• Composites Research
• /
• v.24 no.2
• /
• pp.46-50
• /
• 2011

This paper describes the state of the art for the development of metallic armor materials which are mainly used as armor plates of the combat vehicles. Several important micro-structural features affecting ballistic properties of the metallic armor are discussed. Optimization of the strength and toughness balance of the metallic armor is necessary for the improvement of the ballistic performance resulting from maximizing the resistance to the penetration of the bullet and also to brittle failure of the plates. Understanding and control of the adiabatic shearing phenomenon developed remarkably during high strain rate deformation is needed to prevent brittle failure of the metallic armor materials.

• Tunnel and Underground Space
• /
• v.18 no.6
• /
• pp.457-464
• /
• 2008

Dynamic fracture mechanism of rock is important to improve rapid excavation method and develop precise damage assesment of rock mass in the vicinity of an excavation. In order to investigate dynamic fracture characteristics and dynamic damage mechanism of brittle materials, this study employed pulse shape-controlled Split Hopkinson Pressure Bar (SHPB) system. The P- and S-wave velocities of the tested samples were measured before and after tests to examine damage of the samples. The decay ratios of the Ultrasonic wave velocities increased with impart velocities and the samples which have lower strength showed higher permanent strain significantly.

• Proceedings of the Korea Concrete Institute Conference
• /
• 2008.11a
• /
• pp.137-140
• /
• 2008

If the flexural member of concrete is designed using the FRP rebar, suddenly brittleness destruction resulted from the fracture of FRP rebar is generated in the extreme situation because of brittleness characteristics of FRP rebar and concrete when designed to be less than balanced reinforcement ratio, so it is recommended to design the flexural member of concrete to be more than balanced reinforcement ratio. In ACI 440.1R-06 proposes the different bending strength decrease coefficient according to destructive form of concrete flexural member using the FRP rebar. However, ACI 440.1R-06 applies the same strength decrease coeffient to all FRP rebars made of diverse materials. If the same strength decrease coefficient is applied to all FRP rebars, effect of increasing the reinforcement ratio and selection of FRP rebar will be considerably limited. In this regard, we are to propose the formula to calculate the bending strength decrease coefficient in consideration of change in characteristics of FRP rebar and L/D through the reliability analysis in this paper.

• Journal of the Korean Society for Heat Treatment
• /
• v.5 no.4
• /
• pp.209-214
• /
• 1992

A Study on the Interface Behavior of Aluminium Alloy Matrix Composites Reinforced with W Fiber. In this study the tungsten fiber reinforced 7072 aluminium alloy martrixcomposites were fabricated using vacuum hot press. The fiber has been aligned on the aluminium alloy sheets and these sheets were bonded with diffustion at 873K and 49Mpa. The behaviors of interface layer and mechanical properites have been investigated as a function of holding time at 873K. Several compounds were formed at the interface layer. These compounds were growing in propotion to holding time. XRD analysis showed that these compounds were $WAl_{12}$ $WAl_5$. The tensile strength decreased as the heat treatment time over 10hr gradually. The ductle fractur mode was turned to brittle mode after heat treatment.

• Journal of the Korean Society for Nondestructive Testing
• /
• v.20 no.5
• /
• pp.373-380
• /
• 2000

It is useful to use NDE methods to assess the mechanical properties of materials since destructive methods are time-consuming and usually require cutting of sample from the material/component. In the present research, ultrasonic characteristics have been utilized to evaluate changes of mechanical properties due to heat treatment temperature and condition. The attenuation coefficient of ultrasonic wave increased as the heat treatment temperature because the grain size increased in size as the temperature. The attenuation coefficient decreased as the heat treatment has been progressed (quenched, tempered, PWHT). In the case of ultrasonic velocity measurement, velocity difference between quenched and tempered/PWHT was 40 m/s. There was a good relationship between the attenuation coefficient and the toughness. The relationship can be used for the nondestructive evaluation of the forged reactor vessels. Moreover, the method may be effectively used in the field application.

• Transactions of the Korean Society of Mechanical Engineers A
• /
• v.37 no.12
• /
• pp.1503-1512
• /
• 2013

In this study, we proposed an indentation evaluation method for fracture toughness using cohesive finite element simulations. First, we examined the effect of material properties (yield strain, Poisson's ratio, and elastic modulus) on crack size during Vickers indentation and then generated a regression formula that explains the relations among fracture toughness, indentation load, and crack size. We also proposed another indentation formula for fracture toughness evaluation using the contact size a and E/H (H: hardness). Finally, we examined the relation between the crack size and the indenter shapes. Based on this, we can generate from the formula obtained using the Vickers indenter a formula for an indenter of different shapes. Using the proposed method, fracture toughness is directly estimated from indentation data.

• Korean Journal of Materials Research
• /
• v.6 no.3
• /
• pp.253-263
• /
• 1996

고온.고압하에서 장시간 사용되는 고온부재용 구조물은 경년열화현상을 나타낸다 그러므로 구조물의 안정성 측면에서 재질열화의 정도를 정량적으로 평가하는 것이 중요하다. 그러나 실기 구조물에서 채취할 시험편의 크기와 수는 제한이 되기 때문에 새로운 비파괴적인 평가법이 요구되고 있다. 따라서 본 연구에서는 화력발전설비에 이용되는 탄소강과 페라이트강에 대한 열화도의 평가를 위해 입계부식법의 적용 가능성을 조사한다. 시험결과, 재질열화의 정도는 사용시간보다 사용온도에 더 큰 영향을 받았으며, Larson-Miller인자와 열화도([$\Delta$DBTT]SP)사이의 관계는 선형적이었으나, 강종에 따라 다른 기울기를 보였다. 반면, 연성-취성천이온도 ([$\Delta$DBTT]SP)와 격자절단비(Ni/No)와 관계는 강종에 무관하게 선형적인 비례관계를 나타내었다. 또한, [$\Delta$DBTT]SP와 Ni/No 의 관계로부터 입계부식법은 페라이트계 강뿐만아니라 탄소강에 대해서도 유용한 재질열화 평가 방법임을 알 수 있었다.

• Korean Journal of Materials Research
• /
• v.18 no.7
• /
• pp.394-399
• /
• 2008

The hydrogen embrittlement susceptibility of high strength TRIP/TWIP steels with the tensile strength of 600Mpa to 900Mpa grade was investigated using cathodically hydrogen charged specimens. TWIP steels with full austenite structure show a lower hydrogen content than do TRIP steels. The uniform distribution of strong traps throughout the matrix in the form of austenite is considered beneficial to reduce the hydrogen embrittlement susceptibility of TWIP steels. Moreover, an austenite structure with very fine deformation twins formed during straining could also improve the ductility and reduce notch sensitivity. In Ubend and deep drawing cup tests, TWIP steels show a good resistance to hydrogen embrittlement compared with TRIP steels.

• Proceedings of the Korean Society of Precision Engineering Conference
• /
• 2000.11a
• /
• pp.939-942
• /
• 2000

Single point diamond turning technique for optical crystals is reported in this paper. The main factors influencing the machined surface quality are discovered and regularities of machining process are drawn. Optical crystals have found more and more important applications in the field of modern optics. Optical crystals are mostly brittle materials of poor machinability. The traditional machining method is polishing which has many shortcomings such as low production efficiency, poor ability to be automatically controlled and edge effect of the workpiece. The purpose of our research is to find the optimum machining conditions for ductile cutting of optical crystals and apply the SPDT technique to the manufacturing of ultra precision optical components of brittle material.

• Journal of Advanced Marine Engineering and Technology
• /
• v.30 no.6
• /
• pp.716-723
• /
• 2006

A meso-scale analysis method using the natural element method is proposed for the analysis of material damage of brittle microcracking solids. The microcracking is assumed to occur along Voronoi edges in the Voronoi diagram generated using the nodal points as the generators. The mechanical effect of microcracks is considered by controlling the material constants in the neighborhood of the micorcracks. The macro elastic moduli of isotropic solids containing a number of randomly distributed microcracks are calculated considering the effect of microcrack closure to demonstrate the validity of the proposed method.