PSI - Issue 68

Berkehan Tatli et al. / Procedia Structural Integrity 68 (2025) 1140–1146 Tatli and Yalcinkaya / Structural Integrity Procedia 00 (2024) 000–000

1141

2

Various theories have been proposed to explain hydrogen-induced failure, but no unified mechanism has been established to fully describe the degradation process. Even after 150 years since hydrogen’s e ff ects on metals were first recognized, the debate persists. Key mechanisms include hydrogen-enhanced decohesion (HEDE), hydrogen enhanced localized plasticity (HELP), hydrogen-induced phase transformation (HIPT), and nanovoid coalescence (NVC). Some studies suggest that a combination of these mechanisms contributes to material embrittlement and the degradation of mechanical properties Djukic et al. (2019). HEDE mechanism is mainly based on the degradation of cohesive strength due to the introduction of hydrogen atoms inside the metal. Hydrogen weakens interatomic metallic bonds, leading to separation at lower stress and strain levels. Both theoretical studies and experimental evidence support this mechanism, where hydrogen, similar to sulfur and phosphorus, segregates at grain boundaries, weakening atomic cohesion and ultimately causing intergranular fracture Martin et al. (2012). Hydrogen tends to accumulate in regions of high hydrostatic stress, dislocation shielding zones near the crack tip, and grain boundaries. Gao et al. (1994) reports that hydrogen concentration at the crack tip is driven by lattice concentration, hydrostatic stress, and plastic strain, with hydrostatic stress playing the primary role in high-strength steels, even at low plastic strain levels Lufrano and Sofronis (1998). One of the most widely utilized computational framework in the literature for simulating HEDE mechanism is cohesive zone modelling. This framework is applied in both microscale studies, such as simulating representative volume elements (RVEs) to investigate the influence of microstructural features on macroscopic behavior Lin et al. (2022), and in macroscale studies, such as estimating fatigue crack growth in gaseous hydrogen environments Mori coni et al. (2014), reproducing fracture mechanics tests to obtain fracture resistance curves Brocks et al. (2012). This paper investigates the HEDE mechanism using a PPR potential-based mixed-mode cohesive zone model, coupled with mechanical deformation and stress-driven hydrogen di ff usion. Crystal plasticity (CP) and strain gradient crystal plas ticity (SGCP) constitutive models are employed for microscale analysis. Polycrystalline microstructures, generated via Voronoi tessellation for varying grain sizes, are analyzed to assess the impact of microstructure on macroscopic material behavior. Comparisons between local CP and nonlocal SGCP simulations focus on stress-strain behavior, hydrogen accumulation, and localized stress concentrations. The crucial role of GNDs, particularly near crack tips, is highlighted. By incorporating strain gradient e ff ects, the study draws key conclusions on hydrogen-induced failure and intergranular crack propagation patterns in polycrystalline RVEs.

2. Methodology

This section outlines the constitutive laws employed in the numerical findings presented in section 3, along with the formulation of the potential-based mixed-mode cohesive zone model used to predict intergranular failure.

2.1. Lower Order Strain Gradient Crystal Plasticity Theory

This paper employs the mechanism-based strain gradient crystal plasticity theory proposed by Han et al. (2005) to capture size e ff ects and incorporate nonlocal crystal plasticity behavior into the numerical framework. In this formu lation, geometrically necessary dislocations (GNDs) are integrated using the Taylor relation. The slip law from the local crystal plasticity formulation is modified to include the hardening e ff ect due to the GNDs, as follows:

TOT = g ˆ α

0 τ ˆ α g ˆ α

TOT

N sign ( τ ˆ α ) , g ˆ α

2

2

(1)

˙ γ ˆ α = ˙ γ

+ g ˆ α

,

SSD

GND

where g ˆ α TOT is the e ff ective slip resistance that is defined by the addition of the hardening due to the GNDs on top of the statistically stored dislocations (SSDs), g ˆ α SSD is the slip resistance through the SSDs and g ˆ α GND is the slip resistance through the GNDs on a slip system ˆ α . Evolution of the slip resistance due to the SSDs and GNDs are given as:

g ˆ α GND = g 0 l η ˆ α

ˆ α ˆ β ˙ γ

ˆ β ,

SSD = ˆ β h

˙ g ˆ α

(2)

GND