Initial Stress Values Research Articles

Fuzzy logic model for the prediction failure analysis of composite plates under various cure temperatures

In this study, stress values were examined in pin/bolt connected composite plates which were applied preload and non-preload moment. Composite plates were produced with different curing cycles and geometric dimensions. Failure analysis of composite plates was carried out by applying different preload moments. The results obtained from experimental study were processed using fuzzy model. The results of experiments and fuzzy model were presented as a comparative. As a result of the comparison, it can be said that the initial stress values of failure were approximately determined using fuzzy model.

Design optimization of a laser printer cleaning blade for minimizing permanent set

When a cleaning blade in a laser printer is excessively deformed, immoderate permanent set can result, leading to weaker nip pressure between the cleaning blade and OPC drum that worsens its cleaning performance and printing quality. In this study, the correlation of the permanent set with stress and strain was investigated through tensile tests with rubber test specimens. Based on the experimental results, the maximum von-Mises stress value was used to quantify the permanent set. A design optimization problem was formulated to minimize the maximum von-Mises stress while satisfying the design constraints for maintaining appropriate contact between the cleaning blade and the OPC drum. We employed metamodel-based design optimization using design of experiments, metamodeling and an optimization algorithm to circumvent the difficulty of structural analyses at some design points. Using the proposed design approach, the optimal maximum von-Mises stress was reduced by 40.2 % compared to the initial stress value while all the design constraints were satisfied. In order to verify the validity of our design optimization result, we manufactured the cleaning blades according to the optimum design solution and performed permanent set and printer tests. The test results clearly showed the validity of our design optimization result.

Earing Prediction in Drawing and Ironing Processes Using an Advanced Yield Criterion

This work aims to contribute to the understanding of the role/influence of advanced yield criteria on the earing profile prediction after drawing and ironing, for a cylindrical cup benchmark proposed at the NUMISHEET 2011 conference [1]. Two typical materials used for can-making were considered and studied: an AA5042 aluminum alloy and an AKDQ steel. The drawing and ironing operations are performed on a special die which allows drawing and ironing in one single punch stroke in order to simplify the real process. The benchmark results report include, for each material: (i) the earing profile after drawing and ironing, presenting the cup height evolution with the angle from the rolling direction, and (ii) the evolution of punch force with punch stroke. This work presents a comparison between experimental and numerical results obtained for the aforesaid benchmark with DD3IMP in-house solver, using two sets of parameters for the Cazacu and Barlat 2001 [2] yield criterion, identified based on uniaxial tensile, equi-biaxial tension and disc compression test results. The first set uses the initial yield stress values while the second one used the flow stress values for an accumulated plastic work of 20 MPa. The results highlight the different impact of the experimental data in the earing prediction for both materials: the results for the second set are slightly improved for the AKDQ steel while for AA5042 the effect is negligible. The improved earing prediction obtained with the second set for the AKDQ steel seems to result from a better description of the stress states that occur in the flange zone.

The reflection phenomena of quasi-vertical transverse waves in piezoelectric medium under initial stresses

The propagation of plane vertical transverse waves at an interface of a semi-infinite piezoelectric elastic medium under the influence of the initial stresses is discussed. The free surface of the piezoelectric elastic medium is considered to be adjacent to vacuum. We assumed that the piezoelectric material is anisotropic of the type of a transversely isotropic crystals (hexagonal crystal structure, class 6 mm). For an incident of vertical transverse plane wave, four types (two for the displacement and two for the electric potential) of reflected plane waves, called quasi-longitudinal (qP) and quasi-shear vertical (qSV) waves are shown to be exist. The relations governing the reflection coefficients of these reflected waves for various boundary conditions (mixed-free-fixed) are derived. It has been shown analytically that reflected coefficients of (qP) and (qSV) waves depend upon the angle of incidence, the parameters of electric potential, the material constants of the medium as well as the initial stresses presented in the medium. The numerical computations of reflection coefficients for different values of initial stresses have been carried out by computer for aluminum nitride (AlN) as an example and the results are given in the form of graphs. Finally, particular cases are considered in the absence of the initial stresses and the electric potential. Some of earlier studies have been compared to the special cases and shown good agreement with them.

Thermo-dependent characteristics of polyimide–graphene composites

Polyimide–graphene composites (PIG) were prepared with variable amounts of graphene, and their thermal properties were analyzed in films on substrates or sheet states. The thermal conductivities of PIG composite sheets gradually moved upwards with increase of graphene loading. Coefficient of thermal expansion of composite sheet was higher in out-of-plane mode than in-plane mode. The residual stress of a composite film was monotonously changed in accordance with the variation of temperature and lowered with increase of graphene. In addition, the residual stress of a composite film reached to the initial stress value during cooling process after heating. The stress profiles on further heating and cooling runs closely followed the stress profile during the first cooing run.

Stability and Relaxation of Welding Residual Stresses

Residual stress relaxation of butt-welded small scale steel specimens under static and cyclic mechanical loading was investigated. The experiments were carried out on different types of steel with yield strengths between 300 and 1200MPa. The x-ray and neutron diffraction techniques were applied for the residual stresses analysis. The maximum values of initial residual stresses were measured at the weld bead centerline and were not as frequently assumed as high as the yield strength of the material. From fatigue point of view the internal stresses at the weld toe are of importance. It was observed that only a fraction of maximum residual stress accommodated in the weld bead centerline, is available at the weld toe. Under static tensile and compressive loading by increasing the load level the residual stress field relaxes continuously. The relaxation sets in with delay under compression since the tensile residual stresses should be first overcome. Under cyclic loading, once the first relaxation takes place further cyclic relaxation is either not considerable or continues moderately depending on loading conditions. In high cycle fatigue the residual stresses are stable until 2x106 cycles. In low cycle fatigue however the variation of the residual stresses continues until failure. This variation is partly related to crack initiation and propagation. The von Misses failure criterion with the local yield strength as material resistance against plastic deformation was able to describe the relaxation of surface welding residual stresses in low cycle fatigue.

Comparing Three Equations Used for Modeling the Tensile Flow Behavior of Compacted Graphite Cast Irons at Elevated Temperatures

A comparison between three constituent relationships used to approximate the plastic part of a tensile test curve was performed on compacted graphite cast iron (CGI) samples at temperatures between room temperature and 873 K (600 °C). The investigated relationships were the Hollomon, Ludwigson, and Voce equations. The investigated CGI materials were alloyed with four different amounts of molybdenum, and each chemical composition was cast with three different solidification rates. The two coefficients in the Hollomon equation \( \sigma_{\text{H}} = K_{\text{H}} \times \varepsilon^{{n_{\text{H}} }} \) showed a temperature dependence, where the strength coefficient KH was temperature stable for temperatures up to 573 K (300 °C) and the strain-hardening exponent nH showed a maximum value at about 473 K (200 °C). Both coefficients were affected by an altered metal matrix and by increased nodularity, and they showed a slight increased value with reduced pearlite interlamellar spacing. Ludwigson added an exponential term \( e^{{K_{\text{L}} + n_{\text{L}} \times \varepsilon }} , \) including two new coefficients to the Hollomon equation, to adjust and improve the approximation. The main purpose of KL was to adjust the stress value at zero plastic strain and was affected little by the metal matrix constituents and microstructure features. The value of nL was greatly dependent on the total plastic strain in which small plastic strains resulted in larger nL values, whereas large plastic strains resulted in smaller values. The deformation behavior was similar for all samples; hence, the total plastic strain also had a large influence on whether the adjustment term was positive or negative as a consequence of how nH was chosen. Compared with the Hollomon and Ludwigson equations, the Voce equation \( \sigma_{\text{V}} = \sigma_{S} - \left( {\sigma_{S} - \sigma_{1} } \right)e^{{n_{\text{V}} \times \varepsilon }} \) included coefficients representing an initial stress value σ1 and a saturation stress value σS. The initial stress values and the saturation stress values showed great linear correlations with yield strength values at 0.2 pct elongation and ultimate tensile strength, respectively. The values of both these coefficients were reduced with increasing temperature but had a plateau or even a slight increase between about 473 K and 573 K (200 °C and 300 °C). nV was reduced constantly with increasing temperature and was affected by the total plastic strain values in the same way as nL. The overall best approximation of the stress values was made by the Ludwigson equation followed by the Hollomon equation and last by the Voce equation. The downside with the Ludwigson equation was that the correction term either could be positive or negative, which made it harder to use as a general equation to approximate stress values, compared with the Hollomon and Voce equations.

Controlling stress and stress gradient during the release process in gold suspended micro-structures

In this paper it will be demonstrated that the conditions of the final release process in oxygen plasma strongly influence the shape of suspended gold micro-structures. Both single-clamped and double-clamped cantilever test devices have been examined. The structures have typically a thickness of 1.8 μm, and are produced by patterned gold electrodeposition above a sacrificial photoresist layer which is then removed by oxygen plasma ashing. In general, deformations are strongly reduced lowering the temperature, but the release time increases abruptly. A simple model is presented to explain the experimental observation, and from it can be deduced that yield and inelastic relaxation are critical phenomena responsible for most of the deviations from the ideal planar shape of MEMS structures. To overcome these unwanted effects, the parameters to be optimized are mainly the initial stress value, the release temperature and the vertical structural homogeneity of the gold layer.

VIBRATION OF INITIALLY STRESSED MICRO- AND NANO-BEAMS

This paper is concerned with the vibration problem of initially stressed micro/nano-beams. The vibration problem is formulated on the basis of Eringen's nonlocal elasticity theory and the Timoshenko beam theory. The small scale effect is taken into consideration in the former theory while the effects of initial stress, transverse shear deformation and rotary inertia are accounted for in the latter theory. The governing equations and the boundary conditions are derived using the principle of virtual work. These equations are solved analytically for the vibration frequencies of micro/nano-beams with different initial stress values and boundary conditions. The effect of the initial stress on the fundamental frequency and vibration mode shape of the beam is investigated. The solutions obtained provide a better representation of the vibration behavior of initially stressed micro/nano-beams which are stubby and short, since the effects of small scale, transverse shear deformation and rotary inertia are significant and cannot be neglected.

Impact of Gelation Conditions and Structural Breakdown on the Physical and Sensory Properties of Stirred Yogurts

The objectives of this research were to understand structure-function relationships between the initial yogurt gels and stirred yogurts made from these gels. Yogurt gels were made from milk preheated at 75 or 85°C for 30min and incubated at 32, 38, or 44°C. Yogurt gels were sheared at 5s−1 for 1min to make stirred yogurts. Small amplitude oscillatory rheology and shear rate sweeps (flow curves) were performed to determine the dynamic moduli and apparent viscosity of yogurts, respectively. Confocal scanning laser microscopy was used to examine the microstructure. The sensory properties of stirred yogurts were determined using quantitative descriptive analysis. Yogurt gels made from high preheating temperatures and low incubation temperatures tended to have an increase in storage modulus, apparent viscosity values at structural breakdown, yield stress, and a decrease in the maximum in loss tangent values and permeability. During the shearing of intact gels, there was an initial increase in apparent viscosity in the low shear rate region, because of the resistance derived from the intact network; commonly used models for stirred yogurts could not adequately describe this type of viscoelastic behavior. Increasing milk preheating temperature and decreasing incubation temperature resulted in stirred yogurts with increased apparent viscosity, oral viscosity, and sensory mouth coating attributes. Storage modulus values of the initial gels were positively correlated with the apparent viscosity at 10s−1 (r = 0.61), oral viscosity (r = 0.77), and mouth coating sensory attributes (r = 0.72) of stirred yogurts. A significant negative correlation was observed between maximum in loss tangent values of initial gels and shear stress values (r = −0.68) of gels at structure breakdown, and in stirred yogurts, the oral viscosity (r = −0.85), and apparent viscosity at 10s−1 (r = −0.78). Permeability was negatively correlated with the oral viscosity (r = −0.84) and apparent viscosity at 10s−1 (r = −0.82) in stirred yogurts. These highly significant correlations demonstrated that the structure of the initial gel network and structural breakdown process had a major impact on the physical and sensory attributes of stirred yogurts. The presence of a yield stress and the elastic nature of gels at low strains resulted in commonly used models that could not adequately predict the behavior in the very low shear rate region, although they were able to predict the remainder of the flow profile.

High temperature mechanical behavior of Zr–2.5% Nb alloy

Zr–2.5% Nb alloy is widely used as structural material for pressure tubes of channel nuclear power reactors cooled by heavy pressured water (CANDU-PHW) and boiler water (RBMK, Fugen). Tensile and creep behavior of Zr–2.5% Nb alloy significantly depend on heat treatment. Zr–2.5% Nb alloy is used after different heat treatments to satisfy design requirements to reactor pressure tubes: α-annealing (reactor RBMK-1000), water or gas quenching and aging (reactor RBMK-1500 and Fugen), cold-work and stress-relief (reactor CANDU). An experimental program has been elaborated to research resistance to plastic flow and rupture of Zr–2.5% Nb alloy at the temperatures 20–1200 °C. Tensile tests were carried out at a constant cross-head speed in the range 10 −2 to 10 −4 s −1 and creep tests under uniaxial load. Temperature dependence of flow and rupture stress were distinguish in different temperature ranges. Three ranges of temperature were observed where the rate of the strength reduction was significantly different. Extremely large elongation was observed in the 600–800 °C range that is manifestation of superplasticity. The temperature corresponding to maximum elongation was equal ∼700 °C. Increase of strain rate from ∼10 −4 to 10 −2 s −1 did not changed the temperature ranges of superplasticity observation but resulted in decrease of elongation values. Creep tests were carried out in the temperature range of 500–1000 °C. It was observed that creep curves at the temperatures above 600 °C are not exhibited the hardening and creep rate is increased up to failure. The total elongation is diminished from increase of initial stress value. Results of creep tests (creep rates and time to failure) have been described by power theory of creep the equations.

Thermal stress relaxation of plasma enhanced chemical vapour deposition silicon nitride

AbstractMechanical and thermal properties of silicon nitride films deposited by different plasma process type have been studied. The initial mechanical stress, thickness and hydrogen content have been evaluated respectively by wafer curvature measurements, ellipsometry and Fourier Transform Infra Red spectrometry. These nitrides presented as-deposited stress values ranging from compressive to tensile. High temperature Rapid Thermal Anneal (RTA) at 1100°C or longer thermal treatments at medium temperature, from 700°C to 850°C were carried out on these materials. The evolution of their properties along the different anneals have been measured and compared to the behaviour of high temperature thermal nitride. One can observe that these stoechiometric plasma nitrides have shifted to an equilibrium tensile stress around 1100-1200 Mpa when submitted to the RTA, independently of their initial stress values. Results are interpreted in terms of H desorption and Si-N bond formation. Chemical reaction Si2-N-H + 2 N-H → 2 Si-N + NH3 appears to be the best candidate to figure out the phenomena.

Plasticity Modeling of the Effect of Sample Preparation Method on Sand Response

ABSTRACT Experimental evidence shows that different preparation methods produce sand samples with distinctly different stress-strain response. This paper explores and draws conclusions from the measured differences on the monotonic undrained triaxial response of Toyoura sand, prepared by different methods at the same values of void ratio and initial effective stresses. This is achieved by comparing data and simulations performed with a recently developed plasticity constitutive model, which accounts for the effect of inherent fabric anisotropy on the mechanical response. The inherent fabric anisotropy is represented by a second order symmetric fabric tensor, and its effect on the response at different loading directions is expressed by an appropriate dependence of certain constitutive ingredients on a joint isotropic invariant of the loading direction and the fabric tensor. Use of this constitutive scheme to simulate the aforementioned data on Toyoura sand exploits the fact that the preparation method affects both the dilatancy and the hardening response in a systematic manner. Under the premise that these effects are due to the different inherent fabric created by the preparation method, it follows that the foregoing simulations do not require changes in constitutive equations or entirely different sets of model constants. On the contrary, only model constants related to this inherent fabric anisotropy scheme need readjustment, providing insight to how the sample preparation method affects the response and what can be done to model it.

Dynamic and static softening behaviors of aluminum alloys during multistage hot deformation

The dynamic and static softening behaviors during multistage hot deformation of aluminum alloys (1050, 5182 and 7075 alloys) have been studied by isothermal interrupted hot compression tests in 300 and 400 °C at a strain rate of 0.5 s −1 on Gleeble 1500. The interrupted deformations were conducted with delay times varying between 30 and 120 s after achieving a strain of approximately 0.4 in the first stage. It was indicated that the considerable dynamic softening and exceptional softening associated with a structure softening exist in both 1050 and 7075 alloy deformation at 400 °C, leading to an initial flow stress value at the second deformation lower than that at the first deformation. For all alloys studied, the static softening increases with increasing deformation temperature and holding temperature and delay times, but the increment for 5182 alloy is much higher than that for both 1050 and 7075 alloys in the same hot deformation conditions.

Effect of initial stress on the propagation behavior of SH-waves in multilayered piezoelectric composite structures

The propagation behavior of horizontally polarized shear waves (SH-waves) in a periodic multilayered piezoelectric composite structure with initial stress is taken into account. The multilayered piezoelectric composite structure consists of piezoelectric layer bonded perfectly with polymer layer alternately. Solutions of the mechanical displacement and electrical potential function are obtained by solving the coupled electromechanical field equations for the piezoelectric layer and the polymeric layer, respectively. The phase velocity equation of the SH-waves in the periodic multilayered piezoelectric structure is obtained for the case of SH-waves propagation along the direction normal to the layering. Influence of initial stress on the stop band effect and the dispersion relation of the SH-waves are discussed in detail, respectively. One set of piezoelectric layer-polymeric layer combination system is considered, i.e. PZP-5H ceramic layer–polythene layer. It can be seen from our research results that the stop band effect of the SH-waves propagation is dramatically changed with the increase of the magnitude of the absolute value of initial stress. The dispersion relation changes remarkably also, as the magnitude of absolute value of the initial stress is greater than 100 MPa. The results reported in our research work can be used as the benchmark for the investigation of the wave propagation in the piezoelectric coupled structures and are theoretically significant in the design of wave propagation in the piezoelectric coupled structures as well.

Impact of shipbuilding steel plates with an initial tensile stress

The present investigation is concerned with the impact of thin steel plates used in shipbuilding. The problem is studied numerically with the commercial code LS-DYNA and experimentally with an apparatus propelling a 200g projectile up to 30m/s with a crossbow. The clamped sample of sheet metal is rectangular (250 mm wide; 20 mm width and 4 mm thick), in common shipbuilding material with a 360MPa elastic limit. Impact takes place at the center of the sample. Moreover, an initial tensile stress can be applied to the plate, up to 300 MPa. The aim of this study is first to illustrate the influences of the boundary conditions and the initial tensile stress value on the deflection of the central point of the plate. The second part is concerned with wave propagation. The objective is to explain analytically the dispersive form of the measured strains at different locations and to establish the relationship between the flexural waves and the observed losses of contact.

Influence of Silicate- and Magnesium-Specific Adsorption and Particle Shape on the Rheological Behavior of Mixed Serpentine-Goethite Suspensions

AbstractThe influence of dissolved species and particle morphology on the electrokinetic behavior and the initial yield stress values of Cuban lateritic aqueous suspensions was studied. The lateritic samples were mixtures of serpentine and goethite in different relative proportions. The addition of silicate and Mg ionic species, which are normally found in natural waters used in industrial processes, affected the electrokinetic and flow behavior of the lateritic suspensions. Specific adsorption of these species on particle surfaces was shown by a shift of the isoelectric point and the maximum of the initial yield stress to more acidic pH (Si ionic species adsorption) and more basic pH (Mg ionic species adsorption), when compared to suspensions containing only non-adsorbing electrolytes. The initial yield-stress values determined in samples consisting entirely of goethite varied from sample to sample. A detailed crystallochemical characterization revealed that these changes were associated with the axial ratio (i.e. ratio of particle length to width) of the mineral particles. Goethite samples with larger particle size (smaller number of particles for a given solid concentration) and greater axial ratios presented initial yield-stress values greater than those goethites with smaller particle size and lower axial ratio.

Stress relaxation effect in porous 3C-SiC/Si heterostructure by micro-Raman spectroscopy

Cubic (3C) SiC layers with initial biaxial stress value of about 0.4 GPa are observed by micro-Raman scattering measurements to be completely relaxed after formation of a porous network in the 3C-SiC/Si heterostructure. The porous heterostructure is obtained by an anodization procedure in HF acid solutions usually used for porous Si fabrication. The influence of some anodization parameters such as the anodization current density, HF concentration, and anodization depth on the stress relaxation effect is described. The relaxation is found to be mainly due to pore formation at the 3C-SiC/Si interface.

Analytical solutions for stresses and displacements around tunnels driven in cross-anisotropic rocks

In this paper, closed-form solutions for the stresses and displacements around unlined circular tunnels excavated in an elastic, orthotropic (cross-anisotropic) medium are developed. The effect of both the elastic parameters characterizing the behaviour of the medium and the anisotropy of the initial stress system on the stresses and displacements induced are evaluated. An example of utilizing the theoretical solution for design analysis is given. For convenience of application, design charts are prepared for the determination of stresses and displacements for given values of initial stresses and the elastic parameters. Copyright © 1999 John Wiley & Sons, Ltd.

Stress relaxation behaviour in bending of high strength copper alloys in the Cu–Ni–Sn system

The microstructure of spinodally hardened, stress relaxation tested Cu–Ni–Sn alloys is characterized. The main mechanism of relaxation is recrystallization. At 150°C, initial stress values of 480–600 N mm −2 cause significant microstructural changes even if up to 90% of the initial stress remains after 1000 h of testing. Elevated temperature stability depends on the uniformity of deformation.