Literature Review: Open Source Mechanical Tester

This page is a work in progress. This review covers open source mechanical tester projects that are formally published, and projects posted online without official peer review.

Key words terms (KWT)

1. "Open Source" OR "Open-Source"
2. "Mechanical Test"
3. "Tensile Test"
4. "Material Test"
5. "Stress-Strain"

Strategies

1. Searches were performed through Google Scholar, Scopus, and Web of Science using KWT1 and various permutations of KWT2-KWT5.
2. To find open source designs that were not formally published, a similar search was carried out in standard Google Search and YouTube.

The term mechanical testers refers to a wide range of devices used in the material sciences to determine the physical properties or characterize the behaviour of material samples. This review focuses on open source devices capable of tensile and compression testing. In these tests materials are stretched or compressed until failure while the forces and elongation are measured. Devices capable of these tests are often denoted as universal mechanical testers. All literature included contains enough open source documentation and software for the devices to be replicated with beginner to intermediate experience in electrical and mechanical fabrication.

Mechanical testers are almost always active and destructive; rather than passively recording, the device applies some force or stimulation and measures the response. Furthermore, the sample being tested is permanently altered in the process. A mechanical tester require both actuators and sensors. On a universal tester, the actuators must stretch or compress the test sample. On high-end commercial devices this is often achieved with hydraulics; however, cheaper devices mostly make use of electric motors. In addition, the sensors must measure the stress and strain in the material. Despite the simplicity of the core functionality, significant challenges arise in the design. For the results to be accurate, these devices require exceptionally high precision with a high sampling frequency, all in the presence of extreme forces.

Mechanical testing is vital in both manufacturing and research. These devices are most associated with testing the properties of pure material samples, but they are also applicable for testing operating limits of devices, such as crutches, wheelchairs, or anything with a frame supporting heavy loads. Mechanical testers provide meaningful insight for researchers in material science and engineering. These tests are the most accurate and often exclusive way to meaningfully quantify material properties such as Young's modulus, stiffness, hardness, and ultimate tensile strength.

Destructive mechanical tests have been the standard method of determining material properties since the 1800s. Throughout the entire history, the underlying principles have remained the same; however, significant advances have been made in the precision of the machines and the quality of the sensors. Currently, material testing is standardized by various organizations such as ASTM, ISO, and Euronorm. These bodies ensure that material tests are repeatable and scientifically rigorous; however, conforming to these standards often requires the use of prohibitively expensive commercial machines. The recent rise in popularity and accessibility of 3D printing has provided hobbyists both improved means and the desire to create DIY testers.

The advancements of open source mechanical testing are relevant to anyone working in manufacturing or development who does not have access to prohibitively expensive industrial equipment. This includes researchers, startups, small companies, and even hobbyists for applications from testing 3D printing filaments to finding the operational rating of novel devices.

Open source testing devices are highly applicable in the scientific community, increasing financial accessibility in the material sciences. They are also applicable in industry, potentially cutting costs and increasing efficiency in manufacturing.

• Review open-source computer vision methods of strain measurements.
• Other forms of mechanical testing, such as impact and hardness testing.

Abstract: We present a low cost, desktop size, open source, universal testing machine, designed for inexpensive high-throughput material testing. The tester can apply tensile and compressive loads up to 5 kN at rates ranging from 2 mm/min to 30 mm/min. Force measurements are achieved with ±1.8 N accuracy. The parts list for this machine represents an order of magnitude reduction in the cost per testing unit as compared to commercial systems. We describe the design and construction of the tester and validate its performance. The design, parts list, control software, and user manual are made available freely online under the open source BSD license.

• Approximately a 0.5m test range
• $4000 USD
• Position measurements had a resolution of 0.0005 mm and error of ±1 mm/min
• Force readings had an accuracy of ±1.8N
• Was validated in a comparative test against a commercial MTS 858 Mini Bionix

Abstract: The mechanical properties of soft materials are critically important for a wide range of applications ranging from packaging to biomedical purposes. We have constructed a simple mechanical testing apparatus using off-the-shelf materials and open-source software for a total cost of less than $100. The device consists of a wooden frame supporting a central loading apparatus attached via drawer slides. To perform a mechanical test, a sample is secured within two custom-made 3D-printed clamps affixed to brackets on the base of the frame and the load cell. The extension force is applied by the user pulling on a rope, moving the central loading apparatus up (thereby stretching the sample) while recording the force (measured by a load cell) and the displacement (measured by an ultrasonic sensor). The load cell and ultrasonic sensor are linked to an Arduino microcontroller connected to a laptop through a USB port for data acquisition and analysis. This instrument was easy to assemble and enabled students to better grasp the meaning of tensile testing while promoting experimentation with electronics, computer programming, and mechanical design. Because of its low cost and ease of use, this Arduino-based uniaxial tensile tester can be an ideal device to introduce the concepts of mechanical properties, among other concepts, to students in numerous fields.

• Forces up to 5kg
• Designed for soft material testing

Abstract: The thesis paper will be covering the grounds of what, how and why a Universal Testing Machine is used, an alternative to the generic Universal Testing Machine designs and how it is built. The primary objective of this thesis work is to design and build a Universal Testing Machine which can handle a load up to 10kN for polymer materials which is cost effective and modular. The operating system and electronic components are open-source leaving room for further development and compression test. The mechanical properties of material can be measured through tensile test. It gives the the characteristic of tensile strength, yield strength, modulus of elasticity, ductility, resilience, and toughness. The thesis paper covers the use of real time image processing for calculating and plotting of a stress strain curve. It also covers the implementation of open source code, using a MATLAB user interface to control, analyse and compile the results of tests done using the new machine. These results have been compared to values obtained from a standard Universal Testing machine and thus validated. As the machine is modular, the parts can be swapped with better components that ts the requirement, leaving the possibility for easy upgrades in the future.

• 1.3m Frame
• Strain measurement with computer vision, resolution of 1% of total length
• Stress readings achieved 2% difference to industrial tester
• Validation with PLA samples tested based on ASTM standards compared to a Zwick / Roell UTM

Abstract (Translated to English by Google): This paper presents the design and construction of a portable, educational, low-cost, and easy-to-build Universal Mechanical Testing Machine, following the open-source philosophy, with the aim of supporting the teaching of Mechanics and Strength of Materials. The equipment was designed to be built with modular aluminum profiles and implemented in a technical training workshop in mechanics. The machine is capable of safely applying a maximum load of 10 kN in both tension and compression, moving at speeds ranging from 0.5 to 70 mm/min with a displacement resolution of 0.57 μm at a total cost of US$ 1,000.00 including material and manufacturing. The machine is controlled by an Arduino® UNO board, operates with electromechanical drive, the load application is done by means of trapezoidal spindles, the acquisition of the displacement and speed of the beam is done by quadrature encoders and the speed control is done by PID control (Proportional, Integral and Derivative) and is capable of performing tensile tests, but can be adapted for other tests such as compression, flexion and folding, among others.

• Calibrated the load cell by placing it in a different mechanical testing device
• Validated by testings material samples and comparing the results to published ASTM standards

Abstract: Laboratory work teaches students how technical knowledge is applied in practice and has long been recognized as a crucial component of a complete undergraduate engineering experience. The deployment of new laboratory activities is challenging for several reasons, particularly due to resource constraints. Providing students with meaningful hands-on experience became even more challenging in the remote learning environment during the COVID-19 global pandemic. In the work presented, two low-cost, open-source mechanical testing machines were designed, and implemented in engineering education and research. The Universal Mechanical Testing Kit was launched as lab activities in a first-year introduction to materials science class. The Miniature Mechanical Testing Kit was designed as a remote learning tool and used as a lab kit in a third-year engineering design class during the pandemic to provide students with meaningful hands-on experience while learning from home. Other design variations were also implemented in engineering education and research.

• Focused on the educational application rather than design, optimization, or validation
• Created two different mechanical testers:

The Universal Mechanical Testing Kit:

• 300mm x 300mm x 680mm
• 1kN
• Speed 0.1-240mm/min
• Measured strain with linear scale with ± 0.1mm error
• 3% stress error
• $730 CAD

The Miniature Mechanical Testing Kit:

• 400mm x 140mm x 120mm
• 500N
• Speed: 1-750 mm/min
• Measured strain with linear slide with ± 0.1mm error
• 3% stress error
• $176 CAD

Abstract: The mechanical properties of agriculture materials are critically important for a wide range of applications such as packaging, transporting, sorting, ...etc. A simple mechanical testing apparatus using off-the-shelf materials and open-source software for a total cost of less than 1000 Egyptian pounds was fabricated. The device consists of a steel frame supporting a stepper motor and its cooling fans, apart for converting motor rotary motion to linear motion, drill chuck fixed to screw that move linear motion, stage to place the tested sample and load cell to determine the force. To perform a mechanical test, a sample is placed on the base of the frame and the load cell. The required force is applied by the stepper motor; many types of grips can be fixed in the drill chuck. moving the main shaft down leads to pressing the sample. The displacement (deformation) precisely controlled by controlling step of stepper motor in the code of the program, while recording the force (measured by a load cell) in a SD card memory and also in a computer serial monitor. The load cell and stepper motor are linked to an Arduino microcontroller connected to a laptop through a USB port for data acquisition and analysis. NEMA 23 stepper motor can holding torque up to 19 kg-cm witch fit testing many of agriculture materials This device was easy to assemble. Because of its low cost and ease of use, this Arduino-based universal material tester can be an acceptable device to introduce the concepts of mechanical properties, among other concepts, to researchers in numerous fields.

• The device was meant to be widely applicable, but focus on applications to agriculture
• Maximum sample size of 30cm x 15cm
• Max stress of 100N
• Speed: 0.31 m/s
• Performed an experiment on peanuts
  • Referenced comparable results from other studies

Abstract: Most commercially-available mechanical testing devices are bulky, expensive, and unable to evaluate changes in sample microstructure under load. This leaves a crucial gap in understanding between material structure and bulk mechanical properties. Our objective was to fabricate a mechanical testing device small enough to fit in most upright or inverted microscopy stages and able to position samples to allow for simultaneous mechanical and microstructural characterization. Parts were 3D printed using the hobbyist-friendly Fused Filament Fabrication technique, then assembled with commercial fasteners and translation components to create a mechanical testing device that utilized the deflection of plastic posts to determine sample reaction forces under applied strain. Video of sample deformation was analyzed using a custom processing script to calculate stress and strain behavior in an automated and high-throughput manner. This device was able to perform mechanical characterization with an accuracy comparable to commercial mechanical testing devices for a wide range of nonlinear and viscoelastic samples under dry and hydrated conditions. Additionally, the device showed compatibility with different upright and inverted microscopes and was able to demonstrate accurate mechanical testing results when used with these instruments. We successfully developed a device capable of accurately testing a majority of soft materials in the field of Biomedical Engineering with the ability to perform additional microstructural characterization using microscopy at a price point of $600.

• Designed specifically for microscopy, cannot perform general material tests
• Speed of 0.1-0.6mm/s
• Measured strain with computer vision
  • Resolution: 50-200 μm
  • Error: 0.1-31% deviation depending on the loading conditions and distance
• Measured stress through the deflection of 3D printed plastic posts
• Was validated through a comparative test to commercial testers

Abstract: The mechanical aspects of canvas painting conservation and the study of the effects of conservation treatments benefit greatly from quantifying the mechanical characteristics of the materials. However, this is seldom possible as only few labs have the necessary equipment. This paper presents the development of a biaxial tester to be used for samples of canvas paintings which exhibit orthotropic behavior under biaxial loads. The machine was built as the first step of ongoing Ph.D. research on the mechanics of canvas paintings. An effort was made to create a system that is easy to assemble, with parts that are easy to source and with an overall cost well below the commercial units available. The control software includes the function of automated pre-tensioning to improve the accuracy of the measurement. Our broader purpose here is to make an easy-to-replicate machine available to help conservators and conservation scientists perform tensile tests to make informed choices in materials science.

• Designed for biaxail canvas testing
• Test ranges up to 350mm
• 1000N rating
• 100 mm/min
• Measured strain by counting motor pulses
  • Resolution: 0.00156mm
  • Average error: 0.004mm
• Used four motors and four load cells for stress measurements with 0.782% error
• € 2,500
• Validated the accuracy and repeatability of the motion using a 0.001mm resolution digital dial indicator
• Load cells validated with precision weights

Abstract: In this thesis, a universal testing machine called MT-02 was designed, built, and tested. The goal was to create a machine that can be used for academic training and demonstration purposes. The specifications of the design include: affordability, portability, ease of extending the soft- and hardware, open-source spirit, and the capability to perform tensile tests as well as compression tests with loads up to 3.5 kN. Standardized aluminum profiles are used to build the frame of the machine. Low loaded parts were 3D printed. The control is based on an Arduino board. The machine is operated via a touch display and the measurement data is transmitted via a USB interface to a PC.A tensile test series with plastic samples was performed on the MT-02 as well as on a commercial universal testing machine. After successfully testing all samples, the results were compared and interpreted regarding the accuracy and precision of the built machine. They indicated a good agreement concerning the strength. The strain of the sample, which is estimated by the displacement of the crosshead, deviates by a factor of two from the results of the commercial universal testing machine with a video extensometer. Thus, the Young’s modulus is underestimated by 48%. Optical strain measurement could resolve this issue. In summary, this thesis shows that a universal testing machine can be built at well-equipped workshops with a relatively low budget and effort. Measurement outcomes are comparable to commercial test systems.

• 0.5m in size
• Speed of 0.5-6mm/min
• Used computer vision for strain measurement
• An experiment was conducted where five material samples were tested on the novel device and on a commercial ZwickRoell
  • There was a difference of 28-48%

Abstract: Hyperelastic materials are extensively incorporated in medical implants and microelectromechanical systems due to their large, elastic, recoverable strains. However, their mechanical properties are sensitive to processing parameters that may lead to inconsistent characterization. Various test setups have been employed for characterizing hyperelastic materials; however, they are often costly. Recent advancements in additive manufacturing and opensource software/hardware suggest the possibility of simpler solutions in research settings for characterizing them; raising the question of whether one can characterize these materials with low-cost tools and tests that take advantage of soft and small form-factor samples. Here, the authors investigate the potential of an open-source, 3D-printed test system designed for characterizing such materials. This system is tailored for small form-factor samples (sub-mm thickness) and large elastic deformations, common in polymeric parts of minimally invasive implants. The authors developed parts using additive manufacturing for uniaxial and planar tension testing, with a low-cost image correlation method adapted for measuring large strains. Polydimethylsiloxane was chosen for demonstration of a two-parameter Mooney–Rivlin model, due to its documentation and use in biocompatible devices. The estimated Young’s and shear moduli were repeatable and consistent with the literature. Curve-fitting was challenging and dependent on the optimization choices, when data points were limited, consistent with prior reports. However, with a large number of data points and ideal optimization error choice, 𝐶1 and 𝐶2 were found to be close to those reported previously. This work demonstrates a low-cost, 3D-printed, open-source test setup for characterizing hyperelastic materials using a two-parameter Mooney–Rivlin model with reasonable accuracy.

• Designed for both uniaxial and planar tests of hyperelastic materials with two-parameter Mooney–Rivlin model
• Rated for 1 kg uniaxial, 5 kg planar
• Measured strain with both a linear actuator and computer vision
  • ± 0.03mm overshoot
  • 1% error in measurements
  • camera validated by comparison to calipers
• ± 0.01kg error is stress measurements
• $1471.16 USD
• Used to determine Young's Modulus of Polydimethylsiloxane within 5% of other published results

Author's Overview: TestrBot is a $300 Universal Test Machine (UTM) and can be used to perform any type of static or dynamic testing in tension or compression up to 200 lbs. It was designed to allow me to run an array of physical tests on 3D printed specimens.

• 0.6m test range
• Max speed of 6.35 mm/min
• Measured strain by counting the pulses sent to the stepper motors for a resolution of 3.556μm
• Frame mostly constructed out of wood
• Not formally validated or tested

Author's Overview: This is my DIY Universal Test Machine that I use in many of my videos to test the strength of different materials and parts. I finally gathered all the data and made it available open source so that everyone can build on on their own. I'll go through the whole setup and also show how I managed to build an optical extensometer to measure the strain in the test samples.

• Test range approximately 1m
• Applies up to 2.5kN
• Run at either 1 or 25 mm/min
• Measures strain with a camera and motion tracking
• Cost of £100
• No information on testing or validation of the device

Author's Overview: A $300 low cost electronic universal testing machine for tensile testing applications with loads under 1000N made from 3D printed and commercial parts.

• Claims to use a method of strain measurement inspired by the CNCKitchen device above, but details are not provided
• Was used in a publication on optimizing 3D printing parameters^[14]

Authors' Overview: This instrument is a open-source mechanical tester for compression and tensile testing of specimens, with a design load up to 20kN. To the team's knowledge, this is at the height of non-hydraulic open source instruments. Mechanical assembly has been designed to require a minimum of specialist engineering knowledge and externally machined parts (~1), leaning heavily on outsourced laser cutting of steel components. The main in-house requirement is a good quality drill press in a workshop setting. A novel lever-based design approach is used to take full advantage of a common off-the-shelf electrical linear actuator, providing a versatile load and strain capability. Videos show many of the more complex aspects of component preparation. Electronics assembly has been designed around easily solderable through hole components and readily available electronics modules, with PCB designs provided. A Raspberry Pi and Arduino control the instrument operation and drive the user interface.

• The documentation seems to be abandoned in the late stages of completion
• £3000
• Used both a digital linear scale and a camera for strain measurement
• 1.6m frame
• Constructed mainly out of laser-cut steel
• The verification details are not complete. It seems that they compared the results to a commercial machine

Author's Overview: This is a simple and cheap tensile strength tester for measuring relative tensile strength from filament samples. It's used to test the layer adhesion strength across various filaments or settings. Does increasing the temperature on your favorite filament create stronger parts? Does high speed printing create weaker parts? This mini tester is designed to answer these questions.

• Extremely cheap and simple
• Involves no electrical design or fabrication
• Frame is 3D printed
• Uses a luggage scale to measure force. One of two devices in this review to not use a load cell