Features controlling the early stages of creep deformation of Waspaloy

A model has been presented for describing primary and second stage creep. General equations were derived for the amount and time of primary creep. It was shown how the model can be used to extrapolate creep data. Applicability of the model was demonstrated for Waspaloy with gamma prime particle sizes from 75 - 1000 A creep tested in the temperature range 1000 - 1400 F (538 - 760 C). Equations were developed showing the dependence of creep parameters on dislocation mechanism, gamma prime volume fraction and size

Preliminary studies of the time-dependent shear and uniaxial tensile behaviour of oriented polymers

Summary The work reported in this memo is the initial stages of an investigation of the time-dependent behaviour of certain anisotropic polymers. In the first instance low density polyethylene with a transversely isotropic symmetry is being examined. Different degrees of anisotropy have been induced by cold drawing and the time dependent material parameters necessary to describe the stiffness of the anisotropic polyethylene have been determined. This involved the measurement of uniaxial tensile creep: lateral contraction creep, and torsional creep under conditions of constant load at 20°C ± 0.5°C. The tensile creep and contraction creep apparatus has been described elsewhere (Darlington (a) 1968) and only the principle of the apparatus is discussed here. The torsional creep apparatus is described in detail. Analysis of the experimental data is not yet complete. The data is tabulated in section 5 and a preliminary analysis is presented in section G. Details of proposed future work are discussed in section 7

Creep and tensile behaviour of austenitic Fe–Cr–Ni stainless steels

The control of creep behaviour during service of reformer tubes made of HP-40 austenitic stainless steels is still limited by the knowledge of creep mechanisms in these alloys. Two different HP-40 alloys modified with a low-level addition ofNbwere studied. Creep testswere carried out at 980 and 1050 ◦C with different stress levels, in the range of 20–50MPa, and their resultswere plotted in a Norton-type diagram. Also, low strain rate tensile tests were performed at temperature of 950, 980 or 1000 ◦C. As low strain rate tensile tests showed a plateau at nearly constant stress for a given strain rate, they could be somehow linked with creep tests. Accordingly, tensile and creep results were plotted together on a Larson–Miller (LMP) diagram. The fracture modes of tensile and creep samples were investigated and the effect of different parameters such as sample dimensions, temperature and atmosphere, was also studied

Advances on creep–fatigue damage assessment in notched components

In this paper, the extended Direct Steady Cyclic Analysis method (eDSCA) within the Linear Matching Method Framework (LMMF) is combined with the Stress Modified Ductility Exhaustion method and the modified Cavity Growth Factor (CGF) for the first time. This new procedure is used to systematically investigate the effect of several load parameters including load level, load type and creep dwell duration on the creep–fatigue crack initiation process in a notched specimen. The results obtained are verified through a direct comparison with experimental results available in the literature demonstrating great accuracy in predicting the crack initiation life and the driving mechanisms. Furthermore, this extensive numerical study highlighted the possible detrimental effect of the creep–ratchetting mechanism on the crack growth process. This work has a significant impact on structural integrity assessments of complex industrial components and for the better understanding of creep–fatigue lab scale tests

A continuous damage model based on stepwise-stress creep rupture tests

A creep damage accumulation model is presented that makes use of the Kachanov damage rate concept with a provision accounting for damage that results from a variable stress history. This is accomplished through the introduction of an additional term in the Kachanov rate equation that is linear in the stress rate. Specification of the material functions and parameters in the model requires two types of constituting a data base: (1) standard constant-stress creep rupture tests, and (2) a sequence of two-step creep rupture tests

Relationship between fatigue life in the creep-fatigue region and stress-strain response

On the basis of mechanical tests and metallographic studies, strainrange partitioned lives were predicted by introducing stress-strain materials parameters into the Universal Slopes Equation. This was the result of correlating fatigue damage mechanisms and deformation mechanisms operating at elevated temperatures on the basis of observed mechanical and microstructural behavior. Correlation between high temperature fatigue and stress strain properties for nickel base superalloys and stainless steel substantiated the method. Parameters which must be evaluated for PP- and CC- life are the maximum stress achievable under entirely plastic and creep conditions respectively and corresponding inelastic strains, and the elastic modulus. For plasticity/creep interaction conditions (PC and CP) two more pairs of stress strain parameters must be ascertained

The creep behaviour of fibre reinforced plastics

Synopsis: This paper describes the tensile creep behaviour of fibre reinforced plastics under a uniaxial stress system. In particular, unidirectionally and bidirectionally reinforced plastics are considered for the relative ease with which samples of them can be manufactured. A composite model that has previously been described 1, 2 is used as the basis of theoretical predictions of the variation of the creep compliance with certain material parameters. The parameters whose effects are examined are the volume fraction of the fibres, the distribution of fibre orientations, and the angle between a specified direction and the direction of application of a uniaxial stress system. Specimen experimental results are included to show the accuracy of these predictions and any shortcomings of the theoretical model are examined in the light of this comparison

A new creep model directly using tabulated test data and implemented in ansys

Nowadays plastics are increasingly used in highly stressed structures in all kinds of constructions. The time dependency, the so-called viscosity, is a crucial part of the material behavior of plastics. A typical form of viscosity is creep. Creep is the increase of deformation under constant load. In the FE-simulation creep behavior is usually described by creep law functions. The commercial software provide many creep law functions depending on time, stress, strain, temperature and multiple material parameters. To run a creep simulation, the user must define all the parameters which requires a certain effort. Curve-fitting procedures might be of help, the results, however, often are not precise enough. For these reasons, we introduce our new creep model doing the similar job as the creep law functions but being able to directly use the tabulated data of the creep tests without curve-fitting procedures. In this paper, we use the model to create a 3D stress-creep strain-time surface based on the tabulated data like isochronous curves, which is represented by bicubically blended Coons patches to provide a good convergence due to their differentiability. This creep model supports strain hardening, which shows more realistic behavior when the load changes significantly during the simulated proces