Experimental investigation on static and fatigue behavior of welded sm490a steel under low temperature

Steel Structures 9 (2009) 85-91 www.ijoss.org

Experimental Investigation on Static and Fatigue Behavior of

Welded SM490A Steel Under Low Temperature

Ki-Weon Kang1,*, Byeong-Choon Goo2, Jae-Hoon Kim2, Doo-Kie Kim3, and Jung-Kyu Kim4

1Professor, School of Mechanical and Automobile Engineering, Kunsan National University,

68, Miryong-dong, Kunsan, Jeonbuk 573-701, Korea2Ph.D., Advanced Railroad Technologies Application Research Center, Korea Railroad Research Institute, Gyeonggi 437-757, Korea

3Professor, Department of Civil Engineering, Kunsan National University, 68, Miryong-dong, Kunsan, Jeonbuk 573-701, Korea4Professor, Department of Mechanical Engineering, Hanyang University, 17, Haengdang-dong, Seongdong-gu, Seoul 133-791, Korea

Abstract

The paper aims to evaluate the fatigue behavior and its probabilistic properties in welded SM490A steels, which is utilizedin high speed train, under low temperatures. For the goal, the tensile and fatigue tests are performed under displacement controlmode and constant amplitude loading cycles, respectively at various temperatures (293K, 263K and 233K). The static strengthsfor base and welded materials are increased with the decrease in temperature but, the welded material has a considerable amountof scatter in strength. Also, the fatigue behaviors are greatly influenced by the test temperature for both materials. In particular,the welded material exhibits severe reduction of the fatigue limit compared with the base material. The probabilistic propertiesof fatigue life are investigated through P-S-N (probabilistic S-N) approach and the predicted results are well in conformancewith the experimental results. Also, the variations of fatigue life are greatly influenced by the temperature and this tendencyis more remarkable in the welded material.

Keywords: Fatigue Behavior, Low Temperature, Probabilistic S-N curve, Static Behavior, Welded SM490A Steel

1. Introduction

Most of the structural components, either on the

automotive, pipeline and railway traffic industries, should

be prepared to operate in a range of temperature, which

may vary from low temperature, for example 233K, to

temperatures well above ambient temperature. Recently,

the world-wide trade is rapidly increased and accordingly,

it is more required to establish the intercontinental

transportation system by using the railway transportation

systems such as the TKR (Trans-Korean Railway) or TSR

(Trans-Siberian Railway). However, these may make the

railway vehicle expose the severe environmental condition

such as the extreme low temperature, which hardly occurs

in the local region. It is well known that the increase of

temperature above ambient temperature causes a decrease

in strength of materials, and that the decrease of

temperature below ambient temperature originates the

opposite effect (Silva, 2004). This is true for almost all

mechanical and fatigue properties in various materials.

However, the welded components which are utilized in

the main frame of railway vehicle may exhibit different

features due to their weld imperfection, defects, the

initiation of hot and cold cracks and microstructure

changes in the HAZ (heat affected zone) of the metallic

materials (Miki, 2001; Wahab, 2003). In particular, these

under lower temperatures may quite show special features

due to their ductile-brittle transition behavior.

Due to their inherent defects and non-homogeneity of

structural materials, the materials exhibit a large variation

of properties mainly in fatigue strength or life (Kim, 2003)

which makes it indispensable to employ a statistical

analysis. It is well known that the probabilistic stress-life

approach (P-S-N curve) is useful to evaluate the variation

of fatigue life or strength of machines and mechanical

structures: hence, many researchers have analyzed the

probabilistic properties of fatigue life in metallic materials

through P-S-N approach (Ling, 1997; Zhao, 2000).

However, as mentioned above, it cannot be avoided that

the structures or materials are subjected to severe

environment condition such as low temperatures and, for

temperatures below ambient temperatures, the materials

shows a definite different behaviors. Therefore, because

the subsequent probabilistic properties of fatigue life may

be quite different, it is of necessity to investigate the

probabilistic properties in fatigue behavior of structural

materials at low temperature to improve the safety and

Note.-Discussion open until August 1, 2009. This manuscript for thispaper was submitted for review and possible publication on Septem-ber 5, 2008; approved on December 1, 2008

*Corresponding authorTel: +82-63-469-4872; Fax: +82-63-469-4727E-mail: [email protected]

86 Ki-Weon Kang et al.

reliability of welded structures.

The present study aims to identify the effect of low

temperature on the static and fatigue behavior and its

probabilistic properties in welded SM490A steel that is

utilized in the structural members of railway vehicle. For

these goals, the static and fatigue tests are performed on

the SM490A steel under various temperatures (293K,

263K and 233K). The fatigue limits are determined from

the staircase method and the probabilistic properties of

fatigue life are evaluated through the probabilistic stress-

life (P-S-N) approach.

2. Experimental Procedure

2.1. Materials and specimen

The material used here was SM490A steel which is

utilized in structural member in railway vehicle and the

typical mechanical properties and chemical composition

are summarized in Table 1 and Table 2, respectively. The

materials were welded through X-grooved 3Pass flash

butt method, as shown in Fig. 1 and heat-treated to

minimize the internal stress in the welded specimens. The

welding and heat treatment conditions of welded

specimens are summarized in Table 3.

The base and welded specimens for tensile and fatigue

tests were prepared according to ASTM E8M (ASTM,

2001) and ASTM E466 (ASTM, 1996), respectively, as

shown in Fig. 2. The thickness was 8mm for base and

welded specimens but the weld bead was not removed in

the welded specimens.

2.2. Tensile and Fatigue test

Prior to the tensile and fatigue tests, the specimens

were cooled to test temperatures, 293 K, 263 K and 233

K, in the environmental conditioning chamber (Instron

3119-408) during three hours to assure the thermal

equilibrium with the environment.

The static and fatigue tests were conducted at various

temperatures in the environmental chamber by using a

servo-hydraulic fatigue testing machine (Instron 8801).

The static tests were performed under displacement control

mode with a crosshead speed of 2 mm/min and the

elongation was monitored by using an extensometer with

a gage length of 50 mm.

To obtain the fatigue properties of SM490A steel, the

fatigue tests were performed according to the ISO 12107

(International Organization for Standardization, 2003). At

least 14 specimens were prepared for each test condition.

Eight of these were used for estimating the S-N curve in

the finite fatigue life range and six or more for the fatigue

strength at the infinite fatigue life regime. And the

loading condition was the constant amplitude under T-T

(tension to tension) sine wave load cycles. A stress ratio

R (R=σmin/σmax) of 0.1 was adopted; all the tests were

performed with a frequency of 25 Hz under the above

mentioned temperatures. When the specimen did not fail

after being subjected to the set number of cycles (about

two million cycles), the fatigue test was terminated.

Table 1. Mechanical properties of SM490A steel (typical)

Yield strength Tensile strength Elongation

≥325 MPa 490∼610 MPa ≥17%

Table 2. Chemical composition of SM490A steel (wt%)

C Si Mn P S

0.20 0.55 1.60 0.035 0.035

Figure 1. Schematic diagram of welded joints.

Table 3. Welding and heat treatment conditions

Specification

Method Gas Metal Arc Welding

ConditionsCurrentVoltageSpeed

: 180 A: 105 V: 18 cm/min

Wire size Diameter 1.2 mm

MaterialsFiller metal specification: A5.18Classification: AWS ER 70S-6

Shielding CO2 Flow rate: 15-20 l/min

Heat treatmentHolding temperature: 500±20 oCHolding time: 1 hourHeating & Cooling rate: 120oC/hour

Figure 2. Specimen configuration (unit: mm).

Experimental Investigation on Static and Fatigue Behavior of Welded SM490A Steel Under Low Temperature 87

3. Results And Discussion

3.1. Static and fatigue behavior

To identify the effect of temperature on the static

behavior of base and welded SM490A steel, their stress-

strain curves and summarized results are shown in Fig. 3

and Table 4, respectively. In Table 4, the parentheses

indicates the standard deviation of corresponding mechanical

properties. From the results of base specimen, the stress-

strain behavior is moderately increased with the decreasing

of temperature. In detail, the yield strength at temperatures

of 263 K and 233 K increase 3.2% and 6.6 % from the

results at ambient temperature, respectively. The similar

tendency occurs for the tensile strengths. These behaviors

may be caused by the increasing of the brittleness of the

materials under lower temperatures. And, from the results

of welded specimen, the curves exhibit similar behaviors

for the yield and tensile strength with those of base

specimens. The strain, however, shows a quite different

behavior compared with the base specimen: the strain of

welded materials greatly decreases from the result of base

specimen regardless of test temperatures and moreover,

the severe dispersion of stress-strain curves is present in

welded specimen.

For further understanding these behaviors, the micro-

Vickers hardness (AFFRI model DM-2S) is measured for

the base and welded specimen at the centerline and 1mm

below surface of specimen, respectively, as shown in Fig.

4. The hardness in the base specimen is almost same

along the length of specimen. The hardness in the welded

specimen is, however, increased near heat affected zone

(HAZ): this is caused by the fine grains due to re-

crystallization in HAZ (ASM, 1996). It is, therefore,

reasonable to conclude that the static behavior of both

base and welded SM490A steel is deteriorated at lower

temperatures. Also, for welded specimens, the stress-

strain behavior is affected by the welding and dispersion

in data is severely occurred for all the test temperature.

The large scatter in stress-strain behavior in the welded

specimens may result from the irregular distribution of

flaws and/or bead due to welding.

It is widely accepted that the mechanical properties of

the metallic materials increases when the temperature is

lower than ambient temperature and the same tendency

occurs for overall fatigue strength (Silva, 2004). It is also

clear that there is a huge increase on fatigue strength

Figure 3. Static stress-strain behaviors.

Table 4. Mechanical properties of base and welded SM490A steels

PropertiesTemperature: 293 K Temperature: 263 K Temperature: 233 K

base welded base welded base welded

Yield Strength [MPa] 378.68(3.07) 377.60(6.38) 391.19(4.61) 383.03(9.77) 405.45(1.92) 391.28(10.0)

Tensile Strength [MPa] 538.67(2.31) 541.19(6.95) 564.12(2.12) 541.28(7.71) 580.69(3.84) 566.83(5.91)

Elongation [%] 34.32(0.84) 22.99(2.31) 35.28(1.01) 22.14(3.30) 35.47(1.62) 25.66(3.33)

Table 5. Material constants in Basquin’s equation

ConstantsTemperature: 293 K Temperature: 263 K Temperature: 233 K

base welded base welded base welded

A (MPa) 329.85 592.15 313.16 400.23 311.54 553.30

B -0.04322 -0.10717 -0.03524 -0.06383 -0.03176 -0.09551


when the temperature lowers below ambient temperature

and that fatigue life and strength is more sensitive to low

temperature than the static strength. However, because

the microstructure and brittle-ductile transition behavior

can be changed due to re-crystallization in the welded

specimens, it is necessary to identify the fatigue behavior

of welded specimen at lower temperatures.

On Fig. 5, it is clearly shown that the fatigue behavior

is affected by both the test temperature and welding. Here

the lines indicate the results calculated by Basquin’s

equation σamp=ANfB and their coefficients A and B are

summarized in Table 5. In detail, for the base specimens,

the fatigue life at the finite life region is gradually

increased with the decreasing of the temperature and this

behavior is much more obvious at the long life region.

The fatigue limits are also increased with the decrease of

the test temperature: the limits for various temperatures

(293 K; 263 K; 233 K) are 170.8 MPa, 180.6 MPa and

184.2MPa, respectively. However, the effect of test

temperature on fatigue limit of the welded materials is

relatively small against the base materials (the limits for

293 K; 263 K; 233 K are 114.5 MPa, 102.3 MPa and

Figure 4. Micro-Vickers hardness distribution.

Figure 5. Fatigue behavior of SM490A steel.


113.1 MPa, respectively). And there is a wider dispersion

of fatigue life or strength in the welded materials rather

than the base materials. Also, at the same temperature

level, although the welded materials are heat-treated to

minimize the internal stress, the fatigue life and limits of

the welded are greatly reduced compared to those of the

base materials. Also, for the welded specimens, they have

failed at the base material or edge of bead section. These

may come from the flaws or the irregular configuration of

welding bead and resulting stress concentration factor

(Fricke, 2006). In particular, at the higher stress level,

they have mainly failed at bead region. It is, therefore,

reasonable to conclude that the welded materials are

greatly deteriorated for all the temperature compared with

the base materials and this behavior is more sensitive to

the fatigue loading than the static loading.

3.2. Probabilistic properties of fatigue life

From Fig. 5, one can know that a considerable amount

of scatter is present in the fatigue life of SM490A steel

and moreover, the variation of fatigue life is significantly

affected by the welding as well as test temperatures: it is,

therefore, desirable to conduct a probabilistic analysis to

determine the variation in the fatigue life with the

welding and test temperatures.

To analyze the probabilistic properties in fatigue life of

SM490A steel with the welding and test temperatures, the

authors have adopted the probabilistic stress-life (P-S-N)

approach (Kim, 2003) to accurately define the probability

distribution of fatigue life. Fig. 6 and Fig. 7 indicate the

distributions of the fatigue life of base and welded

SM490A steels under various temperatures, respectively.

Here the dotted lines present the predicted fatigue life at

Pf=5% and Pf=95% on the normal distribution, respectively.

It is evident from the figures that the predicted results

well describe the experimental results regardless of specimen

types (base and welded specimens) and test temperatures.

From Fig. 6 and Fig. 7, it is worthy to note that the

variation of fatigue life depends on both the specimen

types and temperature. To identify these behaviors, the

random variable Z is adopted, which describes the

difference between the experimental and predicted fatigue

life by Basquin’s equation, that is Nf =(σamp/A)1/B×Z. And

the random variable Z can be simply calculated by logZ

=logNf -log(σamp/A)1/B. Here the first term in right hand

indicates the experimental fatigue life and the second

means the predicted fatigue life by Basequin’s equation.

Since the Basquin’s equation presents the fatigue life at Pf

Figure 6. Predicted results for base material.


=50%, it is reasonable to assume that the random variable

with zero means and describes the variation of fatigue

life: therefore, it is also reasonable to assume that the

probabilistic distribution of Z follows the normal distribution.

The median rank method and normal distribution

(Kececioglu, 1993) are applied to analyze the statistical

distribution of the random variable.

Figure 8 indicates the experimental results analyzed

with the median rank method and theoretical prediction

from the normal distribution. The theoretical analyses are

well in conformance with the experimental results for all

cases and the variation of fatigue life at lower temperature

Figure 7. Predicted results for welded material.

Figure 8. CDF for Fatigue life.


is larger than that of ambient temperature for the base and

welded materials. For further understanding this behavior,

the variance of the random variable, σ2ZT

was evaluated

for the lower temperature. The experimental results in

Fig. 9 indicate the σ2ZT

normalized by the σ2Z20, which is

the variance at ambient temperature for the base and

welded specimens. The result tells us that the variance of

random variable, namely, the dispersion of fatigue life

increases rapidly as the temperature decreases and this

behavior is more remarkable in the welded material. It

should, therefore, be noted that when the welded

structures are used out-of-doors, more careful attention is

needed due to their severer variation in the fatigue life.

4. Conclusions

To evaluate the effect of temperature on static and

fatigue behavior and its probabilistic properties of SM490A

steel that is utilized in the structural members of railway

vehicle, the static and fatigue tests are performed on the

base and welded SM490A steel under various temperatures

(293 K, 263 K and 233 K). The following conclusions

have been drawn.

(1) The static strength of the base material moderately

increases with the decreasing of the temperature. For the

welded material, the effect of temperature on static

strength is negligible but the strain shows a quite different

behavior compared with the base specimen due to their

re-crystallization and heat affected zone (HAZ).

(2) The fatigue behavior is greatly influenced by the

welding: the fatigue life and limits of base material is

greatly increased with the decreasing in temperature but

the temperature effect on these is relatively small in

welded material.

(3) To investigate the variation of the fatigue life, the

probabilistic stress-life (P-S-N) approach was adopted

and the predicted results were in conformance with the

experimental results. Additionally, as the temperature

decreased, the variation of fatigue life in SM490A steel

increased rapidly and this behavior is more remarkable in

welded material.

References

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Figure 9. Variation of fatigue life.

Documents

Experimental investigation on static and fatigue behavior of welded sm490a steel under low temperature