• Journal of the Korean Society of Manufacturing Process Engineers
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• v.9 no.1
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• pp.25-34
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• 2010

Fretting damage reduces fatigue life of the material due to low amplitude cyclic sliding and changes in the contact surfaces of strongly connected machine and structures such as bolt, key, fixed rivet and connected shaft, which have relative slip of repeatedly very low frequency amplitude. In this study, the fretting fatigue behavior of 7050-T7451 aluminum alloys used mainly in aircraft and automobile industry were evaluated. The plain fatigue test and fretting fatigue test under cyclic bending load carried out commercial bending fatigue tester and specially devised equipments to cause fretting damage. From these experimental work, the following results obtained: (1) The plain fatigue limit for stress ratio R=-l was about 151MPa. (2) In case of fretting fatigue, fatigue limit for stress ratio R=-l about 72MPa, the fatigue limit for R=0 about 81MPa, and the fatigue limit for R=0.3 about 93MPa. (3) The fatigue limit reduction rates by the fretting damage were about 52%(R=-1), 46%(R=0) and 38%(R=0.3) respectively. (4) The fatigue limit reduction rate decreased with stress ratio increase. In fretting bending test, as stress ratio increased, occurrence of initial oblique crack by fretting decreased or phased out, so that fracture surfaces were formed by plain fatigue crack occurrence, and such tendency was notable as stress amplitude increased. (5) Tire tracks and rubbed scars were observed in the fracture surface and contacted surface.

• The Journal of Advanced Prosthodontics
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• v.6 no.6
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• pp.462-467
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• 2014

PURPOSE. This study investigated the influence of surface conditioning procedures and repeated firings on monoclinic content and strength of zirconia before cementation. MATERIALS AND METHODS. Sintered bar-shaped zirconia specimens were subjected to no surface treatment (control), air abrasion, or grinding (n=21). Their roughness was evaluated using a profilometer, and microscope analysis was performed on one specimen of each group. Then, 2 or 10 repeated firings (n=10) were executed, the monoclinic content of specimens was analyzed by X-ray diffraction, and a three-point flexural strength test was performed. Surface roughness values were compared using one-way analysis of variance (ANOVA) and Tukey honestly significant difference (HSD) tests, the monoclinic content values were tested using Kruskal-Wallis and Mann-Whitney U tests, and the flexural strength values were tested using two-way ANOVA and Tukey HSD tests (P=.05). Spearman's correlation test was performed to define relationships among measured parameters. RESULTS. Surface-treated specimens were rougher than untreated specimens and had a higher monoclinic content (P<.005), and the relationship between roughness and monoclinic content was significant (P<.000). Neither surface treatment nor firing significantly affected the flexural strength, but Weibull analysis showed that for the air-abraded samples the characteristic strength was significantly lower after the $10^{th}$ firing than after the $2^{nd}$ firing. CONCLUSION. After firing, a negligible amount of monoclinic content remained on the zirconia surfaces, and rougher surfaces had higher monoclinic contents than untreated surfaces. Multiple firings could be performed if necessary, but the fracture probability could increase after multiple firings for rougher surfaces.

• Journal of Dental Rehabilitation and Applied Science
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• v.22 no.4
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• pp.329-340
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• 2006

Statement of problem: Damping of the peak force transmitted to implants has been reported by in vitro studies using impact forces on resin-veneered superstructures. Theoretical assumptions suggest that use of acrylic resin for the occlusal surfaces of a prosthesis would protect the connection between implant and bone. Therefore, the relationship between prosthesis materials and the force transmitted through the implant system also needs to be investigated under conditions that resemble the intraoral mechanical environment. Purpose: The purpose of this study was to analyze the fracture strength and modes of temporary prosthesis when a flange or occlusally extended structure were connected on the top of the abutment. Material and method: Modified abutments of winged and bulk design were made by casting the desired wax pattern which is made on the UCLA type plastic cylinder. Temporary crowns were made using templates on the modified abutments, and its fracture toughness and strain were compared to the traditional temporary prosthesis. To evaluate the effect of aging, 5.000 times of thermocycling were performed, and their result was compared to the 24hours specimen result. Results: The following conclusions were drawn from this study: 1. In the fracture toughness test, temporary crown's fracture line located next to the screw hole while modified designs with metal support showed fracture line on the metal and its propagation along the metal-resin interface. 2. Wing and bulk structure didn't show significant difference in the fracture toughness (p>0.05), but wing structure showed stress concentration on the screw hole area compared to bulk structure which showed even stress distribution. 3. In the fracture toughness test after thermocycling, wing and bulk structure showed increased or similar results in metal supported area while off-metal area and temporary crown showed decreased results. 4. In the strain measurement after thermocycling, its value increased in the temporary and bulk structure. However, wing structure showed decreased value in the loading point while increased value in the screw hole area. Conclusion: Wing type design showed compatible result to the bulk type that its application with composite resin prosthesis to the implant dentistry is considered promising.

• Journal of Dental Rehabilitation and Applied Science
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• v.36 no.2
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• pp.61-69
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• 2020

Purpose: Zirconia fixed partial dentures with mandibular 2^nd premolar and 2^nd molar as abutments are fabricated and then the effects of inter-abutment distance on fracture resistance of zirconia fixed partial dentures is studied. Materials and Methods: The materials used in this study are Cameleon S zirconia block and S2 zirconia block, which are divided into CS Group and S2 Group applying different inter-abutment distance for each material, and the sintered zirconia fixed partial denture was luted to the epoxy resin die using a temporary luting cement, and then the fracture resistance was measured by placing a 6 mm diameter hardened steel ball on the occlusal surfaces of the pontics and applying pressure at a cross head speed of 1.0 mm/min on a universal testing machine with a load cell of 5.0 kN. Results: The fracture resistance of zirconia fixed partial dentures is not significantly affected by inter-abutment distance The fracture resistance of zirconia fixed partial dentures in CS Group was significantly higher in 15 mm of inter-abutment distance than in 13 mm and 17 mm of inter-abutment distance (P < 0.05). The fracture resistance of zirconia fixed partial dentures in S2 Group was not significantly different between the three groups (P > 0.05). Conclusion: The fracture resistance of zirconia fixed partial dentures with mandibular 2^nd premolar and 2^nd molar as abutments does not significantly affected by the inter-abutment distance.

• Transactions of Materials Processing
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• v.29 no.2
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• pp.103-112
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• 2020

This study explores the effects of different pre-machining conditions on the deposition characteristics and mechanical properties of austenitic stainless steel samples repaired using direct energy deposition (DED). In the DED repair process, defects such as pores and cracks can occur at the interface between the substrate and deposited material. In this study, we varied the shape of the pre-machined zone for repair in order to prevent cracks from occurring at the slope surface. After repairs by the DED process, macro-scale cracks were observed in samples that had been pre-machined with elliptic and trapezoidal grooves. In addition, it was not possible to completely prevent micro-crack generation on the sloped interfaces, even in the capsule-type grooved sample. From observation of the fracture surfaces, it was found that the cracks around the inclined interface were due to a lack of fusion between the substrate and the powder material, which led to low tensile properties. The specimen with the capsule-type groove provided the highest tensile strength and elongation (respective of 46% and 571% compared to the trapezoidal grooved specimen). However, the tensile properties were degraded compared to the non-repaired specimen (as-hot rolled material). The fracture characteristics of the repaired specimens were determined by the cracks at the sloped interfaces. These cracks grew and coalesced with each other to form macro-cracks, they then coalesced with other cracks and propagated to the substrate, causing final fracture.

• Korean Journal of Materials Research
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• v.2 no.3
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• pp.172-179
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• 1992

$Al_2O_3/ZrO_2$ composites were fabricated by pressureless sintering from commercial powders and/or nano composite powder of $Al_2O_3/ZrO_2$. The Properties of the composites such as density, strength, hardness and fracture toughness were evaluated. Microstructures and fracture surfaces ware also examined. The flexural strength remains unchanged(~640 MPa) as long as the content of commercial powders is not extreamly high, and depends on microstructures of the composites. Fracture toughness(4.3-5.3 $Mpa{\cdot}m^{1/2}$) increases with increasing content of commercial powders. Fractography shows that failure-initiating sources are 1)surface flaws resulting from machining damage, 2)crack-shaped voids formed due to $ZrO_2$ agglomeration, and 3)surface separation caused by inhomogeneous blending and by sinterability difference between nato composite powder and commercial powders of $Al_2O_3/ZrO_2$. Failure mode of the composites was mainly transgranular.

• Composites Research
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• v.17 no.4
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• pp.53-60
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• 2004

This paper describes a method for a falling weight impact test to estimate the impact energy absorbing characteristics and impact strength of CFRP laminate plates based on considerations of stress wave propagation theory. The absorbed energy of T300 orthotropic composites is higher than that of quasi-isotropic specimen over impact energy 6.8J, but in case of using T700 fiber, much difference does not show. Also, absorbed energy of T300 orthotropic composites, which are composed of the same stacking number and orientation became more than that of T700 fiber specimen; however there was no big difference in case of quasi-isotropic specimens. The delamination areas of the impacted specimen were measured with the ultrasonic C-scanner to find correlation between impact energy and delamination area. The fracture surfaces were observed by using the SEM (scanning electron microscope) through a low-velocity impact test in order to confirm the fracture mechanism.

• Journal of Welding and Joining
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• v.34 no.2
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• pp.16-21
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• 2016

Recent keywords of the heavy industries are large-scale structure and productivity. Especially, the sizes of the commercial vessels and the offshore structures have been gradually increased to deliver goods and explore or produce oil and natural gas in the Arctic. High heat input welding processes such as electro gas welding (EGW) have been widely used for welding thick steel plates with flux-cored arc welding (FCAW), especially in the shipbuilding industries. Because high heat input welding may cause the detrimental effects on the fracture toughness of the welded joint and the heat affected zone, it is essential to obtain the sufficient toughness of welded joint. There are well known that the fracture toughness like CTOD, CVN, and KIC were very important factors in order to secure the safety of the structures. Furthermore, the welding residual stress should be considered to estimate the unstable fracture in both EGW and FCAW. However, there are no references on the welding residual stress distribution of EGW and FCAW with thick steel plates. Therefore the welding residual stresses were very important elements to evaluate the safety of the welded structure. Based on the measurement results, the characteristics of residual stress distribution through thickness were compared between one-pass electron gas welding and multi-pass flux-cored arc welding. The longitudinal residual stress in the multi-pass flux-cored arc welding is tensile through all thicknesses in the welding fusion zone. Meanwhile, longitudinal residual stress of EGW is tensile on both surfaces and compressive at the inside of the plate. The magnitude of residual stresses by electron gas welding is lower than that by flux-cored arc welding.

• Journal of the Microelectronics and Packaging Society
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• v.12 no.1 s.34
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• pp.41-46
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• 2005

A systematic investigation of shear testing was conducted to find a relationship between Ni-Sn intermetallic spatting and the brittle fracture observed in electroless Ni(P)/solder interconnection. Brittle fracture was found in the solder joints made of Sn-3.5Ag, while only ductile fracture was observed in a Cu-containing solder (Sn-3.0Ag-0.5Cu). For Sn-3.0Ag-0.5Cu joints, $(Ni,Cu)_3Sn_4$ and/or $(Cu,Ni)_6Sn_5$ compound were formed at the interface without spatting from the Ni(P) film. For Sn-3.5Ag, $Ni_3Sn_4$ compound was formed and brittle fracture occurred in solder pads where $Ni_3Sn_4$ had spalled. From the analysis of fractured surfaces, it was found that the brittle fracture occurs through the $Ni_3SnP$ layer formed between $Ni_3Sn_4$ intermetallic layer and the Ni(P) film. Since the $Ni_3SnP$ layer is getting thicker during/ after $Ni_3Sn_4$ spatting, suppression of $Ni_3Sn_4$ spatting is crucial to ensure the reliability of Ni(P)/solder system.

• Journal of Dental Rehabilitation and Applied Science
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• v.19 no.4
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• pp.269-279
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• 2003

This study investigated the mechanical properties of precious metal-ceramic alloy joined by the laser-welding and the soldering compared with the parent metal. Twenty-four tensile specimens were cast in precious metal-ceramic alloy and divided into three groups of eight. All specimens in the control group(group 1) were left in the as-cast condition. Group 2 and 3 were the test specimens, which were sectioned at the center. Eight of sectioned specimens were joined by soldering with a propane-oxygen torch, and the remaining specimens were joined by laser-welding. After joining, each joint diameter was measured, and then tested to tensile failure on an Instron machine. Failure loads were recorded, and then fracture stress(ultimate tensile strength), 0.2% yield strength and % elongation calculated. These data for three groups were subjected to a one-way analysis of variance(ANOVA). Neuman-Keuls post hoc test was then used to determine any significant differences between groups. The fracture locations, fracture surfaces were examined by SEM(scanning electron microscope). The results were as follows: 1) The tensile strength and 0.2% yield strength of the soldered group($280.28{\pm}49.35MPa$, $160.24{\pm}26.67MPa$) were significantly less than both the as-cast group($410.99{\pm}13.07MPa$, $217.82{\pm}17.99MPa$) and the laser-welded group($383.56{\pm}59.08MPa$, $217.18{\pm}12.96MPa$). 2) The tensile strength and 0.2% yield strength of the laser-welded group were about each 98%, 99.7% of the as-cast group. There were no statistically significant differences in these two groups(p<0.05). 3) The percentage elongations of the soldered group($3.94{\pm}2.32%$) and the laser-welded group($5.06{\pm}1.08%$) were significantly less than the as-cast group($14.25{\pm}4.05%$) (p<0.05). 4) The fracture of the soldered specimens occurred in the solder material and many porosities were showed at the fracture site. 5) The fracture of the laser-welded specimens occurred also in the welding area, and lack of fusion and a large void was observed at the center of the fracture surface. However, the laser-welded specimens showed a ductile failure mode like the as- cast specimens. The results of this study indicated that the tensile strengths of the laser-welded joints were comparable to those of the as-cast joints and superior to those of the soldered joints.