Static Stress Changes Research Articles

Rheological study of methane gas hydrates in the presence of micron-sized sand particles

Natural gas hydrates, intricate crystalline structures formed by water molecules and small gas molecules, have emerged as a significant and globally impactful clean energy resource. However, their commercial exploitation faces challenges, particularly operational disruptions caused by sand-related blockages. Understanding the rheological properties of hydrate slurry, especially in the presence of micron-sized sand particles, is imperative for ensuring the flow assurance of subsea hydrate exploitation. This study extensively investigates the rheological properties of sand-containing hydrate slurries. The findings reveal that these slurries exhibit non-Newtonian fluid characteristics, including yield stress, thixotropy, and shear-thinning behavior. Solid-like elastic features are observed in sand-containing hydrate slurries before yielding, transitioning to viscous behavior after yielding. Even with a minimal amount of sand, both static yield stress and yield strain experience substantial changes, correlating with the increase in sand concentration. The research conclusively establishes the thixotropic nature of sand–hydrate slurries, where the viscosity decay rate is directly influenced by the shear rate. These insights aim to contribute comprehensively to the development of effective flow assurance strategies, ensuring the safe and stable operation of subsea hydrate exploitation.

Hydrological constraints on the potential of enhanced geothermal systems in the ductile crust

Continental crust at temperatures > 400 °C and depths > 10–20 km normally deforms in a ductile manner, but can become brittle and permeable in response to changes in temperature or stress state induced by fluid injection. In this study, we quantify the theoretical power generation potential of an enhanced geothermal system (EGS) at 15–17 km depth using a numerical model considering the dynamic response of the rock to injection-induced pressurization and cooling. Our simulations suggest that an EGS circulating 80 kg s−1 of water through initially 425 ℃ hot rock can produce thermal energy at a rate of ~ 120 MWth (~ 20 MWe) for up to two decades. As the fluid temperature decreases (less than 400 ℃), the corresponding thermal energy output decreases to around 40 MWth after a century of fluid circulation. However, exploiting these resources requires that temporal embrittlement of nominally ductile rock achieves bulk permeability values of ~ 10–15–10–14 m2 in a volume of rock with dimensions ~ 0.1 km3, as lower permeabilities result in unreasonably high injection pressures and higher permeabilities accelerate thermal drawdown. After cooling of the reservoir, the model assumes that the rock behaves in a brittle manner, which may lead to decreased fluid pressures due to a lowering of thresholds for failure in a critically stressed crust. However, such an evolution may also increase the risk for short-circuiting of fluid pathways, as in regular EGS systems. Although our theoretical investigation sheds light on the roles of geologic and operational parameters, realizing the potential of the ductile crust as an energy source requires cost-effective deep drilling technology as well as further research describing rock behavior at elevated temperatures and pressures.

Prediction of geomechanical changes in faulted rock mass around underground structures subjected to earthquakes

Faults are a type of geological structure that often leads to geological disasters in underground structures under normal conditions and especially when earthquakes occur, thereby requiring careful study.This article introduces some numerical simulation results on the seismic wave propagation process, on changes in stress and displacement states in the rock mass with a small fault of a dip angle of 45o, around a circular cross-section tunnel, using the discrete element method (Universal distinct element Code - UDEC). Three cases are simulated: a) The tunnel is located in the hanging wall rock, the fault is located 10m from the centre of the tunnel on the vertical axis; b) The fault runs through the centre of the tunnel and c) The tunnel is located in the footing wall rock, the fault is located 10m from the centre of the tunnel on the vertical axis. The results obtained show a very clear influence of the position and location of the fault on the geomechanical changes in the rock mass around the tunnel, contributing to the reasonable design of tunnels subjected to earthquakes.

Pore Fluid Pressure in St. Gallen Geothermal Field (Switzerland) Based on Earthquake Focal Mechanisms

AbstractInduced seismicity represents a negative drawback during subsurface exploitation for geothermal energy production. Understanding the triggering mechanisms of induced earthquakes can help implement effective seismic hazard mitigation actions. Among the triggering mechanisms, the pore fluid pressure is of primary importance. Here we provide a static picture of the excess pore fluid pressure at the hypocenters of a seismic sequence induced at the deep geothermal field in St. Gallen, Switzerland, in July 2013. We find that in addition to the Coulomb static stress change, fluids play a key role in promoting the sequence. The estimated excess pore fluid pressure for approximately half of the earthquakes is higher than the injection pressure necessary during the well control phase to fight the unexpected gas kick, that accidently occurred during field operations when a trap of overpressured gas was broken.

Slip-weakening friction controls coseismic displacements in a 3D fracture network: Implications for the long-term safety of nuclear waste repositories

During the long-term operational lifespan of nuclear waste repositories in crystalline rock formations, large earthquakes along nearby seismogenic fault zones may occur, coseismically triggering shear displacements of secondary fractures within the respository site. In addition, these secondary fractures that may be associated with slip-weakening friction could accommodate significant slip instabilities and large shear displacements. A cumulative shear displacement exceeding 50 mm could affect the integrity of waste canisters, potentially resulting in the escape of hazardous radionuclides into the groundwater system. To investigate this problem, we develop a novel 3D seismo-mechanical model to simulate the transient rupture of a primary seismogenic fault zone and coseismic slips in a network of secondary fractures located around the primary fault. A plausible postglacial earthquake scenario is studied, where the rupture along the seismogenic fault propagates outward from a predefined hypocenter, with the resulting static stress changes and dynamic ground vibrations captured. We explore different cases with secondary fractures having different degrees of slip-weakening friction, which is found to strongly control the spatial decay of coseismic fracture displacements in the system. The findings derived from our study have significant implications for assessing the long-term safety of nuclear waste repositories in faulted and fractured crystalline rocks.

Spatiotemporal Variations and Postseismic Relaxation Process Around Mt. Fuji, Japan, During and After the 2011 Tohoku‐Oki Earthquake

AbstractTo monitor the volcanoes at a high spatiotemporal resolution, we introduce the singular value decomposition‐based Wiener filter and the three‐component waveforms in ambient noise velocity monitoring. The continuous ambient noise data from 63 stations around Mt. Fuji and Mt. Hakone, Japan, during the January‐September 2011 were analyzed to estimate the seismic velocity variations at a 1‐day temporal resolution, allowing us to distinguish the velocity drops caused by the 2011 Mw 9.0 Tohoku‐oki and the Mw 6.0 East Shizuoka earthquake. The velocity drop during the Tohoku‐oki earthquake was large in volcanic areas and was larger around Mt. Hakone than Mt. Fuji. This difference is possibly due to the existence of fluid‐ and gas‐rich zones at shallower depths and a higher crack density around Mt. Hakone. In addition, the velocity drop at Mt. Fuji during the Tohoku‐oki and the East Shizuoka earthquake was the same level, despite larger static stress changes beneath Mt. Fuji during the East Shizuoka earthquake. We interpret this inconsistency between the velocity drops and static stress changes to arise from incomplete recovery of the generated cracks during the Tohoku‐oki earthquake when the East Shizuoka earthquake occurred. This study also investigates the spatial variations in recovery speed and recovery amount, finding slow recovery speeds in the volcanic areas and fault areas, possibly due to larger crack densities in the crust. Furthermore, we observe the lowest velocity recovery amount in the volcanic areas, which is likely attributed to the maintained increase in pore pressure due to the volcanic gas bubbles.

Superior Yield Strength and High‐Tensile‐Toughness Matching of Medium‐Mn Steel with Gradient Structure Developed by Cyclic Torsion

A multiphase‐gradient‐structured medium‐Mn steel with dislocation/stacking fault (SF) density gradient, austenite fraction gradient, and grain size gradient is designed and fabricated by cyclic torsion. It exhibits higher yield strength (YS) (942 MPa), ultimate tensile strength (1295 MPa), and tensile toughness (290 mJ mm−3) than its homogeneous counterpart (672, 1273 MPa, and 278 mJ mm−3, respectively). High‐density‐interlaced SFs are generated at the surface region because of the change in strain path and stress state during cyclic torsion. These massive interactive SFs promote the persistent martensite transformation of the retained austenite in the surface region within the entire uniform strain range. Therefore, in addition to the central layer, the surface region in the gradient structures provides strong strain hardening. Moreover, the continuous martensite transformation in the surface region allows the gradient structure to be retained even after tensile failure, which is beneficial for improving the heterogeneous‐deformation‐induced (HDI) hardening of the gradient‐structured sample during tensile deformation. Thus, the combination of active and persistent transformation‐induced plasticity effect and HDI hardening contributes to its excellent tensile toughness and ductility. Quantitative analysis indicates that HDI strengthening and dislocation strengthening play a dominant role in the sample's ultrahigh YS.

3D active fault kinematic behaviour reveals rapidly alternating near-surface stress states in the Eastern Alps

Abstract Stress variations in the Earth's crust need to be understood in both the spatial and temporal domains to address a number of pressing societal issues. In this paper, precise three-dimensional records of fault kinematic behaviour obtained by mechanical extensometers are used to investigate changes in stress states along major faults in the Eastern Alps. The monitored faults are fractures with evident Upper Quaternary displacement and are directly attributed to their master tectonic structures. The results demonstrate that activity at the submillimetric scale is highly episodic; periods of repose are punctuated by conspicuous reactivation events affecting one or more of the displacement components. An original approach named the SMB2018 method is used to define the stress state associated with each fault reactivation event. The outputs evidence significant short-term changes in the local stress regime. The directions of the principal normal stresses calculated from these reactivation events present generally similar patterns for both compressional and extensional stress states. Consequently, submillimetric fault activity cannot be controlled by a rotating stress field; such shifts can only be caused by a change in the magnitude of the individual principal normal stresses so that the maximum compression changes to the minimum and vice versa.

Phase field fracture in elasto-plastic solids: Considering complex loading history for crushing simulations

Phase field models have gained growing popularity in fracture analysis. However, phase field modelling remains lacking for complex real-life applications such as structural crushing analyses to date. The novelty of this study is to propose an explicit phase field modelling framework considering complex loading history to predict the crushing behaviour of additively manufactured metallic structures. First, the explicit phase field model is formulated by considering stress state dependent fracture initiations with non-proportional loading. Then, five types of material specimens with a wide range of stress states are experimentally tested to calibrate material parameters for additively manufactured 316L steel and Ti-6Al-4V titanium. Last, the square tubes are 3D-printed and tested under three-point bending and axial compression to demonstrate the capacity of the proposed model. The modelling results unveil that the crushing behaviour of both materials could be properly reproduced in terms of force-displacement curves and crack paths. Finally, the fracture mechanism of crushing deformation is analysed. For the three-point bending of Ti-6Al-4V tubes, cracks are mainly induced by the medium stress triaxiality tension and shear loading. For the axial compression of 316L tubes, the stress states of critical elements are relatively diverse yet slightly concentrated at the shear, compression-shear, and high stress triaxiality tension regions. Remarkably, non-proportional loading is significant in crushing deformation as a material point may experience a significant change in stress state with loading. By considering a non-proportional loading dependent threshold, the proposed phase field model can predict the crushing behaviour of structures even when the fracture initiations do not locate on the fracture locus. This study provides an effective explicit phase field framework featured with an insightful fracture mechanism for quasi-static crushing of additively manufactured metallic materials.

Detection of thermo-mechanical damages by in-process Barkhausen Noise Analysis combined with Grinding Power Evaluation

Grinding of workpieces from hardened steel, especially in case of safety relevant components, usually requires a reliable method for the detection of thermo-mechanical surface damages. Compared to the common post-process methods, in-process detection promises saving of detection time and, particularly, a fast reaction to negative influences on the surface integrity. The present work shows the suitability of magnetic Barkhausen noise (BN) for the in-process detection of thermo-mechanically influenced regions during an outer diameter cylindrical grinding process. Different from other non-destructive inspection methods, BN allows the detection of changes of the residual stress state even before the occurrence of visible tempering zones. However, in case of pronounced tempering or rehardening zones, a single BN value does not enable an unambiguous statement about the surface state. Therefore, additionally the specific grinding power Pc” in dependence of the contact time Δt is considered, which allows to exclude grinding burn starting from light tempering zones. A particular focus of this study is the BN analysis during grinding with different types of grinding wheels.

Machining-Induced Distortion during Peripheral Milling of High Strength Aluminum Parts

In the aerospace and automotive industries aluminum sheet metal components are often produced by forming and post-machining. Due to the change in the complex stress state during manufacturing, part distortion is a major challenge. The overall objective of this work is to streamline the process chain to minimize production costs. For this, a determination of the material properties and analysis of the machining process parameters and their influence on the resulting distortion are required.First, tensile tests of the aluminum alloy EN AW-7075-T651 were performed. Subsequently, a series of machining tests were conducted to examine the effect of the machining process on the resulting part distortion. This paper presents machining-induced part distortion depending on the variation of the process parameters for down milling operations of thick aluminum plate material. It was found that the radial depth of cut and the cutting speed have a high impact on the resulting distortion, while the feed rate has a small influence.

Rockburst Hazard Control Using the Excavation Compensation Method (ECM): A Case Study in the Qinling Water Conveyance Tunnel

Rockburst disasters occur frequently during deep underground excavation, yet traditional concepts and methods can hardly meet the requirements for support under high geo-stress conditions. Consequently, rockburst control remains challenging in the engineering field. In this study, the mechanism of excavation-induced rockburst was briefly described, and it was proposed to apply the excavation compensation method (ECM) to rockburst control. Moreover, a field test was carried out on the Qinling Water Conveyance Tunnel. The following beneficial findings were obtained: Excavation leads to changes in the engineering stress state of surrounding rock and results in the generation of excess energy ΔE, which is the fundamental cause of rockburst. The ECM, which aims to offset the deep excavation effect and lower the risk of rockburst, is an active support strategy based on high pre-stress compensation. The new negative Poisson’s ratio (NPR) bolt developed has the mechanical characteristics of high strength, high toughness, and impact resistance, serving as the material basis for the ECM. The field test results reveal that the ECM and the NPR bolt succeed in controlling rockburst disasters effectively. The research results are expected to provide guidance for rockburst support in deep underground projects such as Sichuan–Xizang Railway.

Micromechanical investigation of the effect of an imposed stress state change during drained cyclic loading using DEM

This study investigates the effect of a stress state change at an intermediate stage during cyclic loading on the drained cyclic deformation of soil using Discrete Element Method (DEM) simulations. Drained cyclic triaxial simulations were conducted on an assembly of irregularly shaped particles, with the stress state shifting along different stress paths at an intermediate stage of cyclic loading. The analysis of the macro- and microscopic responses reveals a link between the cyclic stress–dilatancy behaviour and the anisotropy of the granular assembly. In the simulations without the stress state change or with a stress ratio increase, the strain accumulation direction aligns with the prediction of the Modified Cam Clay (MCC) flow rule; and the strain accumulation is faster at higher stress ratios when the contact distribution is more anisotropic. However, when the particle assembly experiences a stress ratio decrease during cyclic loading, the subsequent strain accumulation direction deviates from the MCC flow rule, and strains accumulate more rapidly at lower stress ratios when the contact distribution is nearly isotropic. A tentative explanation is proposed to explain the altered strain accumulation direction after the stress ratio decrease.

Innovative stress analysis and machine learning forecasting for semi-trailer truck body durability

This article presents an in-depth analysis of the stress-deformation state (SDS) in the bottom structure of a semi-trailer truck body. Engineering analysis was conducted utilizing the SolidWorks software, focusing on a comprehensive CAD model of the semi-trailer truck body. The study explored variations in SDS parameters resulting from alterations in the geometric parameters of the body bottom elements. The research investigated alterations in static stress and displacement relative to changes in the proportions of the cross-section of the channel while maintaining fixed geometric dimensions of the workpiece, thickness of the workpiece, and the material of the body bottom. Graphical representations were generated to illustrate the variations in static stress, displacement, and safety margin concerning the thickness of the shelf and channel. Additionally, dependencies were derived that correlate static stresses in the channel with the thickness of the channel wall and the thickness of the body bottom sheet. The study results were compiled and summarized, offering valuable insights into the stress-deformation state of the semi-trailer truck body's bottom. Furthermore, machine learning techniques, specifically the RandomForest algorithm, were implemented in a Python environment to predict changes in static stress based on various factors. The model's predictions were validated by comparing predicted static stress values with actual values on a test sample. These findings facilitate efficient selection of appropriately sized elements by predicting static stress values, employing the RandomForest machine learning algorithm.

Soil disturbance evaluation of soft clay based on stress-normalized small-strain stiffness

Soil disturbance includes the change of stress state and the damage of soil structure. The field testing indices reflect the combined effect of both changes and it is difficult to identify the soil structure disturbance directly from these indices. In the present study, the small-strain shear modulus is used to characterize soil structure disturbance by normalizing the effective stress and void ratio based on Hardin equation. The procedure for evaluating soil sampling disturbance in the field and the further disturbance during the subsequent consolidation process in laboratory test is proposed, and then validated by a case study of soft clay ground. Downhole seismic testing in the field, portable piezoelectric bender elements for the drilled sample and bender elements in triaxial apparatus for the consolidated sample were used to monitor the shear wave velocity of the soil from intact to disturbed and even remolded states. It is found that soil sampling disturbance degree by conventional thin-wall sampler is about 30% according to the proposed procedure, which is slightly higher than that from the modified volume compression method proposed by Hong and Onitsuka (1998). And the additional soil disturbance induced by consolidation in laboratory could reach about 50% when the consolidation pressure is far beyond the structural yield stress, and it follows the plastic volumetric strain quite well.

Study of the process of connecting rod liners’ shape changing in the pre-failure operation period of transport and technological machines and equipment

BACKGROUND: During the operation of transport and technological machines and equipment, the most frequent failure is loose running of connecting rod liners of a crankshaft. Previous studies on tractor, locomotive and automotive diesel engines did not sufficiently disclose the reasons for loose running of liners. AIMS: Study of the stress-strain state of the steel base of connecting rod liners as the initial and developing factors of the process leading to failure. METHODS: The research is based on the analysis of numerous works devoted to the study of the causes of loose running of connecting rod liners of various internal combustion engines. The methods and results on the operational and bench studies of the stress state and shape changing of connecting rod liners for the KAMAZ-740 (8Ch 12/12) engine obtained by the team of authors are used as the basic data. RESULTS: The process of change in stress state and consequent change in geometric parameters of the steel base, accompanied by change in the shape of connecting rod liners of the KAMAZ-740 engines has been revealed. It is established with the study that the occurrence of failure is not sudden. The process proceeds according to a certain pattern in three stages. The first stage is characterized by the appearance of deformation of a liner due to excessive stress state; the second stage is caused by development of obtained deformations up to critical values; the third stage is loose running of a connecting rod due to obtained deformations and violations of liner’s hydrodynamics conditions. The main result is justification of requirements and conditions for ensuring stability of parameters and shape of liners during long-term operation of engine. CONCLUSIONS: Knowledge of the process leading to the change in shape of crankshaft bearing liners makes it possible to develop design and technological solutions and recommendations to eliminate failures by optimizing the parameters of liners.

Investigation of ductile damage in dual phase steel during tensile deformation by in situ X-ray computed tomography

In situ synchrotron radiation X-ray computed tomography (CT) is used to understand the void nucleation and growth behaviour of dual phase (DP) steel, DP800, during uniaxial tensile deformation. The voids were identified post mortem as having nucleated primarily in the (41 vol%) martensite phase. The behaviour of individual voids was found to be broadly in line with a Rice and Tracey style growth model (their volume increasing by a factor of 10), while the mean void size was broadly in line with a model that included the continuous nucleation of new voids, as well as the growth of existing voids, throughout straining. Voids tended to elongate during uniaxial (low stress triaxiality) straining, but then to dilate more isotropically as necking led to increased triaxiality. One large pre-existing void was found to elongate quicky at first, but then for growth to slow, presumably because it was not located in the highest triaxial stress (necked) region. Perhaps surprisingly the average size of the void population grows only slowly (by a factor of 2) during straining; this is because while the size of each void grows, new small voids are always being nucleated. Finally, the changes of stress triaxiality and shear stress state during tensile deformation of the square cross-section smooth tensile specimen are investigated via finite element modelling, to qualitatively assess the impact of sample geometry on void nucleation behaviour and fracture strain in the current study.

Enhanced Microseismicity During Production Pumping Cessation at the San Emidio Geothermal Field (Nevada, USA) in December 2016

AbstractTectonic activity, geothermal fluids, and microseismic events (MSEs) tend to occur in similar locations as a result of spatiotemporal changes in the subsurface stress state. To quantify this association, we analyze data from a dense seismic array deployed at the San Emidio geothermal field, Nevada for 1 week in December 2016 to coincide with a 19.45‐hr shutdown of all injection and production pumping operations. 123 MSEs were detected, of which 101 occurred during the shutdown. The spatial association of the MSEs with the production wells suggests a causal relationship between the production cessation and the MSEs. Here we performed a detailed analysis to investigate reservoir material properties, distribution of seismically activated faults, and local stress state. We determined the hypocenters, magnitudes, and focal mechanisms for the MSEs, P‐wave tomographic velocity model, and local stress tensor. The results show that most MSEs occurred near the production wells. Magnitudes fall between −2.2 and 0.0 with larger events located closer to the production wells. Most MSEs occurred within a westward‐dipping normal fault zone in the reservoir associated with anomalously low P‐wave velocity values. The focal mechanism and stress inversion results show predominantly normal faulting with the maximum horizontal stress oriented north‐south. We suggest that the MSEs during shutdown were triggered on pre‐existing, small‐scale, critically stressed fault patches in the reservoir as the pore pressure increased around the production wells when the production pumping ceased. We interpret the larger MSE magnitudes closer to the production wells as a result of higher pore pressure increase.

Magmatic connectivity among six Galápagos volcanoes revealed by satellite geodesy

Shallow magmatic reservoirs that produce measurable volcanic surface deformation are often considered as discrete independent systems. However, petrological analyses of erupted products suggest that these may be the shallowest expression of extensive, heterogeneous magmatic systems that we show may be interconnected. We analyse time series of satellite-radar-measured displacements at Western Galápagos volcanoes from 2017 to 2022 and revisit historical displacements. We demonstrate that these volcanoes consistently experience correlated displacements during periods of heightened magma supply to the shallow crust. We rule out changes in static stress, shallow hydraulic connections, and data processing and analysis artefacts. We propose that episodic surges of magma into interconnected magmatic systems affect neighbouring volcanoes, simultaneously causing correlations in volcanic uplift and subsidence. While expected to occur globally, such processes are uniquely observable at the dense cluster of Western Galápagos volcanoes, thanks to the high rate of surface displacements and the wealth of geodetic measurements.

Effects of CPAP and FiO2 on respiratory effort and lung stress in early COVID-19 pneumonia: a randomized, crossover study

Backgroundin COVID-19 acute respiratory failure, the effects of CPAP and FiO2 on respiratory effort and lung stress are unclear. We hypothesize that, in the compliant lungs of early Sars-CoV-2 pneumonia, the application of positive pressure through Helmet-CPAP may not decrease respiratory effort, and rather worsen lung stress and oxygenation when compared to higher FiO2 delivered via oxygen masks.MethodsIn this single-center (S.Luigi Gonzaga University-Hospital, Turin, Italy), randomized, crossover study, we included patients receiving Helmet-CPAP for early (< 48 h) COVID-19 pneumonia without additional cardiac or respiratory disease. Healthy subjects were included as controls. Participants were equipped with an esophageal catheter, a non-invasive cardiac output monitor, and an arterial catheter. The protocol consisted of a random sequence of non-rebreather mask (NRB), Helmet-CPAP (with variable positive pressure and FiO2) and Venturi mask (FiO2 0.5), each delivered for 20 min. Study outcomes were changes in respiratory effort (esophageal swing), total lung stress (dynamic + static transpulmonary pressure), gas-exchange and hemodynamics.ResultsWe enrolled 28 COVID-19 patients and 7 healthy controls. In all patients, respiratory effort increased from NRB to Helmet-CPAP (5.0 ± 3.7 vs 8.3 ± 3.9 cmH2O, p < 0.01). However, Helmet’s pressure decreased by a comparable amount during inspiration (− 3.1 ± 1.0 cmH2O, p = 0.16), therefore dynamic stress remained stable (p = 0.97). Changes in static and total lung stress from NRB to Helmet-CPAP were overall not significant (p = 0.07 and p = 0.09, respectively), but showed high interpatient variability, ranging from − 4.5 to + 6.1 cmH2O, and from − 5.8 to + 5.7 cmH2O, respectively. All findings were confirmed in healthy subjects, except for an increase in dynamic stress (p < 0.01). PaO2 decreased from NRB to Helmet-CPAP with FiO2 0.5 (107 ± 55 vs 86 ± 30 mmHg, p < 0.01), irrespective of positive pressure levels (p = 0.64). Conversely, with Helmet’s FiO2 0.9, PaO2 increased (p < 0.01), but oxygen delivery remained stable (p = 0.48) as cardiac output decreased (p = 0.02). When PaO2 fell below 60 mmHg with VM, respiratory effort increased proportionally (p < 0.01, r = 0.81).ConclusionsIn early COVID-19 pneumonia, Helmet-CPAP increases respiratory effort without altering dynamic stress, while the effects upon static and total stress are variable, requiring individual assessment. Oxygen masks with higher FiO2 provide better oxygenation with lower respiratory effort.Trial registration Retrospectively registered (13-May-2021): clinicaltrials.gov (NCT04885517), https://clinicaltrials.gov/ct2/show/NCT04885517.