Microsoft Word - numero_66_art_07_4332.docx J. G. D. Rodríguez et alii, Frattura ed Integrità Strutturale, 66 (2023) 127-139; DOI: 10.3221/IGF-ESIS.66.07 127 Notch sensitivity study in u-notched polymers built by Additive Manufacturing (AM) Jorge Guillermo Díaz Rodríguez, Alberto David Pertuz Comas, Juan David Sandoval Herrera Universidad Industrial de Santander. Escuela de ingeniería mecánica, Carrera 27, calle 9. Bucaramanga, Colombia jgdiazro@correo.uis.edu.co, https://orcid.org/0000-0002-0479-4827 apertuzc@uis.edu.co, https://orcid.org/0000-0002-9130-6528 juan.sandoval3@correo.uis.edu.co ABSTRACT. Onyx ® is a new material composed of Polyamide 6 reinforced with short carbon fiber, used in the novel additive manufacturing composites technique. This paper aims to presents the axial fatigue performance of this material with and without U-notches. The experimentally determined S-N fatigue curve was obtained under axial load with a load inversion ratio, R = 0.1, and compared to fatigue performance of U-notched samples ranging from 0.25 to 2 mm radius. In addition, the stress concentration factor was compared for static and alternative loading to obtain the notch sensitivity in terms of the U-notch radius, showing that there is indeed a difference in stress concentration between them. The advantage of the approach is that it permits using commonly used dimensioning methods for this AM material. KEYWORDS. Notch sensitivity, Uniaxial fatigue, Additive manufacturing, polymers. Citation: Rodriguez, J. G. D., Comas, A. D. P., Herrera, J. D. S., Notch sensitivity study in u-notched polymers built by Additive Manufacturing (AM), Frattura ed Integrità Strutturale, 66 (2023) 127-139. Received: 26.05.2023 Accepted: 02.08.2023 Online first: 06.08.2023 Published: 01.10.2023 Copyright: © 2023 This is an open access article under the terms of the CC-BY 4.0, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. INTRODUCTION atigue notch sensitivity is a key factor to consider when designing a component. Notch sensitivity can be described as the material´s susceptibility to crack initiation and propagation under variable loading at notches, scratches, and other defects where stress concentration is higher [3]. On the other hand, Additive Manufacturing (AM) is a group of technologies that produce functional parts building components layer by layer, allowing complex shapes and geometries that may not be possible to manufacture by another method [8]. However, AM has not reached maturity [3], so there is room for improvement. From these technologies, material extrusion, MEX-TRB/P/Onyx, according to ISO/ASTM 52900:2021 [12], uses a thermoplastic polymer, which is probably the most popular [20]. However, due to the melting, deposition, and solidification of the thermoplastic polymer matrix, MEX gives a particular mechanical behavior with characteristics to consider while projecting a component [4,7,13]. Moreover, a recent review [6] identified missing mechanical characterization in these polymers. So, a research project was launched to find the notch sensitivity of a thermoplastic polymer used in MEX so parts can be projected with commonly known design methods. AM processes generate time and material savings because producing one-part costs the same as producing many [14]. For example, the MEX technology developed by Markforged [31] uses a thermoplastic polymer as the matrix for continuous F https://youtu.be/qKBeHbJlfeQ J. G. D. Rodríguez et alii, Frattura ed Integrità Strutturale, 66 (2023) 127-139; DOI: 10.3221/IGF-ESIS.66.07 128 fiber-reinforced composites. Onyx, which is polyamide 6 (commonly known as Nylon) with embedded chopped carbon fiber, is one of the polymers used as a matrix. It goes through a heated circular nozzle and is bonded by thermal reaction recreating the part layer by layer. That subsequent deposition leaves gaps between passes, giving an anisotropy intrinsic to most AM technologies, and was identified in the early years of AM development by [24]. A schematic of such gaps is shown in Fig. 1a for the material deposited in the Z direction and a SEM photograph of the actual gaps [22]. Figure 1: Porosity in MEX; a) schematics, b) actual porosity, adapted from [22]. Such gaps might behave as stress concentration sites on which the material may respond differently to static and alternative loads. The fatigue behavior of Nylon is complex and influenced by several factors, such as applied stress, notches, fiber content, and temperature [18]. A notch increases the stress field, see Fig. 2; the sharper the notch, the higher the stress gradient. Furthermore, a U-notch is a deeper than wider rounded-tip notch, whereas a V-notch end shows a null radius. U- notches show higher reproducibility than V-notches [15]. U-notches show higher reproducibility than V-notches [15]. Recently, Marsavina et al. [17] showed that, indeed V-notches require less energy than U-notches to fracture. Therefore, fatigue life decreases significantly with a reduced notch radius [11]. Hence, it is important to investigate the fatigue and fracture behavior of AM components in the presence of such notches [28]. There are different methods to predict the fatigue life of a notched component. For example, the method developed independently by Neuber (line method uses the average stress over a distance from r=0 to 2L) and Peterson (point method, uses r=L/2) to calculate the stress before the crack in the notch of the component, see Fig. 2a for the notation and Fig 2b for an example of an experimentally measured strain field where one can see how the strains go up radially as one gets closer to the notch. Another method, the theory of critical distances (TCD) developed by Taylor [30] considers as a material parameter the point where the fracture toughness, Kc, overcomes the strength of the material (σ0) using Irwin´s plastic zone, see Eqn. (1). 2 0 1 Kc L          (1) Although it was initially devised for metals, the TCD has given good agreements with experimental data for ceramics and brittle polymers such PPMA, PC or PS. On the other hand, there are three fatigue methods components under variable stress: the stress-life (S-N) for components subjected over 1000 cycles, the strain-life (-N) method for components subjected up to 1000 cycles, and the da/dN method for cracked components. The notch sensitivity, q, is a parameter that tells the difference notch between static and alternating stress and is used in the S-N. The fatigue strength limit is generally measured by stress tests [3]. Fatigue tests for MEX polymers have been studied. In [7], a study was conducted on the fatigue resistance in notched polylactic polymer (PLA) samples printed by MEX. They used the critical distance theory (TCD). Pertuz et al. [21] studied the static and fatigue behavior of reinforced and non-reinforced Nylon printed by MEX in uniaxial tests, using the Weibull distribution to adjust the data obtained at each load level. Recently, Bandeira et al. [2] experimentally evaluated the flexural notch sensitivity of high-strength wires. First, they established tensile mechanical properties, and then they used the staircase method [3] to obtain the fatigue endurance limit for the notched and notched-free specimens. The difference in fatigue Molted polymer layers Matrix porosity x y z Nozzle Melted polymer. Infill Gaps Printing bed a) b) J. G. D. Rodríguez et alii, Frattura ed Integrità Strutturale, 66 (2023) 127-139; DOI: 10.3221/IGF-ESIS.66.07 129 performance between notched and notched-free specimens gives the notch sensitivity. Finally, Nijs et al. [19] used full-field strain measures to determine the notch sensitivity in a PP matrix carbon composite. Figure 2: a) Stress distribution in front of a U-notch, b) Experimental strain in front of a round notch [5]. Several fatigue models have been used to predict the fatigue life of thermoplastic materials. For example, the Basquin model adjusts the data in uniaxial fatigue tests for notch and smooth samples [3]. To study smooth and notched Onyx ® fatigue behavior, Nylon based Polymer Matrix Composite (PMC) specimens used in AM were printed by MEX. Fatigue tests were performed under load reversal ratio R = 0.1. Five types of "U" notch with different radii were fatigue tested, and the data obtained by each were compared with uniaxial fatigue S-N from smooth specimens. Then, the difference in static and fatigue stress concentration is shown to extract the notch sensitivity. To the best of our knowledge, this is the first study on notch sensitivity for AM polymers reinforced with short carbon fiber. The paper shows the failure modes of the fatigue tests. This paper aims to find the notch sensitivity to use in accepted design methods. THEORETICAL BACKGROUND his section gives the baseline information for the reader to understand the paper. Material extrusion In 2015, Markforged [31] obtained a patent to manufacture MEX composites reinforced with continuous fibers. MEX is the most popular of AM. A schematic of the composite MEX printing process is shown in Fig. 3. Figure 3: Schematics of composites MEX printing. Adapted from Ref. [14]. T J. G. D. Rodríguez et alii, Frattura ed Integrità Strutturale, 66 (2023) 127-139; DOI: 10.3221/IGF-ESIS.66.07 130 MEX is part of the AM family in which a slicer software layer reconstructs the geometry by layer from an STL file. For Markforged ®, this process is done by software in the cloud called Eiger® [31]. The mechanical properties for the matrix (Onyx ® and Nylon) are shown in Tab.1. First, it must be noted that Onyx ® is a composite itself, a nylon matrix reinforced with chopped carbon fiber. Material E [GPa] σ [MPa] Relative density % Elongation Nylon 1.7 33.5 1.1 4.5 Onyx 1.4 36 1.2 33 Standard ASTM D638 ASTM D638 NA ASTM D638 Table 1: Mechanical properties as provided by Markforged, [23]. Fatigue and stress concentration When a discontinuity appears in a component and is subjected to fluctuating loads, it does not behave in the same way as a smooth component. This represents a change of cross section in the material, concentrating stress in the area where the notch is. The opening of a crack in a notch usually starts at the tip because that is where the maximum stress occurs, see Fig. 2. The stress concentration factor, Kt, was proposed by Inglis in 1913 for a plate with an elliptical hole under uniaxial load, and it can be estimated with Eqn. (2) with the notation depicted in Fig. 1. Kt depends only on the geometry and the load; therefore, it is a purely mechanical parameter. 1 2t a K c   (2) The analysis of geometric and applied load to determine Kt is a complex problem, and one may not find a solution for a particular combination. Instead, it is determined by experimental techniques such as those compiled by Peterson, using an Airy stress function numerically or a combination of said methods [5]. In the case of an experimental approach, Kt can be calculated experimentally using Eqn. (3) [3]. L tK    (3) where σL is the local stress at the notch and σ∞ is the remotely applied static stress. However, if the remotely applied stress is alternative, the material might not behave the same. Therefore, the fatigue stress concentration factor, Kf, is used instead. The notch sensitivity, q shown in Eqn. (4), is a factor that tells the difference in stress concentrators due primarily to the geometry of the notch, the type of load, and the material. In this case, we also investigated if it is affected by the manufacturing method. The notch sensitivity for isotropic materials, such as metals, and selected polymers, can be found in the literature [3]. 1 1 f t K q K    (4) On the other hand, several fatigue models have been used to predict the S-N behavior of thermoplastic materials. The Basquin model is probably the most known, as seen in Eqn. (6).  bA Nf  (5) where A and b are material constants that are determined experimentally. Furthermore, the Walker model introduces the effect of load ratio, R, as seen in Eqn. (6). 2 1eq a R         (6) J. G. D. Rodríguez et alii, Frattura ed Integrità Strutturale, 66 (2023) 127-139; DOI: 10.3221/IGF-ESIS.66.07 131 where γ represents sensitivity to the load ratio, R, ranging between 0 and 1; for γ =0, the material is not sensitive, and for γ =1 is the most influenced by R. Furthermore, Ezeh and Susmel [7] recommended rules for estimating the S-N curve under different load ratios from static tests. Lu [16] tested several polymers with different stress ratios under alternative loads. MATERIALS AND METHODS he experimental procedure was divided into sample printing, static characterization, smooth and notched fatigue testing, and comparison of results. The samples were manufactured with the MEX technique using the Mark Two printer [31] using ONYX which is Nylon embedded with short carbon fiber. Five smooth and 14 U-notched samples with 3 mm thickness were printed with the geometry shown in Fig. 4. The specimen geometry and notch radius values were determined according to the literature [2,7,10]. Finally, anisotropy is a well-documented effect for AM polymers [25] and AM in general [8]. Samples were printed following parameters from previous results [9], which included a solid matrix, and matrix deposition angle parallel to the load. Tensile tests were performed within ASTM D639 guidelines on smooth samples at a 5 mm/min strain rate under displacement control. In addition, both static and fatigue tests were performed under displacement control using Eqn. (7). PL d AE  (7) where d is displacement, L is the initial calibrated length, A is the transversal area, E is elastic modulus, and P is applied load. Figure 4: Samples geometry and measurements in (mm) of specimens. Both static and fatigue tests used an MTS Bionix 370.02 equipped with a 25 kN load cell and an MTS 634.12F-25 extensometer measured axial deformation. Finally, fatigue tests were performed under controlled displacement using the linear elastic relation described in Eqn. (8). The mechanical properties of ONYX thermoplastic were determined in tests under static load. Smooth samples were tested at constant amplitude at a 5Hz rate to estimate the S-N curve. After the curve was established, a nominal stress was applied in each notched sample to produce a local stress that made the sample last about 5000 cycles. The notched specimen’s description is shown in Tab. 2. Kt values were calculated according to Peterson´s estimations [3]. A fractographic evaluation was performed to analyze the failure surface as well as the failure mechanisms under tensile and fatigue loading. First, samples were plated with gold and placed into a Vega 3 Tescan SEM between 7 and 10 keV equipped with a tungsten filament. However, after the gold plating was done in some samples, it made no difference for the SEM. This may be attributed to the chopped carbon fiber in the PA6 matrix that made the sample conductive. T J. G. D. Rodríguez et alii, Frattura ed Integrità Strutturale, 66 (2023) 127-139; DOI: 10.3221/IGF-ESIS.66.07 132 Notch radius [mm] Sample number Quantity Kt 0.25 1 2 2.92 0.5 2 3 2.78 1 3 3 2.52 1.5 4 3 2.27 2 5 3 2.03 Table 2: Notched specimens. RESULTS AND DISCUSSION he results of the tests and a first analysis are shown in this section. Tensile tests The tensile tests were performed for a solid configuration and Elastic modulus and strength were calculated. Fig. 5 shows the Stress vs. Strain curve; the material's ultimate stress (σut) is approximately 45.8 MPa, and the Elastic modulus is 1094 MPa, with 99.94% of data represented by that value. [1] reported similar results, whereas [29] reported slightly lower values for PA6. Figure 5: Static stress vs strain for Onyx Fatigue tests on notched Onyx The applied load P was determined by a percentage of the σut to each sample for the fatigue tests. It is documented that polymers are time-dependent materials [4] they soften under variable loads [18]. According to a literature search [22,26] failure in polymer under fatigue loading can be described when the load fails below 85% of the initial load. The stress–life curve is shown in Fig. 6 for the stress produced when the applied load fell at 85% of the initial load. Furthermore, the data was adjusted to Basquin´s model rendering Eqn. (8), with 93.3% of the data represented by the constants. As a comparison, fatigue results [21] for MEX printed Nylon were added to Fig. 6 for different infill patterns (hexagonal with 50% density, triangular with 20 and 50% density). The results are very different below 100000 cycles, whereas for lives after 100000 cycles, the infill pattern seems to make no sizeable effect. Furthermore, the literature data reported and plotted in Fig. 6 is for Nylon, which is Onyx without the short carbon reinforcement.   0.514 2231.2 fN   (8) E = 1094.2 R² = 0.9994 0 5 10 15 20 25 30 35 40 45 50 0% 2% 4% 6% 8% 10% 12% [MPa] % T J. G. D. Rodríguez et alii, Frattura ed Integrità Strutturale, 66 (2023) 127-139; DOI: 10.3221/IGF-ESIS.66.07 133 Figure 6: Experimental stress - life curve with comparison from [21]. It was also found how the thickness of the specimen changed over axial load, as depicted in Fig. 7, where the thickness went almost uniformly from 3 mm to about 1.95 mm after 1.18 million cycles. Figure 7: Exemplary results of change in thickness after 1181865 cycles. Figure 8: Stress - life curve for each notch sample. Fatigue tests on notched Onyx The "U" notch specimen description for each radius is shown in Tab. 2. When a specimen presented irregularities along the cross-section, it produced a stress concentration; therefore, such samples were rejected for testing. Moreover, with the Basquin constants, Eqn. (8), it was calculated the displacement, Eqn. (7), that caused a life of 5000 cycles for each notch with a stress concentration factor, Eqn. (3). The reference of 5000 cycles gave a nominal stress of 13.5 MPa. The S-N curve 0 5 10 15 20 25 30 35 1.000 10.000 100.000 1.000.000 MPa Log N Exp Tri 50% Tri 20 % Hex 50% after 1'181,865 cycles. No load. J. G. D. Rodríguez et alii, Frattura ed Integrità Strutturale, 66 (2023) 127-139; DOI: 10.3221/IGF-ESIS.66.07 134 is constructed for each notch configuration shown in Fig. 8. For the larger notch, the S-N curve overlaps with the notchless sample. On the other hand, the smaller the radii are, the lower the specimen´s life. However, if the AM printed material had no notch sensitivity, the line would fall directly under the results for the larger notch. In this case, when the radii change, both the stress and life change simultaneously, meaning the material has notch sensitivity. It was observed throughout the test that the crack starts at the notch´s corner, as shown in [7]. Fig. 9 shows how the crack began; Fig. 9a is at a side view, whereas Fig. 9b shows the front view. As time passed, the crack grew to run along the entire side of the notch, as shown in Fig. 9c. Notably, the crack initially grew on only one side of the notch, as shown in Fig. 9b. Figure 9: Crack growth in a U-notch for an initial 3 mm thick sample; a) lateral view of corner crack, b) front view of corner crack, c) through crack. The presence of a notch considerably increases the stress in the vicinity of the discontinuity of the material. If a material behaves differently to the stress concentration under an alternative load, this is called notch sensitivity. This sensitivity depends on the material's mechanical properties as well as the type and stress on the notch. Therefore, one may estimate a stress concentration factor under an alternative load using Eqn. (9). notch smooth fK    (9) So, the Kf value for each notch was estimated by comparing the average fatigue stress for U-notch samples with the Basquin stress at about 5000 cycles, as shown in Tab. 3. The value of Kf is always less than or equal to Kt [3]. So, the notch sensitivity q was estimated using Eqn. (4). The notch sensitivity is between 0 and 1. If q = 0, the material has no sensitivity to the notch, and Kf is equal to 1. On the other hand, if q = 1, the material has full notch sensitivity, and Kf is equal to Kt. When the notch sensitivity parameter is not known, the value of Kt is Kf or q=1 [3]. Tab. 3 shows the notch sensitivity for each configuration. "U" sample U notch radius, mm σ smooth [MPa] Average σ U- notch [MPa] Kt Kf q 1 0.25 27.4 28.6 2.92 1.04 0.02 2 0.5 26.7 29.1 2.78 1.09 0.05 3 1 23.9 28.7 2.52 1.20 0.13 4 1.5 23.1 30.7 2.27 1.33 0.26 5 2.0 20.3 30.5 2.03 1.50 0.49 Table 3: Fatigue stress concentration factor, Kf. Looking at the results in Tab. 3, one sees greater notch sensitivity for the larger notches. Fig. 10 shows the relationship between Kf, Kt, and q with notch radius. If there was no notch sensitivity, the relation between Kt and Kf should be parallel. The notch with a larger notch sensitivity value was the smallest notch radius because the fatigue concentration factor a) b) c) J. G. D. Rodríguez et alii, Frattura ed Integrità Strutturale, 66 (2023) 127-139; DOI: 10.3221/IGF-ESIS.66.07 135 increased. The sample with the sharpest notch had a higher stress concentrator and the lowest lower notch sensitivity. If q = 1 the material fully sensible to the notch and Kf is equal to Kt. Figure 10: Stress concentration and notch sensitivity vs notch radius. Figure 11: SEM image for the notched specimen with 2 mm radius. Fractographic analysis Polymers under cyclic load show a loss in elastic modulus [27]. Therefore, it was considered that the specimen had already failed when the load fell to 85% of the initial load [26]. Whether there was a crack or not, the sample had accumulated damage, so samples were broken under liquid nitrogen to reveal the failure surface. It is observed in Fig. 11 the fracture surface for a specimen with a notch radius of 2 mm is an area of many interstices observing the separation of the matrix from the short fibers, passing the nucleation process, helping the crack growth, and reaching the fracture of the specimen. One also sees parallel lines formed by the circular nozzle extruding molten polymer during the solidification process. Crazing is formed by the unraveling of polymeric chains, which macroscopically is perceived as a whitened area [4]. The alternative loading either causes a progressive rupture of fibrils at the point of the highest stresses or separation of the polymer matrix and fiber, thus becoming a crack. This rupture of fibrils is seen as a matrix micro-ductile failure. Finally, fatigue crack growth locations exposed fiber pull-out and matrix brittle failure, while crack initiation areas in the corners of fracture surfaces exhibited matrix micro-ductile deformation as reported in [18]. 0 0,1 0,2 0,3 0,4 0,5 0,6 0 0,5 1 1,5 2 2,5 3 3,5 0 0,5 1 1,5 2 2,5 qKt, Kf r, mm Kt Kf q FCG Matrix micro-ductile deformation Fiber pull-out and matrix brittle failure Notch J. G. D. Rodríguez et alii, Frattura ed Integrità Strutturale, 66 (2023) 127-139; DOI: 10.3221/IGF-ESIS.66.07 136 In Fig. 12, one can see how the shot carbon fiber sticks out of the nylon matrix and, simultaneously, the void spaces left by missing chopped fiber. Porosity is formed by the unfilled spaces left by the circular nozzle when the polymer is extruded. This porosity may be responsible for the material to exhibit lower strength and stiffness than a subtractive fabrication process. Although under cyclic loading the strain is redistributed by rearrangement of the polymer molecules at the microscopic level [18], polymers exhibit cyclic softening [21]. In Fig. 12, it can be seen that the nylon matrix and short carbon fibers are pulled out from each other, as reported in literature [18]. These are entirely broken due to the dynamic load applied. This is a drawback for AM materials because it causes fatigue resistance to be affected. Figure 12: Molten layers showing short fiber pull-out. Figure 13: SEM image of the fracture zone for a specimen with a notch radius of 0.25 mm showing fiber breakage and fiber-matrix separation. Fig. 13 shows that the nylon matrix looks deformed and somewhat crushed due to the forces it endured until it reached fracture. It also shows in more detail the short carbon fibers together with the nylon matrix. It shows the plastic deformation made by the fluctuating forces applied, as a material with a low plasticity can be considered fragile. The diameter of carbon fibers is approximately 8.4 microns [4]. Finally, it has to be said that a chopped fiber gets loose and under the SEM electric charge one can see it moving and get deformed while focusing the beam on it. a) b) a) b) J. G. D. Rodríguez et alii, Frattura ed Integrità Strutturale, 66 (2023) 127-139; DOI: 10.3221/IGF-ESIS.66.07 137 Finally, Fig. 14 shows different failure mechanics. The matrix layers that may leave porosity during solidification at different times; beach marks which were most likely produced by the alternating stress; and short fiber – voids which reveals how the material absorbed energy by pulling fiber out of the matrix. It is observed that porosity produced by adjacent matrix layers may act as a fatigue crack growth arrester. The growing crack finds a barrier in such discontinuities. This effect of crack arrest by printer layers was also noticed in [17], even for static loading. Figure 14: Close up of fracture surface in 1mm notch radius fatigued sample. CONCLUSION tatic tests on a polymer matrix composite printed by material extrusion with solid configuration gave 45.8 MPa for ultimate stress and 1094 MPa for Elastic modulus. Furthermore, constant displacement tests produced stress-life data under a 0.1 load inversion ratio. Results were data fitted to the Basquin model rendering 0.5142231.2 fN  for a smooth configuration. Finally, fractography revealed fatigue crack growth mechanisms, fiber pull-out, and matrix brittle failure. It was seen the phenomena of load relaxation which has been documented for polymers. In this case, it was observed for a polymer matrix composite. The load relaxation began at an initial force and decreased as time passed until it remained constant for a long period. If one has a high Kt it will produce a greater stress riser; consequently, causing material failure. At the end of the tests, it was concluded that the number of cycles to fracture the Onyx material increased as the notch radius increased and vice versa. The stress concentrator factor, Kf, made a difference for notched specimens under fatigue load. Kf roses as the notched radii grew. Notch sensitivity increases when the notch radius increases, but Kt decreases, and when Kt increases, notch sensitivity decreases as Kf. If this relationship between these variables did not occur, it would be very difficult to design parts capable of withstanding a nominal level of stress when they are in the presence of notches. It is concluded that Onyx is a fatigue-resistant material when the notch radius increases due to the softness of the notch. On the other hand, with a small notch, the notch sensitivity decreases, and a high concentration of stresses produces the material's failure faster than a larger notch. S J. G. D. Rodríguez et alii, Frattura ed Integrità Strutturale, 66 (2023) 127-139; DOI: 10.3221/IGF-ESIS.66.07 138 ACKNOWLEDGMENT he authors acknowledge financial support from project VIE 2827, and the VIE mobility program at Universidad Industrial de Santander. The help of Tecnoparque SENA—Bucaramanga for printing some of the samples and to the center for electron microscopy at UIS is greatly appreciated. REFERENCES [1] Ahmadifar, M., Benfriha, K., Shirinbayan, M. (2023). Thermal, Tensile and Fatigue Behaviors of the PA6, Short Carbon Fiber-Reinforced PA6, and Continuous Glass Fiber-Reinforced PA6 Materials in Fused Filament Fabrication (FFF), Polymers (Basel), 15(3), p. 507. DOI: 10.3390/polym15030507. [2] Bandeira, C.F.C., Kenedi, P.P., Souza, L.F.G. de. (2017). Assessing the Notch Sensitivity of High Strength Wires, Latin American Journal of Solids and Structures, 14(13), pp. 2423–2438. DOI: 10.1590/1679-78253728. [3] Castro, J.T.P., Meggiolaro, M.A. (2016). 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