ADVANCED MATERIALS IN BRIDGES

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1 ADVANCED MATERIALS IN BRIDGES FATIGUE BEHAVIOUR OF MMFX CORROSION-RESISTANT REINFORCING STEEL Siebren J. DeJong and Colin MacDougall Department of Civil Engineering, Queen's University, Ontario, Canada Patrick J. Heffernan Department of Mechanical Engineering, Royal Military College of Canada, Ontario, Canada Abstract Corrosion-resistant MMFX steel used for reinforced concrete was investigated in this study to determine its fatigue characteristics in air under constant amplitude cyclic load. Standard fatigue specimens were machined from MMFX reinforcing bars and subjected to fully reversed (R = -1) loading in load and strain control. In addition, as-received MMFX bars were subjected to load controlled R = 0 constant amplitude loading. The MMFX smooth specimens had a stress range fatigue limit of 1150 MPa, which agrees with rule-of-thumb estimates. The strain-controlled tests were used to obtain the strainlife (ε-2n f ) curve and cyclic fatigue properties of MMFX. The strain-life properties were then incorporated into a Neuber analysis used to predict the fatigue performance of reinforcing bars by considering the effect of stress concentration factors. A strain-life prediction model incorporating a Neuber analysis to account for plasticity and stress concentration factors of 1.6 and 2.6 successfully bounded MMFX rebar stress-life data. A fatigue life of about 310 MPa at 1 x 10 6 fatigue cycles was observed for MMFX in constant amplitude loading. 1. Introduction Civil engineers have recognized that conventional carbon steel reinforcing bars are susceptible to corrosion when used in marine environments or in bridges exposed to deicing salts. In response, designers have used alternative reinforcement materials such as epoxy-coated reinforcing bars, fibre-reinforced polymers (FRP s), and chromium bearing steels such as stainless steel. Recently, a new steel called MMFX has been This paper is part of the 7th International Conference on Short and Medium Span Bridges, Montreal, Canada, AM-011-1

2 introduced to the market. MMFX steel is a proprietary chromium bearing steel with a predominantly martensitic microstructure combined with thin films of retained austenite [1], that combines high strength, ductility, and corrosion resistance. Bridge designers must ensure that fatigue limit states are met, generally limiting the stress range that a design vehicle will apply to the reinforcing steel. Some fatigue tests have been done on MMFX reinforcing bars [2], although the test program was stopped when gripping problems were encountered. The authors recommended testing machined specimens. Further testing of this corrosion resistant material is required in order to ensure its fatigue performance is at least as good as conventional grades of reinforcing steel, thus demonstrating that current code clauses dealing with fatigue limits can be applied safely to the new material. Much of our understanding of rebar fatigue is based on a number of studies on a wide variety of bar grades, sizes, and deformation patterns from several manufacturers [3; 4; 5]. Although there have been attempts to relate geometric details of rebars to stress-life (S-N) data [6; 3; 7], the strain-life (ε-2n f ) approach is better suited to deal with areas of high plasticity such as would occur near the notches found in rebars [8; 9]. In addition, the ε-2n f method overcomes the difficulties associated with testing rebars directly, as results apply to a variety of bar shapes and sizes and can account for the load sequence events found in variable amplitude loading. Recently, Heffernan et al. [9] was able to make reasonably accurate fatigue predictions for reinforcing bars and reinforced concrete beams using the strain-life (ε-2n f ) approach. The main feature of this paper is a strain-life analysis that can be applied to various surface conditions and bar geometries, improving the accuracy of fatigue life predictions at higher strain regimes. Predictions can be compared to current fatigue design curves for reinforcing bars to determine if this new material performs as well as or better than conventional steel in fatigue. 2. Experimental program For this investigation, smooth fatigue specimens as shown in Figure 1, were machined from MMFX reinforcing bars. The material had a chemical composition which is summarized in Table 1. Additional monotonic tension tests were performed on several smooth fatigue specimens, and the mechanical properties are listed in Table 2. All smooth specimens were mechanically polished to give a mirror polish. The first series of tests on smooth specimens was performed in load control according to ASTM E 466 [10] to generate a stress-life (S-N) curve, while the second series was straincontrolled according to ASTM E 606 [11] to generate a strain-life curve and material AM-011-2

3 fatigue life properties. A third series of tests was conducted on as-received MMFX reinforcing bars in load control to obtain the reinforcing bar s S-N curve. Figure 1: Fatigue Specimen (dimensions in mm). Table 1: Chemical composition (wt %) of MMFX. C Si Mn Cr Ni Mo P S N AM-011-3

4 Table 2: Mechanical properties of MMFX. Ultimate Strength (MPa) 1215 Yield Strength: 0.2 % Offset (MPa) 800 Monotonic Elongation (%) 40 E monotonic (MPa) E cyclic (MPa) Hardness: HRA (kgf) 65.6 Testing was done on servo-controlled electrohydraulic testing systems, using constant amplitude fully reversed (R = -1) loading spectra for the smooth specimens and tension only (R = 0) loading spectra for the rebar specimens. Specimens were gripped using hydraulic V-wedge grips, and tests were controlled electronically with FLEX fatigue software. A total of twenty-six smooth specimens were tested in load-control, eleven smooth specimens were tested in strain-control, and twelve rebar specimens in load control. All of these tests were performed in lab air conditions. An axial extensometer was used for strain-controlled tests; although the extensometer was removed for several long-life tests that were converted to load-control once stabilized. Load peak voltages were continuously monitored throughout the strain-life tests, and cyclic hysteresis loops were recorded at logarithmic intervals to observe cyclic material properties. Failure was determined when tensile load peaks dropped below 50 % of stabilized cyclic values. If the fatigue crack initiated at an extensometer knife edge, the test was deemed invalid. Load controlled tests were performed between 5 and 20 Hz depending on the load magnitude and whether specimens experienced noticeable heating. Strain controlled tests ran at rates below 3 Hz. 3. Results 3.1 Stress-life tests The stress-life data for MMFX along with that for a conventional reinforcing steel, CSA Gr. 400, with the same specimen geometry [12] is shown in Figure 2. Runout tests are indicated by an arrow. It is clear that MMFX performs better in constant amplitude fatigue than conventional CSA Gr. 400 in both the short and long life regimes. Both materials appear to exhibit a fatigue limit below which fatigue failure is unlikely to occur when subjected to constant amplitude cyclic loading. While the conventional steel sample did not have any runout points, its long-life data endures cycles at a stress range of 630 MPa. The MMFX fatigue limit is approximately 1150 Mpa Each of these stress range fatigue limits is approximately equal to the ultimate tensile strengths for the respective materials, which is a common observation in steels [8]. AM-011-4

5 Stress Range, ΔS (MPa) MMFX CSA Gr 400 (Heffernan 1997) 0 1.E+03 1.E+04 1.E+05 1.E+06 1.E+07 Cycles to Failure, N f Figure 2: Stress-Life (S-N) curve for MMFX, stainless steel, and CSA Gr Cyclic stress-strain behaviour The cyclic stress-strain loop tips were recorded throughout each test and the hysteresis loop recorded at approximately half of the fatigue life was used for further analysis. The half-life stress-strain loops from the entire series of strain-life tests were fit to a power function: 1 n σ σ ε = + E K (1) where ε is strain, σ is stress, E is the modulus of elasticity, K' is the cyclic strength coefficient, and n' is the cyclic strain-hardening coefficient. Eq. (1) is modified according to Masing s hypothesis to describe the reversed stress-strain path (Bannantine 1990): Δσ Δσ Δε = + 2 E 2K 1 n (2) where Δε is the strain range of a closed hysteresis loop, and Δσ is the stress range. This fitting procedure resulted in K = 1372 MPa and n = AM-011-5

6 The monotonic and cyclic stress strain curves are compared in Figure 3. Monotonic yielding (0.2 % offset) occurred at about 800 MPa for MMFX with non-linear stressstrain response beginning at about 300 MPa stress amplitude. It did not have a welldefined yield point, and there was no apparent yield plateau to the monotonic curve. The cyclic curve indicates that up to stress amplitudes of 600 MPa, the cyclic stressstrain behaviour of MMFX shows little hysteresis Stress Amplitude, Δσ/2 (MPa) Monotonic Stress-Strain Cyclic Stress-Strain: Eq. [3-1] Strain Amplitude, Δε/2 (mm/mm) Figure 3: Monotonic and cyclic stress-strain curves for MMFX. 3.3 Strain-life behaviour The cyclic properties recorded at half-life were used to separate the elastic strains, Δε e, from the total strains: Δ ε e Δσ = (3) 2 2E Plastic strains, Δε p, represent the remaining portion of the total strain: Δ ε p Δε Δε = e (4) The elastic and plastic strains were subsequently fit with power law equations using least squares regression, with elastic and plastic strains as the independent variables and reversals to failure as the dependent variable. Runout tests were not included in the line fits. AM-011-6

7 The strain-life curve with lines fit to the elastic and plastic strain portions is shown in Figure 4. It should be noted that the strain-life curves plot 2N f, the number of reversals until failure (two times the number of cycles to failure, with failure defined as 50 % peak load drop). 0.1 Strain Amplitude, Δε / 2 (mm/mm) Total Strain Amplitude Plastic Strain Amplitude Elastic Strain Amplitude E+03 1.E+04 1.E+05 1.E+06 1.E+07 1.E+08 Reversals to Failure, 2N f Figure 4: MMFX strain-life curve. The strain-life curves can be described mathematically by the strain-life relation as follows: b ( 2N ) + ε ( 2N ) c Δ ε f = (5) f f f 2 σ E where σ f is the fatigue strength coefficient, ε f is the fatigue ductility coefficient, b is the fatigue strength exponent, and c fatigue ductility exponent. These strain-life parameters come from the power law regression fits described above are listed in Table 3. Table 3: Strain-Life Parameters. Elastic Strains Plastic Strains σ f /E b ε f c AM-011-7

8 4. Rebar predictions The fatigue life of a concrete reinforcing bar depends in part on the local stresses and strains that occur near stress concentrations such as the root of a lug or an identification mark. Even if the average strains of a rebar are elastic, the deformations cause stress concentrations resulting in plastic strains in these vulnerable regions, the most likely location for a fatigue crack to initiate. Performing a Neuber analysis is one way to account for these stress concentrations and to predict the fatigue behaviour of a rebar [9] based on its base metal properties. Neuber s rule describes the notch root response to an applied stress (ΔS): K t ΔS = ΔσΔεE (6) where K t is the theoretical stress concentration factor, ΔS is the nominal stress range, Δσ is the local stress range at the stress concentration, and Δε is local strain range. Other researchers [4, 7] have found typical stress concentration factors for reinforcing bars are in a range between 1.6 and 2.6. The nominal strains are assumed to be elastic, which has been shown to give good predictions for lives greater than 10 3 cycles where there is not an excessive amount of plasticity [13]. The strain-life approach separates the material fatigue properties from geometric stress concentrations. By using a Neuber approach, fatigue predictions can be performed for any geometric condition provided that the appropriate stress concentration factor is known. Re-arranging Eq. (6) to isolate local strains and substitution into Eq. (2) results in: 2 2 ( K ΔS) Δσ Δσ n t 2E = + Δσ 2E 2K 1 (7) For a nominal stress range, ΔS, applied to a reinforcing bar, Equation (7) can be solved numerically to find the corresponding local stress range, Δσ. Equation (2) is used to find the local strain. The final step is to solve the strain-life relation, Equation (5) by numerical iteration. Equation (5) can be modified to account for mean stresses, σ 0, in the elastic portion of the fatigue life: b ( 2N ) + ε ( 2N ) c Δ ε f 0 = (8) f f f 2 σ σ E AM-011-8

9 Figure 5 is a comparison of re-bar stress-life results and predicted fatigue life assuming a lower and an upper K t of 1.6 and 2.6, respectively. It includes test data for MMFX reinforcing bars performed by El-Hacha and Rizkalla [2]. Up to 1 x 10 5 cycles, the data from the El-Hacha and Rizkalla [2] study and the current study are consistent. However, for fatigue lives longer than 1 x 10 5 cycles, the data from the current study falls consistently below the El-Hacha and Rizkalla [2] data. The reason for this difference is not clear, however, it may result from slightly different metallurgy or manufacturing processes for the different batches of MMFX used in each study. Stress Range Applied to Rebar, ΔS (MPa) Rebar Prediction Kt=1.6 Rebar Prediction Kt= No. 4 (El-Hacha & Rizkalla) No. 6 (El-Hacha & Rizkalla) No. 4 (DeJong et al.) 100 No. 3 (DeJong et al.) No. 5 (DeJong et al.) 0 1.E+03 1.E+04 1.E+05 1.E+06 1.E+07 Predicted Cycles to Failure, N f Figure 5: MMFX rebar test results and predictions. The data is consistent with predictions using K t = 1.6 up to about 5 x 10 4 cycles. For fatigue lives longer than about 2 x 10 5 cycles, the data is consistent with the predictions using K t = 2.6. Between these two limits, there appears to be a transition from K t = 1.6 to K t = 2.6. The reason for this transition is not clear, but may be related to surface effects. These tend to be eliminated at high strains, which would account for the lower apparent K t at high stress ranges [15]. Conventional steel reinforcing bars typically exhibit a fatigue life of 1 x 10 6 cycles at a stress range of approximately 166 MPa [14]. In the current study, a fatigue life of 1 x 10 6 cycles is observed at a stress range of approximately 310 MPa. Thus, MMFX exhibits superior fatigue resistance under constant amplitude loading in an air environment than conventional steel reinforcing bars. AM-011-9

10 5. Conclusions Based on the reinforcing bar test data, the procedure outlined in this paper gives a conservative preliminary fatigue prediction for MMFX reinforcing bars, although additional tests of other bar diameters and at longer fatigue lives would be useful. Designers who are concerned with fatigue must know that new materials have sufficient fatigue resistance. The results of this study suggest that MMFX reinforcing bars have significantly higher fatigue strength than conventional reinforcement in constant amplitude fatigue in an air environment. Results can be summarized as follows: 1. Constant amplitude load-controlled tests were performed on MMFX smooth specimens giving a fatigue limit of about 1150 MPa stress range at 5 x 10 6 cycles. 2. Up to stress amplitudes of 600 MPa, the cyclic stress-strain behaviour of MMFX shows little hysteresis. 3. A strain-life prediction model incorporating a Neuber analysis to account for plasticity and stress concentration factors of 1.6 and 2.6 successfully bounded MMFX rebar stress-life data. 4. A fatigue life of 1 x 10 6 cycles is observed at a stress range of about 310 MPa for MMFX in constant amplitude loading. 6. References 1. MMFX Steel Corp. of America. (Sept. 2001). MMFX Product Bulletin. 2. El-Hacha, R., Rizkalla, S.H. (2002), "Fundamental Material Properties of MMFX Steel Rebars." Research Report: RD-02/04, North Carolina State University, Constructed Facilities Laboratory, Raleigh, North Carolina, USA, July 2002, 62pp. 3. Helgason, T., Hanson, J. M., Somes, N. F., Corlew, W. G., and Hognestad, E. (1976). Fatigue Strength of High-Yield Reinforcing Bars. Transportation Research Board, Washington, D.C. 4. Jhamb, I. C., and MacGregor, J. G. (1974). "Stress Concentrations Caused by Reinforcing Bar Deformations." Abeles Symposium: Fatigue of Concrete, ACI SP Publication 41-8, American Concrete Institute, Detroit, MI, MacGregor, J. G., Jhamb, I. C., and Nuttall, N. (1971). "Fatigue Strength of Hot Rolled Deformed Reinforcing Bars." ACI J., 68(3), Jhamb, I. C., and MacGregor, J. G. (1974). "Effect of Surface Characteristics on Fatigue Strength of Reinforcing Steel." Abeles Symposium: Fatigue of Concrete, ACI SP Publication 41-7, American Concrete Institute, Detroit, MI, Zheng, H., and Abel, A. (1998). "Stress Concentration and Fatigue of Profiled Reinforcing Steels." International Journal of Fatigue, 20(10), Bannantine, J. A., Comer, J. J., and Handrock, J. L. (1990). Fundamentals of Metal Fatigue Analysis. Prentice-Hall, Inc., Englewood Cliffs, NJ. AM

11 9. Heffernan, P. J., Erki, M., and DuQuesnay, D. L. (2004). "Stress Redistribution in Cyclically Loaded Reinforced Concrete Beams." ACI Structural Journal, 101(2), ASTM Committee E-8. (2002). "E466-96: Standard Practice for Conducting Force Controlled Constant Amplitude Axial Fatigue Tests of Metallic Materials." 11. ASTM Committee E-8. (1998). "E606-92: Standard Practice for Strain-Controlled Fatigue Testing." 12. Heffernan, P. J. (1997). "Fatigue Behaviour of Reinforced Concrete Beams Strengthened with CFRP Laminates." PhD thesis, Royal Military College of Canada, Kingston, ON. 157 pp. 13. Dowling, N. E. (1982). "A Discussion of Methods for Estimating Fatigue Life." Proceedings of the SAE Fatigue Conference, Society of Automotive Engineers, Warrendale, PA, MacGregor, J. G., and Bartlett, F.M. (2000). Reinforced concrete: mechanics and design. 1 st Can. ed. Prentice Hall, Scarborough, ON. 15. DuQuesnay, D.L., Heffernan, P.J. and Howard, A.B., Cyclic and Fatigue Behaviour of a Micro-alloyed Reinforcing Steel. Proceedings of the 15th Canadian Congress of Applied Mechanics. Univ. Victoria, Victoria, B.C., Tabarrok, B. and Dost, S. (Eds.) Vol. 1, pp AM