Stress State Of Material Research Articles

Measurement of the stress state of materials by reflection of polarization-modulated light

A method for measuring mechanical stresses of photoelastic materials from the difference between the reflection coefficients of orthogonally polarized light waves incident on the surface of the stressed medium is considered. Comparative results of measurements of the stress state of polymethylmethacrylate in the conventional transmission polarization optical scheme and by the proposed refraction method are presented. A case of normal light incidence is considered.

Оценка изменений напряженного состояния материалов по отражению света

Оценка изменений напряженного состояния материалов по отражению света

A Micromechanically-motivated Phenomenological Model for Predicting Ductile Fracture Initiation

Abstract A phenomenological ductile fracture initiation model is developed based on the assumption that onset of fracture is a limit state of void evolution followed by void coalescence at microscopic level. In the model, void grows in two typical deformation modes: a) void dilation and b) void elongation/rotation. The fracture initiation model is transformed from stress space into the space of equivalent plastic strain, stress triaxiality and Lode parameter. A parametric study is carried out to better understand the effects of model parameters. The comparison with the experimental data (DP590, TRIP690, 5083-O aluminum alloy) shows a good prediction capability of the proposed model in a large range of stress state for various materials.

Feasibility of Residual Stress Nondestructive Estimation Using the Nonlinear Property of Critical Refraction Longitudinal Wave

Residual stress has significant influence on the performance of mechanical components, and the nondestructive estimation of residual stress is always a difficult problem. This study applies the relative nonlinear coefficient of critical refraction longitudinal (LCR) wave to nondestructively characterize the stress state of materials; the feasibility of residual stress estimation using the nonlinear property of LCR wave is verified. The nonlinear ultrasonic measurements based on LCR wave are conducted on components with known stress state to calculate the relative nonlinear coefficient. Experimental results indicate that the relative nonlinear coefficient monotonically increases with prestress and the increment of relative nonlinear coefficient is about 80%, while the wave velocity only decreases about 0.2%. The sensitivity of the relative nonlinear coefficient for stress is much higher than wave velocity. Furthermore, the dependence between the relative nonlinear coefficient and deformation state of components is found. The stress detection resolution based on the nonlinear property of LCR wave is 10 MPa, which has higher resolution than wave velocity. These results demonstrate that the nonlinear property of LCR wave is more suitable for stress characterization than wave velocity, and this quantitative information could be used for residual stress estimation.

Effect of freezing and thawing on $$\hbox {K}_{0}$$ K 0 geostatic stress state for granular materials

The coefficient of lateral stress at rest, \(\hbox {K}_{0}\), is an important soil parameter that characterizes the geostatic stress state of soils. In this study, an experimental testing program was established to investigate the values of \(\hbox {K}_{0}\) for granular materials that are subject to freezing and thawing. Various material conditions, including different fines contents, relative densities and freezing-thawing cycles, were considered in the testing program. The values of \(\hbox {K}_{0}\) for thawed condition were higher than for unfrozen condition with net volume decrease. The effect of freezing and thawing became more pronounced as fines content increased while the effect of relative density was small. From the multi-cycled freezing-thawing tests, it was observed that the increase in \(\hbox {K}_{0}\) was most significant during the first freezing-thawing cycle. For frozen condition, the values of \(\hbox {K}_{0}\) were very low up to a certain limit stress level. Beyond the limit stress level, the values of \(\hbox {K}_{0}\) became higher and close to those of unfrozen condition. The increases in \(\hbox {K}_{0}\) with net volume decrease after freezing and thawing process was explained with internal changes in microstructure and stress state of soils.

Practical considerations for reliable stress and oxygen surface exchange coefficients from bilayer curvature relaxation measurements

The curvature relaxation technique is a new electrode-free method for simultaneously measuring a material’s chemical oxygen surface exchange coefficient and stress state under controlled atmosphere and temperature conditions. Provided certain conditions are met, this in situ/in operando technique can be used to accurately measure the oxygen surface exchange coefficients and stress states of dense, porous, thin, or thick film oxygen exchange materials. The present paper provides a detailed, practical discussion of these conditions and compares the curvature relaxation technique to alternative oxygen surface exchange coefficient measurement approaches.

Stochastic behavior of nanoscale dielectric wall buckling.

The random buckling patterns of nanoscale dielectric walls are analyzed using a nonlinear multi-scale stochastic method that combines experimental measurements with simulations. The dielectric walls, approximately 200 nm tall and 20 nm wide, consist of compliant, low dielectric constant (low-k) fins capped with stiff, compressively stressed TiN lines that provide the driving force for buckling. The deflections of the buckled lines exhibit sinusoidal pseudoperiodicity with amplitude fluctuation and phase decorrelation arising from stochastic variations in wall geometry, properties, and stress state at length scales shorter than the characteristic deflection wavelength of about 1000 nm. The buckling patterns are analyzed and modeled at two length scales: a longer scale (up to 5000 nm) that treats randomness as a longer-scale measurable quantity, and a shorter-scale (down to 20 nm) that treats buckling as a deterministic phenomenon. Statistical simulation is used to join the two length scales. Through this approach, the buckling model is validated and material properties and stress states are inferred. In particular, the stress state of TiN lines in three different systems is determined, along with the elastic moduli of low-k fins and the amplitudes of the small-scale random fluctuations in wall properties-all in the as-processed state. The important case of stochastic effects giving rise to buckling in a deterministically sub-critical buckling state is demonstrated. The nonlinear multiscale stochastic analysis provides guidance for design of low-k structures with acceptable buckling behavior and serves as a template for how randomness that is common to nanoscale phenomena might be measured and analyzed in other contexts.

Pressure Interaction of Mixing Particulate Material Along the Blade Length

Abstract To assess the energy intensity of particulate materials mixing, it is necessary to know the state of stress in the particulate material in front of mixing elements. The theoretical background of this process results from the theory of the equilibrium limit of the particulate material, and this state may by described by Mohr’s circle theory and the Novosad model. Based on the above assumptions, it is possible to derive the pressure distribution along the blade height, but only for an infinitely long blade. A measuring system implemented on the homogenizer blade was constructed to examine the effects of the distribution of the mixed material on the mixing element along its length.

КОМПЛЕКС ИСПЫТАНИЙ ДЛЯ ИССЛЕДОВАНИЯ ВЛИЯНИЯ НАПРЯЖЕННОГО СОСТОЯНИЯ НА ПРЕДЕЛЬНУЮ ПЛАСТИЧНОСТЬ МЕТАЛЛА ПРИ ПОВЫШЕННОЙ ТЕМПЕРАТУРЕ

A set of tests that can be used tostudy the effect of the stress state on ultimate metal plasticity at high temperature is proposed. Ultimate plasticity can be characterized quantitatively by the strain accumulated to failure. Dimensionless invariant parameters, such as the triaxiality parameter k and the Lode–Nadai parameter μσ, are used as a characteristic of the stress state. A set of the parameters k and μσ unambiguously characterizes the stress state under plastic deformation. Being dimensionless, these parameters can be used to compare the stress state of materials with different levels of strength properties. The triaxiality parameter k characterizes the level of normal stresses. If k > 0, then normal tensile stresses dominate. If k < 0, then normal compressive stresses dominate. The Lode–Nadai parameter μσ characterizes the type of the stress state. The value of μσ = +1/–1 corresponds to the stress state of axisymmetric compression/tension, and μσ = 0 under the plane stress state. The set of tests consists of conventional tension tests of cylindrical specimens, tension tests of notched cylindrical specimens, newly developed tests of “bell” specimens and tests of a thick–walled cup with a thinned bottom. This types of tests has allowed us to investigate ultimate plasticity under monotonic deformation in the range of parameters –1,2 < k < 1,2 and –1< μσ <+1 without applying high pressure technique. The stress–strain state under tests was evaluated by the results of mathematical simulation by means of the finite element method in ANSYS. The tests performed on 01570 aluminium alloy specimens. The test temperature was 360 о C. The shear strain rate was H = 0,1; 0,3; 0,5 s –1 . The fracture locus of the 01570 aluminium alloy has been obtained from the test results. The fracture locus can be used in damage mechanics models to predict alloy fracture under metal forming processes.

Метод оценки напряженного состояния материалов по данным инфракрасного излучения

The solution of a number of geological and engineering problems require knowledge of the stress state of the structural member material. Non-destructive testing methods are used to evaluate the stress state of materials. They are based on such criteria as sound pressure, temperature, ultrasound wave’s characteristics etc., as well as methods evaluating the stress state of the material after destruction. Development of methods of this group is caused by insufficient reliability of theoretical modelling not providing in some case stability of engineering facilities in practice. To avoid the emergencies, it is important to obtain the information on the actual load (stress) at which the structure destruction occurred. These methods are especially important as a tool for experts identifying the cause of the accidents at engineering facilities. Additionally, this information will provide a correction of calculation models improving the safe exploitation of facilities. The aim of this study was to develop a method of materials stress state assessment using an infrared thermography data. Experimental studies showed that there exists a relationship between the temperature on the surface of material displacement and the normal stress affecting the area of failure. The temperature on the surface of material increases with an increase in normal stress in a fracture area. On a basis of the revealed relationship, a method of determination of the material stress state using the infrared thermography data was worked out.

Inspection of birefringent media by photoelasticity: from diffuse light polariscope to laser conoscopic technique

A reliable and noninvasive measurement method for the inspection of naturally birefringent transparent media is presented in this paper. It allows us to achieve a spatially resolved analysis of the stress state of birefringent materials. The developed system is based on photoelasticity and exploits a controlled laser conoscopy technique and a scanning system to perform local measurements in the volume dimensions of the media, which can be displaced over a grid of points. The configuration of the proposed laser conoscopic system is presented, and a dedicated algorithm, designed to perform digital analysis of the fringe patterns, is also described. The design and the realization of the system are discussed, as well as the advantages of the proposed system over the classic diffuse light polariscope technique. The method has been experimentally validated through laboratory tests on birefringent samples with known induced stress. The system has demonstrated its sensitivity to stress and its capability to achieve a spatial resolution on the order of 0.1 mm to resolve stress gradients (uncertainty on the stress amplitude of ± 0.1 MPa ).

Application of the whole powder pattern decomposition procedure in the residual stress analysis of layers and coatings

The X-ray investigation of stress states in materials, based on the determination of elastic lattice strains which are converted to stresses by means of theory of elasticity, is a necessity in quality control of thin layers and coatings for optimizing manufacturing steps and process parameters.This work introduces the evaluation of residual stress from complex and overlapping diffraction patterns using a whole-powder pattern decomposition procedure defining a 2θ-offset caused by residual stresses. Furthermore corrections for sample displacement and refraction are directly implemented in the calculation procedure. The correlation matrices of the least square fitting routines have been analyzed for parameter interactions and obvious interdependencies have been decoupled by the introduction of an internal standard within the diffraction experiment. This decomposition based evaluation has been developed on tungsten as a model material system and its efficiency was demonstrated by X-ray diffraction analysis of a solid oxide fuel cell multilayer system. The results are compared with those obtained by the classical sin2Ψ-method.

Texture evolution in deformed AZ31 magnesium sheets: Experiments and phase-field study

Experimental and phase-field studies are performed to investigate mechanisms of preferential growth which lead to improved formability in AZ31 Mg sheets. A compression/annealing treatment is specialized to modify the initial texture in thin sheets. The texture and stress states of materials are studied via electron back scattered diffraction (EBSD) technique before and after annealing. Using the EBSD data on microstructure and residual stresses, a phase-field model is constructed to simulate the texture evolution after initial compression. The results suggest that the residual-stresses induced by in-plane compression are the main driving force for recrystallization and grain growth. The inhomogeneous stress distribution leads to preferential growth of {21¯1¯0} texture along the normal to the sheet, which are at lowest stress state, at the expense of initial basal texture. Limited mobility of twin boundaries changes the mixture of textures but the non-basal textures are still preferred. The formability tests confirm a significant enhancement of the final product compared to as-received sheets.

Review of photoelastic image analysis applied to structural birefringent materials: glass and polymers

Photoelasticity is particularly suitable for the analysis of the stress state in structural materials that are transparent and birefringent. Some techniques of digital photoelasticity (phase shifting and RGB) are applied to the analysis of stress field in two classes of structural materials. The first one consists of tempered glasses, such as those used in the automotive and architectural fields. The second one consists of thermoset polymers, typically used as matrices in fiber reinforced plastic structural composites. The birefringence of such resins is, in particular, exploited to investigate the development of swelling stresses and changes in fracture toughness as induced by water uptake aging.

The influence of mechanical and microstructural properties on the rate-dependent fracture strength of ceramics in uniaxial compression

The objective of the article is to improve our understanding of how material properties and stress state affect the performance of ceramics at both low and high rates of deformation. Accordingly, studies are performed using finite element simulations and a validated constitutive model, due to Deshpande and Evans (V.S. Deshpande and A.G. Evans, Inelastic deformation and energy dissipation in ceramics: A mechanism-based constitutive model, Journal of the Mechanics and Physics of Solids 56 pp. 3077–3100 (2008)), to investigate the relationship between the mechanical and microstructural properties of ceramics and their rate-dependent fracture strengths in uniaxial compression. The compression simulations consider two different cases, delineated by the boundary condition imposed on the circumferential surface of the cylindrical test specimen: (Case I) A traction-free boundary, allowing for radial confining stresses to develop due to radial inertia; and (Case II) a radial velocity boundary condition with a magnitude that prevents radial stresses from developing. Thus, in the Case II, as in the split Hopkinson pressure bar test, the stress state is uniaxial stress for all applied strain rates. Consistent with experimental results, the model predicts two regimes of rate-dependent fracture strength: (i) A quasistatic regime, for strain rates below a characteristic strain rate, in which the fracture strength is constant and thus insensitive to strain rate; and (ii) a dynamic regime, at strain rates above a characteristic strain rate, in which the fracture strength exceeds quasistatic values and is strain rate sensitive. Dynamic strengthening at high rates is shown to be due to two mechanisms that delay ceramic fracture: (i) Inertia-induced radial confinement and (ii) the limited growth velocities of microcracks. Quasistatic strength σo and characteristic strain rate ε˙o are found to depend on three mechanical properties and two microstructural properties; through control of these parameters the rate-dependent fracture strength, and thus ballistic performance, of manufactured ceramics can be manipulated. Scaling the results by these characteristic parameters reveals self-similarity among all uniaxial stress results (Case II): If the applied strain rate ε˙ is normalized by ε˙o and the fracture strength σf is normalized by σo, all the results collapse down onto a single universal curve described by a power law. Next, we show that experimental results for the compressive behavior of a large number of brittle materials is well described by the derived scaling law. Finally, the model is calibrated to SiC–N, by fitting the scaling law to empirical data, and its response validated.

Parameter estimation approach for particle flow model of rockfill materials using response surface method

Particle flow code (PFC) is widely used to model deformation and stress states of rockfill materials. The accuracy of numerical modeling with PFC is dependent upon the model parameter values. How to accurately determine model parameters remains one of the main challenges. In order to determine model parameters of particle flow model of rockfill materials, some triaxial compression experiments are performed, and the inversion procedure of model parameters based on response surface method is proposed. Parameters of particle flow model of rockfill materials are determined according to the observed data in triaxial compression tests for rockfill materials. The investigation shows that the normal stiffness, tangent stiffness and friction coefficient of rockfill materials will slightly increase with increase of confining pressure in triaxial compression tests. The experiments in laboratory show that the proposed inversion procedure behaves higher computing efficiency and the forecasted stress–strain relations agree well with observed values.

Effect of coherent and incoherent precipitates upon the stress and strain fields of 6xxx aluminium alloys: a numerical analysis

This paper investigates the stress and strain fields induced by the coherent/incoherent precipitates of type MgxSiy in the 6xxx series aluminium alloys using numerical means. During welding of 6xxx series alloys, the heat-affected zone experiences high temperature heating and cooling cycles and hence results in the non-uniform stress and strain states both at macro- and micro-scales. In the present work, the macro-scale finite element (FE) simulations were carried out to calculate and compare the bulk material properties with those of experimental findings. Since the microstructure of 6xxx alloys comprises mostly of soft α-matrix and coherent/incoherent MgxSiy precipitates, the properties of the individual constituents were used to perform micro-scale FE simulations. Temperature dependent theoretical yield strengths for the precipitates were also determined. The simulated micro-stress and strain fields showed strong dependency over the nature (coherent/incoherent), shape morphology (needle-like, random etc.) and distribution of precipitates. Additionally, unlike coherent precipitates, the plastic deformation of the matrix due to incoherent impurities was found to be highly non-uniform and localized. FE analysis was also performed to characterize the effect of grain size upon the stress state of material.

Viscoelastic Model of Asphalt Mixtures under Repeated Load

AbstractRepeated loading test has been widely used to simulate the true stress state of pavement materials under traffic loading with more accuracy. In this paper, a half-sinusoidal load with 0.1-s loading duration and 0.9-s rest period was selected to simulate the dynamic traffic loading. A generalized Kelvin viscoelastic model of asphalt mixtures based on linear-superposition principle under repeated load is developed. The Prony-series coefficients of the generalized Kelvin model are obtained by the static creep tests of AC-13C and AC-20C unmodified and modified asphalt mixtures. Then, the strain of AC-13C and AC-20C asphalt mixtures under repeated load could be predicted by the proposed viscoelastic model. Comparing the predicted strain obtained by retardation-time spectrum having and not having fixed time with the measured strain, it is believed that the retardation-time spectrum covering both the loading and unloading time processes should be selected in predicting strain of asphalt mixtures under re...

Study on Stress State of Low-Frequency Vibration Cutting Process Based on Finite Element Simulation

Proposed a concept, “cutting degree” as one of the parameters indicating the surface characteristics of the machined layer integrity. According to material nonlinearity, geometric nonlinearity, thermal coupling theory, established models of the vibratory cutting simulation, constitutive J-C model, shear model, boundary conditions, made finite element simulation of low-frequency vibration cutting by using software. Accessing to a visualization process, which is the change of material stress state in local area of the workpiece in cutting process. Analyzed the change of material stress states in local area of the workpiece, between low-frequency vibration cutting and non-vibration cutting machining process, other cutting parameters being equal. It is found that the values of cutting degree were apparently different in vibration cutting from those in non-vibration cutting. Contrasting experiments were done and SEM was observed of the machined surfaces. Findings of the experiment supported the simulation and to some extent validated the feasibility of the vision of 'cutting degree' as a term for expression of vibration cutting feature.

Photoelastic Analysis of Partially Occluded Objects With an Integral-Imaging Polariscope

Polariscopes are the basic instruments used for the analysis of the stress state of transparent materials. Polarized light passing through a 3D object carries the integrated effect of the stress field along the light path. Therefore, conventional polariscopes are not able to discern the stress state of objects involving multiple plates with mutual occlusions. In this paper we propose a novel experimental system for three-dimensional stress analysis based on the combination of a polariscope and Synthetic Aperture Integral Imaging technique. Experimental results show the system's ability to recover the information of the stress distribution of a set of plates located at different depths having mutual occlusions.