A comparison of the fatigue behavior between S355 and S690 steel grades

e ur -Mo Mild steel tee ngt lica ents des rol mooth small size specimens allow the assessment of the fatigue crack initiation sults show that the S690 steel grade presents a higher resistance to fatigue crack s the d ural pe oice of application. In general, the use of high stren strength steels [1]. High strength steels are gaining competitiveness weldability and improved weathering ability [6]. Journal of Constructional Steel Research 79 (2012) 140–150 Contents lists available at SciVerse ScienceDirect Journal of Constructio with respect to the mild structural steels. The application of high strength steels on steel bridges is becom- ing attractive. According to Miki et al. [2] and Jensen and Bloomstine [3], the number of new bridges made of high strength steels is in- creasing signiﬁcantly in the last decades. New applications of high strength steels are also being considered, such as windmill tower pro- duction [4]. The use of high strength steels allows the construction of taller windmill towers with simple and cost effective joining systems for tower assembling, contributing to the increase of the competitive- ness of the wind energy generation. Fatigue resistance of the high strength and HPS is a major concern, since it is well known that fatigue resistance does not increase pro- portionally to the static strength of these steels. This is very often the case for welded components [7]. The fatigue resistance of welded joints made of high strength steels may be even lower than for welded joints made of mild steels [2]. Nevertheless, S–N curves pro- posed in design codes (e.g. Eurocode 3 [8]) do not show dependency on material, which implies signiﬁcant safety margins. In general, high strength steels and HPS are still less investigated than construction mild steels, leading to a deﬁcient understanding of the fatigue behav- Despite the important advantages of the provided by the high strength steel grades, ⁎ Corresponding author at: School of Sciences an Trás-os-Montes and Alto Douro, Quinta de Prados, 500 +351 259 350 306; fax: +351 259 350 356. E-mail address: [email protected] (A.M.P. de Jesus). 0143-974X/$ – see front matter © 2012 Elsevier Ltd. Al http://dx.doi.org/10.1016/j.jcsr.2012.07.021 gth steels contributes to higher cost of the high ticular group of high strength steels is the high performance steels (HPS) that combine high strength with enhanced ductility, toughness, weight reduction which compensates the 1. Introduction The use of high strength steels allow and simpler structures with high struct factors are decisive concerning the ch grade, which justiﬁes an inverse dependence between static strength and fatigue life, for applications where fatigue crack propagation is the governing phenomenon. Consequently, the design of structural details with the S690 steel should avoid sharp notches that signiﬁcantly reduce the fatigue crack initiation process. © 2012 Elsevier Ltd. All rights reserved. esign of lighter, slenderer rformance. The economic the steel for a structural faces important challenges. The weldability of the high strength steels is lower than theweldability of mild steels, and decreaseswith strength increasing [5]. The carbon and alloy element contents are, therefore, limited to ensure weldability. Ductility, toughness and corrosion resis- tance are also desired characteristics for the high strength steels. A par- S690 steel grade S355 steel grade High strength steel initiation than the S355 steel. However, the resistance to fatigue crack propagation is lower for the S690 steel Crack propagation ization, the fatigue tests of s behavior of the materials. Re A comparison of the fatigue behavior betw Abílio M.P. de Jesus a,⁎, Rui Matos b, Bruno F.C. Fonto Luis Simões da Silva b, Milan Veljkovic c a IDMEC/Engineering Department, School of Sciences and Technology, University of Trás-os b ISISE/Civil Engineering Department, University of Coimbra, Coimbra, Portugal c Luleå University of Technology, Luleå, Sweden a b s t r a c ta r t i c l e i n f o Article history: Received 2 November 2011 Accepted 24 July 2012 Available online 30 August 2012 Keywords: Fatigue The use of higher strength s the increase of the yield stre tance, which makes the app the design. This paper pres S690 high strength steel gra performed under strain cont increased yield strength the use of these steels d Technology, University of 1-801 Vila Real, Portugal. Tel.: l rights reserved. en S355 and S690 steel grades a a, Carlos Rebelo b, ntes and Alto Douro, Vila Real, Portugal ls allows the design of lighter, slenderer and simpler structures. Nevertheless, h of the steels does not correspond to a proportional increase of fatigue resis- tion of high strength steels on structures prone to fatigue, a major concern of a comparison of the fatigue behavior between the S355 mild steel and the , supported by an experimental program of fatigue tests of smooth specimens, , and fatigue crack propagation tests. Besides the cyclic elastoplastic character- nal Steel Research ior of the high strength steels and HPS. However, this topic has been gaining much interest in the last decade [9,10]. Many fatigue studies are focused on testing structural details rather than investigating the plain material, which limits the extent of the ﬁndings to the spe- ciﬁc geometries under investigation. The investigation on plain mate- rial allows the assessment of the basic fatigue properties of the materials, which are required to model the fatigue behavior of struc- tural components. Particularly, the assessment of the fatigue crack initiation and propagation behaviors is essential to model the fatigue behavior of structural components [9,10]. This paper provides experimental assessment of the fatigue prop- erties of two competing steel grades, namely the S355 mild steel and the S690 high strength steel, both speciﬁed in the EN 10025 standard [11]. Both fatigue crack initiation and fatigue crack propagation be- haviors are investigated. The fatigue crack initiation behavior is eval- uated through fatigue tests of smooth and small size specimens. The fatigue crack propagation behavior is characterized by means of fa- tigue tests of compact tension (CT) specimens, covering several stress Fig. 1. Typical yielding and strain hardening be A.M.P. de Jesus et al. / Journal of Constructio ratios. Fig. 2.Microstructures of the investigated structural steels: a) S355 steel grade; b) S690 steel grade. 2. Overview of current approaches to fatigue The fatigue approaches may be classed into S–N, local and fracture mechanics based approaches. The S–N approach is the basis of current design codes such as the Eurocode 3, part 1-9 [8]. This is a global ap- proach that relates the stress range (e.g. nominal, structural or geomet- ric) applied to the component with the fatigue life. With respect to Eurocode 3, part 1-9, no distinction is made in procedures for welded and non-welded components. The procedures do not account for the material inﬂuence. Also, the classiﬁcation of complex details may be problematic. Local approaches to fatigue and fracture mechanics can be used as alternatives to the global S–N approaches, which requires the knowledge of the basic fatigue properties of the base materials. The local approaches, recognizing the localized nature of the fa- tigue damage, propose the correlation of a local damage parameter (e.g. strain, energy) with the number of cycles required to initiate a macroscopic crack. The most well-known relations in this area are the proposals by Basquin [12], Eq. (1), Cofﬁn [13] and Manson [14], Eq. (2) and Morrow [15], Eq. (3): Δσ ¼ σ ′ f 2Nf � �b ð1Þ haviors of the S355 and S690 steel grades. 141nal Steel Research 79 (2012) 140–150 2 ΔεP 2 ¼ ε′ f 2Nf � �c ð2Þ Δε 2 ¼ Δε E 2 þ Δε P 2 ¼ σ ′ f E 2Nf � �b þ ε′ f 2Nf � �c ð3Þ where σ ′f and b are, respectively, the fatigue strength coefﬁcient and ex- ponent; ε′f and c are, respectively, the fatigue ductility coefﬁcient and ex- ponent; 2Nf is the number of reversals to failure; Δε, ΔεE and ΔεP are, respectively, the total, elastic and plastic strain ranges; Δσ is the stress range and E is the Young's modulus. The constants in these relations may be determined from fatigue tests of smooth specimens under strain-controlled conditions. These tests also allow the identiﬁcation of the cyclic curve of the material which relates the stress amplitude with the strain amplitude, corresponding to the stabilized behavior of thema- terial. This relation is usually expressedusing theRamberg–Osgood equa- tion [16]: Δε 2 ¼ Δε E 2 þ Δε P 2 ¼ Δσ 2E þ Δσ 2K ′ � �1=n′ ð4Þ Table 1 Comparison of the chemical composition between the S355 and S690 steels: EN 10025 standard recommendations [11] versus measured values (% weight). Steel grade C (%) Si (%) Mn (%) Cr (%) Cu (%) Mo (%) Ni (%) V (%) Nb (%) Ti (%) Al (%) P (%) S (%) EN 10025 standard S355 0.2 max 0.5 max 0.9–1.65 max 0.3 max 0.55 max 0.10 max 0.5 max 0.12 max 0.05 max 0.05 max 0.02 min 0.030 max 0.025 max S690 0.2 max 0.8 max 1.7 max 1.5 max 0.50 max 0.70 max 2.0 max 0.12 max 0.06 max 0.05 max 0.015 min 0.025 max 0.015 max 0.095 0.003 – – – 0.022 0.041 0.036 – 0.042 0.086 0.036 0.009 0.005 pec 142 A.M.P. de Jesus et al. / Journal of Constructional Steel Research 79 (2012) 140–150 where K′ and n′ are, respectively, the strain hardening coefﬁcient and ex- ponent. The cyclic curve is required to performan elastoplastic analysis of components, either using a ﬁnite element formulation or using a simpli- ﬁed approach as proposed by Neuber [17]: Δε⋅Δσ ¼ Δσ 2 nom⋅Kt2 E ð5Þ whereΔσnom is the nominal stress range,Δε andΔσ are, respectively, the local strain and stress ranges and Kt is the elastic stress concentration factor. The strain–life Eq. (3) is a general relation that does not account for mean stress effects. In order to account for mean stress effects, Smith, Watson and Topper [18] proposed the following alternative: σ max Δε 2 E ¼ σ ′f � �2 2Nf � �2b þ σ ′f ε′ f E 2Nf � �bþc ð6Þ where σmax is the maximum stress of the cycle and the other nomen- clatures are the same as Eq. (3). Fracture mechanics may be also used as an alternative approach to fatigue, based on the fatigue crack propagation phenomena. This ap- Measured values S355 0.10 0.15 0.64 0.076 0.38 0.014 S690 0.077 0.048 1.35 0.025 – – Fig. 3. Geometry of the smooth s proach may be used to complement the local approaches to fatigue [9,10] allowing the residual life computation of a structural compo- nent with an initial crack. This approach is based on crack propaga- tion laws, with Paris' law [19] being the most used: da=dN ¼ C ΔKð Þm ð7Þ where C and m are material constants, da/dN is the fatigue crack growth rate and ΔK is the stress intensity factor range. The number of cycles to failure is computed by integrating the crack propagation law between an initial crack size (ai) and a critical crack size (af): Nf ¼ ∫ af ai da C ΔKð Þm: ð8Þ Table 2 Nominal dimensions of the smooth specimens. Material W (mm) T (mm) L (mm) L1 (mm) H (mm) R (mm) S355 30 7.5 26 200 12.5 8 S690 16 4 13 110 8 4.5 3. Materials and experimental details 3.1. Basic materials' descriptions A comparison of the fatigue properties between an S355 mild steel and an S690 high strength steel is proposed in this research. These steel grades are speciﬁed according to the EN 10025 standard [11]. Min- imum yield stresses of 355 and 690 MPa are speciﬁed, respectively, for the S355 and S690 steel grades, for thicknesses below 16 mm. The S355 steel grade should exhibit a tensile strength within the range of 470 and 630 MPa and the S690 steel grade should present a tensile strength between 770 and 940 MPa, also for thicknesses below 16 mm. In order to verify the actual static strength properties of the two steel grades used in the experimental program, quasi-static monotonic tensile tests were performed, covering both steel grades. Average yield stresses of 419 MPa and 765.7 MPaweremeasured, respectively for the S355 and S690 steel grades. Average tensile strengths of 732 MPa and 823 MPa were obtained, respectively, for the S355 and S690 steel grades. In gener- al, these strength properties agree with the limits speciﬁed in the EN 10025 standard. However, the sample of the S355 steel grade used in this research exhibited a tensile strength above the range speciﬁed in imens used in the fatigue tests. the standard. Nevertheless, the trend of the monotonic stress–strain curves, the chemical composition and the material microstructure are typical of the S355 steel grade as will be veriﬁed hereafter. The tensile tests were instrumented with strain gauges which allowed the assess- ment of the Young's modulus. Average values of 210.5 and 209.4 GPa were measured, respectively, for the S355 and S690 steel grades. Fig. 1 shows typical monotonic stress–strain curves that were obtained for Table 3 Summary of the fatigue tests of smooth specimens. Specimens (S355) S (mm2) f (Hz) Δε (%) Specimens (S690) S (mm2) f (Hz) Δε (%) S355_100_01 55.94 0.400 1.00 S690_200_01 18.35 0.200 2.00 S355_050_01 56.08 0.800 0.50 S690_100_01 18.23 0.400 1.00 S355_200_01 60.16 0.200 2.00 S690_050_01 17.80 1.000 0.50 S355_040_01 57.65 1.000 0.40 S690_050_02 18.63 1.000 0.50 S355_030_01 57.86 1.333 0.30 S690_100_02 18.51 0.400 1.00 S355_035_01 59.48 1.143 0.35 S690_200_02 17.92 0.200 2.00 S355_030_02 61.06 1.333 0.30 S690_040_01 17.98 5.000 0.40 S355_040_02 60.75 1.000 0.40 S690_040_02 18.25 5.000 0.40 S355_100_02 56.61 0.400 1.00 S690_800_01 17.93 15.000 0.36 S355_200_02 57.75 0.200 2.00 S690_840_01 18.14 15.000 0.38 S690_150_01 17.93 1.000 1.50 of th 143A.M.P. de Jesus et al. / Journal of Constructional Steel Research 79 (2012) 140–150 the S355 and S690 steel grades. These curves are truncated since strains were measured using glued strain gauges that were not able to monitor the test until ﬁnal failure. Nevertheless, the curves allow the comparison of the yield region of both steels aswell as the initial strain hardening be- havior. It is clear that the S355 steel shows a yield plateau, after which a very signiﬁcant strain hardening is veriﬁed. The S690 steel does not show that yield plateau, and a relatively small strain hardening is observed. The yield stresswas determined for the S355 steel as themaximumstress ob- served in the yield plateau. For the S690 steel, the yield stress was de- ﬁned as the stress corresponding to 0.2% permanent strain. According to EN 10025, the S690 steel shows aminimumelongation after a fracture Fig. 4. Geometry of 14%; the S355 steel shows an elongation after a fracture of 22%. Themicrostructures of both steel gradeswere observed using an op- tical microscope. Fig. 2 compares the microstructures of both steel grades. It is very clear that the microstructure of the S690 steel grade is signiﬁcantly more reﬁned than the microstructure of the S355 steel, which has a signiﬁcant impact on mechanical properties, including fa- tigue properties as will be discussed later. The S690 steel is obtained by thermomechanical rolling and supplied in quenched and tempered condition. The S355 steel shows a microstructure of ferrite and perlite which is typical of non-alloyed structural mild steels. Both steels are weldable steels; however the weldability of the high strength steel is in general poorer than the weldability of mid steel due to the higher level of alloy elements. Table 1 presents a comparison of the typical chemical composition for both steels, according to EN 10025 [11]. The table also presents the actual chemical composition measured on steel samples using the spark emission spectrometry. The chemical composi- tions are in general according to the standard recommendations. It is clear that there is a signiﬁcantly higher amount of manganese on the S690 steel, contributing to the higher strength and hardenability of this steel with respect to the S355 steel. The S690 steel also includes Table 4 Nominal dimensions of the CT specimens. Material W (mm) B (mm) L (mm) H (mm) h (mm) D (mm) he (mm) an (mm) α (°) S355 50 8 62.5 60 27.5 12.5 3 10 60 S690 40 5 50 48 22 10 1.6 8 60 grain-reﬁning elements such as aluminum, niobium and titanium. The phosphorus and sulfur contents are lower in the S690 steel grade, since these elements have adverse effects on ductility and toughness, especially on quenched and tempered steels. The hardness of both steel grades weremeasured using an INDENTEC hardness machine, resulting in the values of 27.4±3.9% and 33.8±4.2% HRC, respectively for the S355 and S690 steel grades. The S690 steel grade exhibits a Rockwell C hardness that is 23% higher than the S355 steel grade, which is consistent with the higher yield stress of the S690 steel grade. e CT specimens. 3.2. Experimental details This research aims at comparing the fatigue behavior between the S355mid steel and the S690 high strength steel, based on experimental results from fatigue tests of smooth specimens and fatigue crack propa- gation tests. The fatigue tests of smooth specimens were carried out according to the ASTM E606 standard [20], under strain controlled con- ditions. The crack propagation tests were performed using CT speci- mens, in accordance with the procedures of the ASTM E647 standard [21], under load controlled conditions. Fig. 3 shows the general geometry of the smooth specimens and Table 2 indicates the dimensions adopted for each steel grade. Distinct sizes of specimenswere considered for each steel grade, since specimens were extracted from plates with different thicknesses. Nevertheless both geometries are in accordancewith the ASTME606 recommendation [20]. The gauge length of the specimens was polished with an appropriate se- quence of sandpapers. One series of 10 specimens were prepared for each material. The strain was controlled using a dynamic clip gauge, model INSTRON 2620-202, with a range of ±2.5 mm. Specimens of the S355 steel were instrumented with a reference gauge length of 25 mm; specimens of the S690 steel were instrumented with a reference gauge length of 12.5 mm. The fatigue tests of the smooth specimens were conducted for a strain ratio, Rε, equal to −1, following a sinusoidal waveform with a frequency adjusted to result in an average strain rate of 0.8%/s−1. Table 3 summarizes the parameters adopted in each test, for both steel grades. The table includes the actual section size of each specimen, S, the frequency of the tests, f, and the applied strain range, Δε. Exceptionally, specimens S690_800_01 and S690_840_01 were tested under stress control in the elastic regime. For these two soon as the crack achieved high crack growth rates (approximately 0.3 mm/1000 cycles). the elastic and plastic strain components. These hysteresis loops were deﬁned for half-life. Table 5 Summary of fatigue crack propagation tests. Specimens (S355) B (mm) Fmax (N) Fmin (N) Rσ Specimens (S690) B (mm) Fmax (N) Fmin (N) Rσ S355_00_01 7.79 5764 58 0.0 S690_00_01 4.36 3292.8 32.9 0.0 S355_00_03 7.81 6118.6 61.8 0.0 S690_00_02 4.37 3089.9 30.9 0.0 S355_25_01 7.47 7246.2 1811.5 0.25 S690_25_01 4.36 3842.5 960.6 0.25 S355_25_02 7.37 7288.3 1822.1 0.25 S690_25_02 4.36 3575.4 893.9 0.25 S355_50_01 7.52 9872.4 4936.2 0.50 S690_50_01 4.36 4967.2 2483.6 0.5 S355_50_02 7.41 9345.9 4672.9 0.50 S690_50_02 4.34 4524.6 2262.3 0.5 S355_75_01 7.80 19938.7 14954.0 0.75 S690_75_01 4.36 7636.7 5727.5 0.75 S690_75_02 4.37 6862.5 5146.9 0.75 144 A.M.P. de Jesus et al. / Journal of Constructional Steel Research 79 (2012) 140–150 Both types of fatigue tests (smooth specimens and crack propaga- tion tests) were performed in an INSTRON 8801 servohydraulic ma- chine, rated to 100 kN, at room temperature and in air. 4. Results and discussion The results of the experimental work are presented and discussed in this section. In particular, using the results of the fatigue tests on smooth specimens, a comparison between the mid and high strength steels is performed taking into account their cyclic elastoplastic and strain–life behaviors. Also, the fatigue crack growth behaviors are compared for several stress ratios, taking into account data from the fatigue crack propagation tests. Finally, comparisons with data available in the litera- ture are performed. Tables 6 and 7 summarize the results of the fatigue tests carried out with smooth specimens, under strain controlled conditions, re- spectively for the S355 and S690 steel grades. These tables include the controlled strain range and the resulting number of cycles to fail- ure, Nf, for each specimen. Also, the parameters of the stabilized cyclic stress–strain hysteresis loops are shown namely, the stress range and specimens, the testing frequency was signiﬁcantly increased to result in an appropriate testing time. Specimen S690_150_01 was tested pre- viously as specimen S690_800-01 but, since it did not fail, the specimen was re-tested under a high strain range. Fig. 4 illustrates the geometry of the CT specimens adopted in the crack propagation tests. The nominal dimensions of the specimens are summarized in Table 4. Distinct specimen dimensions were adopted for each material, due to the reasons mentioned before. The crack propaga- tion tests were carried out covering four stress ratios, namely Rσ=0.0, Rσ=0.25, Rσ=0.5 and Rσ=0.75. Two specimens were tested per stress ratio, with one exception: only one specimen made of S355 steel was tested under Rσ=0.75. Table 5 summarizes the experimental program of fatigue crack propagation tests. During tests, cracks were measured on both side faces of the CT specimens, by direct observation through a magniﬁcation system (resolution of 1 μm). The crack propagation tests were performed under a frequency of 20 Hz, which was reduced as Table 6 Fatigue test results obtained using smooth specimens of the S355 steel (Rε=–1). Specimens (S355) Δε (%) ΔεP (%) ΔεE (%) Δσ (MPa) Nf (cycles) S355_100_01 1.00 0.570 0.429 817.39 4805 S355_050_01 0.50 0.219 0.281 569.54 16,175 S355_200_01 2.00 1.443 0.557 975.47 336 S355_040_01 0.40 0.093 0.307 615.40 29,501 S355_030_01 0.30 0.021 0.279 536.34 861,304 S355_035_01 0.35 0.051 0.299 581.51 278,243 S355_030_02 0.30 0.003 0.297 646.56 191,940 S355_040_02 0.40 0.072 0.328 661.21 64,244 S355_100_02 1.00 0.637 0.363 663.57 2009 S355_200_02 2.00 1.427 0.573 968.21 542 4.1. Cyclic elastoplastic behavior Fig. 5 presents the stabilized cyclic stress–strain hysteresis loops obtained for both steel grades. It is clear that the S355 steel shows a higher scatter than observed in the S690 steel. The scatter increases in the S355 steel with the decrease in the amount of cyclic plasticity. The hysteresis loops presented in Fig. 5 are superimposed in Fig. 6, making their lower tips coincident with the origin of the graph. This alternative representation of the hysteresis loops allows the assessment of the Masing behavior [22]. The Masing behavior is observed if the upper branches of the hysteresis loops are all coincident. For a material obey- ing the Masing behavior, the relationship between the cyclic stress and elastoplastic strain ranges and the shape of the hysteresis loops may be both described by the cyclic curve of the material. Both steels show some degree of deviation from the Masing behavior. However, the S690 steel may be considered a quasi Masing material, since the small deviations observed in the upper branches of the hysteresis loops may be attributed to scatter in the material, rather than a phenomenological characteristic. In the case of the S355 steel, the deviations from the Masing behavior are signiﬁcant, this material being considered a non- Masing material. The hysteresis loops presented in Figs. 5 and 6 were determined using a half-life criterion. This criterionmay coincidewith a cyclic stabi- lized behavior criterion, for those tests that showed stabilization. How- ever, some tests did not show stabilization and, in those cases, the half-life criterion corresponds to a pseudo-stabilized behavior. Fig. 7 shows the evolution of the stress amplitudes with the number of cycles and applied strain range. It is clear that for some applied strain ranges, no stabilization of the cyclic behavior is observed, mainly for the S355 steel. The S690 steel shows a quasi-stabilized cyclic behavior just after the ﬁrst cycles. Only small amounts of cyclic hardening is observed for high strain ranges (Δε>1%). Using the stabilized cyclic stress–strain hysteresis loops (see Fig. 5), the elastic andplastic strain rangeswere computed and results are listed in Tables 6 and 7. The plastic strain amplitude was evaluated as the width of the hysteresis loops measured over the axis, σ=0. The elastic Table 7 Fatigue test results obtained using smooth specimens of the S690 steel (Rε=–1). Specimens (S690) Δε (%) ΔεP (%) ΔεE (%) Δσ (MPa) Nf (cycles) S690_200_01 2.00 1.199 0.800 1555.20 190 S690_100_01 1.00 0.295 0.707 1401.88 1272 S690_050_01 0.50 0.015 0.484 1105.02 60,505 S690_050_02 0.50 0.010 0.490 1062.68 44,819 S690_100_02 1.00 0.307 0.692 1435.58 1920 S690_200_02 2.00 1.194 0.807 1590.57 160 S690_040_01 0.40 0.001 0.399 867.39 131,000 S690_040_02 0.40 0.003 0.396 879.30 371,000 S690_800_01 0.36 4.72E−04 0.360 797.95 3,807,939→ S690_840_01 0.38 1.48E−03 0.382 835.56 1,545,579 S690_150_01 1.50 0.760 0.739 1565.25 410 Fig. 5. Stabilized stress–strain hysteresis loops: a) S355 steel; b) S690 steel. Fig. 6. Superposition of the hysteresis loops with the lower tip at the origin: a) S355 steel; b) S690 steel. 145A.M.P. de Jesus et al. / Journal of Constructional Steel Research 79 (2012) 140–150 strain amplitudewas evaluated from the total strain decomposition into elastic and plastic components. The stabilized hysteresis loops were also used to determine the stress range. The relation between the plastic strain amplitude and the stress range deﬁnes the cyclic curve of thema- terial. Fig. 8 compares the cyclic curves of the S355 and S690 steel grades. The cyclic curve of the S690 steel is determined with a high de- termination coefﬁcient; the determination coefﬁcient of the cyclic curve of the S355 steel is relatively low, which is consistent with the scatter in the hysteresis loops observed for this material. The comparison of the two cyclic curves shows a signiﬁcantly higher cyclic strain hardening (cyclic strain hardening coefﬁcient) of the S690 steel. Concerning the slopes of the cyclic curves, they are essentially parallel, which means very similar slopes (cyclic strain hardening exponent). Fig. 7. Evolution of the stress amplitude with the number of cycles and applied strain range: a) S355 steel; b) S690 steel. Fig. 8. Comparison of the cyclic curves between the S355 and S690 steel grades. 4.2. Strain–life behavior Table 8 summarizes the cyclic constants and the parameters of the Morrow's relation for the two steel grades under investigation — the S355 and S690 steel grades. The Morrow's constants resulted from the individual ﬁtting of the Basquin [12] and Cofﬁn–Manson [13,14] Table 8 Summary of cyclic elastoplastic and fatigue properties. Material K′ (MPa) n′ σ′f (MPa) b ε′f c 2NT S355 595.85 0.0757 952.2 −0.089 0.7371 −0.664 7095 S690 1282.65 0.0921 1403 −0.087 0.7396 −0.809 675 S690 [7] – – 1191 −0.09 0.9113 −0.674 5809 HPS 485W [10] 956 0.113 851 −0.069 0.775 −0.701 3686 A7 [10] 1139 0.248 760 −0.121 0.196 −0.486 50,119 Fig. 10. S–N curve prediction for generic structural components. 146 A.M.P. de Jesus et al. / Journal of Constructional Steel Research 79 (2012) 140–150 Fig. 9. Comparison of the strain–life data between the S355 and S690 steel grades: a) elastic strain–life data; b) plastic strain–life data; c) total strain–life data. relations. The analysis of the results shows that the number of transi- tion reversals (2NT) is very distinct between the two steels. The S690 steel shows a very small number of transition reversals, which means that for fatigue lives above 337 cycles the fatigue behavior of the steel is governed by fatigue strength properties rather than fatigue ductil- ity properties. As a consequence, plastic deformation is more fatigue damaging for the S690 steel than for the S355 steel. Fig. 9 compares the effects of the elastic, plastic and total strains on fatigue lives, be- tween the S355 and S690 steels. Fig. 9a) illustrates the higher fatigue resistance of the S690 steel; Fig. 9b) shows the higher fatigue ductility of the S355 steel; ﬁnally, Fig. 9c) shows an improved fatigue behavior of the S690 steel only for total strain amplitudes below 3.27E−3 (lives above 6720 cycles). The extrapolation of the comparisons based on strain–life data to the fatigue behavior of structural components, in particular with re- spect to the S–N curves of those components is not straightforward. However, adopting a simpliﬁed approach based on Neuber's analysis [17] supported by the Ramberg–Osgood description of the cyclic curve of the material [16], it is possible to derive S–N curves, for ge- neric structural components, characterized by an elastic stress con- centration factor, Kt: Δσ2 E þ 2Δσ Δσ 2K ′ � �1=n′ ¼ K 2 t Δσ 2 nom E Δε 2 ¼ Δσ 2E þ Δσ 2K ′ � �1=n′ : ð9Þ For a given nominal stress range applied to a structural component characterized by an elastic stress concentration factor, Kt, onemay com- pute the total strain range, Δε, at the notch root. Using this total strain range, the cycles to failure are computed using the strain–life relations of the materials. Fig. 10 shows the resulting S–N curves generated for generic structural components, considering both steel grades under in- vestigation. The signiﬁcant advantage of the S690 high strength steel Fig. 11. Comparison of the Smith–Topper–Watson relations between the S355 and S690 steel grades. the stress intensity factor range, ΔK. The crack growth rates were Fig. 12. Comparison of strain–life relations between several construction steels. Fig. 13. Fatigue crack propagation rates obtained for the S355 steel: analysis of stress ratio effects. 147A.M.P. de Jesus et al. / Journal of Constructional Steel Research 79 (2012) 140–150 over the mild steel is clear, in terms of fatigue performance, for a wide range of applied nominal stress ranges and elastic concentration factors. However, the beneﬁt of using the S690 is higher for lower elastic stress concentration factors, which means that components made of S690 should present smoother notches. This result is justiﬁed by the signiﬁ- cantly higher yield stress of the S690 steel, with respect to the S355 steel, which reduces the plastic deformation on components made of this steel. The strain–life approach is usually applied to assess the crack initiation life for structural details. Therefore, the beneﬁcial effect of the S690 steel grade will be limited to structural components with dominating fatigue crack initiation. Welded joints made of S690 steel may not show any advantage over welded joints made of S355 steel since crack initiation may have a marginal impact on the total fatigue life. TheMorrow's equation plotted in Fig. 9c) does not account formean stress effects. One alternative is the relation proposed by Smith,Watson and Topper [18] for positive maximum stresses. This model was assessed using the data available for the two steels. Fig. 11 compares the Smith–Watson–Topper relationwith the experimental data. A satis- factory agreement is veriﬁed between the model and the available ex- perimental data for both steel grades. In this comparison, the high strength steel shows better performance for fatigue lives above, approx- imately, 500 cycles. Table 8 also presents the constants proposed in Ref. [7], for the S690 steel grade. Also, data concerning two structural steel grades speciﬁed in ASTM standards are presented, from Ref. [10], including one high performance steel — the HPS 485W steel (ASTM A709 [23]) and one carbon structural steel (relatively low yield strength) as speciﬁed in the former ASTM A7 standard (replaced by ASTM A36 standard [24]). Fig. 12 illustrates the strain–life relations generat- ed using the Morrow's equation with constants from Table 8, allowing a more precise comparison between materials. Table 9 Constants of Paris' law for the S355 and S690 steel grades. Material Rσ Ca m R2 S355 0.0 2.5893E−15 3.5622 0.9716 0.25 2.5491E−15 3.7159 0.9841 0.50 8.2764E−16 3.8907 0.9810 0.75 4.9643E−14 3.2328 0.9504 0.25+0.50+0.75 2.1111E−15 3.7447 0.9872 S690 0.0 6.8261E−13 2.8789 0.9713 0.25 2.3196E−12 2.7592 0.9549 0.50 2.9529E−14 3.4517 0.9703 0.75 1.2956E−15 3.9595 0.9902 0.25+0.50+0.75 2.2607E−13 3.1255 0.9533 HPS 485W [10] 0 6.39E−14 3.12 – 0.5 1.07E−13 3.14 – a da/dN in mm/cycle and ΔK in N.mm−1.5. The comparison of the results shows a discrepancy between the properties proposed in Ref. [7] for the S690 steel and the properties that resulted from this investigation. The discrepancy is more marked for the fatigue ductility properties. Nevertheless, properties proposed in Ref. [7] did not result from an experimental program on S690 steel. Instead, the proposed fatigue properties were extrapolated from other materials with similar monotonic strength properties, reducing the conﬁdence on those properties. Excluding the data from Ref. [7], an analysis of the number of transi- tion reversals, 2NT, shows an inverse dependency between the yield strength and the number of transition reversals. The A7 steel which is characterized by a minimum yield strength of about 250 MPa, shows a very high number of transition reversals, meaning a fatigue behavior dominated by ductility properties for a wider fatigue domain than other steels. A comparison of the fatigue behavior between the S355, HPS 495W and S690 steel grades, shows a remarkable trend. The strain–life curves intersect each other at about 1.4×104 reversals (7×103 cycles) (see Fig. 12). For lives above this intersection point, the fatigue resistance in- creases with the yield strength of the material; inversely, for lives bel- low the intersection point the fatigue performance increases with decreasing yield strength of the materials. It is worth noting that the HPS 495W steel exhibits a minimum yield strength of 495 MPa which represents an intermediate strength between the S555 and S690 steels. 4.3. Fatigue crack propagation rates The results of the fatigue crack propagation tests are presented in this section. The crack growth rates, da/dN, are plotted as a function of Fig. 14. Fatigue crack propagation rates obtained for the S690 steel: analysis of stress ratio effects. 148 A.M.P. de Jesus et al. / Journal of Constructional Steel Research 79 (2012) 140–150 computed using the seven point incremental polynomial technique, as proposed in the ASTM E647 standard [21]. The stress intensity fac- tor ranges were computed using the formulation proposed in ASTM E647 for the CT specimens [21]: ΔK ¼ ΔF B ﬃﬃﬃﬃﬃﬃ W p 2þ αð Þ 1−αð Þ3=2 0:886þ 4:64α−13:32α 2 þ 14:72α3−5:6α4 � � ð10Þ where: α=a /W, a is the crack size, B is the thickness of the specimen, W is the width of the specimen and ΔF is the applied load range. The experimental crack sizes are computed as the average value of the two measurements performed on both faces of the CT specimens. The experimental crack propagation data was correlated with Paris' law, since only the crack propagation regime II was covered by the present experimental research. Table 9 summarizes the constants of Paris' law for both steel grades and for combinations of several stress ratios. The table also in- cludes the determination coefﬁcients resulting from the adjustment of Paris' law to the experimental data. All determination coefﬁcients are above 0.95, which represents very high correlations. Fig. 13 illustrates the effects of the stress ratio on fatigue crack prop- agation rates, for the S355 steel grade. An increase in fatigue crack prop- agation rates is clear, when the stress ratio changes from 0 to any positive stress ratios considered in the experimental program. Also, it is clear that all the positive stress ratios resulted in similar crack propa- gation rates. This behavior is consistent with a crack closure effect that Fig. 15. Comparison of fracture surfaces for the compact tension specimens at the re- gion of stable propagation (Rσ=0.0): a) S355 steel; b) S690 steel. occurs between Rσ=0.0 and Rσ=0.25. For Rσ=0.0 there is some crack closure, the applied stress intensity factor range being not fully ef- fective. For Rσ=0.25 and higher, there is no crack closure, the applied stress intensity factor range being fully effective. Fig. 14 shows the effects of the stress ratios on fatigue crack growth rates for the S690 steel grade. Similar to the S355 steel, the S690 steel shows distinct behaviors between Rσ=0.0 and the other tested positive stress ratios. No signiﬁcant differences are found on crack growth rates for Rσ=0.25, Rσ=0.5 and Rσ=0.75. This behavior is consistent with the crack closure phenomena. Nevertheless, the crack closure effect is less accentuated thanobserved for the S355 steel. One possible explana- tion is the fact that the crack closure may be induced by the roughness of crack surfaces, which facilitates the crack closure before the load reaches the null value. This roughness is closely related to the material grain sizes, which is higher for the S355 steel. Effectively, Fig. 15 com- pares the fracture surfaces obtained for the CT specimens, under stable crack propagation, for Rσ=0.0. It is clear that the fracture surface of the S355 steel shows a higher roughness than that observed for the S690 steel. Therefore, the fatigue crack growth rates of the S355 steel are much more inﬂuenced by the crack closure effects than expected for the S690 steel. Fig. 16 compares the fatigue crack propagation rates between the two steel grades under investigation. The comparison is performed in- dividually for each stress ratio. It is clear that the S690 steel shows the highest fatigue crack growth rates for all tested stress ratios. This result may be justiﬁed by the ﬁner grain of the S690 steel which facilitates the fatigue crack propagation. Fig. 17 compares all crack growth data gener- ated for the S355 and S690 steels. Besides the higher crack growth rates observed for the S690 steel, it is clear that the crack growth rates of the S690 steel, for Rσ=0.0, are similar to the crack growth rates of the S355 steel for Rσ=0.25 and higher. The crack propagation constants for the HPS 485W steel are also in- cluded in Table 9, which were determined by Chen et al. [10]. Fig. 18 compares the fatigue crack propagation rates between the S355, HPS 485W and S690 steels, for a wide range of stress intensity factors and for two stress ratios, namely Rσ=0 and Rσ=0.5. It is interesting to note that fatigue crack propagation rates increase with the yield strength of steels, for both stress ratios. In order to allow a further comparison between the two steels, three notched details were considered with distinct stress concentrations (Kt=1.90, 3.26, 4.52) and the same thicknesses of the respective tested CT specimens. These details were subjected to a remote uniform stress with sinusoidal shape and Rσ=0. Using available crack propagation data for Rσ=0, S–N curves were generated for the details. Paris' law was integrated assuming an initial crack size of 0.5 mm (see Eq. (8)). The required stress intensity factors were evaluated using ﬁnite ele- ment analysis and the J-integral method. In the three details, the same resisting section length was considered (95 mm). Fig. 19a) compares the resulting S–N curves that take into consideration the crack propaga- tion only. Unstable crack propagation was accounted considering the maximum stress intensity factor registered in crack propagation tests for each material (see Fig. 16a). The lower crack growth rates of the S355 steel leads to higher fatigue lives for medium to low stress ranges. However, for high stress ranges, a reverse condition due to the lower toughness of the S355 with respect to the S690 steel is observed. Fig. 19b) presents the simulations of the S–N curves considering both crack initiation and crack propagation phases. The crack initiation was computed using the strain–life approach presented in the previous sec- tion and was assumed a crack initiation criterion of a 0.5 mm deep crack. The resulting global S–N curves show the same trend observed in Fig. 10 for the crack initiation curves — the notched details made of the S690 steel show a higher fatigue resistance than the details made of the S355 steel. This result means that crack initiation is dominant for details under consideration. Crack propagation signiﬁcance is limit- ed for the notched details under consideration, but its importance in- creases if larger member sections are assumed. 149A.M.P. de Jesus et al. / Journal of Constructional Steel Research 79 (2012) 140–150 5. Conclusions The fatigue behavior of the S355 mild steel and S690 high strength steel grades were evaluated by means of an experimental program which included fatigue tests of smooth specimens as well as fatigue crack propagation tests. The analysis of the results leads to the following conclusions: – The fatigue tests on smooth specimens showed that the S690 high strength steel grade exhibits a lower fatigue resistance than the S355 steel, for strain amplitudes higher than 0.33% or fatigue lives bellow 6720 cycles, which represents the low cycle fatigue regime. – In the high cycle fatigue regime, the S690 steel shows a higher fa- tigue resistance than the S355 steel. This superior fatigue resistance, Fig. 16. Comparison of the crack growth rates between the S355 and S69 Fig. 17. Comparison of all crack propagation data obtained for the S355 and S690 steel grades. based on smooth specimen test data, corresponds to a higher resis- tance to fatigue crack initiation. – Concerning the fatigue crack propagation rates, the S690 steel shows systematically higher propagation rates than the S355 steel, for any tested stress ratio. This implies that there is no advantage of using the S690 steel for structural components whose fatigue life is dom- inated by crack propagation, rather than crack initiation. Neverthe- less, the fracture toughness of the S690 steel was demonstrated to be superior to that of the S355 steel (for the tested thicknesses) which beneﬁciate the crack propagation resistance of the S690 steel for high stress levels. – Both S355 and S690 structural steels showed crack propagation rates clearly affected by the crack closure, the S355 steel being more sen- sitive to this phenomenon due to the higher grain size which leads to higher roughness on fracture surfaces. – The fatigue properties of the S690 steel were assessed with a lower 0 steel grades: a) Rσ=0.0; b) Rσ=0.25; c) Rσ=0.50; d) Rσ=0.75. Fig. 18. Comparison of fatigue crack propagation data between three structural steels with distinct strength properties. As a concluding remark, the design of structural details in high strength steels should take advantage of the superior resistance of these steels to fatigue crack initiation. The utilization of high strength steels increases the fatigue sensitivity of the structural detail to sharp notches reducing the fatigue life with respect to details made of mild steel. References [1] Sperle J-O. High strength sheet steels for optimum structural performance. Con- ference on Iron and Steel — Today, Yesterday and Tomorrow, Proc. 250th Anni- versary of The Swedish Ironmasters Association; 1997. [2] Miki C, Homma K, Tominaga T. High strength and high performance steels and 150 A.M.P. de Jesus et al. / Journal of Constructional Steel Research 79 (2012) 140–150 scatter than resulted for the S355 steel, which means higher quality of the high strength steel with respect to the mild steel. – Despite the fact that strain–life data have shown a better fatigue per- formance of the S355 steel for low-cycle fatigue regimes, this result may not be extrapolated directly to structural components under load/stress control due to the superior yield strength of the S690 Fig. 19. S–N curve prediction for three generic notched components: a) accounting fa- tigue crack propagation; b) accounting fatigue crack initiation and propagation. steel. It was shown that the S690 steel, due to its superior yield strength, shows a higher resistance to fatigue crack initiation for structural components under stress control, for awide range of stress concentration factors and applied stress ranges, covering both low and high cycle fatigue regimes. The superior fatigue crack initiation resistance of the S690 steel grade for structural details may not be relevant for welded joints, since fatigue life is often dominated by fa- tigue crack propagation. The comparison of the fatigue data from this study with the fatigue data published in literature for the HPS 485W steel, which is a steel grade with intermediate yield strength between the S355 mild steel and the S690 high strength steel, lead to the following conclusions: – The increase of the yield strength of the steel promotes the rotation of the strain–life curves around the fatigue life of about 7×103 cycles, increasing the fatigue resistance in the high cycle regime and decreas- ing the fatigue resistance in the low-cycle fatigue regime. – The fatigue crack propagation rates increase with the yield strength of the structural steels, independently of the stress ratio. their use in bridge structures. J Constr Steel Res 2002;58:3–20. [3] Jensen L, Bloomstine ML. Application of high strength steel in super long span modern suspension bridge design. Proceedings of the Nordic Steel Construction Conference (NSCC 2009), September 2–4, Malmö, Sweden; 2009. [4] Veljkovic M, Feldmann M, Naumes J, Pak D, Rebelo C, Simões da Silva L. Friction connection in tubular towers for a wind turbine. Stahlbau 2010;79:660-8. [5] Willms R. High strength steel for steel constructions. Proceedings of the Nordic Steel Construction Conference (NSCC 2009), Malmö, Sweden; 2009. p. 597-604. [6] Kayser CR, Swanson JA, Linzel DG. Characterization ofmaterial properties of HPS-485W (70W) TMCP for bridge girder applications. J Bridg Eng 2006;11(1):99–108. [7] Beretta S, Bernasconi A, Carboni M. Fatigue assessment of root failures in HSLA steel welded joints: a comparison among local approaches. Int J Fatigue 2009;31:102-10. [8] European Committee for Standardization (CEN). EN1993-1-9: Eurocode 3: design of steel structures, part 1‐9: fatigue. Brussels: European Standard; 2004. [9] Chen H, Grondin GY, Driver RG. Fatigue resistance of high performance steel. Structural engineering report no 258. Canada: University of Alberta; 2005. [10] Chen H, Grondin GY, Driver RG. Characterization of fatigue properties of ASTM A709 high performance steel. J Constr Steel Res 2007;63:838-48. [11] European Committee for Standardization (CEN). EN 10025: hot rolled products of structural steels. Brussels: European Standard; 2004. [12] Basquin OH. The exponential law of endurance tests. Proc Am Soc Test Mater 1910;10:625-30. [13] Cofﬁn LF. A study of the effects of the cyclic thermal stresses on a ductile metal. Trans ASME 1954;76:931-50. [14] Manson SS. “Behaviour of materials under conditions of thermal stress”, NACA TN-2933. USA: National Advisory Committee for Aeronautics; 1954. [15] Morrow JD. Cyclic plastic strain energy and fatigue of metals. Int. Friction, Damping and Cyclic Plasticity, ASTM, STP 378; 1965. p. 45-87. [16] Ramberg W, Osgood WR. “Description of stress–strain curves by three parame- ters”, NACA TN-902. USA: National Advisory Committee for Aeronautics; 1943. [17] Neuber H. Theory of stress concentration for shear-strained prismatic bodies with arbitrary nonlinear stress–strain law. J Appl Mech 1961;28:544-50. [18] Smith KN, Watson P, Topper TH. A stress–strain function for the fatigue of metals. J Mater 1970;5(4):767-78. [19] Paris PC, Gomez MP, Anderson WE. A rational analytic theory of fatigue. Trend Eng 1961;13(1):9–14. [20] American Society for Testing, Materials. ASTM E606: standard practice for strain controlled fatigue testing. Annual Book of ASTM Standards, Vol. 03.01; 1998. p. 557-71. West Conshohocken, PA. [21] American Society for Testing, Materials. ASTM E647: standard test method for measurement of fatigue crack growth rates. Annual Book of ASTM Standards, Vol. 03.01; 1999. p. 591-629. West Conshohocken, PA. [22] Abdel-Raouf HA, Plumtree A. Cyclic stress–strain response and substructure. Int J Fatigue 2001;23:799-805. [23] American Society for Testing, Materials. ASTM A709/A709M: standard speciﬁca- tion for carbon and high-strength low-alloy structural steel shapes, plates, and bars and quenched-and-tempered alloy structural steel plates for bridges. Annual Book of ASTM Standards, Vol. 01.04; 2004. West Conshohocken, PA, USA. [24] American Society for Testing, Materials. ASTM A36/A36M: standard speciﬁcation for carbon structural steel. Annual Book of ASTM Standards, Vol. 01.04; 2004. West Conshohocken, PA, USA. A comparison of the fatigue behavior between S355 and S690 steel grades 1. Introduction 2. Overview of current approaches to fatigue 3. Materials and experimental details 3.1. Basic materials' descriptions 3.2. Experimental details 4. Results and discussion 4.1. Cyclic elastoplastic behavior 4.2. Strain–life behavior 4.3. Fatigue crack propagation rates 5. Conclusions References

A comparison of the fatigue behavior between S355 and S690 steel grades

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