Affordable Laboratory Grade Tensile Tester

We couldn't afford an industrial tensile tester (~$15k) so we built our own and named it Tensy! This project was design by two senior undergraduates (Nicholas Leonard BME '26 and Justus Badu ME '26) with funding from the Case Western Reserve University Department of Biomedical Engineering.

All purchases are outlined in this updated BOM. All tools required can be found at CWRU Think[Box]. We machined many parts out of aluminum but there is no reason a hobbyist can't use 3D printing or wood.

This Instructables was designed for lab students to follow along with. Download "TensyCode.ino" attached below then follow my detailed instruction guide for setting up and using Tensy!

Tensy Setup and Operation Instructions

Software Setup

Download “TensyCode.ino” attached to Step 1 of the Instructable.

Navigate to https://support.arduino.cc/hc/en-us/articles/360019833020-Download-and-install-Arduino-IDE#installation-instructions and download and install the Arduino IDE if you don’t already have it.

Open the Arduino IDE and click File → Open, then select the “TensyCode.ino” file you downloaded earlier.

Hardware Connection

Connect Tensy to your computer via USB.

Plug Tensy in. You should see “Arduino Nano on …” in the bottom right corner of the Arduino IDE. If it says “[Not Connected]”, try reconnecting the power cord, reconnecting the USB cord, or closing and reopening the application.

Library Installation

Click the books icon on the left middle of the screen titled “Library Manager.”

Search “HX711 Arduino Library” and click Install.

Repeat this process with “AccelStepper.”

Click the books icon again to return to the full screen.

Uploading Code and Serial Setup

Click the forward-pointing arrow in the top left titled “Upload.”

Wait until it says “Done”, then click the “Serial Monitor” tab in the bottom left corner.

Ensure the baud rate in the top right corner of the window is set to 115200 using the dropdown menu.

You will likely see corrupted data or text at first; this is normal. Type “help” into the textbox and click Enter.

Operating Tensy

Follow the serial-displayed directions to set up and operate Tensy. At some point you will need to load your sample into the machine, but this can occur after jogging if necessary.

Data Collection and Analysis

Once you collect data, you will see a long line of numbers separated by commas and semicolons.

Copy this text and paste it into MATLAB with square brackets around it. This will create a table with columns representing time elapsed in milliseconds, force in newtons, and distance traveled in millimeters.

Analyze this data however you please in MATLAB.

For updates check here.

Instead of building our own linear actuator, we decided to buy a complete system that already included an integrated NEMA 17 stepper motor, ball screw, and carriage. This dramatically simplifies the design process, and from our research, actually works out cheaper than making it from scratch.

Similar to the industrial tensile testers, we wanted our test samples to be loaded in the vertical direction, but the linear actuator was made to be used horizontally. This meant that we had to build a sturdy base to hold the system upright, with space for clamps to hold the samples, and space for the electronics to sit.

The base frame was constructed out of 2020 aluminum extrusion for its versatility, rigidity, and machinability. We used a horizontal band saw to cut the extrusion down to size, and connected each piece using corner brackets, sliding t-nuts, and bolts. The assembly process of the base was somewhat finicky because of all the small fastening components, but the finished result is a very sturdy frame that will hold up over time.

We also added a ¾” plywood base to go on the underside of the extruded frame, simply using a two-part epoxy to bond the two together. Finally, we cut out four rubber circles and, again using some epoxy, secured them to the bottom of the wood. This was to prevent the device from rocking on slightly uneven surfaces and give it some more grip.

We wanted the side of the linear actuator with the motor attached to be placed at the top; this meant that any potentially delicate electronics inside would not be damaged if a liquid were accidentally spilled on Tensy. Since all the mounting holes were on the motor side, we had to find another way to attach the actuator. We machined an aluminum back plate featuring six holes; the bottom two to allow bolts and t-nuts to secure to the 2020 extrusion, and the top four to secure to the backside of the linear actuator by utilizing the built-in t-slot channels.

This by itself was very stable, but for our own sanity, we wanted to secure down the front too. This time, we machined two toe clamps made from small aluminum blocks with holes drilled into them. They attach to the aluminum extrusion with a bolt and t-nut, and tighten down with another nut on the top to hold the linear actuator system in place.

Next, we machined a stainless steel corner bracket using a manual milling machine for optimal precision. Four holes on the backside allowed the bracket to be secured down with bolts that screwed into the pre-installed t-nuts already in the carriage. We also machined one hole on the top of the bracket, where the load cell’s thread pokes through, and added a wing nut on top to easily tighten it down by hand.

To test the tensile strength of the desired sample we are testing, we would need a way to secure it on either side. We opted for some simple clamps that could be tightened by hand. The two clamps were machined down to the desired geometry using a manual mill, with a tapped hole on the underside of each to connect them to the 2020 aluminum base and the load cell. Behind each clamp is also a nut that can tighten down to lock the clamp into position. The clamps have another threaded hole horizontally, making room for a long bolt to pass through, and once tightened down, it pinches the sample against the clamp. We used a lathe to make small discs that threaded onto the end of the bolts, which increased the area of the clamping surface. You could also easily 3D print these clamps for other intended applications.

The electronics for the system were built to both measure force and control the motion of the linear actuator through a single Arduino Nano-based architecture, as shown in the wiring diagram. The load cell was wired into a load cell amplifier module, which amplifies the millivolt-level Wheatstone bridge signal and converts it to a digital output. The amplifier was powered from the Nano’s regulated 5V and GND pins, and its DATA and CLOCK pins were connected to designated digital I/O pins on the Nano to allow serial force readings through the HX711 library. The Nano was then connected to a stepper motor driver (integrated on the motor shield), which receives STEP and DIR signals from the Nano and supplies the required current to the NEMA 17 stepper motor. The motor driver’s VMOT terminal was powered by the external 10V supply to provide sufficient torque for the actuator, while all grounds (power supply, Nano, amplifier, and driver) were tied together to ensure a common reference. The Nano streams force data to a computer via USB for MATLAB processing while simultaneously generating step pulses to control actuator speed and direction.

The electronics enclosure was designed in CAD and 3D printed to securely house the Arduino Nano, stepper driver shield, load cell amplifier, and wiring connections in a compact and organized layout. The box was printed in PLA with a wall thickness of 1.5 mm to provide sufficient rigidity. Internal standoffs were integrated into the design to allow the Nano and shield to be mounted using small machine screws, preventing movement during operation. Wire pass-through openings were added for the motor leads, load cell cable, power input, and USB connection. The enclosure was designed as a two-piece assembly with a removable lid to allow easy access for debugging and future modifications while protecting the electronics from accidental contact and mechanical interference during testing.

For updated construction instructions check here.

Fig 1. Tensy demonstrates excellent linearity with an R2 of 1 across a large range of known forces applied by hanging calibration weights. Tensy also demonstrates excellent accuracy with a linear regression RMSE of 22.6mN across 16 measurements ranging from 0 to ~15N.

Fig 2. Tensy demonstrates an excellent resolution of approximately 10mN when calculated with a limit of quantification defined by a SNR>=10.

Fig 3. Tensy demonstrates excellent repeatability and accuracy when compared to expected results. After repeatedly straining and relaxing a single o-ring, the calculated young’s modulus was tightly grouped between trails and falls within the expected range for that material.

Fig 4, 5 & 6. Individual trail data. Best fit region calculated between strains of 0.05 and 0.11.

Click here for updated reports.