Abstract:This paper deals with fatigue of closed cell foams under compression loading. Testing is performed on three densities of Divinycell H-grade and three densities of Rohacell WF-grade foam under cyclic compression loading. The fatigue failure is in all cases described as localised crushing of cell layers. The two main observations from this are that the slopes of the stress-life curves are almost all different, both between the foam types and relative density. Secondly, the stress-life relation slopes are conside… Show more
“…The observed phenomenon of a sudden increase in displacement in the early stage, followed by stabilization in all groups, is consistent with the literature and the behavior of foam-like materials under load. 34,43 Though the displacements in the three groups are not large, they are generally similar to those biomechanical studies having tensioncompression-bending load, which also shows the validity of the model to some extent. 44,45 Also, tension-compressionbending load used in osteoporotic vertebra has been proven a risk factor for pedicle screw loosening, 46 which indicates that usage of 5 pcf SRPF replacing osteoporotic bone not only conforms to common disease clinically, but also makes the results easier to observe.…”
Section: Analysis Of Model Used In the Studysupporting
ObjectivesIt is now understood that pedicle screw loosening at the implant‐bone interface can lead to poor screw–bone interface purchase and decreased fixation stability. Previous biomechanical tests used cadaveric vertebrae and pull‐out or torque loads to assess the effect of the insertional direction of pedicle screws on screw loosening. However, these tests faced challenges in matching biomechanical differences among specimens and simulating in vivo loads applied on pedicle screws. This study aimed to evaluate the effect of the insertional direction of pedicle screws on screw loosening using tension‐compression‐bending loads and synthetic bone vertebrae.MethodsPolyaxial pedicle screws were inserted into nine synthetic bone vertebrae in three directions (three samples per group): cranial, parallel, and caudad (−10°, 0°, +10° of the pedicle screw rod to the upper plane of the vertebra, respectively). Pedicle screws in the vertebrae were loaded using a polyethylene block connected to a material testing machine. Tension‐compression‐bending loads (100N–250N) with 30,000 cycles were applied to the pedicle screws, and displacements were recorded and then cycle‐displacement curve was drawn based on cycle number. Micro‐CT scans were performed on the vertebrae after removing the pedicle screws to obtain images of the screw hole, and the screw hole volume was measured using imaging analysis software. Direct comparison of displacements was conducted via cycle‐displacement curve. Screw hole volume was analyzed using analysis of variance. The correlation between the displacement, screw hole volume and the direction of pedicle screw was assessed by Spearman correlation analysis.ResultsThe smallest displacements were observed in the caudad group, followed by the parallel and cranial groups. The caudad group had the smallest screw hole volume (p < 0.001 and p = 0.009 compared to the cranial and parallel groups, respectively), while the volume in the parallel group was greater than that in the cranial group (p = 0.003). Correlation analysis revealed that the insertional direction of the pedicle screw was associated with the displacement (p = −0.949, p < 0.001) and screw hole volume (p = −0.944, p < 0.001).ConclusionStrong correlations were found between the insertional direction of the pedicle screw and relevant parameters, including displacement and screw hole volume. Pedicle screw insertion in the caudad direction resulted in the least pedicle screw loosening.
“…The observed phenomenon of a sudden increase in displacement in the early stage, followed by stabilization in all groups, is consistent with the literature and the behavior of foam-like materials under load. 34,43 Though the displacements in the three groups are not large, they are generally similar to those biomechanical studies having tensioncompression-bending load, which also shows the validity of the model to some extent. 44,45 Also, tension-compressionbending load used in osteoporotic vertebra has been proven a risk factor for pedicle screw loosening, 46 which indicates that usage of 5 pcf SRPF replacing osteoporotic bone not only conforms to common disease clinically, but also makes the results easier to observe.…”
Section: Analysis Of Model Used In the Studysupporting
ObjectivesIt is now understood that pedicle screw loosening at the implant‐bone interface can lead to poor screw–bone interface purchase and decreased fixation stability. Previous biomechanical tests used cadaveric vertebrae and pull‐out or torque loads to assess the effect of the insertional direction of pedicle screws on screw loosening. However, these tests faced challenges in matching biomechanical differences among specimens and simulating in vivo loads applied on pedicle screws. This study aimed to evaluate the effect of the insertional direction of pedicle screws on screw loosening using tension‐compression‐bending loads and synthetic bone vertebrae.MethodsPolyaxial pedicle screws were inserted into nine synthetic bone vertebrae in three directions (three samples per group): cranial, parallel, and caudad (−10°, 0°, +10° of the pedicle screw rod to the upper plane of the vertebra, respectively). Pedicle screws in the vertebrae were loaded using a polyethylene block connected to a material testing machine. Tension‐compression‐bending loads (100N–250N) with 30,000 cycles were applied to the pedicle screws, and displacements were recorded and then cycle‐displacement curve was drawn based on cycle number. Micro‐CT scans were performed on the vertebrae after removing the pedicle screws to obtain images of the screw hole, and the screw hole volume was measured using imaging analysis software. Direct comparison of displacements was conducted via cycle‐displacement curve. Screw hole volume was analyzed using analysis of variance. The correlation between the displacement, screw hole volume and the direction of pedicle screw was assessed by Spearman correlation analysis.ResultsThe smallest displacements were observed in the caudad group, followed by the parallel and cranial groups. The caudad group had the smallest screw hole volume (p < 0.001 and p = 0.009 compared to the cranial and parallel groups, respectively), while the volume in the parallel group was greater than that in the cranial group (p = 0.003). Correlation analysis revealed that the insertional direction of the pedicle screw was associated with the displacement (p = −0.949, p < 0.001) and screw hole volume (p = −0.944, p < 0.001).ConclusionStrong correlations were found between the insertional direction of the pedicle screw and relevant parameters, including displacement and screw hole volume. Pedicle screw insertion in the caudad direction resulted in the least pedicle screw loosening.
“…In addition, information on the material properties of the foam core, which was obtained from the manufacturing company website [13] and employed in the analysis, is shown in Table 3. It is noted that the properties of foam core are quite compatible to those obtained from experimental data [14]. Figure 10.(a) Finite element mesh for the debonded sandwich specimens, (b) enlarged fine mesh near the debond tip.…”
This research investigated the failure behaviors of the composite sandwich structures with face sheet and core debond subjected to compressive loading. Experiments and numerical simulations were conducted to determine the effects of the face sheet thickness and debond length on the compressive strength and failure mechanisms of the composite sandwich structures. Experimental results revealed that sandwich structures with either thicker face sheets or a shorter debond length exhibited a higher buckling load and greater failure strength. Moreover, when the debond length was short, failure was primarily caused by global buckling; conversely, when the debond length was long, failure was caused by local buckling and crack extension from the debond edge. To characterize the failure mechanism, a nonlinear finite element simulation of sandwich specimens containing imperfections was conducted. The simulation revealed that, when the dominant failure mode was global buckling, failure occurred at the intermediate portion of the foam core and strength could be characterized using the maximum principal strain criterion. However, when the failure mode was local buckling, failure was initiated at the debond tip, and strength could be predicted using the damage zone method.
Fruit and nut shells can exhibit high hardness and toughness. In the peninsula of Yucatan, Mexico, the fruit of the Cocoyol palm tree (Acrocomia mexicana) is well known to be very difficult to break. Its hardness has been documented since the 1500 s, and is even mentioned in the popular Maya legend The Dwarf of Uxmal. However, until now, no scientific studies quantifying the mechanical performance of the Cocoyol endocarp has been found in the literature to prove or disprove that this fruit shell is indeed “very hard”. Here we report the mechanical properties, microstructure and hardness of this material. The mechanical measurements showed compressive strength values of up to ~150 and ~250 MPa under quasi-static and high strain rate loading conditions, respectively, and microhardness of up to ~0.36 GPa. Our findings reveal a complex hierarchical structure showing that the Cocoyol shell is a functionally graded material with distinctive layers along the radial directions. These findings demonstrate that structure-property relationships make this material hard and tough. The mechanical results and the microstructure presented herein encourage designing new types of bioinspired superior synthetic materials.
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