Full Length ArticleThe impact of print orientation and raster pattern on fracture toughness in additively manufactured ABS
Introduction
Polymer based additive manufacturing has been increasingly utilized in a wide range of fields for the last several decades. The ability to quickly design, alter, and print complex geometries produces a cost-effective manufacturing process to reduce the overall number of parts used in space hardware [1], [2], [3]. Past applications using this technology have focused mostly on prototype utilization; however, advances in materials and processing are bringing this adaptable technique to a more mature stage of production.
The National Aeronautics and Space Administration (NASA) and United Launch Alliance (ULA) have both begun to incorporate 3D printed thermoplastic parts into their space vehicles and satellites. An example is the recent development of an antenna array for the FormoSat-7/COSMIC-2 satellite mission which was manufactured using fused deposition modeling (FDM) ULTEM® 9085 [4], [5].
Production of parts by FDM relies on the coordinated movement between an extruding nozzle and a building platform, schematically shown in Fig. 1. The nozzle is responsible for heating and extruding the thermoplastic material in predetermined raster patterns that define the geometry of a single cross-sectional slice of a part. The nozzle traverses back and forth in the XY plane until a single layer is complete, then the build platform moves down one layer step in the Z direction and the process repeats until all layers have been printed. Based upon a part’s physical orientation inside of the printer, the cross-sectional slices that determine the layers will be different. These variations in geometry can produce parts of the same net shape, but with entirely different mesostructures.
Anisotropy that stems from the layered structure, raster patterns, and print orientations has been observed to impact mechanical properties [6], [7], [8], [9]. Zaldivar et al. [6] quantified print orientation dependent tensile properties for ULTEM® 9085. They found mechanical properties reached 85% of the ideal injection molded ultimate tensile strength (UTS) when the gauge lengths were aligned parallel to the build platform in the X or Y orientation. Conversely, samples with a gauge length normal to the build platform in the Z orientation only achieved 45% of the UTS. The X and Y specimens had more extruded filaments in line with the tensile load, where these continuous filaments can efficiently carry and transfer load. It becomes apparent from these results that polymeric FDM parts have anisotropic properties much like continuous fiber composites, where inferior transverse properties must be accommodated. Ahn et al. [7] observed differences in the mechanical properties of FDM ABS when the raster patterns of individual layers were varied. They found that certain patterns can create areas of stress concentration and change the load distribution across a sample. Fabricating parts with an optimized raster orientation where more extruded filaments are in line with the tensile load improved mechanical properties; samples consisting of a [0°]12 lay-up orientation exhibited a 40% increase in tensile strength compared to those of a [+45/−45°]6 pattern [7].
The thermoplastic polymer ABS is used in a variety of FDM machines ranging from commercial to desktop devices [10], [11]. As a result of this ubiquitous application, ABS is used in this study. The effect that these processing parameters may have on tensile and compressive mechanical properties of FDM ABS has been well studied, but more work in the area of fracture toughness is still necessary [7], [8]. In the present work, the physical changes to the mesostructure that result from the print orientation and raster pattern are studied for their effect on fracture toughness. To assess the likelihood of fracture in a specific component, it is useful to know the stress intensity factor, K, which defines the magnitude of the stress field around the crack tip. K contains information regarding the geometry of the crack as well as the current level of stress at the crack tip. Upon reaching a critical value of K called the critical stress intensity factor (KC), a material will fracture. This value is commonly known as the fracture toughness of a material, as it defines the resistance to fracture in the presence of a sharp crack. To quantify the fracture toughness of a material, it is necessary to first define the mode of loading. Mode I, or opening mode, is encountered in most engineering applications, so it is the most commonly used metric of fracture toughness as opposed to Mode II, in-plane shear mode, or Mode III, tearing mode [12]. Compact tension (CT) specimens were chosen to achieve Mode I crack opening and thereby enable determination of fracture toughness. The variation in toughness was compared with micrographs of fracture surfaces to draw novel conclusions about the mechanisms that drive fracture toughness in additively manufactured polymer components as they relate to the raster pattern and sample orientation within the printer.
Section snippets
Materials
All 3D printed parts were manufactured on the Stratasys Fortus 250mc system using ABSplus-P430 material in conjunction with SR-30 soluble support material. Solid files of each part were loaded into Stratasys’s Insight software. The layer height was 0.018 cm and contour width was 0.051 cm. Standard parameters from the software were used for the build density, raster width, and raster delta angle air gap.
CT samples were produced according to ASTM D5045 [13]. Their dimensions were 6.35 cm (W) × 6.10 cm
Results and discussion
The tabulated fracture toughness results of the three print orientations and two raster schemes are provided in Table 1. Additionally, representative load versus crack opening curves are provided for each of these configurations in Fig. 3. Samples in the XYZ and ZXY orientations did not meet the requirements for linear elastic plane strain fracture toughness measurements, and therefore fracture toughness results are reported as KQ. The samples in the XZY orientation met the requirements and
Conclusion
Print and raster orientation in FDM CT samples were shown to significantly impact fracture toughness. Samples printed in the ZXY and XYZ directions contained half or more of their filaments in a direction that was orthogonal to the crack plane, which resulted in a significant obstacle to crack propagation and a fracture toughness that reached 1.97 MPa*m1/2 for the ZXY orientation. The XZY orientation did not have any filaments aligned orthogonally to the crack plane. Instead, it relied upon weak
Acknowledgments
We would like to thank The Aerospace Corporation’s Research Program and Development Office for their support. We would also like to thank Dr. D. Witkin and J. Lee for the opportunity to do this research.
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