• 한국정밀공학회지
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• 제21권5호
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• pp.158-165
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• 2004

Up to now, many crack repair techniques have been developed for inhibiting crack growth in structural components. However, the simplest way for inhibiting crack growth is to apply a indentation at the crack tip or at some distance ahead of the expected crack growth path so as to produce residual compressive stresses that can reduce the effective stresses around the crack tip. In spite of its importance to the aerospace industry, little attention has been devoted to evaluation of the indentation residual stress effect on the fatigue crack initiation life quantitatively. Therefore, in the present work, the magnitude and distribution of the indentation residual stresses were investigated in order to estimate the beneficial effect on fatigue crack initiation by using finite element method. Furthermore, to examine the validity of finite element analysis results, residual stress distribution in the indented specimen was measured by using X-ray diffraction technique, and fatigue crack behavior at fastener hole in aluminum alloy 7075-T6 before and after indentation processes was investigated.

• 한국군사과학기술학회지
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• 제6권4호
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• pp.73-79
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• 2003

Fatigue test is conducted to see the effect of pre-indentation on the fatigue crack retardation of Al5052-H18 plate. Metallographic observation is introduced to deduce the relationship between fatigue crack retardation and fracture appearance with indentation. The grain size of the specimen becomes smaller with the increase of indentation force and the plastic zone is formed with the decrease of grain size. The fatigue striations are appeared densely as the Indentation force becomes higher. Metallographic observation and fatigue test results show that the indentation force has the limited value in improving fatigue crack retardation. Important point to retard the fatigue crack growth is that the crack growth path should pass through the indented area.

• 동력기계공학회지
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• 제16권1호
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• pp.72-77
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• 2012

Acoustic emission (AE) is known to be sensitive to fracture process and so it was expected that AE data may propose as a means of monitoring the fracture information. The aim of this study is to analyze the characteristic of AE signal emitted from glass during Vickers indentation. To observe AE characteristics by surface effect, both glass and coating glass were studied. During Vickers indentation loading, AE signal resulted from penny-like crack is detected. During Vickers indentation unloading, AE signal resulted from both radial/median crack and lateral crack is detected. In case of indentation on glass, the emission energy(${\epsilon}$) is found to be approximately proportional to the fourth power of the crack length. In case of indentation on coating glass, the emission energy(${\epsilon}$) is approximately proportional to the crack length.

• 항공우주시스템공학회지
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• 제1권1호
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• pp.45-52
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• 2007

This paper analyzed the extension effects of fatigue life and the application of pre-indentation in aircraft structural material such as aluminum alloys. The test specimen used the thin sheet of aluminum alloy with a single-edged notch. The experiments were conducted after making the pre-crack under a constant amplitude loading. As the fatigue life extension technique, the pre-indentation making an indent on the predicted path of crack propagation was applied. The work presented here discussed about a proper mathematical relation between crack growth rate and the range of stress intensity factor and about the generalization of crack growth mechanism with large retardation effect. A technique to enhance the applicability of pre-indentation if also mentioned.

• 한국정밀공학회지
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• 제12권2호
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• pp.5-13
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• 1995

Various techniques to measure fracture toughness which is an important parameter to predict fracture behavious of structural materials have been reported. Among these mathods, this paper describes the micro-Vickers indentation crack method to estimate the fracture toughness of some WC-Co composites. Two indentation crack patterns (such as radial-median cracks (orhalf-penny cracks) and Palmqvist cracks generated during indentation) are referred precisely. The fracture toughness of WC-4.7wt%Co, WC-6wr%Co and WE-9wr%Co composites were estimated by using some equations given by Shetty et al., Nihara et al. in this study. We show the reliability of indentation method by comparing the results with those from literatures. The appropriate equation to estimate the fracture toughness in the case of WC-Co composite is given. In addition, some technical informations in terms of the crack length by indentation in estimating the existence of the surface residual stress that prevents to obtain an accurate fracture toughness are presented.

• 한국군사과학기술학회지
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• 제4권2호
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• pp.42-51
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• 2001

The effects of pre-indentation technique are presented for A12024-T3 Alclad alloy using as skin material for aircraft fuselage and wing. Indentations were applied to specimens to be placed on the presumed path of fatigue crack growth before fatigue tests. Tension-tension fatigue tests were conducted on the edge cracked specimens in the L-T orientation. Test results were analyzed to investigate the effectiveness of pre-indentation with the variation of specimen's thickness, position of indentation and applied maximum stress. Fatigue crack retardation by pre-indentation is well recognized in the various conditions.

• Corrosion Science and Technology
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• 제7권6호
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• pp.307-314
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• 2008

The influence of a moist atmosphere on $90^{\circ}$ domain switching under a sustained electric field, stress corrosion cracking of an indentation crack in water and an aggressive solution, and the relation between penetrating crack propagation and domain switching were studied using $BaTiO_3$ single crystal. The results indicate that enlarging the domain switching zone and crack propagation could be facilitated by a moist atmosphere or an aggressive solution due to the indentation residual stress. A moist atmosphere exerts remarkable influence upon the polarization of $BaTiO_3$ single crystal under a sustained electric field, and the surface energy of the c domain was much lower than that of the a domain. Domain switching ahead of a penetrating indentation crack tip was an essential requirement for crack propagation under constant stress.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2001년도 춘계학술대회논문집A
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• pp.597-602
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• 2001

A method for the retardation of fatigue crack growth using ring indentation at the vicinity of a crack is examined. Residual stresses near crack tip are evaluated using fracture mechanics approach. The motivation is to develop a simple and effective method for obtaining an increase in fatigue lives to total failure of materials with crack. Fatigue testing of aluminum specimen showed that the retardation effects are observed after the application of the method.

• 한국항공우주학회지
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• 제31권10호
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• pp.27-33
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• 2003

균열주위에 링압인을 부가함으로서 피로균열을 지연하는 수법을 제시하였다. 균열주위의 잔류응력분포는 Bueckner가중함수를 이용한 파괴역학적 수법으로 평가하였다. 본 연구는 균열을 가진 재료에 대한 파단시까지의 피로수명을 향상시키기 위한 단순하면서도 표과적인 수법을 개발하는데 있다. 알루미늄재료에 대한 피로실험결과 본 수법이 피로지연효과를 효율적으로 얻을 수 있는 것으로 나타났다.

• 대한치과보철학회지
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• 제31권4호
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• pp.482-505
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• 1993

The purpose of this study was to evaluate the fracture characteristics and the effect of resin bonding of laminate porcelain. In order to characterize the indentation-induced crack, Young's moduli and characteristic indentation dimensions were measured. The fatigue life under three point flexure test was measured using the electro-dynamic type fatigue machine, and the crack propagation with thermocycling was investigated on the condition of 15 second dwell time each in $5^{\circ}C\;and\;55^{\circ}C$ bath. The Vickers indentation pattern and the fracture surface were examined by an optical microscope and a scanning electron microscope (SEM). The results obtained were summarized as follows ; 1. Young's moduli(E) of the laminate porcelain and the resin cement used in this experiment were $62.56{\pm}3.79GPa$ and $15.01{\pm}0.12GPa$, respectively. 2. The initial crack size of the laminate porcelain was $69.19{\pm}5.94{\mu}m$ when an indentation load of 9.8N was applied, and the fracture toughness was $1.065{\pm}0.156MPa\;m^{1/2}$. 3. The fatigue life of laminate porcelain showed the constant fracture range at the stress level 27.46-35.30MPa. 4. When a cyclic flexure load was applied, the fatigue life of resin-bonded laminate porcelain was more decreased than that of laminate porcelain. 5. When a thermocycling was conducted, the crack growth rate of resin-bonded laminate porcelain was more increased than that of laminate porcelain. 6. Fracture surface showed the radial crack, the lateral crack, and the macroscopic crack branching region beneath the plastic deformation region when an indentation load of 9.8N was applied.