Specific Stress States Research Articles

Resilient modulus prediction of soft low-plasticity Piedmont residual soil using dynamic cone penetrometer

Dynamic cone penetrometer (DCP) has been used for decades to estimate the shear strength and stiffness properties of the subgrade soils. There are several empirical correlations in the literature to predict the resilient modulus values at only a specific stress state from DCP data, corresponding to the predefined thicknesses of pavement layers (a 50 mm asphalt wearing course, a 100 mm asphalt binder course and a 200 mm aggregate base course). In this study, field-measured DCP data were utilized to estimate the resilient modulus of low-plasticity subgrade Piedmont residual soil. Piedmont residual soils are in-place weathered soils from igneous and metamorphic rocks, as opposed to transported or compacted soils. Hence the existing empirical correlations might not be applicable for these soils. An experimental program was conducted incorporating field DCP and laboratory resilient modulus tests on “undisturbed” soil specimens. The DCP tests were carried out at various locations in four test sections to evaluate subgrade stiffness variation laterally and with depth. Laboratory resilient modulus test results were analyzed in the context of the mechanistic-empirical pavement design guide (MEPDG) recommended universal constitutive model. A new approach for predicting the resilient modulus from DCP by estimating MEPDG constitutive model coefficients (k1, k2 and k3) was developed through statistical analyses. The new model is capable of not only taking into account the in situ soil condition on the basis of field measurements, but also representing the resilient modulus at any stress state which addresses a limitation with existing empirical DCP models and its applicability for a specific case. Validation of the model is demonstrated by using data that were not used for model development, as well as data reported in the literature.

Adaptive multicomponent nanocomposite coatings in surface engineering

This paper reviews experimental research on nanocomposite protective coatings of various chemical compositions and structure. For adaptive multielement and multilayer systems with specific phase composition, structure, substructure, stress state, and high functional properties, formation conditions are considered; the behavior of such systems under extreme operating conditions and in tribological applications is examined; the structural, phase, and chemical composition are discussed as well as the hardness, friction and wear at elevated temperatures; and the adhesive strength of hierarchical protective coatings is analyzed. Finally, the adaptive behavior under different tribological test conditions of multifunctional, multilayer coatings as a function of their properties and structure is examined.

Experimental and theoretical methods on determination of stress-dependent natural frequency of marine sedimentary clay

ABSTRACTExcited by the vibration sources in dynamic engineering, the natural frequency and damping factor of the saturated marine sedimentary clay are key dynamic parameters that influence the responses under cyclic loads. Experimental and theoretical methods are proposed in this paper to analyze the natural frequency and the stress-dependent nonlinearity. The experimental method shows that the natural frequency of soils with specific stress state subjected to large cyclic shear strain can be estimated from the data of dynamic triaxial tests based on the amplitude–frequency response curve. Trial and error by the criterion from the half-power bandwidth method is used to determine the optimal fitting. The results of a theoretical study on the free vibration of soil layers are then presented to derive the analytic solution of natural frequency. In addition to the two frequency-independent elements (a lumped mass matrix and a stiffness matrix), the system’s equivalent damping coefficient matrix is iteratively determined based upon the forced vibration experimentally. Finally, the impacts of the resonance phenomenon on the dynamic shear modulus and hysteretic loop are discussed.

Mechanical Behaviors and Elastic Parameters of Laminated Fabric URETEK3216LV Subjected to Uniaxial and Biaxial Loading

A comprehensive experimental study of the laminated fabric URETEK3216LV subjected to mono-uniaxial, uniaxial cyclic and biaxial cyclic loading was performed to expose the detailed mechanical behaviors and determine proper elastic parameters for the laminated fabrics under specific stress states. The elastic modulus-strain curves and elastic parameter response surfaces were used to reveal the mechanical behaviors, and a weighted average method of integrals was proposed to calculate the elastic parameters for different stress states. Results show that typical stress-strain curves consist of three distinct regions during loading: crimp region, nonlinear transition region and yarn extension region, which is consistent with those of the constitutive yarns. The elastic parameters and mechanical behaviors of the laminated fabric are stress-state specific, and they vary noticeably with the experimental protocols, stress ratios and stress levels. The proposed method is feasible to evaluate the elastic parameters no matter what stress states the materials are subjected to, and thus it may offer potential access to obtain accurate design and analysis of the airship structures under different loading conditions.

Influence of material modeling on earing prediction in cup drawing of AA3104 aluminum alloy sheet

In-plane biaxial tension and combined tension-compression tests are carried out for AA3104 aluminum alloy sheets. Linear stress paths are applied to cruciform specimens to measure the contours of plastic work in the stress space and the directions of plastic strain rates at each stress path. Coefficients α1- α8 and exponent M of the Yld2000-2d yield function are determined to minimize the mean square error of the analytical yield locus from a measured work contour. The values of the weighting coefficients in the evaluation of the error are varied to check the effect of a specific stress state on the earing behavior. The effects of the combinations of the weighting coefficients on the accuracy of earing prediction in the cup drawing process are discussed.

Ultrahardness: Measurement and Enhancement

In the quest for novel superhard materials with hardness comparable to or even higher than that of natural diamond, two issues are of central concern: how hardness can be reliably measured and how hardness can be enhanced. Here, we analyze the specific stress states of the indenter and the tested sample during an indentation hardness measurement and reveal that the traditional prerequisite that the indenter should be harder than the measured sample is not necessary to ensure a reliable hardness measurement. In fact, the indentation hardness can be reliably measured as long as the shear strength of the sample is less than the compressive strength of the diamond indenter. Also, we suggest nanostructuring (nanocrystallinity and nanotwinning) as an effect means of hardness enhancement.

A finite elasto‐plasticity model for thick adhesive layers based on generalized stress‐strain measure

AbstractThe collective term adhesives includes a wide field of materials with a diversity of different material properties. Regarding high‐strength adhesives, the assumption of small strains often holds according to their brittle behavior. The experience with plasticity models based on the additive decomposition into elastic and inelastic strains indicates an appropriate approach to characterize such materials. In some cases, due to a more ductile material response, the assumption of infinitesimal strains is not valid anymore. In particular this is the case for high‐strength adhesives with additives like rubber. But ductile behavior is also observed for specific stress states in one adhesive, e.g. when the behavior for tensile is quite brittle while large shear‐strains could appear. The objective of this work is to overcome the theoretical restriction of small strains and to archive the practical experiences. For the failure criterion two stress invariants are used, which involves the hydrostatic pressure as well as the deviator stress state. The flowrule is introduced for the evolution of the inelastic variables. Herein the flow rule has to be of non‐associated type to ensure the thermodynamical consistency of the model. The plasticity model also includes hardening as well as softening. The presented finite strain model makes use of the fact that the eigenvalues for Green‐Lagrange strains and generalized strains are the same. Thus the limit of applicability is extended to finite strains. (© 2014 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Non-unique response of Mooney–Rivlin model in bi-axial membrane stress

The commonly used two-parameter Mooney–Rivlin incompressible hyper-elastic material model can show non-intuitive responses under certain conditions. This paper shows that critical states with non-unique responses occur at least at very specific bi-axial stress states. This can happen for cases where the constant related to the second invariant of strain is positive, but not for the case with this constant equal to zero (the Neo-Hookean case). The dependence of the instabilities on the ratio between the two constitutive constants is shown by evaluated fold lines. The instability is shown to be related to the imposed boundary conditions. An analytical treatment of the problem shows that dynamic edge effects correspond to the static instability.

Ageing of corrosion resistant steel/rubber/composite hybrid structures

One of the major challenges when preparing reliable hybrid structures is the adhesion between different components. Besides enduring the specific stress state, hybrid structures should maintain the required properties in the service environment without degradation. In this study, the environmental resistance of stainless steel/rubber/GFRP (glass fibre reinforced plastic) hybrid structures were tested by exposure to hot, moist and hot/moist environments and after the ageing by peel testing. Two different stainless steel surface finishes and two different rubber grades were investigated. The results were compared with the properties of a mild steel/rubber/GFRP structure. Both mild steel/rubber and composite/rubber structures are used in industrial applications, such as in vibration damping devices and in automotive components.The peel tests showed that with right rubber compounds, stainless steel/rubber and GFRP/rubber interfaces can maintain their properties even in harsh hot/moist environments to such an extent that the interfacial strength of the joint is higher than the cohesive strength of the rubber. This enables the use of rubber's cohesive fracture properties instead of the substrate/rubber interfacial properties when estimating the strength of the steel/rubber/GFRP hybrid structure. In addition, based on the current study, time-consuming stainless steel pre-treatments are not needed but the stainless steel can be in the as-received stage. According to the chemical analysis even before and after the harsh hot/moist exposure used, none of the studied rubber grades had degraded. Thus, we conclude that it is possible to manufacture environmental resistant stainless steel/GFRP hybrid structures with the aid of EPDM rubbers.

Limitations in Measuring the Flow Stress

In designing of plastic metalworking technologies, one determining amount used is the flow stress (kf ) of the material to be formed. A few experimental methods are used to measure it, but in all case it is intended to establish a specific stress and strain state in the whole volume of the specimen, or at least in its majority. In our work the limitations of upsetting in axial symmetrical and plane strain states were investigated by FE modeling and experimental methods within cold working conditions. Both methods were executed on traditional testing machines and Gleeble 3800 thermo-mechanical simulator. It was intended to determine the flow stress up to as high strain values as possible. The processing of the experimental results showed, reliable flow stress values were obtainable in the equivalent strain range of 0 to 3 using the step-by-step upsetting in plane strain, and in the 0 to 1.2 range by cylindrical upsetting.

Methodology to Produce Locally Heat-Treated EN AW-5182 Aluminum Alloy Sheet Metal Parts

Compared to steel, aluminum has a reduced formability. The consequence is that the drawability of aluminum needs to be extended. This can be achieved by a material recovery that takes place near the zones in which a material failure is initiated during deep drawing. In the considered process, first the aluminum component will be preformed to a specific stress state. In the second step, it will be partial heat treated, before the component is getting finished. Based on the selective intermediate introduction of heat, the material flow of the pre-drawn part is influenced in such a manner that the most highly stressed zones are subjected to further reduction in sheet thickness. This is possible by sacrificing material out of zones near the crack. These areas are referred to below as “sacrificial zones”. They depend on the position of the critical region as a result of the material pre-strain. In these regions, the temperature can be varied. This paper focuses on the development of a methodology to determine a layout of intermediate heat treatment of preformed aluminum sheet metal components. In order to determine such a layout, a principal part must be designed on which the methodology can be reviewed.

Invariant x-ray elastic constants and their use in determining hydrostatic stress

The measurement of stress using x-ray diffraction requires knowledge of the constitutive equation linking strain and stress for the sample under investigation. With the exception of single-crystal materials, a unique constitutive relation does not exist requiring an average over the diffracting ensemble. Such averaging techniques provide bounds to the range of x-ray elastic constants, whose values depend on the elastic anisotropy of the material, the mechanical response of the crystallites and often the x-ray reflection chosen. In fact, for materials possessing cubic symmetry, the general x-ray stress equation can be reduced to a form that relates the measured strain to the hydrostatic stress and the bulk modulus, quantities that are invariant with respect to crystal orientation. These orientations do not correspond to stress-free values but rather to lattice parameters that are independent of the x-ray reflection used. Examples include the application of this technique to a test case of an isotropic, biaxially stressed film, and determining the triaxial stress states in capped features. The validity of these orientations, which are developed for specific stress states, relies on the assumption that the polycrystalline aggregate behaves as a quasi-isotropic ensemble.

Numerical and experimental studies of polyurethane foam under impact loading

Proper predictions of the behaviour of shock absorber materials are of utmost importance in safety assessments for licensing casks for transport and storage of highly active waste. After having identified significant discrepancies between numerical results and the actual response of polyurethane foam limiters subjected to accidental scenarios, a new research project ENREA was established by BAM. A major objective is to enhance and to develop advanced material models intended to simulate limiters under impact loading. They should account for all major factors influencing the load–deformation relationship like temperature, strain rate and specific stress state. The corresponding test program, applicable plasticity models, the overall parameter identification strategy based on local and global optimization techniques as well as experimental and numerical results are presented here in particular for closed cell foams.

Investigation of surface near residual stress states after micro-cutting by finite element simulation

Surface layer properties, namely residual stresses, surface work hardening and surface roughness, have a great influence on the service life of components. Especially the residual stress state of the surface layer after cutting is of great importance and shows a strong material specific behavior. Therefore the dependence of the residual stresses on the cutting edge radius, which plays a crucial role for micro-cutting, is investigated in chip forming simulations using ABAQUS/Standard. The residual stress states are evaluated for the reference material normalized AISI 1045 and are compared with a model material representing a hardened material state. The process knowledge will be increased by the systematic separation of physical effects leading to material specific residual stress states after cutting. The simulation results are validated by the comparison with experimentally determined residual stress depth profiles, using X-ray diffraction method, showing a good correlation.

A Reliable Tool for the Improvement of Micro Powder Injection Moulds Made of Steel

Micro powder injection moulding (μPIM) is a common technique for an economic large-scale production of micro parts. To ensure a long service life a high wear resistance is required which can better be fulfilled by moulds made of steel than by those made of brass. In this study the focus will be on the development of specific residual stress states at and below the surface of a mechanically processed Toolox 44 sample.

Microstructure and abrasive wear in silicon nitride ceramics

It is well known that abrasive wear resistance is not strictly a materials property, but also depends upon the specific conditions of the wear environment. Nonetheless, characteristics of the ceramic microstructure do influence its hardness and fracture toughness and must, therefore, play an active role in determining how a ceramic will respond to the specific stress states imposed upon it by the wear environment. In this study, the ways in which composition and microstructure influence the abrasive wear behavior of six commercially-produced silicon nitride based ceramics are examined. Results indicate that microstructural parameters, such as matrix grain size and orientation, porosity, and grain boundary microstructure, and thermal expansion mismatch stresses created as the result of second phase formation, influence the wear rate through their effect on wear sheet formation and subsurface fracture. It is also noted that the potential impact of these variables on the wear rate may not be reflected in conventional fracture toughness measurements.

Parameter determination for nonlinear stress criteria using a simple regression tool

In geotechnique, laboratory test results are often employed to estimate the parameter values required for the application of criteria commonly used to define specific stress states such as yielding, failure, and residual strength. Many of these criteria rely on a nonlinear formulation relating the different stress tensor components (or the corresponding invariants). There are many regression techniques that can be selected to obtain corresponding parameters based on experimental results. Some of these can be fairly complex but, considering the intrinsic variability of the available data on geomaterials, relatively simple methods are usually deemed sufficient. In that regard, one should favor methods directly available from common software packages. In this paper, the authors present a simple analysis procedure based on Microsoft Excel®which can be applied to different two- and three-dimensional stress criteria. As an example, the technique is applied to the failure strength of rock samples using two nonlinear criteria.Key words: rock mechanics, failure criterion, curve fitting, regression analyses, parameter determination.

Pharmacological properties of traditional medicines. XXVI. Effects of Sansohnin-to on pentobarbital sleep in stressed mice.

We investigated the effect of Sansohnin-to (SAT) on changes of duration in sodium pentobarbital (PB)-induced sleeping time caused by five types of stress. SAT reversed shortened PB sleep in repeated cold stress or 45 min-restraint stress tests and the prolonged PB sleep in 120 min-restraint stress. SAT did not reverse the shortened PB sleep in the specific stress state caused by an alternating rhythm in temperature stress or social isolation stress. In addition, SAT influenced both shortened PB sleep in 45 min-restraint stress and prolonged PB sleep in 120 min-restraint stress. SAT had no effect on PB sleep in unstressed control mice. These findings suggest that SAT has unusual activity, different from synthetic narcoleptics such as benzodiazepine. This is because SAT had no effect on PB sleep in unstressed mice, and it reverses stress-induced decrease and/or increase in PB sleep by improving stress-induced functional changes in the central nervous system, rather than by acting like a synthetic hypnotic on the gamma-aminobutyric acidA (GABA(A)) receptor.

Parameter determination for nonlinear stress criteria using a simple regression tool

In geotechnique, laboratory test results are often employed to estimate the parameter values required for the application of criteria commonly used to define specific stress states such as yielding, failure, and residual strength. Many of these criteria rely on a nonlinear formulation relating the different stress tensor components (or the corresponding invariants). There are many regression techniques that can be selected to obtain corresponding parameters based on experimental results. Some of these can be fairly complex but, considering the intrinsic variability of the available data on geomaterials, relatively simple methods are usually deemed sufficient. In that regard, one should favor methods directly available from common software packages. In this paper, the authors present a simple analysis procedure based on Microsoft Excel® which can be applied to different two- and three-dimensional stress criteria. As an example, the technique is applied to the failure strength of rock samples using two nonlinear criteria.Key words: rock mechanics, failure criterion, curve fitting, regression analyses, parameter determination.

Micromechanics model for elastic stiffness of non-spherical granular assembly

This paper presents a micromechanics model for the elastic stiffness of a non-spherical granular assembly. The microstructural continuum model of ideal spherical assembly is extended for non-ideal assembly. The presented work takes the effects of gradation, shape, and preferred orientation into account by introducing a directional distribution function of branch-vector length. The microstructure of a granular assembly is described by the distributions of packing structure, branch-vector length, and particle number per unit volume. These distributions account or the random nature of a realistic granular material. The microfeatures relevant to the description of non-deal particle assembly are elaborated. The influences of various direction-dependent and direction-independent microfeatures on the elastic stiffness are demonstrated. Hypothetical non-ideal granular assemblies are used to study the effects of gradation, shape and preferred orientation. Based on the proposed model, the paper discusses the inherent anisotropy in a non-ideal granular assembly. The presented work also makes use of a generalized static hypothesis to estimate the contact-force distribution for specific microstructure and stress state. With the estimated contact-force and the Hertz–Mindlin contact theory, the elastic stiffness of a particulate assembly can be evaluated. Hence, the effects of geometric fabric and anisotropic stress state on the elastic stiffness can be deliberated. Consequently, the effects of geometric fabric and kinetic fabric of a natural granular material can be evaluated independently. It is shown that the proposed model can reasonably capture the phenomena of inherent anisotropy and stress-induced anisotropy of a non-spherical granular assembly under small strain. Copyright © 1999 John Wiley & Sons, Ltd.