• Structural Engineering and Mechanics
• /
• 제53권5호
• /
• pp.867-880
• /
• 2015

Mechanical fastening is still one of the main methods used for joining components. Different techniques have been applied to reduce the effect of stress concentration of notches like fastener holes. In this work we evaluate the feasibility of combining laser shock peening (LSP) and cold expansion to improve fatigue crack initiation and propagation of open hole specimens made of 6061-T6 aluminum alloy. LSP is a new and competitive technique for strengthening metals, and like cold expansion, induces a compressive residual stress field that improves fatigue, wear and corrosion resistance. For LSP treatment, a Q-switched Nd:YAG laser with infrared radiation was used. Residual stress distribution as a function of depth was determined by the contour method. Compact tension specimens with a hole at the notch tip were subjected to LSP process and cold expansion and then tested under cyclic loading with R=0.1 generating fatigue cracks on the hole surface. Fatigue crack initiation and growth is analyzed and associated with the residual stress distribution generated by both treatments. It is observed that both methods are complementary; cold expansion increases fatigue crack initiation life, while LSP reduces fatigue crack growth rate.

• 소성∙가공
• /
• 제31권1호
• /
• pp.23-28
• /
• 2022

Although the hole expansion test is currently the most commonly used method to evaluate the stretch flangeability of HSS, it has been criticized due to its poor repeatability and reproducibility for test results. This paper focuses on the development of a new measurement method to investigate the stretch flangeability of HSS. Two materials (DP590, DP980) were investigated with a hole expansion test and a developed test method. Test results showed that the developed test method could be used as one stretch flangeability test to help identify relevant parameters of the shearing process to avoid edge cracking.

• 한국항공우주학회지
• /
• 제31권3호
• /
• pp.79-84
• /
• 2003

홀 확장법은 홀 주위에 압축 잔류응력층을 생성시킴으로서 균열 발생을 지연시키는 방법으로 항공산업 분야에서 널리 사용되고 있다. 홀 확장 잔류응력의 분포는 홀 확장률, 맨드럴 삽입방향, 재료 물성치 등 여러 가지 홀 확장 피라미터에 따라 달라진다. 홀 확장 잔류응력층의 정확한 파악은 항공산업의 실제 설계에 있어서 매우 중요함에도 불구하고 이에 대한 연구는 상대적으로 부족하였다. 본 연구에서는 유한요소해석을 통하여 두 개의 인전한 홀에 홀 확장법을 적용하는 경우 대한 잔류응력 분포를 구하였다. 연구 결과 두개 홀에 동시에 홀 확장을 하는 경우가 순차적으로 홀 확장으로 하는 경우보다 더 유리한 잔류응력 분포를 얻을 수 있었다.

• 한국항공우주학회지
• /
• 제31권2호
• /
• pp.35-40
• /
• 2003

홀 확장법은 홀 주위에 압축 잔류응력을 생성시켜 홀에서의 균열발생을 지연시키고 피로수명을 향상시키기 위한 효과적인 방법이다. 여러 연구들이 이러한 이로운 효과에 초점을 맞추어 온 반면, 홀 확장 잔류응력이 존재하는 홀 쪽으로 성장해 가는 균열의 피로균열 성장에 관해서는 연구가 거의 이루어져 있지 않다. 본 논문의 목적은 홀 확장 잔류응력하의 피로특성을 평가하는 것이다. 중앙 홀을 갖는 평판에 대해 홀 확장법을 적용한 후 피로시험을 수행하여 홀 확장 잔류응력이 피로수명에 미치는 영향을 평가하였다. 또한 CT 시험편에 홀 확장법을 적용하여 잔류응력을 생성시킨 후 피로균열성장 시험을 수행하여, 홀 확장 잔류응력에 의한 피로균열성장 특성의 변화를 고찰하였고, 홀 확장률의 영향을 평가하였다.

• 한국전기전자재료학회:학술대회논문집
• /
• 한국전기전자재료학회 2006년도 추계학술대회 논문집 Vol.19
• /
• pp.233-234
• /
• 2006

In this paper, Via hole pore were investigated during PLAS (PessureLess Assisted Constrained Sintering) process of LTCC. Ag and Ag-Pd paste mixture were tested for via paste. Ag paste with 10~25% Ag-Pd paste showed no via hole pore, but further increase of Ag-Pd contents in via paste increased via pore. From shrinkage curve, 10~25% Ag-Pd paste showed expansion behaviors before shrink and this phenomena result in the reduction of via hole pore during PLAS process.

• 대한기계학회:학술대회논문집
• /
• 대한기계학회 2000년도 추계학술대회논문집A
• /
• pp.470-474
• /
• 2000

Cold expansion of fastener holes is a mechanical process widely used in the aerospace industry. This treatment leads to an improvement of fatigue behavior due to the developed compressive residual stresses on the hole surface. The residual stress profile depends on the parameters of cold expansion, which are, expanding rate, inserting direction of mandrel, material properties etc. and the method to confirm this profile is only measurement by X-ray diffractometer. Despite its importance to aerospace industries, little attention has been devoted to the accurate modelling of the process. In this paper, Two-dimensional axisymmetric finite element simulations have been conducted for the cold expansion in an aluminium plate in order to predict the magnitude and distribution of the residual stress and plastic deformation. Maximum compressive residual stress could be increase about 7 percent using the 2-step cold expansion method.

• 한국항공우주학회지
• /
• 제30권2호
• /
• pp.46-51
• /
• 2002

홀 확장법은 홀 주위에 압축 잔류응력 층을 생성시킴으로서 균열 발생을 지연시키는 방법으로 항공산업 분야에서 널리 사용되고 있다. 홀 확장잔류응력의 분포는 홀 확장률, 맨드럴 삽입방향, 재료 물성치 등 여러 가지 홀 확장 파라미터에 따라 달리진다. 홀 확장 잔류응력 층의 정확한 파악이 항공사업의 실제 설계에 있어서 매우 중요함에도 불구하고 이에 대한 연구는 상대적으로 부족하였다. 본 연구에서는 3차원 유한요소해석을 통하여 알루미늄 평판에 홀 확장법을 적용하는 경우의 잔류응력 분포를 예측하고자 하였다. 유한요소해석의 타당성을 검증하기 위하여 스트레인 게이지를 이용한 홀 확장 잔류변형률을 측정하였으며, 홀 확장률과 2단 홀 확장이 잔류응력 분포에 미치는 영향을 파악하였다. 2단 홀 확장을 적용함으로써 최대 압축 잔류응력의 크기가 약 7% 향상됨을 알 수 있었다.

• 한국정밀공학회지
• /
• 제20권3호
• /
• pp.165-171
• /
• 2003

In a structure, many components are fastened together using bolts, nuts and rivets through drilled holes. Not only do these fastened joints enable easy assembly and dismantling, they are also able to transfer as well as to distribute loads applied onto the structures. The major drawback of such connections, however, is that the fatigue life of the components is reduced by the presence of the hole since the stress concentration around the hole is increased. In addition, the hole drilling process itself may introduce defects or roughness at the surfaces of the hole that may cause further decrease in fatigue performance of the components. For applications where fatigue loadings are important, one way to compensate the decrease in fatigue life of the components is by introduction of beneficial compressive residual stress around the hole using cold working. The material used for this research were A12024-T351 and A17050-T7451 using the primary member of aircraft. We present, In this paper, the characteristics of coldworking by evaluation of the hole expansion ratio, residual stress distribution, and fatigue properties.

• 한국기계가공학회지
• /
• 제11권3호
• /
• pp.73-79
• /
• 2012

This paper describes the machining characteristics of the sintered carbide and die steel(STD-11) by electric discharge drilling with various tubular electrodes. Electrical discharge machining(EDM) removes material from the workpiece by a series of electrical sparks that cause localized temperatures high enough to melt or vapourize the vicinity of the charge. In the experiment. four types of electrode which have different diameter are used with the application of continuous direct current and axial electrode feed. The controlled factors include the dimension of the electrode. In drilling by EDM, the dielectric flushed down the interior of the rotating tube electrode, in order to order to facilitate the removal of machining debris the hole. The expansion increase with increasing the thickness of material and the diameter of electrode and the expansion of sintered carbide is 1.75 times large then that of die steel. The taper of machined hole decrease with increasing the thickness of material. The crater sixe of die steel is larger then thet of sintered carbide and the surface roughness of sintered carbide is 1.58 tims larger then that of die steel.

• 대한기계학회논문집A
• /
• 제26권1호
• /
• pp.147-152
• /
• 2002

Effect of lip shape on the hole flangeability of high strength steel sheets is investigated. Circular plates of various hole sizes are tested and the variation of lip length as well as the variation of thickness on the sectional views of the finished lip were studied. The conventional hole flanging process is limited to a certain limit hole diameter below which failure will ensue during the hole expansion. The intention of this work is to examine the effect of lip shape on the flangeability of TRIP steel and Ferrite-Bainite duplex steel and find out major parameters which can affect flanging shape of high strength hot rolled steels. Over the ranges of conditions investigated, the minimum hole diameter of F+B steel is better than TRIP steel. while, the lip-shape accuracy of TRIP steel is better than that of F+B steel. although the tensile strength and elongation of %P steel are superior than those of Ferrite-Bainite duplex steel, the flangeability is found to be not so strongly sensitive to the tensile properties but sensitive to displacement on the circumferential direction of hole edge.