Representative Loading Conditions Research Articles

A multiscale modeling for progressive failure behavior of unidirectional fiber-reinforced composites based on phase-field method

The effective macroscopic properties of composites are determined by the intricate interactions among the individual components within their microstructure. Preserving these microscopic details during the failure simulation of macrostructures presents significant challenges. This work proposes a multiscale modeling framework to numerically predict the macroscopic fracture properties of unidirectional fiber-reinforced composites based on micromechanical analysis. In this study, 2D representative volume elements (RVEs) combined with the phase-field method are utilized to simulate fiber-reinforced composites under transverse loadings. A series of representative loading conditions are employed to investigate cracking patterns and to construct failure strength envelopes of the composites subjected to different multiaxial proportional loadings. By extracting the softening curve from the uniaxial tensile simulation of the RVE and fitting it with a tenth-order polynomial, the homogenized cohesive law, combined with the phase-field method, is applied to the damage analysis of macroscopic heterogeneous materials. The homogenized model of unidirectional fiber reinforced composites is numerically validated through simulations of a 2D flat plate. The simulation results demonstrate the excellent potential of the proposed multiscale modeling framework to accurately and efficiently predict the progressive failure and fracture behavior of fiber-reinforced composites in engineering applications.

Determination of distal fibula fracture for prediction of ankle injuries

Objective This study quantified the local fracture tolerance of the distal fibula. The purpose of this data is to refine the understanding of ankle fracture tolerance in the population and enhance injury prediction by computational tools. Methods Fracture patterns of the fibula were obtained from the US National Highway Traffic Safety Administration (NHTSA) Crash Injury Research Engineering Network (CIREN). Of 143 cases of ankle injury, 120 included fibula fracture, many of which accompanied by radiology images. The most common fibula fracture type was a Weber C fracture, which was then used as the testing target to replicate with postmortem human surrogates. Isolated distal fibulae (male and female, across a wide anthropometric range) were subjected to quasi-static lateral-medial four-point bending superimposed on axial precompression. Results Of the 20 specimens tested, 17 fractured in compression and bending, two fractured in compression, bending, and shear, and one did not fracture upon the imposed displacement. Fractures occurring outside of the target testing span were treated as right-censored data points. Fracture patterns varied among specimens, with oblique fractures being most common, followed by segmental fractures. At failure, compressive force ranged from 77 N to 370 N and bending moment from 17 Nm to 47 Nm. Conclusions From the 19 fractured specimens, the variety of fracture patterns observed were generally consistent with the range of fibula fracture types and locations commonly observed in the field. While comparative studies with fibula specimens are largely absent from literature, comparison of this study to other long bones (radius and ulna) show that bending moment at fracture is similar. To the author’s knowledge, this is the first study to quantify the fracture tolerance of isolated distal fibulae under loading conditions representative of ankle injury mechanisms in motor vehicle collisions. By combining the results of this study with complementary results from parallel tibia-focused experiments (currently in-press), these results will aid development of tissue-level lower leg fracture prediction of human body models, thus enhancing prediction of ankle injury during safety assessment simulations. Specifically, the fracture tolerance information might generate an injury risk function to guide tissue-level injury prediction with human body models.

Proposal of a New Control System Making Use of AI Tools to Predict a Ship’s Behaviour When Approaching the Synchronism Phenomenon

In spite of the IMO’s efforts to improve the safety of intact stability on ships, synchronous rolling still causes large rolling angles, resulting in a very dangerous situation on board. The implementation of automatic tools to predict this undesirable situation has not been widely implemented. Furthermore, the safety of ship stability is not the only responsibility of people involved in the design process, so ship operators must have enough knowledge to predict and avoid these dangerous situations well in advance. Therefore, from a theoretical point of view, in the first part, this paper aims to present a valuable guiding tool for ship operators in order to predict synchronous rolling and avoid undesirable situations on board by making use of only empirical observations of the wave profile and moments. With this purpose, mathematical models are first proposed, with the ship sailing in the worst condition, i.e., with and without considering the damping factor, at zero speed and considering the influence of any pure beam and trochoidal waves. Relevant results shown provide the exact time and wave profile at which the maximum rolling angles are reached. In the second part of the paper, a new control system making use of AI tools is proposed in order to be used by ship operators on board, avoiding dangerous situations. Finally, the results are validated using a set of ship rolling simulations for the most common and representative ship loading conditions and wave periods.

Numerical Investigation of an Experimental Setup for Thermoplastic Fuselage Panel Testing in Combined Loading

The main purpose of this study comprises the design and the development of a novel experimental configuration for carrying out tests on a full-scale stiffened panel manufactured of fiber-reinforced thermoplastic material. Two different test-bench design concepts were evaluated through a numerical modeling strategy, which will be validated at the next stage using a targeted series of mechanical tests. A baseline experimental setup was developed after a number of candidate configurations were numerically investigated. The supporting elements along with the load introduction systems were defined in such a way as to represent the stiffness of a fuselage barrel section and its representative loading scenarios. The test rig and the investigated thermoplastic panel were numerically simulated to acquire valuable data pertaining to deformations and stresses when subjected to different loading combinations. Two distinct load cases were numerically examined: the first case was the in-plane compression of the thermoplastic panel, while the second case consisted of an internally applied pressure load introduced via an inflatable airbag, installed under the panel. Both loading scenarios were recreated inside the numerical virtual environment in order to examine two distinct stiffening configurations as well as to determine the maximum/limit loads to be used in the planned future experimental campaign. It was concluded that the designed test rig could successfully be used for the structural evaluation of fuselage panels under representative loading conditions.

3D microscale investigation of active deformation mechanisms of halite under conditions representative of underground hydrogen storage

To tackle the challenges raised by climate change, we need to rapidly switch our energy sources to low-carbon ones for all economic sectors. As renewable energies are intermittent, efficient energy carriers, such as hydrogen, will be needed to meet the energy demand. Green hydrogen is considered to be a promising energy vector for the future. However, in addition to problematics related to its production, safe and large scale storage solutions still need to be developed. The geological formations including saline aquifers and former depleted gas fields offer the largest storage capacities but are more adapted to seasonal or mid-to-long term storage. Conversely, underground salt caverns are well suited for storage/withdrawal cycles as short as daily cycling. These artificial structures, offering exceptional tightness, have already been used for decades for hydrocarbons storage but at seasonal storage/withdrawal cycles. The adaptation to short-term hydrogen storage still requires further studies to ensure the stability of the caverns under such loading conditions. Indeed, rapid cyclic loading conditions may impact the tightness and the integrity of the cavern [1, 6].
 Rock salt is polycrystalline material with an essentially viscoplastic behaviour, involving different micro-mechanisms such as crystal slip plasticity and grain boundary sliding. At mechanical loading conditions representative of those operated in storage caverns, the rock salt is characterized by non-linear viscous flow. The activation of grain boundary sliding is necessary to accommodate local plastic incompatibilities between neighbouring grains. It has been shown in uniaxial loading conditions but has not been verified in triaxial conditions yet [2, 4]. The presence of brine also affects the micro-mechanisms involved, with for example phenomena like dissolution-precipitation or diffusional mass transfer along grain boundaries, and can modify the mechanical behaviour [5].
 In our studies, we investigate the active micro-mechanisms in synthetic halite through in situ X-ray microcomputed tomography (XR-µCT) analysis and digital volume correlation (DVC) and damage quantification. To reproduce loading conditions representative of those in real salt caverns, we apply different confining pressures with a triaxial cell. This triaxial device, developed recently [3], is adapted to in situ XR-µCT tests. We study the development of damage networks and the evolution of pores during the deformation of rock salt under different confining pressures. Samples of halite are prepared by compaction of pure NaCl powder in dry and humid conditions. It gives samples with different brine contents and allows us to study the effect of brine on the deformation mechanims.
 An effect of brine is visible, as the cracks seem to appear earlier in the dry samples. On the µCT scans, cracks start to be visible for a lower strain in the case of dry samples compared to the case of wet samples. For a dry sample in uniaxial loading conditions, the cracks were already clearly visible and well formed in most of the sample at 1.3% axial strain. For a wet sample, very few cracks only start to become slightly visible in some areas at 1.45% axial strain. This could be due to the activity of mechanisms involving brine, such as dissolution-precipitation allowing for crack healing.
 The effect of the confining pressure is also noticeable. After unloading the wet sample and removing the confining pressure, we observed that the few cracks formed during the deformation of the sample opened up, indicating that the healing process was not
 complete. Dissolution and precipitation are kinetically slaw processes, which are mostly active at low strain rates. Therefore, further deformation experiments must also explore the effect of strain rate.
 Natural salt always contains some humidity. Therefore, the effect of brine in deformation and damage formation/healing mechanisms is important to understand, especially in the context of gas storage in salt caverns.

The effect of biopolymer treatment on the potential instability of a soft soil under cyclic loading

Mud pumping is frequently reported as a serious concern in soft soil foundations subjected to repeated train loads. Although conventional methods such as lime and cement treatment can be used to alleviate this problem, they can cause considerable environmental damage. This paper presents the novel use of an eco-friendly biopolymer, Xanthan Gum (XG), to enhance the performance of soft subgrade under cyclic loading in the context of railways. A series of cyclic triaxial tests were carried out on natural subgrade soil with a history of mud pumping and the results were compared with XG-treated specimens subjected to typical loading conditions representative of heavy-haul trains. The efficacy of XG was examined by varying cyclic stress ratio (CSR) and XG content for different test specimens. The results indicate that XG, with its hydrophilic nature and densification of soil specimens through the pore filling and particle bonding process, can help reduce the accumulated excess pore pressure (EPP) effectively, thereby substantially improving soil resistance to cyclic loading and mitigating the migration of pore water within the test specimen. The permanent strains are found to be less than 3% despite using a small amount of XG content (0.5%) under a high cyclic stress ratio (CSR = 0.8) and the number of loading cycles up to 50,000 cycles, while the resilient modulus of soil can increase significantly as XG content increases.

On the failure behavior of a GFRP composite cylinder under Brazier crushing force – An experimental and computational study

The induced ovalization or out-of-plane deformation in the circular and transition airfoils of modern MegaWatt (MW) size wind turbine rotor blades is caused by the combined effect of the applied far-field bending loads and the local structural curvature. In general, the phenomenon of the ovalization or out-of-plane deformation of the circular elastic cross sections under the applied bending load or the internal/external pressure is termed the Brazier effect [1 2]. Considering the poor mechanical performance in the out-of-plane direction of the typically used laminated glass composites for wind turbine rotor blade construction, the current research work aims to develop a thoroughly validated computational methodology to understand the ovalization (out-of-plane deformation) induced failure behavior in circular cross-sections located between the root and transition region of a rotor blade, using a substructure level testing and modeling under representative load conditions. To this end, detailed manufacturing, testing, and a computational modeling campaign is devised and executed at different length scales starting from coupon to cylinder (substructure) level. The required elastic, strength, and fracture properties for the cylinder progressive damage analysis are evaluated using coupon-level experiments following various ASTM standards.The developed numerical methodology is thoroughly validated using the substructure level experimental load–displacement curves, strain, and damage profiles. Detailed experimental and numerical studies reveal that the induced ovalization leads to a non-linear relation between the applied crushing force and the local radius of curvature until the unstable collapse of the cylinder. Based on the thorough local stress and damage analysis, it is evident that along with the dominant interlaminar shear stress (ILSS), the presence of the interlaminar tensile (ILT) stress component is the key to the delamination induced collapse of the cylinder.

Load condition determination for efficient fatigue analysis of floating offshore wind turbines using a GF-discrepancy-based point selection method

Fatigue failure is one of the decisive factors to be considered in the structural safety design of floating offshore wind turbines (FOWTs). In the fatigue assessment of FOWTs, adequate simulations are required to account for the variability of long-term environmental load conditions. Although the number of potential environmental conditions can be infinite, only a limited number of representative load conditions are used in practice. This inevitably introduces uncertainty into the fatigue estimation, which is usually referred to as the finite sampling-induced uncertainty. To reduce this uncertainty, numerous simulations are required at the expense of unbearable computation burdens. The present study proposes a method to determine the representative load conditions for efficient fatigue evaluation of FOWTs based on the GF-discrepancy of point set. The copula method is adopted to establish the joint probabilistic model of long-term wind and wave conditions. The in-house developed StoDRAFOWT model and the SN-curve approach are employed for the fatigue evaluation. The fatigue uncertainty due to finite sampling under different load condition selection methods is then assessed. The comparison results indicate that the proposed method can significantly reduce the uncertainty in the fatigue estimation of FOWTs against conventional methods, thus improving the fatigue analysis efficiency of FOWTs.

Analytical prediction of stress and strain in adhesive tube-to-tube joints under thermal expansion/contraction

ABSTRACT Adhesive joints are widely applied and studied for various industrial applications. The interest in adhesive joints has expanded to include heating, ventilation, air conditioning, and refrigeration (HVAC&R) systems having a significant number of joints employed for manufacturing. This study investigates an analytical modeling approach for predicting joint stress and strain distribution under static loading with thermal strain. A review of modeling techniques identified the need to develop a joint analytical model under loading conditions representative of HVAC&R applications. The details of the model, governing equations, assumptions, boundary conditions, and solution techniques are first reported. The model is validated via comparison to existing results before performing parametric studies to provide insights on the influences of thermal expansion and inner tube pressure on possible failure. It is found that the joint overlap length plays an important role in stress distribution, while the adhesive thickness has less impact. Overall, the results indicate that static loading failure is not likely a concern for joints in HVAC&R systems, but the thermal strain and stress induced by temperature fluctuations must be carefully considered. This modeling effort establishes a framework that can be used to generate criteria and instructions on designing adhesive joints across different HVAC&R

FEA-Based Investigation of Fatigue Life and Durability of Materials and Structures in Automotive Applications

In the rapidly evolving automotive industry, the longevity and reliability of materials and structures are paramount. This research paper presents a comprehensive Finite Element Analysis (FEA)-based investigation into the fatigue life and durability of materials and structures commonly employed in automotive applications. Utilizing state-of-the-art FEA tools, the study evaluates the stress distributions, strain concentrations, and fatigue-induced deformations under cyclic loading conditions representative of real-world automotive scenarios. A comparative analysis of various automotive materials, including advanced high-strength steels, aluminium alloys, and novel composite materials, is conducted to discern their fatigue performance. The results elucidate the critical regions susceptible to fatigue failure and provide insights into the underlying mechanisms governing material degradation. Furthermore, the study introduces a novel fatigue life prediction model, calibrated against experimental data, offering enhanced accuracy in predicting the lifespan of automotive components. The findings of this research not only contribute to the fundamental understanding of fatigue phenomena in automotive materials but also pave the way for the development of more durable and sustainable vehicles in the future. This work serves as a cornerstone for engineers and researchers aiming to optimize material selection and design strategies, ensuring safer and longer-lasting automotive structures.

Mechanisms and kinetics of pore closure in thick aluminum plate

Interrupted in situ tests at high temperature characterized with X-ray microtomography were employed to study porosity evolution under loading conditions representative of aluminum hot rolling. Coupling digital volume correlation and simulations, it is possible to access, for each pore, its morphological evolution, and the mechanical fields it experienced. The closure kinetics of hundreds of pores were studied, which is described as the pore volume as a function of the hydrostatic integration. Closure kinetics are a function of the shape of the pore. Focus is put on pores with a complex shape which are more detrimental to mechanical properties. Their closure kinetics are exponential due to the pore fragmentation during closure. The closure of complex pores is fostered by an initial orientation perpendicular to the loading direction and a low initial fragmentation. Based on the closure kinetics of 243 complex pores, a new pore closure model is proposed.

A two-degree-of-freedom tuned mass damper for offshore wind turbines on floating spar supports

This paper presents an original approach to mitigate the motion of floating spar supports for offshore wind turbines. A two-degree-of-freedom tuned mass damper device, consisting of a chain of masses, linearly-elastic springs and viscous dashpots, is connected to the system within the nacelle. Considering a four-degree-of-freedom model of the system under simplified aerodynamic rotor loads, as well as Morison hydrodynamic loads, the proposed approach involves tuning the two natural frequencies of the device to the pitch natural frequency of the system and a relevant frequency within the range of the wave loads. Extensive numerical simulations on the OC3-Hywind spar floating wind turbine, for different load cases, prove that the proposed two-degree-of-freedom tuned mass damper is more effective than a standard single-degree-of-freedom one, assuming the two devices feature the same total mass. Further, design parameters are identified for representative loading conditions, in terms of ratio between the two masses within the device and ratio between the total masses of device and system.

Allowables Evaluation for the Design of a Thermoplastic Fuselage

Carbon fibre reinforced polymers are commonly used in the primary and secondary structures of aircraft, mainly due to their excellent specific mechanical properties. Currently, there is a growing interest in the use of carbon fibre and thermoplastic resin composite materials in aeronautical applications, since they offer multiple advantages compared to thermosetting equivalents. This study presents the results of the mechanical tests carried out at the coupon and element levels for the characterization of the properties of a thermoplastic composite that will form a fuselage panel. In addition to standardized tests, tests are carried out to evaluate the behaviour of critical areas of the panel, such as the radius of the frames, the stringers, and the joint between the frame and the skin around the stringers (mouse-hole). In these latest tests, experimental procedures are developed for their execution, including the design of the necessary tools to reproduce the representative boundary conditions. As a result of this research, the allowables have been obtained for the design of the curved thermoplastic fuselage panel. This work is developed as part of the European project DELTA, within the framework of the Clean Sky 2 JU program. The objective of this project is to develop and execute innovative experimental test procedures that allow the validation of thermoplastic composite aircraft fuselage panels, in order to apply these solutions in medium and long-range aircraft (LPA). The ultimate goal of this project will be achieved through the execution of a curved thermoplastic fuselage panel test under representative load conditions.

Effects of Loading and Boundary Conditions on the Performance of Ultrasound Compressional Viscoelastography: A Computational Simulation Study to Guide Experimental Design.

Most biomaterials and tissues are viscoelastic; thus, evaluating viscoelastic properties is important for numerous biomedical applications. Compressional viscoelastography is an ultrasound imaging technique used for measuring the viscoelastic properties of biomaterials and tissues. It analyzes the creep behavior of a material under an external mechanical compression. The aim of this study is to use finite element analysis to investigate how loading conditions (the distribution of the applied compressional pressure on the surface of the sample) and boundary conditions (the fixation method used to stabilize the sample) can affect the measurement accuracy of compressional viscoelastography. The results show that loading and boundary conditions in computational simulations of compressional viscoelastography can severely affect the measurement accuracy of the viscoelastic properties of materials. The measurement can only be accurate if the compressional pressure is exerted on the entire top surface of the sample, as well as if the bottom of the sample is fixed only along the vertical direction. These findings imply that, in an experimental validation study, the phantom design should take into account that the surface area of the pressure plate must be equal to or larger than that of the top surface of the sample, and the sample should be placed directly on the testing platform without any fixation (such as a sample container). The findings indicate that when applying compressional viscoelastography to real tissues in vivo, consideration should be given to the representative loading and boundary conditions. The findings of the present simulation study will provide a reference for experimental phantom designs regarding loading and boundary conditions, as well as guidance towards validating the experimental results of compressional viscoelastography.

Autoignition and preliminary heat release of gasoline surrogates and their blends with ethanol at engine-relevant conditions: Experiments and comprehensive kinetic modeling

This work utilizes a rapid compression machine (RCM) to experimentally quantify autoignition and preliminary heat release characteristics for blends of 0 to 30% ethanol by volume into two surrogates (FGF-LLNL and FGF-KAUST) that represent a full boiling range gasoline (FACE-F). Experimental conditions cover pressures from 15 to 100 bar, temperatures from 700 to 1000 K, and diluted/stoichiometric and undiluted/lean fuel loading conditions representative of boosted spark-ignition and advanced compression ignition engines, respectively. Direct comparison is made with previously reported results for FACE-F/E0–E30 blends. A detailed gasoline surrogate chemistry model is also proposed, and chemical kinetic modeling is undertaken using the proposed model to generate chemical insights into the compositional effects and ethanol blending effects.Although experiments show similar qualitative trends between the surrogates, quantitative differences between the surrogates are obvious, where FGF-LLNL displays greater low-temperature reactivity and faster evolution of low-temperature heat release (LTHR) than FGF-KAUST, with such differences being significantly muted by ethanol blending. Flux analyses reveal the compositional effects on surrogate reactivity at the diluted/stoichiometric condition, where n-heptane facilitates first-stage ignition reactivity for FGF-LLNL/E0 by initiating earlier and more rapid ȮH branching than n-butane for FGF-KAUST/E0. Sensitivity analyses highlight the importance of non-fuel-specific interactions between ethanol and surrogate sub-chemistries in controlling the reactivity of ethanol-blended surrogates. Direct experimental comparisons between the surrogates and FACE-F, as well as between the surrogate/EtOH and FACE-F/EtOH blends highlight the need of high-fidelity surrogates that can fully replicate the target gasoline in properties including ignition reactivity and LTHR characteristics at extended conditions, as well as their response to ethanol blending. Overall, the model captures the experiments reasonably well. Nevertheless, the model displays increasing disagreement with experiments for the two surrogates at higher levels of ethanol blending, and this is found to be caused primarily by non-fuel-specific interactions between ethanol and surrogate component sub-chemistries. Futhermore, the inadequacy of the kinetic model to capture surrogate-to-surrogate differences at the diluted/stoichiometric condition suggests more physical testing is needed to facilitate more extensive model validation.

Phytoliths can cause tooth wear.

Comparative laboratory sliding wear tests on extracted human molar teeth in artificial saliva with third-body particulates demonstrate that phytoliths can be as effective as silica grit in the abrasion of enamel. A pin-on-disc wear testing configuration is employed, with an extracted molar cusp as a pin on a hard disc antagonist, under loading conditions representative of normal chewing forces. Concentrations and sizes of phytoliths in the wear test media match those of silica particles. Cusp geometries and ensuing abrasion volumes are measured by digital profilometry. The wear data are considered in relation to a debate by evolutionary biologists concerning the relative capacities of intrinsic mineral bodies within plant tissue and exogenous grit in the atmosphere to act as agents of tooth wear in various animal species.

A Two-Phased Approach to Quantifying Head Impact Sensor Accuracy: In-Laboratory and On-Field Assessments.

Measuring head impacts in sports can further our understanding of brain injury biomechanics and, hopefully, advance concussion diagnostics and prevention. Although there are many head impact sensors available, skepticism on their utility exists over concerns related to measurement error. Previous studies report mixed reliability in head impact sensor measurements, but there is no uniform approach to assessing accuracy, making comparisons between sensors and studies difficult. The objective of this paper is to introduce a two-phased approach to evaluating head impact sensor accuracy. The first phase consists of in-lab impact testing on a dummy headform at varying impact severities under loading conditions representative of each sensor's intended use. We quantify in-lab accuracy by calculating the concordance correlation coefficient (CCC) between a sensor's kinematic measurements and headform reference measurements. For sensors that performed reasonably well in the lab (CCC ≥ 0.80), we completed a second phase of evaluation on-field. Through video validation of impacts measured by sensors on athletes, we classified each sensor measurement as either true-positive and false-positive impact events and computed positive predictive value (PPV) to summarize real-world accuracy. Eight sensors were tested in phase one, but only four sensors were assessed in phase two. Sensor accuracy varied greatly. CCC from phase one ranged from 0.13 to 0.97, with an average value of 0.72. Overall, the four devices that were implemented on-field had PPV that ranged from 16.3 to 91.2%, with an average value of 60.8%. Performance in-lab was not always indicative of the device's performance on-field. The methods proposed in this paper aim to establish a comprehensive approach to the evaluation of sensors so that users can better interpret data collected from athletes.

Experimental and modeling study of C2–C4 alcohol autoignition at intermediate temperature conditions

C2–C4 alcohols are advantageous blendstocks identified by many research groups, including the U.S. Department of Energy Co-Optima Initiative, towards enabling efficient, boosted Spark-Ignition (SI) engines. Their use in advanced engine applications requires a comprehensive understanding of their intermediate-temperature autoignition behavior. This work reports an experimental and modeling study covering their fundamental autoignition characteristics in a twin-piston rapid compression machine at pressures of 20 and 40 bar, intermediate temperatures from 750 to 980 K, and two fuel loading conditions representative of boosted SI engines. Direct comparison between these alcohols is made, where the order of reactivity is established across different thermodynamic and fuel loading conditions. Changes in preliminary exothermicity (or intermediate-temperature heat release) displayed in single-stage autoignition across different alcohols and conditions are also quantified. This provides insight into fuel-to-fuel differences, and how these could affect advanced combustion concepts such as spark-assisted compression ignition. Kinetic models are used to simulate the experiments, and reasonable agreement is obtained. The sensitivity analysis results demonstrate the importance of accurately capturing the autoignition kinetics, particularly H-abstraction reactions on the parent fuels by OH and HO2, and the branching ratio associated with these.

Hydraulic Transport Through Calcite Bearing Faults With Customized Roughness: Effects of Normal and Shear Loading

AbstractUnderstanding fluid flow in rough fractures is of high importance to large scale geologic processes and to most anthropogenic geo‐energy activities. Here we conducted fluid transport experiments on Carrara marble fractures with a novel customized surface topography. Transmissivity measurements were conducted under mechanical loading conditions representative of deep geothermal reservoirs (normal stresses from 20 to 70 MPa and shear stresses from 0 to 30 MPa). A numerical procedure simulating normal contact and fluid flow through fractures with complex geometries was validated toward experiments. Using it, we isolated the effects of roughness parameters on fracture fluid flow. Under normal loading, we find that (i) the transmissivity decreases with normal loading and is strongly dependent on fault surface geometry and (ii) the standard deviation of heights (hRMS) and macroscopic wavelength of the surface asperities control fracture transmissivity. Transmissivity evolution is nonmonotonic, with more than 4 orders of magnitude difference for small variations of macroscopic wavelength and hRMS roughness. Reversible elastic shear loading has little effect on transmissivity; it can increase or decrease depending on contact geometry and overall stress state on the fault. Irreversible shear displacement (up to 1 mm offset) slightly decreases transmissivity and its variation with irreversible shear displacements can be predicted numerically and geometrically at low normal stress only. Finally, irreversible changes in surface roughness (plasticity and wear) due to shear displacement result in a permanent decrease of transmissivity when decreasing differential stress. Generally, reduction of a carbonate fault's effective stress increases its transmissivity while inducing small shear displacements does not.

Elasto-Viscoplastic Material Model of a Directly-Cast Low-Carbon Steel at High Temperatures.

A model-based process control of material production processes demands realistic material models describing the local evolution of the thermal and mechanical state variables, i.e., temperature, stress, strain, or plastic strain, for the relevant microstructure state. In the present work, a material model for the specific microstructure in a continuously cast strand shell, viable for reproducing cyclic viscoplastic effects, was developed for a 0.17 wt.% C steel. Experimental data was generated using directly-cast samples and a well-controllable testing facility to apply representative loading conditions. Displacement- and force-controlled experiments in the temperature range of 700–1100 °C were conducted, with a special focus on the relevant strain rates documented for the straightening operation. A temperature-dependent constitutive material model combining elastic, plastic, and viscoplastic effects was parameterized to fit the whole set of experimentally-determined material response curves. In order to account for the cyclic plastic material response, a combination of isotropic and kinematic hardening was considered. The material model sets a new standard for the material description of a continuously cast strand shell, and it can be applied in elaborate continuous casting simulations.