Abstract: Formable ultra high-strength TRIP-aided steel with bainitic ferrite matrix structure (TBF
steel) contributes to a drastic weight reduction and an improvement of crash safety of automobile.
In this study, fatigue strength of 0.2%C-1.5%Si-1.5%Mn TBF steels was investigated. High fatigue
limit was achieved in TBF steels austempered at 400-450oC, containing a large amount of stable
retained austenite. The fatigue limit was linearly related with mobile dislocation density, as well as
TRIP effect of retained austenite. When compared to conventional martensitic steel, the TBF steel
exhibited lower notch-sensitivity or higher notched fatigue performance. Complex additions of
0.5%Al, 0.05%Nb and 0.2%Mo considerably improved the notched fatigue performance, as well as
the smooth fatigue strength. This was associated with the stabilized retained austenite and refined
microstructure which suppress fatigue crack initiation and/or propagation.
Abstract: The welding quality of spot weldment is an important factor that significantly affects the
strength, stiffness, safety, and other performance characteristics of vehicles. Therefore, quality
control and fatigue life evaluation of spot weldment are necessary processes. This paper presents a
method for determining the fatigue life of multi-lap spot weldment of a high strength steel sheet. In
this method, the fatigue life is estimated by using the lethargy coefficient, which is the total defect
coefficient according to rupture stress and time obtained by the quasi static tensile-shear test. Also,
in this study, we modified the lethargy coefficient by using the welding current. And, we define a
specific lapping constant, which is a characteristic constant of 2 or 3 lap weldments. The fatigue life
obtained by the fatigue estimate equation, which contains a specific lapping constant was compared
and verified with an experimental value. And we analyzed the relation of lap number, welding
current and fatigue life. This method can save processing time and cost for predicting the life cycle
of a structure.
Abstract: In order to investigate effects of low energy laser peening (LP) without protective
coating on surface fatigue crack growth behavior, rotating bending fatigue tests were carried out on
cast Al-Si-Mg aluminum alloy with a pre-cracked round bar type specimens. As the results, the
fatigue crack growth was restrained by the compressive residual stress induced by laser peening
treatment. And also, the three dimensional (3D) image of surface fatigue cracks was reconstructed
by using a micro computed tomography (μCT) with phase contrast technique. It was also shown
that the surface crack growth was restrained for the laser peened materials.
Abstract: In the present study, fatigue tests of sharp-notched Zr-based bulk metallic glass (BGM),
were conducted under fully reversed cyclic bending, and the fatigue crack initiation mechanisms
were clarified by using AFM. The fracture surface was also observed to examine the crack
propagation mechanism. The fatigue notch factor was 2.0, while the elastic stress concentration
factor is 2.7. From the macroscopic observations of fractured specimen, either tension mode or
shear mode fracture morphologies were observed. Either in smooth specimens or notched
specimens, no prodigious sign of crack initiation were observed, i.e., fatigue cracks were initiated
from shear bands those were formed just before the crack initiation.
Abstract: This paper describes the fatigue behaviour at elevated temperatures in a ferritic stainless
steel, type 444. Test temperatures evaluated were ambient temperature, 673K and 773K in
laboratory air. Fatigue strength decreased at elevated temperatures compared with at ambient
temperature. At all temperatures, cracks were generated at the specimen surface due to cyclic slip
deformation, but fractographic analysis revealed a brittle features in fracture surface near the crack
initiation site at elevated temperatures. Cracks initiated earlier at elevated temperatures than at
ambient temperature and subsequent small cracks grew faster at elevated temperatures even though
the difference in elastic modulus was taken into account, indicating the decrease in crack initiation
resistance and crack growth resistance. The observed decrease in both resistances was discussed in
relation to the 748K(475C) embrittlement in ferritic stainless steels.
Abstract: Mg-Y2O3 nanocomposites were synthesized by disintegrated melt deposition method
(DMD). Tensile tests and constant stress amplitude fatigue tests were carried out on Mg
nanocomposites with 0.5, 1.0 and 2.0 volume percentages of reinforcements. Yield strengths of the
nanocomposites were found to increase with higher volume fraction of Y2O3. Both unreinforced Mg
and the Mg nanocomposites were found to strain harden upon cycling. The hardening behaviour
was more intense in the nanocomposites.
Abstract: Study was attempted to produce stress-life curve of the zirconium tube under the cyclic
pressurization and to analyze its behavior. The internal pressurization machine was devised and the
low cycle fatigue test at the zirconium cladding tube was carried out. The result showed that the
O’Donnell and Langer relationship can be fitted to the fatigue behavior of the zirconium cladding
under the cyclic pressurization, where the fatigue limit of the Zircaloy-4 at 350oC was shown at
341.6MPa. From the analysis of the diametral changes and the fractographic observation, a
combined creep-fatigue interaction rather than a pure fatigue had an influence on the failure of the
zirconium cladding under the cyclic pressurization at 350oC.
Abstract: Tensile and fatigue properties were evaluated for base and welded type 316LN stainless
steel. Welding methods were GTAW (308L, Ar environment) and GTAWN (316L, Ar + N2
environment). Yield strength of weld joint was higher than that of base metal but elongation of weld
joint was lower than that of base metal. UTS of weld joint was slightly lower than that of base metal.
Yield strength and elongation with welding method were almost same. Fatigue life of weld joint
was lower than that of base metal but fatigue strength of weld joint was higher than that of base
metal. Ferrite content was increased with welding. Fatigue life welded by GTAWN was better than
that of GTAW at RT and 600°C. This fatigue life behavior was consistent with the behavior of
Abstract: In this study, we investigated the fatigue characteristics of SMA40 materials as a
potential application in connecting rod material for an automobile. Average fatigue life of
168,700 cycles and fatigue limit of 437 MPa was determined from SMA40 specimens. However,
when tested under fretting condition, decrease in fatigue lifetime was 20% and fatigue limit was
reduced to 350 MPa.
Abstract: In these days, most urban railway vehicles have been serviced under the random load
application. However, it is considered to be a major factor of safety to predict the fatigue life for
structures. It is thus required that fatigue assessment method for cumulative damage approach while
Korea domestic regulations practices only the only has endurance limit approach. With this endurance
limit approach, fatigue life prediction is impossible. In this research, the fatigue assessment for urban
transit structure by using of cumulative damage approach method and related theories are presented.