TRANSIENT STATE TENSILE TEST RESULTS OF STRUCTURAL STEEL

S355 (RAEX 37-52) AT ELEVATED TEMPERATURES

Jyri Outinen

Pentti Makelainen

ABSTRACT

Rakenteiden Mekaniikka, Vol.28

No.1, 1994,pp. 3-18

An experimental research is carried out for investigating mechanical properties of

structural steel S355 at elevated temperatures. Test values of thermal elongation,

modulus of elasticity and yield stress and stress-strain curves are compared with the

values and curves given in Eurocode 3 Ill, in the European Recommendations for the

Fire Safety of Steel Structures /2/ and in the Finnish Code of Steel Structures /10/.

INTRODUCTION

The main purpose of this research was to examine the mechanical properties of structural

steel S355 at temperatures 20°C .. 700°C using transient state tensile test method.

Transient state tensile tests have not been done before this in Finland for structural steel

S3 55 and therefore a comprehensive experimental study was of great importance.

3

Experimental work was carried out during September and October 1994 at Helsinki

University of Technology in the Laboratory of Steel Structures. Financing for the

research was provided by Rautaruukki Oy.

Results of this study are compared with the values of mechanical properties given in

Eurocode 3 Ill, in the European Recommendations for the Fire Safety of Steel

Structures /2/ and in the Finnish Code of Steel Structures /10/.

STEEL MATERIAL S355 (RAEX 37-52)

Test material used in this research was hot-rolled structural steel S355 (RAEX 37-52)

manufactured by Rautaruukki Oy. Test specimens were cut out from a hot-rolled steel

sheet with nominal thickness of 12l1llll, longitudinally to rolling direction. Structural

steel material is in accordance with the requirements of the European standard SFS-EN

10 025 /9/ for structural steel S355.

Mechanical properties of the test material at room temperature

Four tensile tests were carried out at room temperature to determine the mechanical

properties of the test material at room temperature. Results from the transient and

steady state tests were compared with these results. Six tensile tests were carried out to

determine modulus of elasticity for the test material. Tensile tests were carried out as

stress rate-controlled. The rate of loading was 0.52 (N/mm2)/s which caused a rate of

strain of 0.003 min"1 to the test specimen. Stress-strain curves were used to determine

the tensile properties for each test specimen.

Test results from the tensile tests are compared in Table 1 with the minimum values

given by the manufacturer and the measured values given in the test report of the

inspection certificate of the test material .

4

Table I. Test results, minimum values given by the manufacturer and measured values given in the test report of the inspection certificate.

Measured value Minimum Reported test value requirement of inspection

certificate Modulus of elasticity 210 600 206 000 not measured E (N/mm2

)

Yield stress 406.1 355 416 ReH(N/mm

2)

Ultimate stress 526.9 490-630 548 R,(N/mm2

)

Chemical C()mposition of the test material

Chemical composition of the test material including comparisons with maximum values

given by the manufacturer is presented in Table 2.

Table 2. Chemical composition of the steel material S355 (RAEX 37-52).

Elementary Measured value Maximum value substance (%) (%)

Carbon c 0.150 0.180 Silicon Si 0.180 0.500 Manganese Mn 1.410 1.600 Niobium Nb 0.010 0.050 Sulphur s 0.011 0.020 Phosporus p 0.019 0.025 Aluminium AI 0.041 minimum 0. 020

TESTING FACILITIES

Test specimen

The test specimen having so called proportional circular cross-section was in accordance with the standard EN 10 002-5 /51. The test specimen is shown in Fig. 1. Dimensions of the test specimen are given in Table 3.

5

A-A 8-8

• W to

Fig. 1. Test specimen

Table 3. Dimensions of the test specimen.

Total length Lt 70mm Parallel length Lc 40mm Original gauge length Lo 25mm Diameter d 5 ±0.040 mm Metric screw thread M10 10mm Radius of curvature r 6mm

Testing device

The tensile testing machine used in this research was manufactured by the German

company Roeii+Korthaus. Testing machine is verified in accordance with the standard

EN 10 002-2 /6/. Loading range can be chosen between 0 .. 50 kN and 0 .. 250 kN.

Loading range used in these tests was 0 .. 50 kN. Maximum load used was about 11 kN.

Maximum loading capacity of the machine is 250 kN. Maximum error of the load cell is

±0.05kN by the use of maximum loading capacity.

Gauge length of the extensometer used in this research was 25mm with elongation range

of 0 .. 2.5mm. The extensometer has an accuracy of ±0.003mm. The extensometer is in

accordance with the standard EN 10 002-4/7/.

The heating device was manufactured by the German company Maytec GmbH. The

device is in accordance with the standard EN 10 002-5 /8/. The oven in which the test

specimen is situated during the tests was heated by using three separately controlled

resistor elements. The temperature-controlling unit is manufactured by the British

6

company Eurotherm Ltd. The air temperature in the oven was measured with three

separate temperature-detecting elements. The steel temperature was measured accurately

from the test specimen using temperature-detecting element that was fastened to the

specimen during the heating. The temperature-measuring device has an accuracy of ±

3°C. The testing device is shown in Fig.2. The difference between the air temperature

and the steel temperature during the tests is given in Table 4.

Table 4. Measured temperature of the test specimen compared with the measured air temperature in the heating oven.

Steel temperature Air temperature Steel temperature Air temperature (OC) (OC) (OC) (OC)

20 20 400 432 50 96 450 470 100 173 500 509 150 226 550 551 200 268 600 600 250 310 650 650 300 352 700 700 350 392 750 750

Resistor elements

Fig.2. The transient state tensile testing device.

7

TEST RESULTS

Transient state tensile tests

Transient state tensile tests were carried out with two equal tests at each load level of 3,

20, 50, 80, 110, 140, 170, 200,230,260,290, 320 and 350 N/mm2. Thermal elongation

of the structural steel was determined with five tests at load level of 3N/mm2 .

Heating rate in the transient state tests was l0°C min-I Temperature was measured

accurately from the test specimen during the heating.

Test specimen was heated until the temperature was 750°C or until breaking of the test

specimen. In the temperature-strain relationship, thermal elongation was subtracted from

the total strain. Final results were converted into stress-strain curves using the method

given in /5/ . The test method was in accordance with the standard /8/.

Temperature-strain relationships

Thermal elongation Elh has been considered by subtracting it from the total strain Etot :

Er; = Etot,Ti - Eth,Ti

where

Er; IS elongation at temperature T; ,

Etot,ri is total elongation at temperature T; and

Eth,Ti is thermal elongation at temperature T;.

(I)

Measured temperature-strain curves at stress levels of20-200 N/mm2 are shown in Fig. 3

and at stress levels of230-380 N/mm2 in Fig. 4.

8

800

700

600

Esoo Cl)

5 400 ~

~ 300 E Cl)

1- 200

100

0

1. 2

-e:

/~ ~ ------/((; :::::---

}f

I

0 O.S

~-

5.

-1 . 20 N/mm2

2. 50 N/mm2

3. 80 Nlmm2 I

4.110 Nlmm2

5.140 Nlmm2

6.170 Nlmm2

7. 200 Nlmm2

1.s 2 2s 3 as 4 ~s s ~s 6 as 7 ~s 8 as Strain(%)

Fig. 3. Temperature-strain curves at stress levels of 20-200 Nlmnl

600

E4oo Cl)

5 300

l

I~ / !-----~

( !7 l/ v ::--f---- f--

/ v--~

~ )j 1/ I 1 0 230 Nlmrrf -

2. 260 Ntmrrr

'r- - 5. _j 3. 290 Nlmrrf -6.

4. 320 Nlmrrf

soo

E 200 ~

5. 350 Nlmm2 '-----

6. 380 Nlmrrf ' ,________ ~

100

0 0 O.S 1.S 2 2.S 3 3.S 4 4.S s S.S 6 6.S 7

Strain(%)

Fig.4. Temperature-strain curves at stress levels of 230-380 Nlmnl.

9

Thermal elongation

Thermal elongation of the structural steel was determined with five tests at load level of

3N/mm2 Test specimen was heated with heating rate of 10°C/min until temperature was

750°C. Thermal elongation was measured during the heating process.

An analytical expression was fitted into the average test values by following equation:

(2)

where

~1/1 is relative thermal elongation and

T s is steel temperature.

The regression coefficient for equation (1) is 0. 9981. Results of the five tests and the

average are shown in Fig. 5. Average thermal elongation values of the five tests are

compared with the analytical expression in Fig. 6.

0.9

0.8

~ e... 0.7

~ 0.6

c 0.5 0 ~ 0.4 en c 0 0.3 Qi iii 0.2 e Gl .c 0.1 1-

0 ....,.,.., .--

0 100

~ ~ v-----

d ~ .4ii ~ - Test1 .

~ - Test2.

~ ~ - Test3.

~ ~ - Test4. ,. - Test5.

~ - Average value

200 300 400

Temperature (°C)

500 600 700

-

~

c--

-

800

Fig. 5. Temperature dependence of thermal elongation of structural steel S355 (RAEX

37-52): Cun,esfromfive tests and the m 1erage curve.

10

0.9 ~ e... 0.8 5

0.7 <I

0.6 c

0.5 ~ C) 0.4 c 0 0.3 Gi ;;; 0.2 E CJ 0.1 .c 1- 0

.A>

_a/ _of)f} )!Y

"'~ ~

~ ~

~-o Average (5 tests)

~ ~·

--t.tn = 5x1o·•r,2 + 0.8x1o·5r, -0.000197 ~ ~

0 100 200 300 400 500 600 700 800

Temperature (°C)

Fig. 6. Temperature dependence of thermal elongation of structural steel S355 (RAEX 37-52): The average curve and an analytical expression fitted into the average curve.

In the Laboratory of Steel Structures at Helsinki University of Technology, the1mal

elongation has been measured earlier for structural steel S235 (Fe360) and for cold­formed sheet steel Z32. In this research it was measured for structural steel S355 (RAEX

37-52). The thermal elongation of all these steels differs from the values given in Eurocode 3: Part 1.2 for structural steels. Comparison of the thermal elongation of these

three materials is shown in Fig. 7. The results for structural steels S235 and S355 are compared with the thermal strain according to Eurocode 3: Part 1.2 in Fig. 8.

0.9 +----+----lf-----+-----1----f-----+----+-- --1

~ 0.8 +----+------+----+---1---+-----+-~-......,!.~""--l

~ 0.7 +----+----lf-----+----+---r---~----+/~~~----~~~-~~--t

5 °o.65 +----+----+------+---+-----+~--.:;;- ~""'-"'"~--c:j....-<-:::.._-+------1

~ . ~-

;;; E 0.2 CJ .c 0.1 1-

0

8 0.

4

t======t=====jt=====1~:=.........-::~~~~t--~-::: ·:--::-·=-=· ~=~~~~~i~~~~~~~~~~--1 Gi 0.3 + ~~~: :-;.. 8355 (RAEX 37-52) -

- ·- - 5235 (Fe3608) -

t-~~~~~~t~~-~~-'-~_-'_· j----t---t~·~-~--~·~·Z;32~--.-_j ~r::::---· -

0 100 200 300 400 500 600 700 800

Temperature (°C)

Fig 7. Thennal elongation determined for structural steels S235 (Fe360) and S355 (RAEX 37-52) and for cold-formed steel Z32.

11

1.1 -

1

~ ~

0.9

5 0.8 <]

0.7 c

0.6 0

i 0.5 Cl c 0 0.4 Qj ;;; 0.3 E

-- - -- Eurocode 3: Part 1.2

~ 0.2 --S355 (RAEX 37-52) .c 1- 0.1

0

'-- - - - S235(Fe360B) -~-::: - - ·

-~-

0 100 200 300 400 500 600 700 800

Temperature ('C)

Fig. 8. Thermal elongation of structural steels S235 (Fe360) and S355 (RAEX 37-52) compared with thermal elongation according to Eurocode 3: Part 1.2.

Stress-strain relationships

The transient state tensile test results converted into stress-strain curves at temperatures

100°C-700°C are shown in Fig. 9. Stress-strain curves for test material S3 55 (RAEX 3 7-

52) at temperatures 300°C-700°C are compared in Fig.l 0 with stress-strain curves given

in Eurocode 3: Part 1.2 11/.

400

350

300 N" E 250 .e ~ 200

"' ~ 150 U5

100

50

0

1.

! 2. 3. I--

I v ~ r--4. --

II v-::: ~ !----'/ v f; -f/

---------

V---~ '/

0 0.2 0.4 0.6 0.8

~

J1.100' C 2. 200'C 3. 300'C 4.400'C r I 5. 500'C 6. 600'C 7. 700'C

1.2 1.4 1.6 1.8 2

strain (%)

Fig.9. Stress-strain cun,esjor test material S355 (RAEX 37-52) at temperatures IOOOC-7000C.

12

400 .----,,----.----~----.-----,----,-----,----,-----,---·-,

--+--30o·c - - - EC3t3oo·c -4oo•c - -- -EC3t4oo•c -&-soo·c - --- EC3t5oo·c --o--6oo·c ----- · EC3t6oo·c

0 0.2 0.4 0.6 0.8 1.2 1.4 1.6 1.8 2

Strain (%)

Fig. 10. Stress-strain cun,es for test material S355(RAEX 37-52) at temperatures 3000C-7000C compared with stress-strain curves of Eurocode 3: Part 1.2.

Modulus of elasticity

Modulus of elasticity of structural steel S355 (RAEX 37-52) was determined from the

stress-strain curves which were converted from the transient state test results. The

modulus of elasticity was determined as an initial slope of the stress-strain curves.

The modulus of elasticity for the steel material S355 (RAEX 37-52) at room temperature

was determined with six tensile tests and the average value of these tests was 210 600

N/mm2. The value given by the manufacturer is 206 000 N/mm2

. The experimental value

is used as a reference value in analysing the transient state test results .

Test results at temperatures 20°C - 700°C are compared in Fig. 11 with the values of

mechanical properties given in Eurocode 3 Ill, in the European Recommendations for

the Fire Safety of Steel Structures /2/ and in the Finnish Code of Steel Structures /8/ .

13

--- RakMK B 7

- - - - ECCS fT C3

- --- EC 3: Part1. 2

Test results

--=::: :

~ 0.9

~ 0.8 >-

w 0.7

0 0 .6 ~ II) 0.5

---~ __ - , - ~~

"'--- --- --'-:..::: -_

:I '3 0.4 '0 0 E 0.3 ,., "' .l! 0.2 t; .. 0. 1 jjj ..... ....... ...... """"'·

0 0 100 200 300 400 500 600 700

Temperature ('C)

Fig. 11. Comparison of elasticity modulus ratio ETIE2o'C at temperatures 20 'C -700 'C.

Yield stress

Yield stress was determined for the test material S355 (RAEX 37-52) from the stress­

strain curves based on the transient state test results. Test results for yield stress cr0.2

(Rpo.2), based upon 0.2% non-proportional extension as defined in /8/, are shown in

Fig.1 2. Test results are compared with the values for cr0.2 according to the Eurocode 3

/1/ and with the values according to the Finnish Code /10/. Test results for yield stress

cro.s ,cr1.o and cr2.o , based upon total extension of 0.5%, 1.0% and 2.0 % , are shown in

Figs. 13, 14 and 15. Test results are compared with the values for cro.s ,cr1.o and cr2.0 according to the Eurocode 3: Part 1.2 /11. The reduction factors for effective yield

strength given in Eurocode 3 are based on values of yield stress cr2.o.

450

400

N' 350 E

~ 300

::1 250 t>

II) 200

II)

~ 150

¥ 100 >

--Test results

-~ - -- EC 3 :Part 1.2 - ....... . -<:"'--::. ::::--........ - RakMK B7

-----~ - --- ~.::- I'.. -- -~ -- ·-._ .....................

~ ----~--:::..- ...........

50 -~ .......

0 0 100 200 300 400 500 600 700

Tern perature ('C)

Fig 12. Yield stress ao.lRpo2) for steel S355(RAEX 37-52) at temperatures 20 OC-700 OC.

14

450

N' 400 E

£ 350

300

J 250 ~

t1 200

~ 150

i 100 :!! ..

50 >=

-- - Eurocode 3 : Part 1.2 r---........ -~ --Test results I--

~ ---... ~

'~ --~

...... _~

0 0 100 200 300 400 500 600 700

T emperature ("C)

Fig. 13. Yield stress cr05 (R10.5) for steel S355(RAEX 37-52) at temperatures 20CC-700CC.

450 I I

N' 400 E

£ 350 ............... I

Test results t Eurocode 3: Part 1.2 ,._ _ ,_

300 ~ . ~ 250 ~

b 200

~ 150 ~

~ 100 :!! ..

~ '~

'~ ....... '~

.... >= 50

0 1-·

0 100 200 300 400 500 600 700

Temperature ("C)

Fig. 14. Yield stress O"to(RtJ.o) for steel S355(RAEX 37-52) at temperatures 20 CC-700 CC.

450

400 N' E 350 E ~ 300 -, r! 250 ~

" 200 ~ ~

i 150

:!! 100 ..

I --Test results

Part 1.2 ~ - I Eurocode 3 :

~~

-- ~ '~

'~ '<~

>= 50

0 100 200 300 400 500 600 700

Temperature ("C)

Fig. 15. Yield stress cr2.0 {R12.0) for steel S355(RAEX 37-52) at temperatures 20 CC-700CC.

15

Steady state tests

Steady state tensile tests were carried out for the test material S355 (RAEX 37-52) with

two equal tests at each temperature 400°C, 500°C and 600°C.

Tensile tests were carried out as stress rate-controlled. The rate of loading was 0.52

(N/mm2)/s which caused a rate of strain of0.003 min-1 in the test specimen.

Stress-strain relationships

Stress-strain curves measured from the steady state tests at temperatures 400°C, 500°C

and 600°C are shown in Fig. 16. Stress-strain curves from the steady state tests are

compared with transient state test curves in Fig. 17.

500

450

400

~ 350 N

E 300 .E ~ 250 1/)

~ 200 iii

150

100

50

0

------~ r-- ,.--o---' ............ _...o---'

/ v/ ~

if ~ ~ rv r-a-

ILf ~

0 0.2 0.4 06 0.8 Strain(%)

-----steady state test 4oo•c ,----

--o-- steady state test 500•c ~

-~r- steady state test 6oo·c -

1.2 1.4 1.6 1.8 2

Fig. 16. Stress-strain curves for steel S355 (RAEX 37-52) measured in the steady state

tests at temperatures 400 "C, 500 't.' and 600 't.'.

16

450

400

350

N" 300 E .E 250 ~ Ill 200 Ill Ql ... ...

150 (/)

100

50

~-

l 1--~--

~ --¥ - - -x-~--- ------ -

-x- --- --x-x- --~ ~- ~ - .. .--- ----- -

....->t- - .::.:o--' ----- -----~::o:-? ~-- ---·

~ ,. .. -. -· Ill v-- .. .. . ...... -------- ---··-·- ..........

/ ...---..- . .. ..... - . ........... ... ...

rv ~--· · ----·· -x-Steady st. 4oo•c - - - Trans.st. 4oo•c F.-···

'!.· -o-Steady st. soo·c - · - · Trans.st. soo·c --fr- Steady st. soo·c · · · · · · Trans.st. 6oo•c

0

0 0.2 0.4 0.6 0.8 1.2 1.4 1.6 1.8 2

strain (%)

Fig. 17. Measured steady state stress-strain curves for steel S355 (RAEX 37-52) at

temperatures 400 't:', 500 't:' and 600 't:' compared with the measured transient state

curves.

Mechanical properties of the test material based on steady state data

Mechanical properties of the test material S355 (RAEX 37-52) determined with the

steady state data are given in Table 5.

Table 5. Mechanical properties of the test material S355 (RAEX 37-52) at temperatures

400°C, 500°C and 600°C based on steady state data.

Temperature (°C) Yield stress cro.2 Yield stress cro.s Ultimate stress fu

(N/mm2) (N/mm2

) (N/mm2)

400 249.5 280.0 515.0

500 230.2 245.0 380.0

600 167.8 170.0 226.0

17

ACKNOWLEDGEMENTS

The authors wish to acknowledge the support of the company Rautaruukki Oy in making

this experimental research project possible. The authors are especially grateful to Mr. Jouko Kanerva M.Sc.(Eng) from the company Rautaruukki Oy for his efforts in promoting and organizing this research programme.

REFERENCES

Ill European Committee for Standardisation (CEN), Eurocode 3: Design of steel structures, Part 1.2 : Structural fire design, Brussels 1993.

/2/ European Convention for Constructional Steelwork (ECCS), European Recommendations for the Fire Safety of Steel Structures, Elsevier Scientific Publishing Company, Amsterdam 1983 .

/3/ Preston R.R. ,ECCS, Technical Committee 3, The thermal expansion of structural steels at elevated temperatures; A review of current data, Cleveland 1993 .

/4/ Miller K.,Makelainen P., Mechanical properties of cold-rolled hot-dip zinc coated sheet steel at elevated temperatures, Research report nr.58, Helsinki University of Technology, Department of Civil Engineering, Espoo 1983.

!51 Narinen Pekka, Mechanical properties of structural steel at fire temperatures (in Finnish), Master's Thesis, Helsinki University ofTechnology, Faculty of Civil Engineering and Surveying, Espoo 1994.

/6/ Standard EN 10 002-2: Metallic materials. Tensile testing. Part 2: Verification on the load cell of tensile testing machine, Brussels 1992.

171 Standard EN 10 002-4: Metallic materials. Tensile testing. Part 4: Verification of extensometers used in uniaxial testing, Brussels 1992.

/8/ Standard SFS-EN 10 002-5: Metallic materials. Tensile testing. Part 5: Method of testing at elevated temperature (in Finnish), Helsinki 1992.

/9/ Standard SFS-EN 10 025: Hot-rolled products ofnon-alloy structural steel (in Finnish), Helsinki 1993.

11 Of Rakentarnismaarayskokoelma, ohjeet B7, Terasrakenteet (Finnish Codes of Building Regulations, Code B7, Steel Structures), Helsinki 1987.

Jyri Outinen, Graduating student Laboratory of Steel Structures Helsinki University of Technology

18

Pentti Makelainen, Professor, Dr. Tech. Laboratory of Steel Structures Helsinki University of Technology