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通过对带钢窄边裂纹缺陷的金相显微组织观察和扫描电镜与能谱分析,确定了带钢窄边裂纹缺陷的形成原因,并针对原因提出了相应的预防措施,有效地控制了带钢窄边裂纹缺陷的重复发生。 相似文献
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通过对带钢窄边裂纹缺陷的金相显微组织观察和扫描电镜与能谱分析,确定了带钢窄边裂纹缺陷的形成原因.并针对原因提出了相应的预防措施.有效地控制了带钢窄边裂纹缺陷的重复发生。 相似文献
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通过对带钢Q235B边裂缺陷进行检验和分析,认为造成此次带钢边裂的主要原因是铸坯边部细小裂纹、深振痕、皮下气泡、针孔以及轧钢划伤引起的,通过采取措施减少铸坯边部缺陷和划伤,可以减少边裂缺陷。 相似文献
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针对通化钢铁股份有限公司生产热轧Q195带钢过程中存在边部裂纹缺陷的难点问题,通过对边裂的统计分析、原料坯的成分检验,对其形成原因及机理进行了研究.结果表明:原料坯中C、Mn、Si等元素的含量对热轧带钢边裂缺陷的影响明显,采用了钢水成分调整工艺措施后,有效地控制了带钢边裂质量问题. 相似文献
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制品受到热冲击后内部会出现热应力,当热应力过大时制品中会产生裂纹。硅砖中裂纹包括表面裂纹和层裂。表面裂纹又分为横向裂纹、竖向裂纹和网状裂纹。通常层裂和竖向裂纹是机压所致,横向裂纹和网状裂纹是烧成造成的。影响裂纹生成的主要因素是烧成和机压,但与生产过程有关的其它因素的作用也不容忽视。 相似文献
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Effects of crack aspect ratio on the behavior of small surface cracks in fatigue: Part I. Simulation
K. S. Ravichandran 《Metallurgical and Materials Transactions A》1997,28(1):149-156
A simple simulation of alternate growth of a small surface crack in the surface and depth directions was performed to illustrate
the changes in crack aspect ratio, induced by grain boundaries, as a function of crack size. It is shown that at small crack
sizes, large variations in aspect ratio, a/c (a is the crack depth and c is the half-surface length), occur, due to local crack front perturbations induced by grains that are oriented for crack
growth. At these crack sizes, the assumption of a semicircular crack shape (a/c=1.0) was found to cause errors in stress intensity range (ΔK) calculations. This, in turn, led to significant scatter or “anomaly” in small crack growth rates relative to large cracks.
At large crack sizes, the effects of local crack front perturbations on crack aspect ratio and ΔK were found to be insignificant. As a result, the scatter in crack growth data was found to decrease to a negligible level
at large crack sizes. It is suggested that the limiting crack size above which the small crack behaves as a large crack, l
2=10d (d = grain size), proposed by Taylor and Knott, is related to the crack size above which the effects due to aspect ratio variations
are small. 相似文献
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W. J. Drury Arun M. Gokhale S. D. Antolovich 《Metallurgical and Materials Transactions A》1995,26(10):2651-2663
The geometry of crack faces often plays a critical role in reducing crack extension forces when crack closure occurs during
fatigue crack growth. Most previous studies of fatigue crack closure are concerned with mechanical measures of closure as
related to the crack growth rate; very little attention has been given to the geometry of the crack surfaces. Our objective
is to identify those aspects of crack surface geometry that are important in the closure process, to develop quantitative
fractographic techniques to estimate such attributes in a statistically significant and robust manner, and to correlate them
to the physical process of crack closure. For this purpose, fatigue crack propagation experiments were performed on a Ni-base
superalloy and crack growth rates and crack closure loads were measured. Digital image profilometry and software-based analysis
techniques were used for statistically reliable and detailed quantitative characterization of fatigue crack profiles. It is
shown that the dimensionless, scale-independent attributes, such as height-to-width ratio of asperities, fractal dimensions,
dimensionless roughness parameters,etc., do not represent the aspects of crack geometry that are of primary importance in the crack closure phenomena. Furthermore,
it is shown that the scaledependent characteristics, such as average asperity height, do represent the aspects of crack geometry
that play an interactive role in the closure process. These observations have implications concerning the validity of geometry-dependent,
closure-based models for fatigue crack growth. 相似文献
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A fracture mechanics-based numerical simulation of a crack propagating under mode I loading through discontinuously reinforced
metal matrix composites (MMCs) is presented. Microcrack initiation due to debonding and breakage of reinforcements, shielding
and antishielding effects caused by both microcracks and the reinforcements, the effect of crack deflection, and growing crack
singularity are considered in the calculation of local crack tip driving forces. Statistical variations of spatial distribution
and strength of the reinforcements are also considered. The essential feature of the model is to predict crack initiation
toughness and crack path morphologies using a mixed-mode crack propagation criterion. Application of the program to predict
crack growth behavior in an SiC whisker/Al alloy composite is presented. Microcracking far ahead of the tip of a main crack,
crack deflection toward the microcracks, and subsequent incorporation of the microcracks which most affect the main crack
are well simulated. The predicted microcrack distribution and variation of mixed-mode crack tip driving forces with crack
growth are also evaluated. 相似文献
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《Acta Metallurgica》1987,35(7):1415-1432
Fatigue crack growth behavior of 4340 steels was investigated in four gaseous environments; laboratory air, wet hydrogen, dry hydrogen and dry helium. Specimen orientation does not affect crack propagation rate results. The effects of R-ratio (load ratio) and environment on crack growth rate properties are interrelated. Increasing R -ratio increases the rates of near-threshold crack propagation. Nevertheless, the effect of R-ratio on crack growth rates in air is much more significant than that in the two dry environments. Interestingly, the R-ratio effect in wet hydrogen is comparable to that in dry environments. At an R-ratio of 0.1, the rates of crack propagation in air are slower than those in dry environments while crack growth rates are essentially identical in wet hydrogen and dry environments. Increasing R -ratio was found to decrease the environmental effect. Furthermore, increasing yield strength from 700 to 1040 MPa does not affect crack propagation behavior. While surface roughness-induced crack closure is thought to be minimal in affecting gaseous-environment near-threshold crack growth behavior of 4340 steels, oxide-induced crack closure governs crack propagation kinetics. It is suggested that in moisture-containing environments, thick oxide deposits measured on fracture surfaces may not result in high crack closure levels. Nevertheless, oxide-induced crack closure rationalized the effects of R-ratio and environment on near-threshold crack growth rate properties. Furthermore, hydrogen embrittlement is believed not to play an important role in influencing wet-hydrogen environment near-threshold crack propagation behavior. At higher ΔK levels (⩾ 12 MPa √m), an “intrinsic” dry hydrogen effect seems to be present, and crack closure, however, cannot account for the environmental effect. 相似文献
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Md. Shahnewaz Bhuiyan Hiroyuki Toda Kazuyuki Shimizu Hang Su Kentaro Uesugi Akihisa Takeuchi Yoshio Watanabe 《Metallurgical and Materials Transactions A》2018,49(11):5368-5381
High-resolution synchrotron X-ray microtomography has been successfully used to evaluate the local crack driving force at arbitrary crack tip locations as a form of CTOD. This is to our knowledge the first experimental evidence in supporting a correlation between the local fracture toughness associated with the corresponding hydrogen-assisted fracture mode including quasi-cleavage, intergranular, and dimple. Our results have revealed that very small CTOD, of about 1.26 μm, is observed when the crack tip is located in the quasi-cleavage fracture. Compared to quasi-cleavage fracture, the CTOD values increase by a factor of 5 when the crack tip is located in intergranular fracture mode and even greater increase in CTOD (of about 18 times) is observed when the crack tip is located in dimple fracture mode. We also observed that the crack propagation process under the influence of hydrogen deviates greatly from that of standard behavior, where stable crack growth is accompanied by a change in crack tip singularity from the HRR to the RDS. It was concluded that the presence of high concentration of hydrogen ahead of the crack tip increases the slip localization, and thereby reduces crack tip blunting. Hence crack continues to grow before the crack tip becomes fully blunt. 相似文献
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Different stages of the Very High Cycle Fatigue (VHCF) crack evolution in tool steels have been explored using a 20 kHz ultrasonic fatigue testing equipment. Extensive experimental data is presented describing VHCF behaviour, strength and crack initiating defects in an AISI H11 tool steel. Striation measurements are used to estimate fatigue crack growth rate, between 10?8 and 10?6 m/cycle, and the number of load cycles required for a crack to grow to critical dimensions. The growth of small fatigue cracks within the “fish‐eye” is shown to be distinctively different from the crack propagation behaviour of larger cracks. More importantly, the crack initiation stage is shown to determine the total fatigue life, which emphasizes the inherent difficulty to detect VHCF cracks prior to failure. Several mechanisms for initiation and early crack growth are possible. Some of them are discussed here: crack development by local accumulation of fatigue damage at the inclusion – matrix interface, hydrogen assisted crack growth and crack initiation by decohesion of carbides from the matrix. 相似文献
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Haibo Xie Zhengyi Jiang W. Y. Daniel Yuen 《Metallurgical and Materials Transactions B》2011,42(6):1244-1252
Edge cracks in cold rolling of the thin strip affect the strip quality and productivity significantly. In this study, an experimental
and mechanical investigation on microstructures has been carried out to study the edge crack formation during cold rolling
of the thin strip. The effects of the feed material microstructures on the edge crack evolution were studied employing optical
microscopy and scanning electron microscopy (SEM). Experimental observation indicates that fine grain occurs in hot-rolled
microstructure and coarse grain is produced in ferritic rolled microstructure. Different grain sizes affect significantly
the formation mechanics of the microcrack, crack initiation, and orientation of crack extension. The grain size and grain
boundaries effects on crack retardation are discussed also during edge crack initiation. During the crack growth in coarse
grain, most edge crack tips will blunt, which improves the crack toughness by causing less stress concentration. Overall,
the fine microstructure shows a good crack initiation resistance, whereas the coarse microstructure has a better resistance
to crack propagation. This research provides additional understanding of the mechanism of microstructure influence on edge
crack evolution of cold strip rolling, which could be helpful for developing defect-free thin strip. 相似文献