Greater Stress Concentration Research Articles

Effect of Labial Frenum Notch Size and Palatal Vault Depth on Stress Concentration in Maxillary Complete Dentures: A Finite Element Study

Fracture is a common cause of acrylic resin denture failure due to crack development in a site of excessive concentration of stress in the denture. The purpose of this study was to evaluate the effect of labial frenum notch size and palatal depth on stress concentration in maxillary complete dentures. Three-dimensional finite element models of maxillary complete dentures with different palatal vault depths (shallow, medium, and deep) and different frenum notch sizes (small, medium, and large) were constructed. The stress concentration was analyzed using Nastran software and displayed in terms of von Mises stress generated under two conditions: dropping on a hard flat surface from a standard distance and applying vertical load to the occlusal surface. The greatest stress concentration was observed in the labial frenum notch. Stress increased with an increase in the size of the labial frenum notch and a decrease in the depth of the palatal vault. The stress concentration in maxillary denture bases was greater when a vertical load was applied compared with dropping the denture on a hard surface from a standard distance. Using finite element analysis, the presence of a large labial frenum notch and a shallow palatal vault increased the concentration of stress in maxillary complete dentures. Stress concentration in a maxillary denture base is much greater during the application of a vertical load than when dropping the denture on a hard flat surface. The concentration of stress at the tip of a large frenum notch and in a shallow palatal vault could have a weakening effect on the maxillary complete acrylic resin denture base making it more vulnerable to fracture. In this scenario alternative denture base strengthening strategies, such as using a metal framework, should be considered in the fabrication of maxillary complete dentures.

Comparison of Stress and Strain States in Pavements with and without Reflective Cracks

Comparison of stresses and strains in pavements with and without reflective cracking is presented. The series of finite-element numerical simulations were carried out for several types of flexible pavements and overlays placed on these pavements. The influence of such parameters as: the load position, the overlay thickness, and subgrade properties on stress and strain concentrations was investigated. Numerical results confirmed the fact that stresses in cracking pavements strongly differ from corresponding values in the uncracked ones. The greatest stress concentration was observed in the zone above the crack. In the case of asymmetrical position of loading with regard to the crack a high level of shear stresses appeared near the crack. The stresses and displacements in the pavement did not differ much for overall effective modulus in the range between 1,000 and 4,500 MPa. Below this range displacements increased strongly.

Stress Analysis of All-on-4 System Using a 3-Dimensional Finite Element Method

The purpose of this study was to investigate the mechanical risk factors of All-on-4 system by analyzing the stress distribution induced on implants and surrounding bone tissue using a 3-dimensional finite element method (3D-FEM). The effects of bone quality and superstructure material property were also investigated.A 3D-FEM model of an edentulous mandible was constructed from computerized tomographic images. The two anterior-most implants were placed in the right and left lateral incisor areas. Two additional implants were placed anterior to the mental foramen with a distal inclination of 40 degrees to the occlusal plane. The 3D-FEM All-on-4 model included a fixed mandibular complete-arch superstructure supported by four implants. Four types of bone (1 to 4) were simulated by varying the Young's modulus for cancellous bone. In addition, three superstructure materials were prepared: acrylic resin, gold alloy, and Co-Cr alloy. A 100 N load was applied on the left first molar region. Stress concentration was observed around the left posterior inclined implants, especially on the distal side. When the cancellous bone is low elastic, greater stress concentration was observed. Of the three materials, the acrylic resin superstructure showed the greatest stress concentration around the inclined implant.The present study indicated that the use of the superstructure with higher elastic material is useful for reducing the mechanical risk of All-on-4 system.

Finite Element Analysis of Undermining of Pressure Ulcer with a Simple Cylinder Model

With pressure sores, surface damage indicates that subcutaneous fat tissue necrosis has occurred. We hypothesized that formation of necrosis under a pressure sore changes the stress distribution, which in turn affects further extension of the necrosis. In the present study, two-dimensional finite element models were used to perform analysis under different undermining size conditions. Greater stress concentration was observed in the larger undermining model. This may be the reason that, in clinical situations, a large area of undermining necrosis is sometimes observed under the skin of sores with a small area of damage.

The influence of surface roughness on porcelain strength

Objectives: In order to adjust occlusion, the functional surfaces of porcelain restorations are often ground and mechanical machining is even an essential part of the CAD–CAM process for these restorations. The aim of this study was to investigate the influence of the finishing procedures on the biaxial flexure strength of four commercial porcelains. Methods: Four commercial porcelains of which two are used for metal–ceramic restorations (Flexo Ceram Dentine and Vita VM K68) and two for veneers and inlays (Duceram LFC Dentine and Cerinate BODY) are used in this study. For each porcelain, sixty discs (∅=22 mm, h=±2.0 mm) were produced using twelve different finishing procedures. Twenty discs were left untreated, twenty discs were milled, using a high-speed diamond disc, and twenty discs were machined in a high-speed grinding/polishing device. Half of the samples were glazed. In each of these six groups, half of the samples were stored for 16 h at 80°C in a 4% acetic acid solution. The biaxial flexure strength was determined using the ball-on-ring method. In each group the roughness of the surface was determined and examined via SEM. Results: With the exception of Flexo Ceram Dentine, a significant correlation was found between the roughness of the surface and the biaxial strength: the smoother the surface, the stronger the sample. The differences in biaxial strength may be attributed to the stress concentration of an applied load due to the roughness of the surface caused by mechanical finishing or chemical action. The fact that the strength of Flexo Ceram Dentine was not affected by the different surface treatments is probably due to the size of the leucite particles, which apparently induce more stress concentration than the surface flaws and the roughness of the surface. Significance: It was concluded that surface roughness determines the strength of a porcelain material, except where the inner structure of the material causes greater stress concentration than that caused by the combination of surface roughness and surface flaws.

The effect of dispersed paint particles on the mechanical properties of rubber toughened polypropylene composites

The influence of dispersed paint particles on the mechanical properties of rubber toughened PP was investigated. The matrix was basically a hybrid of PP, rubber and talc. Model systems with spherical glass bead filled matrix were also studied to examine the effect of filler shape and size. Properties like tensile strength, strain at break, impact strength, and fracture toughness were influenced by the dispersed inclusions. Tensile strength at yield decreased linearly according to Piggott and Leinder's equation. Strain at break decreased more drastically with paint particles than glass beads, revealing that irregularly shaped particles offered greater stress concentrations. The tensile strength and strain at break were less influenced by the size of paint particles whereas a slight decrease in the modulus values was observed with decreasing particle size. Impact strength and fracture toughness also decreased with increasing filler fraction. Lack of stress transfer between filler and matrix aided in reduction of impact strength. Decrease in fracture toughness was influenced by volume replacement and constraints posed by fillers. The size of paint particles had little effect on the impact strength and fracture properties at the filler concentration levels used in this investigation.

Stress Field at a Transcurrent Plate Boundary in the Presence of Frictional Heat Production at Depth

—A model is proposed to study the modification of the stress field at a transcurrent plate boundary due to frictional heat production at depth. Two cases are considered a stable and a stretched lithosphere. The model is applied to those weak faults where the dynamic friction is small compared to a static one; if the deformation along the brittle portion of the fault is entirely accommodated by a series of seismic ruptures in a quasi-static state where the fault has been moving for millions of years, the long-term thermal field perturbation due to these ruptures results in only a few degrees and can be neglected. The boundary zone is considered as a viscoelastic body subject to a constant strain rate. The lower section of the boundary is assumed to slip aseismically along a vertical transcurrent fault and to completely accommodate the plate motion, while the upper section is locked. The slipping zone is divided into a semi-brittle zone, placed between the isothermal surfaces of 300°C and 450°C, and a ductile zone beneath. The frictional heat is calculated by assuming a linearly decreasing friction in the semi-brittle and a constant friction in the ductile zones. The heat modifies the temperature field, producing an upward movement of the semi-brittle and ductile fault sections. As a consequence, the thickness of the brittle fault section is reduced and friction at the base of this section is less. The stress field in the boundary zone is calculated as a function of time for different friction profiles and slip rates on the fault. Owing to heat production, a greater stress concentration is produced on the brittle fault section, while shear stress is lowered in regions occupied by the uplifted semi-brittle layer. These effects are found to be remarkable only in the case of a stable zone, with a standard unperturbed geotherm, while they are irrelevant in a stretched zone with a high geothermal gradient. In any case, the role of the semi-brittle layer appears to be more prominent in the case of boundaries with higher slip rates, due to the presence of higher stress values.

ＩＭＺインプラントの緩衝機構に関する実験的研究 第１報 静的荷重下における単独インプラント周囲の歪み

Recently, osseointegrated dental implants have come to be used more and more in the treatment of edentulism. Osseointegration has been found to achieve better results than fibro osseous integration however, it has been suggested that excessive and/or concentrated stresses may cause resorption of the surrounding bone.To address this problem, the IMZ implant system incorporates stress-absorbing elements, such as the intramobile element (IME). There are varying reports regarding the characteristics of the IME, however, and its effectiveness is still controversial.This study compares the stress-transfer characteristics around a single freestanding IMZ implant with three types of internal elements: an IME made of polyoxymethylene, an intramobile connector (IMC) consisting of a polyoxymethylene head and titanium screw, and a metal element (Metal-E) made of stainless steel. An IMZ implant (4×13mm) and five strain gauges (Kyowa Inc.) around the implant were embedded in an acrylic resin block (3×3×3cm). The vertical (10 and 15kgf) and horizontal (3 and 5kgf) static forces applied to the surface of the superstructure and the strain values around the implant were measured.The results are as follows:1. Under vertical forces, among the three types of elements no significant difference in the stresses around a single freestanding implant was found.2. Under horizontal forces, a more uniform stress around a single freestanding implant with the Metal-E was observed. When using the IME, there was a greater stress concentration on the upper-oppressive side of the implant. The action of the IMC appeared to be midway between the IME and the Metal-E.3. The IME fractured after several applications of 5-kgf horizontal force.

Visualization of stress and strain related to removable partial denture abutments

The following conclusions are based on the conditions set forth in this investigation and apply to forces acting through a vertical direction of application only: 1. The unilateral removable partial denture produced no more stress than the bilateral removable partial denture. 2. The fully active Dalbo removable partial denture resulted in the greatest stressbreaking action. 3. The rigid Dalbo removable partial denture resulted in the least ridge displacement. 4. The rigid Dalbo removable partial denture resulted in the greatest stress concentrations in the distal abutment. 5. None of the attachments tested resulted in a distribution of the stresses along the edentulous ridge. 6. The use of two abutments (double abutments) resulted in greater stress concentrations in the distal abutment than the use of single abutments. 7. Forces in the single abutment were resisted along the long axis of the root. 8. Forces in the double abutments were resisted along the long axis of the distal abutment.

A biomathematical study on implant design and stress distribution.

Three types of implant were simulated by the two-dimensional finite element method using the mechanical properties of human mandible and implant material as input data. Type 1 was a blade implant, type 2 was blade implant with subperiosteal wings in buccolingual direction, and type 3 was subperiosteal implant with no blade. The effect of bucco-lingual subperiosteal wings in decreasing the stress concentration around the neck portion of the blade implant was investigated under three kinds of occlusal force. In conclusion, the greatest stress concentration was observed at the cortical bone around the neck portion of T-1 blade implant. It was effective in decreasing the stress concentration at this neck portion to change the implant design from T-1 to T-2 or T-3.

Reinforced discontinuous bend subjected to uniform internal pressure: Effect of reinforcing ring stiffness

This paper deals with the results of an experimental study undertaken to assess the stress-reducing effect of internally bracing a traditional constant-depth elliptical reinforcing ring. Stress profiles, calculated from strain readings recorded by electrical-resistance foil strain gauges bonded to inner and outer surfaces of the cylindrical limbs of model Araldite bends, are presented for four reinforcement conditions: constant-depth (traditional) elliptical ring; constant-depth ring with internal cruciform bracing; constant-depth ring with a single brace across the minor diameter; perfectly rigid reinforcing plate. When a traditional reinforcing ring was used the greatest stress-concentration factor (referring to circumferential membrane stress remote from the discontinuity as unity) was found to be 4.15. This was reduced to 2.90 by the use of a ring with a single minor-diameter brace and was further reduced to 2.70 when a ring having a cruciform brace set across major and minor diameters was used. The effect of an infinitely rigid reinforcement was to reduce the maximum stress-concentration factor to 2.60.