Uniaxial Research Articles

Bi-axial and shear buckling of laminated composite rhombic hypar shells

The bi-axial and shear buckling behavior of laminated hypar shells having rhombic planforms are studied for various boundary conditions using the present mathematical model. In the present mathematical model, the variation of transverse shear stresses is represented by a second-order function across the thickness and the cross curvature effect in hypar shells is also included via strain relations. The transverse shear stresses free condition at the shell top and bottom surfaces are also satisfied. In this mathematical model having a realistic second-order distribution of transverse shear strains across the thickness of the shell requires unknown parameters only at the reference plane. For generality in the present analysis, nine nodes curved isoparametric element is used. So far, there exists no solution for the bi-axial and shear buckling problem of laminated composite rhombic (skew) hypar shells. As no result is available for the present problem, the present model is compared with suitable published results (experimental, FEM, analytical and 3D elasticity) and then it is extended to analyze bi-axial and shear buckling of laminated composite rhombic hypar shells. A C0 finite element (FE) coding in FORTRAN is developed to generate many new results for different boundary conditions, skew angles, lamination schemes, etc. It is seen that the dimensionless buckling load of rhombic hypar increases with an increase in c/a ratio (curvature). Between symmetric and anti-symmetric laminations, the symmetric laminates have a relatively higher value of dimensionless buckling load. The dimensionless buckling load of the hypar shell increases with an increase in skew angle.

A new rock brittleness index on the basis of punch penetration test data

Brittleness is one of the most important properties of rock which has a major impact not only on the failure process of intact rock but also on the response of rock mass to tunneling and mining projects. Due to the lack of a universally accepted definition of rock brittleness, a wide range of methods, including direct and indirect methods, have been developed for its measurement. Measuring rock brittleness by direct methods requires special equipment which may lead to financial inconveniences and is usually unavailable in most of rock mechanic laboratories. Accordingly, this study aimed to develop a new strength-based index for predicting rock brittleness based on the obtained base form. To this end, an innovative algorithm was developed in Matlab environment. The utilized algorithm finds the optimal index based on the open access dataset including the results of punch penetration test (PPT), uniaxial compressive and Brazilian tensile strength. Validation of proposed index was checked by the coefficient of determination (R2), the root mean square error (RMSE), and also the variance for account (VAF). The results indicated that among the different brittleness indices, the suggested equation is the most accurate one, since it has the optimal R2, RMSE and VAF as 0.912, 3.47 and 89.8%, respectively. It could finally be concluded that, using the proposed brittleness index, rock brittleness can be reliably predicted with a high level of accuracy.

Mangani-pargasite, NaCa2(Mg4Mn3+)(Si6Al2)O22(OH)2 , a new mineral species of the amphibole

Mangani-pargasite, ideally NaCa 2 (Mg 4 Mn 3+ )(Si 6 Al 2 )O 22 (OH) 2 , is a new mineral species of the calcium amphibole subgroup of the amphibole supergroup. The type specimen was found on the mine dump of the Langban Fe-Mn-(Ba-As-Pb-Sb) deposit in Varmland, Bergslagen ore province, Sweden. Crystal chemical analyses resulted in the empirical chemical formula: A (Na 0.90 Pb 0.07 K 0.03 ) Σ1.00 B (Ca 1.93 Mn 2+ 0.07 ) Σ2.00 C (Mg 4.25 Mn 3+ 0.39 Al 0.26 Fe 3+ 0.10 ) Σ5.00 T (Si 6.35 Al 1.65 ) Σ8.00 O 22 W (OH) 2 . The mineral is biaxial positive, with a = 1.635(5), b = 1.645(5), c = 1.660(5) and the measured optic angle 2 V = 85(5)°. The dispersion is weak (r > v), and the optic orientation is: Y || b ; Z ^ c = 25(3)°. Mangani-pargasite is red to brownish red with weak pleochroism; X = pale reddish brown, Y = pale reddish brown and Z = pale brownish red; X ≈ Y > Z . The unit-cell parameters are a = 9.9448(5), b = 18.0171(9), c = 5.2829(3) A, b = 105.445(3)°, V = 912.39(9) A 3 , Z = 2, space group C 2/ m . The ten strongest reflections in the X-ray powder diffraction pattern [ d -values in A, I , ( h k l )] are: 8.420, 29, (1 1 0); 3.368, 17, (1 3 1), 3.279, 49, (2 4 0); 3.141, 100, (3 1 0); 2.817, 44, (3 3 0); 2.698, 21, (1 5 1); 2.389, 18, (3 5 0); 1.904, 29, (5 1 0); 1.650, 22, (4 6 1) and 1.448, 46, (6 6 -1).

Correction to: A class of biaxial micro/meso-scale structures for isotropic in-plane inertial sensing and actuation: design, fabrication and experiments

Correction to: A class of biaxial micro/meso-scale structures for isotropic in-plane inertial sensing and actuation: design, fabrication and experiments

Effect of biaxial stress state on seismic fragility of concrete gravity dams

Dams are important structures for management of water supply for irrigation or drinking, flood control, and electricity generation. In seismic regions, the structural safety of concrete gravity dams is important due to the high potential of life and economic loss if they fail. Therefore, the seismic analysis of existing dams in seismically active regions is crucial for predicting responses of dams to ground motions. In this paper, earthquake response of concrete gravity dams is investigated using the finite element (FE) method. The FE model accounts for dam-water-foundation rock interaction by considering compressible water, flexible foundation effects, and absorptive reservoir bottom materials. Several uncertainties regarding structural attributes of the dam and external actions are considered to obtain the fragility curves of the dam-water-foundation rock system. The structural uncertainties are sampled using the Latin Hypercube Sampling method. The Pine Flat Dam in the Central Valley of Fresno County, California, is selected to demonstrate the methodology for several limit states. The fragility curves for base sliding, and excessive deformation limit states are obtained by performing non-linear time history analyses. Tensile cracking including the complex state of stress that occurs in dams was also considered. Normal, Log-Normal and Weibull distribution types are considered as possible fits for fragility curves. It was found that the effect of the minimum principal stress on tensile strength is insignificant. It is also found that the probability of failure of tensile cracking is higher than that for base sliding of the dam. Furthermore, the loss of reservoir control is unlikely for a moderate earthquake.

Energy analysis-based core drilling method for the prediction of rock uniaxial compressive strength

The uniaxial compressive strength (UCS) of rock is a basic parameter in underground engineering design. The disadvantages of this commonly employed laboratory testing method are untimely testing, difficulty in performing core testing of broken rock mass and long and complicated onsite testing processes. Therefore, the development of a fast and simple in situ rock UCS testing method for field use is urgent. In this study, a multi-function digital rock drilling and testing system and a digital core bit dedicated to the system are independently developed and employed in digital drilling tests on rock specimens with different strengths. The energy analysis is performed during rock cutting to estimate the energy consumed by the drill bit to remove a unit volume of rock. Two quantitative relationship models of energy analysis-based core drilling parameters (ECD) and rock UCS (ECD-UCS models) are established in this manuscript by the methods of regression analysis and support vector machine (SVM). The predictive abilities of the two models are comparatively analysed. The results show that the mean value of relative difference between the predicted rock UCS values and the UCS values measured by the laboratory uniaxial compression test in the prediction set are 3.76 MPa and 4.30 MPa, respectively, and the standard deviations are 2.08 MPa and 4.14 MPa, respectively. The regression analysis-based ECD-UCS model has a more stable predictive ability. The energy analysis-based rock drilling method for the prediction of UCS is proposed. This method realized the quick and convenient in situ test of rock UCS.

Numerical investigation on the response of circular double-skin concrete-filled steel tubular slender columns subjected to biaxial bending

Recently, Concrete-filled double skin steel tubular (CFDST) columns have proven an exceptional structural resistance in terms of strength, stiffness, and ductility. However, the resistance of these column members can be severely affected by the type of loading in which bending stresses increase in direct proportion with axial load and eccentricity value. This paper presents a non-linear finite element based modeling approach that studies the behavior of slender CFDST columns under biaxial loading. Finite element models were calibrated based on the outcomes of experimental work done by other researchers. Results from simulations of slender CFDST columns under axial loading eccentric in one direction showed good agreement with the experimental response. The calibrated models are expanded to a total of thirty models that studies the behavior of slender CFDST columns under combined compression and biaxial bending. The influences of parameters that are usually found in practice are taken into consideration in this paper, namely, eccentricity-to-diameter (e/D) ratios, slenderness ratios, diameter-to-thickness (D/t) ratios, and steel contribution ratios. Finally, an analytical study based on current code provisions is conducted. It is concluded that South African national standards (2011) provided the most accurate results contrasted with the Eurocode 4 (2004) and American Institute of Steel Construction (2016) that are found to be conservative. Accordingly, correction factors are proposed to the current design guidelines to provide more satisfactory results.

Thermographic analysis of failure for different rock types under uniaxial loading

Mining activities focus on the production of mineral resources for energy generation and raw material requirements worldwide and it is a known fact that shallow reserves become scarce. For this reason, exploration of new resources proceeds consistently to meet the increasing energy and raw material demand of industrial activities. Rock mechanics has a vital role in underground mining and surface mining. Devices and instruments used in laboratory testing to determine rock mechanics related parameters might have limited sensing capability of the failure behavior. However, methodologies such as, thermal cameras, digital speckle correlation method and acoustic emission might enable to investigate the initial crack formation in detail. Regarding this, in this study, thermographic analysis was performed to analyze the failure behaviors of different types of rock specimens during uniaxial compressive strength experiments. The energy dissipation profiles of different types of rocks were characterized by the temperature difference recorded with an infrared thermal camera during experiments. The temperature increase at the failure moment was detected as 4.45oC and 9.58oC for andesite and gneiss-schist specimens, respectively. Higher temperature increase was observed with respect to higher UCS value. Besides, a temperature decreases of about 0.5-0.6oC was recorded during the experiments of the marble specimens. The temperature change on the specimen is related to release of radiation energy. As a result of the porosity tests, it was observed that increase in the porosity rate from 5.65% to 20.97% can be associated to higher radiation energy released, from 12.68 kJ to 297.18 kJ.

Efficient flexible boundary algorithms for DEM simulations of biaxial and triaxial tests

The accurate modeling of boundary conditions is important in simulations of the discrete element method (DEM) and can affect the numerical results significantly. In conventional triaxial compression (CTC) tests, the specimens are wrapped by flexible membranes allowing to deform freely. To accurately model the boundary conditions of CTC, new flexible boundary algorithms for 2D and 3D DEM simulations are proposed. The new algorithms are computationally efficient and easy to implement. Moreover, both horizontal and vertical component of confining pressure are considered in the 2D and 3D algorithms, which can ensure that the directions of confining pressure are always perpendicular to the specimen surfaces. Furthermore, the boundaries are continuous and closed in the new algorithms, which can prevent the escape of particles from the specimens. The effectiveness of the proposed algorithms is validated by biaxial and triaxial simulations of granular materials. The results show that the algorithms allow the boundaries to deform non-uniformly on the premise of maintaining high control accuracy of confining pressure. Meanwhile, the influences of different lateral boundary conditions on the numerical results are discussed. It is indicated that the flexible boundary is more appropriate for the models with large strain or significant localization than rigid boundary.

Bearing capacity of shallow foundations on the bilayer rock

The traditional formulations for estimation of bearing capacity in rock mechanics assume a homogeneous and isotropic rock mass. However, it is common that the rock mass consists of different layers of different rock properties or of the same rock matrix with distinct geotechnical quality levels. The bearing capacity of a heterogeneous rock is estimated traditionally through the weighted average. In this paper, the solution of the weighted average is compared to the finite difference method applied to a bilayer rock mass. The influence of different parameters such as the thickness of the layers, the rock type, the uniaxial compressive strength and the overall geotechnical quality of the rock mass on the bearing capacity of a bilayer rock mass is analyzed. A parametric study by finite difference method is carried out to develop a bearing capacity factor in function of the layer thickness and the rock mass quality expressed in terms of the geological strength index, which is presented in a form of a chart. Therefore, this correlation factor allows estimating the bearing capacity of a rock mass that is formed by two layers with distinct GSI, depending on the bearing capacity of the rock mass formed only by the upper layer and considered by that way as homogenous and isotropic rock mass.

Strength characteristics and fracture evolution of rock with different shapes inclusions based on particle flow code

Natural rock mass contains defects of different shapes, usually filled with inclusions such as clay or gravel. The presence of inclusions affects the failure characteristics and mechanical properties of rock mass. In this study, the strength and failure characteristics of rock with inclusions were studied using the particle flow code under uniaxial compression. The results show that the presence of inclusions not only improves the mechanical properties of rock with defects but also increases the bearing capacity of rock. Circular inclusion has the most obvious effect on improving model strength. The inclusions affect the stress distribution, development of initial crack, change in crack propagation characteristics, and failure mode of rock. In defect models, concentration area of the maximum tensile stress is generated at the top and bottom of defect, and the maximum compressive stress is distributed on the left and right sides of defect. In filled models, the tensile stress and compressive stress are uniformly distributed. Failing mode of defect models is mainly tensile failure, while that of filled models is mainly shear failure.

Scale effect of mechanical properties of jointed rock mass: A numerical study based on particle flow code

The synthetic rock mass (SRM) were used to investigate the influence of specimen size on the mechanical properties of jointed rock mass. The SRM were established based on parallel bond model (PBM) and smooth joint model (SJM) and the scaled rock specimens were sampled in two SRMs considering three sampling locations. The research results show that the smaller the initial fracture density is, the greater the uniaxial compressive strength (UCS), elastic modulus (E) is when compared with the same sampling location. The mechanical properties of rock specimens obtained by different sampling methods in different SRMs have different scale effects. The strength of rock specimens with more new cracks is not necessarily less than that of rock specimens with fewer new cracks and the failure of rock is caused by the formation of macro-fracture surface.

Горные породы – сегнетоэластики

Purpose. Identification of the features and the properties of segnetoelastics in rocks. Research Methods. Laboratory experiments on the deformation of rocks in a volume field of compressive stresses, macro- and microscopic studies of the deformations and the orientations of an optical indicatrix, analysis of the data obtained. Research results. As a result of microscopic and optical studies of rocks in thin sections on a polarizing microscope, tests of rocks under the conditions of unequal triaxial compression, analysis of the results obtained, it was found that the rock-forming minerals of the sedimentary sequence at a normal temperature and in the absence of an external load have a domain structure. Under the conditions of a triaxial unequal component compression, the volumetric deformations at the micro and macro levels do not exceed hundredths, while the relative micro deformations fully correspond to the dilatancy volume at the macro level. Shear rotational deformations reach 15–20%, their resonant increment increases several times over three periods before the destruction. During the loading, the direction of the rotation is changed 3-4 or more times, an optical indicatrix changes the direction. All deformations are alternating in sign and, until resonance, the amplitude of their increment does not exceed 0.5%. During the loading, the deformation, elastic, optical, electromagnetic, and other parameters are changed abruptly (structural-phase transitions of the second, and upon the destruction, of the first and third kind). The interconnection between the load and the deformations is nonlinear, a number of characteristic deformations are distinguished at which these jumps occur. All said indicate that rocks are classical distortion segnetoelastics. The following parameters can be taken as parameters of a segnoelastic transition: the value or increase in the value of relative deformation; an elastic modules varying within ± ∞; the sign and the magnitude of the increment of the Lode angle for the deformations; the angle of rotation of the optical indicatrix, the presence of structural-phase transitions in the minerals. Scientific novelty. It has been shown for the first time that all five signs and traits of segnetoelastics are present in sedimentary rocks. Practical significance. The possibility for isolating of segnetoelastics without laborious microscopic and electromagnetic studies.

Modeling of anisotropic hardening and grain size effects based on advanced numerical methods and crystal plasticity

Modeling of anisotropic behavior as well as hardening behavior based on micromechanical quantities in combination with a spectral solver is the focus of this study. A deep drawing steel as well as two different aluminum alloys are investigated. Prediction capabilities of the proposed modeling strategy are discussed and the benefits of the micromechanical model are highlighted. Further, a comparison of the crystal plasticity (CP) results with the well established macroscopic model YLD2000-2d underlines the importance of the CP as a complementary modeling technique to the macroscopic modeling. Both models – the microscopic as well as the macroscopic – are validated on experimental data mainly gained from uniaxial and biaxial tests. In the second part of this study, strong inhomogeneous microstructures are investigated from a modeling point of view. For this purpose, a Hall–Petch phenomenological model is implemented in the CP open-source code DAMASK to take the grain size effects into account. Appropriate combinations of the grain sizes in a bimodal microstructure are presented in order to increase the strength as well as ductility of a generic aluminium alloy. The proposed numerical strategy of coupling the CP and efficient FFT-based spectral solver supports the development of new materials in an optimal way.

Determination of acousto-optical effect maxima for optically isotropic crystalline material on the example of GaP cubic crystal

The peculiarities of the acousto-optic interaction geometry optimization in the optically isotropic crystalline material are demonstrated on the example of GaP cubic crystal. The calculation algorithm takes into account the change of the optical indicatrix form during acoustic wave propagation through the crystal. On this basis, the highest achievable value of the acousto-optic figure of merit M2=98·10−15 s3/kg is determined by the extreme surfaces method. This value is achieved in the case of the isotropic diffraction of the light beam on the longitudinal acoustic wave. The approach developed in this work can be used for optimization of the acousto-optic interaction in other optically isotropic crystalline materials.

Chiral Microwrinkles: Controlled Fabrication of 3D Chiral Microwrinkles via Asymmetrical and Biaxial Bucklings (Adv. Funct. Mater. 29/2019)

Chiral Microwrinkles: Controlled Fabrication of 3D Chiral Microwrinkles via Asymmetrical and Biaxial Bucklings (Adv. Funct. Mater. 29/2019)

Acquisition and analysis of aluminum alloy property in thermal environment based on biaxial tension

Acquisition and analysis of aluminum alloy property in thermal environment based on biaxial tension

Triclinic Pedial Sarcosinium Hydrogen l(+)‐Tartrate—Crystal Growth, Pyroelectricity, Thermal Expansion, and Linear Optical Properties

AbstractSingle crystals of optical quality and dimensions up to 25 × 25 × 40 mm3 of sarcosinium hydrogen l(+)‐tartrate and up to 15 × 20 × 30 mm3 of sarcosinium hydrogen d(−)‐tartrate are grown from aqueous solution during growth periods of 10–14 weeks. For sarcosinium hydrogen l(+)‐tartrate refractive indices and their dispersion in the wavelength range 365–1083 nm are determined. The biaxial positive crystals show marked optical anisotropy with maximum birefringence at 365 nm of Δn = 0.089 and only small orientation dispersion of the principal axes of the optical indicatrix. Based on the linear optical data, phase matching conditions for collinear second harmonic generation (SHG) are analyzed. The optical properties allow SHG phase matching for both, type I and type II processes. Temperature‐dependent coefficients of the tensor of thermal expansion [αij] between 145 and 300 K show pronounced anisotropy and moderate orientation dispersion of thermal expansion, with principal values at 293 K of = −3.3(1) × 10−6 K−1, = 35.7(1) × 10−6 K−1, and = 66.6(1) × 10−6 K−1. The absolute value of the pyroelectric vector |pσ| of sarcosinium hydrogen l(+)‐tartrate amounts to about 20 µC K−1 m−2 and varies only slightly with temperature in the temperature range 100–320 K.

A new uniaxial compression tester: development and application

Powder flow characterization with some of the conventional shear testers can be costly, time consuming and requires a trained operator, therefore the application of a cheaper, simpler, and sometimes faster uniaxial compaction tester (UCT) have often been suggested as an alternative. However, it has been known for many years that the results derived from the two methods are not necessarily the same, due to the lower state of compaction attained in the traditional design of UCT. Additionally, the traditional design of UCT tends to give a wide scatter in the results. To overcome these limitations of the UCT, this work developed an easy-to-use uniaxial tester in order to reduce the difference between flow properties reported by the two techniques. In this regard, flow functions of four powders in the cohesive and very cohesive range were measured with shear testers, conventional uniaxial compaction tester (UCT) and the new uniaxial tester at the University of Greenwich (“Greenwich Uniaxial Tester” or GUT). A method in the style of Janssen approach for correcting wall friction effect on the compaction stress of UCT was applied. Results showed the unconfined yield strength attained from GUT is in line with the results obtained from shear testers while the results from UCT are well below the shear tester results suggesting the advantage of the new GUT.

КОНЦЕНТРАЦІЯ НАПРУЖЕНЬ ПРИ ПРУЖНО-ПЛАСТИЧНОМУ ДЕФОРМУВАННІ

Розглядається квазістатична задача про складне двохосьове навантаження квадратної пружно-пластичної пластинки з круговим отвором. В якості фізичних рівнянь використаний диференціально-нелінійний варіант теорії пластичності, що враховує мікродеформації. З використанням методу скінченних елементів отримані числові результати, що свідчать про те, що вплив історії навантаження на концентрацію напружень пластинки може бути значним.