NSCC2009

1 INTRODUCTION

In the frame of the unification of the European technical rules for the design of steel structures (Eurocode 3) a method has been developed to select steels to avoid brittle fracture. This concept is implemented in EN 1993-1-10:2005 and applicable to structural steel members subject to fatigue loads (e.g. bridges) with details covered by EN 1993-1-9:2005. It is based on a safety assessment using fracture mechanics and assumes surface cracks with design sizes which may have developed from initial flaws which were not found by non-destructive testing (NDT) and propagated under fatigue load. In assessment the K-concept is applied using stress intensity factors. The subject of this paper are recent practical engineering applications of these rules in different fields of applicability. In fact the rules are developed with a main focus on bridge structures. Therefore some of the basic assumptions of the standard simplified procedure using tabulated values are not transferable to other fatigue loaded steel structures than bridges. However using the basic design formula and using sufficiently chosen assumptions also other structures can be designed. The paper starts with a summary of the background of the design rules. Thereafter an example shows a “standard” application using the simplified procedure. The paper ends with other examples of application using the basic design formula.

ABSTRACT: The new European unified technical design rules for steel structures (Eurocode 3) offer a method in EN 1993-1-10 to select steels to avoid brittle fracture. This concept, based on a safety assessment using fracture mechanics, is available for members subject to fa-tigue loads (e.g. bridges). This paper present the wide range of applicability of these rules.

Innovative rules in Eurocode 3, Part 1-10 for the choice of material toughness and the wide range of applicability

B. Kühn1 & G. Sedlacek2 1Verheyen-Ingenieure GmbH & Co. KG, Bad Kreuznach, Germany

2c/o Institute of Steel Construction, RWTH University, Aachen, Germany

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2 BACKGROUND OF THE RULES

2.1 Fracture-behavior of steel under low temperature

For ferritic steels, the fracture behavior of tensile loaded components, in particular the extent, to which they exhibit a non-linear load-deformation curve by yielding, depends strongly on the temperature. Figure 1 shows in a schematic way the fracture behavior of tensile loaded components which bear a crack-like flaw. The figure contains different information which are related to the fracture behavior. Characteristic temperatures are also defined which enable the distinction of fracture behavior into brittle and ductile: 1. The fracture mechanism (on a microscopic scale) being cleavage at low temperatures and

becoming shear or ductile above a temperature Ti. 2. The fracture stress depending on temperature and increasing from low temperatures to a

temperature Tgy, where net section yielding is observed before fracture and going further up to a temperature Tm where the full plastic behavior in the gross section and the ultimate load is reached.

3. The macroscopic description of the fracture behavior is defined as brittle if fracture occurs before net section yielding and where the global behavior is linear elastic or as ductile behind this point, where plasticity can be observed in the cross section and the load displacement deviates from linearity.

The temperature region above Tm characterises the region with large plastic strains which enable plastic redistribution of stress concentrations in the cross-section and the formation of plastic hinges for plastic mechanisms. In the upper shelf region above Ta the ultimate tension strength results from the stability criterion and is not controlled by toughness. In the range T 7 Tm (room temperature) all member tests have been carried out, from which the resistance functions and design rules for steel structures in Eurocode 3 have been derived. Below Tm is the temperature transition range that leads to the lower shelf behavior, where the material toughness decreases with temperature and the fail-ure modes change from ductile to brittle. Below Tm the macroscopic plastic deformations are smaller than those above Tm. They suffice to reduce stress concentrations in the cross-sections so that the nominal stress concept can be applied. They are, however, no longer sufficient for plastic hinge rota-tions, so that global analysis should be made on an elastic basis.

Figure 1. Fracture behavior of components depending on temperature (schematic view). A limit that separates this macroscopic ductile failure mode from the brittle failure mode is the tem-perature Tgy, at which net section yielding is reached before failure. The brittle fracture avoidance concept presented here is related to this area. Below Tgy the plastic deformations are restricted to local crack tip zones, which can be quantified with fracture mechanics parameters like K, CTOD or J-Integral.

2.2 Fracture mechanics method in background of the design rules

Fracture assessments in the brittle area below the temperature Ti below which no stable crack growth may occur could be performed with fracture mechanical parameters as J-integrals or CTOD-values that take both the elastic and the plastic strains into account. However for practical reasons, the stress intensity functions, initially valid for the fully elastic range T < TIc only, can be used in a more practical way because of their availability from handbooks

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where solutions can be found for most relevant cases. The stress intensity factor K is taken for mode I actions and has been derived from a stress field around the crack tip. Its validity is limited to elastic behavior where plasticity even in the vicinity of the crack tip is limited. The error resulting from neglecting the local plasticity at the crack tip is considered by a correction factor kR6 from the CEB6-R6-Failure Assessment Diagram (FAD) (Harrison et al. 1986) applied to the elastic value of the action effect Kappl,d. The corresponding resistance is KMat,d depending on temperature TEd, which may be determined from J-Integral, CTOD or valid KIc-values from CT-tests. The basic verification format with these values reads:

Ed (K) > Rd (K) or Kappl,d > KMat,d (1) which, however, needs further processing to achieve two goals: 1. Correlation between the resistance KIc raised from CT-tests and the standard values TKV

raised from Charpy-V-tests, 2. Transformation to a format for verifying with temperatures on the action side TEd and as well

as on the resistance side TRd (based on TKV). The first goal is reached in two steps. First by expressing KMat,d as a function of TEd by the standardized K-(TK100 - TEd)-Master curve from Wallin (Wallin 1994), which refers to the temperature TK100, for which KMat takes the value 100 MPa. Second by correlating the temperature TK100 for the fracture mechanical parameter K = 100 MPa(m)0.5 with the temperature T27J for the Charpy-impact energy KV = 27J (modified Sanz-correlation (Marandat et al.1976, Sanz 1980)). The verification formula based on K-values may be transferred to a formula based on temperature values T by applying logarithms, so that the final assessment scheme reads, see also Figure 2:

Figure 2. Safety assessment for limit state “brittle fracture” in EN 1993-1-10

( )EddMatdappl T K K ,*

, ≤

( )EddMat

R

dappl

dappl T K k

KK ,

6

,*, ≤

−=

ρ

41

100

6

,*,

251052

exp7020

⋅

+

∆+−

+≤−

=eff

REd

R

dappl

dapplb

TTT

k

KK

ρ

41

27

6

,*,

25105218exp7020

⋅

+

∆+°+−

+≤−

=eff

RJEd

R

dappl

dapplb

TCTT

k

KK

ρ

T100

100

T in °C

K inMPaοm

Sanz-T100-T27J-Correlation

T27J

27

T in °C

AV in J

T27J in °C

T100in °C

T100 = T27J - 18°C

σσσσ = 13°C

T100

100

TEd in °C

KMat inMPaοm

Wallin-Toughness-Curve

1

1

LR

kR6

R6-FAD

Wallin-Master-Kurve

Modifizierte Sanz-T100-T27J-KorrelationKV in J

( )R

eff

dappl

EdJ T

b K

TCT ∆+

−

⋅−

⋅+≤°−70

1025

20ln52 18

25,0*

,

27

27

For more information about the background of the design rules, see (Kühn 2005, Stranghöner 2006, Sedlacek et al. 2008).

3 EXAMPLE FOR STANDARD APPLICATION

3.1 Input parameters

For a composite road bridge with the cross-section in Figure 3 the choice of material for the bottom flange of the steel girder is questioned. The dimensions of the steel girder are also given in Figure 3.

Figure 3. Cross-section of composite bridge at mid-span (continuous over 2 spans; location Magde-burg-Germany) and dimension of cross-section of the steel beam at mid-span The reference temperature is determined in Table 1. Table 1. Determination of reference temperature. No Effect Value 1 2 3 4 5 6

Minimum air temperature Tmd Radiation loss of member, ∆Tr ∆Tσ (detail: transverse stiffener welded to bottom flange covered by EN 1993-1-9) ∆TR (National Annex)

1005.0 −= sε& (from project specification): ε&T∆ DCF = 0 (no cold-forming): DCFT∆

- 25 °C - 5 K 0 K 0 K - 16 K*) 0 K

7 TEd - 46 °C *) calculated with fy(t) = fy0 – 0.25 · t/t0 = 355 – 0,25 · 26/1 = 349 N/mm² and

(2.1)

For more information on input parameters used in formula (2.1) see (Kühn 2005). The relevant stress σEd is calculated with ψ1 = 0.7 from the “accidental” load combination (γ = 1.0) and is given to 215 N/mm². That lead to a stress level of σEd = 215/349 • fy(t) = 0.62 • fy(t).

3.2 Simplified choice of material

The use of table 2.1 of EN 1993-1-10:2005 gives the minimum toughness requirement T27J = -20C, or S355J2, see Figure 4, where

( )KKnn

tfT

y 16~3.150001.0005.0

5503491440

5501440 5.15.1

0

−=

−−=

−−=∆ l

&

&l&

ε

εε

Material S355

[cm/m]

2.6

1.0

1.2

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tpermissible(0.62•fy(t)) = 39 mm > tavailable = 26 mm (3).

Figure 4. Interpolation of steel grade from Table 2-1 of EN 1993-1-10

4 OTHER POSSIBILITIES OF APPLICATION

4.1 Components of wind power plants (Specific: strain rate effects and other metallic materials than structural steel)

In general members of the towers of wind power plants (see Figure 5) are made of structural steel and therefore it can deal with like shown in the section 3. However what should be kept in mind, is

the strain rate effect. According to Kühn 2005 (section 3.2.2.5.3) non-neglectable strain rates (ε& = 0.0001 to 0.1 s-1) may occur during squalls. However in most cases sufficient ductile steel grade may be chosen also for members which are critical ones form the point of view of brittle fracture (e.g. braces, built-in part of foundation, etc.). Therefore often fine grain steel grades (e.g. S355 NL or ML) were taken to ensure sufficient safety against brittle fracture also in cases where power plants are built in cold climate regions. But what to do with elements not made of structural steel, e.g. rotor hub and shaft ? These elements are made of different metallic materials. E.g. regular shafts are made of forged steel as 30CrNiMo8. In such cases only de-tailed investigations should be carried out using the basic verification format, see formula (1). The main problem of these investigations is to gather sufficient information on the material in particular toughness characteristics considering also strain rate effects. Technical delivery conditions, e.g. as EN 10083:1996, do not provide toughness require-ments or, if so, the requirements are useless for a brittle fracture inves-tigations. Additionally it has to be noticed that the load on such mem-bers may be high in particular if stress concentration effects due to change in stiffness have to be taken into account (e.g. in the area around the connection between shaft and rotor hub).

Figure 5. Wind power plant Finally it has to be considered that wind power plants in general have different load histories comparing with bridges. On one hand a high number of load cycles are close to the cut-off limit defined by the fatigue design procedure. But on the other hand the number of these load cycles is much higher (x10 to x100) than on bridges and some of the load cycles are in a range not much less than yield strength, which is also not typical for bridges. Therefore special assumptions have to be

10 0 -10 -20 -30 -40 -50 10 0 -10 -20 -30 -40 -50 10 0 -10 -20 -30 -40 -50

at T °C Jmin.

JR 20 27 60 50 40 35 30 25 20 90 75 65 55 45 40 35 135 115 100 85 75 65 60

J0 0 27 90 75 60 50 40 35 30 125 105 90 75 65 55 45 175 155 135 115 100 85 75

J2 -20 27 125 105 90 75 60 50 40 170 145 125 105 90 75 65 200 200 175 155 135 115 100

JR 20 27 55 45 35 30 25 20 15 80 70 55 50 40 35 30 125 110 95 80 70 60 55

J0 0 27 75 65 55 45 35 30 25 115 95 80 70 55 50 40 165 145 125 110 95 80 70

J2 -20 27 110 95 75 65 55 45 35 155 130 115 95 80 70 55 200 190 165 145 125 110 95

M, N -20 40 135 110 95 75 65 55 45 180 155 130 115 95 80 70 200 200 190 165 145 125 110

ML, NL -50 27 185 160 135 110 95 75 65 200 200 180 155 130 115 95 230 200 200 200 190 165 145

JR 20 27 40 35 25 20 15 15 0 65 55 45 40 30 25 25 110 95 80 70 60 55 45

J0 0 27 60 50 40 35 25 20 15 95 80 65 55 45 40 30 150 130 110 95 80 70 60

J2 -20 27 90 75 60 50 40 35 25 135 110 95 80 65 55 45 200 175 150 130 110 95 80

K2, M, N -20 40 110 90 75 60 50 40 35 155 135 110 95 80 65 55 200 200 175 150 130 110 95

ML, NL -50 27 155 130 110 90 75 60 50 200 180 155 135 110 95 80 210 200 200 200 175 150 130

M, N -20 40 95 80 65 55 45 35 30 140 120 100 85 70 60 50 200 185 160 140 120 100 85

ML, NL -50 27 135 115 95 80 65 55 45 190 165 140 120 100 85 70 200 200 200 185 160 140 120

Q -20 30 70 60 50 40 30 25 20 110 95 75 65 55 45 35 175 155 130 115 95 80 70

M, N -20 40 90 70 60 50 40 30 25 130 110 95 75 65 55 45 200 175 155 130 115 95 80

QL -40 30 105 90 70 60 50 40 30 155 130 110 95 75 65 55 200 200 175 155 130 115 95

ML, NL -50 27 125 105 90 70 60 50 40 180 155 130 110 95 75 65 200 200 200 175 155 130 115

QL1 -60 30 150 125 105 90 70 60 50 200 180 155 130 110 95 75 215 200 200 200 175 155 130

Q 0 40 40 30 25 20 0 0 0 65 55 45 35 30 20 20 120 100 85 75 60 50 45

Q -20 30 50 40 30 25 20 0 0 80 65 55 45 35 30 20 140 120 100 85 75 60 50

QL -20 40 60 50 40 30 25 20 0 95 80 65 55 45 35 30 165 140 120 100 85 75 60

QL -40 30 75 60 50 40 30 25 20 115 95 80 65 55 45 35 190 165 140 120 100 85 75

QL1 -40 40 90 75 60 50 40 30 25 135 115 95 80 65 55 45 200 190 165 140 120 100 85

QL1 -60 30 110 90 75 60 50 40 30 160 135 115 95 80 65 55 200 200 190 165 140 120 100

S420

S460

S690

steel

grade

S235

S275

S355

sub

grade

Charpy energy reference temperature TEd in °C

KV

σEd=0.25 x fy(t)σσσσEd=0.50 x fy(t)σσσσEd=0.75 x fy(t)

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drawn for calculation and special requirements have to be pointed out for fabrication. Both have to be combined to ensure that crack sizes assumed for the design and used in the fracture mechanics calculation are safe. In brittle fracture assessments “safe” means that an initial flaw with a defined initial size can surely be found by the required quality checks (e.g. by using NDT). And in the “accidental” case that such a flaw will be not found it has to be ensured that the time between two inspections or the life time (optimum) is shorter than the time which the initial crack needs to grow under the specific load history to a critical crack size, where brittle fracture may occur under frequent loads. Proper requirements for fabrication of shafts can be: − No welding allowed (also no repair welding) − Stress concentration effects have to be minimised (e.g. by smooth change of stiffness) − No cold forming − 100% NDT with special regard on the surface (no cracks allowed, other defects must have a

specific distance to the surface and surface roughness is limited) Based on such requirements an “accidental” initial crack size has to be assumed and crack growth calculations have to be carried out to show whether there is enough time to find the crack before it becomes a critical one. If there is sufficient time a fracture mechanics calculation has to show that available toughness is higher than the toughness requirements due to the crack, the frequent load and under consideration of strain rate effects. Simplified assumptions concerning the model used for calculation of stress intensity factors may be made, e.g. see Figure 6. Figure 6. Shaft with semi-elliptically surface crack simplified by a plate (width W and thickness t) with an edge crack on one side (crack depth a) Depending on the boundary condition brittle fracture investigations on such components often lead to toughness requierments of Kmat = 45 – 100 MPa(m)0.5, which can be fulfilled by forged steel as 30CrNiMo8. However the calculated requirement should be agreed by contract, because most of the available technical delivery conditions do not provide it.

4.2 Stop devices (Specific: strain rate and cold forming effects)

At some steel structures working platforms are needed on a high level. If rails can not be built in stop devices are used as fall protection for the workers, see Figure 7. Figure 7. Examples for stop devices

Simplified model: plate with edge crack on one side t

W

Top-view

View

section mast

30

In the case a worker is fixed on such stop devices and falls down his or her fall will be stopped by it. Also if energy absorbers are used to damp the fall, the load-time-history of the stop device will often show strain rates ε& 7 10-4 s-1. Therefore strain rate effects have to be considered in brittle fracture investigations of such components. Till now only few types of stop devices were investigated to find out their specific maximum of strain rate. However strain rates up to ε& = 10 s-1 have been found. But it has to be noticed that these few results can not be transferred to other types of stop devices due to their different load-time- and load-deformation-behaviour. Additionally some types of stop devices (e.g. the one right hand side in Figure 7) show huge plastic deformation after a fall was stopped, see Figure 8.

Figure 8. Example for plastic deformation of a stop device after a fall was stopped These plastic deformations have to be taken into account, too, if brittle fracture calculations were carried out for such stop devices. According to Kühn 2005 the degree of cold forming can be calculated using the following equation:

( )100

116

max ⋅⋅+⋅

⋅==

b

h

nL

fDCF ε (4)

with f, L, h, b and n according to Figure 9.

Figure 9. Definition of dimensions used in equation (4) This equation leads for typical dimensions of the type of stop device as shown in Figure 7 (right hand side) to degrees of cold forming (DCF) of 30% to 35%. Using equation (2.3) and (2.4) of EN 1993-1-10 : 2005 both effects (strain rate and cold forming) lead to a temperature shift of transition temperature of ∆Tε& ≈ -75 K and ∆Tεcf ≈ -105 K. With regard on these results it is obviously that: brittle fracture investigations are recommended for all stop devices and not only for as they are used in cold climate environments, and if normal structural steel should be used further detailed investigations are recommended to find more realistic assumptions for both effects than the above mentioned theoretically estimated values. For the discussed typ of stop device tests were carried out, where the load-time-history during the damping of a fall as well as the final plastic deformation were measured. It has been shown that both strain rate (ε& > 4 s-1) and plastic deformation (DCF > 15%) are much lower than theoretically estimated. It has to be considered, too, that both effects have their maximum not at the same time (maximum of strain rate at the beginning of damping fall and maximum degree of cold forming at the end) and in general the point of the member, where strain rate and plastic deformation have their

e e e e e

L

r

f

α

ϕ/2

b

h

plastische Dehnungen

χ

ε

max

max

h/2

Plastic strain

31

maximum, did not have other negative effects concerning brittle fracture, e.g. welds, change of stiffness, etc.. Therefore generally such typs of stop devices may be made of steel grade S355 J2.

5 CONCLUSIONS

The new European unified technical design rules for steel structures (Eurocode 3) offer a method in EN 1993-1-10 to select steels to avoid brittle fracture. This concept, based on a safety assessment using fracture mechanics, is available for various members made of structural steel. In general the method is used for members subject to fatigue loads, but it is also applicable for members under quasi static loads, see Höhler et al. 2004. As shown in the present paper the method may be used in cases with specific toughness requirements due to high strain rate and / or high cold deformation effects, too. Also other metallic materials can be dealt with. This method was used sufficently for the choice of material for following applications: boat hulls, bogies of trains, crane structures made of high performace steel grades up to S1100, hangers of arch bridges with thicknesses larger than 180mm and huge inhomogenities of the material in through thickness direction, cast nodes, castor containers concidering also the negative effect of nuclear radiation on the toughness, pressure vessels, pressure pipe lines, etc.

6 REFERENCES

Eurocode 3: Design of steel structures - Part 1-10: Material toughness and through-thickness properties, 2005. Eurocode 3: Design of steel structures - Part 1-9: Fatigue, 2005. EN 10083: Steels for quenching and tempering - Part 1: General technical delivery conditions, 2006 Harrison, R.P., K. Loosemore and I. Milne, Assessment of the integrity of structures containing defects, CEGB-Report R/H/R6, Revision 3, 1986. Höhler, S., Kühn, B., Sedlacek, G., Brittle fracture mechanical concept for welded connections in steel struc-tures, Proc. of the 10th Nordic Steel Conference, Copenhagen (Denmark), 7-9 June 2004, pp 65 to 76 Kühn, B., Beitrag zur Vereinheitlichung der europäischen Regelungen zur Vermeidung von Sprödbruch, Dissertation am Lehrstuhl für Stahlbau, RWTH Aachen, 2005, ISBN 3-8322-3901-4. Marandat et al. 1976 Marandat, B., Sanz, G., Étude par la mécanique de la rupture de la ténacité d’aciers à résistance moyenne fournis en forte épaisseur, Revue de Métallurgie, pp. 359-383, April 1976. Sanz, G., Essai de mise au point d’une méthode quantitive de choix des qualités d’aciers vis-à-vis du risque de rupture fragile, Revue de Métallurgie, CIT, pp. 621-642, July 1980. Sedlacek, G., Feldmann, M., Kühn, B., Tschickardt, D., Höhler, S., Müller, C., Hensen, W., Stranghöner, N., Dahl, W., Langenberg, P., Münstermann, S., Brozetti, J., Raoul, J., Pope, R., Bijlaard, F., Commentary and worked examples to EN 1993-1-10 “Material toughness and through thickness properties“ and other tough-ness oriented rules in EN 1993, JRC – ECCS Joint Report, 1st Edition, Sept. 2008, EUR 23510 EN. Stranghöner, N., Werkstoffwahl im Stahlbrücken, DASt (Deutscher Ausschuß für Stahlbau), Forschungs-bericht 4/2006, Stahlbau Verlags- und Service GmbH, Düsseldorf, 2006 Wallin K., Methodology for Selecting Charpy Toughness Criteria for Thin High Strength Steels, Part 1, 2 and 3, Jernkontorets Forskning, Nr. 4013/89, VTT Manufacturing Technology, VTT Espoo, 1994.

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