Mechanical Properties of Structural Steel under Post

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Mechanical Properties of Structural Steel under Post-Impact Fire Mahsa Mirmomeni*,1; Amin Heidarpour1; Xiao-Ling Zhao1; Jeffrey A. Packer2; and Chengqing Wu3 1

Department of Civil Engineering, Monash University, Melbourne, VIC 3800, Australia. 2 Department of Civil Engineering, University of Toronto, Toronto, ON, Canada, M5S 1A4. 3 School of Civil, Environmental and Mining Engineering, The University of Adelaide, SA 5005, Australia.

Abstract Accurate prediction of material properties under combined high strain rate and elevated temperature are essential for safe design of structures to withstand postimpact fire situations such as collision by heavy vehicles followed by fire. Numerous material tests performed in recent years do not address the influence of such sequential loading on the mechanical properties of mild steel. An inclusive test program is carried out in the Civil Engineering Lab at Monash University to investigate the post-impact fire properties of Grade 350 structural steel and the results are presented here. Specimens have undergone interrupting high strain rate tensile loading, controlled locally at defined levels of elongation, to account for different deformation states. Different damage levels are introduced for each rate of strain with respect to the displacement corresponding to the ultimate stress (fu). Subsequently, the partly damaged specimens are subjected to static tensile loading to failure at high temperature conditions. Material behaviour of pre-damaged steel is compared to those of each individual loading scenario and to design code expressions. The test results demonstrate that the combined actions are profoundly different from that in which the structure is subjected to either high strain rate or thermal loading and notably vary from those predicted in different codes. Moreover, it is shown that the strength and ductility of mild steel are significantly dependent on the rate of loading, the pre-deformation history and the temperature it is subsequently exposed to. The experimental results can be used by researchers and structural engineers as benchmark data for calibrating current material model constants and/or developing new material models which take into account the coupled effect of high strain rate and temperature for rational fire analysis and design of steel structures. Key Words: Mild steel; partial damage; post-impact fire; combined actions.

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INTRODUCTION The unfortunate rise of terrorist attacks (such as the 2001 World Trade Centre incident, the 2005 London underground bombings, the 2013 Pakistan bombings, etc.) and road side accidents causing explosions or high speed crashes, has urged the engineering communities to give significant attention to investigating the response of structures subject to impact, explosion, and fire. A growing amount of experimental work has been published reflecting the effect of strain rate upon the behaviour of structural steel [1-3] and the mechanical properties of mild steel at elevated temperatures [4, 5]. However, there is limited research on the combined effect of these extreme actions, for instance post-impact fire, which is profoundly different from that in which the structure is exposed to either loading [6-11]. The Final Report published in 2005 by the National Construction Safety Team of the National Institute of Standards and Technology (NIST) on the collapse of the World Trade Center Twin Towers reflected that the fire following the aircraft impact and explosion caused catastrophic damage to the structure and led to the progressive collapse of the towers [12]. In the majority of the existing impact and fire research and practise, the effect of high strain rate is considered using well known rate dependent material models such as the JohnsonCook model [13], and the Zerrielli-Armstrong model [14] whereas the effect of temperature is considered through empirical equations recommended by codes of practice [15, 16]. In other words, while the rapid deformation induced by high strain rate loading cannot be neglected in the fire resistance of material, the existing constitutive models do not reflect this coupling effect. In this study a broad experimental investigation under well-defined conditions has been carried out aiming to investigate the complex behaviour of high strain rate induced partially damaged structural steel at elevated temperatures. The results of these fully coupled experiments can be used as benchmark data for developing material models which can predict the nonlinear behaviour of structural steel material under post-impact fire loading.

EXPERIMENTAL PROGRAM Test material and specimen Test specimens were taken from AS3678-Grade 350 hot rolled mild steel structural plates [17], approximately equivalent to ASTM A572 Gr50 [18], with a nominal thickness of 8 mm. Well-established standards are available for tensile testing at quasi-static conditions at ambient temperature [19] and also elevated temperatures [20]. However, with regard to dynamic tensile testing, no published guidelines are available to date for the testing method and specimen dimensions [21]. For this testing program, the specimen geometry is determined so as to ensure higher strain rates and homogeneous deformation of the specimen in the gauge section and more importantly to meet the specific demands of the utilized testing device. Therefore, the specimen shown in Figure 1 is used.

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Figure 1. Test specimen geometry

Test setup The dual-phase test set-up, simulating the impact and subsequent fire conditions is presented in Figure 2.

Figure 2. Two-phase experimental program setup, (a) specimen material (b) first phase: interrupted high strain rate testing, (c) second phase: elevated temperature quasi-static tensile test In Phase I, two strain rates in the lower impact ranges of the strain rate regime namely 1 s-1 and 10 s-1, were chosen to induce the initial partial deformation via a servo-hydraulic testing machine. In Phase II, elevated temperatures were imposed

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using a temperature-controlled environmental chamber while steady-state quasi-static tensile load was applied to failure. It should be noted that despite the time-consuming process of mounting and demounting the specimen in two separate test setups, the heating up time, as well as the long duration required for the cool down of the whole system used for the second phase, the time interval between the two consecutive phases of the tests was kept to less than 24 hours. The number of type tests conducted in this study is presented in Table 1. For each type test, at least 2 specimens were tested. Where the consistency of the two tests was not proven to be satisfactory (a deviation of more than 5% from the average value of the two samples), more tests were carried out till data consistency was achieved. Table 1. Number of type tests carried out Type of test Mono-phase uninterrupted/interrupted high strain rate test, ambient Two-Phase test ( $ = 1 Two-Phase test ( $ = 10

elevated temperatures) elevated temperatures)

Mono-phase quasi-static test at elevated temperatures

Number of Type tests 8 15 15 9

Phase I

Interrupted high strain rate tensile testing was carried out at room temperature using an Instron 8802 servo-hydraulic testing machine with a load capacity of 250kN as shown in Figure 2(b). Load and position were determined using built-in transducers while direct measurement of strain was achieved using a MTS LX500 non-contact laser extensometer with a strain resolution of 1 µm. Test Parameters - Strain rate The maximum crosshead speed of the Instron 8802 is 150mm/sec. The nominal rate of strain is calculated by the ratio of V/LP,0, where LP,0 is the initial parallel length of the specimen (as defined in Figure 1) and V is the crosshead displacement rate. Hence, to achieve a specific nominal strain rate during a test, the machine was operated at a relevant constant crosshead velocity.

- Damage level Uninterrupted uniaxial high strain rate (HSR) tensile tests at strain rates of 1 and 10 s-1 (denoted as HSR ( ˙=1) and HSR ( ˙=10)) were carried out and the obtained loadelongation curves were used to define three distinct damage levels for each strain rate. As it can be seen in Figure 3, the primary damage level is defined as PD=u. At this damage level, the specimen is deformed up to Du which is the elongation at the point corresponding to its ultimate tensile strength (UTS). The other two damage levels were chosen in the upper and lower vicinity of Du. The lower damage level

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(PDu) is past the ultimate stress, where the deformation is localized and a neck starts forming with a significantly less elongation than the fracture elongation.

Figure 3. Elongation-controlled damage levels Procedure Interrupted HSR tests were performed, controlled locally at different predetermined elongation values corresponding to one of the defined damage levels. Tests were abruptly terminated by the operating software at the designated elongation. Subsequently, specimens were taken out of the grips ready for the second Phase. The changed cross-section dimension of each partially damaged specimen was hence measured at the mid-gauge length for calculating the accurate stress and strain values in Phase II. The reduced cross sections are proportional to one another and proportional to the level of damage as presented in Table 2. Table 2. Ratio of the reduced cross-sectional area (Ar) to the original crosssectional area (A0) after the interrupted high strain rate test (Post Phase I) Rate of strain (s-1) 1 10 Ar / Ao 0.92 0.92 0.83 0.89 0.72 0.84

Level of deformation PDu

Phase II In the second phase, quasi-static tensile tests at elevated temperature were carried out on high strain rate induced partially damaged specimens to understand the

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influence of elevated temperature on the mechanical properties of damaged steel. Specimens were tested using an Instron environmental chamber mounted onto an Instron 5982 testing machine with a load capacity of 100kN, as shown in Figure 2(c). Test Parameter - Temperature The partially damaged specimens were tested under temperatures of ambient, 150°C, 300°C, 450°C, and 600°C. In this study, the steady-state test method was adopted as the testing technique for investigating the material properties at elevated temperatures since it is more applicable and easier to conduct. Tests were carried out in accordance with the standard test condition requirements of AS 2291 [20] and ASTM E21 [22]. The temperature of the specimen was measured by means of three K-type thermocouples positioned in intimate contact with the surface of the specimen gauge length. The same non-contact laser extensometer used in the first phase of the test was utilized for strain measurements. Procedure The specimen was initially heated up to the specified temperature with a heating rate of 10 °C/min and maintained at that constant temperature until the temperature was stabilized. During the heating process, the load on the specimen was manually maintained at zero such that free thermal expansion of the specimen was allowed. On stabilization of the temperature, uniaxial tensile load was applied at displacement rate of 0.3 mm/min until failure. RESULTS AND DISCUSSION

Table 3 shows the results of the uninterrupted uniaxial tensile tests at three strain rates of quasi-static (3.3× 10-4), 1 s-1 and 10 s-1 which were performed primarily to indicate the rate dependency characteristics of the test material at room temperature. As it is anticipated, the behaviour of mild steel is noticeably strain rate sensitive. Note that since a yield point is not easily distinguishable in the test results, it is customary to adopt and report the 0.2% proof stress as the yield stress. Table 3. Mechanical properties of mild steel for different strain rates at ambient temperature Nominal strain rate

0.2% proof stress

Ultimate stress

y (UYS)

y (0.2%)

(MPa)

(MPa)

%

3.3e-4

366.6

345.7

503.6

19.6

1

437.5

390.8

546.3

16.1

10

500.1

512.8

580.6

2.2

6

u

Ultimate strain

(MPa)

1/sec

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As it can be seen from this table, the proof stress (f0.2%) shows a significant 13% rise when the uninterrupted tensile test is conducted at a strain rate of 1 s-1, and a 48% increase at the highest rate of 10 s-1 in comparison to that of quasi-static loading condition. However, in this study, the coupling and sequential effect of elevated temperature on pre-damaged material under high strain rate loading is of interest. Design codes utilize reduction factors to account for the loss of material properties at elevated temperatures compared to that of ambient state. The same approach is adopted here. The ratio ( ) of yield stress (f0.2%), f2%, ultimate stress (fu), and ultimate strain ( u) (strain corresponding to the ultimate stress) for the high strain rate induced partially damaged mild steel to that of non-pre-damaged material are presented in Figure 4 at various temperatures. is defined as the magnitude of the variable extracted from a dual-phase test at elevated temperatures (denoted as (PDi, Ti)) to the corresponding value extracted from the non-pre-damaged value at ambient temperature (denoted as (PD0, T0)). The 0.2% proof stress (f0.2%) is defined as the stress corresponding to the intersection of the stress-strain curve with the 0.2% strain offset of the proportional line. The f2% stress represents the stress values at 2 percent strain. Elevated temperature reduction factors are available for similar stresses in different design codes; namely AS 4100 [15], AISC [23], Eurocode-3 [16] and BS 5950-8 [24] which can serve as bases for comparison. As can be seen from Figure 4, although fu increases with increasing predeformation, ductility of the material reduces significantly with increasing predeformation. In other words, the ultimate strain ( u) occurs at relatively close strain values for the two higher damage levels (PD=u and PD>u). When comparing the effect of strain rate on the material behaviour which has been previously subject to the main damage level (PD=u), it can be seen that although the partially damaged specimen can endure more load and has more strength since it has initially undergone a lighter dynamic load, and has had a more ductile deformation process, once it reaches this residual ultimate strength, its properties decay more rapidly and failure will occur in a shorter span of time. This low residual deformation capacity maybe critical for a partially damaged structural element if subject to post-impact fire. The values of proof stress, ultimate stress, and ultimate strain for different damage levels tend to converge as the temperature rises, thus the effect of pre-deformation loses its significance to the more dominating effect of elevated temperature.

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2.5

綱 = 1 (s-1)

2

f´0.2% (PDi, Ti) / f0.2% (PD0, T0)

f´0.2% (PDi, Ti) / f0.2% (PD0, T0)

2.5

1.5 1 0.5 0

綱 = 10 (s-1)

2 1.5 1 0.5 0

0

150

300

450

600

0

Temperature (°C)

150

300

450

600

Temperature (°C)

(a) 2.5

綱 = 1 (s-1)

2

f´2% (PDi, Ti) / f2% (PD0, T0)

f´2% (PDi, Ti) / f2% (PD0, T0)

2.5

1.5 1 0.5 0

綱 = 10 (s-1)

2 1.5 1 0.5 0

0

150

300

450

600

0

Temperature (°C)

150

300

450

600

Temperature (°C)

(b) 綱 =1

2

2.5

(s-1) fú (PDi, Ti ) / fu (PD0, T0)

fú (PDi, Ti ) / fu (PD0, T0)

2.5

1.5 1 0.5

2 1.5 1 0.5 0

0 0

150

300

450

0

600

Temperature (°C)

150

300

450

Temperature (°C)

(c)

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2.5

2

(PD0, T0)

綱 = 1 (s-1)

u

1.5

ú (PDi, Ti ) /

ú (PDi, Ti ) /

u

(PD0, T0)

2.5

1 0.5

綱 = 10 (s-1)

2 1.5 1 0.5 0

0 0

150

300

450

0

600

150

300

450

600

Temperature (°C)

Temperature (°C)

(d) No Pre-Damage

AISC

Pre-Damage Level < DU

AS 4100

Pre-Damage Level = DU

Eurocode 3:1-2

Pre-Damage Level > DU

BS 5950-8

Figure 4. Ratio of (a) proof stress (f0.2%), (b) f2%, (c) ultimate stress (fu), and (d) ultimate strain ( u) for partially damaged mild steel induced by high strain rate loading at elevated temperature to that of non-pre-damaged material at ambient

The imperative feature of these curves is the significant difference between the results of HSR-induced tests at elevated temperature to the fire resistance expression values of different design codes. The reduction factors presented by AS 4100, AISC and Eurocode-3 for f0.2% and BS 5950 for f2% are only able to give reasonable predictions for the non-pre-damaged material and cannot properly reflect the behaviour of the material under combined loading effects. Moreover, the increase in ultimate strength (fu) of the material due to elevated temperatures has been neglected by the AISC specification [23] and the observed variation highlights the cogent difference between the results obtained for the combined actions and those given by codes for individual effects.

CONCLUSION

The mechanical behaviour of high strain rate induced partially damaged mild steel under temperatures ranging from room temperature to 600°C test conditions has been presented here. The obtained results indicate that the damage induced by impact load significantly alters the high temperature mechanical behaviour of the partially deformed steel and such combined effects do not follow the individual effect trends available in literature. At very high temperatures experienced under fire the strain

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rate or pre-damage do not have a consequential effect on the mechanical properties of mild steel material and temperature effects are dominate. Moreover, due to the initial increase in the ultimate strength of the material at elevated temperatures prior to reaching very high thermal conditions, it can be argued that design codes (which consider an overall reduction in the material strength) do not entirely reflect the behaviour of the material. Although with such increase in strength comes a significant reduction in ductility which should be contemplated. Results indicate that the behaviour of mild steel which has been partially damaged by high strain rate loading and is subject to subsequent elevated temperatures is decisively different to the values predicted in current design codes for yield stress and ultimate stress variations with temperature. Therefore, developing new material models or generating material constraints for existing models which can reflect the material behaviour under multi-phase loading history is a necessity. ACKNOWLEDGEMENT

The work presented in this research is supported by the Australian Research Council through a Discovery Project (DP130100181).

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[8] S. Sinaie, A. Heidarpour, X.L. Zhao, A multi-objective optimization approach to the parameter determination of constitutive plasticity models for the simulation of multi-phase load histories, Computers & Structures, 138 (2014) 112-132. [9] Q.-Y. Song, A. Heidarpour, X.-L. Zhao, L.-H. Han, Performance of Double-Angle Bolted Steel I-Beam to Hollow Square Column Connections Under Static and Cyclic Loadings, International Journal of Structural Stability and Dynamics, (2014) 1450098. [10] S. Hosseini, A. Heidarpour, F. Collins, C.R. Hutchinson, Effect of strain ageing on the mechanical properties of partially damaged structural mild steel, Construction and Building Materials, 77 (2015) 83-93. [11] Q.-Y. Song, A. Heidarpour, X.-L. Zhao, L.-H. Han, Performance of Unstiffened Welded Steel I-Beam to Hollow Tubular Column Connections Under Seismic Loading, International Journal of Structural Stability and Dynamics, (2014) 1450033. [12] NIST-NCSTAR-1, Federal Building and Fire Safety Investigation of the World Trade Center Disaster: Final Report of the National Construction Safety Team on the Collapses of the World Trade Center Towers (NIST NCSTAR 1), in: S. Shyam-Sunder (Ed.), 2005. [13] G.R. Johnson, W.H. Cook, A constitutive model and data for metals subjected to large strains, high strain rates and high temperatures, in: Proceedings of the 7th International Symposium on Ballistics, 1983, pp. 541-547. [14] F.J. Zerilli, R.W. Armstrong, Dislocation-mechanics-based constitutive relations for material dynamics calculations, Journal of Applied Physics, 61 (1987) 1816-1825. [15] AS4100. Steel structures, Standards Australia, (1998). [16] EN 1993-1-2. Eurocode 3: Design of steel structures – Part 1–2: General rules – structural fire design, in, European Committee for Standardization, 2005. [17] AS 3678. Structural steel - Hot-rolled plates, floorplates and slabs, Standards Australia, (1990). [18] ASTM A572. Standard Specification for High-Strength Low-Alloy ColumbiumVanadium Structural Steel, American Society of Testing and Materials, (2013). [19] AS1391, Metallic materials - Tensile testing at ambient temperature. Standards Australia, (2007). [20] AS 2291. Metallic materials - Tensile testing at elevated temperatures, Standards Australia, (2007). [21] M. Borsutzki, D. Cornette, Y. Kuriyama, A. Uenishi, B. Yan, E. Opbroek, Recommendations for dynamic tensile testing of sheet steels, in, International Iron and Steel Institute, 2005. [22] ASTM E21. Standard Test Methods for Elevated Temperature Tension Tests of Metallic Materials, American Society of Testing and Materials, (2009).

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[23] AISC 360. Specification for structural steel buildings, American Institute of Steel Construction, (2010). [24] BS 5950-8. Structural use of steelwork in building Code of practice for fire resistant design, British Standards, (2003).

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