Explanation of the FFF Printing Failure Criteria Project
The primary focus of this project is to determine the tensile failure criteria for additive manufactured parts made with an FFF (fused filament fabrication) printer. As additive manufacturing techniques are usually anisotropic, traditional failure criteria do not apply to them. This lack of information is particularly detrimental to fields where safety is key, such as aerospace and automotive. In order to safely use parts made with additive manufacturing in these fields, criteria must be established. This project will do this by testing the tensile strength of FFF printed parts with the grain direction at degrees varying between 0 and 90 degree from the vertical direction, at 15 degree increments.
Preliminary Design and Manufacturing Considerations
We will be performing tensile tests with specimens created based on specifications listed in the ASTM D-638 and reference [1]. Using previously specified dimensions for the tensile test specimen will allow other similar research the use of data from this project. SABIC MG94 ABS filament will be used, however with the MakerSpace not permitting ABS and to help provide further data for UW-Madison research into this polymer, we will be 3D printing these coupons from the Lulzbot TAZ5, a MG94 designated FFF printer in the polymer lab. 3D printing software specific to the polymer lab will be used and provides the necessary control to print beads in the required changing orientations from 0 to 90 degrees. The test specimens will be printed with parameters listed in Table 1 which are based on parameters outlined in reference [1].
Table 1: Printing Parameters [1]
| Printing Parameter | Value |
| Nozzle Temperature | 220 C |
| Bed Temperature | 100 C |
| Printing Speed | 2000 mm/min |
| Layer Height | 0.2 mm |
| Path Width | 0.5 mm |
| Extrusion Factor | 1 |
Proposed first trials
In the experiment, we will be measuring tensile yield strengths of 3D printed dog-bone specimens with 6 different filament orientations: 0, 15, 30, 45, 60, 75, and 90 degrees from the load direction. We will test five specimens for each filament orientation, so there will be 35 (5 specimens x 7 orientations) samples in total. To achieve the end goal of understanding the failure criteria of FFF materials, this project will be divided into three parts: printing test pieces, performing tensile tests, and analyzing test data.
Since the variable of interest in this project is the filament orientation, we need to minimize the variations of all other parameters that may affect the mechanical properties of our test specimens. The dimensions of all the specimens will strictly follow the ASTM D-638 Type 1 coupon standard [1]. The specimens will be printed using the FFF printers in ME1001 using the SABIC MG94 ABS filament produced by Polymer Research Center. The printing parameters (e.g. in-fill density, printing speed, and layer thickness) will be controlled by following the conditions specified in Table 3.2 in “Defining a failure surface for Fused Filament Fabrication parts using a novel failure criterion”. Each dog-bone specimen will be inspected for any defect after printing, and the dimensions will be measured using a vernier caliper. If the measured dimensions are not within the tolerances specified by ASTM D-638 [1], the specimens will be discarded. In the end, we expect to have the total of thirty-five test pieces for the tensile tests (five specimens for each filament orientation).
The tensile tests will be performed on the MTS Sintech 10/GL Testing Machine [1] in Engineering Hall. The testing methodology used in Section 3.3.1 of “Defining a failure surface for Fused Filament Fabrication parts using a novel failure criterion” [1] will be followed. After all the data is collected, we will create a plot of maximum tensile stresses versus filament angles. Incorporating the shear test data from [1], we will apply failure criteria to our experimental results. One possible criterion would be the Tsai-Hill criteria [2] which is widely used for anisotropic materials.
Sources
[1] G. A. Mazzei Capote, “Defining a failure surface for Fused Filament Fabrication parts using a novel failure criterion,” Sep. 2018.
[2] C. H. Wang and C. N. Duong, “Bonded Joints and Repairs to Composite Airframe Structures,” p. 33, Oct. 2015.