Dr. Jeffery Touchet joined The Joint Chiropractic team of local chiropractors in 2015 with his arrival at the Lexington clinic location. He earned his doctorate of chiropractic from Life College in Marietta, Ga.
The strength of a porcelain material was largely determined by its surface roughness and the inner structure of the material may cause a larger stress concentration than that caused by the surface roughness in combination with the surface flaws present on the material.[33] If the material is given an adequate surface treatment, it will not require properties that stop cracking and the surface of the material would remain smooth, which in turn results in a restoration that will be long lasting.
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In general, ceramic strength is limited by the size and distribution of an inherent flaw population. Fracture may occur without measurable plastic deformation and failure can also start from small flaws prior to plastic deformation. This fact is expressed by a low resistance against crack extension, which is characterized by the parameter fracture toughness Kc.[43] Combination of bending and torsion forces produce surface flaws in ceramics and once critical dimensions are reached, the fracture occurs. Numerous studies have shown that catastrophic failure may occur far below the short-time fracture strength due to a slow growth of a subcritical crack up to the critical crack length.[44,45] This indicates that the strength degradation is measured during a period of a lifetime.[46] Slow crack growth is strongly influenced by the amount and composition of a glass phase in the ceramic microstructure[47] and the deleterious effect of slow crack propagation may be attributed to the stress-enhanced chemical reaction occurring in the presence of water vapor at a crack tip. This occurs preferentially in silicate base glasses resulting in bond rupture.[48] Studies indicate that even moisture levels of 0.017% may cause stress corrosion.[49] Charles[50] explained that cracks present in ceramics tend to grow at a slow rate first under the influence of stress. This slow growth of cracks continues until the intensity of stress reaches a critical value for a particular ceramic restorative material.
Several factors including powder compaction, process of forming, firing and also shaping can also cause flaws in ceramics. During these laboratory processes, the flaws may become inherited in the micro structure of the ceramic. Damage caused during grinding; pull-out caused during polishing, micro-porosity present on the subsurface and the introduction of large pores by technicians during restoration manufacture are common technical laboratory flaws.[20,51] Other flaws may be inherent which include cracking around grains with thermal expansion and porosities, which are developed during the process of ceramic firing.[52]
The failure of many materials, including ceramics, is attributed to the propagation of densely distributed cracks, rather than to a single precisely defined the fracture.[53] The number of cracks and micro cracks is extremely large, and their location and orientation are random. Irwin[54] demonstrated that stress intensity is related to a crack shape in a particular location with respect to the loading geometry. The finishing procedures influence the existence of micro cracks and residual stress. For example, glazing could round the crack tip of possible micro crack and these changes in length and tip would in turn change the strength of the material. Surface roughness will lead to a non-uniform stress distribution and concentrate locally an applied stress due to the shape differences in the surface layer.[33] Due to the presence of surface roughness, the developed cracks may not propagate randomly, but occur or propagate at points with higher stress. The theory that initiation of cracks starts at stress concentration points caused by surface roughness was given by Mecholsky et al.[55] who loaded samples with grinding grooves and gouges both perpendicular and parallel to the loading direction.
It may be well-known that less plaque accumulates on ceramic or porcelain restorations; a rough surface accelerates plaque accumulation.[88] Increased amount of plaque on the rough surfaces of ceramics will exert not only caries-causing virulence, but also a harmful influence on periodontal tissue. For a full-coverage crown or a bridge, caries incidence risk would be slight, but instead much attention has to be given to the gingival tissues. Kawai et al.,[88] concluded that more plaque was adhered over glazed surfaces of ceramics as compared with their polished surfaces. This means that a glazed surface would not be clinically acceptable from a biologic point of view. Glazing can produce an undulating and rough surface that, usually, has irregularities, inducing more adhesion of bacteria and other substances. Rashid[27] also concluded that glazed surfaces are rougher as compared to the polished surfaces. Although polished surfaces have been reported to have voids and micro cracks on the subsurface of porcelain,[89] these superficial defects did not contribute to the Average Roughness (Ra) values or the amount of plaque adhesion. Contrary to other reports, polishing with diamond paste is helpful for obtaining a smoother surface that will prevent plaque from accumulating.
FT showed the highest fluoride content at the surface of the material. The lowest amounts of fluoride ions were detected at the surfaces of the FT disks stored at low pH environments, and this difference was statistically significant (p FVIII>KN>FII>FIX). SEM analysis of the surface morphology revealed numerous voids, cracks and microporosities in all experimental groups, except for KN and HSF. More homogenous material structure with more discrete cracks was observed in samples stored at neutral pH environment, compared to disks stored in acidic solutions.
The surface of each disk was rinsed with 2 ml of deionized water. The specimens were mounted on aluminium stubs, sputter-coated with gold (Bal-Tec SCD 005 Sputter Coater, Bal-Tec AG, Balzers, Liechtenstein), and then examined using scanning electron microscope (Jeol JSM-6460LV, Jeol Industries Ltd., Tokyo, Japan) equipped with energy dispersive spectrometer (SEM/EDS). In each disk, SEM/EDS analysis was performed in three randomly selected spots. Quantitative changes of the material surface and the recharge ability after the NaF treatment were evaluated using EDS. SEM was used to determine the effects of different storage media and different pH environments on morphological characteristics of cured cement disks. The criteria for evaluation included the presence of cracks and micropores at the surface of the material.
SEM showed morphological differences in all materials, in the samples stored at pH 2.5, pH 5.5 and in saline (figure 1). More homogenous material structure with more discrete cracks was observed in samples stored at neutral pH environment. Destruction of the material surface was evident in samples stored at pH 2.5. SEM analysis revealed the presence of a large number of voids, cracks and microporosities in FT, FII, FVIII and FIX specimens at pH 2.5, yet they were not detected in KN and HSF specimens.
3. Acidic environment affects the material surface, resulting in less homogenous structure, with voids, gaps and microporosities. The amount of cracks and microporosities correlates with decreasing pH.
Weft-knitted textiles offer many advantages over conventional woven fabrics since they allow the fabrication of doubly curved geometries without the need of stitching multiple patches together. This study investigated the use of high-strength continuous fibres as knitted textile reinforcement, focusing on various knitting patterns, fibre materials, coating types and spatial features to enhance the bond conditions between concrete and reinforcement. The bond is of particular interest since the contact surface of knitted textiles is fundamentally different due to their closed surface, compared to commercially available textile reinforcement, which is normally formed as orthogonally woven grids of rovings. An experimental campaign consisting of 28 textile-concrete composites was conducted, where digital image correlation-based measurements were used to assess the load-deformation behaviour and to analyse the crack kinematics. The results showed a beneficial post-cracking behaviour for epoxy coated configurations with straight inlays. The comparison of these configurations with conventional textile reinforcement generally showed a similar behaviour, but with higher utilisation compared to the filament strength. The Tension Chord Model, which assumes a constant bond stress-slip relationship, was adapted for the specific geometry of the knitted reinforcement, and it was used for the estimation of bond stresses and a post-diction of the experimental results, generally showing a good agreement.
This study addresses the structural behaviour of weft-knitted textile reinforced concrete. To this end, an experimental study by means of uniaxial tension tests on thin concrete elements with weft-knitted textile reinforcement was conducted. The investigations focused on various knitting patterns, fibre materials, types of coating and spatial features within the textile to enhance the bond at the interface between concrete and reinforcement, which considerably influences the post-cracking behaviour and deformation capacity. Digital image correlation-based measurements were used to assess the deformations, cracking pattern and crack kinematics. The experimental data allowed the back-calculation of bond stresses using the Tension Chord Model [28]. Furthermore, the performance of the fibres is compared to experimental data of conventional textile reinforcement from existing literature, and their potential use in structural applications is discussed. 2ff7e9595c
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