

Bill of materials

Pressure gauge
[a,b]: Car jack, 2 tons
Steel square tube, length (30 cm), width (10 cm)
Threaded steel rod, length (30 cm), Ø (10mm)
Hex nuts (8)
Washers (8)

Here is a fun video demonstrating the fabrication process.

Fabrication steps

Caution: When working with metal, be very careful of your environment and work piece. You cannot see/feel if metal is scorching warm before it’s too late, and you get 2nd degree burn on your fingers. The machinery is spinning very fast, and has A LOT of torque. It wont stop or even slow down if your hair or sleeves get caught. Wear protective eyewear. After every cut in metal, make sure to deburr ALL edges. The metal is razor sharp after a cut/drill. Frame: [Square tube, threaded rod, 8 hex-nuts, 8 washers]

Cut a 30 cm long piece of square tubing.
Cut your piece in half, lengthwise. You now have the top and bottom of the frame.

If you are not able to use a saw for this step, a grinder with a cut-wheel will work. However; if you do this, you’ll need to mill the edges in order to make them square.

Measure out where you’ll have to drill holes. The edge of the drilled hole should be 2.5 cm from any edge. Use one tap with a center-punch.
Drill the holes. The holes should be drilled in steps in order to not destroy the drill. Start with 4mm → 6mm → 8 → 10mm.
Cut two, 30 cm long pieces from a threaded rod. Ø10mm
Assemble according to the schematic.

Car jack: [Car jack, pressure gauge]

This step will depend heavily on which car jack you acquire. We bought a car jack without a pressure gauge. If you buy a jack with a pressure gauge, disregard this part of the instructions. I’ll explain how to hack the car jack in general terms, and how to determine which pressure gauge to buy.

Completely disassemble the car jack, everything, even the ball valves.

There will be some hydraulic oil; save it. Note how you disassemble it, you’ll have to assemble it again later.

Measure the diameter at the widest part of the innermost main piston (inside the red square on the figure).

This is the point at which the piston is experiencing hydraulic force. You need this area to determine which pressure gauge to buy.

Measure where to drill (Ø1mm) an L-shaped channel in the base of the jack. This means that you’ll have to make 2 plunges that meet inside the base of the car jack.

This step might be a bit tricky. Just be sure to make straight plunges with the drill-press and not to drill all the way through. Make well measured and well considered plunges. Look at the illustration for visual aids.

Enlarge the outer hole to be able to accommodate the threads of your pressure gauge.
Tap the enlarged hole with the appropriate thread pitch.

This mostly follows a standard e.g. American Standard Thread.

Clean All of the car jack parts in engine cleaner/ethanol.
Screw in the pressure gauge after having taped the threads with teflon/plumbers tape. Optionally use an O-ring.
Assemble the hydraulic press, fill it with the hydraulic oil.

Deciding on an appropriate pressure gauge

In order to determine what pressure gauge to buy/order, you’ll need to do some calculations and measurements. Don’t worry, it will be easy. I’ll use our car jack as an example. we used these youtube videos as references: video 1: ,video 2.

You’ll need to know the area of the innermost piston in order to calculate the maximum pressure the gauge will need to be able to withstand. The formula is as follows: \begin{equation} \frac{\textrm{Maximum lifting capacity of the jack}}{\textrm{Areal of the innermost piston}} \end{equation} The car jack our team bought had a maximum press capacity of 2000 kg (4409.25 lbs). The radius of the innermost piston was 0.47 in. This makes the area 0.70 in ($\textrm{area}=\textrm{radius}\cdot\pi^2$). \begin{equation} \frac{\textrm{Maximum lifting capacity of the jack}}{\textrm{Areal of the innermost piston}}=\frac{4409.25 \textrm{lbs}}{0.70 \textrm{in}^2}=6288.1 \textrm{psi} \end{equation} Now you know that your pressure gauge will need to be able to go to 6300 $\psi$. Buy a pressure gauge with its maximum pressure rating as close to 6300 as possible, while making sure that the dial has a high resolution.
We opted to buy a pressure gauge that could go to 3626 $\psi$. This means that our maximum pressing capacity is lower, but the minimum resolution is higher since the dial has more digits.
To determine the resolution of the hydraulic press we’ll need to multiply the minimum dial feature/step with the area factor: 1 bar (14.50 $\psi$). Multipli this with the area of the piston 0.7. \begin{equation} 14.50 \psi \cdot 0.7 = 10.150 lbs \end{equation} This makes our press’s minimum resolution 4.60 kg/bar and the maximum load 1150 kg (2535 lbs) after having converted the units to metric.

Characterization of the hydraulic press

In order to verify the hydraulic presses real accuracy, we tried to characterize it using a normal bathroom scale. This is not optimal, since we are only able to verify the very bottom of the range of the press (0-100 kg). The press goes to 1150kg. In theory, the press should be able to press with 4.60 kg/bar. This was verified. At higher pressures, the hydraulic press seemed to lose some of its power. This most likely stems from air bubbles on the inside of the pressure chamber. We were offered to have it certified with a calibration load-cell through its whole range, but weren’t able to make it in time before the deadline.

Limitations

The machine we used to characterize our fungi with was the Instron 6022. Its maximum load capacity is 10,000 newton, or 1 ton. Its smallest step size is approximately 0.5 $\%$ of its maximum load capacity, which is 50 newton. This load, on our test samples, results in a pressure of 0.8 bar, because the area of the sample is 6.25 $cm^{2}$ \begin{equation} 10,000N \cdot\ 0.5 \% =50N\\ \textrm{area of test sample:} 2.5^{2}=6.25 cm^{2}=0.000625m^{2}\\ 50N/0.000625m^{2}=80,000Pa=0.8bar \end{equation} Using the website convertunits.com, it states that 1 bar exerts 0.816 $kg/cm^{2}$. Our samples size is 6.25 $cm^{2}$, this gives us a resolution of: \begin{equation} 6.25 cm^2 \cdot 0.816 \frac{kg}{cm^2}=5.10kg \end{equation} 5.10 kg is the smallest resolvable load the machine can perform. Under this value, there is simply too much noise in the signal. Our homemade hydraulic press had a resolution of 4.60 kg. Our \$50 hydraulic press has a finer resolution than a \$93.000 machine.

This all sounds very nice, but there are some major caveats though. The homemade hydraulic can only be used to test the yield strength of any given material, that will likely break before 1150 kg. This is done by closely observing the pressure gauge and looking for when the pressure drops on the gauge. You’ll usually also be able to hear when the yield strength has been surpassed. This is not the case for foam-materials though, because these do not have a defined “drop-off” in their compressional strain/stress curve. This is a limiting factor, since we cannot record the materials continuous deformation/strain. We are essentially only able to “maybe” determine a yield point, using the hydraulic press. The Instron 6022 is able to electrically and continuously record the materials response to stresses, which makes it the more suitable machine in the end. What makes these machines expensive is their certification and calibration, making them very accurate throughout the entire range of the load-cell.

Team:DTU-Denmark/Hardware