A Comprehensive Creep Model

1967 ◽  
Vol 89 (3) ◽  
pp. 496-502 ◽  
Author(s):  
T. Z. Harmathy
Keyword(s):  
Creep Curve ◽  
Equations Of State ◽  
Tertiary Creep ◽  
Creep Model ◽  
Polycrystalline Materials ◽  
Creep Theory ◽  
Deformation Processes ◽  
Stress Dependent ◽  
Explicit Expressions

Based on Dorn’s creep theory, a comprehensive creep model has been developed that is applicable to the calculation of deformation processes at steadily increasing temperatures and slowly varying load. It is shown that the entire course of the creep curve (excepting the tertiary creep) of structural (polycrystalline) materials is uniquely determined by two stress-dependent parameters, εt0 and Z. Explicit expressions for the description of creep processes are proposed. These expressions cannot strictly be regarded as equations of state, but yield an accuracy acceptable for engineering calculations even if dσ/dt ≠ 0.

Computational Design of Microstructures With Stochastic Property Closures

2020 ◽  
Vol 143 (1) ◽  
Author(s):  
Pinar Acar
Keyword(s):  
Material Properties ◽  
Computational Design ◽  
Polycrystalline Materials ◽  
Stochastic Property ◽  
Analytical Uncertainty ◽  
Magnetostrictive Strain ◽  
Material Systems ◽  
Stochastic Solution ◽  
Deformation Processes ◽  
Computational Solution

Abstract The present work addresses a stochastic computational solution to define the property closures of polycrystalline materials under uncertainty. The uncertainty in material systems arises from the natural stochasticity of the microstructures as a result of the fluctuations in deformation processes. The microstructural uncertainty impacts the performance of engineering components by causing unanticipated anisotropy in properties. We utilize an analytical uncertainty quantification algorithm to describe the microstructural stochasticity and model its propagation on the volume-averaged material properties. The stochastic solution will be integrated into linear programming to generate the property closure that shows all possible values of the volume-averaged material properties under the uncertainty. We demonstrate example applications for stiffness parameters of α-Titanium, and multi-physics parameters (stiffness, yield strength, magnetostrictive strain) of Galfenol. Significant differences observed between stochastic and deterministic closures imply the importance of considering the microstructural uncertainty when modeling and designing materials.

Life Fraction Hardening Applied to a Modified Theta Projection Creep Model for a Nickel-Based Super-Alloy

2014 ◽  
Author(s):  
W. David Day ◽  
Ali P. Gordon
Keyword(s):  
Strain Hardening ◽  
Analytical Calculation ◽  
Tertiary Creep ◽  
Creep Model ◽  
Integration Scheme ◽  
Hardening Rule ◽  
Creep Models ◽  
Computationally Intensive ◽  
Super Alloy ◽  
Life Fraction

This paper presents the application of a life fraction hardening rule to the analytical calculation of creep in hot section components. Accurate prediction of creep is critical to assuring the mechanical integrity of heavy-duty, industrial gas turbine (IGT) hardware. The accuracy of such predictions depend upon both the creep models assumed and how those models are implemented in a finite element solution. A modified theta projection creep model for a nickel-based super alloy was presented in a previous paper as an accurate simulation of creep behavior [1]. Application of such a user defined creep law depends upon definition of a hardening rule in the form of either an explicit or an implicit integration scheme in order to calculate incremental strains during any time increment. Time hardening is the simplest and least computationally intensive of the two most common hardening rules, but does not correctly show the effect of changing stresses or temperatures. Strain hardening may provide the most accurate solution, but the creep models are too complex to invert, which results in highly iterative and computationally intensive solutions. A life fraction hardening rule has been presented in other works [2] as a compromise between time hardening and strain hardening. Life fraction hardening is presented here as a highly efficient and accurate means of calculating incremental creep strain when applied to a modified theta projection creep model. A user creep subroutine was defined using a state variable to represent the strain life fraction at any time. By using the time to tertiary creep as the denominator for the life fraction, no new material constants are needed to relate to creep failure. The start of tertiary creep is effectively considered to be a failure. Additional design insight can be provided through the inclusion of other state variables to calculate temperature margins at current conditions. Material testing with changing stress levels will be used to help validate the technique. A simplified example of the technique is presented in the paper. More accurate creep predictions allow our company to improve the structural integrity of its turbine blades and vanes.

scholarly journals Model Of Relaxation Of Residual Stresses In Hot-Rolled Strips

2015 ◽  
Vol 60 (3) ◽  
pp. 1935-1940 ◽  
Author(s):  
A. Milenin ◽  
R. Kuziak ◽  
V. Pidvysots'kyy ◽  
P. Kustra ◽  
Sz. Witek ◽  
...  
Keyword(s):  
Stress Relaxation ◽  
Residual Stresses ◽  
Thermal Stresses ◽  
Plastic Material ◽  
Material Model ◽  
Creep Model ◽  
Creep Theory ◽  
Elastic Plastic ◽  
Elastic Plastic Material ◽  
Hot Rolled

Abstract Residual stresses in hot-rolled strips are of practical importance when the laser cutting of these strip is applied. The factors influencing the residual stresses include the non uniform distribution of elastic-plastic deformations, phase transformation occurring during cooling and stress relaxation during rolling and cooling. The latter factor, despite its significant effect on the residual stress, is scarcely considered in the scientific literature. The goal of the present study was development of a model of residual stresses in hot-rolled strips based on the elastic-plastic material model, taking into account the stress relaxation. Residual stresses in hot-rolled strips were evaluated using the FEM model for cooling in the laminar cooling line and in the coil. Relaxation of thermal stresses was considered based on the creep theory. Coefficients of elastic-plastic material model and of the creep model for steels S235 and S355 were obtained from the experiments performed on the Gleeble 3800 simulator for the temperatures 35-1100°C. Experiments composed small tensile deformations of the sample (0.01-0.02) and subsequent shutter speed without removing the load. Model of the thermal deformation during cooling was obtained on the basis of the dilatometric tests at cooling rates of 0.057°C/s to 60°C/s. Physical simulations of the cooling process were performed to validate the model. Samples were fixed in the simulator Gleeble 3800, then heated to the temperature of 1200°C and cooled to the room temperature at a rate of 1-50°C/s. Changes of stresses were recorded. Good agreement between calculated and experimental values of stresses was observed. However, due to neglecting the effect of stress relaxation the stress at high temperatures was overestimated. Due to the change of their stress sign during the unloading process the resulting residual stresses were underestimated. Simulation of residual stresses in rolling and cooling were performed on the basis of the developed model. It was shown that the effect of stress relaxation and phase transformations on the distribution of residual stresses in strips is essential and neglecting these factors could lead to an underestimation of residual stresses.

scholarly journals Creep rupture of wallaby tail tendons.

1995 ◽  
Vol 198 (3) ◽  
pp. 831-845 ◽  
Author(s):  
X T Wang ◽  
R F Ker
Keyword(s):  
Dynamic Test ◽  
Primary Creep ◽  
Creep Rupture ◽  
Tertiary Creep ◽  
Dynamic Tests ◽  
Specimen Length ◽  
Creep Theory ◽  
Time To Rupture ◽  
Tail Tendon ◽  
Strength And Stiffness

The tail tendons from wallabies (Macropus rufogriseus) suffer creep rupture at stresses of 10 MPa or above, whereas their yield stress in a dynamic test is about 144 MPa. At stresses between 20 and 80 MPa, the time-to-rupture decreases exponentially with stress, but at 10 MPa, the lifetime is well above this exponential. For comparison, the stress on a wallaby tail tendon, when its muscle contracts isometrically, is about 13.5 MPa. Creep lifetime depends sharply on temperature and on specimen length, in contrast to strength and stiffness as observed in dynamic tests. The creep curve (strain versus time) can be considered as a combination of primary creep (decelerating strain) and tertiary creep (accelerating strain). Primary creep is non-damaging, but tertiary creep is accompanied by accumulating damage, with loss of stiffness and strength. 'Damage' is quantitatively defined as the fractional loss of stiffness. A creep theory is developed in which the whole of tertiary creep and, in particular, the creep lifetime are predicted from measurements made at the onset of creep, when the tendon is undamaged. This theory is based on a 'damage hypothesis', which can be stated as: damaged material no longer contributes to stiffness and strength, whereas intact material makes its full contribution to both.

scholarly journals FATIGUE FAILURE AS A COMPLEX OF RELAXATION PROCESSES OCCURRED AT THE VERTICES OF THE STRESS RISERS

InterConf ◽  
2021 ◽  
pp. 341-348
Author(s):  
P. Volosevish ◽  
B. Mordyuk
Keyword(s):  
Fatigue Life ◽  
Fatigue Failure ◽  
Grain Boundary Sliding ◽  
Polycrystalline Materials ◽  
Second Phase ◽  
Relaxation Processes ◽  
Stress Risers ◽  
Eye Fatigue ◽  
Surface Modified ◽  
Stress Dependent

This paper considers the stress-dependent fatigue life of polycrystalline materials and their fatigue failure as a result of the relaxation processes that occurred on the stress risers of various scales: macroscopic stress risers of technological nature (pores, cracks, surface roughness, etc.), and microscopic stress risers at the grain/subgrain boundaries and/or second phase particles. Participation of the relaxation mechanisms plastic (vacancies and dislocation activities, grain boundary sliding) and brittle (cracks) nature in the process of the ‘fish eye’ fatigue crack formation is also addressed. The model described the parabolic dependencies of the densities of elementary carriers of plastic and brittle relaxations on the load change rate (i.e., on the growth rate of the stresses concentrated at the vertices of the stress risers) correlates well to the fatigue life data observed for the surface-modified metallic materials.

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