Ronald L. Poveda

Tilted and Short Implants Supporting Fixed Prosthesis in an Atrophic Maxilla: A 3D‐FEA Biomechanical Evaluation

PURPOSE This study compared the biomechanical behavior of tilted long implant and vertical short implants to support fixed prosthesis in an atrophic maxilla. MATERIALS AND METHODS The maxilla model was built based on a tomographic image of the patient. Implant models were based on micro-computer tomography imaging of implants. The different configurations considered were M4S, four vertical anterior implants; M4T, two mesial vertical implants and two distal tilted (45°) implants in the anterior region of the maxilla; and M6S, four vertical anterior implants and two vertical posterior implants. Numerical simulation was carried out under bilateral 150 N loads applied in the cantilever region in axial (L1) and oblique (45°) (L2) direction. Bone was analyzed using the maximum and minimum principal stress (σmax and σmin ), and von Mises stress (σvM ) assessments. Implants were analyzed using the σvM . RESULTS The higher σmax was observed at: M4T, followed by M6S/L1, M6S/L2, M4S/L2, and M4S/L1 and the higher σvM : M4T/L1, M4T/L2 and M4S/L2, M6S/L2, M4S/L1, and M6S/L1. CONCLUSIONS The presence of distal tilted (all-on-four) and distal short implants (all-on-six) resulted in higher stresses in both situations in the maxillary bone in comparison to the presence of vertical implants (all-on-four).

Carbon Nanofibers: Structure and Fabrication

In this chapter, vapor-grown and electrospun carbon nanofibers (CNFs) are emphasized. Fabrication processes and surface modification methods for CNFs are presented. Microstructure of CNFs is discussed based on the reported observations in various studies. CNFs have a complex structure compared to the structure of carbon nanotubes . The orientation of carbon layers in CNFs affects their mechanical properties. Experimental analyses and resulting trends are discussed from various published works, such as studies that investigate the tensile and flexural properties of individual CNFs. The range of measured properties is rather wide, which is likely due to the difference in the structure of the fibers that were tested and the presence of defects. Molecular dynamic simulation results on single nanofibers are also presented to understand their potential of reinforcing composites.

Dynamic Mechanical Analysis of CNF/Polymer Composites

The viscoelastic properties of carbon nanofiber (CNF) reinforced polymer matrix composites are examined in this chapter. Temperature-dependent variations in the properties from subzero to above glass transition temperature (T g ) is studied. Different compositions of CNF/polymer composites are evaluated for parameters such as storage modulus , loss modulus , and damping parameter using a dynamic mechanical analyzer. The maximum use temperature and T g of such composites are also determined. The ability to tailor the properties by means of CNFs and improve the stability of reinforced polymer composites at high temperatures is important for aerospace applications that handle mechanical load under thermal extremes.

Post-impact residual high strain rate compressive properties of carbon fiber laminates

The purpose of the present study is to investigate the residual high strain rate compressive properties of cross-ply carbon fiber laminates after drop-weight impact. The tested laminate is divided into four quadrants and specimens are extracted from each quadrant for high strain rate compression testing using a split–Hopkinson pressure bar. The testing is conducted in the strain rate range of 1000–1450 s−1. A comparison between pre- and post-impact carbon fiber laminates subjected to high strain rate compression suggested that there is a maximum decrease of 76.1% and 71.6% in the peak stress and absorbed energy of the laminate, respectively, at high strain rate compression. Microscopy was conducted on the failed specimens to determine the failure mode. Extensive delamination observed at the impact point causes significant decrease in compressive strength and energy absorption of the laminate.

Weight-saving potential of open- and closed-cell functionally graded foams under compressive loading

The weight-saving potential of open- and closed-cell functionally graded foams in structural applications is studied. Optimisation of material microstructures can lead to the design of lightweight foams that can effectively withstand applied loads and mitigate damage. A tetrakaidecahedron-shaped cell, which packs to fill space in three dimensions, is used to create open- and closed-cell foam models. Four functionally graded models and a plain foam model, all containing three vertically stacked cells, are studied for both open- and closed-cell foams. A density gradient is applied along the axial direction of the structures. The relative stiffness per unit mass for the closed-cell foams is found to be several orders of magnitude higher compared with that of the open-cell foams. The relative stiffness per unit mass is observed to change more rapidly for the open-cell foams than the closed-cell foams as the gradient decreases. This indicates retention of specific stiffness for closed-cell foams over a wide spectrum of density gradients. This study demonstrates the weight-saving potential of functionally graded foams in designing damage-tolerant structures and helps in optimising the geometrical parameters of foams for obtaining the desired set of properties.

Mechanical Properties of CNF/Polymer Composites

This chapter discusses the effect of composition of CNF reinforced nanocomposites on their mechanical properties measured under tensile, compressive, and flexural loading. The structure of the CNFs plays an important role in determining the reinforcement efficiency. In most cases, random CNF dispersed nancomposites have been studied. Thermoplastic resin, thermosetting resin, and elastomer matrix nanocomposites have been studied. Polypropylene is the most common thermoplastic resin that is reinforced with CNFs. Among thermosets, epoxy and vinyl ester resin matrix nanocomposites are studied. The strength and stiffness improve, depending on the volume fractions of CNF dispersed within several different polymer matrices. It is noted that compared to the mechanical properties of single CNFs reported in Chap. 2, the level of enhancement of mechanical properties of nanocomposites is only moderate. The bending of long aspect ratio fibers and stacked-cup structures are potential reasons for this outcome. Nevertheless, combined with other properties, such as electrical and thermal conductivity increase in otherwise insulating resins, the moderately enhanced nanocomposites can develop new applications.

Electrical Properties of CNF/Polymer Composites

The electrical properties of carbon nanofiber (CNF) reinforced epoxy matrix nanocomposites are summarized in this chapter. The effect of CNF weight fraction on the conductivity, impedance , and dielectric constant is considered. The results show that the impedance decreases and the dielectric constant increases with increasing CNF content in the composites. Nanocomposites containing 10 wt% CNFs showed significantly higher dielectric constant because of the presence of a continuous network of CNFs in the composite. The resistance, capacitance, and dielectric constants of the neat resin are found to be lower at the test frequency of 105 Hz compared to the values measured at 1 Hz. The dielectric constant of CNF/epoxy composites increases monotonically from 12.4 to 16.1 with increasing CNF content from 1 to 10 wt%.

Thermal Expansion of CNF/Polymer Composites

Improved dimensional stability of composites is desired in applications where they are exposed to varying temperature conditions. Polymers inherently have a high coefficient of thermal expansion (CTE) , which complicates their inclusion in structural designs exposed to temperature extremes. However, through low-CTE reinforcements dispersed in a polymeric matrix, the CTE of the composite can be lowered to a degree suitable for use in service. This chapter aims at summarizing the studies that analyze the effect of vapor-grown carbon nanofibers (CNFs) on the thermal expansion behavior of polymer matrix composites. CNFs typically have a significantly lower CTE than epoxy resins and other widely used polymer matrices, which result in composites with increased dimensional stability as the dispersed CNF content is increased. The use of nanofibers has resulted in the ability to tailor the thermal expansion of the composite over a wide range. Schapery’s model is used to estimate the upper and lower bounds on the CTE of CNF/epoxy nanocomposites. The experimental results are found to be within the bounds.

CNF Reinforced Multiscale Composites

Given the advantages of utilizing CNFs in polymers for improvement in mechanical properties, current research has progressed to the reinforcement of composites with three or more phases in order to elicit specified composite property enhancements and support tailorability and multifunctionality within composite systems. Since there are several possible categories in which to classify such composites, the following sections will focus on two types of composites that have been prominent in the literature: (1) particle reinforced composites with CNFs and (2) continuous fiber composite laminates with CNFs. In this chapter, the processing methods used to enhance the mechanical properties of the composite are discussed first, and then the studies involving composites containing polymer matrices, CNFs, and additional phases are reviewed.

Environmental Effects on CNF/Polymer Composites

A crucial factor that affects all composite materials exposed to typical service conditions is environmental degradation . It has been shown that over time the combination of moisture and fluctuating temperatures degrades the structure of polymeric composites through chemical and morphological degradation mechanisms. Experimental investigations have not only demonstrated degradation of the polymeric matrix but also of the CNF–matrix interface due to surface morphology variation caused by weathering. This chapter aims to focus on the moisture effects on the structure and mechanical properties of CNF/polymer composites in an effort to offer insight into the mechanisms of degradation and failure under load after moisture exposure. However, information on this aspect of CNF/polymer composites is in the nascent stage and extensive future work is required.