Hypotension associated with spinal anesthesia for cesarean section is still a clinical problem. The role of crystalloid preloading to prevent hypotension associated with spinal anesthesia in parturients during cesarean section has been challenged. However, studies with crystalloids predict that fluid loading should be more efficacious if administered immediately after induction of spinal anesthesia. The effects of colloid loading after spinal anesthesia in cesarean section have not been studied enough. The aim of this study was to compare pre and co-loading of hetastarch for the prevention of hypotension following spinal anesthesia for cesarean delivery.Materials and Methods: This randomized clinical trial study was performed in 112 parturients (ASA I or II) undergoing elective cesarean section. Patients were randomly allocated to one of the two groups to receive rapid infusion of 500 ml of 6% hydroxyethylstarch (HES) before spinal anesthesia (preloading group, n=56), or rapid infusion of 500 ml of HES after induction of spinal anesthesia (co-loading group, n=56). The incidence of hypotension and the amount of vasopressor, (ephedrine 5 mg/mL+phenylephrine 25 micg/mL) were compared in the treatment of hypotension.Results: There was no significant difference in hypotension between the two groups (P=0.58). The preloading group used 2.2±1 ml of vasopressor mixture compared with 1.7±0.7 ml in the co-loading group (P=0.04) and the difference was significant.Conclusion: Colloid loading after induction of spinal anesthesia is as effective as preloading in reducing hypotension in cesarean section.

Background and Aim: A key factor in restoring the endodontically treated teeth is ferrule preparation. When the ferrule is absent, occlusal loads may cause the post or root to fracture. The purpose of this in vitro study was to evaluate the effect of ferrule preparation on fatigue resistance of teeth restored with quartz-fiber posts.Materials and Methods: Twenty single-rooted lower premolars having similar dimentions were randomly devided into two groups of 10. In control group the teeth were prepared with 3-mm of remaining coronal tooth structure and in test group teeth were prepared with 1-mm of remaining coronal tooth structure. The teeth were endodontically treated. 9-mm long Post holes were prepared, and D.T. light quartz-fiber(RTD,France) posts were cemented with Panavia F2(Kuraray,Japan).Then the core build up was done with Bisco core build up composite(Bisco,USA) and full metal crowns were cemented with Zinc phosphate(Harvard cement, Germany). All specimens were mounted in acrylic resin blocks and intermittently loaded (180 N) at an angulation of 45- degree to the long axis of the teeth at a frequency of 4 loads per second, until failure occurred.Results: There was significant difference between the loads cycles of two groups studied. (p<0.0001).Conclusion: The results of this study showed that an increased amount of coronal dentin significantly increases the fracture resistance of teeth restored with quartz-fiber posts.

The rock masses have naturally a large number of cracks and discontinuities which cause to be prone to damage under dynamic loading. The earthquake as a kind of dynamic loading may propagate the inherent cracks in rock masses and leads to failure of the rock structures such as mines, oil and gas wells, tunnels, dams, etc. Therefore, the fracture of rock masses due to cracks under earthquake should be considered by civil, mining and even mechanical engineers and researchers. Fracture mechanics as a branch of mechanical engineering science has been frequently employed for investigating the fracture behaviour of cracked rock structures. According to the orientation of crack relative to applied load, the cracked parts may be subjected to pure or combined mode loading I, II and III. The underground rock masses are often subjected to a compressive loading due to the pressure of upper rock masses. In the first sight, the crack flanks under compression are pressured together and the geometry discontinuity is vanished. However, the crack faces may be subjected to sliding loading and the vulnerability of cracked rock masses is still remained. Therefore, the fracture analysis of cracked rock masses under compressionshear loading should be investigated. Similar to the mixed mode I/II loading, there are several studies in the literature investigated the fracture of cracked specimens under Compression-shear loading both experimentally (Al-Shayea, 2005) and theoretically (Li et al., 2009). From the theoretical viewpoint, the compression is considered as a compressive stress in the stress field around the crack tip and then the fracture criteria based on new stress field is utilized for predicting the fracture resistance of cracked specimens. The aim of this paper is to present a new approach for predicting the fracture load of cracked rock samples under Compression-shear loading. The new approach is based on the maximum tangential stress (MTS) criterion which is one of the classical fracture criteria in fracture mechanics.

Liquefaction triggered by earthquakes is a source of damage to structures resting on loose to medium sandy soils. During earthquakes soil structures and the ground are subjected to vibratory excitations which are to a high degree erratic and extremely irregular. The magnitude of shear stress that is applied to a soil element in a real earthquake will, therefore, vary at random from one second to another. In order to evaluate the effect of such randomness, averaging procedures have been employed. In this averaging method a complex stress time history can be converted to an equivalent number of constant harmonic stress cycles. The equivalent load pattern thus determined has been considered as a representative load. This concept tends itself considerably to the simplification of the laboratory method. The soil selected for this thesis is known as Firouzkouh Sand No. 161, that exerted by load pattern that recorded in Kojaeli earthquake in 1999. Finally by using the stress averaging method, the reduction coefficient of stress is proposed and compared to those of existing studies and codes.

Background and Aims: Failure of bonding between artificial teeth and denture base material is a considerable problem for patients who wear dentures. According to the different impact of artificial teeth and different information about resistance force of mastication and also with deficiency in researchs, this study was designed to compare the bond strength of composite and acrylic artificial teeth to auto-polymerized denture base resins with and without cyclic loading.Materials and Methods: In this experimental and in vitro study, an acrylic resin auto-polymerized (Rapid Repair, Dentsply) and four artificial teeth (Acrylic Marjan new, Composite Glamour teeth and Ivoclar acrylic and composite teeth) were used. Therefore, 8 groups of 10 specimens each were evaluated. All specimens were thermocycled for 5000 cycles, in water baths between 5 and 55oC. Half the specimens in each group were treated with cyclic loading at 50N for 14, 400 cycles at 1.2 Hz.The shear bond strengths were measured using a Universal Testing Machine. Data were analyzed using Two-way ANOVA test.Results: Statistical analysis demonstrated no significant effect of cyclic loading on the shear bond strength, but the type of artificial tooth affected the shear bond strength (P=0.006). Also, the interaction between Cyclic loading and the type of artificial tooth showed no significant difference (P=0.98). Tukey test showed that acrylic teeth (Ivoclar) had statistically higher bond strength values than that of other teeth (PGlamour=0.02), (PComposite ivoclar=0.01) and (PMarjan new=0.02).Conclusion: Within the limitation of this study, the predominant type of fracture in all groups was cohesive, therefore the bond strength was adequate in all teeth and the type of artificial tooth may influence the bond strength of denture teeth to denture base resin. Cyclic loading had no significant effect on the bond strength of denture teeth to the auto-polymerized acrylic resin.

Background and Aim: Microgap in the implant-abutment interface is one of the main challenges in the treatment of two-piece implants. This study aimed to investigate the effect of two types of abutments (zirconia and titanium) on microgap at implant-abutment interface area under oblique cyclic loading in vitro. Methods and Materials In this in vitro study, 12 implant-abutment assemblies were used, each containing six sets with either zirconia or titanium abutments and vertically mounted in the modified resin blocks of auto-polymerized polyester base. The specimens were then subjected to oblique cyclic loading of 75 N at a 30 ± 2 degrees angle to the longitudinal axis of the implant with a frequency of 1 Hz in 500, 000 cycles, equivalent to 20 months of human mastication. For the microgap analysis, direct observation from the top was used with a scanning electron microscopy (SEM) with a magnification of 5000×. Statistical analysis was used to compare the microgap before and after the application of loading with the paired t-test. Results: The amount of microgap before force application in the titanium abutment group was (2. 6± 0. 7) significantly higher than the zirconia abutments (1. 9± 0. 5) (P = 0. 033). The dimension of the microgap in the titanium abutment group significantly decreased following cyclic loading (P = 0. 047), but in the zirconia group showed a significant increase (P = 0. 035). Finally, the dimension of the microgap following oblique cyclic loading in the titanium abutment group (2. 0± 0. 8) was not significantly different with the zirconia abutments (2. 7± 0. 9) (P = 0. 262). Conclusion: The difference of microgap after oblique cyclic loading between two types of titanium abutment and zirconium abutment is not significant, and both are clinically acceptable.

Summary Failure mechanism of rocks is one of the fundamental aspects to study rock engineering stability. The macroscopic deformation and failure of rock is a dynamic, gradual and cumulative process of nucleation, growth, penetration, coalescence of micro-cracks, which is a non-equilibrium, non-linear evolutionary process. In the present study, the effect of loading rate on rock failure mechanism was considered. For this purpose, some experimental tests were conducted on Brazilian disk specimens of a homogeneous and isotropic sandstone at six different loading rate (0. 3, 0. 6, 1. 2, 2. 4, 4. 8 and 9. 6 mm/min). During the tests, acoustic emission (AE) sensors were used to monitor the fracturing process. AE monitoring showed that micro-crack density induced by the applied loads during different stages of the failure processes increases as loading rate increases. Also, it is found that loading rate influences the mode of induced fracture, so that the number of tensile fractures decreases when loading rate increases.

The accurate prediction of crack initiation and growth in manufacturing processes is crucial for minimizing production costs and enhancing the reliability of components. This study focuses on integrated experimental investigation and fracture modeling approach for ductile metals, particularly addressing the mechanisms of ductile fracture and shear localization. The importance of establishing robust damage criteria for accurate reliable numerical simulations cannot be denied. Current literature reveals a significant lack of data on shear and ductile fracture criteria for materials like stainless steel alloy 304. To address this gap, a series of experimental tests was conducted to extract the necessary coefficients for these criteria. Various sample geometries were analyzed to investigate the effects of different triaxiality stress states and loading rates on fracture initiation. The triaxiality stress states were chosen within a range of 0.2 to 2 and strain rates were applied at values of 0.02 s-1, 4.5 s-1, and 30 s-1. A set of coefficients for modeling ductile and shear fracture was derived, taking into account the effects of loading rate and orientation. This research not only provides critical coefficients for fracture modeling but also supports the optimization of manufacturing processes in the automotive industry and other sectors, ultimately contributing to improved material performance and component reliability