• Journal of the Korean Society of Safety
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• v.35 no.5
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• pp.1-8
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• 2020

Mobile elevating work platforms (MEWPs) consist of a work platform, extending structure, and chassis, and are used to move persons to working positions. MEWPs are useful but are composed of pieces of equipment, and accidents do occur owing to equipment defects. Among these defects, accidents caused by the fracture of bolts fixed to the extension structure and swing system are increasing. This paper presents a failure analysis of the fixing bolts of MEWP. Standard procedure for failure analysis was employed in this investigation. Visual inspection, chemical analysis, tensile strength measurement, microstructural characterization, fractography analysis by Optical Microscope(OM) and Scanning Electron Microscopy(SEM), and finite element analysis (FEA) were used to analyze the failure of the fixing bolts. Using this failure analysis approach, we found the root cause of failure and proposed a means for solving this type of failure in the future. First, the chemical composition of the fixing bolt is obtained by a spectroscopy chemical analysis method, which determined that the chemical composition matched the required standard. The tensile test showed that the tensile and yield strengths were within the required capacity. The stress analysis was carried out at five different boom angles, and it was determined that the fixing bolt of MEWP can withstand the loads at all the boom angles. The outcomes of the fatigue analysis revealed that the fixing bolt fails before reaching the design requirements. The results of the fatigue analysis showed primarily that the failure of the fixing bolt was due to fatigue. A visual inspection of the fractured section of the fixing bolt also confirmed the fatigue failure. We propose a method to prevent failure of the fixing bolt of the MEWP from four different standpoints: the manufacturer, safety certification authority, safety inspection agency, and owner.

• Journal of Welding and Joining
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• v.7 no.3
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• pp.19-27
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• 1989

Both the SS41 steel and the M.E.F(martensite encapsulated islands of frrite) dual phase steel made of SS41 steel by heat treatment were welded by friction welding, and then manufactured machinemade Vnotch standard Charpy impact specimens and precracked with a fatigue system at BM(base metal), HAZ(heat affected zone) and WZ(weld interface Zone). The impact test of them was performed with an instrumented impact test machine at a number of temperatures in constant loading velocity and the dynamic fracture characteristics were studied on bases of the absorbed energy, dynamic fracture toughness and fractography from the test. The results obtained are as follows; At the room temperature, the absorbed energy is HAZ.geq.WZ.geq.BM in case of the M.E.F. dual phase steel: BM.geq.HAZ.geq.WZ in case of the SS41 steel, HAZ.geq.BM.geq.WZ at the low temperature. The absorbed energy is decreased markedly with the temperature lowering; it is highly dependent on the temperature. The dynamic fracture toughness of the M.E.F. dual phase steel is HAZ.geq.WZ.geq.BM at the room temperature; BM.geq.WZ.geq.HAZ below-60.deg. C. Therefore the reliability of friction welding is uncertain at the low temperature(below-60.deg. C). The dynamic fracture toughness of the SS41 steel; HZA.geq.WZ.geq.BM at overall temperature region. The flaw formed by rotational upsetting pressure was shown y SEM; in this region. The absorbed energy per unit area and dynamic fracture toughness were low relative to other region.

• Korean Journal of Materials Research
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• v.6 no.8
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• pp.841-851
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• 1996

In a recent investigation, the formation of bainite phase in direct-quenched low carbon non heat-treated steel was reported. In this study the effects of bainite phase on the mechanical properties of direct-quenched microalloying steels were investigated. By isothermal transformation at $480^{\circ}C$ for 7 sec., volume fraction of bainite lath was 15~20%, and the UTS and impact energy were increased. In this case $B_{ll}$ and $B_{lll}$ type bainite was observed and the fractography of impact test specimen showed a ductile fracture tendency. Isothermal transformation for 100sec., yielded 30% volume fraction of granular bainite and the mechanical properties were decreased. The f ractography of impact test specimen showed a brittle fracture tendency. The addition of Mo was more effective than B for improving impact energy because amounts of boron aditions were restricted to considerably lower levels, typically 10~ 30ppm. From this study, it is predicted that 15~20% volume fraction of lath bainite on the direct quenching process is procduced by addition of Mo up to 1.2wt. % and controlling the finish forging proc¬ess at $1000^{\circ}C$ and using oil as direct quenching media. This will improve mechanical properties of the direct- quenched steel.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.25 no.6
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• pp.996-1003
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• 2001

In order to investigate the fatigue fracture characteristics by corrosion degradation of 12Cr alloy steel, both the fatigue characteristics in air of them artificially degraded during long period and the corrosion fatigue characteristics were experimentally evaluated in various environments which were determined from electro-chemical polarization tests. And also, their fracture mechanisms were analyzed and compared, fractographyically. From their results, the fracture mechanical characteristics of it artificially degraded during long period in the distilled water, 3.5 wt.% NaCl solution and 12.7wt.%(1M) Na$_2$SO$_4$solution of 25, 60 and 90$\^{C}$ did not show distinguishable difference comparing with non-corroded one in regardless of temperature and degradation period. It means that degradation of the material by just surface corrosion does not remarkably affect to fatigue crack growth. On the other hand, the crack growth rates by corrosion fatigue increased due to activity increase of corrosive factors such as OH(sub)-,Cl(sup)- and SO$_4$(sup)- at the crack tip with temperature increase. Therefore, the crack growth rates by corrosion fatigue were more faster than that in air of the artificially degraded specimen due to the such difference of crack growth mechanism.

• Korean Journal of Metals and Materials
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• v.47 no.10
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• pp.605-612
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• 2009

The suitability of twin roll strip casting for Ag-27.5%Cu-20.5%Zn-2.5%Mn-0.5%Ni brazing alloy (known as HS-49D) was examined in the present work and the mechanical properties and microstructure of the strip were also investigated. The effect of annealing heat treatment on the properties was also studied. The new manufacturing process has applications in the production of the brazing alloy. XRD and microstructural analyses of the Ag-27.5%Cu-20.5%Zn-2.5%Mn-0.5%Ni strip revealed a eutectic microstructure of an Ag-rich matrix (FCC) and a Cu-rich phase (FCC) regardless of heat treatment. The results of mechanical tests showed tensile strength of 434 MPa and 80% elongation for the twin roll casted strip. Tensile results showed decreasing strengths and increasing elongation with annealing heat treatment. Microstructural evolution and fractography were also investigated and related to the mechanical properties.

• Korean Journal of Metals and Materials
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• v.48 no.1
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• pp.28-38
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• 2010

High temperature high cycle and low cycle fatigue deformation behavior of automotive heat resistant aluminum alloys (A356 and A319 based) were investigated in this study. The microstructures of both alloys were composed of primary Al-Si dendrite and eutectic Si phase. However, the size and distribution for eutectic Si phase varied: a coarse and inhomogeneous distributed was observed in alloy B (A319 based). A brittle intermethallic phase of ${\alpha}-Fe\;Al_{12}(Fe,Mn)_3Si_2$ was detected only in B alloy. Alloy B exhibited high fatigue life only under a high stress amplitued condition in the high cycle fatigue results, whereas alloy A showed high fatigue life when stress was lowered. With regard to the low-cycle fatigue result ($250^{\circ}C$) showing higher fatigue life as ductility increased, alloy A demonstrated higher fatigue life under all of the strain amplitude conditions. Fractographic observations showed that large porosities and pores near the outside surface could be the main factor in the formation of fatigue cracks. In alloy B. micro-cracks were formed in both the brittle intermetallic and coarse Si phasese. These micro-cracks then coalesced together and provided a path for fatigue crack propagation. From the observation of the differences in microstructure and fractography of these two automotive alloys, the authors attempt to explain the high-temperature fatigue deformation behavior of heat resistant aluminum alloys.

• Journal of the Korean Society for Heat Treatment
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• v.37 no.3
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• pp.121-127
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• 2024

We investigate the effect of T6 heat treatment on the microstructure and mechanical properties of AA365 (Al-10.3Si-0.37Mg-0.6Mn-0.11Fe, wt.%) alloy fabricated by vacuum-assisted high pressure die casting by means of thermodynamic calculation, X-ray diffraction, scanning and transmission electron microscopy, and tensile tests. The as-cast alloy consists of primary Al (with dendrite arm spacing of 10~15 ㎛), needle-like eutectic Si, and blocky α-AlFeMnSi phases. The solution treatment at 490 ℃ induces the spheroidization of eutectic Si and increase in the fraction of eutectic Si and α-AlFeMnSi phases. While as-cast alloy does not contain nano-sized precipitates, the T6-treated alloy contains fine β' and β' precipitates less than 20 nm that formed during aging at 190℃. T6 heat treatment improves the yield strength from 165 to 186 MPa due to the strengthening effect of β' and β' precipitates. However, the β' and β' precipitates reduce the strain hardening rate and accelerate the necking phenomenon, degrading the tensile strength (from 290 to 244 MPa) and fracture elongation (from 6.6 to 5.0%). Fractography reveals that the coarse α-AlFeMnSi and eutectic Si phases act as crack sites in both the as-cast and T6 treated alloys.

• Restorative Dentistry and Endodontics
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• v.32 no.2
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• pp.130-137
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• 2007

This in vitro study examined the effect of surface defects on cutting blades on the extent of the cyclic fatigue fracture of HEROShaper Ni-Ti rotary files using fractographic analysis of the fractured surfaces. A total of 45 HEROShaper (MicroMega) Ni-Ti rotary flies with a #30/.04 taper were divided into three groups of 15 each. Group 1 contained new HEROShapers without any surface defects. Group 2 contained HEROShapers with manufacturing defects such as metal rollover and machining marks. Croup 3 contained HEROShapers that had been clinically used for the canal preparation of 4-6 molars A fatigue-testing device was designed to allow cyclic tension and compressive stress on the tip of the instrument whilst maintaining similar conditions to those experienced in a clinic. The level of fatigue fracture time was measured using a computer connected the system. Statistical analysis was performed using a Tukey's test. Scanning electron microscopy (SEM) was used for fractographic analysis of the fractured surfaces. The fatigue fracture time between groups 1 and 2, and between groups 1 and 3 was significantly different (p<0.05) but there was no significant difference between groups 2 and 3 (p>0.05). A low magnification SEM views show brittle fracture as the main initial failure mode At higher magnification, the brittle fracture region showed clusters of fatigue striations and a large number of secondary cracks. These fractures typically led to a central region of catastrophic ductile failure. Qualitatively, the ductile fracture region was characterized by the formation of microvoids and dimpling. The fractured surfaces of the HEROShapers in groups 2 and 3 were always associated with pre-existing surface defects. Typically, the fractured surface in the brittle fracture region showed evidence of cleavage (transgranular) facets across the grains, as well as intergranular facets along the grain boundaries. These results show that surface defects on cutting blades of Ni-Ti rotary files might be the preferred sites for the origin of fatigue fracture under experimental conditions. Furthermore this work demonstrates the utility of fractography in evaluating the failure of Ni-Ti rotary flies.

• Journal of the Korean Society for Nondestructive Testing
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• v.18 no.3
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• pp.172-180
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• 1998

It is attempted to verify the Quantitative relationship between fracture mechanical parameters (${\Delta}K$, $K_{max}$) and X-ray parameters (residual stress, half-value breadth) of A12009-15v/o $SiC_w$ composite, and normalized SS41 steel. In this study, fatigue crack propagation test were carried out and X-ray diffraction was applied to fatigue fractured surface in order to investigate the change of residual stress and half-value breadth on fatigue fractured surface. And it is loaded prestrain to each tensile specimen, A12009-15v/o $SiC_w$ composite(0.3, 0.5, 1, 1.5, 2%) and normalized SS41 steel(0.63, 2.25, 7.50, 13.7, 20%), for investigating plastic strain rate using nondestructive measurement method. X-ray diffraction was applied to the prestrained tensile specimens in order to measure the change of residual stress and half-value breadth.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.36 no.11
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• pp.1381-1389
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• 2012

The object of this study considers corrosion fatigue improvement of 7075-T6 aluminum by using shot peening treatment on 3.5% NaCl solution at room temperature. Aluminum alloy is generally used in aerospace structural components because of the light weight and high strength characteristics. Many studies have shown that an aluminum alloy can be approximately 50% lighter than other materials. Mostly, corrosion leads to earlier fatigue crack propagation under tensile conditions and severely reduces the life of structures. Therefore, the technique to improve material resistance to corrosion fatigue is required. Shot peening technology is widely used to improve fatigue life and other mechanical properties by induced compressive residual stress. Even the roughness of treated surface causes pitting corrosion, the compressive residual stress, which is induced under the surface layer of material by shot peening, suppresses the corrosion and increases the corrosion resistance. The experimental results for shot peened specimens were compared with previous work for non treated aluminum alloy. The results show that the shot peening treatment affects the corrosion fatigue improvement of aluminum alloys and the induced compressive residual stress by shot peening treatment improves the resistance to corrosion fatigue.