PSI - Issue 42

Guilherme Opinião et al. / Procedia Structural Integrity 42 (2022) 1266–1273 Guilherme Opinião / Structural Integrity Procedia 00 (2022) 000 – 000

1270

5

pullout resistance was made considering that the screw is fixed by a cylinder (intended to simulate the bone) and is pulled by an axial force of 450 N of magnitude. Mesh convergence studies were performed for all load scenarios and all designed geometric cases, considering a 10% variation in maximum von Mises stress as a convergence criterion. For tightening studies, 0.3 mm CTETRA (4) mesh elements were used, and for the pullout test analysis, 0.5 mm CTETRA (4) mesh elements were used for the screw models while 1.0 mm CTETRA (4) mesh elements were used for the bone cylinder model. For the tightening simulation, it was considered that the screw was fully embedded allowing only the interior of the screw to rotate on itself and, thus, determine the stresses generated on the internal surface of the implant. To simulate the pullout of the screw from the bone tunnel where it is inserted, an axial force of 450 N was applied to the top of the screw. Regarding movement constraints, the bone cylinder was fixed both at the bottom and top faces, not allowing it to move in any direction. At the screw-bone cylinder interface, a surface-to-surface contact was created between the external surface of the screw thread and the internal surface of the bone block in contact with the model and a coefficient of friction of 0.37 was assigned. The comparison between hexagonal and Torx plus drive geometries under torsional efforts aimed to analyze the influence of these geometries on the stress distribution along the interference screw. Generally, the screw with hexagonal drive geometry presents lower stresses than the screw with the Torx plus drive geometry. Furthermore, results indicate that models with inclined and elliptical-shaped holes develop greater stresses in the screw body around the holes (region that does not include the screw thread) and the highest stress concentration peaks are found in the regions around the holes. Models with smaller pitch (4 mm) appear to develop greater stresses than models with a pitch of 5 mm. Regarding the models with vertical holes, the symmetric thread geometry models, and the ones with asymmetric thread with proximal angle of 30º, distal angle of 40º and 1.5 mm of height are subjected to higher stresses regardless the pitch of the screw. For the models with 5 mm of pitch, the model with asymmetric thread with proximal angle of 45º, distal angle of 40º and depth of 1.5 mm seems to be subjected to lower stresses. Regarding the type of hole, models with elliptical and inclined holes develop higher stress concentration in the areas close to the holes but they present little significance differences when compared to models with vertical holes. The analysis of the developed stresses in each one of the designs allowed a pre-selection of screws to be fused filament fabricated. Details of this analysis can be consulted in detail in the work by Guilherme Opinião [28]. 3.3. Fused filament fabrication Among the evaluated geometric combinations, four screw models were selected to be tested in the torque tightening scenario. Table 1 shows the geometric characteristics of each one of the four chosen screws.

Table 1. Selected geometries produced by fused filament fabrication (FFF). See Figure 2 for nomenclature.

Pitch [mm]

Thread geometry

Holes geometry

Number of holes

4

Asymmetric, PA: 45º, DA: 40º, T: 0,3mm, H: 1,5mm Asymmetric, PA: 45º, DA: 40º, T: 0,3mm, H: 1,5mm Asymmetric, PA: 45º, DA: 40º, T: 0,3mm, H: 1,5mm Asymmetric, PA: 45º, DA: 40º, T: 0,3mm, H: 1,5mm

Elliptical and inclined that follow the thread

12

4

Elliptical and inclined that follow the thread

21

5

Elliptical and inclined that follow the thread

10

5

Elliptical and inclined that follow the thread

17

The screws were printed in a Ultimaker® 3 with an AA 0.4 print core, and the slicing software used to prepare the gcode file was Ultimaker Cura 4.6. Three replicas of each model were produced. Since the use of materials such as PLLA or PLDA was not available, screw prototypes were produced with PLA, which still allows a relative comparison between samples. Concerning build orientation, samples were printed with an inclination of 15º relative to the plane of the print bed and slightly raised so as not to cause defects in the area in contact with it, as is demonstrated in Figure 3(A). This