Dike Propagation During Global Contraction: Making Sense of Conflicting Stress Histories on Mercury

Thrust fault-related landforms, smooth plains units, and impact craters and basins have all been observed on the surface of Mercury. While tectonic landforms point to a long-lived history of global cooling and contraction, smooth plains units have been inferred to represent more punctuated periods of effusive volcanism. The timings of these processes are inferred through impact cratering records to have overlapped, yet the stress regimes implied by the processes are contradictory. Effusive volcanism on Mercury is believed to have produced flood basalts through dikes, the propagation of which is dependent on being able to open and fill vertical tensile cracks when horizontal stresses are small. On the contrary, thrust faults propagate when at least one horizontal stress is very large relative to the vertical compressive stress. We made sense of conflicting stress regimes through modeling with frictional faulting theory and Earth analogue work. Frictional faulting theory equations predict that the minimum and maximum principal stresses have a predictable relationship when thrust faulting is observed. The Griffith Criterion and Kirsch equations similarly predict a relationship between these stresses when tensile fractures are observed. Together, both sets of equations limit the range of stresses possible when dikes and thrusts are observed and permitted us to calculate deviatoric stresses for regions of Earth and Mercury. Deviatoric stress was applied to test a physical model for dike propagation distance in the horizontally compressive stress regime of the Columbia River Flood Basalt Province, an Earth analogue for Borealis Planitia, the northern smooth plains, of Mercury. By confirming that dike propagation distances from sources observed in the province can be generated with the physical model, we confidently apply the model to confirm that dikes on Mercury can propagate in a horizontally compressive stress regime and calculate the depth to the source for the plains materials. Results imply that dikes could travel from ∼89 km depth to bring material from deep within the lithosphere to the surface, and that Mercury’s lithosphere is mechanically layered, with only the uppermost layer being weak.

The Potential Use of Brick Wastes in the Generation of New Materials for Construction through Geopolymerization Processes

The calcined clay bricks are the second most used materials in construction that, after the demolition processes, tends to become rubble, generating a negative visual and environmental impact, in addition to the fact that the brick-making process has not been industrialized in Ecuador, for that, its properties are deficient; in this way, the present research aims to study the physical, chemical and mineralogical characteristics of brick waste from the Southern part of ​​this country, for the elaboration of ecological bricks through geopolymerization processes, using as alkaline activator Sodium Hydroxide at temperature ranged between 90 °C and 200 °C, obtaining an optimal mixture at the combination 12.5 M, 26 wt% Cs, 150 °C. The mechanical properties of bricks as simple compression and flexural strength, respectively, applying the experimental Griffith criterion method by finite element simulation method. These ecological bricks obtained are suitable for use in construction.

Effect of point defects and nanopores on the fracture behaviors in single-layer MoS2 nanosheets

Point defects and nanopores are inevitable and particularly noticeable in single-layer (SL) MoS2. Molecular dynamics (MD) simulations have been done to comprehensively study the influences of point defects and nanopores on tensile deformation behaviors of SLMoS2 nanosheets, and the dependences of fracture properties on defect type and concentration, pore size, temperature and strain rate are discussed. The formation energy of S vacancy (VS) is the lowest one, but that of VMoS6 is the highest one, corresponding to the highest and lowest fracture stress, respectively. The local stress concentration around point defects and nanopores might lead to the early bond breaking and subsequent nucleation of cracks and brittle fracture upon tensile loading. A modified Griffith criterion is proposed to describe the defect concentration and pore size dependent fracture stress and strain. These findings provide us an important guideline for the structural design of 2D materials in future applications.

A computational framework for crack propagation in spatially heterogeneous materials

This paper presents a mathematical formulation and numerical modelling framework for brittle crack propagation in heterogeneous elastic solids. Such materials are present in both natural and engineered scenarios. The formulation is developed in the framework of configurational mechanics and solved numerically using the finite-element method. We show the methodology previously established for homogeneous materials without the need for any further assumptions. The proposed model is based on the assumption of maximal dissipation of energy and uses the Griffith criterion; we show that this is sufficient to predict crack propagation in brittle heterogeneous materials, with spatially varying Young’s modulus and fracture energy. Furthermore, we show that the crack path trajectory orientates itself such that it is always subject to Mode-I. The configurational forces and fracture energy release rate are both expressed exclusively in terms of nodal quantities, avoiding the need for post-processing and enabling a fully implicit formulation for modelling the evolving crack front and creation of new crack surfaces. The proposed formulation is verified and validated by comparing numerical results with both analytical solutions and experimental results. Both the predicted crack path and load–displacement response show very good agreement with experiments where the crack path was independent of material heterogeneity for those cases. Finally, the model is successfully used to consider the real and challenging scenario of fracture of an equine bone, with spatially varying material properties obtained from CT scanning. This article is part of a discussion meeting issue ‘A cracking approach to inventing new tough materials: fracture stranger than friction’.

COMPARISON OF PREDICTED FAILURE AREA AROUND THE BOREHOLES IN THE STRIKE-SLIP FAULTING STRESS REGIME WITH HOEK-BROWN AND FAIRHURST GENERALIZED CRITERIA

Breakout is a shear failure due to compression that forms around the borehole due to stress concentration. In this paper, the breakout theory model is investigated by combining the equilibrium elasticity equations of stress around the borehole with two Hoek-Brown and Fairhurst generalized fracture criteria, both of which are based on the Griffith criterion. This theory model provides an explicit equation for the breakout failure width, but the depth of failure is obtained by solving a quartic equation. According to the results and in general, in situ stresses and rock strength characteristics are effective in developing the breakout failure area, As the ratio of in-situ stresses increases, the breakout area becomes deeper and wider. Because in the shear zone, the failure envelope of the Fairhurst criterion is lower than the Hoek-Brown failure criterion, the Fairhurst criterion provides more depth for breakout than the Hoek-Brown criterion. However, due to the same compressive strength of the rock in these two criteria, the same failure width for breakout is obtained from these two criteria. Also, the results obtained for the depth of failure from the theoretical model based on the Fairhurst criterion are in good agreement with the laboratory results on Westerly granite.

Innovative Materials for Sustainable Construction

The construction industry has focused on trying to minimize and control the environmental impacts caused within the process of production and manufacture of fired bricks, for this reason the present research proposes five different alternative mixtures for the elaboration of ecological bricks, four of these based on soil-cement and one obtained through a geopolymerization process, using raw materials from the amazon region and the southern highlands of Ecuador, such as soil from the Centza mine (MC), sand from the Quiringue mine (MQ), organic correctors of husk rice (RH ), peanut shell (PS), natural gypsum (G) from the Malacatos sector and fired brick residues from the same sector. The raw materials were characterized (analysis: physicochemical and mineralogical); the soil-cement-based combinations used different percentages of substitution of organic correctors and gypsum, the optimum percentage of water and cement was determined through the compaction test and resistance to simple compression respectively, the samples were cured and tested at ages of 7, 14 and 28 days. In the geopolymerization process, an alkaline solution NaOH was used in different concentrations of molarity and solution contents, the specimens were cured at temperatures of 90 °C, 120 °C, 150 °C, 180 °C and 200 °C. The different combinations were subjected to indirect traction with the purpose to determine the optimal mixture and subsequent estimation of the compressive strength of bricks applying the Griffith criterion, the results were validated by the finite element method, obtaining strengths of 4 MPa in the combination soil-cement sand (SC_Ar1), in soil-cement rice husk (SC_RH2) and soil-cement peanut shell (SC_PS2) mixtures its resistance is 3 MPa, while in the soil-cement gypsum (SC_G4) mixture the resistance is 6.90 MPa and finally the resistance in geopolymeric mixture (GBW) is 13.75 MPa; In this way, the optimal combinations comply and increase the resistance to simple compression of bricks by 35% the SC_Ar1 mixture, 130% in the SC_G mixture with respect to the spanish standard and 129% the GBW mixture with respect to the ecuadorian standard.

Influence of Carbon Nanotubes on Traditional Material

Brick as a material is of vital importance in the construction industry, however, the burning processes for its preparation contribute to environmental pollution and the generation of greenhouse gases; for this reason, the present research has as aims to propose quality traditional materials for sustainable buildings through the design of soil-cement mixtures in making brick using raw materials from the amazon region of Ecuador: Centza mine (MC) and Quiringue mine (MQ) and improve the mechanical properties of the brick by incorporating carbon nanotubes, which have been dispersed in two aqueous media, sodium naphthalene sulfonate (NSS) and calcium chloride (CC) in percentages of 0.5%, 1% and 1.5%. The characterization of the raw material (analysis: physicochemical and mineralogical) was of great help. The optimum percentage of cement and water was determined through simple compression tests and soil compaction respectively. The different combinations were tested at indirect traction strength at ages 7, 14 and 28 days, determining an optimal mixture for each group of combinations, in this way the simple compressive strength of bricks has been estimated using the Griffith criterion and validation of results by finite element method applying the CivilFEM software, obtaining a resistance of 4 MPa in mixtures of SC-Ar1, 6.3 MPa in combinations of MWCNTs NSS-9 and 5.3 MPa in mixtures of CC-4 MWCNTs, increasing resistance by 57.5% and 32.5% with respect to soil-cement bricks and qualifying them as suitable for use in construction according to standars.

How collective asperity detachments nucleate slip at frictional interfaces

Sliding at a quasi-statically loaded frictional interface can occur via macroscopic slip events, which nucleate locally before propagating as rupture fronts very similar to fracture. We introduce a microscopic model of a frictional interface that includes asperity-level disorder, elastic interaction between local slip events, and inertia. For a perfectly flat and homogeneously loaded interface, we find that slip is nucleated by avalanches of asperity detachments of extension larger than a critical radius Ac governed by a Griffith criterion. We find that after slip, the density of asperities at a local distance to yielding xσ presents a pseudogap P(xσ)∼(xσ)θ, where θ is a nonuniversal exponent that depends on the statistics of the disorder. This result makes a link between friction and the plasticity of amorphous materials where a pseudogap is also present. For friction, we find that a consequence is that stick–slip is an extremely slowly decaying finite-size effect, while the slip nucleation radius Ac diverges as a θ-dependent power law of the system size. We discuss how these predictions can be tested experimentally.

Kinetic Theory of the Fracture of the Coal (Rock) Edge by the Gas-Filled Cracks. Instantaneous Loading-Out

The kinetic theory of the fracture of brittle materials is applied to the study of undercritical and critical growths of cracks in gas-filled rocks. In this type of the materials, the gas filtration from the environment to the cavity of a growing crack plays an important role. The proper account for this factor combined with the dynamics of the stressed state of the bed allows the estimation of the rate of growth of the main crack on the assumption of the Griffith criterion validity. It is found that, immediately after the instantaneous loading-out of the bed in the course of excavation, the cracks of certain size and orientation are exploded with the succeeding growth dependent on the gas entry into the cracks. The time of the filtration growth of the cracks has been estimated. The intervals of the control parameters (formational gas pressure, crack size, overburden pressure, surface energy of coal/rock, modulus of elasticity), where the spontaneous fracture of the bed becomes possible, have been found. The results open a way to the forecast of instantaneous outbursts of coal, rock, and gas.

Theoretical Investigation on Failure Strength and Fracture Toughness of Precracked Single-Layer Graphene Sheets

Young’s modulus, failure strength, and failure strain of precracked graphene are investigated via finite element method based on molecular structure mechanics in this research. The influence of distribution, length, and orientation of precrack and graphene sizes on these mechanical properties is analyzed. The ratio of precrack length and graphene width is defined as P value, and its particular value Pc can be found, at which the variation trends of Young’s modulus, failure strength, and strain have changes with increasing P value. In addition, the fracture toughness of precracked graphene is investigated, and the stress intensity factor (SIF) is calculated according to the Griffith criterion in classical fracture mechanics. The numerical values of the SIF are about 3.20-3.37 MPa√m, which are compared with the experimental results, and the simulations verify the applicability of the classical fracture mechanics to graphene.