Values Of Normal Stress Research Articles

Thermal Shock Test of Ceramics by Helium Gas Cooling through a Narrow Slit

In order to evaluate the reliability of ceramics under transient thermal stress, thermal shock test equipment was designed and fabricated. Cylindrical specimens made of Si3N4 and SiC were tested by the test equipment. Each specimen was preheated uniformly at 1573K in an electric furnace and then pulled down rapidly into a cooling chamber beneath the furnace. At the same time, it was cooled locally by high-velocity helium gas that was passed through a narrow slit. The transient temperature distributions were measured by optical pyrometers and thermocouples. The moment of fracture was confirmed by the sound of the fracture itself. Fracture originated at the outer surface immediately under the cooling nozzle. Consequently, the average value of nondimensional fracture stress of Si3N4 specimens was about two times that of SiC specimens.

On solving the plastoelastic problen in space of the commutative matrixes

A method for calculating mechanical parameters of multilayer structural elements (MSE) and their layers in elastic and plastoelastic zones are presented and grounded in the case of axial stret­ching. The method of diagonal matrices is proposed to define the parameters of MSE's and their layers. In the elastic zone, MCE's are completely defined by two matrices: that of the modulus of elasticity and layer cross-section areas. In the plastoelastic zone, by the matrix of the layer cross­section areas and a diagonal matrix-function that defines σ - ε diagrams of the layers. In the case of stretching, the above mentioned matrices make up a commutative group with respect to product operation which makes it possible to obtain compact expressions for the required parameters that do not depend on the number of layers and are analogous to scalar ones (a single-layer case). This kind of calculation methods enables us to compute the values of axial stiffness and normal stress as well as the quantity of limiting axial load or the zones areas of elastic and plastoelastic deformation, when the diagrams σ - ε of deformation materials composing it correspond to that diagram that of plastoelastic and elastically-strengthening materials.

Internal field sources, their screening and the flow stress

The experimental investigation of the internal elastic stress fields in polycrystals of deformed Cu–Al and Cu–Mn alloys was carried out. The basic sources of the internal stress field were identified and the decrease of the field with increasing distance from the sources was determined. The average values of the internal stresses as a function of strain were measured. On this basis, the separation of sequences of dislocation substructure transformations into low-energy and the high-energy configurations was carried out.

A numerical study of fluid flow and solute transport in a variable-aperture fracture using geostatistical method

This paper presents the numerical studies for the fluid flow and solute transport in a variable-aperture fracture, generated by using the geostatistical method (turning band method). In order to represent the dependence between the effective normal stress and the geometry of aperture distribution, we combined a simple mechanical model with the flow model. The solute transport was simulated using the particle following algorithm. As the results obtained from these numerical simulations, the fracture permeability depended strongly on the fracture roughness and zones in contact, which are varied with the effective normal stress. In comparison between the spatial correlation lengths λ/L=0.2 and 0.4, the fracture with λ/L=0.4 was more permeable than that with λ/L=0.2. The particle displacement depended also on the variation of the fracture roughness and zones in contact according to the effective normal stress. The most efficient path for flow was varied with the effective normal stress. However, from a certain value of the effective normal stress, for example 11.95 MPa in this study, this path maintains almost the same form. Moreover the particles displaced only along reduced channels (i.e. one or two channels) and the spatial dispersion of particles becomes thus constant.

Modeling of the thermomechanical behavior of porous shape memory alloys

Abstract Two methods are used in this work to estimate the porous shape memory alloy (SMA) thermomechanical behavior. The porous SMA is assumed to be made of two components, the dense SMA matrix and the pores. An existing rate-independent type constitutive model is employed to describe the matrix behavior. Two contrasting strategies are used to estimate the overall thermomechanical behavior: (1) the unit cell finite element method (UCFEM) to account for periodic distribution of pores in the SMA matrix, and (2) an averaging micromechanics method based on the incremental formulation of the Mori–Tanaka method to account for random distribution of pores in the matrix. Cylindrical and spherical shapes are considered as approximations of open and closed pores, respectively, in both methods. Results are presented for both types of pores and comparisons are made between the two methods under various loading conditions. Both methods compare well in predicting the isothermal elastic material properties and pseudoelastic response under axial and out-of-plane shear loading. However, the transformation results differ under transverse and in-plane shear loading. This difference is found to be due to the use of an average value of stress for the SMA matrix in the micromechanics averaging method, which diminishes the effect of local stress concentration thereby delaying the onset of phase transformation caused by an applied load. On the other hand, the actual values of stress at all material points in the SMA matrix are used in the UCFEM causing phase transformation in regions near the pores at smaller applied load values than what is calculated by the micromechanics averaging method.

Material database optimization using Monte Carlo method in engineering application

Ceramic materials were simulated numerically. The practical goal of the research was to assess numerically the average residual stress value in such materials. The scientific goal of the work was to establish a basic analytical apparatus. One of the main aspects was to create material database, enabling simulation of polycrystalline structure with random orientations of thermal expansion orthotropy axes. Monte Carlo method was adopted for this purpose. The calculation results were analyzed with the use of statistical method, which is analogous to experimental measure analysis. However, the developed methods can be readily used to analyze both ceramics and ceramic composites. Currently, the methods can take advantage of more precise geometrical description to achieve more accurate models reflecting real internal structure of the material.

Rheology of PVC Plastisol: IV. Force Balance among Osmotic Pressure, Normal Stress, and Dilatational Stress

The normal stress values have been calculated for the pseudo-plastic region and the dilatant region of the steady-state flow of PVC plastisols. The viscosity–shear rate data had been taken from the literature and analyzed previously to quantitatively describe the presence of an immobilized layer. In the present work the normal stress in the pseudo-plastic region was evaluated as a force acting against the expansion of the immobilized layer caused by the osmotic pressure. The osmotic pressure is that which arises from the concentration difference between the mobile phase and the immobilized layer. The calculated normal stress increases with increasing shear rate. The trend is similar to that observed with other systems such as polymer solutions and polymer melts. In the dilatant region the dilatational stress acting on the immobilized layer is present in addition to the above osmotic pressure; the normal stress is counteracting the dilatational stress and the osmotic pressure. Therefore, the calculated sum of the latter two forces gives the magnitude of the normal stress. The normal stress values have been found to be constant, independent of shear rate.

Soil behavior during freezing and thawing using variable and constant confining pressure triaxial tests

Although variable confining pressure (VCP) triaxial tests are generally preferred to constant confining pressure (CCP) triaxial tests due to the more realistic stress application, VCP tests have never been utilized when investigating freeze–thaw effects on unbound road materials. In this study, three soils were investigated for resilient behavior during freezing and thawing utilizing both VCP and CCP triaxial testing. The soils were tested at selected temperatures between +20 and –10°C during one full freeze–thaw cycle. The results were analyzed in terms of the traditionally used resilient modulus and Poisson's ratio, as well as volumetric and shear components, and indicate a significant difference in moduli computed from CCP and VCP data. However, resilient moduli display compatible values when interpreted in terms of mean values of deviator stress and mean normal stress. With regard to freezevthaw effects on resilient moduli, the results are inconsistent with previous findings. However, this can be explained by the different test conditions applied.Key words: freeze–thaw, triaxial tests, unbound pavement materials, subgrade soils, resilient modulus.

An application of the J‐Q model for estimating cleavage stress in the brittle‐to‐ductile transition

A recent model was proposed by the authors to predict cleavage failure for steels based on a weak link mechanism and a crack tip stress field modified for planar constraint by the J–Q theory. The model uses the distribution of toughness results at a single temperature to predict a toughness distribution at a different temperature and/or geometry. In this model a material cleavage stress is needed to predict when the weak link fracture is triggered. This cleavage stress is a key input for the application of the model but it is not a property that is routinely measured and it is hence not available for most steel alloys. In this paper, a method to estimate the average value of the cleavage stress is presented, based on a characteristic of the model to predict cleavage failure. Examples of cleavage stress are given for several steels and these results are used to predict the toughness distributions for structural component models.

THE SCATTER OF SURFACE RESIDUAL STRESSES PRODUCED BY FACE-TURNING AND GRINDING

Conventional studies on residual stresses induced by manufacturing processes have focused on the average residual stress value over the processed surface area. However, what dictates the fatigue life of a manufactured surface is its weakest point. Thus, it is not the average value of the stress but the local extreme that is most relevant for safety considerations. Therefore, it is very important to study the variations of residual stresses over the machined surface. This paper is the continuation of the work [1] investigating the magnitude of surface residual stress scatter between the face-turned and ground samples. The objective of this research is to test the hypotheses that the scatter of surface residual stresses over the faced samples is smaller than that of the ground ones and that the scatter of surface residual stresses varies significantly among ground samples while it does not vary in a statistically significant sense among faced specimens for the given cutting conditions. In order to compare the surface residual stress variations, two sets of the specimens of Ti 6Al-4V bar are ground while the other two sets faced. The residual stresses over a small surface area (5mm × 8 mm) are measured at four locations of each machined sample using an X-ray diffraction technique. Statistical analysis of the measured residual stresses shows that the proposed hypotheses hold. Experimental data also show that a different number of grinding passes may induce a different scatter of microhardness. The possible causes and ramifications of the foregoing results are discussed. It is suggested that the variations of residual stress be included as a surface integrity parameter, joining its average value.

New micromechanical experimental approach for the characterization of fibre–matrix adhesion in composite materials

The evaluation of interfacial adhesion between fibres and matrix in composite materials can be approached by the use of micromechanical tests such as the pull-out test. One of the principal limits of this experimental technique is that it makes possible to obtain only one average value of the interfacial shear stress and that an extrapolation is generally necessary to obtain the maximum value of this stress. In this paper, we propose a new technique of characterisation based on the use of DMA. The geometry of the samples remains the same as the one used for the pull-out, but it allows to considerably limit the number of samples necessary for the measurement of a parameter characteristic of adhesion. At the present time, the pull-out technique by DMA has many advantages compared to the conventional pull-out test: elastomeric matrices can be investigated with great reliability, and the composite samples can be classified according to their interfacial stress transfer ability.

Experimental Investigation of the Interaction Mechanism at the EPS Geofoam-Sand Interface by Direct Shear Testing

The results of laboratory direct shear tests conducted at the expanded polystyrene (EPS) geofoam-sand interface are presented in this paper. The sands used in the tests were composed of rounded to subangular particles with values of mean particle size ranging from 0.28 to 2.17 mm and void ratios ranging from 0.51 to 0.72. The EPS geofoam was of two different densities: 10 and 20 kg/m3. The test results are expressed in the form of shear stress versus normal stress in direct shear failure envelopes, which were found to be nonlinear. The curved interface failure envelopes were approximated by piecewise linear envelopes composed of three linear segments corresponding to three types of phenomenological interaction mechanism, which develop successively for increasing values of normal interface stress: purely frictional, frictional-adhesional, and purely adhesional. A simple conceptual framework is proposed that qualitatively explains the observed interface behavior in terms of the normal interface stress, EPS geofoam hardness (or density), interface relative roughness (which incorporates the mean particle size), shape of sand particles, and void ratio of sand. The experimental results are compared to similar results reported in the literature for HDPE geomembrane-sand interfaces. Apparent values of interface friction angle, δ, and adhesion, ca, are proposed in the paper for the tested sands and for specific ranges of normal interface stresses and the two EPS geofoam densities.

Micromechanical modelling of the static and cyclic loading of an Al 2124-SiC MMC

Abstract The objective of this study was to use micromechanical finite element models to simulate both the static and cyclic mechanical behaviour of a metal matrix composite: a forged Al 2124 alloy reinforced with 17% SiC particles, at two different temperatures: room temperature and 150°C. In the simulations, periodic unit cell models incorporating the explicit representation of the matrix and the reinforcing particles in both 2D and 3D, were used. Micromechanical models with both idealised and realistic reinforcing particle shapes and distributions were generated. The realistic particle shapes and distributions were inferred from experimental SEM micrographs. The pattern and intensity of the plastic deformation within the matrix was studied and the macroscale behaviour of the composite was inferred from average stress and strain values. In order to include the effects of residual stresses due to the processing of the material, a quenching simulation was performed, prior to the mechanical loading, and its effects on the macroscopic tensile behaviour of the MMC was assessed. The effects of removing the periodicity constraint on the models by using a cell embedding technique was investigated. In order to try and model the deformation behaviour of the matrix more accurately, crystal plasticity models, which included the explicit representation of individual grains were examined for different matrix grain morphologies. The results of the simulations were compared with experimental results for the MMC in terms of macroscopic tensile stress–strain curves. Finally, the effects of different matrix strain hardening models were examined in order to investigate the cyclic behaviour of the MMC.

Modeling of composite/concrete interface of RC beams strengthened with composite laminates

In this paper a finite element model for predicting shear and normal stresses in the adhesive layer of plated reinforced concrete beams has been developed. The numerical results carried out agree with those obtained in previous studies by other authors. It is found that shear stresses and high concentrations of peeling forces are present at the ends of the plates when the composite beam is loaded in flexure. These concentrations can produce premature failure of the strengthened beam because of debonding of the plate or cracking of the concrete cover along the level of internal steel reinforcement. The numerical simulation captures the actual interfacial stresses and, in particular, the maximum values of shear and normal stresses.

Shear test results for cohesion and friction coefficients for different granular materials: scaling implications for their usage in analogue modelling

Laboratory tests have been carried out on dry granular materials such as quartz sand, glass microspheres and sugar with different grain size, rounding and sphericity. The measurements have been made with a simple shear test machine for different values of normal stress (∼50–900 Pa). Shear stress has been plotted against normal stress in order to determine the cohesion and coefficient of internal friction for the investigated materials. Resulting values of cohesion and coefficient of internal friction are mainly dependent on rounding and sphericity, while grain size has a less significant influence. Further, the behaviour of the materials for very small normal stresses (∼0–400 Pa) is more complex than previously assumed. The fracture envelopes for all materials investigated are convex-outward for this small range and converge towards a straight failure envelope with increasing normal stress. Finally, in extensional faulting experiments, there is no significant change in fault dip with increasing depth. Therefore, the non-linear behaviour for small normal stresses is best described as a dependence of the cohesion on the normal stress and not as a dependence of the coefficient of internal friction on the normal stress. Values for cohesion increase from ∼0 Pa (±15 Pa) at zero normal stress to 137–247 Pa (±15 Pa) for normal stresses greater than ∼250–400 Pa. The results show that well-rounded, spherical material is better suited to model brittle behaviour of rocks in crustal and lithospheric scale analogue models than less well-rounded material, since it has a smaller cohesion and a coefficient of internal friction, which is closer to values of natural rocks.

Indentation failure behavior of honeycomb sandwich panels

The failure behavior of simply supported honeycomb sandwich panels subjected to indentation loading on the top skin has been studied. A plate-on-elastic-foundation approach based on distributed load and panel-specific considerations (finite thickness and transverse shear) is developed to analyze the panel stress distribution. Panels with various glass-fibre/epoxy skins and aramid cores are analyzed by using this approach together with the finite-element analysis. Surprisingly good agreement is found between the theoretical and FEA solutions, suggesting that the model satisfactorily describes the panel indentation behavior. Indentation-induced stresses in the core relevant to the onset of indentation failure are found to depend on skin bending stiffness, core stiffness and indenter size. The calculated maximum shear and normal stress values are compared with the experimental data to assess the core failure mechanisms involved. The comparisons clearly show that core shear failure dominates the onset of indentation failure.

ANALYSIS OF METHODS FOR DETERMINING SOIL SHEAR STRENGTH DESIGN VALUES/GRUNTO STIPRUMO RODIKLIŲ SKAIČIUOJAMŲJŲ REIKŠMIŲ NUSTATYMO METODŲ ANALIZĖ

Designing foundations by soil safety factors is used for obtaining soil strength design values. The methods [10, 11, 12, 13] used for their calculations and minimal values of investigated coefficients are provided in the standards. The design values of soil characteristics computed by different methods do not match. In reality they should ensure the unique reliability of soil shear strength design values. In the present work, the soil shear strength values are computed by different methods. The experimental data are obtained for sands using direct shear strength apparatus. The comparative analysis of obtained results and methods for predicting the design values of soil strength characteristics is applied. When designing foundations, the relation between soil shear strength and normal stress is treated as linear (1, 2), but the dependency of the lower control limit on normal stress is nonlinear (5). Therefore, methods for getting experimental data of soil shear strength give different results. In corresponding standards, different additional conditions are considered for approximation of the investigated non-linear dependency to the linear one. The soil safety factors used for computing design values of tangent of internal friction angle and cohesion depend on these additional conditions. The approximation of non-linear dependency to the linear one does not allow ensuring the unique probability of soil shear strength design values. It is impossible to ensure the desired unique probability for different values of normal stress using traditional methods and constant soil safety factors. In order to obtain the same probability for soil shear strength values lower than the designed value, the probability-statistical methods should be applied. In this article, the method for determining soil shear strength design values is presented (10, 11). This method ensures the same probability of the soil shear strength design values for any values of normal stress. The probability-statistical methods should be applied for design of foundations when soil strength design values depend on normal stress. In order to apply the investigated methods, the probability of foundation collapse or reliability index β, the mean values of soil strength and their variance should be known. Dimensions of the foundation are obtained from mean value sand variance of soil strength characteristics solving the system of equations (13). The solution of this system is a combination of arguments of foundation design conditions that are investigated as normal random variable. The investigated combination corresponds to the limit state.

MOHR APPROACH TO PREDICTION OF BEAM SHEAR STRENGTH.

Keywords QUEENS UNIVERSITY BELFAST, NORTHERN IRELAND TECHNOLOGY CENTRE PILKINGTON GROUP ENGINEERING BEAMS, SHEAR, STRENGTH, PREDICTION, ULTIMATE, REINFORCED, CONCRETE, FAILURES, CRITERION, INTERACTION, RELATIONSHIPS, LIMITING, VALUE, STRESS, NORMAL, COMPRESSION, ZONES, T BEAMS, HIGH, HIGH STRENGTH, CONTINUOUS, PROPERTIES, LOADS, TESTS, RATIOS, SLENDERNESS, CODES, PRACTICE, BIAXIAL, ELASTIC, DISTRIBUTION, BEHAVIOUR, FLEXURE, RESISTANCE, MECHANISMS, BENDING, MOMENTS, INFLUENCE, STANDARDS, BRITISH... Show All

Cyclic Deformation Behavior and Dislocation Substructures of Hexagonal Zircaloy-4 Under Out-of-Phase Loading

Macroscopic response and microscopic dislocation structures of Zr-4 subjected to biaxial fatigue under different phase angles of 30°, 60°, 90° and different equivalent strain ranges of 0.8%, 0.6%, 0.4% were studied. The testing results show that the delay angle between the stress deviators and strain increment tensors is strongly dependent on phase angle and the equivalent strain range. When phase angle equals 60°, the delay angle has the minimum variation range for all specimens. The mean value of the delay angle decreases with increasing phase angle or the equivalent strain range. The variation range and average value of the Mises equivalent stress have the maximum in S3 with the phase angle of 90°. They decrease as the equivalent strain range decreases. Zr-4 displays a pronounced initial hardening followed by a continuous softening for all specimens during out-of-phase cycling. The stabilized saturation stresses of Zr-4 under out-of-phase cycling are much higher than that under uniaxial cycling. It indicates that Zr-4 displays an obvious additional hardening. As the phase angle increases, the typical dislocation structure changes from dislocation cells to tangles. The dislocation-dislocation interactions increase resulting in an additional hardening. In essence, the degree of additional hardening depends, among other factors, on the maximum shear stress ratio of resolved shear stresses and critical resolved shear stresses (RSS/CRSS). [S0094-4289(00)00601-0]

Stable Width and Depth of Gravel-Bed Rivers with Cohesive Banks

An analytical model is developed to determine the influence of the bank stability on the stable width and depth of alluvial gravel-bed rivers with cohesive banks. The formulation of the model is based on the assumption that the stable width corresponds with an optimum condition that is equivalent to the maximum bed load transporting capacity. The optimum condition develops when the channel banks are at their limiting stability with respect to either mass failure or fluvial erosion. Two basic channel types are identified: bank-height and bank-shear constrained. Mass failure stability is estimated using a simple total stress approach. A method for estimating the critical bank shear stress based on model calibration is proposed. Analysis of field data indicates that the effect of the bank vegetation on bank stability can be expressed in terms of the critical bank shear stress. The average critical bank shear stress value calculated for riverbanks covered by vegetation with well-developed root networks was fo...