Largest Inclusion Size Research Articles

Relationship between non-inclusion induced crack initiation and microstructure on fatigue behavior of bainite/martensite steel in high cycle fatigue/very high cycle (HCF/VHCF) regime

High cycle and very high cycle fatigue (HCF/VHCF) in a bainite/martensite (B/M) multiphase steel with varying inclusion size and microstructural features was studied using ultrasonic axial cycling test. The fatigue crack initiation was predominantly induced by inclusions in specimens with large inclusion size, whereas fatigue crack initiated from the sub-surface microstructure in specimens with coarse microstructure. The fatigue life from granular bright facet (GBF) to fish-eye and from fish-eye to the critical crack size was calculated to obtain an estimate of the contribution to fatigue life by GBF, for the two modes of crack initiation within the HCF/VHCF regime. The results demonstrated that the majority of fatigue life was consumed by the crack initiation process along with the formation of GBF irrespective of whether the crack initiated from inclusions or from sub-surface microstructure. In the case of crack initiation from sub-surface microstructure, the ratio of fatigue crack initiation life to total fatigue life (Ni/Nf) had a wide scatter, which is attributed to varying B/M hierarchical structure in individual prior austenite grains.

Effect of inclusion and microstructure on the very high cycle fatigue behaviors of high strength bainite/martensite multiphase steels

We studied the mutual and relative effect of inclusion and microstructure on very high cycle fatigue (VHCF) behavior of bainite/martensite (B/M) multiphase steel by designing four combinations of various inclusion sizes and microstructures. Results show that the fatigue crack initiations are mostly induced by inclusions in the specimens with large inclusion size, whereas the “non-inclusion induced crack initiations” (NIICI) are observed in the specimens whose microstructure is coarse. Unlike the inclusion induced cracks that grow along the plane of maximum tensile stress, the early non-inclusion induced cracks within micro-facet propagate along the plane of maximum shear stress (i.e., Stage I crack), which is related to the strain localization in oriented B/M laths during cyclic loading. Finally, with the assistance of scanning electron microscopy, electron backscatter diffraction, 3-Dimensional surface profile, focused ion beam and transmission electron microscopy, we proposed the NIICI mechanism of coarse B/M microstructure, and found the orientation and size of coarse B/M block and blocky retained austenite (RA) are crucial microstructural factors in facilitating the formation of NIICI within VHCF regime.

Solution of temperature distribution under frictional heating with consideration of material inhomogeneity

Abstract Nonmetallic inclusions and defects can be commonly found in the initial metallurgical process of metal. This paper studies the influence of the size, position and interval of inhomogeneities on temperature field of inhomogeneous materials under frictional heating. A thermo-mechanical model of point contact is established based on the semi-analytical method, the equivalent inclusion method and three-dimensional high-order theory. The developed algorithm has good stability and convergence. The results show that the large inclusion size near the contact surface has significant effect on the temperature field. When the thermal conductivity of inhomogeneity is higher than that of the substrate, the temperature rise in the contact zone is lower; otherwise, opposite trend is observed.

Fatigue Strength and Crack Initiation Mechanism of Very-High-Cycle Fatigue for Low Alloy Steels

The fatigue strength and crack initiation mechanisms of very-high-cycle fatigue (VHCF) for two low alloy steels were investigated. Rotary bending tests at 52.5 Hz with hour-glass type specimens were carried out to obtain the fatigue propensity of the test steels, for which the failure occurred up to the VHCF regime of 108 cycles with the S-N curves of stepwise tendency. Fractography observations show that the crack initiation of VHCF is at subsurface inclusion with “fish-eye” pattern. The fish-eye is of equiaxed shape and tends to tangent the specimen surface. The size of the fish-eye becomes large with the increasing depth of related inclusion from the surface. The fish-eye crack grows faster outward to the specimen surface than inward. The values of the stress intensity factor (K I ) at different regions of fracture surface were calculated, indicating that the K I value of fish-eye crack is close to the value of relevant fatigue threshold (ΔK th ). A new parameter was proposed to interpret the competition mechanism of fatigue crack initiation at the specimen surface or at the subsurface. The simulation results indicate that large inclusion size, small grain size, and high strength of material will promote fatigue crack initiation at the specimen subsurface, which are in agreement with experimental observations.

SPring-8 X-Ray Micro-Tomography Observations of Zirconium Inclusions in CO$_{2}$ Laser Fusion Splice for Single Mode Optical Fibers

Laser fusion splicing has been realized for the simple splicing of optical fibers in a small board area, and for accommodating the increasing number of optical fibers that must be mounted on a board. For the splicing, zirconia ceramics are conventionally used to fabricate V-groove substrates because of their high machining performance. In this study, the existence of zirconium inclusions between the spliced optical fibers could be clarified with SPring-8 micro-computed tomography (SP-μCT) for the first time. The large inclusion size was found to increase the splicing loss. Moreover, the average splicing loss could be reduced from 0.18 dB to 0.15 dB by the adoption of a cleaning process to remove the zirconium inclusions that originate from the V-groove substrate used for the fiber.

Extreme value models for the assessment of steels containing multiple types of inclusion

Statistical methods based on extreme value theory have recently proved useful for estimating the sizes of large inclusions in clean steels. Observational evidence, however, suggests that for some steels the methods need modification to account for the presence of different types of inclusions. A new model on which estimations may be based is proposed for cases when two or more types of inclusion are present. The paper shows that the new model is more realistic than an earlier mixture model and that it fits data equally well. It may be used to give extended guidance about sampling strategies when the presence of two or more types of inclusion is suspected. The new model also enables the rate of occurrence of different inclusion types to be estimated without identification of the types of individual inclusions, and it allows the effect on maximum inclusion size to be predicted when inclusion characteristics are modified.

Methods for estimating the sizes of large inclusions in clean steels

The sizes of large inclusions within a cast of hard steel have a major influence on fatigue characteristics, but are not directly measurable by routine means. Recently, two methods have been proposed for estimation of the size distribution of large inclusions on the basis of measurements made on the sections of inclusions revealed in samples from a polished plane surface of the material. This paper reviews the two methods, showing that they are closely related and that properties found by each may be deduced from those found by the other. The paper also discusses the problem of inferring the distribution of the projected (three-dimensional) size of large inclusions from measurements made by either of the methods on sectional (two-dimensional) sizes. A simple new approximate solution to this stereological problem is proposed and is compared to existing approaches.

Interrelationship between statistical methods for estimating the size of the maximum inclusion in clean steels

Two types of approach based on the statistics of extremes have been developed recently to estimate the sizes of large inclusions in clean steel. The first type (termed here “threshold” approaches) includes methods based on the Generalized Pareto distribution (GPD) and the Exponential distribution (EXPGPD). Both of these methods use measurements of the sizes of all inclusions larger than a certain threshold size in a sample. The second type of approach (termed here the “extreme values” type) includes the Statistics of Extreme Values (SEV) method and the Generalized Extreme Values (GEV) method. In these, only the size of the largest inclusion in each of a set of samples is measured. This paper compares the four methods and describes their inter-relationship. The distribution of large sizes depends on a shape parameter ξ. The influence of ξ on confidence intervals for the characteristic size of the maximum inclusion is studied by considering the shape of the likelihood function. The value of ξ is found to have a considerable effect on the precision of estimation. In the methods based on the GPD and GEV the value of ξ is not specified in advance. In such a case the GPD method gives more precise estimation of the characteristic size of the maximum inclusion than the GEV method. On the other hand in the EXPGPD and SEV methods ξ is assumed to be zero. In this case the EXPGPD method gives more precise estimation than the SEV method. The choice of a method of estimation of the characteristic size of the maximum inclusion is discussed in the light of these findings.

The Largest Inclusions in a Piece of Steel

Fatigue properties of steels are strongly influenced by the presence of microscopic particles of oxides or foreign material known as inclusions. The size of the largest inclusion is an important determinant of fatigue strength. This paper studies the problem of estimating the sizes of large inclusions from measurements made on a two-dimensional section of the steel. The approach combines traditional stereological ideas with more recent extreme value modeling. It is shown that both classical likelihood and Bayesian approaches are useful in the inference.

Prediction of fatigue strength in HSS-Co, S55C and the friction welded materials based on statistical evaluation of inclusion size

In this paper, the effects of nonmetallic inclusions on the fatigue strength of medium and high strength steels are quantitatively investigated by considering the relationship between the Vickers hardness, Hv and the maximum size of nonmetallic inclusion,\(\sqrt {(area)_{\max } } \). The maximum size of nonmetallic inclusion defined by the square root of the projected area in an inclusion can be estimated by the statistics of extreme values with a definite number of specimens or structural components. While most of other studies using the parameter\(\sqrt {(area)_{\max } } \) have performed the experiments with the specimens with small artificial defects or notches, this study investigates the possibility of prediction on fotigue strength in the unnotched smooth specimens. The results show that strength of the friction welded joints of HSS-Co to S55C carbon steel is almost equal to that of the weaker material in the optimum welding conditions. Under the limiting condition for the nonpropagating cracks emanating from defects or inclusions, the threshold stress intensity factor range °Kth and the lower limit of fatigue strength σw1 were successfully estimated from the largest inclusion size\(\sqrt {(area)_{\max } } \). From the analysis of SEM fractographs, it can be concluded that the fatigue fractures of the specimens are associated primarily with the inclusions located at the outer periphery of the specimens.