• Transactions of Materials Processing
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• v.30 no.1
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• pp.5-15
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• 2021

The effects of heat treatments (T6 and T73) on the microstructure, mechanical properties, and high cycle fatigue behavior of modified AA7075 alloys were investigated. A modified 7075 alloy was manufactured using modified-Mg (Mg-Al2Ca) instead of the conventional element Mg. Based on the microstructure, the average grain size was 4.5 ㎛ (T6) and 5.2 ㎛ (T73). Regardless of heat treatment, the modified AA7075 alloys consisted of Al matrix containing homogeneously distributed Al2CuMg and MgZn2 phases with reduced Fe-intermetallic compound. Room temperature tensile tests showed that the properties of modified 7075-T6 (Y.S.: 622MPa, T.S: 675MPa, elongation: 15.4%) were superior to those of T73 alloy (Y.S.: 492MPa, T.S: 548MPa, elongation: 12.8%). Experimental data show that the fatigue life of T6 was 400 MPa, about 64% of its yield strength. However, the fatigue life of T73 alloy was 330 MPa and 67%. Irrespective of the stress level, all crack initiation points were located on the specimen surface, and no inclusions acting as stress concentrators were seen. Superior mechanical properties and high cycle fatigue behavior of modified AA7075-T6 alloy are attributed to the fine grains and homogeneous distribution of small second phases such as MgZn2 and Al2CuMg, in addition to reduced Fe-intermetallic compounds.

• Korean Journal of Construction Engineering and Management
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• v.25 no.2
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• pp.36-44
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• 2024

This paper presents the application of Convolutional Neural Networks (CNNs) and Region of Interest (ROI) techniques for concrete crack analysis. Surfaces of concrete structures, such as beams, etc., are exposed to fatigue stress and cyclic loads, typically resulting in the initiation of cracks at a microscopic level on the structure's surface. Early detection enables preventative measures to mitigate potential damage and failures. Conventional manual inspections often yield subpar results, especially for large-scale infrastructure where access is challenging and detecting cracks can be difficult. This paper presents data collection, edge segmentation and ROI techniques application, and analysis of concrete cracks using Convolutional Neural Networks. This paper aims to achieve the following objectives: Firstly, achieving improved accuracy in crack detection using image-based technology compared to traditional manual inspection methods. Secondly, developing an algorithm that utilizes enhanced Sobel edge segmentation and ROI techniques. The algorithm provides automated crack detection capabilities for non-destructive testing.

• Journal of the Korean Society of Safety
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• v.30 no.4
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• pp.1-7
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• 2015

Self-piercing riveting (SPR) process is gaining popularity due to its many advantages. The SPR does not require a pre-drilled hole and has capability to join a wide range of similar or dissimilar materials and combinations of materials. This study investigated the fatigue strength of self-piercing rivet joint with aluminum alloy (Al-5052) and steel (SPCC) sheets. Static and fatigue tests on tensile-shear specimens were conducted. From the static strength aspect, the optimal punching force for the specimen with upper SPCC (U.S) sheet and lower aluminum alloy(L.A) sheets was 34 kN. During static test the specimens fractured in pull-out fracture mode due to influence of plastic deformation of joining area. There was a relationship between applied load amplitude $P_{amp}$ and number of cycles N ; $P_{amp}=19588N_f^{-0.211}$ and $P_{amp}=4885N_f^{-0.083}$ for U.S-L.A and U.A-L.S specimens, respectively. U.A-L.S fatigue specimens failed due to fretting crack initiation around the rivet neck between upper and lower sheets.

• Composites Research
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• v.12 no.5
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• pp.1-11
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• 1999

Cyclic deformation and fatigue behavior of $Al/$Al_2O_3$ metal matrix composites and matrix alloy were studied. Hatigue strength Al/$Al_2O_3$ composites was about 210MPa, and that of Al matrix alloy was 170MPa. Most of the resultant displacement due to permanent plastic deformation occurred in less than the first 5% of fatigue life. In case of composites, decrease of cyclic displacement was smaller than that of matrix because the reinforcements acted as barriers to dislocation movement. Consequently, cyclic stress-displacement response curve can be considered to have these atages ; an initial few cycles of rapid hardening, followed by progressive hardening for most the fatigue life, and then just prior to failure, an instantaneous drop in stress carrying capability of the material due to multiple microcrack initiation, eventual coalescence of microcrack to form a macrocrack and then rapid macroscopic crack growth.

• Journal of Powder Materials
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• v.25 no.4
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• pp.340-345
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• 2018

Additive manufacturing by electron beam melting is an affordable process for fabricating near net shaped parts of titanium and its alloys. 3D additive-manufactured parts have various kinds of voids, lack of fusion, etc., and they may affect crack initiation and propagation. Post process is necessary to eliminate or minimize these defects. Hot isostatic pressing (HIP) is the main method, which is expensive. The objective of this paper is to achieve an optimum and simple post heat treatment process without the HIP process. Various post heat treatments are conducted for the 3D-printed Ti-6Al-4V specimen below and above the beta transus temperature ($996^{\circ}C$). The as-fabricated EBM Ti-6Al-4V alloy has an ${\alpha}^{\prime}$-martensite structure and transforms into the ${\alpha}+{\beta}$ duplex phase during the post heat treatment. The fatigue strength of the as-fabricated specimen is 400 MPa. The post heat treatment at $1000^{\circ}C/30min/AC$ increases the fatigue strength to 420 MPa. By post heat treatment, the interior pore size and the pore volume fraction are reduced and this can increase the fatigue limit.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.34 no.10
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• pp.1411-1417
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• 2010

CFRP (Carbon Fiber Reinforced Plastic) has received considerable attention in various fields as a structural material, because of its high specific strength, high specific stiffness, excellent design flexibility, favorable chemical properties, etc. Most products consisting of several parts are generally assembled by mechanical joining methods (using rivets, bolts, pins, etc.). Holes must be drilled in the parts to be joined, and the strength of the components subjected to static and fatigue loads caused by stress concentration must be decreased. In this study, we experimentally evaluated the variation in the residual strength of a holenotched CFRP plate subjected to fatigue load. We repeatedly subjected the hole-notched specimen to fatigue load for a certain number of cycles, and then we investigated the residual strength of the hole-notched specimen by performing the fracture test. From the results of the test, we can observe the initiation of a directional crack caused by the applied fatigue load. Further, we observed that the residual strength increases with a decrease in the notch effect due to this crack. It was evaluated that the residual strength increases to a certain level and subsequently decreases. This variation in the residual strength was represented by a simple equation by using a model of the decrease in residual strength for plain plate, which was developed by Reifsnider and a stress redistribution model for hole-notched plate, which was developed by Yip.

• Transactions of the Korean Society for Noise and Vibration Engineering
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• v.22 no.10
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• pp.968-973
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• 2012

Slam testing is a mandatory testing process to evaluate the fatigue resistance of a door plate module before delivering it to car makers. This process is very hard job to complete it since the testing facilities are considerably expensive and the required testing time is relatively very long, i.e. more than eight days for a single specimen. In this paper, an accelerated testing method of a door plate module is proposed using vibration test equipment instead of the current one by exposing to the critical excitation of a door glass. Under the proposed excitation method, the similar testing result can be evaluated within less than two hours. The suitability of the proposed testing method was demonstrated by comparing failure modes of both the current testing method and the proposed one.

• Corrosion Science and Technology
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• v.18 no.2
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• pp.49-54
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• 2019

The purpose of this paper is to present failure analysis, of the heat exchanger tube in a district heating system. SS304 stainless steel is used, as material for the heat exchanger tube. The heat exchanger operates in a soft water environment containing a small amount of chloride ions, and regularly repeats operation and standstill period. This causes concentration of chloride ions on the outer surface of the tube, as well as repeat of thermal expansion, and shrinkage of the tube. As a result of microscopic examination, cracks showed transgranular as well as branched propagation, and many pits were present, at the initiation point of each crack. Energy disperstive spectroscopy analysis showed Fe and O peak, as well as Cl peak, meaning that cracks were affected by Cl ion. Failure of the tube was caused by chloride-induced stress corrosion cracking by thermal stress, high temperature, and localized enrichment of chloride ions.

• Journal of Power System Engineering
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• v.13 no.5
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• pp.52-58
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• 2009

A gas turbine consists of an upstream compressor and a downstream turbine with a combustion chamber, and also the compressor and the turbine are generally coupled using a single shaft. Many casing bolts are used to assemble two horizontally separated casings, the gas turbine casing and the compressor casing, in both of axial and vertical directions. Because drilled holes for casing bolts in vertical direction are often too close to drilled holes for casing bolts in axial direction, one can observe cracks in the area frequently during operations of a gas turbine. In this study of the root cause analysis for the cracking initiating from the drilled holes of the casings of a gas turbine, the finite element analysis(FEA) was applied to evaluate the thermal and mechanical characteristics of the casings. By applying the field operation data recorded from combined cycle power plants for FEA, thermal and thermo-mechanical characteristics of a gas turbine are analyzed. The crack is initiated at the geometrical weak point, but it is found that the maximum stress is relieved when the same type of cracks is introduced on purpose during FEA. So, it is verified that the local fracture could be delayed by machining the same type of defects near the hole for casing flange bolts of the gas turbine, where the crack is initiated.

• Journal of Korean Society of Steel Construction
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• v.25 no.5
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• pp.451-461
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• 2013

In steel bridges, there are several details that constrain the deformation such as buckling by external forces. Most of these details which are composed of the intersection members have scallops in order to exclude the weld defects inherently and to get the ease of fabrication and also to decrease the stress concentration. In this study, stress relief hole (SRH) near stress concentration zone with detail category D or under is proposed as a method to improve the resistance on the fatigue crack initiation to detail category C. And the effects of the appropriate size and location of SRH were examined and the applicability to improve the fatigue resistance of the floorbeam web and the rib wall at rib/floorbeam intersection in the orthotropic steel deck bridge was evaluated.