Characterization of hydrogen embrittlement in high-strength fastener steels: mechanical testing and numerical modeling

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High strength and ultrahigh strength mechanical fasteners are broadly characterized by tensile strengths ranging from 1,000 to 2,000 MPa, and are often used in critical applications. Damage resulting from hydrogen embrittlement can occur by a complex interaction of material characteristics, environmental conditions, manufacturing flaws, installation conditions, and joint design criteria. The consequences of failures can range from minimal to catastrophic, even resulting in loss of life. Hydrogen embrittlement (HE) is a common problem in high strength steel caused by the absorption of elemental hydrogen into the material matrix. Hydrogen damage in high strength steel fasteners can be summarized as a migration of mobile hydrogen from lattice sites to points of stress concentration, resulting in crack initiation followed by crack propagation by atomic bond decohesion at the crack tip. The effect of hydrogen embrittlement in materials is generally characterized by various types of slow strain rate tests and incremental step load tests which quantify the loss of mechanical properties such as strength and ductility.This project builds on previous work as it aims to characterize and quantify the individual effects ofmicrostructural defects in hydrogen transport of high strength steel components such as fasteners. A multiscale modeling approach that combines atomistic and continuum numerical simulations will be employed, creatively bridging nanoscale trapping energetics with macroscopic hydrogen diffusion. In parallel to the modeling, dedicated experiments such as measurements of hydrogen in bulk materials by thermal desorption are being explored to validate and parameterize the model. The above modeling and experimental efforts will be further integrated with measurements from incremental step load tests to provide predictive assessments on the HE susceptibility of the material based on the trapping and transport characteristics of hydrogen.