Revised ASTM Tensile Test Specimen
by MKreiger, published
The change for this specimen is an added cylinder on one corner to create a starting/end point for the layers. This tensile specimen is for a current research project & is associated with benefits for anyone who wishes to collaborate by contributing sets of 10 of tensile specimen.
This a specimen for tensile testing in accordance with ASTM D638 "Standard Test Method for Tensile Properties of Plastics". It is a Type 1 specimen to be made from rigid plastic.
Tensile testing of RepRap printed specimens is being performed at the Michigan Tech Open Sustainability Technology Research Group mse.mtu.edu/~pearce/Index.html. Specific information about the testing can be found here appropedia.org/Mechanical_testing_of_polymer_components_made_with_the_RepRap_3-D_printer.
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A few thoughts of how the strength of a 3D printed part may have increased.
With some plastics (e.g. semicrystalline polypropylene) you can increase the strength of a fusion weld by retracting (i.e. pulling apart) the fused zone by a small amount.
Applying tension to the fusion zone will help align the “spherulites” (chain molecules). A living hinge takes advantage of the spherulites alignment to resist fatigue. Aligned spherulites increase the strength of the bond area. Think of it as though you were straightening out a pile of noodles into orderly columns.
Spherulites appear to be a type of kruptoendo (i.e. hidden within) composite:
Spherulites are composed of highly ordered lamellae, which result in higher density, hardness, but also brittleness of the spherulites as compared to disordered polymer. The lamellae are connected by amorphous regions which provide certain elasticity and impact resistance.
Crystallization of polymers
Assumption: the plastic has spherulites (i.e. semicrystalline).
I believe that the extrudate is being stretched during the 3D printing process, and this stretching of the fusion weld zone is aligning the spherulites which increase the strength. This might explain why a 3D printed part is stronger than what the manufacturers have suggested.
If this is not the case, I would like to know what is going on!!! :)
Wikipedia indicates that PLA has crystallinity.
Use of stereospecific catalysts can lead to heterotactic PLA which has been found to show crystallinity.
ABS is amorphous, so we shouldn’t see the strength increase via 3D printing.
Rigid Nylon strength should increase (if you could 3D print it?). If it's a stretchy Nylon it's not testable under the same conditions.
The filament manufacturers need to offer material certifications, so engineers can accurately specify a filament material, and incoming Q.C. inspection can test against it.
What plastic materials are you testing, and from what manufactures?
Thank you for testing the strength of 3D printed parts!!
Hi -- The aim was to pull out some concrete values for printed strength on RepRap like machines so that engineers could start to do some real engineering on them.
We still have not posted our results because we are doing a controlled and pretty massive study and want to ensure everything is done right. As of now - the values from our RepRaps are coming in stronger than those published in the literature for commercial machines - so we are double checking everything before we make any big claims.
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