PSI - Issue 13

Rodrygo Figueiredo Moço et al. / Procedia Structural Integrity 13 (2018) 1915–1923 Rodrygo F. Moço, Fábio G. Cavalcante and Gustavo H. B. Donato / Structural Integrity Procedia 00 (2018) 000–000

1916

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Keywords: Structural steels, Fatigue crack growth, Effects of plastic prestrain, Pressure vessels, Pipelines.

Nomenclature APS

Accumulated Plastic Strain

American Society for Testing and Materials

ASTM

C

Coefficient of Paris law

Compact under Tension specimen Fatigue crack growth rate Finite Element Method Stress intensity factor Leak Before Break Exponent of Paris law Number of fatigue cycles Linear-Elastic Fracture Mechanics

C(T) da/dN FEM

K

LBB

LEFM

m N

Prestrain

PS S ys S uts

Yield strength

Ultimate tensile strength Pipes manufacturing process

UOE

Applied range of K

Δ K

1. Introduction The increasing demand for efficiency in engineering components motivates the search for materials and/or geometries that provide mass reduction and cost optimization, while at the same time increased strength, stiffness, lifetime and simple installation/operation. In this context, varying metal forming techniques are necessary to provide the desired geometries combined to enhanced mechanical properties (highly influenced by the thermomecanical conception of the manufacturing processes). Such metal forming techniques range from simple bending and drawing to more complex processes such as hydroforming, superplastic and creep-age forming (RAMEZANI, RIPLIN, 2012). Considering pipelines and flow lines manufactured by UOE, continuous or spiral forming processes, methods like stamping, bending and calendering are usually employed generating different microstructures (desirable or deleterious), stress states and thus mechanical performance under fatigue and fracture. Pipelines, pressure vessels and related structural components are highly relevant for the expanding chemical, oil and gas industries 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%) should not be neglected and the resulting altered mechanical properties and residual stresses should be taken into account for structural integrity assessments (Kang et al., 2007; PETINOVAND MELNIKOV, 2011). Studies from Raffo et al. (2007) reveal that accumulated plastic strains for UOE process can reach 10%. Additional plastic cold straining can also occur during installation procedures such as pipe reeling, where pipes are stored on a drum for the deposition on the seabed for supporting oil and gas applications. Although advantageous in economic terms, this technique imposes plastic straining during each step, which can represent accumulated fatigue damage. Several studies from the 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 (VILAS BÔAS, 2012). International standards, in its turn, reveal APS between 2% and 3% for the whole process, combined to requirements for installation that limit maximum straining during each step to 1% (DNV-RP-F108, 2006; DNV-OS-F101). One must take into account that such installation stresses and strains will be superimposed to preexisting ones. In this context, the main objective is to assess the effects of plastic prestraining on fatigue performance of pipelines and pressure vessels (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 – ANDERSON, 2005; SURESH, 1998; BARSONAND ROLFE, 1977), which were taken into account to elaborate a nonlinear model to predict crack growth rates ( da/dN ) as a function of Δ K and varying plastic prestrain levels for the studied geometries made of