Mean Value Of Stress Research Articles

Transition in Failure Mechanism Under Cyclic Creep in 316LN Austenitic Stainless Steel

Cyclic creep behavior of a type 316LN austenitic stainless steel was investigated in the temperature range from 823 K to 923 K (550 °C to 650 °C). A transition from fatigue-dominated to creep-dominated failure mode was observed with an increase in the mean stress. The threshold value of mean stress for the transition was seen to be a strong function of the test temperature. Occurrence of dynamic strain aging proved beneficial owing to a substantial reduction in the strain accumulation during cyclic loading.

Generalized versus Patient-Specific Inflow Boundary Conditions in Computational Fluid Dynamics Simulations of Cerebral Aneurysmal Hemodynamics

Attempts have been made to associate intracranial aneurysmal hemodynamics with aneurysm growth and rupture status. Hemodynamics in aneurysms is traditionally determined with computational fluid dynamics by using generalized inflow boundary conditions in a parent artery. Recently, patient-specific inflow boundary conditions are being implemented more frequently. Our purpose was to compare intracranial aneurysm hemodynamics based on generalized versus patient-specific inflow boundary conditions. For 36 patients, geometric models of aneurysms were determined by using 3D rotational angiography. 2D phase-contrast MR imaging velocity measurements of the parent artery were performed. Computational fluid dynamics simulations were performed twice: once by using patient-specific phase-contrast MR imaging velocity profiles and once by using generalized Womersley profiles as inflow boundary conditions. Resulting mean and maximum wall shear stress and oscillatory shear index values were analyzed, and hemodynamic characteristics were qualitatively compared. Quantitative analysis showed statistically significant differences for mean and maximum wall shear stress values between both inflow boundary conditions (P < .001). Qualitative assessment of hemodynamic characteristics showed differences in 21 cases: high wall shear stress location (n = 8), deflection location (n = 3), lobulation wall shear stress (n = 12), and/or vortex and inflow jet stability (n = 9). The latter showed more instability for the generalized inflow boundary conditions in 7 of 9 patients. Using generalized and patient-specific inflow boundary conditions for computational fluid dynamics results in different wall shear stress magnitudes and hemodynamic characteristics. Generalized inflow boundary conditions result in more vortices and inflow jet instabilities. This study emphasizes the necessity of patient-specific inflow boundary conditions for calculation of hemodynamics in cerebral aneurysms by using computational fluid dynamics techniques.

Low-cycle Thermal Fatigue and High-cycle Vibration Fatigue Life Estimation of a Diesel Engine Exhaust Manifold

This paper aims at estimating the low-cycle and high-cycle fatigue life of a turbocharged Diesel engine exhaust manifold. First, a decoupled thermo-structural Finite Element analysis has been performed to investigate low-cycle fatigue phenomena due to the thermal loadings applied to the exhaust manifold. High/low temperature cycles causes stress-strain hysteresis loops in the manifold material whose related dissipated energy can be directly correlated to low-cycle thermal fatigue. Afterwards, a dynamic harmonic analysis has been performed aiming at investigating the existence of high-cycle fatigue phenomena due to vibrational loading applied to the exhaust manifold during the duty cycle. Three direction acceleration experimental loadings have been applied to the model. An ad-hoc methodology has been developed to superimpose thermo-structural results to dynamic harmonic analysis results. In particular, quasi-static thermo-structural results have been employed to identify the mean stress values of vibration fatigue cycles, while alternate stress values have been derived from harmonic analysis. Different combinations of frequencies and phases of the acceleration input signals have been considered to create different high-cycle fatigue loadings. Each cyclic load case has been processed employing the multiaxial Dang Van fatigue criterion.

Development of the positive mean stress diagrams using genetic algorithm approach

ABSTRACTIn this numerical study, a new optimum positive mean stress fatigue failure equation is developed using previous experimental data and genetic algorithm. Two independent curve fitting coefficients are implemented in the equation to supply better correlations with experimental data of the failure strength envelope. In the literature, Gerber, Goodman, Soderberg, Morrow, Bagci, ASME (elliptic line), Clemson, Sekercioglu and so on suggested different equations for estimating fatigue strength envelope under mean stress condition. In these models, the effect of the materials was not considered in details. Some of these mean stress linear expressions are very conservative or have stress area bigger than the yield limit. The yield strength and the effect of materials are considered in the proposed model. The values of the positive mean stress, which correspond to fatigue failure, are obtained, and a minimum average absolute error among the models presented in the literature is remarked. The proposed model, which has less conservative structure, can be effectively used in the mechanical design process.

Determination of Gas Pipeline Soil Overburden Compaction Multiplier in CAESAR II

Backfill is needed after putting pipes in the trench and the stress changes with the change of overburden compaction multiplier. We use CAESAR II to do stress analysis and draw the reasonable overburden compaction multiplier on a gas pipeline. By changing the soil overburden compaction multiplier and comparing pipes mean stress value, when the overburden compaction multiplier is set to 3, operation, primary and secondary stress ratio would reach minimum. And with the increase of overburden compaction multiplier, the stress ratio increases slightly.

Comparative Study of Fatigue Damage Models Using Different Number of Classes Combined with the Rainflow Method

Fatigue damage increases with applied load cycles in a cumulative manner. Fatigue damage models play a key role in life prediction of components and structures subjected to random loading. The aim of this paper is the examination of the performance of the “Damaged Stress Model”, proposed and validated, against other fatigue models under random loading before and after reconstruction of the load histories. To achieve this objective, some linear and nonlinear models proposed for fatigue life estimation and a batch of specimens made of 6082T6 aluminum alloy is subjected to random loading. The damage was cumulated by Miner’s rule, Damaged Stress Model (DSM), Henry model and Unified Theory (UT) and random cycles were counted with a rain-flow algorithm. Experimental data on high-cycle fatigue by complex loading histories with different mean and amplitude stress values are analyzed for life calculation and model predictions are compared.

Ratcheting assessment of steel alloys under step-loading conditions

The present study examines the capability of a recently modified hardening rule to characterize ratcheting response of materials subjected to multi-step uniaxial stress cycles. The modified hardening rule was developed based on Armstrong–Frederick (A–F) hardening rule through implementing new ratcheting rate dependent coefficients γ2 and δ. These coefficients were estimated by means of calibrated curves for any given stress levels defined from the uniaxial single-step ratcheting response at various cyclic stress levels. At a constant mean stress, ratcheting strain progressively increased as stress amplitude over steps of loading history increased. Similar response was also evident for step-loading with constant stress amplitude while the values of mean stress increased. For high–low histories, the trend of predicted ratcheting strain from higher to lower magnitudes found agreeable with that of experimental data. The discrepancy of the predicted and experimentally ratcheting strain values in the high–low step loading however was due to constancy in the shape and size of translating yield surface in the modified kinematic hardening rule. The modified hardening rule was employed to assess ratcheting response of SS316L, SA333, SS316L(N) and 1070 steel alloys under various step-loading conditions. Predicted ratcheting data at various stress level were found in good agreements as compared with the experimental ratcheting strains.

Mean stress effect correction using constant stress ratio S–N curves

This paper presents a stress based approach to take into account the influence of the mean stress value on fatigue strength of constructional materials. Elaborated model uses two S–N curves, i.e. for alternating stress (R=−1) and another one obtained under stress ratio R≠−1, for calibrating the equations of boundary condition. Two particular equations for the coefficient of intensification in stress transformations were proposed. The main advantage of the proposed solution is that the mean stress effect correction depends on the number of cycles to failure, what corresponds to the observed changes in experimental results presented in the literature. Proposed relations were compared with popular models for mean stress correction. The verification was made using selected series of experimental results taken from the literature. It was shown that the proposed solution is well correlated with experimental results.

Prediction of failure of cast iron with dynamic loading and high temperature

The fracture of compressor wheel in turbocharger leads wheel pieces impacting into the container. A design method is needed to assure full containment of turbocharger failures within the cast iron housings that operate at high temperatures. During the impacting the container of turbocharger is under critical condition: dynamic loading from wheel pieces and high temperature (in some cases it can reach 1000°C). In this project the material of container is cast iron GGG400SiMo. To investigate the damage behaviour of the material, several groups of experiments were undertaken under specific conditions: quasi-static loading and room temperature, high strain rate dynamic loading and high temperature (up to 600°C). The tests took a wide range of triaxiality (σm/σe; σm and σe are mean stress and equivalent stress) value into account. Extended Cast Iron model was adopted to simulate the elastoplastic property of the cast iron; Vöhringen model was used to consider the effect of strain rate; and proper failure criteria (including maximum and minimum limits) were proposed and applied into all the tests. The simulations with failure prediction mostly matched the experiment very well except for the ones under temperature of 200°C and 400°C with quasi static loading. In the end the finite element model of turbocharger was used to simulate the damage behaviour of the structure with proposed failure criteria.

Thermo-mechanical FE model with memory effect for 304L austenitic stainless steel presenting microstructure gradient

The main purpose of this study is to determine, via a three dimensions Finite Element analysis (FE), the stress and strain fields at the inner surface of a tubular specimen submitted to thermo-mechanical fatigue. To investigate the surface finish effect on fatigue behavior at this inner surface, mechanical tests were carried out on real size tubular specimens under various thermal loadings. X-ray measurements, Transmission Electron Microscopy observations and micro-hardness tests performed at and under the inner surface of the specimen before testing, revealed residual internal stresses and a large dislocation microstructure gradient in correlation with hardening gradients due to machining. A memory effect, bound to the pre-hardening gradient, was introduced into an elasto–visco-plastic model in order to determine the stress and strain fields at the inner surface. The temperature evolution on the inner surface of the tubular specimen was first computed via a thermo-elastic model and then used for our thermo-mechanical simulations. Identification of the thermo-mechanical model parameters was based on the experimental stabilized cyclic tension–compression tests performed at 20°C and 300°C. A good agreement was obtained between numerical stabilized traction–compression cycle curves (with and without pre-straining) and experimental ones. This three dimensional simulation gave access to the evolution of the axial and tangential internal stresses and local strains during the tests. Numerical results showed: a decreasing of the tangential stress and stabilization after 40 cycles, whereas the axial stress showed weaker decreasing with the number of cycles. The results also pointed out a ratcheting and a slightly nonproportional loading at the inner surface. The computed mean stress and strain values of the stabilized cycle being far from the initial ones, they could be used to get the safety margins of standard design related to fatigue, as well as to get accurate loading conditions needed for the use of more advanced fatigue analysis and criteria.

Durability evaluation of a composite bogie frame with bow-shaped side beams

In the current study, the static structural safety and durability of two composite bogie frame models were evaluated using a finite element analysis. Prior to evaluation, the in-plane and out-of-plane mechanical properties of the base material were measured. The fatigue property was also measured to evaluate the durability of two composite bogie frames. Based on material data, the static structural safety was examined using the Tsai-Wu failure criterion under ten different loading conditions. Stress combination data were used to evaluate durability of two composite bogie frames. Analysis results indicate that the maximum Tsai-Wu failure indices of the two composite bogie frames under a twisting load were 0.48 and 0.57. Based on the durability evaluation using Goodman diagrams, all combined stress data of the two models were within the endurance limit. In the bogie frames with a side beam height of 50 mm, the side beam bottom area was the most affected region. The joint center area revealed higher mean stress and stress amplitude values in the bogie frames with a side beam height of 150 mm.

On the low-cycle fatigue response of pre-strained austenitic Fe61Mn24Ni6.5Cr8.5 alloy showing TWIP effect

The low-cycle fatigue behavior of a high manganese austenitic alloy showing TWIP (Twinning-Induced Plasticity) in its homogenized as-received (AsRec) and in pre-strained conditions was determined. Pre-strains were applied both in tension and compression. The fatigue experiments, conducted in total strain control under fully reversed push–pull loading, were accompanied by thorough microstructural investigations utilizing electron backscatter diffraction and transmission electron microscopy. The results clearly lay out the different microstructural evolutions during cyclic loading of the AsRec and the pre-deformed conditions. Cyclic hardening prevails for the AsRec condition due to the increase in dislocation density and the subsequent rearrangement of dislocations into labyrinth and cell structures upon further cycling, whereas pronounced softening accompanied by a decrease in absolute value of mean stress dominated the cyclic deformation response of the pre-deformed conditions.

Left ventricular wall stress compendium

Left ventricular (LV) wall stress has intrigued scientists and cardiologists since the time of Lame and Laplace in 1800s. The left ventricle is an intriguing organ structure, whose intrinsic design enables it to fill and contract. The development of wall stress is intriguing to cardiologists and biomedical engineers. The role of left ventricle wall stress in cardiac perfusion and pumping as well as in cardiac pathophysiology is a relatively unexplored phenomenon. But even for us to assess this role, we first need accurate determination of in vivo wall stress. However, at this point, 150 years after Lame estimated left ventricle wall stress using the elasticity theory, we are still in the exploratory stage of (i) developing left ventricle models that properly represent left ventricle anatomy and physiology and (ii) obtaining data on left ventricle dynamics. In this paper, we are responding to the need for a comprehensive survey of left ventricle wall stress models, their mechanics, stress computation and results. We have provided herein a compendium of major type of wall stress models: thin-wall models based on the Laplace law, thick-wall shell models, elasticity theory model, thick-wall large deformation models and finite element models. We have compared the mean stress values of these models as well as the variation of stress across the wall. All of the thin-wall and thick-wall shell models are based on idealised ellipsoidal and spherical geometries. However, the elasticity model's shape can vary through the cycle, to simulate the more ellipsoidal shape of the left ventricle in the systolic phase. The finite element models have more representative geometries, but are generally based on animal data, which limits their medical relevance. This paper can enable readers to obtain a comprehensive perspective of left ventricle wall stress models, of how to employ them to determine wall stresses, and be cognizant of the assumptions involved in the use of specific models.

A macroscopic damage‐plastic constitutive law for modeling quasi‐brittle fracture and ductile behavior of concrete

SUMMARYA new phenomenological macroscopic constitutive model for the numerical simulation of quasi‐brittle fracture and ductile concrete behavior, under general triaxial stress conditions, is presented. The model is particularly addressed to simulate a wide range of confinement stress states, as also, to capture the strong influence of the mean stress value in the concrete failure mechanisms.The model is based on a two‐surface damage‐plastic formulation. The mechanical behavior in different domains of the stress space is separately described by means of a quasi‐brittle or ductile material response: (i) For positive values of the mean stress (tensile states), an isotropic continuum damage model with strain softening is considered. In this context, and in order to avoid the Boundary Value Problem ill‐posedness induced by the softening law, a regularization technique based on the Continuum Strong Discontinuity Approach (CSDA) is adopted, which results equivalent to a damage model with embedded cohesive cracks providing anisotropic responses. (ii) A plastic model governs the material behavior when the mean stress is negative (confinement states). It is based on the classical plastic flow theory. In particular, a yield criterion similar to that of Willam and coauthors, which depends on the three stress invariants, is used. Additional features defining the plastic response are: an isotropic strain hardening law and a non‐associative flow rule.The paper presents the numerical implementation of the model using an efficient integration algorithm, namely, the Impl‐Ex scheme. Several widely known experimental tests (such as uniaxial, biaxial and triaxial tests) carried out on concrete specimens are used to calibrate and validate the performance of the proposed formulation. Finally, a classical 2D reinforced concrete beam example is analyzed in order to show the predictive capability of the model in structural analysis applications. Copyright © 2011 John Wiley &amp; Sons, Ltd.

Multiaxial fatigue assessment using a simplified critical plane-based criterion

In the present paper, a simplified critical plane-based criterion is employed to evaluate the multiaxial high-cycle fatigue strength of smooth metallic specimens. According to such a criterion, the critical plane orientation is correlated to the weighted average directions of the principal stresses, and the multiaxial fatigue strength is assessed through an equivalent stress expressed by a quadratic combination of the shear stress amplitude and the amplitude of an equivalent normal stress acting on the critical plane. The criterion is a simplified version of the original Carpinteri–Spagnoli (C–S) criterion. The proposed modifications are related to: the weighting procedure of the principal stress axes; the definition of the equivalent normal stress by taking into account the mean normal stress effect; the expression of the quadratic combination of stresses. Several experimental results related to the high-cycle fatigue strength for smooth metallic specimens, subjected to in-phase or out-of-phase biaxial loading with different mean stress values and various ratios of the stress amplitudes, are analysed by employing the C–S criterion, its simplified version, and other criteria available in the literature.

Low-Cycle Fatigue Deformation Behavior and Evaluation of Fatigue Life on Extruded Magnesium Alloys

To evaluate fatigue deformation behavior and fatigue life of extruded magnesium alloy, total strain-controlled and stress-controlled low-cycle fatigue test of three extruded magnesium alloys, AZ31, AZ61 and AZ80, were performed in ambient atmosphere at room temperature using smooth round bar specimen. Mean tensile stress during total strain-controlled fatigue process and mean compressive strain during stress-controlled fatigue process appeared due to asymmetry of yield stress between tension and compression. The values of mean stress and mean strain appeared on three alloys were associated with the ratio of compressive yield stress to tensile one, which resulted in mechanical twinning in the compressive phase. Fatigue criteria proposed previously considering the mean stress effect were evaluated in terms of fatigue life predictions based on the experimental results. Also, an energy-based model taken into account of plastic and elastic energy density was discussed to predict the fatigue lives obtained from total strain- and stress-controlled fatigue experiments.

The Prediction of Fatigue Crack Initiation Life in Spot Welds

Abstract: In this paper, strain‐based fatigue life prediction method has been used to estimate the fatigue crack initiation life of spot‐welded joints of Mild Steel JSC270D and Ultra‐High Strength Steel JSC980Y. To do so, the joints were simulated using three‐dimensional finite‐element (FE) models, and then nonlinear FE analysis was performed to obtain the local stress and strain ranges and finally, the Morrow equation was applied to estimate the crack initiation lives. The results have been compared with those obtained from experimental crack growth morphology. In addition, the difference between fatigue limits for smooth specimens and spot‐welded joints for mentioned materials has been briefly discussed. It has been shown that mean stress values in the Ultra‐High Strength Steel can significantly decrease the fatigue limit of spot‐welded joint because even at very low load level the stresses exceed the yield point at the root of nugget of spot‐welded joint, while the amount of mean stress in the Mild Steel for the same load level is much less than that of Ultra‐High Strength Steel. The comparison between numerical results of fatigue crack initiation lives and experimental data provided good agreement between numerical predictions and crack growth morphology observations. The results also shows that in some cases, depending on the joint type, the life spent in the nucleation phase can be an important part of the final failure lifetime.

Determinist-Probabilistic Concept in Modeling Fatigue Damage Through a Micromechanical Approach

Motivated by a micromechanical determinist-probabilistic model coupled with damage recently developed by the authors, a new generalization is proposed to describe the nonlinear elasto-inelastic cyclic strain-stress behavior of polycrystals notably under biaxial cyclic loading paths. In this context, this generalization considers a compressible and linear anisotropic granular elastic strain behavior coupled with damage. The model is expressed in the framework of the time dependent plasticity for a small strain assumption. It is assumed that a damage variable initiates at the mesoscopic (granular) level where the plastic strain localization phenomenon takes place. The associated thermodynamic force of the damage variable is determined using the concept of total granular energy (elastic and inelastic). The transition of the elastic strain from the single to the polycrystal is modified due to its explicit coupling with damage. Comparisons between predicted and experimental results are conducted describing the low-cycle fatigue behavior of the aluminum alloy 2024 under different complex cyclic loading paths. It is demonstrated that the model has a reasonable ability in describing the cyclic behavior of this alloy. Qualitatively, the model is tested under different cyclic loading paths with stress-controlled condition describing especially the ratcheting behavior of the alloy. In fact, the effects of the applied mean stress on the predicted overall elasto-inelastic behavior and on the fatigue life are carefully studied. It shows the dependence of the fatigue life on the mean stress value.

High cycle fatigue of metastable austenitic stainless steels

Metastable austenitic stainless steels are currently used in applications where severe forming operations are required, such as automotive bodies, due to its excellent ductility. They are also gaining interest for its combination of high strength and formability after forming. The biggest disadvantage is the difficulty to predict the mechanical response, which depends heavily on the amount of martensite formed. The martensitic transformation in metastable stainless steels can be induced by plastic deformation at room temperature. In this research, the martensitic transformation was provoked by means of torsion testing. Several torsion angles were selected to achieve different percentages of martensite at the surface of the specimens. The next step was to evaluate their effect on the fatigue life of the steel. Fatigue testing in the high cycle regime was done at different levels of mean stress. As a conclusion, the presence of martensite in the surface of the specimen led to an increase of the fatigue life when high mean stress was imposed. By contrast, at lower values of mean stress, martensitic transformation has no positive effect on the fatigue life.

Mean stress relaxation during cyclic straining of high strength aluminum alloys

Cycle-dependent relaxation may alter mean stress values and thus affect fatigue crack initiation life. This phenomenon is an issue both for accuracy of estimated fatigue lives and for the success of methods of intentionally introducing beneficial mean stresses. Although rigorous plasticity theories may describe cycle-dependent relaxation, it is not feasible to incorporate their complex mathematics into strain-based life estimates when loading histories contain millions of cycles. Empirical models are, on the other hand, more numerically efficient and, therefore, for the present purpose, constitute a better choice for modeling this transient phenomenon. Experiments have been performed to study mean stress relaxation in aluminum alloys 7075-T6511 and 7249-T76511. Preliminary tests were also conducted on alloy 7475-T651. Two empirical models are used to illustrate the experimentally observed cycle-dependent relaxation.