The Effect of Silica Particle Size on the Mechanical Enhancement of Polymer Nanocomposites.

In the present work, SiO2micro/nanocomposites based on poly-lactic acid (PLA) and an epoxy resin were prepared and experimentally studied. The silica particles were of varying sizes from the nano to micro scale at the same loading. The mechanical and thermomechanical performance, in terms of dynamic mechanical analysis, of the composites prepared was studied in combination with scanning electron microscopy (SEM). Finite element analysis (FEA) has been performed to analyze the Young's modulus of the composites. A comparison with the results of a well-known analytical model, taking into account the filler's size and the presence of interphase, was also performed. The general trend is that the reinforcement is higher for the nanosized particles, but it is important to conduct supplementary studies on the combined effect of the matrix type, the size of the nanoparticles, and the dispersion quality. A significant mechanical enhancement was obtained, particularly in the Resin/based nanocomposites.

Mechanical Testing and Modeling of the Time-Temperature Superposition Response in Hybrid Fiber Reinforced Composites.

The purpose of this study was to manufacture hybrid composites from fabrics with superior ballistic performance, and to analyze their viscoelastic and mechanical response. Therefore, composites in hybrid lay-up modes were manufactured from Vectran, Kevlar and aluminum fiber-woven fabrics through a vacuum assisted resin transfer molding. The specimens were consequently analyzed using static three-point bending, as well as by dynamic mechanical analysis (DMA). Apart from DMA, time-temperature superposition (TTS) analysis was performed by all available models. It was possible to study the intrinsic viscoelastic behavior of hybrid ballistic laminates, with TTS analysis gained from creep testing. A polynomic mathematical function was proposed to provide a high accuracy for TTS curves, when shifting out of the linearity regimes is required. The usual Williams-Landel-Ferry and Arrhenius models proved not useful in order to describe and model the shift factors of the acquired curves. In terms of static results, the highly nonlinear stress-strain curve of both composites was obvious, whereas the differential mechanism of failure in relation to stress absorption, at each stage of deformation, was studied. SEM fractography revealed that hybrid specimens with Kevlar plies are prone to tensile side failure, whereas the hybrid specimens with Vectran plies exhibited high performance on the tensile side of the specimens in three-point bending, leading to compressive failure owing to the high stress retained at higher strains after the maximum bending strength was reached.

Finite element simulation of the mechanical impact of computer work on the carpal tunnel syndrome.

Carpal tunnel syndrome (CTS) is a clinical disorder resulting from the compression of the median nerve. The available evidence regarding the association between computer use and CTS is controversial. There is some evidence that computer mouse or keyboard work, or both are associated with the development of CTS. Despite the availability of pressure measurements in the carpal tunnel during computer work (exposure to keyboard or mouse) there are no available data to support a direct effect of the increased intracarpal canal pressure on the median nerve. This study presents an attempt to simulate the direct effects of computer work on the whole carpal area section using finite element analysis. A finite element mesh was produced from computerized tomography scans of the carpal area, involving all tissues present in the carpal tunnel. Two loading scenarios were applied on these models based on biomechanical data measured during computer work. It was found that mouse work can produce large deformation fields on the median nerve region. Also, the high stressing effect of the carpal ligament was verified. Keyboard work produced considerable and heterogeneous elongations along the longitudinal axis of the median nerve. Our study provides evidence that increased intracarpal canal pressures caused by awkward wrist postures imposed during computer work were associated directly with deformation of the median nerve. Despite the limitations of the present study the findings could be considered as a contribution to the understanding of the development of CTS due to exposure to computer work.

Dynamic mechanical properties of a maxillofacial silicone elastomer incorporating a ZnO additive: the effect of artificial aging.

The main objective of the current study was to investigate the dynamic mechanical properties of a room-temperature vulcanizing silicone incorporating different fractions of zinc oxide (ZnO) after indoor and outdoor photoaging. Forty-eight samples were produced by adding different amounts of ZnO into a commercial maxillofacial silicone (EPISIL-E). The samples were divided into 4 groups containing 0.0, 0.2, 0.5, and 1 wt% ZnO additive, respectively. Samples were exposed to sunlight (subgroup 2), ultraviolet (subgroup 3), and fluorescence (subgroup 4) aging, whereas nonaged samples comprised the control subgroup (subgroup 1). Dynamic mechanical analysis was used to determine the storage modulus (E'), loss modulus (E″), and damping capacity (tanδ). General linear statistic model was conducted to evaluate the effects of aging, testing frequency, and composition on the dynamic mechanical properties of the silicone with the ZnO additive. Post hoc analysis was performed using Tukey test. Statistical analysis revealed a significant impact of composition on tanδ (P < 0.05). Aging influenced E' and E″ (P < 0.01). The combination of aging and composition had a significant effect on all dynamic properties (P < 0.01).

Experimental and numerical determination of the mechanical response of teeth with reinforced posts.

The aim of this study was to evaluate the mechanical behavior of endodontically treated teeth restored with fiber reinforced composite posts versus titanium posts, by both experimental testing and numerical simulation (finite element analysis (FEA)). Forty maxillary central incisors were endodontically treated to a size 45 file and then obturated using gutta-percha points and sealer with the lateral condensation technique. The teeth were divided into four groups of ten teeth each. All the posts were of similar dimensions. The first group was restored using carbon fiber reinforced posts (CB), the second and third groups were restored using glass fiber reinforced posts (DP and FW, respectively), and the fourth group (control group) was restored using conventional titanium posts (PP). Half of the specimens of every group were submitted to hydrothermal cycling (2000 cycles, at 5 °C and 55 °C, respectively). All specimens were loaded until failure at a 45° angle with respect to the longitudinal axis at a cross head speed of 0.5 mm min(-1). A two-dimensional finite element model was designed in order to simulate the experimentally obtained results. Mechanical testing revealed that teeth restored with titanium posts exhibited the highest fracture strength. Debonding of the core was the main failure mode observed in glass fiber posts, whereas vertical root fractures were observed in the titanium posts. FEA revealed that the maximum stresses were developed at the interface between the post, dentin and the composite core critical regions in all three cases. Hydrothermal cycling had no significant effect on the fracture behavior of fiber reinforced composite posts.

Viscoelastic property mapping along encrusted polymeric urinary catheters.

PURPOSE: Mapping of the material viscoelastic property variation along the length axis of polymeric stents. MATERIALS AND METHODS: Five pigtail ureteral stents and five percutaneous stents placed in vivo for different periods were studied. Viscoelastic property changes along the length of polymeric stents were measured using the dynamic mechanical analysis technique. The type of encrustations was identified using FT-IR spectroscopy and their morphology by scanning electron microscopy (SEM). RESULTS: Variations in viscoelastic stiffness were confirmed for both types of stents. In a few cases, the variation was quite large (400%). Various encrustation types and/or organic sheathing of the salt were found to be responsible for this effect. In detail, calcium oxalate monohydrate (COM) plaques for percutaneous and COM/organic layer for pigtail stents were recognized by FT-IR spectroscopy and SEM observation to invoke reinforcing effects along the stent axes. CONCLUSIONS: The phenomenon of stiffness variation along the stent axes requires further study to understand the mechanism behind it and consequently to improve the biomaterial and design of specific areas of stents as well as to avoid encrustations. This further research might lead in the near future to the development of new types of polymer stents for drug-eluting specialized areas.

Early failure of a zirconia femoral head prosthesis: fracture or fatigue?

BACKGROUND: Zirconium ceramics are one of the most controversial prosthetic biomaterials with good mechanical properties on the one hand, but early failure incidents on the other hand. A case of an early failure of zirconia femoral head in a 73-year-old clergy man, who underwent a total hip arthroplasty 28 months before, is reported. METHOD: The retrieved femoral head fragments, during revision surgery, were studied using scanning electron microscopy. FINDINGS: Although a randomly occurred fracture without biomechanical significance can be a possible explanation, the collected data give evidence that initial non-critical microcracks or crazes due to contact stresses or strain mismatches or both between the ceramic head and the metallic tapered neck of the prosthesis led to the fatigue of the material. The final outcome was a catastrophic fracture of the head. INTERPRETATION: Fixation of the neck into the zirconia head, with no mismatching or stress differences, would solve the fatigue-induced head destruction.