APPLICATION OF A MODEL OF HYDROGEN-ASSISTED FATIGUE CRACK GROWTH IN 4130 STEEL | NIST

In this work, we applied a finite element model to predict the cyclic lifetime of 4130 steel cylinders under the influence of hydrogen. This example is used to demonstrate the efficacy of a fatigue crack growth (FCG) model we have developed. The model was designed to be robust and incorporate features of stress-assisted hydrogen diffusion, large-scale plasticity, hydrogen gas pressure, loading frequency, and effects of microstructure. The model was calibrated to the 4130 steel material by use of tensile tests and experimental FCG results of a compact tension specimen. We then used the model to predict the hydrogenassisted FCG rate and cycle life of a pressurized cylinder with a deliberate initial thumbnail crack. The results showed good correlation to the cyclic lifetime results of 4130 pressurized cylinders found in the literature. INTRODUCTION If hydrogen is to be used as an energy carrier to provide an alternative to fossil fuels, a vast network of pipelines is required to transport the hydrogen across the country. Furthermore, truck-mounted and loose pressure vessels will likely continue to be employed as short-distance hydrogen transportation and storage solutions. Steel pipelines are likely the most economical means of longdistance hydrogen transportation. The most common materials used in natural gas pipelines are API-5L grade steels that have a specified minimum yield stresses between 52 ksi (358 MPa) and 80 ksi (551 MPa). These steels carry the designation of API-5L X52, X65, X70, X80, etc. Future hydrogen-specific pipe installations may include the use of X100 and X120 pipeline steels. Pressure vessels for hydrogen use are commonly produced with ASTM SA/A516 or AISI 413X steels, where the X is replaced with a 0 or 5 depending upon carbon content. While the difference in geometries and boundary conditions between pipes and the cylinder portion of pressure vessels may be easily represented, the steels used comprise vastly different microstructures. Although the differing microstructures ASME-2017 book.indb 605 7/14/17 11:36 AM 606 may or may not yield significantly different deformation responses under normal operating conditions, they do likely produce varying hydrogen diffusivities [1-4] and, therefore, have the potential for significantly different fatigue and fracture responses in the presence of hydrogen. The pipeline and infrastructure required to transport hydrogen across the United States does not currently exist. Given that steel pipelines have for a long time been the best method to transport fuels long distances, the United States will need a hydrogen transmission network similar to that of the current natural gas transmission network in size. The United States currently has approximately 305,000 miles of natural gas pipelines [5] and only one half of one percent of that quantity is hydrogen-dedicated pipeline [6, 7]. A primary barrier to the use of carbon steel to transport hydrogen, whether by pipeline or pressure vessel, is the deleterious effect that hydrogen has upon the deformation, fracture, and fatigue response of the steel [8, 9]. Design and engineering codes for hydrogen transmission via pipeline [10] and distribution via piping [11], as well as pressure vessels [12], have been developed to ensure safe and effective use of these steels for hydrogen service. The newest versions of the codes that support the transmission and distribution of hydrogen and hydrogen-bearing gasses are being informed by the recent experimental results from laboratories such as the National Institute of Standards and Technology (NIST) and Sandia National Laboratories (SNL) [13-15]. It is nearly universally understood that microstructure-specific physics-based models are required to inform the large-scale infrastructure expansion required to meet the needs of a future network for hydrogen transmission. A phenomenological hydrogen-assisted fatigue crack growth (HA-FCG) model that predicts cycles to failure for given material-specific calibration parameters and known initial and boundary conditions has been created and used to inform the forthcoming 2016 version of the ASME B31.12 Hydrogen Piping and Pipeline code (to be released in 2017). The current HA-FCG model is based upon the understanding that the crack-growth response of a steel results from an interaction between competing damage mechanisms and was initially implemented with closed-form solutions to the crack stress response and the stress-assisted hydrogen diffusion [16, 17]. In our current effort, the elasticplastic deformation response coupled with the hydrogen diffusion model is implemented in the finite element (FE) program ABAQUS1. The predicted hydrogen concentration, as a function of elastic and plastic deformation and gas pressure, is coupled with the existing phenomenological framework to predict the HA-FCG response of the steel. The strength of this modeling framework is that one can predict a steel’s HA-FCG response, for any geometry that can be modeled, based upon laboratory results from simple specimens, e.g. compact tension (CT) specimens. This work details the use of the modeling framework to predict cycles to failure for pipes and pressure vessels that have thumbnail-shaped internal cracks. Ultimately, the model implementation will be updated to include 1 Identified for clarity only, no endorsement by NIST is implied ASME-2017 book.indb 606 7/14/17 11:37 AM 607 microstructure-specific steel domains and their associated deformation and hydrogen diffusion properties. Coupled Hydrogen-Assisted Fatigue Crack Growth Model The phenomenological HA-FCG model, calibrated to X100 pipeline steel is detailed in [16, 17]. The model has also been partially calibrated to X52 and X70 steels [18]. The model has the following functional form: dddd ddddtotal = dddd ddddfatigue + δ(PPH − PPHth) dddd ddddH , EQ. 1 where, a is the crack length, N is the number of cycles, and P is the hydrogen pressure. The model predicts the total fatigue crack growth as the sum of the fatigue crack growth resulting from fatigue only and the HA-FCG. HA-FCG is predicted by dddd ddddH = [( ddddPH ) −1 + ( dddd∆KK) −1 ] −1