Fatigue Strength Depending on Position of Cracks for Weldments

Lee Hae-Woo,Park Won-Jo The Korean Society of Mechanical Engineers 2006 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.20 No.5

This is a study of fatigue strength of weld deposits with transverse cracks in plate up to 50 mm thick. It is concerned with the fatigue properties of welds already with transverse cracks. A previous study of transverse crack occurrence, location and microstructure in accordance with welding conditions was published in the Welding Journal (Lee et al., 1998). A fatigue crack develops as a result of stress concentration and extends with each load cycle until fatigue occurs, or until the cyclic loads are transferred to redundant members. The fatigue performance of a member is more dependent on the localized state of stress than the static strength of the base metal or the weld metal. Fatigue specimens were machined to have transverse cracks located on the surface and inside the specimen. Evaluation of fatigue strength depending on location of transverse cracks was then performed. When transverse cracks were propagated in a quarter-or half-circle shape, the specimen broke at low cycle in the presence of a surface crack. However, when the crack was inside the specimen, it propagated in a circular or elliptical shape and the specimen showed high fatigue strength, enough to reach the fatigue limit within tolerance of design stresses.

Fatigue Strength Depending on Position of Cracks for Weldments

Hae Woo Lee,Won Jo Park 대한기계학회 2006 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.20 No.5

This is a study of fatigue strength of weld deposits with transverse cracks in plate up to 50 ㎜ thick. It is concerned with the fatigue properties of welds already with transverse cracks. A previous study of transverse crack occurrence, location and microstructure in accordance with welding conditions was published in the Welding Journal (Lee et aI., 1998). A fatigue crack develops as a result of stress concentration and extends with each load cycle until fatigue occurs, or until the cyclic loads are transferred to redundant members. The fatigue performance of a member is more dependent on the localized state of stress than the static strength of the base metal or the weld metal. Fatigue specimens were machined to have transverse cracks located on the surface and inside the specimen. Evaluation of fatigue strength depending on location of transverse cracks was then performed. When transverse cracks were propagated in a quarter- or half-circle shape, the specimen broke at low cycle in the presence of a surface crack. However, when the crack was inside the specimen, it propagated in a circular or elliptical shape and the specimen showed high fatigue strength, enough to reach the fatigue limit within tolerance of design stresses.

Behavioral Characteristics of Fatigue Cracks in Small Hole Defects Located on Opposite Sides of the Shaft Cross Section

Sam-Hong Song,Il-Hyuk Ahn,Jeong-Moo Lee 한국정밀공학회 2004 International Journal of Precision Engineering and Vol.5 No.4

The shaft with the circular cross section has symmetric structural combination parts to keep the rotating balance. Hence the crack usually initiates from symmetric combination parts due to the stress concentration of these parts. In this study to estimate the fatigue behavior of symmetric cracks, the fatigue test was performed by using a rotary bending tester and the specimen with symmetric defects in circular cross section. The characteristics of crack initiation and propagation on the symmetric surface cracks in circular cross section were examined. We also observed the internal crack using the oxidation coloring method and investigated the fatigue behavior using the relationship between the surface crack and the internal crack. As a result, the fatigue life of symmetric cracks was reduced by 35% compared to that of a single crack. We examined the characteristics of fatigue behavior of elements with symmetric cracks using internal crack propagation rate and maximum stress intensity factor range that were obtained from an approximation method.

Fretting fatigue crack initiation and propagation behavior of Inconel 690 alloy

Choon Yeol Lee,정한규,JaeDoKwon 대한기계학회 2016 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.30 No.11

Several studies have been conducted on the fretting fatigue limit characteristics of Inconel alloy tube material used in steam generators of nuclear power plants. Nevertheless, additional research on fretting fatigue crack initiation and propagation behavior is necessary in order to evaluate its fretting fatigue life more accurately. In this study, crack growth tests of fretting fatigue are conducted, and the characteristics of fatigue crack initiation and propagation are analyzed on Inconel 690 alloy. Also, plain fatigue crack growth tests are performed on the same material, and the results are compared with those of fretting fatigue crack growth tests. From both of the plain and fretting fatigue crack growth test results, the DK-da/dN diagrams are obtained and the crack growth rates are compared. It is found that the crack growth rate for fretting fatigue tests is faster than that for plain fatigue tests under a certain value of DK. However, over this value of DK, the crack growth rate for fretting fatigue tests becomes slower than that for plain fatigue tests due to debris which is produced by fretting and trapped in the propagated cracks. Finally, the fracture surfaces examined by an optical microscope, and the initiation angles of the oblique cracks are determined under various applied stresses. Also, the microstructure of the fracture surfaces is observed by a Scanning electron microscopy (SEM).

주철의 피로전파거동에 미치는 미세조직의 영향

김석원 ( Sug Won Kim ),박진성 ( Jin Sung Park ),이형철 ( Hyung Chul Lee ),김동건 ( Dong Keun Kim ),우기도 ( Kee Do Woo ) 한국주조공학회 2005 한국주조공학회지 Vol.25 No.1

N/A This study aims to investigate the effects of the microstructures and nodule type on the fatigue characteristics of cast iron. Fatigue tests were carried out in tension-tension mode using a servo-hydraulic testing machine with load control mode operating at a frequency of 15 Hz. The tests were conducted at stress ratio R=Kmin/Kmax, of 0.1. Initial crack ΔK values were highly per-formed with increase in tensile strength of DCI fatigue specimens. ΔK_(th) region, fatigue crack propagation was primarily advanced through cell boundary and in periphery of near nodule. Fatigue crack propagation rate of D2 consisted with 2Phase(Ferrite + Pearlite) was slow due to crack closure; enhanced by crack deflection and occurred crack branching. The generation of crack branch was occurred due to interaction of crack-nodule. At Threshold and Paris zone, the fractographs of the fatigue fracture surface for DCI show typical striations of a ductile fracture and isolated cleavage planes near graphite. The effect of microstructure on fatigue crack propagation of GC strongly depends on the type of take. The generation of crack branch occurred due to interaction of crack-nodule. The fractographs of the fatigue fracture surface for GC show cleavage plane along the flake graphite.

Fatigue Crack Growth Characteristics of Several Kinds of Structural Steels Under Low-Cycle Fatigue Loading

Chao Jiang,Takeshi Hanji,Kazuo Tateishi,Masaru Shimizu 한국강구조학회 2023 International Journal of Steel Structures Vol.23 No.1

Because low-cycle fatigue is one of the failure modes in steel structures during earthquakes, crack growth prediction in the low-cycle fatigue region has become an important factor in the evaluation of the seismic performance of steel structures. This study conducted fatigue crack growth tests with several kinds of structural steels to clarify their crack growth behaviour in the low-cycle fatigue region. These tests employed an elastic compliance technique usually used in the high-cycle fatigue region to determine crack size. Before applying the elastic compliance method to a low-cycle fatigue crack growth test, a correlation between specimen load-line crack-opening elastic compliance and crack size was analytically derived. The test results indicated that crack sizes obtained by the elastic compliance method agree relatively well with those actually measured from images. Moreover, it was revealed that the crack growth rate can be correlated with the cyclic J-integral range calculated by finite element analysis, and that the relationships between the crack growth rate and the cyclic J-integral range are similar regardless of the steel type.

Crack nucleation during mechanical fatigue in thin metal films on flexible substrates

Kim, B.J.,Shin, H.A.S.,Jung, S.Y.,Cho, Y.,Kraft, O.,Choi, I.S.,Joo, Y.C. Elsevier Science 2013 Acta materialia Vol.61 No.9

The evolution of damage due to mechanical fatigue in thin metal films on flexible substrates was investigated by in situ electrical resistance measurements. A tensile fatigue load was applied to the metal films by subjecting a single edge of the curved samples to repeated linear motion. The change in the resistance of the metal films was monitored in situ. Upon the nucleation of a fatigue-induced crack, the electrical resistance of the metal film began to increase. The resistance subsequently continued to increase with crack propagation. Therefore, in situ electrical resistance measurements can be used to identify the fatigue-induced crack nucleation cycle. The number of cycles required for crack nucleation decreased with the increase in the fatigue-stressed area of the samples. This behavior is attributed to an increase in the crack nucleation probability with increasing sample size. The amount of strain applied also modified the number of cycles required for crack nucleation according to the Coffin-Manson relationship. The increase in the electrical resistivity with respect to the number of fatigue cycles can be accurately predicted when the fatigue cycle is normalized by the nucleation cycle. This indicates that the fatigue lifetime is determined by crack nucleation and not by crack propagation.

혼합모드 하중 하에서의 피로균열닫힘과 성장거동

송삼홍(Samhong Song),서기정(Kijeong Seo),이정무(Jeongmoo Lee) 한국자동차공학회 2004 한국자동차공학회 춘 추계 학술대회 논문집 Vol.- No.-

The stress conditions acting on the practical structure are complex, and thus most cracks existing in the practical structures are under mixed-mode loading conditions. The effect of shear load component of mixed-mode loading acts more greatly in the stage of crack initiation than crack propagation stage. Hence, research on the behavior in the stage of crack initiation need to be examined in order to evaluate behavior of mixed-mode fatigue cracks. In this study, the crack opening displacement(COD) was measured by using the direct measuring method(DMM). We examined the behavior at crack tip by determining crack opening load. From the test results, the propagation behavior of mixed-mode fatigue cracks was evaluated. Also, we examined the characteristic of crack propagation under mixed-mode loading by considering propagation directions and paths of mixed-mode fatigue cracks.

Neutron diffraction residual stress analysis during fatigue crack growth retardation of stainless steel

Seo, Sukho,Huang, E-Wen,Woo, Wanchuck,Lee, Soo Yeol Elsevier 2017 International journal of fatigue Vol.104 No.-

<P><B>Abstract</B></P> <P>After tensile overloading during fatigue crack growth, retardation of the crack growth rate was significant. Neutron diffraction was employed to examine the evolution of crack-tip residual stress fields during constant-amplitude cyclic loading and during fatigue crack growth following the overload. It was found that the tensile overload induces larger compressive residual stress and zone size near the crack tip in the crack-growth and crack-opening direction. For the maximum crack growth retardation, the largest compressive residual stresses were measured in the region between an overloading point and the current propagating crack tip, for all three of the orthogonal directions. Such large compressive residual stresses in the crack-wake region are thought to reduce the crack tip driving force, thereby retarding the crack propagation rate significantly. Residual stress mapping was performed to examine the effect of the fatigue stress state on the residual stresses in the three different regions from the centerline to the surface, along the through-thickness direction in the compact-tension specimen. Much larger compressive residual stresses were measured at the surface than at mid-thickness. It is thought that larger compressive residual stresses at the surface result from the plane stress condition, resulting in a slower fatigue crack growth rate at the surface of the specimen.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Neutron diffraction was used to measure residual stress fields near the crack tip. </LI> <LI> A tensile overload induced larger compressive residual stress and zone size. </LI> <LI> The maximum retardation revealed the largest compressive residual stresses. </LI> <LI> Compressive residual stresses were larger at the surface than in the middle. </LI> <LI> Effect of the fatigue stress state on the residual stress was revealed. </LI> </UL> </P>

비드형상 및 복수 표면균열의 확률적 특성을 고려한 필릿 용접부 피로수명 평가

한승호,한정우,Han Seung-Ho,Han Jeong-Woo 대한용접접합학회 2005 대한용접·접합학회지 Vol.23 No.3

The fatigue life of welded joints is associated with crack initiation and propagation life. Theses cannot be easily separated, since the definition of crack initiation is vague due to the initiation of multiple cracks that are distributed randomly along the weld toes. In this paper a method involving a notch strain and fracture mechanical approach, which considers the characteristics of welded joints, e.g. welding residual stress and statistical characteristics of multiple cracks, is proposed, in an attempt to reasonably estimate these fatigue lives. The fatigue crack initiation life was evaluated statistically, e.g. the probability of failure occurrence in 2.3, 50 and $97.7\%$, in which the cyclic response of the local stress/strain hi the vicinity of the weld toes and notch factors derived by the irregular shape of the weld bead are taken into account. The fatigue crack propagation life was simulated by using Monte-Carlo method in consideration of the Ad-factor and the mechanical behavior of mutual interaction/coalescence between two adjacent cracks. The estimated total fatigue life, $(N_T)_{P50\%}$, as a sum of crack initiation and propagation life under the probability of failure occurrence in $50\%$ showed a good agreement with the experimental results. The developed technique for fatigue lift estimation enables to provide a quantitative proportion of crack initiation and propagation life in the total fatigue life due to the nominal stress range, ${\Delta}S$.