• Transactions of the Korean Society of Mechanical Engineers A
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• v.28 no.8 s.227
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• pp.1221-1228
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• 2004

The vertical roller mill is the important machine grinding and mixing various crude materials in the manufacturing process of portland cement. Table liner is one of grinding elements of vertical roller mill and is subjected to the cyclic bending stress by rollers load and the centrifugal force by rotation of table. It demands $4{\times}10^7$ cycle but has $4{\times}10^6{\sim}8{\times}10^6$ cycle. It fractures at the edge of grinding path of outside roller. The repair expense for it amounts to $30\%$ of total maintenance of vertical roller mill. Therefore, this study shows the fracture mechanism of table liner for vertical roller mill using HDM and fatigue analysis and makes the estimation for safety of vertical roller mill.

• Journal of the Korean Society of Manufacturing Process Engineers
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• v.3 no.3
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• pp.79-85
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• 2004

The lightness of components required in automobile and machinery industry is requiring high strength of components. In particular, fatigue failure phenomena, which happen in metal, bring on danger in human life and property. Therefore, antifatigue failure technology takes an important part of current industries. Currently, the shot peening is used for removing the defects from the surface of steel and improving the fatigue strength on surface. Therefore, in this paper the effect of compressive residual stress of spring steel(JISG SUP-9)by shot peening on fatigue crack growth characteristics in stress ratio(R=0 1, R=0 3, R=0 6)was investigated considering fracture mechanics. By using the methods mentioned above, I arrived at the following conclusions: (1) The fatigue crack growth rate(da/dN) of the shot peening material was lower than the unpeening material And in stage I, ${\Delta}K_{th}$, the threshold stress intensity factor, of the shot peening material is high in critical parts unlike the unpeening material. (2) Fatigue life shows more Improvement in the shot peening material than in the unpeening material. And compressive residual stress of surface on the shot peening processed operate the resistance of fatigue crack propagation.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.9 no.3
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• pp.65-73
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• 1989

Continuum formulations for the expressions of dynamic energy release rates and computational methods for dynamic stress intensity factors are developed for the analysis of dynamic fracture problems subjected to stress wave loading. Explicit volume integral expressions for instantaneous dynamic energy release rates are derived by modeling virtual crack extensions with the dynamic Eulerian-Lagrangian kinematic description. In the finite element applications a finite region around a crack-tip is modeled by using quarter-point singular isoparametric elements, and the volume integrals are evaluated for each crack-tip element during virtual crack extensions while the singularity is maintained. It is shown that the use of the present method is more reliable and accurate for the dynamic fracture analysis than that of other path-independent integral methods when the effects of stress waves are significant.

• Journal of Power System Engineering
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• v.8 no.4
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• pp.11-16
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• 2004

In this study, to investigate the effects of omitting low-amplitude cycles from a flight-simulation loading, crack growth tests were conducted on 2124-T851 aluminum alloy specimens. Three test spectra were generated by omitting small load ranges as counted by the rain-flow count method. The crack growth test results were compared with the data obtained from the flight-simulation loading. The experimental results show that the ranges equal to or smaller than 5% of the maximum load do not contribute to crack growth behavior because these are below the initial stress intensity factor range. Omitting these from the flight-simulation loading, test time can be reduced by 54%. However, in the case of omitting the load ranges below 15% of the maximum load, crack growth rates decreased, and crack growth curve deviated from the crack growth data under the flight-simulation loading because loading cycles above fatigue fracture toughness were omitted.

• Transactions of the Korean Society of Mechanical Engineers
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• v.13 no.1
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• pp.87-95
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• 1989

The fatigue crack propagation of random short fiber SMC composite material was investigated. In macroscopic viewpoint, SMC composite material was treated as isotropic material and was analyzed in terms of conventional fracture mechanics. Experiments were conducted on mode I and mixed respectively and various loading level was applied to each mode. Fatigue crack growth can be explained in three steps and most of fatigue life is consumed in initial crack growth. In this experiments, power law, i.e, da/dN=C(C.DELTA.K)$^{m}$ , between fatigue crack growth rate and stress intensity factor range, was valid and the value of the exponent m is about 10, which is much higher than that of other metals. Fracture mechanism was also investigated by SEM fractographic study.

• Journal of Welding and Joining
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• v.3 no.2
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• pp.16-26
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• 1985

Post weld heat treatment(PWHT), at more than 600.deg. C, is essential to remove residual stress and hydrogen in weld HAZ and improve fatigue characteristics. However, residual stress during PWHT is responsible for PWHT embitterment and it promotes precipitation of impurities to grain boundary. In this paper, the effect of stress simulated residual stress on fatigue failure was evaluated by fatigue test, microhardness test and fractograph. The obtained results are summarized as follows; (1) The fatigue crack growth rate(da/dN) of parent and heat treated parent was affected by microstructure due to heat treatment and it depended on stress intensity factor (.DELTA.k). (2) The fatigue strength of weld HAZ was dependent on applied stress during PWHT and da/dN after PWHT was slower than as-weld. (3) Softening amount of weld HAZ was bigger than any other due to PWHT. Hardness value of weld HAZ was affected by heat treatment under the applied stress of 10 $kgf/mm^2$, but beyond 20 $kgf/mm^2$ it was increased by the applied stress rather than heat treatment. (4) Beyond the applied stress of 20 $kgf/mm^2$ during PWHT, intergranular fracture surface was observed and its amount was increased with applied stress during PWHT. (5) Effect of applied stress during PWHT on aspect of fracture surface was larger rather than that on fatigue crack growth behavior.

• Journal of Power System Engineering
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• v.4 no.4
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• pp.41-47
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• 2000

Fatigue crack propagation rate and threshold characteristics of the SA516/70 steel which is used for the low temperature pressure vessels, were studied in the room temperature of $25^{\circ}C$ and low temperature ranges of $-10^{\circ}C,\;-30^{\circ}C,\;-60^{\circ}C\;and\;-80^{\circ}C$ with stress ratio of R=0.1. In the logarithmic relationship between the fatigue crack propagation rate($d{\alpha}/dN$) and stress intensity factor range ${\Delta}K$, the linear relationship was obtained up to $d{\alpha}/dN=4.425{\times}10^4mm/cycle$ in the same of room temperature, but in low temperature case, the relationship was extended to the range of low crack propagation rate. The fractured specimens were examined by SEM. Tested results showed that specimen failed at low temperature exhibit the quasi-cleavage fracture formation however considerable ductility proceed final fracture.

• Journal of Ocean Engineering and Technology
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• v.2 no.2
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• pp.104-111
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• 1988

This paper deals with the micro-surface-cracks behavior on the unnotched smooth specimens of Type 304 stainless steel at $593^{\circ}C$ in air under creep and creep-fatigue conditions that have 10 mim and 1 min load holding times respectively. The behaviors of the micro-surface-cracks have been visualized by means of surface replica method and optical micro-photography. The quantitative characteristics of initiation, growth and coalescence of micro-surface-cracks have been investigated by observing and measuring the crack growth behaviors. some of the important results are as follows: Main crack initiates at grain boundary in the early stage(10 to 20%)of its life time and grows through coalescence and finally leads to fracture. The distribution of micro-surface-crack length, 2a, can be plotted against the composite Weibull distribution. The growth rate of the main crack can be plotted against the stress intensity factor, crack tip opering displacement and J integral.

• Journal of the Society of Naval Architects of Korea
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• v.35 no.1
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• pp.61-71
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• 1998

This paper describes a dynamic fracture behaviors of structural elements under elastic or elasto-plastic stress waves in two dimensional space. The governing equation of this problem has the type of hyperbolic partial differential equation, which consists of the equation of motions and incremental elasto-plastic constitutive equations. To solve this problem we introduce Zwas' method which is based on the finite difference method. Additionally, in order to deal with the dynamic behavior of elasto-plastic problems, an elasto-plastic loading path in the stress space is proposed to model the plastic yield phenomenon. Based on the result of this computation, the dynamic stress intensity factor at the crack tip of an elastic material is calculated, and the time history of a plastic zone of a elasto-plastic material is to be shown.

• Journal of Korea Foundry Society
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• v.12 no.6
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• pp.464-470
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• 1992

In the present study, the effects of Al/SiC interface reaction and the formation of $Al_4C_3$ compounds on the mechanical properties of the Al/SiC composites prepared by squeeze casting were investigated. After squeeze casting, the size of dendrites in Al without whiskers were larger than those with whiskers. The hardness of composite materials (about 72 Hv) was found to be approximating 40% higher than that of matrix metal (29Hv), which gradually increases which heat treatment Time showing maximum hardness at 12hr. The observation of increasing number of compounds in 12hrs heat treatment suggests that these compounds are responsible for the increase of hardness. By X-ray diffraction studies, those compounds were identified as $Al_4C_3$, (Al, Si). And intensity of Si peak increased. The tensile strengh of composite materials was gradually decresed by heat tretment, which was in contrast to the behavior of hardeness. With incresing heat tretment time, the fracture mode of composite materials was changed from large dimples and pull-out form of fiber to the fracture and rupture foum of fiber.