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1.
The effects of microstructure, impurity content, and testing temperature on the fracture toughness (as measured by the crack
tip opening displacement (CTOD)) and microcrack extension resistance curves (R curves) of Ti-6Al-4V alloys were examined. At 0 °C, microstructure is the most influential factor in the toughness-strength
relationship. Acicular microstructure specimens have a higher CTOD than specimens with equiaxed microstructures, regardless
of strength (0.2 pct proof stress) and impurity content. At −196 °C, impurity content becomes a controlling factor in the
toughness-strength relationship. Extra-low impurity (ELI) specimens, which have a lower impurity content, show a higher CTOD,
irrespective of microstructure. Microcracks extended from the notch tip before the maximum load was reached during testing
were investigated, and crack initiation (δ
i) and extension-resistance properties were evaluated by obtaining exact R curves of the microcracks. At 0 °C, specimens with different microstructures and different impurity contents have almost
the same δ
i. But acicular-microstructure specimens with a higher CTOD at a given strength show a greater crack extension resistance.
At −196 °C, ELI specimens, which have a higher CTOD, show a larger crack extension resistance. It is concluded that the crack
extension-resistance property of the microcracks extended from the notch tip before the maximum load is a controlling factor
for the fracture toughness of Ti-6Al-4V alloys. 相似文献
2.
Bin Wang Yingchao Xu Jing Hu Senfeng Zhang Chengwu Cui Hongxia Lan 《Transactions of the Indian Institute of Metals》2018,71(10):2517-2526
Aiming at the security problems of pipeline steel application, the different positions of the welded joints of circumferentially welding pipeline of X80 steel were investigated by microstructure observation, the hardness, Charpy impact toughness and crack tip opening displacement (CTOD) test at low temperature. The Vickers hardness test results show that there are local softened regions in heat-affected zone (HAZ). Charpy impact test indicate that the ductile–brittle transition temperature of weld is below ??60 °C, the ductile–brittle transition temperature of HAZ is around ??38 °C. CTOD test reveal that the fracture toughness of HAZ shows a large fluctuation since it is in the ductile–brittle transition temperature regime. 相似文献
3.
J. H. Zhu P. K. Liaw J. M. Corum J. G. R. Hansen J. A. Cornie 《Metallurgical and Materials Transactions A》1998,29(11):2855-2862
Mechanical behavior and damage mechanisms of an Al2O3 particulate-reinforced Al matrix composite (Al2O3p
/Al) prepared by pressure infiltration are investigated and compared with those of a cast ductile iron. In addition to low
cost and reduced weight, the composite has a Young’s modulus comparable to the ductile iron. However, its fracture toughness
is lower than that of the ductile iron. Interface debonding between the graphite and ferrite is responsible for the crack
initiation behavior of the ductile iron. The crack in the ductile iron is arrested by the ductile ferrite phase surrounding
the graphite, leading to high fracture toughness. For the Al2O3p
/Al composite, the dominating crack initiation mode is particulate cracking. Interface debonding and zigzag cracking of particulates
are additional fracture modes. The high content of Al2O3 particulates and the high thermal and elastic incompatibilities between the Al matrix and Al2O3 particulates result in brittle fracture and low fracture toughness for the composite. Possible ways to increase the fracture
toughness of the Al2O3p
/Al composite material are also outlined. 相似文献
4.
A fractographic investigation of thermal embrittlement in cast duplex stainless steel 总被引:1,自引:0,他引:1
The fracture surface topography analysis (FRASTA) technique under development at SRI was applied to seek an explanation for
severe thermal embrittlement observed in cast duplex stainless steel. By comparing topographic features of conjugate fracture
surfaces, FRASTA showed that fracture in thermally embrittled cast duplex stainless steel occurs by microcrack initiation
at delta-phase grain boundaries and at alpha-phase/gamma-phase interfaces, and by microcrack growth along these boundaries
and interfaces. The critical crack tip opening displacement (CTOD) as measured from cross-sectional views generated by the
FRASTA technique indicated a microcrack initiation toughness,J
Ic, of 287 KJ/m2, in excellent agreement with measurements using conventional fracture mechanics procedures, and significantly less than the
toughness of unaged material.
Formerly with SRI International, Menlo Park, CA, is with the Department of Mechanical Engineering, Nagoya Institute of Technology,
Gokiso-cho, Showa-ku, Nagoya, Japan 466. 相似文献
5.
A great amount of iron grinding balls in tube mills have been consumed. Under this impact abrasive wear working condition, the failure of wear resistant alloying white irons grinding balls is mainly caused by fatigue spalling. The impact wear resistance of martensitic high chromium cast iron (Cr of 15%) is not high sometimes, but its cost is not low. Thus, medium Cr Si wear resistant cast iron is recommended. The influence of the iron on impact fatigue resistance and impact wear resistance is pronounced. Ball on ball impact fatigue test and high stress impact wear test of the grinding balls have been carried out. The results show that the impact fatigue resistance (IFR) and impact wear resistance (IWR) of medium Cr Si cast iron are superior to those of martensitic high chromium cast iron (Cr of 15%). The main reasons are that (1) the stress in medium Cr Si cast iron is released in the as cast state; (2) the matrix is fine pearlite with better toughness and plasticity; (3) the pearlite is more stable compared with a retained austenite under repeated impact load and less phase transformation can take place; (4) high silicon content improves the morphology of eutectic carbide; (5) there is no secondary carbide which results in less crack sources. All these factors are beneficial to improvement of impact fatigue spalling resistance. The eutectic carbide M7C3 is the main constituent to resist wear. 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.
S. Sankaran G. Malakondaiah Gouthama S. Sangal K. A. Padmanabhan 《Metallurgical and Materials Transactions A》2006,37(4):1191-1200
A multiphase ferrite-bainite-martensite (F-B-M) microstructure was developed in an automotive grade V-bearing medium carbon
microalloyed steel, 38MnSiVS5. It was characterized using optical, scanning, and transmission electron microscopy. The tensile,
Charpy impact, and static and dynamic fracture toughness behaviors were evaluated. The results are compared with those of
ferrite-pearlite (F-P) and tempered martensite (T-M) microstructures of the same steel. Although the tensile properties of
the multiphase microstructures were superior, the Charpy impact and static and dynamic fracture toughness properties were
inferior compared with those of the other two microstructures. The F-P condition displayed the highest plane strain fracture
toughness value (KIC), while the T-M condition was characterized by the highest dynamic fracture toughness (conditional) value (KIDQ). The Charpy impact energy of the T-M condition was greater than that for the other two conditions. An examination of the
surfaces of fractured samples revealed predominant ductile crack growth in the F-P microstructure and a mixed mode (ductile
and brittle) crack growth in the T-M and the F-B-M microstructures. Although the Charpy impact energy, plane fracture toughness
(KIC), and conditional dynamic fracture toughness (KIDQ) of the multiphase microstructure were inferior to those of the T-M and the F-P microstructures, the toughness properties
were comparable to those of medium carbon low alloy steels having bainite-martensite (AISI 4340) or tempered martensite microstructures. 相似文献
8.
《Acta Metallurgica Materialia》1994,42(3):893-911
The influence of the type, volume fraction, thickness and orientation of ductile phase reinforcements on the room temperature fatigue and fracture resistance of γ-TiAl intermetallic alloys is investigated. Large improvements in toughness compared to monolithic γ-TiAl are observed in both the TiNb- and Nb-reinforced composites under monotonic loading. Toughness increases with increasing ductile phase content, reinforcement thickness and strength; orientation effects are minimal. Crack-growth behavior is characterized by steep resistance curves primarily due to crack trapping/renucleation and extensive crack bridging by the ductile-phase particles. In contrast, under cyclic loading the influence of ductile phases on fatigue resistance is strongly dependent upon reinforcement orientation. Compared to monolithic γ-TiAl, improvements in fatigue-crack growth resistance are observed in TiNb-reinforced composites only in the face (C-L) orientation; crack-growth rates for the edge (C-R) orientation are actually faster in the composite. In comparison, Nb-particle reinforcements offer less toughening under monotonic loading but enhance the fatigue properties compared to TiNb reinforcements under cyclic loading. 相似文献
9.
为了明确低温用球墨铸铁材料断裂微观机理,针对石墨球对系列温度球墨铸铁冲击断口演变过程的作用机制进行研究。采用SEM、激光共聚焦显微镜等手段系统分析了不同温度下石墨球对球墨铸铁冲击断裂过程的影响。定量断口分析结果表明,与冲击功随温度的变化一致,断口表面粗糙度Sa和空穴扩张比Rc/R0(韧窝与石墨球半径之比)均随温度的下降呈明显降低趋势。随着冲击试验温度的降低,由韧性断裂逐渐过渡到脆性断裂,这种断裂机制的变化导致断面粗糙度指数剧烈下降,空穴扩张比值趋近于1。冲击过程中裂纹总是在石墨 基体界面处发生开裂并沿着石墨 基体界面不断扩展,因此实际生产过程中应该注重改善石墨球与基体界面处组织状态。 相似文献
10.
Hisao Matsunaga Teruki Usuda Keiji Yanase Masahiro Endo 《Metallurgical and Materials Transactions A》2014,45(3):1315-1326
Hydrogen-induced ductility loss in ductile cast iron (DCI) was studied by conducting a series of tensile tests with three different crosshead speeds. By utilizing the thermal desorption spectroscopy and the hydrogen microprint technique, it was found that most of the solute hydrogen was diffusive and mainly segregated at the graphite, graphite/matrix interface zone, and the cementite of pearlite in the matrix. The fracture process of the non-charged specimen was dominated by the ductile dimple fracture, whereas that of the hydrogen-charged specimen became less ductile because of the accompanying interconnecting cracks between the adjacent graphite nodules. Inside the hydrogen-charged specimen, the interspaces generated by the interfacial debonding between graphite and matrix are filled with hydrogen gas in the early stage of the fracture process. In the subsequent fracture process, such a local hydrogen gas atmosphere coupled with a stress-induced diffusion attracts hydrogen to the crack tip, which results in a time-dependent ductility loss. 相似文献
11.
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
12.
This paper presents a study of dynamic fracture initiation behavior of 2124-T6 aluminum matrix composites containing 0, 5.2,
and 13.2 vol pct SiC whiskers. In the experiment, an explosive charge is detonated to produce a tensile stress wave to initiate
the fracture in a modified Kolsky bar (split Hopkinson bar). This stress wave loading provided a stress intensity rate, KI,, of about 2 × 106 MPa√m/s. The recorded data are then analyzed to calculate the critical dynamic stress intensity factor,K
Id, of the composite, and the values obtained are compared with the corresponding quasi-static values. The test temperatures
in this experiment ranged from −196 °C to 100°C, within which range the fracture initiation mode was found to be mostly ductile
in nature. The micromechanical processes involved in void and microcrack formation were investigated using metallographic
techniques. As a general trend, experimental results show a lower toughness as the volume fraction of the SiC whisker reinforcement
increases. The results also show a higher toughness under dynamic than under static loading. These results are interpreted
using a simple dynamic fracture initiation model based on the basic assumption that crack extension initiates at a certain
critical strain developed over some microstructurally significant distance. This model enables us to correlate tensile properties
and microstructural parameters, as, for instance, the interspacing of the SiC whiskers with the plane strain fracture toughness. 相似文献
13.
The dependence of fatigue crack growth rate on the cyclic stress intensity factor was determined for six iron-carbon alloys
ranging in carbon content from 0.23 to 1.08 wt pct carbon. Both ferrite/pearlite and ferrite/free iron carbide microstructures
were studied. Scanning electron microscope fractography studies correlated the fatigue mechanism with microstructure. It was
found that when the predominant mode of crack growth was ductile, the crack growth rateda/dN could be related to the cyclic stress intensity factor ΔK by an equation of the formda/dN = (ΔK)m where andm are constants. The constantm was approximately equal to four when the crack growth mechanism presumably was the blunting and resharpening of the crack
tip by slip processes. The constantm was greater than four when the crack growth mechanism was void coalescence in the interlamella ferrite of pearlite colonies.
The preferred fatigue crack path through the pearlitic alloys was through the free ferrite phase.
formerly Research Assistant at Materials Science and Engineering Department and Materials Research Center, Northwestern University. 相似文献
14.
L. P. Zhang C. L. Davis M. Strangwood 《Metallurgical and Materials Transactions A》1999,30(8):2089-2096
Thermally stable TiN particles can effectively pin austenite grain boundaries in weld heat-affected zones (HAZs), thereby
improving toughness, but can also act as cleavage initiators. The HAZs simulated in a GLEEBLE 1500 TCS using two peak temperatures
(T
p
) and three cooling times (Δ∼
8/5) have determined the effects of matrix microstructure and TiN particle distribution on the fracture toughness (crack tip
opening displacement (CTOD)) of three steels microalloyed with 0.006, 0.045, and 0.1 wt pct Ti. Coarse TiN (0.5 to 6 μm) particles are identified in steels with the two higher levels of Ti, and fine Ti(C, N) (35 to 500 nm) particles were present
in all three steels. Large prior austenite grain size caused by higher T
p
decreased fracture toughness considerably in steels containing coarse TiN particles but had little effect in their absence.
Fracture toughness was largely independent of matrix microstructure in the presence of coarse particles. Cleavage fracture
initiation was observed to occur at coarse TiN particles in the samples with a large prior austenite grain size. Alloy thermodynamics
have been used to rationalize the influence of Ti content on TiN formation and its size. 相似文献
15.
Clean high carbon heavy rail steel was prepared by the process of vacuum induction furnace smelting, forging and rolling. Mechanisms of RE on the impact toughness and fracture toughness for clean high carbon steel were investigated. In addition, the appropriate range of RE content for clean high carbon steel was determined. Both the austenite grain size and pearlite lamellar spacing decreased due to small amount of RE, consequently the impact toughness and fracture toughness were improved evidently. When the RE content exceeded a critical value, the pearlite lamellar spacing was increased, because RE was segregated on the austenite grain boundaries, damaged the orientation relationship of pearlite transformation, caused the disorder growth and morphology degenerating of pearlite. With the increasing of RE content, both the impact toughness and fracture toughness of clean high carbon steel were gradually increased at first and then decreased. It was found that when the RE content was between 00081% and 00088%, both the impact toughness and fracture toughness of clean high carbon heavy rail steel were the best. The maximum ballistic work was 212 J (20 ℃) and 122 J (-20 ℃), respectively. The maximum plane-strain fracture toughness was 4567 MPa·m1/2 (20 ℃) and 3704 MPa·m1/2 (-20 ℃), respectively. 相似文献
16.
17.
In order to understand the fracture toughness of nodular cast iron, the damage zone was studied by Scanning Electron Microscope
(SEM) observations of the polished surface of a CT 25 specimen before and after ductile tearing. Damage is defined as decohesion
at the graphite/matrix interface. It is shown that the damage zone is very large in nodular cast iron (almost throughout the
whole remaining ligament ahead of the crack tip), so linear elastic fracture mechanics (LEFM) are not valid for small specimens.
The size of the damage zone was calculated analytically by introducing a damage initiation criterion which was based both
on observations of the debonding of the interface between matrix and graphite nodules and on measurements of the pressure
sensitivity of cast iron. To take into account the actual boundary conditions, the damage zone was also calculated by numerical
modeling using the modified Gurson’s model and by considering the nodular cast iron as a porous material. The calculated results
led to good agreement with the damage zone observations. Plane stress and plane strain calculations yielded nearly the same
size plastic zone. This result is opposite to those obtained for fully dense materials. 相似文献
18.
19.
Peter Hübner Rene Kiessling Horst Biermann Thomas Hinkel Werner Jungnickel Rudolf Kawalla Heinz-Werner Höppel Johannes May 《Metallurgical and Materials Transactions A》2007,38(9):1926-1933
Fracture-mechanics experiments were carried out on ultrafine-grained (UFG) samples of aluminum and two Al alloys to obtain
the fracture behavior under static and cyclic loading. The UFG materials investigated show crack resistance behavior under
static loading, which was confirmed by ductile fracture surfaces. Under cyclic load, the crack growth rate was described well
by the ESACRACK model. The crack propagation results show no influence of the type of the severe plastic deformation method
in the Paris region but more effect in the threshold region.
This article is based on a presentation made in the symposium entitled “Ultrafine-Grained Materials: from Basics to Application,”
which occurred September 25–27, 2006 in Kloster Irsee, Germany. 相似文献
20.
Giovanni Straffelini 《Metallurgical and Materials Transactions A》2000,31(5):1443-1451
The impact fracture toughness of sintered iron and high-strength sintered steels, with densities between 7.0 and 7.25 g/cm3, have been investigated by means of instrumented impact testing on fatigueprecracked as well as 0.17-mm-notched specimens.
Experimental results show that the fracture behavior is controlled by the properties of the resisting necks at the crack/notch
tip. The materials with impact yield strengths of up to 700 MPa display an increase in fracture toughness as the yield strength
is increased. These materials undergo continuous yielding during loading, and ductile fracture takes place once the critical
plastic strain is attained within a large process zone. A process-zone model, physically consistent with the fractographic
observations, correctly rationalizes their impact fracture toughness. The materials with higher impact yield strengths display
an impact curve which is linear up to fracture and are characterized by a fracture toughness which is independent of the yield
strength. For these materials, the process zone reduces to the first necks at the crack/notch tip, and fracture takes place
once the local applied stress-intensity factor reaches the fracture toughness of the matrix. 相似文献