PSI - Issue 68

P. Langourani-Kosteletou et al. / Procedia Structural Integrity 68 (2025) 112–118 P. Langourani-Kosteletou et al. / Structural Integrity Procedia 00 (2025) 000–000

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screw had been properly inserted. To prevent the screws from subsiding into the sawbones during insertion and loading of the construct, a washer was utilized. All experiments were carried out using an MTS (Material Testing System) Insight loading frame (of capacity 10 kN) under displacement-controlled mode at a rate equal to 0.5 mm/s. The load was applied on one of the lateral surfaces of block 1, i.e., normal to the axis of the screw, while block 2 was rigidly clamped with the aid of a stiff metallic frame (Fig. 1). The force-displacement curve was obtained for each construct, and the stiffness, proportional ity limit, conventional yield load, and yield displacement were calculated for each test piece. The stiffness of each construct was defined as the slope of the line that best fits the linear region of the force-displacement curve. The proportionality limit was defined as the load beyond which the relationship between force and displacement ceased to be linear. This point marked the end of the linear elasticity region on the force-displacement curve. The yield force was determined by applying a 0.015 mm offset parallel to the stiffness (Schriefer et al., 2005; Downey et al., 2015). The yield displacement was defined as the corresponding displacement of the yield force. After a series of preliminary experiments, it was determined that the loading of the constructs should be terminated when the total displacement, as measured by the loading frame, exceeds 1.5 mm since, in all experiments, deviation from linearity was detected well before the 1.5 mm displacement. Therefore, the criterion for terminating the loading process was established at 1.5 mm. Software IBM SPSS Statistics, Version 29.0, was used to identify statistically significant differences between the two tested groups (Group A and Group B) regarding these properties.

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Fig. 1. (a) Sketch of the experimental setup. TL denotes the Thread Length (16 mm for Group A and 32 mm for Group B, respectively). L denotes Screw Length (75 mm for both groups). The Blue Arrow indicates the applied load . After the two blocks were fixed with either a partially threaded 16 mm TL (Group A) or a 32 mm TL (Group B), block 2 (of 60 mm length) was rigidly clamped, and a vertical downwards displacement was applied on block 1 (of 30 mm length). The specific setup allowed relative displacement of block 1 with respect to block 2 along their interface, leading to shear loading of the screw; (b) Photos of the setup. 3. Results The data were collected from nine constructs of Group A and nine constructs of Group B. The load-displacement graph of construct 5 from Group A is plotted in Fig. 2a, and it is used here to explain the methodology followed in the present study to detect/calculate the quantities under study. More specifically, the linear segment of the curve, re presenting the elastic region of the construct, was detected and isolated (see Fig. 2b) in order for the best-fitted linear equation to be obtained (dotted red line in Fig. 2b) together with the correlation coefficient (or the coefficient of determination (R 2 )). For the specific construct, the linear equation was y=0.4616x−0.0628 and R 2 =0.9995. The slope of the linear equation corresponds to the stiffness of the construct (equal to 461.6 N/mm for the A5 construct). The equation derived from the trendline in the elastic region was then plotted back on the original graph as a red line (Fig. 2a). Based on this red line, the terminal point of linearity is detected or, in other words, the proportionality limit (for the A5 construct, proportionality limit: P l =0.35 kN). As a next step, a straight line parallel to the stiffness (red line), offset by a displacement of 0.015 mm, is drawn (blue line in Fig. 2a). The intersection point of this blue line with the load-displacement curve defines the yield load (yield force: F y =0.4 kN for the A5 construct) and, as a result, the respective yield displacement can be detected on the horizontal axis (yield displacement: D y =1.04 mm for the A5 construct). The aforementioned methodology was followed for all samples of the present study. Some data were excluded from the analysis as they were identified as statistical outliers. Table 1 presents all valid experimental data that were included in the analysis. The load-displacement curves for Groups A and B are displayed in Fig. 3.