Elastoplastic constitutive behavior and strain localization of a low-porosity sandstone during brittle fracturing

We investigated the elastoplastic behavior and strain localization of the Zigong sandstone (porosity: 6.5%) during brittle fracturing based on two series of axisymmetric compression experiments. The experiments were conducted under various confining pressures (σ3 = 0 ~ 80 MPa). For each confining pressure, the sandstone specimens were deformed under constant axial and circumferential strain rates, respectively. When σ3 < 60 MPa, the sandstone first undergoes stable deformation in the post-peak stage and then loses its stability. Before the emergence of instability, the mechanical behavior is hardly affected by the controlling method. When the confining is larger, the sandstone manifests a stable failure process during the whole loading stage. The observed elastoplastic behavior was described by a two-yield surface constitutive model established in the framework of generalized plastic mechanics. The proposed constitutive model incorporates two quadratic yield functions, as well as two linearly independent plastic potential functions, to honor the shear yield and volumetric dilatancy, respectively. Via the return mapping algorithm, the proposed constitutive model was verified by comparing the numerical results with experimental results. In addition, the two-yield surface constitutive model, which is equivalent to the model proposed by Rudnicki and Rice,1 was applied to localization analysis. Assuming that the onset of localization occurs at peak stress, frictional coefficient μ and dilatancy factor β were determined from experimental data. The variations of both plastic parameters predict the transition of localization mode from pure dilation bands under uniaxial compression to pure shear bands at high confining pressures, which is consistent with the experimental observations.

Realizing pure shear mode of dielectric elastomers by tuning biaxial prestress of a deformation controller

In this article, we propose a method to realize the pure shear deformation mode of dielectric elastomer (DE) membranes by tuning two in-plane prestresses. With utilization of carbon grease electrodes, VHB 4905 membranes are prestretched and attached into a retractable device, forming a pure-shear deformation controller. Experimental results demonstrate that, accurate pure shear deformation mode of DEs can be realized by tuning the mechanical loads in the two directions of the deformation controller. Furthermore, large deformation in the direction of free state can be achieved without electromechanical instabilities. The designed deformation controller accurately realizes the specific pure shear deformation mode of DEs and can be utilized to help design the practical soft actuators.

Characteristic Tearing Energy and Fatigue Crack Propagation of Filled Natural Rubber

Below the incipient characteristic tearing energy (T0), cracks will not grow in rubber under fatigue loading. Hence, determination of the characteristic tearing energy T0 is very important in the rubber industry. A rubber cutting experiment was conducted to determine the T0, using the cutting method proposed originally by Lake and Yeoh. Then, a fatigue crack propagation experiment on a edge-notched pure shear specimen under variable amplitude loading was studied. A method to obtain the crack propagation rate da/dN from the relationship of the crack propagation length (Δa) with the number of cycles (N) is proposed. Finally, the T0 obtained from the cutting method is compared with the value decided by the fatigue crack propagation experiment. The values of T0 obtained from the two different methods are a little different.

Piezoreflectance and Electronic Structure of Alloys of Copper with Polyvalent Solutes

<p>Piezoreflectance and other optical measurements have been made on other optical measurements have been made on a phase alloys of Cu with Zn Ga, Al, In and Ge. The samples were evaporated films deposited on the face of a resonant oscillator assembly. The application of this type of strain transducer to piezoref-reflectance alleviates systematic errors and allows the response to pure shear strains to be distinguished, even in amorphous materials, using the polarisation dependence of the results. The energies of the d band -> Fermi level threshold, the interconduction band threshold, and the L2' -> L1 critical point transition were determined for the alloys. Previous optical studies using more conventional methods either have not been able to resolve these features or have not located them with accuracy comparable with piezoreflectance. With increasing alloy concentration the d band threshold is found to shift slowly to higher energies, the inter-conduction band transitions more rapidly to lower energies. Zn impurities produced much smaller shifts than the others, indicating the importance of interactions between impurity d states and the d and conduction states of the host. Significant differences were found between the isovalent solutes Ga and A1. In concentrated alloys with Zn, Ga and Al the interconduction band threshold tended to a common value of about 2.5 eV. This lack of simple dependence on e/a, the electron per atom density in the alloy, is relevant to the understanding of the electronic structure of the Hume-Rothery alloys and the regularity of the [alpha] phase boundary.</p>

Piezoreflectance and Electronic Structure of Alloys of Copper with Polyvalent Solutes

<p>Piezoreflectance and other optical measurements have been made on other optical measurements have been made on a phase alloys of Cu with Zn Ga, Al, In and Ge. The samples were evaporated films deposited on the face of a resonant oscillator assembly. The application of this type of strain transducer to piezoref-reflectance alleviates systematic errors and allows the response to pure shear strains to be distinguished, even in amorphous materials, using the polarisation dependence of the results. The energies of the d band -> Fermi level threshold, the interconduction band threshold, and the L2' -> L1 critical point transition were determined for the alloys. Previous optical studies using more conventional methods either have not been able to resolve these features or have not located them with accuracy comparable with piezoreflectance. With increasing alloy concentration the d band threshold is found to shift slowly to higher energies, the inter-conduction band transitions more rapidly to lower energies. Zn impurities produced much smaller shifts than the others, indicating the importance of interactions between impurity d states and the d and conduction states of the host. Significant differences were found between the isovalent solutes Ga and A1. In concentrated alloys with Zn, Ga and Al the interconduction band threshold tended to a common value of about 2.5 eV. This lack of simple dependence on e/a, the electron per atom density in the alloy, is relevant to the understanding of the electronic structure of the Hume-Rothery alloys and the regularity of the [alpha] phase boundary.</p>

A Thermodynamically Consistent Model of Quasibrittle Elastic Damaged Materials Based on a Novel Helmholtz Potential and Dissipation Function

In the paper, a thermodynamically consistent model of elastic damaged material in the framework of small strain theory is formulated, describing the process of deterioration in quasibrittle materials, concrete in particular. The main goal is to appropriately depict the distinction between material responses in tension and compression. A novel Helmholtz energy and a dissipation potential including three damage parameters are introduced. The Helmholtz function has a continuous first derivative with respect to strain tensor. Based on the assumed functions, the strain–stress relationship, the damage condition, the evolution laws, and the tangent stiffness tensor are derived. The model’s predictions for uniaxial tension, uniaxial compression, uniaxial cyclic compression–tension, and pure shear tests are calculated using Wolfram Mathematica in order to identify the main features of the model and to grasp the physical meaning of an isotropic damage parameter, a tensile damage parameter, and a compressive damage parameter. Their values can be directly bound to changes of secant stiffness and generalized Poisson’s ratio. An interpretation of damage parameters in association with three mechanisms of damage is given. The considered dissipation potential allows a flexible choice of a damage condition. The influence of material parameters included in dissipation function on damage mode interaction is discussed.

Microalgal biomass as renewable biofiller in natural rubber compounds

Microalgal biomasses, consisting of micronized Spirulina Platensis and its low protein fraction, were investigated in this work as possible renewable biofillers in natural rubber compounds, with the aim of replacing the commonly used carbon black. Natural rubber, in some cases blended with 10% of epoxidized natural rubber to improve the matrix-filler affinity, was compounded with 25, 35, 50 and 75 phr of each biomass. Compounds with 25, 35 and 50 phr of carbon black N990 were also prepared as benchmarks. After compounding, vulcanization times were determined by dynamic mechanical analysis. Rubbers were vulcanized by compression moulding and characterized by means of morphological analysis (scanning electron microscopy), thermal analysis (thermogravimetric analysis, dynamic mechanical thermal analysis) and mechanical tests (tensile tests, strain induced crystallization detection by X-ray diffraction, pure shear fracture tests). Microalgal biomass turned out to be homogeneously dispersed in natural rubber matrix and the materials obtained required lower curing times compared to carbon black compounds. It was found that, up to 50 phr, Spirulina has the ability to increase rubber tensile strength and modulus, acting similarly to N990, while decreasing rubber thermal stability and fracture toughness.

Heterogenous late Miocene extension in the northern Walker Lane (California-Nevada, USA) demonstrates vertically decoupled crustal extension

The spatial distribution and kinematics of intracontinental deformation provide insight into the dominant mode of continental tectonics: rigid-body motion versus continuum flow. The discrete San Andreas fault defines the western North America plate boundary, but transtensional deformation is distributed hundreds of kilometers eastward across the Walker Lane–Basin and Range provinces. In particular, distributed Basin and Range extension has been encroaching westward onto the relatively stable Sierra Nevada block since the Miocene, but the timing and style of distributed deformation overprinting the stable Sierra Nevada crust remains poorly resolved. Here we bracket the timing, mag­nitude, and kinematics of overprinting Walker Lane and Basin and Range deformation in the Pine Nut Mountains, Nevada (USA), which are the western­most structural and topographic expression of the Basin and Range, with new geologic mapping and 40Ar/39Ar geochronology. Structural mapping suggests that north-striking normal faults developed during the initiation of Basin and Range extension and were later reactivated as northeast-striking oblique-slip faults following the onset of Walker Lane transtensional deformation. Conformable volcanic and sedimentary rocks, with new ages spanning ca. 14.2 Ma to 6.8 Ma, were tilted 30°–36° northwest by east-dipping normal faults. This relationship demonstrates that dip-slip deformation initiated after ca. 6.8 Ma. A retrodeformed cross section across the range suggests that the range experienced 14% extension. Subsequently, Walker Lane transtension initiated, and clockwise rotation of the Carson domain may have been accommodated by northeast-striking left-slip faults. Our work better defines strain patterns at the western extent of the Basin and Range province across an approximately 150-km-long east-west transect that reveals domains of low strain (~15%) in the Carson Range–Pine Nut Mountains and Gillis Range surrounding high-magnitude extension (~150%–180%) in the Singatse and Wassuk Ranges. There is no evidence for irregular crustal thickness variations across this same transect—either in the Mesozoic, prior to extension, or today—which suggests that strain must be accommodated differently at decoupled crustal levels to result in smooth, homogenous crustal thickness values despite the significantly heterogeneous extensional evolution. This example across an ~150 km transect demonstrates that the use of upper-crust extension estimates to constrain pre-extension crustal thickness, assuming pure shear as commonly done for the Mesozoic Nevadaplano orogenic plateau, may not be reliable.

Experimental Evaluation on Mixed Mode I/II Stress Intensity Factors using CTS welded and non-welded specimen of Aluminum Alloy AA3003

In the present paper, experimental investigation on the fracture of aluminum alloy AA3003 are conducted on the Compact Tension Shear CTS specimen non-welded and CTS specimen welded by FSW process under mixed mode loading by using Arcan loading device based on Richard’s principle suitable for mixed mode. All loading in mixed mode starting from pure tension (mode I) up to pure shear (mode II) can be obtained and tested by varying the loading angles from 0° to 90°. The stress intensity factor for the Compact Tension Shear (CTS) specimen are determined three normalized lengths cracks 0.3, 0.5 and 0.7.The length of notches influence on the variation of stress intensity factor KI, KII. For CTS specimen with notches with a short length, the values of KII are greater than those obtained for notches with a long length.