• Transactions of the Korean Society of Machine Tool Engineers
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• v.11 no.2
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• pp.43-49
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• 2002

SB41 material is welded automatically and is investigated some effects of the welding residual stress in the growth and propagation of fatigue crack, so as to study the fatigue behaviour in the welding residual stress field. The summarized results are as follows; 1) In case of the load amplitude is constant, as the stress ratio is changing to 0.1, 0.33 and 0.5 the propagation life is constant but the growth life decreases. And than, when maximum load or minimum load is constant as the stress ratio increases the growth life and propagation life. 2) It was shown that fatigue crack propagation ratio da/dN was almost constant regardless of the stress ratio change at constant load amplitude and that the larger stress ratio, the slower was the fatigue crack propagation ratio. 3) The opening ratio U is influenced by $K_max$ but it isn't only the function of $K_max$ because data range is very large. 4) The fatigue life of the specimens on tensile compressive residual stress field was decreased and increased respectably more than that of the base metal.

• Journal of Advanced Marine Engineering and Technology
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• v.25 no.6
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• pp.1228-1236
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• 2001

In this study, CT specimens were prepared from ASTH A5l6 steel which was used for pressure vessel plates for moderate and lower temperature service. And we got the fellowing characteristics from fatigue crack growth test carried out in the environment of room and low temperature at $25^{\circ}C$ , $-30^{\circ}C$, $-60^{\circ}C$, $-80^{\circ}C$, $-100^{\circ}C$ and $-120^{\circ}C$ and in the range of stress ratio of 0.1, 0.3 by means of opening mode displacement. At the constant stress ratio, the Threshold stress intensity factor range ΔAKth in the early stage of fatigue crack growth (Region I) and stress intensity factor range $\DeltaK$ in the stable of fatigue crack growth (Region II) was increased in proportion to descend temperature. It assumed that the fatigue resistance characteristics and fracture strength at low temperature is considerable higher than that of room temperature in the early stage and stable of fatigue crack growth region. The straight line slope relation of logarithm da/dN-$\Delta$K in Region II that is, the fatigue clack growth exponent m increased with descending temperature at the constant stress ratio. It assumed that the fatigue crack growth rate da/dN is rapid in proportion to descend temperature in Region II and the cryogenic-brittleness greatly affect a material with decreasing temperature.

• Journal of Ocean Engineering and Technology
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• v.14 no.3
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• pp.78-83
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• 2000

In this study, CT specimens were prepared from AST SA516 Gr. 70 which was used for pressure vessel plates for room and low temperature service. And we got the following characteristics from fatigue crack growth test carried out in the environment of room and low temperature at 25$^{\circ}C $, -60$^{\circ}C $, -80$^{\circ}C $ and -100$^{\circ}C $ and in the range of stress ratio of 0.05, 0.3 by means of opening mode displacement. At the constant stress ratio, the threshold stress intensity factor range ${\delta} K_{th}$ in the early stage of fatigue crack growth (Region I) and stress intensity factor range $\delta $K in the stable of fatigue crack growth (Region II) were increased in proportion to descending temperature. It was assumed that the fatigue resistance characteristics and fracture strength at low temperature is considerable higher than that of room temperature in the early stage and stable of fatigue crack growth region. The straight line slope relation of logarithm da/dN -$\delta $K in Region II, that is, the fatigue crack growth exponent m increased with descending temperature at the constant stress ratio. It was assumed that the fatigue crack growth rate da/dN is rapid in proportion to descending temperature in Region II and the cryogenic-brittleness greatly affect a material with decreasing temperature.

• Proceedings of the KWS Conference
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• 2001.05a
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• pp.220-223
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• 2001

In this study, CT specimens were prepared hem ASTM SA516 which was used for pressure vessel plates for room and low temperature service. And we got the following characteristics from fatigue crack growth test carried out in the environment of room and low temperature at $25^{\circ}C$, -3$0^{\circ}C$, -6$0^{\circ}C$, -8$0^{\circ}C$, -l$0^{\circ}C$ and -l2$0^{\circ}C$ and in the range of stress ratio of 0.1, 0.3 by means of opening mode displacement. At the constant stress ratio, the threshold stress intensity factor range ΔK$_{th}$ in the early stage of fatigue crack growth ( Region I ) and stress intensity factor range ΔK in the stable of fatigue crack growth ( Region II) was increased in proportion to descend temperature. It assumed that the fatigue resistance characteristics and fracture strength at low temperature is considerable higher than that of room temperature in the early stage and stable of fatigue crack growth region. The straight line slope relation of logarithm da/dN - ΔK in Region II, that is, the fatigue crack growth exponent m increased with descending temperature at the constant stress ratio. It assumed that the fatigue crack growth rate da/dN is rapid in proportion to descend temperature in Region H and the cryogenic-brittleness greatly affect a material with decreasing temperature.e.greatly affect a material with decreasing temperature.

• Transactions of the Korean Society of Automotive Engineers
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• v.11 no.1
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• pp.143-150
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• 2003

Experimental research has been carried out to investigate the characteristics of the fatigue crack initiation and propagation behavior of Tailor Welded Blank(TB) sheet used for vehicle body. We used three types of specimens which were machined of the same base metal: one is 1.4mm thick, another is 1.6mm thick, and the third(TB specimen) is laser-welded of two specimens(1.4mm and 1.6mm thick ones). The results of tensile and hardness test indicate that the yield strength of the TB specimen is the highest, and the hardness around welding bead is higher than that of base metal. Fatigue strength and fatigue limit of the TB specimen are much superior to those of the base metal up to $10^6$ cycles. The fatigue crack propagation of the heat-affected zone of the TB specimen is slower than that of the base metal. Welding bead has the fastest crack Propagation in the low stress intensity factor range$(\DeltaK)$ region, but the slowest in the high $\DeltaK$ region. The fatigue propagation characteristic of the TB specimen is relatively stable in comparison with that of the base metal in the high ${\Delta}K$ region around over $28MPa\sqrt{m}$.

• Journal of the Korean Society for Precision Engineering
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• v.13 no.12
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• pp.70-77
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• 1996

Fatigue tests were carried out at high temperature on a Cr-Mo-V steel in order to assess the fatigue life of components used in power plants. The characteristics of high temperature fatigue were divided in terms of cycle-dependent fatigue and time-dependent fatigue, each crack propagation rate was examined with respect to fatigue J-integral range, .DELTA. J$_{f}$and creep J-integral range, .DELTA. J$_{c}$. The fatigue life was evaluated by analysis of J-integral value at the crack tip with a dimensional finite element method. The results obtained from the present study are summarized as follows : The propagation characteristics of high temperature fatigue cracks are determined by .DELTA. J$_{f}$for the PP(tensile plasticity-compressive plasticity deformation) and PC(tensile plasticity - compressive creep deformation) stress waveform types, and by .DELTA. J$_{c}$for the CP(tensile creep- compressive plasticity deformation) stress waveform type. The crack propagation law of high temperature fatigue is obtained by analysis of J-integral value at the crack tip using the finite element method and applied to examine crack propagation behavior. The fatigue life is evaluated using the results of analysis by the finite element method. The predicted life and the actual life are close, within a factor of 2.f 2.f 2.

• Composites Research
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• v.27 no.2
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• pp.52-58
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• 2014

Short fiber reinforced composites manufactured by injection molding have diverse fiber orientations variable with measuring positions even in the same specimen, which is caused by the flow induced fiber orientation. Fiber orientations considerably affect the mechanical and thermal properties of final composite products. In this study, fiber orientation of injection molded carbon fiber reinforced PA6/PPO composite was measured at several points of the specimen by optical microscopy analysis and the corresponding izod impact strength, coefficients of thermal expansion (CTE) were also measured to investigate the influence of local fiber orientation on the mechanical and thermal properties. Izod impact strength where fiber was perpendicular to the direction of crack propagation was higher than where fiber was parallel to the direction, which could be explained be the impact resistance reinforcing mechanism by fiber orientation. CTE was also lower where fiber was parallel to the measurement direction of CTE than where fiber was perpendicular to the direction, which could be also explained by the dimensional stability mechanism by fiber orientation.

• Transactions of the Korean Society of Mechanical Engineers
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• v.11 no.6
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• pp.953-962
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• 1987

Domestic dissimilar structural steels, SM 45 C and SUS304 were friction welded under optimal welding condition and the micro-artificial holes were drilled at SM 45 C base metal, SM 45 C HAZ, welded zone, SUS 304 HAZ, and SUS 304 base metal for fatigue behavior tests. In this study, the fatigue limit and the behavior of micro-crack propagation, crack propagation rate, and its dependency on stress intensity factor under the low stress level and high stress level of bending stress have been investigated. The results obtained are as follows. (1) The fatgiue strength of the portion of SM45C B.M., SM45C HAZ, welded zune, SUS304 HAZ and SUS304 B.M. on notched friction welded specimens are 20 kgf/mm$^{2}$, 32 kgf/mm$^{2}$, 27kgf/mm$^{2}$, 29kgf/mm$^{2}$, and 29kgf/mm$^{2}$, respectively. (2) The fatigue strength of welded zone of unnotched and notched specimens are 32.5kgf/mm$^{2}$, and 27kgf/mm$^{2}$, respectively. (3) Micro-crack initiation in the welded zone, HAZ, and each base metals occurrs simultaneously in front and rear of micro-hole tips in the view of the rotational directions. (4) Fatigue crack propagates more slowly in the welded zone than in another protions of specimen, regardless of the magnitude of the stress level. (5) Fatigue crack propagation rates were plotted as a function of stress intensity range. The value of m in the equation da/dN=C(.DELTA.K)$^{m}$ was found to range from 2.09-2.55 in this study.

• Journal of the Korean Society for Nondestructive Testing
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• v.21 no.2
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• pp.163-168
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• 2001

Due to aging, adhesively bonded and riveted aircraft lap joints can contain distends, cracks around rivet holes, fatigue induced flaws, and corrosion. It is required for the safety of aircraft to inspect these defects through the whole region of mint in rapid speed. Bond quality or adhesively bonded and riveted aluminum lap splice joints is investigated using non-contact remote ultrasonic nondestructive evaluation (NDE). Non-contact ultrasonic tests are performed using laser generation and air-coupled transducer detection. A Q-switched Nd:YAG laser and a periodic transmission mask are used to generate a selected Lamb mode. The Lamb wave is generated on one side of the lap splice joint, propagates along the plate, interacts with the joint and is detected on the other side by a micromachined air-coupled capacitance transducer. Analysis of recorded signals allows to evaluate the condition of the bond.

• Composites Research
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• v.31 no.1
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• pp.8-11
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• 2018

Sheet molding compound (SMC) is one of the most economical fiber reinforced composite fabrication processing for automotive applications. In this study, we studied the optimum formulation for the production of SMC which shows low specific gravity without lowering the mechanical properties by using glass bubble (GB) which is a low specific gravity filler and glass fiber (GF) as a reinforcing material. The tensile strength increased with the increase of the GF in the SMC, and the specific gravity decreased with the increase of the GB. The synergistic effect of improving the mechanical properties as the specific gravity is lowered is found in the optimum formulation. The synergy effect was confirmed by the internal structure analysis that the dispersion effect of the crack propagation of the GB and the improvement of the binding force between the fiber and the matrix due to the incorporation of the GB.