The present disclosure relates to systems and methods to produce tensile test bars for identifying material mechanical property data using material extrusion additive manufacturing processes.
The statements in this section merely provide background information related to the present disclosure and may or may not constitute prior art.
Material extrusion additive manufacturing (ME-AM) produces parts with anisotropic mechanical properties. Therefore, these properties are tested in different directions to provide relevant and useful mechanical property data to part designers. Parts can be defined as having three principal directions: along the length of deposited beads (or roads) in the plane of a layer (the XY direction), across the width of deposited beads in the plane of a layer (the YX direction), and parallel to the length of the deposited beads and parallel to the direction in which layers are deposited (the ZX direction).
Test specimens made according to international standards for plastics in the American Society for Testing Materials (ASTM) D638, and International Organization for Standardization (ISO 527) recommend dogbone-shaped test specimens, with a thinner central region (the gage length) and a wider grip region. When known software is used to generate mechanical test specimens for XY direction testing, the software generates toolpaths with a constant width. There are several methods to produce a sample of this shape with ME-AM. These methods result in stress concentrators in the transition region between a gage length and a grip region. This commonly causes the test specimen to fail in the transition region.
ASTM D638 and ISO 527 require valid test specimens to fail in the gage length to consider the test successful and useable for reporting. XY tensile bars tend to fail at stress concentrators caused by imperfect filling of the bar. The gage length of the bar in the common case of ISO 527 Type 1 bars has a width of 10 mm, while the tab has a width of 20 mm. In the filleted transition region (delta region), there are gaps which cannot be filled using known slicing software. These gaps act as crack initiators and cause early failure in the tensile bars, lowering the reported tensile strength for the material. Because of this failure mode, many mechanical test specimens for the XY direction produced by ME-AM do not produce useable results according to the standards.
Thus, while the current systems and methods to generate mechanical test specimens for XY direction testing are useful for their intended purpose, there is room in the art for an improved system and method for producing material extrusion 3D printed mechanical testing specimens.
This disclosure describes a system and method for generating mechanical test specimens for XY direction testing. Test specimens are three-dimensional (3D) printed using extrusion based additive printing out of one or more thermoplastic materials. The 3D printed part or test specimen may include post-processing such as computer numerical control (CNC) machining to achieve Geometric Dimensioning and Tolerance (GD&T) standards according to the application.
The system and method of the present disclosure produces beads of extruded material having a width that is variable based on predetermined material feed speed and volumetric flow rate as well as material temperature.
The system and method of the present disclosure eliminates stress concentrators and discontinuities in a test bar by continuously varying (increasing) an extrusion width (path width) of individual beads as the beads extend from an initial width in a gage region of the test bar gradually expanding through a transition region and ending at a final width in a grip region of the test bar. Continuously varying the extrusion width eliminates gaps between the beads and precludes having incomplete beads of material in the test bar.
The system and method of the present disclosure may be initiated using computer code to create a test specimen or may be initiated using computer assisted design (CAD) software to create a test specimen.
Further areas of applicability will become apparent from the description provided herein. It should be understood that the description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure.
The drawings described herein are for illustration purposes only and are not intended to limit the scope of the present disclosure in any way.
The following description is merely exemplary in nature and is not intended to limit the present disclosure, application, or uses.
Referring now to
The toolpaths vary the width of the bead 18 from the thinner 0.4-mm extrusion width 16 in the gage region 20 to the thicker or wider 0.6-mm extrusion width 22 in the grip region 24 by continuously increasing the width in the transition region 26 between the gage region 20 and the grip region 24. In aspects, each transition region is in the range of 24 mm in length. This allows production of test specimens that reduce a severity of stress concentrators that are known to cause the specimens to fail in the transition region 26 such that test failure will occur in the gage region 20 as desired. The present system and method mimics the way injection molded specimens are prepared. The flow fields that form when molten polymer is injected into a dogbone-shaped cavity cause the polymer flow front to “neck down” as it enters the gage region 20, then flair out or swell in width as it fills toward the other end at the grip region 24. In the gage region 20 all of the beads 18 have a common initial width. Extending into the transition region 26 all of the beads have a generally increasing width of the beads 18, and in the grip region 24 all of the beads have a common final width wider than the initial width and wider than the widths in the transition region 26. While reference is made to an aspect utilizing a 0.4 mm diameter nozzle opening, a 0.4 thick bead width in a first, gage region and a 0.6 mm thick bead in a second, grip region, it should be appreciated that alternative dimensions may be utilized herein as well, provided that the overall dimensions determined by the testing standard are met. Thus, if a larger diameter nozzle is used, or thicker beads are used, fewer than 25 beads may be necessary. If a smaller diameter opening nozzle is used, or thinner beads are used, more than 25 beads may be necessary.
With continuing reference to
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During preparation of the toolpaths common print parameters are adjusted, including an extrusion or aspect ratio AR defined as the bead width divided by the bead height 38, a material extrusion temperature, and parameters of an acceleration compensation factor to account for acceleration of the nozzle 12 during printing. A print speed may be adjusted dynamically to keep a constant polymer volumetric flow rate. The constant volumetric flow rate allows the extruded material to have the same residence time in the nozzle 12 and have approximately the same deposition temperature. The constant extrusion speed allows the extrusion material to have the same residence time in the nozzle 12 and have approximately the same deposition temperature. Thus, the residence time in the nozzle is maintained the same at a first extrusion speed in the first, gage 20 region and at a second extrusion speed in the second, grip 24 region.
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A comparative analysis was performed of tensile testing bars produced using continuous varying bead width as described herein and illustrated in
According to other aspects, a method of continuously varying an extrusion width (path width) through a transition region between a gage length and a grip region of a test specimen 10 is provided. According to several aspects, the method utilizes computer code, such as a program written in Python, Java, or another programming language to algorithmically generate toolpaths. The program uses known dimensions of a standardized test specimen and calculates coordinates and material deposition amounts. The program produces a toolpath file, generally as G-code, for use on an ME-AM 3D printer.
According to further aspects an operator writes toolpath instructions (generally utilizing G-code) directly, utilizing known dimensions of the standardized test specimen and calculating the coordinates and material deposition amounts.
According to several aspects the functionality of printing standardized test specimens is incorporated into computer software. The designer of the software creates an algorithm allowing the toolpath width to be adjusted according to predetermined criteria; for example, the algorithm eliminates gaps between toolpaths by adjusting the toolpath width according to a predetermined physical range.
According to several aspects the functionality of allowing several variable extrusion width beads in each layer of a part is incorporated into computer software. The designer of the software creates an algorithm allowing the toolpath width to be adjusted according to predetermined criteria; for example, the algorithm allows all beads placed on a perimeter of an object in a predetermined layer to have an equal width, different from a default width used in other areas of a part. The equal width beads eliminate gaps between toolpaths by adjusting the toolpath width according to a predetermined physical range.
The method of the present disclosure moves discontinuities outside of the gage length and outside the shoulder region of the tensile bar. The failure stress is thereby increased. By continuously varying the extrusion width while traveling from the gage to the tab regions, the delta region is eliminated.
The method of the present disclosure produces XY mechanical test specimens using ME-AM.
The method provided herein artificially mimics the flow fields produced from injection molded specimens with the flared toolpath and modulated bead width. This results in an optimized strain field in the transition region which leads to higher quality test data and is more representative of the true mechanical properties of the material.
A method for producing tensile test bars by material extrusion additive manufacturing (ME-AM) of the present disclosure produces tensile test bars which avoid stress concentrators.
Accordingly, several aspects of the present disclosure relate to a 3D printed specimen. The 3D printed specimen includes a plurality of beads, wherein the plurality of beads define a first region and a second region. The first region exhibits a first region width, and the second region exhibits a second region width, wherein the first region width is less than the second region width. Each of said plurality of beads exhibit a first bead dimension in the first region and a second bead dimension in the second region. The first bead dimension and the second bead dimension are both one of a width and a height. In particular aspects, the 3D printed specimen is a dogbone for mechanical testing and, in particular, for tensile testing.
In aspects of the above, the first bead dimension is a first bead width, and the second bead dimension is a second bead width.
In any of the above aspects, the second region includes a first additional set of beads disposed on a first side of the plurality of beads and a second additional set of beads disposed on a second side of the plurality of beads opposing the first set of beads.
In any of the above aspects, the specimen is in the shape of a dog bone and wherein the first region forms a gage region, and the second region forms a grip region.
In any of the above aspects, the 3D printed test specimen further includes a transition region between the first region and the second region and in the transition region the plurality of beads each transition between the first bead width and the second bead width.
In any of the above aspects, the first bead width is 0.4 mm, and the second width is 0.6 mm.
In any of the above aspects, there are twenty five beads in the plurality of beads.
In any of the above aspects, no discontinuities are present in each of the plurality of beads in the first region and the second region.
Additional aspects of the present disclosure relate to a method of forming a specimen, including any of the aspects of the specimen described above. The method includes extruding a plurality of beads from a nozzle; and forming a first region and a second region with the plurality of beads. Each of said plurality of beads exhibit a first bead dimension in the first region and a second bead dimension in the second region. Further, the first region exhibits a first region width, and the second region exhibits a second region width, and the first region width is less than the second region width, and the first bead dimension and the second bead dimension are one of a width and a height.
In any of the above aspects, the first dimension is a first bead width, and the second bead dimension is a second bead width.
In any of the above aspects, the second bead width is greater than an opening diameter of the nozzle.
In any of the above aspects, the method further includes extruding a transition region between the first region and the second region and in the transition region the plurality of beads each transition between the first bead width and the second bead width.
In any of the above aspects, the method further includes increasing the width of the second region by extruding a first additional set of beads adjacent to a first side of the plurality of beads in the second region; and extruding a second additional set of beads adjacent to a second side of the plurality of beads, wherein the first side opposes the second side.
In any of the above aspects, the method further includes varying at least one of an extrusion speed and a volumetric flow rate while forming the first region and the second region.
In aspects of the above, a residence time in the nozzle is maintained the same at the first speed and at the second speed.
In aspects of the above, a deposition temperature of the plurality of beads is maintained the same in the first region and the second region.
In any of the above aspects, the method further includes generating a tool path for extruding the plurality of beads.
In any of the above aspects, the method further includes constraining the height of the plurality of beads between the nozzle and one of a build plate supporting the plurality of beads or a previously extruded bead.
Yet further aspects of the present disclosure relate to system for forming a specimen. The system includes a three-dimensional printer including an extrusion nozzle, wherein the three-dimensional printer is configured to extrude a plurality of beads along a tool path; and a programming script configured for use by the three-dimensional printer, the programming script including instructions defining the tool path, wherein the tool path defines the plurality of beads, wherein the plurality of beads define a first region and a second region and each of said plurality of beads exhibit a first bead dimension in the first region and a second bead dimension in the second region, wherein the first region exhibits a first region width and the second region exhibits a second region width and the first region width is less than the second region width and the first bead dimension and the second bead dimension are both one of a width and a height.
Advantages of the present 3D printed specimen, system, and method include, but are not limited to, a reduction in discontinuities along the bead length reducing stress concentrations that may cause failure of the test specimen, particularly in the gage length and transition region. Further advantages include mimicking the preparation of injection molded test specimens through the 3D printing method, including necking down of the flow front in the gage region and flaring out of the flow front in the grip region.
The description of the present disclosure is merely exemplary in nature and variations that do not depart from the gist of the present disclosure are intended to be within the scope of the present disclosure. Such variations are not to be regarded as a departure from the spirit and scope of the present disclosure.
The present disclosure claims the benefit of the filing date of U.S. Provisional Application No. 63/046,185 filed on Jun. 30, 2020, the teachings of which are incorporated herein by reference.
Filing Document | Filing Date | Country | Kind |
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PCT/US21/39841 | 6/30/2021 | WO |
Number | Date | Country | |
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63046185 | Jun 2020 | US |