Chang Sung Seok

Fatigue Damage Evaluation of Woven Carbon-Fiber-Reinforced Composite Materials by Using Fatigue Damage Model

. 하여 피로손상 누적곡선을 획득하고 해석하였다. Abstract: Owing to the high specific strength and stiffness of composite materials, they are extensively used in mechanical systems and in vehicle industries. However, most mechanical structures experience repeated load and fatigue. Therefore, it is important to perform fatigue analysis of fiber-reinforced composites. The properties of composite laminates vary depending upon the stacking sequence and stacking direction. Fatigue damage of composite laminates occurs according to the following sequence: matrix cracking, delamination, and fiber breakage. In this study, fatigue tests were performed for damage analysis. Fatigue damages, which have to be considered in fatigue analysis, are determined by using the stiffness values calculated from hysteresis loops, and the obtained fatigue damage curve is examined using Maos equation and Abdelals equation.

Prediction of Fracture Strength of Woven CFRP Laminates According to Fiber Orientation

CFRP composite materials have been widely used in various fields of engineering because of their excellent properties. They show high specific stiffness and specific strength compared with metallic materiasl. Woven CFRP composite materials are fabricated from carbon fibers with two orientation angles (0°/90°), which influences the mechanical properties. Therefore, woven CFRP composite materials show different types of fracture behavior according to the load direction. Therefore, the fracture behavior of these materials needs to be evaluated according to the load direction when designing structures using these materials. In this study, we evaluate the fracture strength of plain-woven CFRP composite materials according to the load direction. We performed tests for six different angles (load direction: 0°/90°, 30°/-60°, +45°/-45°) and estimated the fracture strength for an arbitrary fiber angle by using the modified Tans theory and harmonic function.

Evaluation of Fatigue Characteristic of a Real Waterworks Pipe

The fatigue characteristic of a material or a structure is derived from fatigue tests of standard specimens. However, many researches have reported that test results of standard specimens are very different from those of real structures or components. One reason for this difference is the constraint effects according to the geometrical difference. Therefore, to calculate more accurate fatigue life, the constraint effect must be considered by comparing test results of standard specimens with those of real structures or components. Another reason for this difference is the surface condition. All surfaces of a standard specimen are polished to obtain similar conditions in a fatigue test. However, in a piping system, surface conditions of components are different from each other and very different from that of a standard specimen. Because fatigue life is effected by a surface condition, to evaluate the fatigue life of a piping system, fatigue tests must be conducted with a specimen extracted from a pipe with the same surface condition. The objective of this paper is to evaluate the fatigue characteristic of a real waterworks pipe by conducting fatigue tests with standard specimens and non-standard specimens of base metal and weld metal. Standard fatigue specimens and non-standard specimens were extracted from a steel pipe used in waterworks. Also, fatigue tests of pipes used in water service were carried out and then compared with those of standard specimens and non-standard specimens. From these results the relation between the S-N diagram of a specimen and that of a pipe specimen was evaluated.

A Study on the Effect of Welding Residual Stress and Weld Bead Profiles on Fatigue Behavior

In this study, we studied the effect of welding residual stresses and weld bead profiles on fatigue behavior of a material, JIS SM 490 A, with yielding strength of about 350 MPa and tensile strength of about 520 MPa. Using X-grooved butt weld plates, several kinds of test specimens such as parent material, reinforcement removed, as-welded and weld toe ground are made. Tensile tests, instrumented indentation tests and fatigue tests were carried out. And S-N curves for the above specimens were compared and analyzed. Introduction In these days, welding joints are widely used not only in railway industry but also in other industries. As speed of vehicles increases, structures of rolling stock become more and more light. Therefore, fatigue life evaluation is one of the challenging problems. A lot of study on the effect of weld bead shapes and weld details on fatigue [1-4] has been performed and many phenomena were explained. As for the effect of residual stress, some contradictory conclusions have been reported. Some results show the effect of welding residual stresses on fatigue life is significant. On the other hand, other results show specimens with tensile residual stresses have even longer fatigue life. These unclear conclusions seem to be mainly concerned with residual stress relaxation during fatigue tests and the metallurgical difference of the welded materials and parent materials. To clearly understand the effect of residual stress on fatigue, it is necessary to study residual stress relaxation during fatigue tests and metallurgical change due to heat treatment separately. In this study, we investigated the effect of welding residual stresses and weld bead profiles on fatigue behavior of a material, JIS SM 490 A, which is one of the materials used for rolling stock structures. Using X-grooved butt weld plates, several kinds of test specimens such as parent material, reinforcement removed, as-welded and weld toe ground are made. Tensile tests, instrumented indentation tests and fatigue tests were carried out. And S-N curves for the above specimens were compared and analyzed. Specimens The chemical compositions of JIS SM 490 A are shown in Table 1. Table 1. Chemical compositions (wt %) of the specimens, JIS SM 490 A C Si Mn P max. S max. 0.2 max 0.35 max 0.60~1.40 0.035 0.035 Two plates of 300 100 10mm were welded through 3 passes under CO2 gas by a semiautomatic robot. The conditions of GMAW are I=150~190 Ampere and V=105 Voltage. AWS ER Key Engineering Materials Online: 2004-08-15 ISSN: 1662-9795, Vols. 270-273, pp 2302-2307 doi:10.4028/www.scientific.net/KEM.270-273.2302

A Study on the Characteristics of Fracture Resistance Curve of Ferritic Steels

Fracture resistance (J-R) curves are used for elastic-plastic fracture mechanics analyses. The specimen geometry and the reversed cyclic loading affect the fracture resistance (J-R) curve. On the specimen geometry, The effects of specimen size plane size, specimen thinkness, side grooving and crack length were studied. On the reversed cyclic loading, the effects of load ratio and incremental plastic displacement were studied. The results revealed that theJ-R curves increased with increasing plane size and decreased with increasinga/W and theJ-R curves decreased with decreasing the load ratio and the incremental plastic displacement, respectively.

Investigation of welding residual stress of high tensile steel by finite element method and experiment

Due to varying temperature distribution of welding area during welding process, thermal stress is generated. It is known that the thermal stress forms residual stress. The welding residual stress has an important effect on welding deformation, embrittlement fracture, fatigue fracture, etc. In this paper, residual stress due to welding was numerically investigated by finite element method. To verify the results of numerical analysis, the residual stress of high tensile steel was measured by the hole-drilling method. Temperature change experimentally measured at the location of 3-mm-off-weld-bead, in addition, was compared with the numerical analysis. The above methodologies were applied to H-plate with 13.5mm thickness through MIG welding process. The distributions of the residual stress and the temperature distributions from the experimental and the numerical analyses were confirmed to be close.

Study of Failure Criterion of Hole-Notched Plain-Weave Carbon Fiber Reinforced Plastic (CFRP) Composites

초록:CarbonFiberReinforcedPlastic(CFRP)복합재료는높은비강성및우수한화학적특성등으로인하여여러분야에서점점사용이증가하고있다.대부분의CFRP복합재료는여러부품들의조립을통해제작된다.이러한독립된부품들은볼트,핀등과같은기계적인방법을통해체결된다.볼트나핀에의한hole은구조내에서노치로작용하여부품의강도저하의원인이된다.본논문에서는홀의크기와시험편폭이노치재의파괴강도에끼치는영향을평가하여hole을포함하고있는평직CFRP복합재료의정하중파괴강도를실험적으로평가하였다.이를위하여본논문에서는홀크기와시험편폭에따른점응력조건의특성길이를평가하였으며,특성길이와노치재의파괴강도의관계를확인하였다.이를이용하여노치재의정하중파괴기준을재정의하였다.Abstract:Recently,carbonfiberreinforcedplastic(CFRP)havebeenusedinvariousfieldsbecauseofitshighspecific modulus, and chemical properties. Most products in which CFRP composites are used aremanufactured by joining the product components by bolts or pins. Holes for bolts and pins decrease thestrength of the components because these holes act as notches in the structures. In this study, the fracturestrength of CFRP plain-weave composite plates containing holes is experimentally investigated to examine theeffects of hole-size and specimen width on notched tensile strength. The results show that the characteristiclength considered in the point stress criterion depends on the hole size and specimen width. There exists acertain relation between notched tensile strength and characteristic length. Fracture criterion is redefined onbasisofthisrelation.

Evaluation of Pipe Fracture Characteristics for SA312 TP304L Stainless Steel

In order to perform elastic-plastic fracture mechanical analyses, fracture resistance curves for concerned materials are required. A standard CT specimen was used to obtain fracture resistance curves. However the fracture resistance curve by the standard CT specimen was very conservative to evaluate the integrity of the structure. Also the fracture resistance curve was affected by the specimen geometry, crack plane orientation, reverse cyclic loading and dynamic strain aging. The objective of this paper is to be certain the conservativeness of the fracture resistance curve by the standard CT specimen. For these purpose, fracture tests using the real-scale pipe specimen and standard CT specimen test were performed. A 4-point bending jig was manufactured for the pipe test and the direct current potential drop method was used to measure the crack extension and the length for the pipe test. From the result of the pipe and the standard CT specimen tests, it was observed that the fracture resistance curve of the standard CT specimen test was conservative compare to that of the pipe specimen test.

Effect of temperature on the fracture toughness of A516 Gr70 steel

Fracture toughness JIC and KIC tests were performed on A516 Gr70 carbon steel plate at the temperature ranging from −160°C to 600°C, and test results were analyzed according to ASTM E 813 and ASTM E 399. Unloading compliance J-integral tests were performed on ITCT specimens. The relation between the JIC value and the test temperature was obtained. It was concluded that the temperature ranging from −15°C to 600°C is the upper shelf region of ductile-brittle transition temperature, and in this temperature range, fracture toughness JIC values decreased with increasing temperature. The ductile brittle transition temperature of the material may be around −30°C. In the region near −30°C, the tendency of JIC to decrease with decreasing temperature was significant.

Influence of Compressive Stress in TGO Layer on Impedance Spectroscopy from TBC Coatings

Impedance spectroscopy is a non-destructive evaluation (NDE) method first proposed and developed for evaluating TGO layers with compressive stress inside thermally degraded plasma-sprayed thermal barrier coatings (PS TBCs). A bode plot (phase angle (h) vs. frequency ( f )) was used to investigate the TGO layer on electrical responses. In our experimental study, the phase angle of Bode plots is sensitive for detecting TGO layers while applying compressive stress on thermal barrier coatings. It is difficult to detect TGO layers in samples isothermally aged for 100 hrs and 200 hrs without compressive stress, and substantial change of phase was observed these samples with compressive stress. Also, the frequency shift of the phase angle and change of the phase angle are observed in samples isothermally aged for more than 400 hrs.