Multi-Objective Optimization of Support Structures for Metal Additive Manufacturing

Electron-beam melting (EBM) is a rapidly developing metal additive manufacturing (AM) method. It is more effective with complex and customized parts manufactured in low volumes. In contrast to traditional manufacturing it offers reduced lead time and efficient material management. However, this technology has difficulties with regard to the construction of overhang structures. Production of overhangs using EBM without support structures results in distorted objects, and the addition of a support structure increases the material consumption and necessitates post-processing. The objective of this study was to design support structures for metal AM that are easy to remove and consume lower support material without affecting the quality of the part. The design of experiment methodology was incorporated to evaluate the support parameters. The multi-objective optimization minimizing support volume, support removal time along with constrained deformation was performed using multi objective genetic algorithm (MOGA-II). The optimal solution was characterized by a large tooth height (4 mm), large tooth base interval (4 mm), large fragmented separation width (2.5 mm), high beam current (6 mm), and low beam scan speed (1200 mm/s).

Radular morphology by using SEM in Pugilina cochlidium (Gastropoda: Melongenidae) populations, from Thondi coast-Palk Bay in Tamil Nadu-South East coast of India

Radula in all melongeninae species is rather uniform and characterized by bicuspid lateral teeth with strongly curved cusps and sub rectangular rachidians, bearing usually 3 cusps. The aim of the present study was to describe the radula of 2 Pugilina cochlidium populations using SEM. The radula in 2 species proves itself as a rachiglossate type showing the radular formula of 1 + R + 1. The first population hasthe central tooth wide with sharp cusps equal in length, emanate from posterior margin of tooth base. The lateral teeth have 2 cusps and are long, sharp, pointed and bent towards the rachidian tooth. Whereas the second population, the central tooth is narrow with sharp cusps equal in length, emanate from posterior margin of tooth base. The lateral teeth have 2 cusps and are broad, longer, sharp, pointed and bent towards the rachidian tooth. They are typically sickle shaped with broad strong base. In both populations the rachidian tooth is subquadrate with 3 big cusps in the middle, but in the second population the base of the rachidian is concave while in the first population it is straight. In the present study the median rachidian of the second population, has a broad basal region when compared to first. This similar observation has been made in Chicoreus virgineus ponderosus and Siratus virgineus ponderosus. In the present study, since 2 populations exhibit the same generalized rachiglossate pattern it does not offer much scope for systematic diagnosis below generic level.

Reconstructing the archosaur radiation using a Middle Triassic archosauriform tooth assemblage from Tanzania

Following the Permo–Triassic mass extinction, Archosauriformes—the clade that includes crocodylians, birds, and their extinct relatives outside crown Archosauria—rapidly diversified into many distinct lineages, became distributed globally, and, by the Late Triassic, filled a wide array of resource zones. Current scenarios of archosauriform evolution are ambiguous with respect to whether their taxonomic diversification in the Early–Middle Triassic coincided with the initial evolution of dietary specializations that were present by the Late Triassic or if their ecological disparity arose sometime after lineage diversification. Late Triassic archosauriform dietary specialization is recorded by morphological divergence from the plesiomorphic archosauriform tooth condition (laterally-compressed crowns with serrated carinae and a generally triangular lateral profile). Unfortunately, the roots of this diversification are poorly documented, with few known Early­–Middle Triassic tooth assemblages, limiting characterizations of morphological diversity during this critical, early period in archosaur evolution. Recent fieldwork (2007–2017) in the Middle Triassic Manda Beds of the Ruhuhu Basin, Tanzania, recovered a tooth assemblage that provides a window into this poorly sampled interval. To investigate the taxonomic composition of that collection, we built a dataset of continuous quantitative and discrete morphological characters based on in situ teeth of known taxonomic status (e.g., Nundasuchus, Parringtonia: N = 65) and a sample of isolated teeth (N = 31). Using crown heights from known taxa to predict tooth base ratio (= base length/width), we created a quantitative morphospace for the tooth assemblage. The majority of isolated, unassigned teeth fall within a region of morphospace shared by several taxa from the Manda Beds (e.g., Nundasuchus, Parringtonia); two isolated teeth fall exclusively within a “Pallisteria” morphospace. A non-metric multidimensional scaling ordination (N = 67) of 11 binary characters reduced overlap between species. The majority of the isolated teeth from the Manda assemblage fall within the Nundasuchus morphospace. This indicates these teeth are plesiomorphic for archosauriforms as Nundasuchus exhibits the predicted plesiomorphic condition of archosauriform teeth. Our model shows that the conservative tooth morphologies of archosauriforms can be differentiated and assigned to species and/or genus, rendering the model useful for identifying isolated teeth. The large overlap in tooth shape among the species present and their overall similarity indicates that dietary specialization lagged behind species diversification in archosauriforms from the Manda Beds, a pattern predicted by Simpson’s “adaptive zones” model. Although applied to a single geographic region, our methods offer a promising means to reconstruct ecological radiations and are readily transferable across a broad range of vertebrate taxa throughout Earth history.

A Comparison of Shear Bond Strength of Ceramic and Resin Denture Teeth on Different Acrylic Resin Bases

The purpose of this study is to compare the shear bond strength of different resin bases and artificial teeth made of ceramic or acrylic resin materials and whether tooth-base interface may be treated with aluminium oxide sandblasting. Experimental measurements were carried on 80 specimens consisting of a cylinder of acrylic resin into which a single tooth is inserted. An ad hoc metallic frame was realized to measure the shear bond strength at the tooth-base interface. A complete factorial plan was designed and a three-way ANalysis Of VAriance (ANOVA) was carried out to investigate if shear bond strength is affected by the following factors: (i) tooth material (ceramic or resin); (ii) base material (self-curing or thermal-curing resin); (iii) presence or absence of aluminium oxide sandblasting treatment at the tooth-base interface. Tukey post hoc test was also conducted to evaluate any statistically significant difference between shear strength values measured for the dif-ferently prepared samples. It was found from ANOVA that the above mentioned factors all affect shear strength. Furthermore, post hoc analysis indi-cated that there are statistically significant differences (p-value=0.000) between measured shear strength values for: (i) teeth made of ceramic material vs. teeth made of acrylic resin material; (ii) bases made of self-curing resin vs. thermal-curing resin; (iii) specimens treated with aluminium oxide sandblasting vs. untreated specimens. Shear strength values measured for acryl-ic resin teeth were on average 70% higher than those measured for ceramic teeth. The shear bond strength was maximized by preparing samples with thermal-curing resin bases and resin teeth submitted to aluminium oxide sandblasting.

Evaluation of the artificial tooth and polymer-base bond in removable dentures

Introduction: The most commonly used materials for prosthetic denture base are acrylic resins - polymethyl methacrylate, which has all the important physical and chemical properties. The bond between a denture base and an acrylic tooth has a significant functional value. Aim: To evaluate the influence of various procedures on bonding quality between the denture base and the artificial tooth, the measurement of gaps when connecting the tooth and the denture base using an optical or electron microscope, and evaluating the mechanical properties by compressive testing. Materials and methods: Forty eight acrylic models with bonds between acrylic teeth and prosthetic denturebases, and 12 models for compressive testing were created as the materials in this in vitro study. The acrylic models were allocated to four groups (with 12 samples) for microscopy, and three groups for compressive testing. The samples for microscopy contained denture bases with fixed, unprocessed acrylic teeth, mechanically processed acrylic teeth, chemically processed acrylic teeth, and mechanically and chemically processed acrylic teeth, respectively. The acrylic models had the same dimensions, and the surface of each model was prepared using a slow speed saw, abrasive diamond paste, and special paper grinders. A special metal cast was created for compressive testing with internal dimensions of 20 x 9 mm. The size of a gap was measured on the light inverse microscope (Nikon Epiphot 300) and pressure testing was performed on a Zwick/Roell Z010 pressure machine. Results: The average value of the gap on mechanically and chemically treated samples was 68.250 ?m, which was significantly lower than the gap on the untreated samples with the average value of 103.75 ?m. The compressive strength was 3200 N/mm2 on untreated samples and above 6000 N/mm2 on the mechanically and chemically treated ones. Conclusions: It can be concluded that surfaces which come into contact must be mechanically and chemically processed. Although in practice prostheses are made using mechanical tooth-base retention, larger gaps occur when compared to mechanically and chemically processed surfaces.

Express Model for Load Sharing and Stress Analysis in Helical Gears

The performance of a gear set is strongly influenced by the manufacturing and assembly quality. Therefore, detailed analyses at the design stage, where the effects of expected assembly and manufacturing errors can be simulated, are crucial. At an early design stage, when contact conditions are addressed, the widely used finite element method (FEM) may still result in unwanted computing time. The paper presents an Express model developed to serve as a fast design tool offering fine simulation and a high precision level. The model establishes load sharing, fillet stresses and pressure distribution along the contacting surfaces of meshing helical gear teeth. The calculations combine the finite strip method with a pseudo-three-dimensional (3D) model of the tooth base solved with finite differences to calculate tooth bending deflexion and fillet stresses. The accuracy of the procedure is demonstrated through 3D FEM models. A contact cell discretization completes the model. This very fast and accurate approach gives the contact pressure distributions resulting from the roll-slide motion of mating teeth. An analysis of a helical gear set in two different assembly positions reveals the effects of edge contact, and exhibits the influence of tooth stiffness reduction near tooth corners.

Dentitions of Barbclabornia (new genus, Chondrichthyes: Xenacanthiformes) from the Upper Palaeozoic of North America

<i>Barbclabornia luedersensis</i> (Berman, 1970) is defined on the basis of small (2 mm high) isolated teeth that lack an intermediate cusp. It is known from the Lower Permian and possibly the Upper Pennsylvanian of North America. The two principal cusps are slightly curved orally, nearly parallel, and bear cristae mainly on their distal halves. They are cylindrical but become compressed proximally. The long axis of each cusp base is >45° to the labial margin of the tooth base. The base bears a prominent apical button in contact with the cusps; a central foramen is absent. Fewer than ten foramina occur on the aboral surface of the base, which bears a prominent concave basal tubercle. The shape of the base ranges from somewhat triangular to quadrangular. The cusps are composed of orthodentine covered by hypermineralized pallial dentine; the base is composed of orthodentine but may also contain trabecular dentine. Except for the possible occurrence of symphysial teeth, the dentition is homodont. <i>Barbclabornia</i> cf. <i>B. luedersensis</i> is stratigraphically highest in the known range of the genus and is restricted to the nearly lowermost part of the Clear Fork Group (Artinskian) of Texas. The teeth are similar to <i>B. luedersensis</i>, but are more robust and have a quadrangular-shaped base. <i>Barbclabornia</i> was large, based on an undescribed palatoquadrate some 45 cm long. It was probably freshwater and is most closely related to <i>Triodus</i>. <br><br> <i>Barbclabornia luedersensis</i> (Berman, 1970) wird anhand kleiner (2 mm Höhe), isolierter Zähne definiert, denen eine Medianspitze fehlt. Die Art ist aus dem unteren Perm und möglicherweise dem oberen Pennsylvanium von Nordamerika bekannt. Die zwei Hauptspitzen sind schwach oral geneigt, beinahe parallel und zeigen hauptsächlich auf der distalen Hälfte Cristae. Die Spitzen sind zylindrisch, erhalten allerdings proximal durch Kompression einen ovalen Umriss. Der Winkel zwischen der Längsachse jeder Spitze und dem labialen Rand der Zahnbasis beträgt mehr als 45°. Die Basis besitzt einen prominenten, in Kontakt mit den Spitzen stehenden Apicalhöcker, ein zentrales Foramen fehlt. Weniger als zehn Foramina sowie ein prominenter, konkav eingebuchteter Basaltuberkel befinden sich auf der aboralen Seite der Basis. Die Basis zeigt einen etwa tri- bis quadrangulären Umriss. Die Spitzen bestehen aus Orthodentin, ummantelt von einem hypermineralisierten Pallialdentin; die Basis besteht aus Orthodentin, kann aber auch Trabeculardentin beinhalten. Mit Ausnahme des Auftretens von Symphysenzähnen ist die Bezahnung homodent. <i>Barbclabornia</i> cf. <i>B. luedersensis</i> repräsentiert den stratigraphisch jüngsten Vertreter dieser Gattung und ist etwa auf den untersten Abschnitt der Clear Fork Group (Artinskium) von Texas beschränkt. Die Zähne ähneln denen des <i>B. luedersensis</i>, sind aber robuster und besitzen eine Basis mit quadrangulärem Umriss. Ein unbeschriebenes Palatoquadratum von etwa 45 cm Länge weist <i>Barbclabornia</i> als großwüchsig aus. Es handelt sich wahrscheinlich um eine Süßwasserform. Barbclabornia besitzt enge Beziehungen zu <i>Triodus</i>. <br><br> doi:<a href="http://dx.doi.org/10.1002/mmng.20030060106" target="_blank">10.1002/mmng.20030060106</a>

Dynamic Analysis of Gear Meshing Teeth Using Modified Timoshenko Beam Theory

An explicit dynamical formulation of the gear meshing teeth using modified Timoshenko beam theory is presented in this paper. The acting position direction and magnitude of the external force is assumed time variant. The meshing tooth is modeled as a cantilever beam where the inertia forces due to the large rotation of the tooth base, as well as the external equivalent axial force and moment are all included in the equations of motion. Computer algorithms for gear dynamics based on the theory developed is presented. In the numerical simulations, the involute for the gear tooth profile is considered.

Structure, morphology, composition and organization of biogenic minerals in limpet teeth

The structure, morphology, composition, and organization of inorganic solids in the radula teeth of the limpet Patella vulgata have been studied by electron microscopy, electron diffraction, and e. d. X. a. of fractured, acid-treated, and sectioned tissue. Minerals first appear in the tooth base and comprise: amorphous and poorly crystalline granular, particulate, and sheet-like phases of variable composition (Fe, Si, P, Ca); irregular laths of crystalline goethite; and single crystals of prismatic goethite. The presence of localized Si and P may inhibit goethite crystallization in many regions of the tooth base. Mineralization of the tooth cusp begins with goethite impregnation of the posterior region. Crystals are deposited in the form of thin fibrous strands (15–20 nm width) with the [001] crystallographic axis initially parallel to the posterior tooth wall. Miner­alization proceeds by an increase in the number and thickness of the crystals within the posterior region. In contrast, the anterior zone is only partly impregnated with crystals aligned parallel to the long axis of the cusp. The mature crystals are well ordered, acicular in morphology but with extensive growth distortions, and organized along regularly interspaced (30–50 nm) electron-dense filaments within the cusp. Removal of iron reveals the presence of silica-impregnated fibres, folded sheets, and tubular structures (often 30–60 nm in diameter) within essentially intact teeth. We propose that goethite crystallization and organization is regulated, in part, by spatial constraints established by an ordered filamentous organic matrix and that silica impregnates the matrix components at a later stage in mineralization thus maintaining the structural integrity of the organic tissue.