Atsushi Sugeta

Static and fatigue fracture resistances of pulpless teeth restored with post–cores

OBJECTIVES Superior restorative methods for effectively strengthening pulpless teeth need to be identified, since vertical root fractures of pulpless teeth are still a major problem in everyday clinical practice. The present study tested the null hypothesis that there were no differences in static and fatigue fracture resistances of pulpless teeth restored with different types of post-core systems. METHODS Extracted human premolars were restored with a combination of either a fiber post or metallic post and a composite resin core. Teeth with full crown preparations without post-core restorations served as a control. A 90 degrees vertical or 45 degrees oblique static compressive load was applied to restored teeth, and fracture loads and modes of fracture were recorded. Fatigue fracture tests were conducted by applying sinusoidal cyclic loads to restored teeth from vertical or oblique directions. Fatigue limits for each restoration were calculated using the staircase approach. RESULTS In both static and fatigue fracture testing under vertical or oblique loadings, the fracture loads of teeth restored with fiber posts were significantly greater than those of teeth restored with metallic posts. The fatigue limits of teeth restored with fiber and metallic posts were 112 kgf and 82 kgf respectively under vertical loadings and 26 kgf and 20 kgf under oblique loadings. SIGNIFICANCE The combination of a fiber post and a composite resin core showed superior fracture resistance against both static and fatigue loadings compared to restorations using a metallic post, and is therefore recommended in restoring pulpless teeth.

Heat Treatment Strengthens Human Dentin

The flexural strength of Type I collagen, the major organic component of human dentin, increases with heat. We hypothesized that human dentin can be strengthened by heating, which may help prevent fracture of non-vital teeth after restoration. Beam-shaped dentin specimens were obtained from the crowns of human third molars. The dentinal tubular orientations were arranged to run parallel or perpendicular to loading surfaces. The flexural and microtensile strengths of dentin in the parallel specimens were 2- to 2.4-fold greater after being heated between 110°C and 140°C for 1 hr. The stress intensity factors at fracture also increased after specimens were heated. The x-ray diffraction analyses suggested that shrinking of the lateral packing of the collagen triple-helices from 14 Å to 11 Å was the probable cause of the strengthening of heated dentin. We conclude that heat treatment strengthens human dentin.

Fatigue strength of USP treated ASTM CA6NM for hydraulic turbine runner

Abstract In order to investigate the effect of an ultrasonic shot peening (USP) treatment on the fatigue characteristics of structural materials for hydroelectric facilities, plane bending fatigue tests were carried out using USP treated ASTM CA6NM stainless cast steel. The results showed that the fatigue limit of the USP material was approximately 60% higher than that of the untreated material. Haigh’s diagram evaluation suggested that an increase in crack initiation resistance associated with the high hardness and compressive residual stress by USP treatment was the main reason for the improvement in fatigue strength. Furthermore, the simulation of fatigue crack propagation implied that compressive residual stress leads to a low crack propagation rate compared with the untreated material. Consequently, it is clear that the USP treatment is an effective process for improving the fatigue properties of CA6NM.

Effects of Rehydration on Dentin Strengthened by Heating or UV Irradiation

Type I collagen, the major organic component of human dentin, plays an important role in regulating the mechanical strength of dentin. Collagen in dentin can be strengthened by heating. We hypothesized that UV irradiation could produce similar strengthening effects and might maintain the strength of dentin after rehydration. Beam-shaped dentin specimens from the crowns of human third molars were subjected to flexural testing. Flexural strengths were two and three times greater than those in the control group after 5 minutes’ UV irradiation and heating to 140°C, respectively. After 30 days of rehydration, the heated specimens reverted to their original strength, whereas the UV specimens were 69% stronger than the original. Raman spectra of dental collagen were unchanged after heating, whereas several peaks, including a C-C bond in a proline ring, were amplified by UV irradiation. It is concluded that dentin strengthened by UV irradiation retains strength after rehydration because of chemical changes in collagen.

Fatigue Damage Evaluation of Friction Stir Spot Welded Cross-Tension Joints Under Repeated Two-Step Force Amplitudes

This paper investigates an approach to evaluate the fatigue damage of FSSW cross-tension specimens under two-step force amplitude conditions. In fatigue tests with repeated two-step force amplitude, the fatigue limit of the welded joint disappeared. However, the fatigue damage evaluation using the modified Miner’s rule erred too much on the side of safety, as the modified Miner’s rule tends to overestimate the damage by applied forces below the fatigue limit. Thus, it was determined that, within the testing conditions used in this study, the fatigue damage evaluation using Haibach’s method yielded an accurate evaluation. In the case where significant plastic deformation caused by the applied force occurred near the welded zone, the cumulative fatigue damage value based on Miner’s rule was often larger than unity. Therefore, it is important to consider a cumulative damage estimation that takes into account the effect of pre-strain from the high force amplitude.

Effects of loading mode on the fatigue behavior of laser welds in automobile mild steel sheet

In this study, to clarify the effect of the loading mode on the fatigue properties of laser-welded joints, the fatigue properties and the fatigue crack behavior were experimentally investigated by using two types of laser-welded joint: tension-shear type (TS-type) and cross-tension type (CT-type). Furthermore, to clarify the effect of the bead width of laser welding on fatigue properties, CT-type joints with a wider bead width were also prepared (CT-wide-type). From static tensile and fatigue tests, TS-type and CT-type joints possess significant low fatigue resistance given the static strength of both joints. The macroscopic fracture morphologies of TS-type and CT-type joints with a narrow bead width are dependent on the applied force amplitude. However, the macroscopic fracture morphologies of CT-wide-type joints, which have a wider bead width, are not affected by the force amplitude level. In addition, using an external force to organize the fatigue strength of laser-welded joints with different loading modes is difficult. However, our fracture-mechanics approach based on the stress intensity factor at the slit tip, as calculated by FEM analyses, can be used to organize the fatigue strength of various joints irrespective of the loading mode.

3-Dimensional observation of the interior fatigue fracture mechanism on friction stir spot welded using 300 MPa-class automobile steel sheets

The authors have shown an approach to evaluate the mechanical properties of cross-tension type specimen welded by friction stir spot welding (FSSW) with the focus on fatigue crack behaviour. In this study, cold-rolled 0.8-mm-thick low carbon steel sheet JIS G3141 SPCC was used base metal. Results shown that, the hardness test values within the stirred zone in welding joint were much higher than that of the base metal. The fatigue strength of FSSW specimen was very low to compare with the tensile force of base metal and FSSW joint itself. The fatigue crack initiated near the interface between two thin steels sheet was observed in detail by the three-dimensional observation method. This method was used to investigate the crack initiation and propagation behaviour. The typical morphologies of the three-dimensional were observed with constant low force amplitude level of each fatigue life. It shows that a main crack initiated at the end of slit and FSSW specimen used in this study requires the large number of cyclic loading to initiate the fatigue crack. Furthermore, the typical morphologies of the three-dimensional with constant high force amplitude level were observed. From this observation, it is cleared that a main crack initiated at the end of slit and FSSW specimen used in this study requires few number of cyclic loading to initiate the fatigue crack. Therefore, fatigue fracture modes were independent on force amplitude level.

Fatigue properties of friction stir welds of treated Al to carbon fibre-reinforced plastic

ABSTRACT The purpose of this paper is to establish the welding technology for dissimilar materials of carbon fibre-reinforced plastic. Therefore, this study focused on friction stir spot welding, which can be simply substituted for spot welding which is widely used for welding of automobile body. In this study, friction stir spot welding was used to weld joints of carbon fibre-reinforced plastic and aluminium alloy, and the effect of aluminium alloy surface treatments on the fatigue properties and the fracture mechanism of dissimilar material joints were investigated experimentally and statistically. As a result, it is possible to create welding joints with high fatigue durability by using the surface treatments on the aluminium alloy.

A Study on the Mechanism of Small Fatigue Crack Deflection Behavior in Alpha-Brass by Means of In Situ Atomic Force Microscopy and Crystallo-Graphic Orientation Analysis

Successive observation of transgranular small fatigue crack growth behavior of alpha-brass was performed by means of an atomic force microscope (AFM) equipped with small in-plane bending fatigue testing machine. In the low growth rate region after crack initiation, the fatigue crack grew in a zig-zag manner as a result of successive crack branching and deflection. The fatigue crack deflection behavior was investigated by the crystallographic orientation analysis based on the Electron Back Scatter Diffraction (EBSD) technique. The slip factor considering the slip system and singular stress field at the crack tip was introduced in order to evaluate the easiness of slip deformation instead of Schmid factor. The direction of crack deflection was found to be explained well by the slip factor and the geometric relative location between the preferential slip plane and crack front.

Quantitative Analysis of Deformation near Fatigue Crack Tip Using Image Processing Technique.

A computer image processing system incorporated into specially designed servo-hydraulic fatigue loading facilities operating in a scanning electron microscope has been developed. Direct, real-time observations of fatigue crack growth behavior under both constant amplitude and repeated two-step loadings were made, and quantitative analysis of crack opening displacement and deformation near the fatigue crack tip were made using a newly developed image processing technique.