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Strength Test Facility for Open Source Appropriate Technology (OSAT) 3D Printing

by julianh72, published

Strength Test Facility for Open Source Appropriate Technology (OSAT) 3D Printing by julianh72 Nov 29, 2011
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Summary

A Test Bed and sample Test Pieces for strength / stiffness testing of 3D Printed parts

(See attached Word document for a detailed description)

I am recording my development process and experiments in more detail on my blog: http://julianh72.blogspot.com/ I will continue to post relevant files here on Thingiverse, but I will have more space for discussion and debate on my blog. The first entry is here: http://julianh72.blogspot.com/2011/11/testing-strength-of-reprap-printed.html but there will be more discussion on later posts.

EDIT 02 December 2011 - Modified parts (Mk II) uploaded, to make the test assembly more stable.

The strength and stiffness of components manufactured using RepRap and other 3D Printing technologies are affected by a number of independent and inter-dependent parameters, including:
a) the generic stock material used (e.g. PLA, ABS, PE, etc)
b) the actual source and condition of the stock material; e.g. it has been reported that PLA filament will absorb atmospheric humidity over time, and this affects its condition when placed using RepRap-type fabrication; it has also been reported that different coloured batches of otherwise nominally identical filament from the same source can exhibit significantly different fabrication properties, and therefore presumably different mechanical properties
c) the tool-path and build parameters which are used to fabricate the component (e.g. filament diameter, layer thickness, number of layers in the surface “shell”, % solid fill vs. % voids used in the core fill, etc)
d) the orientation of the part during printing (filaments parallel or perpendicular to the main load paths, etc)
e) the attributes of the actual printer (print head speed, precision / repeatability of placing the print head, backlash, etc)
When manufacturing components which are intended to have mechanical strength and function, it is necessary to be able to measure the actual strength of materials which can be printed with a particular machine, using a particular set of printing parameters, so that the strength and service life of the component can be predicted, and to assist in the design of new components which will be subjected to loads whose magnitude can be predicted or estimated.
This project came about partly because I am an engineer, and I “need to know” these sorts of things, but was also spurred on by reading an paper on “3-D Printing of Open Source Appropriate Technologies for Self-Directed Sustainable Development” by Pearce, Blair, Laciak, Andrews, Nosrat & Zelenika-Zovko.
[Ref: www.ccsenet.org/jsd “Journal of Sustainable Development” Vol. 3, No. 4; December 2010 http://www.ccsenet.org/journal/index.php/jsd/article/view/6984/6385 ]

Instructions

 Print one (or more) Test Beds.
 Print sufficient identical Test Pieces for the testing program. Keep records of the build and print parameters used (material, used, machine settings, % solid core fill, etc). Weigh the test pieces as a record of overall effective density. Where scales of only limited precision are available, weigh five or more identical pieces in a single weighing to get an effective mean sample weight.
 It is suggested that a minimum of three identical test pieces should typically be printed whenever undertaking a formal testing and reporting program, although a single test piece can still yield useful insights into material behaviour when trying to determine a “semi-quantitative” assessment of comparative properties yielded by different printer settings etc. If the set of Test Pieces are printed as a nested suite in a single print run (rather than as a series of sequential print runs), this will ensure all pieces are manufactured under identical conditions, minimising variations due to machine preparation, operator skill, environmental conditions (temperature , humidity, dust), etc. It should be possible to arrange a suite of 5 or even 10 identical test pieces to be produced in a single print run in this way.
 Consider printing sets of test pieces in different orientations when it is desired to understand the 3-dimensional orthotropic characteristics of the material.
 Install steel pins on test bed, and place test piece on pins
 Measure the clearance between the underside of the Test Piece and the Deflection Measuring Pad on the Test Bed
 Apply an initial load increment (well below the anticipated failure load) and measure the mid-span deflection
 Apply additional load increments and measure the deflection at each increment. It is recommended that data points should be entered into a graphing Spreadsheet or similar as it is recorded, to allow trends to be identified immediately (e.g. possible deflection reading errors, non-linear behaviour may indicate onset of failure, etc)
 Repeat for each test piece in a suite, so that statistically meaningful results can be obtained.
 Repeat for different manufacturing orientations, so that the influence of orthotropic material properties can be assessed.

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I agree with mike, as the dog-bone tensile test is more of a standard. You wouldn't need to print that large of piece as most houses can handle down to about 6". You may also want to include some "fracture" testing, or is that what you refer to with respect to delamination? Some houses will test for strength via part compression if the part is made of a pliable material.

Also....is there a placeholder for a certificate against the build process? Something that states that the material meets a printed spec?

Have you looked into any of the ASTM standards? A bend test may be the easiest to setup but a dog-bone tensile test would be useful too. I'm sure others has tested RP parts so no need to re-invent the wheel. It would be very helpful to have some idea of what properties make the biggest difference.

Thanks for your input, Mike!

Yes, I looked into standard testing methods - ASTM, ISO, etc - but these procedures suffer from two crucial flaws.

Firstly, the Standards are not
“Open Source", and in my view, an Open Source manufacturing system MUST have Open Source testing methods! E.g. "ASTM D638 - 10 Standard Test Method for Tensile Properties of Plastics" costs $46.00 as an electronic download, and I am not going to spend that sort of money just to find out whether it
is even practical to apply with my own limited resources.
Secondly, and more critically, they all need laboratory-standard testing machines and instruments. For the OSAT concept to apply, it is absolutely essential that the testing gear can be manufactured and used with nothing more sophisticated t
han the RepRap itself, and the basic tools needed to build and maintain the RepRap. Proper laboratory-standard testing should also be employed (perhaps to set the “official” standards and to calibrate the OSAT testing procedures), but RepRappers need the ability to do some practical tests on the
ir own desktops.
Hence the concept that I have floated here - employing the basic methodology and philosophy of standard testing, but scaled down to a "home brew" situation.
This link http://www.appropedia.org/RepRap_Mechanical_Testing_Literature_Reviewhttp://www.appropedia.org/RepR... points to a few relevant papers, but some are
quite old (like “A Study of the State-of-the-Art Rapid Prototyping Technologies” by Chou, Chow
&
amp; Wong, from 1998 - well and truly predating the RepRap phenomenon!), and others are "pay per view".

I really like this, and agree it is a necessary tool. Have you gathered any data already using this approach?

As an engineering PhD student, I have a certain amount of experience with modelling and optimisation and I have a technique in mind that would be ideal for this sort of assessment.

Thanks for your comments!

This is a "work in progress", and I fully expect the designs of the Test Pieces and Test Bed to evolve, both in response to my own testing, and the comments of others in the Thingiverse community.

For example, I have just printed some test pieces as per the uploaded STL files, and I found they are quite unstable because they are so narrow. I will modify the designs to try to make them sit better on the test bed, but keeping the pieces quite slender overall so they are not too stiff and strong for practical desktop testing.

I will upload my first test results as soon as I can, and the spreadsheet I will use to present my results may be used to spark a separate discussion on how to collect and share test results.

That sounds great, I'm keen to help out with generating some data.

How are you measuring everything? It'd be nice to build something to automate this process.

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