1. What will it do?

The eTester is a device for measuring the elastic properties of materials. It uses the method and equations outlined in ASTM standard 1876, the 'Standard Test Method for Dynamic Young's Modulus, Shear Modulus, and Poisson's Ratio by Impulse Excitation of Vibration'. However, the device is simplified in terms of what it is designed to measure. I have programmed the interface to measure dynamic Young’s modulus only. It can be computed if the mass, dimensions, and either flexural or longitudinal resonant frequency of the material is known. Given the ease of measurement of mass and dimensions, the aim of the eTester is to measure the fundamental resonant frequency, and feed it into the equation for calculating elasticity. For simplicity, I focus on measuring the longitudinal resonant frequency only. The purpose of measuring the elastic modulus is to have the required data for modelling the performance of objects fabricated from the material. In particular, it is useful for generating data on new, non-standard materials. This can help Fab Labs who want to use local materials that are either made or sourced, but do not have good data. With greater data transparency, we can use new local materials with greater confidence.

2. Who's done what beforehand?

The measurement technique for determining dynamic elastic properties is standard, and therefore widely used. A good study demonstrating the method being used can be found here. However, I have not found a ‘device’ that is designed to be fabricated and used in a Fab Lab type setting. I aim to create a template for making this device so that it can be replicated without needing to start from the wider set of scientific guidelines.

I then found a Buzzmac professional kit that has been made and is available for sale. The user manual was extremely useful, and it is well documented with an extensive reference list.

3. What materials and components will be required? Where will they come from?

The following is the list of components specified in the test method:

An impulser (e.g. flexible polymer rod with metal ball to excite the sample)

From: purchase a rubber mallet for striking a xylophone like the image below.

Signal detector (e.g. contact transducer such as an accelerometer or non-contact transducer such as an acoustic microphone)

From: We then ordered a few sensors to experiment with from Mouser: MEMS Microphone.: $1.08, Digital Zero Height SiSonic, and this Output Digital Microphone.

Based on these calculations I determined I needed a sensor to cover the range of 0-100 Hz, with an elastomer being the lowest resonance frequency, and aluminium being the highest.

Electronic system (including a signal conditioner/amplifier, signal analyser, and frequency readout device)

From: In order to experiment with different amplifiers, we ordered the following ones:

Audio Amplifiers Sound-Plus Hi-Perf JFET-Inp Aud Op Amp: $2.74 Precision Amplifiers High Prec,Low Noise Op Amp: $9.51 Operational Amplifiers - Op Amps Dual Lo-Noise Hi-Spd JFET-Input: $2.92 Operational Amplifiers - Op Amps Lo Pwr Single-Sply Rail-to-Rail: $1.53

Support system that isolates the specimen from ambient vibrations while also being able to be adapted to control for sample modes of vibration (e.g. wire suspension)

I decided to use the standard recommendation of moveable foam strips. My design is to use a metal base plate, with moveable foam strips glued to magnets.

4. how much will it cost?

The following is an estimated parts list with costs:

Sensors: $16.70

Materials: scrap in Fab Lab, estimated at $10

Magnets: $5

Foam: $5

Electronics components: $25

Total: $61.70

5. what parts and systems will be made?

I need to make the following parts:

Samples of material for testing

Electronics board for sensing material frequencies

Application and interface for calculating elastic moduli from measured frequencies

6. what processes will be used?

Moulding and casting (for sample preparation)

Laser cutting (for sample preparation)

CNC (for preparing moulds)

Electronics design (designing board)

Embedded programming (programing board)

Application and interface design (for designing an application and user interface)

7. what tasks need to be completed?

7.1 what questions need to be answered?

What specific set of components do I need for my electronics board?

How do I program my application to perform a Fast Fourier Transform (FFT) to the measured frequency from the sensor?

How do I program my application to compute the elastic modulus from the resonant frequency, mass and dimensions of the sample?

Can I use serial communication from my board to the computer?

Which sensor best covers the range of material frequencies that need to be measured?

How should I best position the supporting foam strips so as to be located at the correct nodes? How can I make this easy for other users?

How can I make my user interface user-friendly, demonstrating in an intuitive way the relationship between resonant frequencies of a material, and its elastic modulus?

What material samples can I make from materials commonly found in a Fab Lab?

How may I create guidelines for preparing samples from more uncommon, local materials?

7.2 what is the schedule?

I plan for my eTester to be ready for demonstration by Friday, June 16th. The following tasks are scheduled:

May 31-June 2: Design application and interface

June 5th-7th: Design and fabricate my PCB plus components

June 8th: Test board and debug if necessary

June 9th: Test board communication with software and application interface

June 12th: Prepare material samples and apparatus setup

June 13th-14th: Measure samples with eTester and record results

June 15th: Demo day: create video and diagram, upload to website

7.3 how will it be evaluated?"

I plan to evaluate my eTester according to the following criteria:

Does the sensor give an output frequency of samples tested?

Does my application compute the elastic modulus for a given material sample, with the measured resonant frequency, and given mass and dimensions?

Does the calculated elastic modulus of my material samples (using standard materials like acrylic, PC, PVC, ABS), approximately match the mechanical datasheets for these materials? In other words, can I replicate industrial testing data?