Evolution Of Normal Stress Research Articles

Investigating Dynamic Weakening in Laboratory Faults Using Multi‐Scale Flash Heating Coupled With mm‐Scale Contact Evolution

AbstractFlash‐weakening models typically show good agreement with the total magnitude of weakening in high‐speed rock friction experiments, however deviations during the acceleration and deceleration phases, and at low and intermediate sliding velocities, remain unresolved. Here, we incorporate inhomogeneous mm‐scale normal stress evolution into a model for flash heating and weakening to resolve outstanding transient and hysteretic friction observed in laboratory experiments and to identify unique solutions to constitutive parameters. We conduced 37 rock friction experiments on Westerly granite using a high‐speed biaxial apparatus outfitted with a high‐speed infrared camera. We initiated velocity steps from quasi‐static rates of 1 mm/s to sliding velocities ranging from 300 to 900 mm/s and conducted both constant‐ and decreasing‐velocity tests following the velocity step. Two sliding surfaces geometries were used to control mm‐scale life‐times and rest‐times. Constant‐strength sliding is achieved within 2–3 mm of initiating the velocity step in all constant‐velocity experiments. Macroscopic surface temperature is inhomogeneous and increases with slip distance, velocity, and decreasing rest‐time. Weakening increases with sliding velocity and decreasing rest‐time. We combine thermal models with measured surface temperatures to constrain the evolution of local normal stress at the mm‐scale and incorporate this evolution into a flash‐weakening model that considers weakening at both the µm‐ and mm‐scale. The flash‐weakening model improves when the effects of mm‐scale wear processes are incorporated and multi‐scale weakening is considered, however some transient friction remains undescribed. Models will be advanced by further incorporating wear processes and by considering processes at the mm‐scale and above.

Stratification Mechanism in the Bidisperse Colloidal Film Drying Process: Evolution and Decomposition of Normal Stress Correlated with Microstructure.

The evolution of the normal stress and microstructure in the drying process of bidisperse colloidal films is studied using the Brownian dynamics simulation. Here, we show that the formation process of small-on-top stratification can be explained by normal stress development. At high PeL's, a stratified layer with small particles is formed near the interface. The accumulated particles near the interface induce the localization of normal stress so that the normal stress at the interface increases from the beginning of drying. We analyze this stress development from two points of view, on the global length scale and particle length scale. On the global length scale, the localization of normal stress is quantified by the scaled normal stress difference between the interface and substrate. For all PeL's tested in this study, the scaled normal stress difference increases until the accumulation region reaches the substrate. After the maximum, the stress difference remains at the maximum at lower PeL's, while it decreases at higher PeL's. The microstructural analysis shows that this stress development is explained through the evolution of the particle contact number distribution at the interface and substrate. On the particle length scale, we derive the scaled local force applied to each type of particle by decomposing the local normal stress. At high PeL's, the scaled local force for the large particle is large compared to that for the small particle near the interface, indicating that the large particles are strongly pushed away from the interface. Associating the volume fraction profile with the local force field, we suggest that the strong scaled force for the large particle is attributed to the significant increase in the average number of small particles in contact with large ones. This study has significance in probing the drying mechanism of bidisperse colloidal films and the stratification mechanism.

Drying mechanism of monodisperse colloidal film: Evolution of normal stress and its correlation with microstructure

AbstractWe investigate the drying process of monodisperse colloidal film over a wide range of Péclet numbers (Pe) by using the Brownian dynamics simulation. We analyze the detailed process in three aspects; accumulation front, normal stress, and microstructure. The evolution of particle distribution is quantified by tracking the accumulation front. The accumulated particles contribute to the continuous increase of the normal stress at the interface. At the substrate, the normal stress first stays constant and then increases as the accumulation front touches the substrate. We quantitatively analyze the stress development by a scaled normal stress difference between the two boundaries. At all tested Pe, the stress difference increases to the maximum, followed by a decrease during drying. Interestingly, a mismatch is observed between the stress difference maximum and the initial stress increase at the substrate. The microstructural analysis reveals that this mismatch is related to the microstructural development at the substrate.

Effect of strain rate difference between inside and outside groove in M−K model on prediction of forming limit curve of Ti6Al4V at elevated temperatures

Abstract The influence of initial groove angle on strain rate inside and outside groove of Ti6Al4V alloy was investigated. Based on the evolution of strain rate inside and outside groove, the effect of strain rate difference on the evolution of normal stress and effective stress inside and outside groove was also analyzed. The results show that when linear loading path changes from uniaxial tension to equi-biaxial tension, the initial groove angle plays a weaker role in the evolution of strain rate in the M−K model. Due to the constraint of force equilibrium between inside and outside groove, the strain rate difference makes the normal stress inside groove firstly decrease and then increase during calculation, which makes the prediction algorithm of forming limit convergent at elevated temperature. The decrease of normal stress inside groove is mainly caused by high temperature softening effect and the rotation of groove, while the increase of normal stress inside groove is mainly due to strain rate hardening effect.

Structure-Property Relationships via Recovery Rheology in Viscoelastic Materials.

The recoverable strain is shown to correlate to the temporal evolution of microstructure via time-resolved small-angle neutron scattering and dynamic shear rheology. Investigating two distinct polymeric materials of wormlike micelles and fibrin network, we demonstrate that, in addition to the nonlinear structure-property relationships, the shear and normal stress evolution is dictated by the recoverable strain. A distinct sequence of physical processes under large amplitude oscillatory shear (LAOS) is identified that clearly contains information regarding both the steady-state flow curve and the linear-regime frequency sweep, contrary to most interpretations that LAOS responses are either distinct from or somehow intermediate between the two cases. This work provides a physically motivated and straightforward path to further explore the structure-property relationships of viscoelastic materials under dynamic flow conditions.

Contact mechanics of a dilatant region located at a compressed elastic interface

Interfaces possess complex mechanical responses that are governed by several factors including the type of material, the local topography of the interacting surfaces, the stress state and the mode of deformation. This paper examines the mechanics of a mated smooth interface that is subjected to a normal stress and where the contact is perturbed by a circular patch that can experience dilatancy under shear. The analysis of the static stress drop occurring during shear at the interface is examined using a contact mechanics approach that accounts for the separation at the pre-compressed geological interface induced by the development of dilatancy of the patch during relative shear. This paper presents an elementary model of the mechanics that takes into consideration the normal stress evolution during dilatant shearing of the interface. The problem is of particular interest to the modelling of local phenomena that can occur at material interfaces and geological faults that are subjected to steady movement.

Sensitivity analysis on the interaction between hydraulic and natural fractures based on an explicitly coupled hydro-geomechanical model in PFC2D

Fluid-induced fracture nucleation, propagation and interaction with natural fractures are the crucial issues for well understanding the process of hydraulic fracturing in unconventional reservoirs. In this paper, an explicitly coupled hydro-geomechanical model in two-dimensional Particle Flow Code (PFC2D) is employed to investigate the interaction between hydraulic fractures (HFs) and natural fractures (NFs), and the numerical scheme is validated by using analytical results. Several factors affecting the interactions between induced-fractures and NFs are being investigated and discussed. The simulation results show that three fundamental crossing scenarios can be well reproduced: (I) Direct Crossing, (II) Offset Crossing and (III) No Crossing (Arrested), and the crossing types agree well in the evaluation by Blanton's criterion. A sensitivity analysis shows that hydraulic fracture (HF) tend to be arrested by a natural fracture (HF) under low approach angles and prefer to cross the NF under higher in-situ differential stress states. Crossing is likely to be impeded for NFs with lower frictional coefficient and higher permeability. Additionally, lower fracturing fluid injection rates and lower fluid viscosities both are advantageous conditions for NF reactivation, and tend to arrest the HF. Moreover, the mechanism of the interaction of HFs and NFs influenced by the aforementioned factors can be well explained by the evolution of normal stress on the pre-existing interface, which is the key issue for correct simulation of hydraulic fracturing. Compared with the analytical model, the model in this study can reflect the dynamic stress evolution process, which will be a useful tool for optimizing the design of hydraulic fracturing in fractured reservoirs.

Mechanical origin of b-value changes during stimulation of deep geothermal reservoirs

The distribution of induced-earthquake magnitudes in deep geothermal reservoirs is a classical tool for monitoring reservoirs. It typically shows some important fluctuations through time and space. Despite being a very crude information (i.e., a scalar quantity) of very complex mechanical stress evolution, understanding these variations could still give us insights into the mechanics of the reservoir. Here, we analyze the output of a simple quasi-static physical model of a single fault and propose a new way to describe bursts that could be compared to seismic events. Our model is an elastic fiber bundle model describing multiple ruptures along a single major fault of the reservoir. It consists of a set of fibers with various strengths, arranged in a planar L×L two-dimensional array, linking together two elastic half-spaces. It mimics a fault zone with various asperities. During load, the fibers break quasi-statically according to a stress threshold distribution. Contrary to classical fiber bundle model, here, when a fiber breaks, it redistributes the load on the surviving fibers through long range elastic interactions. Interestingly, the elasticity of the half-spaces which changes the range of the stress distribution, characterizes two distinct regimes. In a stiff regime, we find an effective ductile deformation regime at large scale since the damage distribution is very difuse. Conversely in softer systems, the distance between two consecutive breaking fibers gets smaller and failure of the interface is localized, exhibiting an effective brittle regime. We analyze two types of burst distributions: a classical one built from the statistics of the broken bonds during each failure step and a new one defines from a waiting time matrix of the fracture front propagation. The first one reproduces several known results for this type of model. The new one evidences the existence of effective creeping advances of the front with statistics that follow a Gutenberg-Richter distribution, in particular, in the ductile regime (stiff systems). We proposed a new definition of bursts in a fault model, based on the local fracture front velocity. We find that b v -values for the distribution of the velocity clusters are very consistent with Gutenberg-Richter distribution of induced seismicity. We link the b v -value fluctuations in the reservoirs to the influence of the velocity threshold level that could be related to recording limitation. b b -values obtained from the broken bond statistics are hardly comparable to seismic events because of the lack of space contiguousness of broken fibers during bursts. In the light of our results, we discuss the implications of b-value changes in geothermal reservoir in terms of fault asperities and normal stress evolution.

Two-dimensional wood drying stress simulation using control-volume mixed finite element methods (CVFEM)

The work was aimed at simulating two-dimensional wood drying stress using the control-volume finite element method (CVFEM). Stress/strain was modeled by moisture content gradients regarding shrinkage and mechanical sorption in a cross-section of wood. CVFEM was implemented with triangular finite elements and lineal interpolation of the independent variable which were programmed in Fortran 90 language. The model was validated by contrasting results with similar ones available in the specialised literature. The present model's results came from isothermal (20ºC) drying of quaking aspen (Populus tremuloides): two-dimensional distribution of stress/strain and water content, 40, 80, 130, 190 and 260 hour drying time and evolution of normal stress ( -2,5<σxx<1,2, MPa), from the interior to the exterior of wood.

Evolution of normal stress and surface roughness in buckled thin films

In this work we investigate buckling of compressed elastic thin films, which are bonded onto a viscous layer of finite thickness. It is found that the normal stress exerted by the viscous layer on the elastic film evolves with time showing a minimum at early buckling stages, while it increases at later stages. The normal stress also shows a minimum as a function of applied compressive stress, which depends strongly on the viscosity of the underlying layer and strain values. Furthermore, with decreasing viscosity the film roughness amplitude also shows a minimum at early buckling stages. The effect of the viscosity becomes more pronounced with increasing strain in the film. Finally, decreasing elastic film thickness and/or increasing viscous layer thickness also enhance buckling roughness.