Journal of the Society of Materials Science, Japan

Characteristics of Stage 2 Fatigue Crack Growth in Ti-6Al-4V Alloy and Its Fractgraphy

Katsutoshi ASAMI, Kenji IIDA

1994 Volume 43 Issue 488 Pages 502-508
Published: May 15, 1994
Released on J-STAGE: June 03, 2009

DOIhttps://doi.org/10.2472/jsms.43.502

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The characteristics of the stage 2 fatigue crack growth was investigated in Ti-6Al-4V alloy, in order to clarify the influence of microstructure and stress ratio. The fatigue tests were carried out at the stress ratio, R, of 0.1, 0.5 or 0.8 by using annealed (AR) and solution treatment-and-aged (STA) materials. The microstructure of AR material was coarse and anisotrophic in comparision with that of STA material.
The fracture surface and the cross section of fatigue cracks were observed on both materials, and the relationship between the da/dN-ΔK_eff curve and the observed results were discussed.
(1) The da/dN of AR material was lower than that of STA material because the fatigue crack grew zigzag.
(2) The ΔK_eff·th at R=0.8 in AR material was higher in comparison with those at other stress ratios because the secondary cracks with a length of about 100μm were generated.

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Effect of Silicon Additions on Austempered Structure and Mechanical Properties in Ultrahigh Strength Low Alloy Steels

Yoshiyuki TOMITA, Takeyoshi OKAWA

1994 Volume 43 Issue 488 Pages 509-514
Published: May 15, 1994
Released on J-STAGE: June 03, 2009

DOIhttps://doi.org/10.2472/jsms.43.509

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Silicon-modified 4340 (Si-4340) and 4340 steels have been studied to determine the effects of silicon addition on austempered structure and mechanical properties of ultrahigh strength low alloy steels. As a result of austempering Si-4340 steel in the bainitic temperature region (593-673K), the retention of a large amount of austenite (12-25vol.%) was observed in the carbon-free upper-bainitic ferrite region. Compared to the conventionally quenched and tempered steel (CQT), the austemper of Si-4340 steel increased fracture toughness (K_IC) at 623K and below, owing to the increased Charpy impact energy and percent elongation, while the strength decreased. Compared to the CQT and 4340 steel austempered in the same temperature region, the austemper of the steel had detrimental effects on the K_IC and other relevant mechanical properties at 643K and above. The results are described and discussed in terms of metallographic observations, X-ray measurements and fractography.

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Fatigue Crack Initiation and Propagation Behavior and Fracture Surface of Eutectoid Steels

Chikayuki URASHIMA, Shin-ichi NISHIDA

1994 Volume 43 Issue 488 Pages 515-521
Published: May 15, 1994
Released on J-STAGE: June 03, 2009

DOIhttps://doi.org/10.2472/jsms.43.515

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To clarify the mechanism of fatigue failure in eutectoid steel, fatigue crack initiation and propagation behaviors have been studied in detail under rotating bending fatigue tests using rail steels. The fatigue failure surface was also examined by using SEM.
The main results obtained are as follows;
(1) Fatigue cracks initiated at the interface between ferrite and pearlite in case of rail steel with pro-eutectoid ferrite. In rail steel without pro-eutectoid ferrite, fatigue cracks occurred at the inter-lamellar of pearlite due to the slip.
(2) The fatigue crack preferentially propagated along the inter-lamellar of pearlite in the early stage. As it grew, however, it propagated perpendicularly to the stress axis by changing its direction at every pearlite block.
(3) On the fatigue failure surface in eutectoid steel, the pearlite lamellar was observed, which looks apparently alike striation. But the typical striation pattern of fatigue was not observed.

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Tensile and Fatigue Fracture Surface of Laser-Treated Spheroidal Graphite Cast Iron

Hisakichi SUNADA

1994 Volume 43 Issue 488 Pages 522-527
Published: May 15, 1994
Released on J-STAGE: June 03, 2009

DOIhttps://doi.org/10.2472/jsms.43.522

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The effects of laser treatment on tensile and fatigue properties of spheroidal graphite cast iron were studied by fractographic observations. The laser-treatment was carried out using a 800-500W laser power and at scan speed of 0.5-1.2m/min. The results obtained are as follows;
(1) The typical structure of laser-treated specimen is composed of the melted zone which has cellular dendritic structure, the unmelted (heat affected) zone that contains martensite surrounding graphite nodules, and the ferritic matrix zone.
(2) A sharp drop in hardness occurred at the border between the melted zone (HV800) and the HAZ (HV200).
(3) The tensile test indicated that increasing laser input power lowers the tensile strength.
(4) The tensile fracture surface of the untreated specimen exhibited ductile dimple fracture mode, whereas the laser-treated one revealed dendritic pattern in the melted zone, mixture of quasi-cleavage and small dimple modes in the HAZ and ductile dimple mode in the matrix.
(5) The fatigue life of the laser-treated specimen with melted zone became shorter than that of the untreated one, while the life without the melted zone was longer than that of the untreated one. Lowering the frequency resulted in weakening fatigue strength.
(6) The fatigue fracture surface of the untreated specimen showed no striation pattern. Although the fatigue fracture pattern on the laser-treated specimen was similar to the tensile fracture one, the size and number of graphites on the fatigue fracture surface were less than those on the tensile one.

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Strength Evaluation of Gray Cast Iron on the Basis of Quantitative Analysis of Graphite Shape

Masaharu FUJIHARA, Takahiro HATTORI, Takeshi NAITO

1994 Volume 43 Issue 488 Pages 528-532
Published: May 15, 1994
Released on J-STAGE: June 03, 2009

DOIhttps://doi.org/10.2472/jsms.43.528

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For gray cast iron, it is not easy to perform the non-destructive estimation of material strength and failure analysis, such as quantitative evaluation and estimation of failure mode.
The authors applied an image processing technique to measure the graphite size and the area ratio of graphite on the fracture surface, and tried to correlate those with the material strength and failure mode based on the fact that graphite is the key factor for the fracture of cast iron.
The followings were obtained in this report. The tensile strength had a correlation to the maximum length of graphite which was obtained by statistical treatment. This relationship can be used for the non-destructive estimation of tensile strength. Moreover, the area ratio of graphite on the fatigue fracture surface depended on the applied stress level.

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Strength and Fracture Morphology of Co-Based Alloy Powder Compaction by Explosive Method

Minoru NISHIDA, Takao ARAKI, Katsushi MINAKUCHI, Shinji MORITA

1994 Volume 43 Issue 488 Pages 533-538
Published: May 15, 1994
Released on J-STAGE: June 03, 2009

DOIhttps://doi.org/10.2472/jsms.43.533

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Co-based alloy powders were dynamically compacted by using a cylindrical axisymmetric explosive method. The compacted powders were post-heat-treated in evacuated capsules from 3.6ksec to 32.4ksec at 973K and 1273K. The mechanical property was examined by using a bending test.
The bending strength of the as-compacted material was below 120MPa, which is about one-fifth that of the cast material. The fracture pattern of the as-compacted material exhibited interparticle fracture. The strength of the compacted material was improved by post-heat-treatments at 1273K. But the strength was below 500MPa. The fracture pattern of transparticle fracture exhibited the quasi-cleavage fracture and in the case of the interparticle fracture the dimple pattern was observed on the powder surface.

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Influence of Anisotropy on Fatigue Crack Growth Behavior of Ti-6Al-4V Alloy

Yoshio KITSUNAI, Masazumi TANAKA, Etsuji YOSHIHISA, Takashi HONDA

1994 Volume 43 Issue 488 Pages 539-544
Published: May 15, 1994
Released on J-STAGE: June 03, 2009

DOIhttps://doi.org/10.2472/jsms.43.539

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The fatigue crack growth tests under constant amplitude and two-step program loadings were carried out to assess the effect of anisotropy on the crack growth of a forged Ti-6Al-4V alloy. CT specimens were cut from three different orientations with regard to the rolling direction of the material. Crack closure was measured by a compliance method to examine the overloading effect on the crack growth during the two-step program loading. Fracture surface morphologies were examined by a scanning electron microscope to assess the crack growth mechanism of the material tested. It was found that the crack growth rate was almost independent of the orientation of the material, regardless of loading condition. The crack opening stress intensity under the two-step program loading was almost the same in each block and was governed by the maximum stress intensity factor in the block. The fatigue crack growth rate under two-step program loading was correlated well with an effective stress intensity factor range estimated based on the linear accumulation of the stress intensity factor ranges of each step in the block. Cleavage facets occupied the major portion of fracture surface of low ΔK region under constant amplitude loading. Fracture surface under program loading was covered with striations corresponding to the loading sequence. Large striation due to overloading seems to be formed through the blunting and resharpening mechanism at crack tip.

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Acceleration of Fatigue Crack Growth under Intermittent Overstressing

Effect of Mean Stress and Crack Closure

Ryoichi KOTERAZAWA, MUDJIJANA, Qinsheng YU, Tian-jian WANG, Takayoshi ...

1994 Volume 43 Issue 488 Pages 545-550
Published: May 15, 1994
Released on J-STAGE: June 03, 2009

DOIhttps://doi.org/10.2472/jsms.43.545

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Fatigue crack growth behaviour under intermittent overstresses of different mean stress levels was investigated in dry air with a low carbon steel. A very small number of cycles of overstress applied intermittently between a very large number of cycling of understress below threshold caused significant acceleration in crack growth rate amounting to about one hundred times as compared to the case of steady cyclic stress for all cases of mean stress level. The acceleration in the cases with tensile mean stress was slightly smaller than that in the cases without it, and it was related to a little higher crack closure level in the former case. However, correlation in the acceleration versus understress diagram was not improved appreciably by using effective stress intensity taking account of the crack closure. Stress history in precracking had some effect on the crack closure and, consequently, on the acceleration. The correlation in this case was appreciably improved by using the effective stress intensity. Difference was not recognized in fracture surface morphology among the cases with different mean stress levels, and so the micromechanism of crack growth acceleration would be the same throughout all cases of different mean stress. The acceleration under multi-level intermittent overstresses with mean stress could be predicted from the two-level intermittent overstress test data by linear summation in the same way as the case without mean stress.

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Intergranular Corrosion and Stress Corrosion Cracking of Cold-Rolled Zircaloy-2

Hitoshi UCHIDA, Shozo INOUE, Kazuyuki MORIMOTO, Keiji KOTERAZAWA

1994 Volume 43 Issue 488 Pages 551-555
Published: May 15, 1994
Released on J-STAGE: June 03, 2009

DOIhttps://doi.org/10.2472/jsms.43.551

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The susceptibility to intergranular corrosion and stress corrosion cracking (SCC) of cold-rolled Zircaloy-2 in CH₃OH/0.4%HCl solution at room temperature was examined fractographically.
It was found from the stress-free exposure tests that the weight loss due to intergranular corrosion increased gradually with exposure time. From the subsequent tensile tests in air, the mechanical properties deteriorated greatly though there was less dependent on rolling reduction. The resulting appearance consisted of intergranular and ductile fracture and their area fractions were approximately constant, independently of rolling reduction. The threshold stress to SCC occurrence did not exist essentially, and its potential range corresponded with the active region of anodic polarization diagram. However, the uneven general corrosion appeared when the potential was moved to the positive direction. The fracture mode was changed from an intergranular into a cleavage-like fracture. Here, the former area fraction decreased with an increasing rolling reduction and the latter inversely increased.

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Fractographic Evaluation for the Susceptibility to Hydrogen Embrittlement Type Stress Corrosion Cracking in Cold Worked Duplex Stainless Steel and Its Welded Joints

Masato MURATA, Yoshihiko MUKAI, Jinbo WAN

1994 Volume 43 Issue 488 Pages 556-561
Published: May 15, 1994
Released on J-STAGE: June 03, 2009

DOIhttps://doi.org/10.2472/jsms.43.556

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The effects of welding and plastic processing in duplex stainless steel on the susceptibility to hydrogen embrittlement were studied by fractography. In this paper, the crystall graphic fracture planes were determined by the facet pit method and three dimensional images (topographs) were analyzed with Topo-SEM. As the results, the susceptibility of weld metals increased more than that of base metals because of its coherent crystal orientations. The susceptibility to hydrogen embrittlement of base metal subjected plastic deformation decreased and the dominant fracture plane changed from {100} to {110}, {112} with increasing the degree of plastic processing. It is considered that this result is caused by dropping of hydrogen activity due to hydrogen trapping by dislocations.

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Hydrogen Cracking of Duplex Stainless Steel Weldment

Toshio KURODA, Carl D. LUNDIN

1994 Volume 43 Issue 488 Pages 562-566
Published: May 15, 1994
Released on J-STAGE: June 03, 2009

DOIhttps://doi.org/10.2472/jsms.43.562

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The methods of measuring diffusible hydrogen, the mechanical properties and the critical hydrogen level not to crack were investigated on Ferralium 255 and Alloy 2205 duplex stainless steel weldments using Ferralium 259 and 2209/2209W SMAW electrodes.
The diffusible hydrogen in the duplex stainless steel weldment was found to be exactly measurable at 673K for 259.2ks. Fisheyes characterized by circular bright spots with a pore or a slag inclusion at the center were observed on the tensile fracture surfaces of Ferralium 259 weld pad. The weld cracking hardly occurred below 7ppm hydrogen in G-Bop test of Ferralium 259 welds. The weld cracking of 2209/2209W welds hardly occurred below 34ppm hydrogen in Y-Groove test and below 25ppm hydrogen in G-Bop test.

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Tensile Properties of Directionally Solidified MarM247LCDS Ni-Base Superalloy in High Pressure Hydrogen at Room Temperature

Seiji FUKUYAMA, Kiyoshi YOKOGAWA, Tadashi IIDA

1994 Volume 43 Issue 488 Pages 567-572
Published: May 15, 1994
Released on J-STAGE: June 03, 2009

DOIhttps://doi.org/10.2472/jsms.43.567

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Tensile properties of the directionally solidified Ni-base superalloy MarM247LCDS used for the turbine blades of the fuel and oxidizer turbopumps installed in the liquid hydrogen fueled rocket engine, LE-7 engine, of H-II rocket were investigated in a high pressure hydrogen of 19.7MPa at room temperature. The results obtained are as follows;
(1) The 0.2% proof stress, ultimate tensile strength, elongation and reduction of area in hydrogen were smaller than those in argon. The effect of hydrogen on the tensile properties increased with decreasing strain rate. Hydrogen environment embrittlement depended little on the solidified direction.
(2) The crack initiation occurred at carbides in the area between dendrites both in argon and hydrogen. Then the crack propagated in γ matrix between γ' and γ' with ductile fracture in argon and with brittle fracture in hydrogen, respectively.

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Corrosion Fatigue Strength of High Tensile Steels and Its Fractographic Study

Shin-ichi NISHIDA, Chikayuki URASHIMA

1994 Volume 43 Issue 488 Pages 573-579
Published: May 15, 1994
Released on J-STAGE: June 03, 2009

DOIhttps://doi.org/10.2472/jsms.43.573

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Considering that the corrosion fatigue is especially time-dependent, it is necessary to confirm the long-life fatigue properties of structural steels to be adoptable to the practical design curve. Fatigue tests have been performed on three kinds of high tensile steels for offshore structures in air or by dropping fresh water and sea water to the midsection of each specimen under rotating bending load at various cyclic speeds including lower frequencies. In addition, fractographic observation has been carried out on the crack initiation and fracture surface of each specimen.
The main results obtained on the basis of the long-time corrosion fatigue tests (maximum period; 1.45 years/specimen) are as follows:
(1) The fatigue strength decreases with higher cyclic numbers and lower frequencies. There exists a good linear relation between stress amplitude and cyclic frequency. The relation can be expressed by a linear logarithmic equation.
(2) The effect of corrosion liquid on fatigue strength is remarkable in long period tests and the fatigue strength in sea water becomes lower than that in fresh water in the frequency range between 1500 and 4rpm.
(3) The fatigue strength is a little dependent on tensile strength in the low cycle region and the difference in fatigue strength among high tensile steels becomes reversed in the high cycle region. This phenomenon appears not only in fresh water but also in sea water.
(4) The crack initiates from pits and propagates mainly by transgranular fracture. Both quasi-cleavage fracture and intergranular fracture are observed in the heat-treated steels under low cyclic frequencies in the high cycle region. These fracture patterns would be caused by corrosive effect and accelerate the crack propagation rate of the heat-treated steels.

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The Quantitative Analysis of Fatigue Fracture Surfaces of HT50 (TMCP) and Mild Steel in Sour Crude Oil

Ryuichiro EBARA, Yoshikazu YAMADA, Daisuke SAKAI, Akira FUSHIMI, Hiros ...

1994 Volume 43 Issue 488 Pages 580-586
Published: May 15, 1994
Released on J-STAGE: June 03, 2009

DOIhttps://doi.org/10.2472/jsms.43.580

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Corrosion fatigue crack propagation tests of HT50 (TMCP) and mild steel and corrosion fatigue life test of HT50 (TMCP) were conducted in sour crude oil containing 400ppm H₂S. The detailed SEM observations were carried out on these fracture surfaces obtained. The crack propagation rate of these steels in the sour crude oil was accelerated in the stress intensity factor range, ΔK higher than about 16MPa√m. The ratio of acceleration of the crack propagation rate, da/dN in the sour crude oil in the ΔK of 30MPa√m was 7 for HT50 (TMCP) and mild steel.
In the higher stress range of S-N curve for the round notched bar specimens of HT50 (TMCP), the influence of sour crude oil containing 400ppm H₂S on the fatigue life was observed obviously. The fatigue life ratio decreased with decreasing the stress to unity at the lower axial stress.
It was found that the crack propagated predominantly on the cleavage fracture surface in association with brittle striation in the accelerated crack propagation area of fracture surface in the sour crude oil.
The striation spacing per cycle, S obtained from the measured striation spacing ΔS versus ΔK curve was well coincident with the da/dN-ΔK curve in the accelerated crack propagation area.
It might be concluded that the environmental enhancement of the fatigue crack propagation rate in sour crude oil is dependent on hydrogen evolved by reaction between the structural steels and hydrogen sulfide in the sour crude oil.

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Fatigue of Advanced Materials

1. Fundamentals of Fatigue and Advanced Materials

Masahiro JONO

1994 Volume 43 Issue 488 Pages 587-593
Published: May 15, 1994
Released on J-STAGE: June 03, 2009

DOIhttps://doi.org/10.2472/jsms.43.587

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