Stress corrosion cracking mechanism of prestressing steels in bicarbonate solutions

Stress corrosion cracking mechanism of prestressing steels in bicarbonate solutions

Corrosion Science 49 (2007) 4069–4080 www.elsevier.com/locate/corsci Stress corrosion cracking mechanism of prestressing steels in bicarbonate soluti...

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Corrosion Science 49 (2007) 4069–4080 www.elsevier.com/locate/corsci

Stress corrosion cracking mechanism of prestressing steels in bicarbonate solutions J. Sanchez *, J. Fullea, C. Andrade, C. Alonso Institute Eduardo Torroja of Construction Sciences, Serrano Galvache, 4, 28033 Madrid, Spain Received 6 February 2006; accepted 3 May 2007 Available online 5 July 2007

Abstract Prestressing steels occasionally fail by a process named ‘‘stress corrosion cracking’’. This process has not been fully elucidated and several theories exists in order to explain the cases in which real structures have collapsed. This paper briefly mentions the different theories and identifies the progress in understanding whether it is necessary to use a testing method, which is able to separate the different steps and mechanisms contributing to the failures. This paper presents the methodology used for inducing controlled localized attack to study the susceptibility of the high strength steels resistance to stress corrosion cracking (SCC). The method is designed to study the growth of cracks initiated from a mechanical notch; the crack is not produced by fatigue. It consists of several stages: coating of the bar with epoxy resin, generation of a small notch, constant load and controlled potential test in the media, mechanical test in air and fractographic study. It allows us to calculate the crack propagation rate and the fracture toughness in the same test. Finally, it has been possible to apply the surface mobility mechanism (SMM) in order to identify the SCC mechanism that operates.  2007 Published by Elsevier Ltd. Keywords: A. Steel; B. Potentiostatic; B. SEM; C. Hydrogen embrittlement; C. Stress corrosion

*

Corresponding author. E-mail address: [email protected] (J. Sanchez).

0010-938X/$ - see front matter  2007 Published by Elsevier Ltd. doi:10.1016/j.corsci.2007.05.025

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1. Introduction Steel reinforcement in concrete is protected from corrosion by passivation due to the high alkalinity produced by the hydration of the cement. This protection can be maintained indefinitely until an aggressive element in sufficient concentration reaches the bar. The most common causes of corrosion are the carbonatation of the concrete cover, which produces a reduction in the pH of the pore solution, and the penetration of chlorides, which induces pitting corrosion. A particular case of corrosion of the steel embedded in concrete is the stress corrosion cracking (SCC), which can appear in prestressed structures. The SCC is produced by the simultaneous action or synergy of a mechanical tension and a corrosive medium. Nucleation at the steel surface results in the appearance of microscopic cracks that penetrate and induce the brittle failure of the wire due to a triaxial stress condition. The failures due to SCC in prestressed structures are seldom occurring. However due to the brittle failure, the accidents can give rise to collapses and catastrophes as that of the Point Pleasant bridge [1], of the cover of the Conference hall of Berlin [2], or of the pressure pipelines [3]. Up to the present, the mechanism of SCC has not been explained satisfactorily. Numerous mechanisms have been proposed to explain the brittle failure of metal failure under stress, but only some of them, three especially, are considered to be relevant: 1. Mechanism of anodic dissolution; developed by Parkins [4]. 2. Film-induced cleavage model; whose theoretical aspects have been developed by Newman [5]. 3. Surface mobility SCC mechanism; developed by Galvele [6]. The mechanism of anodic dissolution (AD) considers the electrochemical anodic dissolution at the tip of the crack as the fundamental kinetic parameter. The film-induced cleavage model (FC) accepts the anodic dissolution at the crack tip but it places the emphasis to the mechanical properties and the effect of micro-notchs that are the cause of microscopic cracks. The surface mobility mechanism (SMM) proposes a new perspective in which the crack advances, not due to anodic dissolution but due to diffusion of atomic vacancies created in the lips of the crack towards its tip. SMM is the only mechanism that proposes equations enabling the prediction of crack propagation rate and that incorporates the effect of the hydrogen produced during the process, achieving the formulation of an extension of the theory on SCC to hydrogen embrittlement [6]. Coherent with the lack of agreement in the type of mechanism that operates, an agreed testing method does not exist for the study of the susceptibility to SCC or to hydrogen embrittlement either [7]. In the case of high strength steel wires for prestressed concrete, there is a standard with a test type where the aggressiveness is increased to accelerate the process. This test enables one to detect the susceptibility of steel to hydrogen embrittlement and serves as a quality control test to detect defects [3,8]. Other authors suggest a test closer to concrete performance, which is based on the application of the theory of anodic dissolution [9–11] to specimens in alkaline solutions containing chlorides or of sodium bicarbonate [12,13] or in the use of pre-cracked specimens induced by fatigue [10,11,14,15] in whose studies the fracture toughness is calculated by fracture mechanics.

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None of these tests can be generalized and give conclusive results. It has thus seemed, necessary to try to develop a more suitable testing method. The main measurements would be: It should be practical, able to be used for control of production and for making predictions of long term performance. In the present work, some results of a more realistic testing methodology than the present ones are given. It allows us to calculate the fracture toughness of a wire and to predict the speed of the advancement of the crack. It replaces the generation of fatigue cracks by the creation of a natural crack made to grow from one notch, electrochemically by treating in a solution of sodium bicarbonate with the wire under tension. When the crack reaches a certain depth, it is tested in air. The fracture toughness is calculated by Fracture Mechanics (FM). Besides, the theory of the SMM is applied to the fractographic results. 2. Experimental method 2.1. Materials and equipment A steel of eutectic composition has been tested in two conditions: cold drawn steel (1510 MPa yield strength) and the modified parent pearlitic steel (1300 MPa yield strength). The chemical composition of both is therefore the same and it is as shown in Table 1. The machine of slow strain rate tests (SSRT) used in the study has a speed range between 5000 lm/min and 0.10 lm/min. The load limit is 40 kN and the displacement is 10,000 lm. The specimen is fixed to gags by a thread. The SCC cell has been adapted for working in the slow strain rate machine. The cell is sealed at its lower part by a gasket, whereas in the upper part it is not sealed hermetically since it has to allow the displacement of the specimen. On the top of the cell some holes enable to introduce a platinum counter-electrode of platinum, the reference silver–silver chloride electrode and type J thermocouple. Besides, the cell has a temperature control with a refrigerator and a heater with thermostat. The test operates potentiostatically. When wire failure occurs a second potentiostat applies a protective cathodic potential to preserve the fractography by SEM. All the information, current, potential and temperature, are registered with a data logger. 2.2. Testing methodology A set of tests were carried out to localize the generation of a single pit and avoid depassivation on the rest of the surface. After several trials, epoxi coating was used in order to avoid depassivation by the generation of various pits. A notch, artificially made to leave the steel surface in contact with the solution, was used to reproduce a single pit. Actually, the more realistic conditions are based on the generation of a crack by electrochemical Table 1 Chemical composition of pearlitic high strength steel %C

%Si

%Mn

%P

Cr

Ni

%S

0.8

0.2

0.7

60.02

0.20

0.074

60.03

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dissolution from a pit. However, the use of a notch may at least represent better reality than to generate the crack by fatigue. The samples were mechanized to a diameter of 2.5 mm and a length of 13.2 mm the working area. They were prepared as shown in Fig. 1 and submitted to potentials as follows: (a) Smooth test specimen: this was used as a reference and was tested at the open circuit potential (OCP). (b) Notched specimen (Fig. 1b): tests were carried out both to OCP and to controlled potential. (c) Specimens covered with epoxy resin and later notched (Fig. 1c): cracks were induced when submitted to a controlled potential in order to generate anodic corrosion. (d) Specimens covered with epoxy resin except for a small window (Fig. 1d) but not notched: the test was carried out the same as in (c) by applying a controlled potential. The testing method consists of several stages: (1) Mechanical-electrochemical generation of the crack in the two types of steel: • Anotch is produced in the middle of the wire of a length of 13.2 mm (except in the reference and type d). • The specimen is thus strained to 80% of its yield strength. • The specimen is immersed in a solution of sodium bicarbonate at a constant temperature (in the present case 25 C). • A fixed potential is applied, for around 100 h simultaneously the current is registered by a data logger. The coated specimens were covered with epoxy resin and all specimens were notched.

Fig. 1. The different types of tested specimens: a–d according to the text.

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The specimens are then removed from the solution and dried. (2) Slow strain rate test: • SSRT is performed in air at a rate of 3 · 107 s1 in order to determine the fracture toughness. (3) SEM analysis: • SEM is used to identify brittle zones, cleavage, and the different zones of the fractured surface. • The crack dimensions are measured. • The reduction in the area is accounted for. 3. Results 3.1. Selection of testing conditions to localize the pit and generate a crack Due to the application of a fixed potential in the range where corrosion develops, oxides were visible in the mouths of the notch. Having verified this performance, the bars were mechanically tested in air notice how this slight corrosion can affect the strain–stress curve. Both steel types tested show lower values of strain at the failure load and lower reduction in area in the bars coated with the epoxy resin. The uncoated notched specimens that were not covered with epoxy resin (b) (Fig. 2) and those that were covered with epoxy resin leaving a window (d) (Fig. 3), represent a ductile failure and high value of area reduction. The failure, in this case of notched specimen (b), took place from the plane of the notch but any crack was not observed . This was taken as

Fig. 2. Surface of the fracture of the modified parent pearlitic steel not covered by epoxy resin and notched (type b).

Fig. 3. Surface of the fracture of the modified parent pearlitic steel covered with epoxy resin and with a window (type d).

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Fig. 4. Surface of fracture of covered and notched modified parent pearlitic steel (type c).

Fig. 5. Cleavage in modified parent pearlitic steel (type c).

an indication of the need to isolate the notch from the rest of the surface of the specimen in order to localize more generation of the crack at the bottom of the notch. This way, if the specimen is covered with epoxy resin and later notched (c), the anodic current generated by the applied potential is applied in generating a crack at the bottom of the notch and grows from it in a normal direction to the loaded one. In doing so, the surface of the fracture corresponding to the specimens (c) covered with epoxy resin and notched (Fig. 4) shows a brittle failure that has taken place without the reduction of area. In its surface, it is possible to identify a semi-elliptical crack. The failure in this case took place at the moment when the maximum load is reached and without excessive deformation. Brittle zones appear in the whole interior of the surface of the fracture (Fig. 5). These results indicate that only the specimens covered by the epoxy resin and notched exhibit a change in the failure type from ductile to brittle. Once selected the method of concentration of the damage (coating and notching), a few tests were carried out in order to know the influence of the applied potential on the crack propagation at constant load. Cold drawn steel was used for these tests. The resulting potential is more sensitive to induce SCC is 275 mVAg/AgCl in the bicarbonate solution used. This test induces the lowest deformation and reduction of area as a consequence of the crack generated by stress corrosion cracking. 4. Discussion Brittle failures were obtained, thanks to the localization of the crack in the manner described above and by the application of an external potential. For the analysis of the results, the SMM mechanism proposed by Galvele [6] and the theory of fracture mechanics applied to cylindrical wires proposed by Valiente and coworkers [15] has been used.

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From the results obtained, two parameters were calculated: 1. The crack propagation rate obtained from the potentiostatic test. 2. The fracture toughness obtained from the tension test. 4.1. Application of Surface mobility SCC mechanism [6] 4.1.1. Theory background This mechanism calculates the crack propagation rate (cpr) from the following expressions, according Galvele’s theory:   3   Ds ra þ aEb cpr ¼ exp 1 ; ð1Þ L kT where, cpr is the crack propagation rate (m s1), Ds is the surface self-diffusion coefficient (m2 s1), L is the diffusion distance of the vacancies (normally 108 m), r is the elastic surface stress at the tip of the crack (N m2), a is the atom size (m), a is a dimensionless function, Eb is the binding energy for hydrogen (J), k is the Boltzmann constant (J K1), T is the temperature (K).     30T m 13T m Ds ¼ 7:40  104 exp  þ 0:014  104 exp  ; ð2Þ RT RT where, R is the gas constant (8.32 J mol1 K1) and Tm is the melting point of the surfaceadsorbed impurity (K). By using this theory it is possible to calculate the crack propagation rate (cpr) in the function of different variables like temperature or different iron oxides, and when hydrogen evolution is introduced in the calculation. 4.1.2. Calculation of crack propagation rate Applying these equations to the present results, where brittle failures were obtained, thecrack propagation rate, in the 0.05 M bicarbonate solution for both the steel conditions at 80% of their yield strength was calculated from the length of the maximum crack observed in the SEM study and from the duration of the potentiostatic tests carried out at a constant load (Table 2). Plotting the calculations by using the form of Arrhenius plot of cpr versus the different types of oxides, the results (Fig. 6) indicate that ferric oxide and/ or magnetite would be formed, and hydrogen embrittlement would have taken place. An analysis of the oxides has not been made, but the observation of the surface of the fracture presented brittle zones which could be compatible with the presence of hydrogen. 4.2. Fracture toughness calculation 4.2.1. Theoretical background Due to the limited size of the samples, prestressed steels cannot be prepared to obtain standardized specimens for testing fracture toughness of the material (ASTM E399-78). For the case of a cylindrical geometry of the material, the calculation of the stress intensity factor and the criterion of fracture have been proposed by Valiente and Elices [15]. The above mentioned authors have assumed that cracks are formed along the whole perimeter of the specimen and the superficial cracks have a semi-elliptical shape.

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Table 2 Crack propagation rate (m/s) Material

Test number

Experimental cpr (m/s)

Modified parent pearlitic steel

01 02 03 04 05 06 07 08

1.74E09 2.29E09 3.20E09 4.26E09 2.01E09 8.44E08 1.24E09 2.89E09

Cold drawn steel

01 02 03 04 05 06 07 08 09

1.85E09 2.31E09 1.07E08 2.86E09 2.67E08 1.74E08 2.31E09 2.04E09 1.11E09

1.E-04 1.E-05 1.E-06

Fragilización por hidrógeno

1.E-07

FeO

cpr (m/s).

1.E-08

Fe2O3

1.E-09 Fe3O4

1.E-10 1.E-11

Datos experimentales

1.E-12

FeO sin H2

1.E-13

Fe2O3 sin H2

1.E-14 1.E-15 1.E-16 0.0025

Fe3O4 sin H2

0.0027

0.0029

0.0031

0.0033

0.0035

0.0037

0.0039

1/T (K)

Fig. 6. Crack propagation rate (cpr) at 25 C calculated through the SMM mechanism versus experimental results.

4.2.2. Calculation of toughness Valiente’s equation is used to calculate the fracture toughness. In this, the stress intensity factor (KI) is replaced by the fracture toughness (KIC). To obtain the parameters a and b, it is necessary to study the surface of the fracture, the dimensions of the notch and the crack. Both showed a semi-ellipstical form whose semi-axes are a and b. Following this is shown some examples of crack generation in modified parent pearlitic steel (Fig. 7) and cold drawn steel (Fig. 8).

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8.0 7.256

LOAD (kN)

7.0 6.0 5.0 4.0 3.0 2.0 1.0 0.0 0

200

400

600

800

1000

1200

1400

ELONGATION (μm)

Fig. 7. Modified parent pearlitic steel (I). Load–elongation and the surface of the fracture obtained in the type c specimen at a –275 mVAg/AgCl potential.

9.0 8.374

8.0

LOAD (kN)

7.0 6.0 5.0 4.0 3.0 2.0 1.0 0.0 0

200

400

600

800

1000

1200

1400

ELONGATION (μm)

Fig. 8. Cold drawn steel. Load–elongation and the surface of the fracture obtained in the type c specimen at a – 275 mVAg/AgCl potential.

Table 3 shows (a) the values of the semi-axes calculated from the apparent ellipse identified in the crack, (b) the value of the maximum load in the curved load–elongation and Table 3 Crack dimensions, maximum load and fracture toughness a (mm)

KIQ (MPa m1/2)

Cold drawn steel

0.32 0.76 0.8 0.92

53.8 51.4 51.5 44.8

Pearlitic steel

0.6 0.76 0.88 0.92 0.98 1 1.08

46.1 39.8 34.9 38.3 38.0 40.3 40.6

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(c) the stress intensity factor from the load of failure, that is to say, the probable value of fracture toughness (KIQ), determined from Eq. (3). Comparing the values of KIQ thus calculated it should be mentioned that other authors obtained different values. Thus, Ayaso and Toribio [16], obtained a value of 61.2 MPa m1/2 for the modified parent pearlitic steel and 107.9 MPa m1/2 for the cold drawn steel, whereas Lancha [14] reported a value of 53 MPa m1/2 for the modified parent pearlitic steel and 84 MPa m1/2 for the cold drawn steel. Instead of these discrepancies, the value of the fracture toughness is lower than that value which is obtained by fatigue. The average value of the fracture toughness for a crack generated by stress corrosion cracking is 39.7 MPa m1/2 with a standard deviation of 3.4 MPa m1/2 to parent steel and 50.4 MPa m1/2 with a standard deviation of 3.9 MPa m1/2. This value is almost 30% lower to that of pearlitic steel and 40% to that of cold drawn steel. 4.3. Suitability of testing conditions The difference found with previous results on toughness calls for some speculation on the testing conditions. Regarding the suitability of the testing methodology to represent the behaviour of wires for prestressing concrete, it has to be first mentioned that from the three phases in the SCC process [3] crack initiation, crack propagation by SCC and failure (Fig. 9), the last two phases, crack propagation and failure, are the only ones attempted to be quantified in the present study. In the present tests, the initiation phase has not been studied as the crack is generated by fatigue. It seemed to us that it is more realistic to replace the initiation phase by the introduction of a mechanical defect, in this case a notch. The ideal thing would have been to introduce this first phase also in a natural form, which is by generating a pit electrochemically, that will be attempted in further work. It has to be added with regard to the testing conditions that the length of the crack has been calculated on the surface of fracture and the crack propagation rate was estimated by measuring the duration of the test. In both materials tested, modified parent pearlitic steel and cold drawn steel the same crack propagation rate value under the same mechanical and electrochemical conditions has been obtained. However, it has been observed that the fracture that takes place in modified parent pearlitic steel, is different from that observed in cold drawn steel. Due to the anisotropic microstructure of the cold drawn steel, a higher hydrostatic tension appears on the plane that forms an approximate 80 angle with the horizontal [16]. This phenomenon has an influence on the mechanical behaviour and on the fracture toughness value. In this case it is necessary to calculate the fracture toughness by the

Process step Mechanism

Initiation Electrochemical

Grow up Surface mobility + Stress intensity

Fracture Stress intensity

Fig. 9. Surface of the fracture. Phases of the SCC.

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KI + KII mode. In consequence, although present work seems to represent closer than other the real conditions in the unfilled spaces of grouts, further work is needed to try to reproduce better the real conditions in an existing structure. 5. Conclusions In the present study, we have attempted to explore experimental conditions to induce SCC phenomena closer to some real conditions (carbonated) instead of using pre-crack specimens generated by the fatigue test and additionally to calculate the crack propagation rate by the SMM and the metal toughness by fracture mechanics. The first objective has been only partially achieved, because the crack initiation phase has not been naturally obtained. It was necessary to produce a mechanical notch on the surface to generate a crack. However 1. The crack growth rate has been calculated. The crack value obtained is the same in both types of steel tested: cold drawn steel and modified parent pearlitic steel. Nevertheless, the cold drawn steel has a higher yield strength, due to the cold drawn process, and its toughness is higher too. 2. It has been possible to apply the SMM in order to identify of the SCC mechanism that operates. According to this mechanism, crack propagation velocity can be estimated. The theoretical values obtained here are close to experimental ones, which correspond with the formation of the magnetite and the ferric oxide with hydrogen present in the metal lattice. 3. From the results it has been possible to calculate the fracture toughness (pearlitic steel, 39.7 MPa m0.5 and cold drawn steel, 50.4 MPa m0.5), though the values are different from those obtained by other authors. This difference can be due to the conditions of generation of the crack: potentiostatically, this is different from the generation of the crack by fatigue. The measurement of the large reduction of the fracture toughness when the material is immersed in media inducing stress corrosion cracking indicated a possible need to review the damage tolerance of prestressed structures in some contaminated atmospheres. Acknowledgments The authors thank the Ministerio de Fomento for the funding and the accomplishment of the project ‘‘Not destructive methods and strategies for the control of the corrosion in pretested steels’’, the Department of Science and Technology (MAP2003-03912), to the CSIC for the scholarship of investigation I3P and, specially, Prof. Gustavo Guinea (UPM). They are also grateful to Dr. J.R. Galvele for his useful explanation of the SCC theory. References [1] S.T. Rolfe, J.M. Barsom, Fracture and Fatigue Control in Structures, Applications of Fracture Mechanics, Prentice-Hall, New Jersey, 1977. [2] U. Nurnberger, Ana´lisis and evaluation of failures in prestressed steel, in: Proceedings of the Third FIP Symposium, FIP-Berkeley, Wexham Springs, Slough, UK, 1981.

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[3] M. Elices, Problemas de Corrosio´n Bajo Tensio´n en el Hormigo´n Pretensado, I Congreso Iberoamericano de Corrosio´n y Proteccio´n, 1983. [4] R.N. Parkins, Metal Environmental Reactions, vol. 1, Newness Butterworths, 1976. [5] K. Sieradzke, R.C. Newman, Stress corrosion cracking, J. Phys. Chem. Solids 48 (11) (1987). [6] J.R. Galvele, A stress corrosion cracking mechanism based on surface mobility, Corros. Sci. 27 (1) (1987) 1–33. [7] J. Fullea, Experimental methods to evaluate the susceptibility of prestressing steels to stress corrosion cracking, in: Proceedings of the International Workshop on Failures of Post-tensioned Concrete Structures Messina, 2000, p. 17. [8] J. Toribio, M. Elices, Nuevas Aportaciones al Ensayo FIP de Fragilizacio´n por Hidro´geno en Tiocianato Amo´nico, Hormigo´n y acero 27 (168) (1988) 121–130. 2 [9] R.N. Pakins, S. Zhou, The stress corrosion cracking of C–Mn steel in CO2 –HCO 3 –CO3 solutions I: stress corrosion data, Corros. Sci. 39 (1) (1997) 159–173. [10] L. Caballero, M. Elices, Influencia de la velocidad de deformacio´n en la propagacio´n de fisuras por corrosio´n bajo tensio´n, Revista Iberoamericana de Corrosio´n y Proteccio´n 17 (1) (1986) 15–22. [11] L. Caballero, M. Elices, Un me´todo para la medida de la cine´tica de las fisuras de corrosio´n bajo tensio´n en ensayos a velocidad de deformacio´n constante, Revista Iberoamericana de Corrosio´n y Proteccio´n 17 (1) (1986) 43–48. [12] M. Acha Hurtado, Corrosio´n Bajo Tensio´n de Alambres de Acero Pretensado en Medios Neutros con ¼ ´ n ‘‘Eduardo Torroja’’ C.S.I.C. Madrid, HCO 3 y Alcalinos con SO4 , Instituto de Ciencias de la Construccio PhD Thesis, 1993. [13] M.C. Alonso, C. Andrade, R.P.M. Procter, M. Saenz de Santa Marı´a, Susceptibilidad a la Corrosio´n Bajo Tensio´n del Acero Pretensado en Disoluciones de NaHCO3, Hormigo´n y Acero (166) (1988) 121–126. [14] A.M. Lancha, Influencia del Trefilado en la Corrosio´n Bajo Tensio´n de Aceros Eutectoides, Universidad Complutense de Madrid, Facultad de Ciencias Quı´micas, 1987. [15] A. Valiente, M. Elices, Premature failure of prestressed steel bars, Eng. Fail. Anal. 5 (3) (1998) 219–227. [16] F.J. Ayaso, J. Toribio, Anisotropic Fracture Behabiour of Progressively Drawn Pearlitic Steels, Anales de Meca´nica de la Fractura 20 (2003) 261–266.