Themes: Hydrogen Embrittlement and Environmentally Assisted Cracking 

EFFECTS OF CRACK TIP STRESS RELAXATION ON SUBCRITICAL CRACK GROWTH IN SILICATE GLASSES: THRESHOLD AND STOCHASTICITY

Silicate glass is a non-equilibrium material and as such evolves over time to reduce internal energy through thermally activated structural rearrangement. This statement is perhaps especially true in the highly stressed region around a crack tip. At the atomistic scale, structural changes to accommodate crack growth or to mediate stress relaxation become indistinguishable. Here, we present a simple expression for static fatigue threshold using slow crack growth power law parameters and a structural relaxation time scale. Using subcritical crack growth data from the literature and measured threshold data, this model is demonstrated for soda lime silicate glass. In addition, we discuss the impact of crack tip relaxation on statistical lifetime prediction and evolution of flaw populations.
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MODELING OF HYDROGEN EMBRITLLEMENT USING MIXED NONLOCAL FINITE ELEMENTS

Industrial power generation and transmission structures are designed to have a service life of 40 years. Knowledge of the evolution of material behavior over long periods of time is therefore crucial to ensure the safety and reliability of these facilities. Due to the continuously increasing power demand, new energy sources are needed. As part of the decarbonization of these sources, hydrogen will play an important role as an energy vector. However, hydrogen can easily diffuse in materials, inducing premature failure with reduced ductility and toughness. This phenomenon, called hydrogen embrittlement (HE), is a complex mechanism which combines mechanical and chemical loadings. Therefore, this work presents a strategy to simulate HE by the finite element method integrating plasticity and damage coupled to hydrogen diffusion. Since damage is highly dependent on local stresses and hydrostatic pressure mixed formulations in displacement, pressure and volume variation have been proposed to control volumetric locking. To represent ductile rupture, the Gurson-Tvergaard-Needleman (GTN) model based on an implicit gradient nonlocal formulation with two internal lengths is considered, which allows regularizing void growth and strain-controlled nucleation. All the implementations and simulations have been carried out using the Z-set software.
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EFFECTS OF TESTING RATE ON HYDROGEN-ASSISTED FRACTURE OF FERRITIC STEELS

Conventional wisdom suggests hydrogen-assisted fracture occurs principally at slow testing rates as hydrogen requires time to diffuse to the region of elevated stress, such as the crack tip. The effects of testing rate were examined on ferritic-based steels by performing fast rate fracture tests in gaseous hydrogen at testing rates spanning four orders of magnitude.
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MODELING OF STRESS CORROSION CRACK INITIATIONS OF POLYETHYLENE PIPE TRANSPORTING CHLORINATED WATER

Stress corrosion cracking is one of the long-term failure mechanism of thermoplastic pipes when exposed to the oxidative agents, such as the chlorinated water. In this paper, the stress corrosion crack initiation model was suggested based on the diffusion of chlorinated water with oxidation, combining with the energy analysis by cracking. The multiple micro cracking, which is a dominant feature in stress corrosion cracking failure, was successfully simulated by the proposed model.
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MODDELING OF INTERGRANULAR STRESS CORROSION CRACKING MECHANISM THROUGH COUPLING OF SLIP-OXIDATION AND COHESIVE ZONE MODEL

A finite element model was proposed for intergranular stress corrosion cracking modelling (IGSCC). The model is based around a moving integration point formulation which enables the model to track the oxide, dissolution, and crack tip. The formulation is introduced in the cohesive element. The model also relies on an electrochemical model, based on the slip-oxidation model and a diffusion model. The model is dependent on the plastic strain rate and creep strains for oxide rupture to evaluate the effect of creep and plastic strain on crack growth and oxide thickness in IGSCC.
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INTEGRATED MODELING OF STRESS CORROSION CRACKING IN SUPERALLOYS

The reliability of turbine blades is strongly dependent on the humidity, contamination, stress, and temperature to which they are exposed during operation. In many cases, cracks initiate simultaneously at multiple locations, which can result in crack arrest (shielding) or (coalescence). This presentation will explore an integrated computational and experimental approach that evaluates crack interaction in CMSX-4 superalloy using C-Ring tests with a layer of contaminant salt exposed to 550C. A phase-field model calculates the diffusion of species and reduces the material critical energy release rate accordingly. The model, which is parameterised to enable cracking above a threshold stress, predicts the critical crack spacing that results in shielding or coalescence. In addition, the integration of X-Ray microscopy (XRM) characterisation with FEM modelling demonstrates univocally the role of crack interaction in stress corrosion cracking. We conclude discussing the value of integrating models and experiments to understand complex failure mechanisms.
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ENVIRONMENTAL STRESS CRACKING RESISTANCE OF HIPS UNDER CYCLIC LOADING USING CRACKED ROUND BAR SPECIMENS

The service life of polymers depends strongly on their loading conditions and the environment surrounding them. Prolonged contact of a polymer with an oily or fatty environment increases the tendency of crazing and thus shorten the service life. The objective of this paper was to investigate two different high-impact polystyrene polymers (HIPS) in terms of their environmental stress cracking resistance (ESCR) in air and sunflower oil environments by cyclic testing. It was shown that the HIPS grade with bigger rubber particles, even though it has lower short-term mechanical performance in tensile modulus, yield strength, and notched impact strength, is preferrable in terms of ESCR and should be used in fatty environment applications.
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EXPLORING THE PHNOMENOLOGY AND GOVERNING MECHANISMS FOR THE LOADING RATE DEPENDENCE OF ENVIRONMENTALLY ASSISTED CRACKING IN STRUCTURAL ALLOYS

While literature indicates that the applied loading rate (dK/dt) can affect environmentally assisted cracking (EAC) behavior, the quantification of dK/dt dependencies and mechanistic understanding of why the applied dK/dt influences EAC remain limited. In this study, a slow-rising stress intensity (K) framework was utilized to measure EAC kinetics over dK/dt ranging from 0.2 to 20 MPa√m/hr in Beta-C Ti, AA7075-T651, AA5456-H116, Monel K-500, 304L SS, Pyrowear 675, and Custom 465-H900 stainless steel immersed in 0.6 M NaCl at applied potentials known to promote modest EAC susceptibility. Results demonstrate that the crack growth rate (da/dt) exhibits two characteristics regimes of behavior with increasing dK/dt across multiple alloys. In particular, a ‘plateau’ regime where da/dt is independent of dK/dt was observed for elevated dK/dt, while a ‘linear’ regime where da/dt linearly scales with dK/dt was noted for slow dK/dt. These findings are analysied in the context of stress- and strain-controlled failure criteria and the environmentally modified Ritchie-Knott-Rice criteria for crack advance. The implications of these findings on recent testing standardization efforts for HEAC are then discussed.
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