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1.
The fatigue crack initiation and propagation behavior of a niobium bearing HSLA steel heat treated to give two tempered martensitic
microstructures presumably with and without fine niobium carbides has been studied by light microscopy, electron microscopy,
and strain gage measurements of plastic zone deformation. The high cycle, stress controlled fatigue life of the steel in both
heat treated conditions was quite similar with the steel presumably containing the fine niobium carbides having slightly better
resistance at low stress amplitudes. This slightly better high cycle resistance is associated with better resistance to fatigue
crack initiation for this heat treatment. The fatigue crack propagation behavior of the steel was the opposite. The steel
presumably containing the fine niobium carbides exhibited a much faster fatigue crack growth rate than that without them.
The difference in growth rates is explained in terms of the plastic work expended during the propagation of the fatigue crack. 相似文献
2.
C. Fukuoka H. Yoshizawa Y. G. Nakagawa M. E. Lapides 《Metallurgical and Materials Transactions A》1993,24(10):2209-2216
Fatigue tests were performed to examine how microstructural conditioning influences crack initiation and propagation in SA508
class 3 low-carbon steel. A 3-mm-long crack was introduced in compact tension (CT) fatigue test specimens under four different
loads in order to obtain crack tip plastic zones at different stress intensity factor ranges, ΔK = 18, 36, 54, and 72 MPa√m.
The microstructure of the plastic zones around the crack tip were examined by trans- mission electron microscopy (TEM) and
selected area electron diffraction (SAD). Micro- orientation of the dislocation cells in the plastic zones of all of the CT
samples increased to 4 deg from the level of an as-received sample. Four-point bending fatigue tests were performed for plate
shape samples with a large cyclic strain range. The SAD value of the bending samples was also 4 deg in the damaged area where
cracks already initiated at an early stage of the fatigue process. These test results indicate that the microstructural conditioning
is a prerequisite for the fatigue crack initiation and propagation in SA508. These observations may lead to better under-
standing of how fatigue initiation processes transit to cracks. 相似文献
3.
Use of the nanoindentation technique for studying microstructure/crack interactions in the fatigue of 4340 steel 总被引:2,自引:0,他引:2
Fan Yang Ashok Saxena Laura Riester 《Metallurgical and Materials Transactions A》1998,29(12):3029-3036
The objectives of this research are to study the influence of microstructure on the fatigue crack growth behavior in 4340
steel and to explore the application of the nanoindentation technique for determining the plastic deformation zone at a fatigue
crack tip. Two heat treatment conditions were chosen for the steel: annealed and quenched plus tempered. The annealed steel
consists of coarse pearlite and proeutectoid ferrite, while the quenched and tempered steel consists of fine tempered martensite.
Fatigue crack propagation tests were conducted on disklike compact (DCT) specimens. Subsequently, the nanoindentation technique
was applied to quantitatively determine the plastic deformation zone at fatigue crack tips. The plastic deformation zone size
determined by the nanoindentation test seems larger than the cyclic deformation zone calculated using the fracture mechanics
equation, which involves many assumptions. The fatigue crack growth test results show that the annealed steel has a higher
resistance to crack growth than the quenched and tempered steel. The fatigue crack in the annealed steel tends to grow along
pearlite domain boundaries, or the cementite/ferrite interfaces within a pearlite domain. In contrast, the fatigue crack in
the quenched and tempered steel tends to traverse the fine martensite laths. Consequently, the actual crack path in the annealed
steel is rougher than in the quenched and tempered steel and more secondary cracks are observed in the annealed steel. 相似文献
4.
The effect of residual stresses and loading frequencies on corrosion fatigue crack growth behavior under synthetic seawater
with a free corrosion potential was examined using center-cracked tension (CCT) and single edge-cracked tension (SECT) specimens
machined from mild steel butt-welded joints and the parent material. A series of fatigue crack growth tests were carried out
with a sinusoidal loading wave form at a stress ratio of 0.05 with a loading frequency of 0.017 to 6.7 Hz. The results show
that the crack growth resistance of a weld metal in the SECT specimen is higher than that in the CCT specimen regardless of
testing conditions. The discrepancy is attributed to the differences in residual stress distribution at the crack tip in the
two specimen geometries. The crack growth rate of the weld metal in the CCT specimen in seawater increased with decreasing
loading frequency. The acceleration of the crack growth rate may be related to the occurrence of brittle striation or cleavage
due to hydrogen embrittlement. It was found that the corrosion fatigue crack growth rate of a welded joint with tensile residual
stress can be predicted using the effective stress intensity factor range, which takes into account both the residual stress
and the loading frequency effects. 相似文献
5.
《钢铁研究学报(英文版)》2015,(7)
Very high cycle fatigue behavior(107-109 cycles)of 304 Laustenitic stainless steel was studied with ultrasonic fatigue testing system(20kHz).The characteristics of fatigue crack initiation and propagation were discussed based on the observation of surface plastic deformation and heat dissipation.It was found that micro-plasticity(slip markings)could be observed on the specimen surface even at very low stress amplitudes.The persistent slip markings increased clearly along with a remarkable process of heat dissipation just before the fatigue failure.By detailed investigation using a scanning electron microscope and an infrared camera,slip markings appeared at the large grains where the fatigue crack initiation site was located.The surface temperature around the fatigue crack tip and the slip markings close to the fracture surface increased prominently with the propagation of fatigue crack.Finally,the coupling relationship among the fatigue crack propagation,appearance of surface slip markings and heat dissipation was analyzed for a better understanding of ultrasonic fatigue damage behavior. 相似文献
6.
Safe use of welded structures is dependent on fracture mechanics properties of welded joints. In present research, high strength low alloyed HSLA steel in a quenched and tempered condition, corresponding to the grade HT 80, was used. The fluxo cored arc welding process (FCAW), with CO2 as shielding gas, was used and two different tubular wires were selected. The aim of this paper is to analyse fracture behaviour of undermatched welded joints, and also to determine relevant parameters which contribute to higher critical values of fracture toughness. Towards this end three differently undermatched welded joints were analysed using results of testing the composite notched specimens with through thickness crack front positioned partly in the weld metal, partly in heat affected zone (HAZ) and partly in base material (BM).The presence of different microstructures along the pre‐crack fatigue front has an important effect on the critical crack tip opening displacement (CTOD). This value is the relevant parameter for safe service of welded structure. In the case of specimens with through thickness notch partly in the weld metal, partly in the heat affected zone and partly in the base material, i.e. using the composite notched specimen, fracture behaviour strongly depends on a partition of ductile base material, size and distribution of mismatching factor along vicinity of crack front. If local brittle zones occur in the process zone, ductile base metal can not prevent pop‐in instability, but it can reduce it to an insignificant level while the fracture toughness parameter is higher and the weakest link concept can not be applied. 相似文献
7.
8.
The effectiveness of MIG welding with Argo‐shield gas & ER70S‐6 electrode in joining LRS (Grade‐B) steel was investigated through structure–property correlation of the joint region. Microstructure, tensile and fatigue properties, and mode of fracture (SEM fractograph) were correlated. Fatigue behavior has been investigated in air and sea water with thin specimen at near‐endurance stress amplitude up to 105 cycles. The crack growth rate (da/dN) maintained a non‐linear relationship with logarithm of stress intensity factor range (logΔK) for the near‐threshold values of ΔK. Considerable hardness and microstructural variation was observed across the weldment. Weld with more pearlite content was found to possess higher hardness and strength than the parent steel. Though, both in weld and in parent steel, either in air or in sea water, fatigue crack propagated at very slow rate with significant intermittent crack arrest, weld provided much higher resistance to crack growth in air. However, sea water accelerated the crack growth in weld and brought it closer to that in the parent steel. The morphologically complex microstructure of weld suffered much faster crack propagation in sea water than in air. While fatigue fracture in parent steel (both in air and sea water) and weld in air was found to occur through dimple rupture via microvoid coalescence, weld in sea water exhibited a mixed mode of failure. 相似文献
9.
摘要:为提高焊接构件的动载疲劳寿命,以热模拟为试验手段,对Q700D高强钢进行了焊接热模拟,研究了粗晶热影响区的疲劳寿命、小裂纹扩展行为以及组织软化特征。利用Paris方程和轴向拉伸疲劳试验数据,建立了ΔKth值与模拟粗晶区疲劳寿命的对应关系,利用ΔKth值实现了快速预估粗晶区疲劳寿命。研究表明:相同应力幅值下的lgN值与ΔKth值存在一定的线性拟合关系,即ΔKth值越大,则疲劳寿命N越长。小裂纹扩展微观机理在于所形成的大角度晶界(不小于15°)对小裂纹尖端的止裂性较强,可迫使小裂纹尖端转向耗能。CGHAZ的软化与第二相粒子回熔与粗化有关,粗化的第二相粒子易萌生小裂纹,可通过提高大角度晶界抑制裂纹扩展。 相似文献
10.
The microscopic fracture behaviors of a C-Mn steel plate and of two types of weld metal in Charpy V, crack tip opening displacement
(COD), and precracked impact specimens have been studied by observations of the size distribution of the cleavage-initiating
particles, the morphology of the fibrous crack zones, the microstructure in the region initiating the cleavage crack, and
the size distribution of microcracks remaining in the specimens. It has been found that the different values of the microscopic
cleavage fracture stress, σf, measured in notched and precracked specimens of the same metal result from the change in the critical event (critical length)
in the fracture processes. The critical event in the Charpy V-notched specimens at —45 °C to —65 °C is the propagation of
a ferrite grain-sized microcrack into the ferrite matrix; however, in the precracked COD specimens at —110 °C, it is the propagation
of a second-phase particle-sized microcrack into the neighboring ferrite grain. The change of the critical event is considered
to be related to the difference of effective shear stress ahead of a notch root or a crack tip. The different sensitivities
to the notch acuity of the base and weld metal result from their micro-structures. Finally, the prerequisite for establishing
a correlation between Charpy V and COD tests is analyzed. 相似文献
11.
12.
J. Wareing 《Metallurgical and Materials Transactions A》1975,6(8):1367-1377
Fatigue failure in metals and alloys occurs by the nucleation and controlled propagation of a surface crack. At ambient temperature
propagation is transgranular and is controlled, to a large extent, by continuum plasticity effects at the crack tip. At elevated
temperatures this simple process might be affected by oxidation and the tendency towards intergranular propagation. The elevated
temperature fatigue crack propagation behavior of a 20/25/Nb stainless steel and a type 316 stainless steel is studied by
optical measurement of the crack growth rate of artificially-induced notches under conditions where gross plastic straining
is present in the bulk of the material. Tests conducted at ambient temperature are included for comparison with elevated temperature
behavior. By reference to fatigue life data for smooth unnotched specimens, tested under identical conditions to the crack
growth tests, an attempt is made to rationalize the roles of crack initiation and propagation in the fatigue process and indicate
the relevance of crack growth data in predicting fatigue life data. 相似文献
13.
For the study of the DP980 steel high cycle fatigue property, high fatigue tests of DP980 steel and DP980 steel laser welded joints were carried out with fatigue testing machine, the Basquin equation was concluded. Microstructures and fractures were analyzed by optical microscope and scanning electron microscope. The results show that DP980 steel laser welding joints have the weld concavity at the welding root and top, the quality of welded joints is medium. The fatigue limit of DP980 steel is 341MPa, the fatigue limit of DP980 steel laser welded joint is 148MPa, the fatigue limit decreases by 50% compared with the fatigue limit of the base metal. For DP980 steel, the crystal boundary of the ferrite/martensite is the main location of micro cracks initiation, the fatigue fracture of DP980 steel is the quasi cleavage fracture. For DP980 steel laser welding joints, the fatigue cracks initiation is located in the weld concavity, not in the heat affected zone, the fatigue fracture is cleavage fracture. DP980 steel and DP980 steel laser welding joints crack propagation is characterized by the obviously fatigue striations coupled with secondary cracks. 相似文献
14.
摘要:为了研究DP980钢的高周疲劳性能,采用疲劳试验机对DP980钢和DP980钢激光焊接接头进行高周疲劳试验,得到Basquin方程,并利用光学金相显微镜和扫描电镜进行组织和断口分析。结果表明:DP980钢激光焊接接头的焊缝根部和顶部出现形状凹陷,焊接接头的质量为中等。DP980钢疲劳极限为341MPa,DP980钢激光焊接接头的疲劳极限为148MPa,激光焊接接头的疲劳极限较母材的疲劳极限降低约50%。对于DP980钢而言,铁素体/马氏体晶界是裂纹萌生的主要位置,疲劳断口为准解理断口。对于DP980钢激光焊接接头而言,疲劳裂纹源位于焊缝凹陷处,而非热影响区及母材,疲劳断口为解理断口。DP980钢和DP980钢激光焊接接头的疲劳裂纹扩展区均有明显的疲劳条带,并伴随有二次裂纹。 相似文献
15.
Yo Tomota Shin Daikuhara Shun Nagayama Masanori Sugawara Norihiko Ozawa Yoshitaka Adachi Stefanus Harjo Shigeo Hattori 《Metallurgical and Materials Transactions A》2014,45(13):6103-6117
Three-dimensional microstructure observations, macro- to micro-scopic residual stress measurements by three methods and creviced bent beam SCC tests were performed for Inconel/low alloy steel (LAS) weld samples. The possible reasons for the suppression of SCC crack propagation near the weld interface found at a nuclear power plant were estimated to include the crack branching at the grain boundary (GB) parallel to the interface, i.e., Type II GB, compressive residual stresses in the LAS region and crack tip oxidation in the LAS at the interface. The formation mechanism of Type II GB and stress gradient in individual grains in the Inconel are also discussed. 相似文献
16.
Milovi Ljubica Vuherer Tomaz Zrili Milorad Momilovi Dejan Jakovi Dragan 《钢铁研究学报(英文版)》2011,(Z1):888-892
Welded joint is a critical region of a welded structure.The operational safety of welded pressure equipment mostly depends on the behaviour of loaded weldments.Safety of welded structure is dependent on the properties of welded joint as whole and of its constituents (parent metal,heat affected zone and weld metal).In this paper the behaviour of welded joint cracked constituents is considered.Structural integrity assessment procedure is applied to welded pressure vessel produced of high-strength low-alloyed steel,operating at-40°C,comparing crack driving force and material crack resistance,using path-independent contour J-integral as fracture mechanics parameter.The comparison of crack driving force,expressed by J-integral and material resistant curve,J-R curve,provide the possibility to determine the extent of the stable crack as well as the critical crack size for its final fast propagation and also to assess the structural integrity of a cracked pressure vessel. 相似文献
17.
J. T. Al-Haidary A. A. Wahab E. H. Abdul Salam 《Metallurgical and Materials Transactions A》2006,37(11):3205-3214
The fatigue crack propagation rate (FCPR) in 316L austenitic stainless steel (ASS) and its weldments was investigated, at
two loading amplitudes, 7 and 8.5 kN, under tension-tension mode. Two welding techniques, submerged arc welding (SAW) and
manual arc welding (MAW), have been used. Magnetic δ-ferrite, depending upon Ni and Cr content in the metal, in the weld zone
upon solidification was considered. The ferrite number (FN) of δ-ferrite formed in the SAW zone was much higher (maximum 9.6)
compared to the corresponding value (maximum 0.75) in the MAW zone. A fatigue starter notch was positioned at different positions
and directions with respect to the weld zone, in addition to the heat-affected zone (HAZ). Regions of high and low FCPRs as
the fatigue crack propagated through and across the weld zone have been noticed. This is related to the direction of the tensile
residual stresses present in weld zone, resulting from solidification of the weld metal. The FCPR was higher along through
the HAZ and weld zone because of the microstructural change and direction and distribution of tensile residual stresses. The
FCPR was much lower when crack propagated perpendicular to the weld zone, particularly in the case of SAW in which higher
δ-ferrite volume fraction was noticed. A lower FCPR found across the weld zone, in both SAW and MAW, was accompanied by rubbed
areas in their fractures. 相似文献
18.
19.
通过断裂试样断口的宏观和显微分析、显微组织表征、拉伸和冲击试验以及解理断裂应力条件,讨论分析了锌锅用低强度级别钢板弯曲成形断裂的微观解理断裂行为。结果表明,钢板发生解理断裂的微观机制与冲击试样断裂相同,即晶粒尺寸控制的穿过晶界的裂纹扩展是解理断裂的临界事件。粗大的铁素体晶粒的面积分数过高显著降低了裂纹扩展阶段所需的局部解理断裂应力σf。断口宏观分析判断在钢板边部应存在导致应力集中的初始裂纹源,这极大降低了启动解理断裂的断裂应力并同时提高裂纹源前端的正应力σyy,扩大了解理断裂活跃区至初始裂纹前端,从而不可避免地发生脆性解理断裂。 相似文献
20.
Load-controlled fatigue tests were conducted on dual-phase X80 pipeline steel to investigate the effects of stress ratio (R-ratio) on the fatigue crack growth behaviour. Dual-phase X80 pipeline steel showed a non-linear relationship between fatigue crack growth rate (da/dN) and the stress intensity factor range (ΔK) at each R-ratio. Fatigue crack propagation curves of X80 pipeline steel were evaluated using the conventional Paris equation and a new exponential equation named αβ model. In addition, the electron back-scattered diffraction technique was used to study the effects of stress ratio on the fatigue crack growth behaviour. The results indicated that the corresponding ΔK of the transition point decreased with the increase of R-ratio. That was attributed to the variation of the crack path and the fracture mode because of the changes in the size of monotonic plastic zone and cyclic plastic zone at crack tip. Compared to the overall fitting, piecewise fitting by Paris equation and αβ model, piecewise fitting was the most accurate method, and αβ model is more convenient and efficient than the conventional Paris-based equations. 相似文献