21º CBECIMAT - Congresso Brasileiro de Engenharia e Ciência dos Materiais 09 a 13 de Novembro de 2014, Cuiabá, MT, Brasil

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1 21º CBECIMAT - Congresso Brasileiro de Engenharia e Ciência dos Materiais Effects of plastic prestrain on the fatigue crack growth (da/dn vs. K) behavior of ASTM A36 steel Donato, G. H. B. 1,a, Cavalcante, F. G. 1,b, and Oliveira A.C.C. 1,c 1 University Center of FEI, São Bernardo do Campo - SP, Brasil a gdonato@fei.edu.br, b fcavalcanti@metrosp.com.br, c anacoh@terra.com.br Keywords: Structural steels. ASTM A36. Fatigue crack growth. Plastic prestrain. Abstract. High responsibility components operating under cyclic loading can have their performance to the initiation and growth of fatigue cracks highly influenced by previous thermomechanical processing. Within the interest of the present work, different manufacturing processes and installation techniques incorporate cold plastic straining; two typical examples are: i) the offshore pipelines installation method called reeling; ii) the fabrication of pipes using the UOE method. Within this scenario, this work investigates the effects of plastic prestrain (4,3, 8,3, e 14,7%) on the fatigue crack growth rate (da/dn vs. K) for a hot-rolled ASTM A36 steel. C(T) specimens were prepared from original and prestrained materials, submitted to crack growth testing according to ASTM-E647 and fractures were. Results revealed that plastic prestraining reduced crack growth rates for the studied material. Introduction Increasing demands for efficient engineering components have guided the search for materials and/or geometries that provide mass reduction, cost optimization, increased strength, stiffness and lifetime, and simplicity on installation and operation activities. In this context, varying metal forming techniques are necessary in order to provide the desired (sometimes complex) geometries combined to enhanced mechanical properties (highly influenced by the thermomecanical conception of the manufacturing processes). Considering pipelines and flow lines manufactured by UOE, sphere metal forming or spiral processes, methods like stamping, bending and calendering are usually employed generating different microstructures (desirable or deleterious) and performance. Pipelines, pressure vessels and related structural components are highly relevant for the expanding oil and gas industry in Brazil, being critical in case of failures. Most part of such structures are made of flat sheets which are thermomechanically processed providing the desired geometry and properties. Therefore, the applied strains during manufacturing procedures (which can overcome 20%) must be taken into account for structural integrity assessments. Additional plastic straining can also take place during installation procedures such as pipe reeling, where pipes are stored on a drum for the deposition on the sea bed for supporting oil and gas applications. Although advantageous in economic terms, this technique imposes plastic cold straining during each step, which can represent accumulated fatigue damage. Several studies from te literature report local strains between 1 and 5% for winding and unwinding, limited to 2% accumulated plastic strain (APS) at the end of the process (1). International standards, in its turn, reveal APS strain between 2 and 3% for the whole process, combined to requirements for installation that limit maximum straining during each step to 1%. (2,3). The main objective in this context is to assess the effects of plastic prestraining on fatigue performance (those structures can undergo millions of loading cycles throughout its lives), since previous results from the literature demonstrate that both beneficial or deleterious effects can be generated for crack initiation or propagation. This paper focuses on the effects of varying plastic prestrain levels on fatigue crack growth characterized by the theoretical framework of the Linear- Elastic Fracture Mechanics (LEFM based on K). In addition, different from other previous research efforts, the studied material was prestrained before specimens were extracted to try to closely reproduce real conditions. 5450

2 21º CBECIMAT - Congresso Brasileiro de Engenharia e Ciência dos Materiais Studied Material and Experimental Procedures The material was obtained as a hot rolled ASTM A36 steel plate, with 12.7 mm thickness and planar dimensions of 1 m by 1 m. Initially, small samples were extracted for chemical characterization and conventional metallographic analysis. In addition, tensile and compact C(T) specimens were machined respectively for tension and fatigue crack growth testing. The ASTM A36/A36M-08 (4) standard, which determines the basic specifications of the material in terms of chemical composition and monotonic mechanical properties, was taken as a reference. The monotonic tension tests were employed to characterize basic mechanical properties and quantify the anisotropy related to the rolling direction. Because of that, specimens were taken from the plate middle plane at 0, 45 and 90º relative to the rolling direction. The specimens and the tensile tests were developed according to ASTM E8/E8M-11 (5) standard. Before the fatigue crack growth tests, the desired prestrain levels had to be applied. Consequently, 8 strips with 50 mm in width and 500 mm in length were machined from the original plate as presented by figure 1. The strips received special painting and transverse marks each 10 mm to make image analysis feasible and better characterize the obtained plastic prestrain levels. After prestraining, C(T) specimens according to ASTM E (6) standard were machined from the strips in the T-L direction (see figure 1), according to additional arguments provided by ASTM E (7) standard. Figure 1 - Schematic representation of the strips to be prestrained and from which C(T) specimens were extracted. All tension and fatigue crack growth tests, as well as the prestraining procedures, were performed on an MTS 810 universal testing machine with capacity of 250kN. The strain levels were quantified by clip-gages and images of the strips acquired with a Canon EOS T2i camera and EF 50 mm 1:1.8 lens before and after strain, where a scheme of alignment by three points, two at the base and one on the camera display was designed to avoid parallax effects. Due to the overlap of the right and left sides of the images, 3 regions of 10 mm each were eliminated in each side avoiding wide-angle effects. The images were analyzed in Adobe Photoshop CS4 software and merged in CorelDraw X5 software for pixel calibration and final measurements. The crack growth tests were performed according to ASTM E (6) standard and formulations supported by compliance solutions including the rotation corrections of ASTM E (7). All tests employed R = 0.1, constant load range and increasing K. COS special clip-gages with 3mm travel were employed and, together with the MTS load cell, permitted K and crack sizes estimation during the whole tests. The tests were performed in two stages, as usual: first, a precrack was created under K control to guarantee a realistic crack in each sample; second, the fatigue crack growth test was conducted based on a load control scheme. After each test, cracks were heat tinted and fractographic analyses were conducted in each sample. Crack dimensions were measured according to ASTM E (8) standard, with 9 different lengths along the crack front. The dimensioning was performed by image analysis on CorelDRAW X5 software, analogously to the procedure for measuring prestrain levels. Additionally, a special algorithm in Matlab was developed for predicting crack sizes based on compliance solutions from ASTM E (6) standard and from finite element solutions proposed by Moreira (9). The same MatLab code was responsible for calculating da/dn vs. K evolutions based on load (P) vs. Crack Opening Displacement (COD) MTS records. 5451

3 Stress - σ [MPa] 21º CBECIMAT - Congresso Brasileiro de Engenharia e Ciência dos Materiais Results and Discussion The chemical analysis by atomic emission spectrometry presented the components referred in standard, plus additional components not prohibited by the same (see Table 1). Table 1 - Chemical composition (% mass) of ASTM A36 steel by sample studied material. C Mn S Si Cu Ni Mo Cr P Metallographic analysis showed that the microstructure is formed by light regions with ferrite (α) grains and with dark grain boundaries and pearlite, which is composed of ferrite (α) and cementite (Fe3C). Both banded grains and central segregations were not found on the analyzed samples. Also, transverse and longitudinal directions presented essentially similar features. (a) Transverse (b) Longitudinal Figure 2 Microstructure for ASTM A36 steel. (a) Transverse 100x, (b) Longitudinal 100x. From the tension monotonic tests, the mechanical properties shown in the table 2 were obtained. Stress strain curves are presented by figure 3 for different loading speeds no strain-rate effect was obtained for the desired prestraining speeds. Table 2 - Mechanical properties obtained from tensile tests. E [GPa] S YS Offset [MPa] S YS Autographic [MPa] S UTS [MPa] Al% [ ] MEAN VALUES v 50 mm/min v 75 mm/min v 10 mm/min v 1 mm/min Strain - ε [ ] Figure 3 True stress-true strain evolutions for the ASTM A36 studied steel. 5452

4 Taxa de Crescimento da Trinca - da/dn [mm/ciclo] log Taxa de Crescimento da Trinca - da/dn [mm/ciclo] log 21º CBECIMAT - Congresso Brasileiro de Engenharia e Ciência dos Materiais According to ASTM A36/A36M-08 (4) standard, it is evident that the results meet the requirements for categorization of the material as ASTM A36. In addition, an statistical Analysis of Variance (ANOVA) was applied to the monotonic properties, indicating that anisotropy relative to the rolling direction is not relevant. As can be seen in figure 3, the studied material presents remarkable discontinuous flow, which did not allow prestrain levels to be below ~ 2.5 %. Prestrain levels around ~2% were also desired, but could not be implemented since the uniform strain distribution along the strips could not be guaranteed. Besides the original material, three levels of cold plastic strain could be applied, including: 4.3, 8.3, and 14.7%. Together with the original condition, 4 prestrain levels were assessed. The crack growth results obtained directly from the testing machine software presented large data scatter and the expected regions I, II and III could not be identified, even using the recommendations of the ASTM E (6) for data smoothing. Since load and COD records were smooth, it was clear that the problem was originated on the da/dn calculations, which demands a numerical derivation. Motivated by this limitation, the developed Matlab algorithm imported experimental raw loads and displacements from the MTS machine and computed K, compliances and da/dn values for each cycle applying different derivation techniques, which provided enhanced numerical stability. Additional rules were also created to eliminate invalid points (outliers) according to selected criteria from the ASTM E (6) standard. Thus, crack growth da/dn vs. ΔK curves were generated from the post-processed data. Excluding data regarding regions I and III (of no interest for this investigation), it was possible to evaluate the effect of varying prestrain levels on the crack growth rates on region II (see figure 4(a)). Considering Paris law (Equation 1) as applicable to the obtained data (figure 4 shows the linearity of results in a log-log plot), C and m parameters could be fitted by a minimum square approach. (1) Even visually, one can realize from figure 4 that the increase in prestrain levels promoted a reduction in the crack growth rate (da/dn) for the same value of ΔK. In order to better evaluate the effects, figure 4(b) presents a selected amplified region between 35 and 45 MPa m, which comprises the overlapping region II of all the tests. 1.E-2 Curva da/dn vs. ΔK - Região II 0% PS - CP 01 2.E-3 Curva da/dn vs. ΔK - Região II 0% PS - CP 05 0% PS - CP % PS - CP 03 2A 4.3% PS - CP 06 2A 4.3% PS - CP 03 2B 4.3% PS - CP 04 2B 1.E-3 8.3% PS - CP 04 3A 8.3% PS - CP 05 3A 8.3% PS - CP 05 3B 8.3% PS - CP 06 3B 14.7% PS - CP 02 1A 14.7% PS - CP 05 1A 1.E-4 1.E-5 15 (a) Fator de Intensidade de Tensões - ΔK [MPa m] log 65 2.E-4 35 (b) 0% PS - CP 01 0% PS - CP 05 0% PS - CP % PS - CP 03 2A 4.3% PS - CP 06 2A 4.3% PS - CP 03 2B 4.3% PS - CP 04 2B 8.3% PS - CP 04 3A 8.3% PS - CP 05 3A 8.3% PS - CP 05 3B 8.3% PS - CP 06 3B 14.7% PS - CP 02 1A 14.7% PS - CP 05 1A Fator de Intensidade de Tensões - ΔK [MPa m] log Figure 4 - da/dn vs. ΔK curves. (a) Region II. (b) Region II with magnification

5 da/dn Reduction [%] Coefficient - C Exponent - m 21º CBECIMAT - Congresso Brasileiro de Engenharia e Ciência dos Materiais Figures 5(a,b) presents average values for parameters C and m respectively, as a function of the prestrain % level in the x axis. It can be clearly noted an increasing trend for C, combined to a decreasing trend for the m exponent. Both parameters must always be analyzed together and supported by the da/dn vs. K plot, but in this case it is evident that larger prestrains moved the curve to the right and reduced its slope, which are considered benefits for the mechanical performance of the material. 2.5E-8 2.0E-8 Vertical Displacement of the Sigmoidal Curve C Mean Values (a) (b) Slope of the Sigmoidal Curve m Mean Values 1.5E-8 1.0E E E % Prestrain % Prestrain Figure 5 - C and m parameters. (a) Vertical displacement. (b) Slope. A final check based on Analysis of Variance (ANOVA) confirmed the statistical significance of the observed trends. The compilation of the average values (figure 6) reveals the percentage reduction of average crack growth rates (da/dn) as a function of different prestrain levels for different ΔK loading regimes. It means that there is an increasing sensitivity of the crack growth rate decrease with the plastic prestrain for higher values of ΔK. In percentage terms and with reference to the original material (0% prestrain), figure 6) reveals that reductions around 20% in fatigue crack growth rates were observed for prestrain levels between 4,.3 and 14.7% for the ASTM A36 steel % da/dn Reduction for ΔK Values % 0-8.3% % ΔK [MPa m] Figure 6 Percentage reduction of average crack growth rates (da/dn) as a function of different prestrain levels for different ΔK loading regimes. Finally, after the separation of the specimens and photographic record of fractures, crack measurements called the attention to one important phenomenon. Table 4 reveals, based on real measurements of the crack front, that significant underestimates for crack size were obtained when using compliance solution from current standards, especially for the pre-crack size. Despite it does not affect the results of the present study, it calls the attention for the importance of improving such compliance methods. 5454

6 21º CBECIMAT - Congresso Brasileiro de Engenharia e Ciência dos Materiais Table 4 - Cracks measurements and comparison with compliance predictions from standards. MEAN VALUES IMAGE ANALYSIS COM PLIANCE E MEF 1 ERROR % E647 2 ERROR % PRE-CRACK [mm] CRACK [mm] (1) rotation correction of the ASTM E1820 standard and coefficients by FEM. (2) coefficients of the ASTM E647 standard. Conclusions The assessment of the plastic prestrain effect on the fatigue cracks growth rate for ASTM A36 steel in region II, performed between 35 and 45MPa m ΔK, revealed significant reduction in the da/dn values with the increase of prestrain levels, reaching ~20 % for prestrain levels between 4.3 and 14.7 %. It can be concluded that this effect was due to increase in mechanical properties (strain hardening) with the imposed prestrain levels, and is beneficial for structural performance. It is worth mentioning that this work, differently from other previous in the literature, prestrained the material using a uniaxial stress fields prior to machining in order to avoid residual stresses ahead of crack growth. This way, it keeps close relationship with real conditions to which materials are exposed in applications. Measurements of the pre-crack and final crack dimensions showed deviations from the compliance predictions following current standards. Although this occurrence does not compromise the interests of the present work, reinforces the need for further research and improvements in the unloading compliance methods. References [1] VILAS BÔAS, Ana Carolina Caldas. Comportamento Mecânico e de Corrosão do Aço X70 Sob Carregamento Cíclico Simulando Bobinamento de Tubos. Dissertação de Mestrado. Brasil: Universidade Federal de Minas Gerais - Curso de Pós Graduação em Engenharia Metalúrgica e de Minas, 2012, p.44. [2] DET NORSKE VERITAS AS DNV-RP-F108, Recommended Practice: Fracture Control For Pipeline Installation Methods Introducing Cyclic Plastic Strain. Oslo, Norway, p.7 [3] DNV-OS-F101, Offshore Standard: Submarine Pipeline Systems. Oslo, Norway, 2012, p.175 [4] AMERICAN SOCIETY FOR TESTING AND MATERIALS A36/A36M 08: Standard Specification for Carbon Structural Steel. Pennsylvania, USA, [5] E8/E8M 11: Standard Test Methods for Tension Testing of Metallic Materials. Pennsylvania, USA, [6] E647 11: Standard Test Methods for Measurement of Fatique Crack Growth Rates. Pennsylvania, USA, [7] E399 09: Standard Test Method for Linear-Elastic Plane-Strain Fracture Toughness KIc of Metallic Materials. Pennsylvania, USA, [8] E : Standard Test Method for Measurement of Fracture Toughness, Pennsylvania, USA, [9] MOREIRA, Felipe Cavalheiro. Determinação Numérica de Limites de Deformação e Flexibilidades Elásticas Aplicáveis a Geometrias C(T), SE(B) E SE(T). Brasil: Centro Universitário da FEI - Dissertação de Mestrado,