Hydrostatic Stress Research Articles

Failure criterion for concrete under volumetric stress state conditions

Based on the experiments conducted by the authors, a six-parameter failure criterion for concrete has been developed, which makes it possible to take into account the volumetric stress state in strength calculations of massive concrete and reinforced concrete structures. The developed strength criterion is adapted to a spatial eight-node finite element (solid type) and implemented in the PRINS software. To verify the developed criterion, the work provides a com-parison with both experimental data and calculation results that meet other strength criteria widely used for concrete. Thus, the compression and tension meridians of the developed fracture criterion were compared with experimental data, as well as with the Willam & Warnke criterion and the modified Drucker & Prager criterion with Mohr & Coulomb constants. A comparison of compression meridians shows that in the mode of low hydrostatic stresses , these criteria converge with each other and with experimental data. In the mode of average hydrostatic stresses , the criterion proposed by the authors and the Willam & Warnke criterion show similar results, while the modified Drucker & Prager criterion shows on 20% overestimation of the failure value. In the mode of high hydrostatic stresses , the Willam & Warnke criterion in com-parison with the proposed criterion and experimental data, gives an underestimated value of concrete failure.

A phase‐field model for hydrogen‐promoted fracture based on a mixed rate‐type variational setting

AbstractCertain metals experience a substantial deterioration in mechanical properties when exposed to a hydrogen environment, an effect termed hydrogen embrittlement. To understand, predict, and counteract this hydrogen‐assisted material degradation, sufficiently accurate material models are needed. According to the current hypothesis, hydrogen diffusion is driven by gradients of concentration and hydrostatic stress. To capture this, a phase‐field model is formulated as a multi‐field problem coupling deformation, crack propagation, and diffusion to analyze hydrogen‐promoted fracture. Here, the displacements, a fracture‐related phase‐field, the hydrogen lattice occupancy, and the chemical potential are considered as primary field variables. Approaches proposed in the literature often use an extrapolation of the hydrostatic stress calculated at the material point level onto the nodes and later use the B‐matrix to compute the gradient of hydrostatic stress. In order to circumvent this potentially inaccurate extrapolation, the model is recast into a mixed rate‐type variational setting, where the chemical potential—whose gradient governs the hydrogen flux—is obtained from the numerical solution of a saddle point problem. A representative boundary value problem is presented to demonstrate the applicability of the developed numerical framework.

DETERMINATION OF THE STRESSED METAL STATE DURING HOT ROLLING BY THE FINITE ELEMENT METHOD

Purpose. Determination of the stress-strain state of the metal during the rolling of large ingots to prevent the occurrence of internal defects, and determining the effect of forced cooling of the ingot surface during hot rolling on the stress-strain state.
 Research methods. Finite element method, upper estimate method.
 Results. Based on the finite element method, a comparative simulation of the stress-strain state of the ingot with different cooling times was performed. As a result of the study, it was established that the forced cooling of the ingot surface during hot rolling helps to reduce the probability of the internal continuity defect forming. The given results of comparison of the distribution of strain intensity along the rolling cross-section in the basic version and with additional annealing indicate a decrease in the probability of formation of discontinuities in the axial zone of the ingot. This, in turn, proves the effectiveness of forced annealing of the surface layers of the ingot (workpiece).
 Scientific novelty. A mathematical model of the distribution of the main stress state components was developed. It took into account the redistribution of temperatures and, as a result, the mechanical properties of the metal according to the height of the deformation focus during the hot rolling of relatively large blanks.
 Practical value. The use of forced cooling leads to a significant increase in hydrostatic and normal stresses in the axial zone, reducing the probability of the formation and subsequent growth of internal continuity defects. Thus, the quality of finished products increases, in particular, valuable rolled products made of special grades of steel.

Effect of dislocation slip and deformation twinning on the high-pressure phase transformation in Zirconium

Zirconium transforms from the α→ω phase under high hydrostatic pressures. The critical transformation pressure is expected to be affected by internal stresses associated with defects. This study focuses on the effects of dislocations and twins and their associated stress fields on the transformation. Samples are pre-loaded to seed dislocations or twins. In-situ high-hydrostatic-pressure X-ray synchrotron experiments are performed revealing that microstructures with pre-existing prismatic 〈a〉 dislocations and {101¯2} twins promote the transformation more effectively than pyramidal 〈c + a〉 dislocations or {112¯2} twins. Post-mortem electron backscatter diffraction further shows that pre-seeded defects stabilize the ω-phase at ambient conditions. In-situ stress-hold neutron diffraction experiments are also performed combining both hydrostatic and deviatoric stresses capturing the role of deviatoric stresses on phase transformation kinetics. These results support the findings of recent atomistic simulations indicating flow of prismatic 〈a〉 dislocations at an α-ω interface promotes the growth of the ω domain.

The Effect of Deformation Parameters of Clay-Core on Arching Behaviour of Rockfill Dam

In zoned embankment dams, horizontal and vertical cracks developing in the upstream-downstream direction for various reasons cause internal erosion, resulting in serious consequences such as dam failure. Hydraulic fracturing is one of the mechanisms that cause these cracks to develop in the upstream-downstream direction. Hydraulic fracturing occurs when the stresses at the upstream face of the core are less than or equal to the hydrostatic stresses originating from the reservoir. The arching phenomenon creates the stress environment in which hydraulic fracturing can develop. In transverse arching, one of the arching types, stress transfer occurs from the core to the transition and shell zones. As a result of this stress transfer, the vertical stresses on the upstream surface of the clay core decrease. This study examines the effect on zoned dam transverse arching behavior in combinations where the geomechanical characteristics of the clay core (Elasticity modulus and Poisson ratio) change, provided that the material characteristics in the transition and shell zones are constant. Numerical analyses were carried out using the finite element method using the maximum cross-section of Çınarcık Dam. As a result of numerical analysis, it was seen that the increase in the elasticity modulus and Poisson ratio values, which are the deformation parameters of the clay core, was effective in reducing the transverse arching potential.

An incompressible liquid slit between dissimilar anisotropic elastic media

We use the Stroh sextic formalism to solve the generalized plane strain problem associated with a finite incompressible liquid slit lying between dissimilar anisotropic elastic half-planes subjected to a system of uniform remote stresses. The liquid slit admits an internal uniform hydrostatic stress field and is perfectly bonded to the surrounding media. By enforcing the interface conditions on the solid-solid and liquid-solid interfaces, a Riemann-Hilbert problem of vector form is obtained and is solved analytically using a decoupling procedure. An application of the residue theorem following the imposition of the incompressibility condition of the liquid slit leads to the determination of the internal uniform hydrostatic tension within the liquid slit. When the elastic bimaterial is orthotropic, the internal uniform hydrostatic tension is equal to the remote normal stress component perpendicular to the surfaces of the liquid slit. The presence of the liquid slit will inhibit crack growth along the anisotropic elastic bimaterial interface.

Concrete strength criteria for biaxial and triaxial state of stress

Based on the experiments conducted by the authors, a concrete strength criteria has been developed, which allows us to take into account biaxial and triaxial states of stress in strength calculations of massive concrete and reinforced concrete structures. The developed strength criteria is adapted to a spatial eight-node finite element (solid type) and implemented in the PrinCe programme. In order to conduct the developed criteria operational check, both experimental data and calculation results of other widely used strength criteria for concrete were compared. Thus, to verify the proposed criteria under triaxial compression, the calculation results were compared with experimental data as well as with the Willam-Warnke criterion and the modified Drucker-Prager criterion. The comparison shows that in the low hydrostatic stresses mode results of the above criteria converge both with each other and with the experimental data. In the medium hydrostatic stress mode , the criteria proposed by the authors and the Willam-Warnke criterion show close results, while the modified Drucker-Prager criterion, on the contrary, gives 20 % overestimation of concrete strength. Comparison of the results of concrete biaxial tests shows good agreement with the criteria developed by the authors. Thus, for heavy coarse concrete the differences in strength values were not more than 3,5 %, for heavy fineconcrete and expanded clay concrete — not more than 4 %.

A macroscopic viscoelastic viscoplastic constitutive model for porous polymers under multiaxial loading conditions

A macroscopic constitutive model, the Porous Eindhoven Glass Polymer (Porous EGP) model, is presented to describe the deformation behavior of cavitated rubber toughened polymers under multiaxial loading conditions. It is shown that the proposed macroscopic constitutive model is able to describe the non-linear pre-yield regime, strain rate dependence, post-yield behavior (strain softening and hardening) and void evolution for loading conditions ranging from shear to equi-triaxial (pure triaxial) tension and compression. The Porous EGP model is a combination of a well established non-linear viscoelastic viscoplastic model, the Eindhoven Glassy Polymer (EGP) model, and the modified Gurson model. The Gurson model is adopted to determine the equivalent stress and plastic rate of deformation tensor making it depending on the void volume fraction, deviatoric and hydrostatic stress. The macroscopic constitutive model is developed based on the response of realistic 3D representative volume elements (RVEs) containing randomly positioned mono-disperse inclusions. The constitutive behavior of the matrix phase in this full-field model is described by the EGP model, and the cavitated inclusions are idealized as voids. Their response is studied for a range of void volume fractions, multiaxial loading conditions, strain rates and thermodynamic states. The yield behavior of the heterogeneous material depends non-linearly on the macroscopic hydrostatic stress. This response is well captured with the proposed macroscopic constitutive model.

Unraveling the Performance Decay of Micro-sized Silicon Anodes in Sulfide-based Solid-State Batteries

Solid-state batteries (SSBs) containing Si anodes have recently emerged as a promising solution to overcome challenges associated with Li anodes. However, the development of Si anodes is hindered by the requirement of high external pressure and the unclear understanding of failure mechanisms. Herein, with experiments and simulations under varying Si loads and external pressures, we report that the long diffusion path generates a large Li+ concentration gradient and a pronounced hydrostatic stress gradient, thereby restricting the Li storage capacity of Si via the stress-induced chemical potential. Additionally, the resultant uneven distribution of high Von Mises stress leads to easy cracking of Si anodes during delithiation, thus disturbing the electron/ion transport path, altering reaction sites, and deteriorating the cycle performance of SSBs. External pressure was also shown to have minimal benefits on Li+ diffusion, mainly by reducing the Von Mises stress to reduce the cracking degree for ensuring electron/ion transport, so as to realize the balance of Li+ concentration gradient and electrochemical potential, and avoid Li plating and interfacial side reactions. Our findings propose the failure mechanism of Si anodes in SSBs, emphasizes the importance of minimizing the Li+ concentration gradient under low external pressure, and provide valuable insights for the development of Si-containing SSBs.

Lithiation-induced swelling of electrodes

Understanding the correlation between the deformation field and the diffusion of lithium in electrode materials can provide the knowledge of the effects of materials constants on the deformation behavior of lithiated electrode materials. In this work, we study the lithiation-induced swelling and deformation of three different electrodes of plate-like, cylindrical-shell and spherical-shell shapes under the framework of linear elasticity. Linear relationships between the lithium concentration and hydrostatic stress are derived for all the three different electrodes. For the diffusion of lithium with negligible contribution of stress-limited diffusion to the diffusion of lithium in electrode materials, the lithium-induced swelling is proportional to the partial molal volume of lithium and the state of charge for the three different electrodes of plate-like, cylindrical- and spherical shapes. The increase rate of the normalized surface displacement (the swelling) is the largest at the onset of lithiation and decreases gradually to a plateau with the increase of the lithiation time. Both the constraint and geometrical shape of an electrode play important roles in controlling the swelling and stress evolution in the electrode.

Failure characterization of fully grouted rock bolts under triaxial testing

Confining stresses serve as a pivotal determinant in shaping the behavior of grouted rock bolts. Nonetheless, prior investigations have oversimplified the three-dimensional stress state, primarily assuming hydrostatic stress conditions. Under these conditions, it is assumed that the intermediate principal stress (σ2) equals the minimum principal stress (σ3). This assumption overlooks the potential variations in magnitudes of in situ stress conditions along all three directions near an underground opening where a rock bolt is installed. In this study, a series of push tests was meticulously conducted under triaxial conditions. These tests involved applying non-uniform confining stresses (σ2 ≠ σ3) to cubic specimens, aiming to unveil the previously overlooked influence of intermediate principal stresses on the strength properties of rock bolts. The results show that as the confining stresses increase from zero to higher levels, the pre-failure behavior changes from linear to nonlinear forms, resulting in an increase in initial stiffness from 2.08 kN/mm to 32.51 kN/mm. The load-displacement curves further illuminate distinct post-failure behavior at elevated levels of confining stresses, characterized by enhanced stiffness. Notably, the peak load capacity ranged from 27.9 kN to 46.5 kN as confining stresses advanced from σ2 = σ3 = 0 to σ2 = 20 MPa and σ3 = 10 MPa. Additionally, the outcomes highlight an influence of confining stress on the lateral deformation of samples. Lower levels of confinement prompt overall dilation in lateral deformation, while higher confinements maintain a state of shrinkage. Furthermore, diverse failure modes have been identified, intricately tied to the arrangement of confining stresses. Lower confinements tend to induce a splitting mode of failure, whereas higher loads bring about a shift towards a pure interfacial shear-off and shear-crushed failure mechanism.

Pre-strain and hydrogen charging effect on the plastic and fracture behavior of quenching and partitioning (Q&P) steel

This study investigates the fracture behavior of plastically deformed quenching and partitioning (Q&P) steel under hydrogen conditions. The experimental results from slow strain rate tensile (SSRT) tests reveal a considerable decrease in elongation with increase of hydrogen traps induced by pre-strains. The effect of pre-strain on hydrogen susceptibility is analyzed through fractography and microstructure measurements. The relative reduction of area decreased from 0.389 at 0 % pre-strain to 0.181 at 12 % pre-strain with hydrogen. For hydrogen-free case, fracture surfaces show typical ductile fracture with shallow dimples by micro-void coalescence induced by phase transformation. For hydrogen-charged specimens, the fractography indicates quasi-cleavage and transgranular fracture at specimen center and brittle-like fracture can be analyzed by the hydrogen-enhanced decohesion. Additionally, the brittle fracture area is enlarged with pre-strain. The mixed fracture between the edge and center of cross-section can be explained using finite element modeling, which calculates the distributions of dislocation density and flux, hydrostatic stress during SSRT at different pre-strains. As an effect of pre-strain in the hydrogen-charged Q&P steel, micro-cracks initiate at transformed martensite with low level of pre-strain, whereas their propagation is blunted at ferrite interface. For specimens with pre-strains exceeding 8 %, the cracks initiate predominantly at the martensite, which propagate through deformed ferrite matrix with high dislocations and trapped hydrogens. Our findings shed light on a new metallurgical design for improving the resistance to hydrogen embrittlement in Q&P steel when pre-strain, hydrogen transport, and phase transformation are properly controlled.

Modeling of local hydrogen concentration on microscopic scale to characterize the influence of stress states and non-metallic inclusions in pipeline steels

Hydrogen as an energy carrier plays an important role in achieving the ambitious climate targets associated with decarbonization. The main challenge is to ensure safe transport of gaseous respectively liquid hydrogen through the already existing natural gas pipelines. Since experimental methods, such as hydrogen-induced-cracking tests, fail to provide quantitative answers to the question of which local effects/mechanisms trigger failure under hydrogen loading, complex numerical multi-scale approaches are needed. In this work, a phenomenological crystal plasticity model based on dislocation slip mechanisms was implemented in Abaqus/Implicit and coupled with a hydrogen diffusion model to investigate the influence of microstructural characteristics in terms of grain size, texture, phase fraction but also lattice defects such as dislocations, grain boundaries, carbides, and non-metallic inclusions on hydrogen-induced-damage behavior. As a driving force for hydrogen diffusion, both hydrostatic stresses and plastic strains associated with mechanically and thermally induced residual stresses under various stress states were investigated. It was found that the impurity degree of the steel in terms of non-metallic inclusions plays an important role in hydrogen susceptibility in two aspects, namely geometrically (notch-effect) and in the generation of residual stresses. The effect of matrix shrinkage on the inclusions and the resulting stress field after cooling leads to the accumulation of hydrogen atoms around the inclusions, resulting in locally high critical concentrations. The simulations performed on microstructures with and without non-metallic inclusions under different stress states demonstrated the notched effect that the presence of non-metallic inclusions increases the local hydrogen concentration up to 28 %.

Effect of interface on elastic and toughening behavior in poly lactic acid/thermoplastic starch blends: Micromechanical finite element analysis

AbstractIt is frequently emphasized that the action of interfacial adhesion is a critical parameter to improve the stiffness and toughness of polylactic acid/thermoplastic starch (PLA/TS) blends. In this work, the micromechanical behavior of PLA/TS blends with droplet morphology selected from literature is predicted and analyzed systematically by finite element analysis. A quantitative assessment of the effect of interface (perfect or imperfect) on the elastic behavior and craze initiation for toughening of PLA/TS blends is presented. For the elastic behavior, the PLA phase is the blend's load‐bearing component as the TS is more compliant than PLA, so an interface perfectly bonded reduces the blend's elastic modulus when compared to the modulus obtained if the interface is weakly bonded. Regarding the toughening behavior, as a compliant phase, the TS has the potential to nucleate stable crazes in the host PLA matrix independently of the degree of interfacial adhesion because the highly stressed region lies near the equator of the particle; nonetheless, the critical stress for craze initiation is very sensitive to the TS particle size. On the other hand, as the TS is less capable than PLA to develop large hydrostatic stresses, the TS has a low potential to dissipate energy by cavitation.

Stress‐hybrid virtual element method on quadrilateral meshes for compressible and nearly‐incompressible linear elasticity

AbstractIn this article, we propose a robust low‐order stabilization‐free virtual element method on quadrilateral meshes for linear elasticity that is based on the stress‐hybrid principle. We refer to this approach as the stress‐hybrid virtual element method (SH‐VEM). In this method, the Hellinger–Reissner variational principle is adopted, wherein both the equilibrium equations and the strain‐displacement relations are variationally enforced. We consider small‐strain deformations of linear elastic solids in the compressible and near‐incompressible regimes over quadrilateral (convex and nonconvex) meshes. Within an element, the displacement field is approximated as a linear combination of canonical shape functions that are virtual. The stress field, similar to the stress‐hybrid finite element method of Pian and Sumihara, is represented using a linear combination of symmetric tensor polynomials. A 5‐parameter expansion of the stress field is used in each element, with stress transformation equations applied on distorted quadrilaterals. In the variational statement of the strain‐displacement relations, the divergence theorem is invoked to express the stress coefficients in terms of the nodal displacements. This results in a formulation with solely the nodal displacements as unknowns. Numerical results are presented for several benchmark problems from linear elasticity. We show that SH‐VEM is free of volumetric and shear locking, and it converges optimally in the norm and energy seminorm of the displacement field, and in the norm of the hydrostatic stress.

The crucial role of periodontal ligament's Poisson's ratio and tension-compression asymmetric moduli on the evaluation of tooth displacement and stress state of periodontal ligament

The hydrostatic stress in the periodontal ligament (PDL) evaluated by finite element analysis is considered an important indicator for determining an appropriate orthodontic force. The computed result of the hydrostatic stress strongly depends on the PDL material model used in the orthodontic simulation. This study aims to investigate the effects of PDL Poisson's ratio and tension-compression asymmetric moduli on both the simulated tooth displacement and the PDL hydrostatic stress. Three tension-compression symmetric and two asymmetric PDL constitutive models were selected to simulate the tensile and compressive behavior of a PDL specimen under uniaxial loading, and the resulting numerical results were compared with the in-vitro PDL experimental results reported in the literature. Subsequently, a tooth model was established, and the selected constitutive models and parameters were employed to assess the hydrostatic stress state in the PDL under two distinct loading conditions. The simulated results indicate that PDL Poisson's ratio and tension-compression asymmetry exert substantial influences on the simulated PDL hydrostatic stress. Conversely, the elastic modulus exhibits minimal impact on the PDL stress state under the identical loading conditions. Furthermore, the PDL models with tension-compression asymmetric moduli and appropriate Poisson's ratio yield more realistic hydrostatic stress. Hence, it is imperative to employ suitable Poisson's ratio and tension-compression asymmetric moduli for the purpose of characterizing the biomechanical response of the PDL in orthodontic simulations.

Impacts of nanofiller shapes on the interface confinement effect in polymer nanocomposites

Shape effect of nanofillers (NFs) on the properties of nanocomposites has attracted considerable attention in recent research. The present study aims to examine how NF shapes influence the confinement effect of polymer-NF interface on the physical properties of nearby polymer (the interphase). Herein molecular dynamics simulations are performed to capture the shift of interphase properties when NFs change from nanosphere to nanocylinder and nanofilm geometry. A stress analysis is then carried out in the interphase to identify a key parameter for characterizing the stress-property coupling and revealing the pathway of the NF shapes to impact the interphase properties. It is shown that the peak hydrostatic stress σhyd quantifies the effect of internal stresses on the properties of the interphase. Specifically, the shape transition considered can substantially enhance the peak σhyd by flattening the stress space in the interphase without largely affecting the polymer-NF interaction. Increased peak σhyd in turn upshifts the interphase properties and thus, leads to the coupling between the NF shapes and interface confinement effect.

Significant mobilization of REE from calc-alkaline melt into fluid during fluid exsolution: Insights from magmatic amphibole composition from Tieshan skarn Fe-Cu deposit (Hubei Province, China)

Hydrothermal fluids associated with carbonatites and peralkaline silicate rocks typically exhibit high REE content, but this is generally not the case for calc-alkaline rocks. However, the Tieshan skarn Fe-Cu deposit in the Edong district of Hubei Province in China demonstrates economic REE mineralization potential linked to calc-alkaline rocks. To investigate the presence of REE enrichment in the fluid, we present textural and compositional data of rock-forming mineral amphibole from the quartz diorite in the Tieshan deposit. Three distinct types of amphibole are identified in the Tieshan quartz diorite, namely Early amphibole, Main-stage amphibole, and Late amphibole, following the crystallization sequence. Early amphibole (813–864 °C, 115–152 MPa) crystallized when the magma was emplaced at a depth of approximately 4.8 km. Main-stage amphibole (741–806 °C, 58–103 MPa) crystallized at a depth of roughly 3 km. Between the early and main-stage amphibole crystallization, the magma experienced fractional crystallization of pyroxene, amphibole, and plagioclase, resulting in a REE enrichment in the residual melt. Late amphibole crystallized subsequent to the transition from lithostatic to hydrostatic stress states (656–729 °C, 27–52 MPa). Significantly, the transport of REE from the magma system initiated towards the end of Main-stage amphibole crystallization. A comparison with the Tonglvshan granitic rocks in the same district indicates an elevated F content and extended fractional crystallization history in the Tieshan magma, contributing to REE enrichment in its melt. Furthermore, the high F content lowered the solidus temperature of the Tieshan magma, thereby facilitating the more efficient removal of REE into the fluid. The assimilation of evaporate into the magma possibly led to heightened sulfur and chlorine levels in the fluid, enabling the transport of a significant amount of REE.

Interaction between an edge dislocation and a circular incompressible liquid inclusion

We use Muskhelishvili’s complex variable formulation to study the interaction problem associated with a circular incompressible liquid inclusion embedded in an infinite isotropic elastic matrix subjected to the action of an edge dislocation at an arbitrary position. A closed-form solution to the problem is derived largely with the aid of analytic continuation. We obtain, in explicit form, expressions for the internal uniform hydrostatic stresses, nonuniform strains and nonuniform rigid body rotation within the liquid inclusion; the hoop stress along the liquid-solid interface on the matrix side and the image force acting on the edge dislocation. We observe that (1) the internal strains and rigid body rotation within the liquid inclusion are independent of the elastic property of the matrix; (2) the internal hydrostatic stress field within the liquid inclusion is unaffected by Poisson’s ratio of the matrix and is proportional to the shear modulus of the matrix; and (3) an unstable equilibrium position always exists for a climbing dislocation.

Effect of Tensile Loading and Temperature on the Hydrogen Solubility of Steels at High Gas Pressure

The hydrogen solubility in ferritic and martensitic steels is affected by hydrostatic stress, pressure, and temperature. In general, compressive stresses decrease but tensile stresses increase the hydrogen solubility. This important aspect must be considered when qualifying materials for high‐pressure hydrogen applications (e.g., for pipelines or tanks) by using autoclave systems. In this work, a pressure equivalent for compensating the effect of compressive stresses on the hydrogen solubility inside of closed autoclaves is proposed to achieve solubilities that are equivalent to those in pipelines and tanks subjected to tensile stresses. Moreover, it is shown that the temperature effect becomes critical at low temperatures (e.g., under cryogenic conditions for storing liquid hydrogen). Trapping of hydrogen in the microstructure can increase the hydrogen solubility with decreasing temperature, having a solubility minimum at about room temperature. To demonstrate this effect, the generalized law of the hydrogen solubility is parameterized for different steels using measured contents of gaseous hydrogen. The constant parameter sets are verified and critically discussed with respect to the high‐pressure hydrogen experiments.