A Modern Philosophy for Creep Lifing in Engineering Alloys (original) (raw)
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The most important aspect of any engineering material is not only its chemical composition but also its structure, because material properties are closely related to this feature. To be successful in "materials tailoring" a material engineer must have both a good understanding of this relationship between structure and properties and a good understanding of degradation mechanisms operating in materials under conditions of engineering practice (i. e. temperature, stress, corrosion etc.) Determination of the creep mechanisms is extremely difficult in the temperature and applied stress region typical for the exploitation of contemporary heat resistant materials because of experimental tests duration. Optical measurement of creep strain does not affect the stress state of a creep specimen. This advantage is important mainly with the high sensitive helicoid spring specimen technique, which is used for very low stresses and creep strains testing. The "manual" optical measurements of strain using cathetometer with a digital readout provide results with the strain accuracy between 10-7 and 10-6 , depending on the specimen geometry and experimental details. This sensitivity is suitable for detection of the creep rates down to 10-13 s-1 , the rates found obviously in the practical application of creep resistant materials. Creep experiments for several metallic materials from pure metal (Fe) or simple binary alloy (Ni-Cr) to heat resistant steel (AISI 316H type stainless steel) are analysed in close relation to their microstructure. The viscous creep mechanisms responsible for the creep deformation under the given conditions are discussed.
An analysis of modern creep lifing methodologies in the titanium alloy Ti6-4
Materials Science and Engineering: A, 2013
Traditional creep lifing techniques based on power law equations have shown themselves to be extremely limited, particularly in the prediction of long term data based only on short term experiments. More recently, alternative approaches such as the Wilshire equations and hyperbolic tangent methods have been proposed which offer a new insight into the field, with the Wilshire equations in particular showing promise in predicting long term behaviour based on short term results. The aerospace industry however has different requirements to the power generation industry where the Wilshire equations have been thoroughly tested. The current work seeks to investigate the ability of these methods to extrapolate across temperatures and in the case of the Wilshire equations offer a capability to accurately describe the time to specific strains during creep deformation.
Recent progress in the modeling of high-temperature creep and its application to alloy development
Journal of Materials Engineering and Performance, 1995
Recent progress in the understanding of high-temperature creep of alloys is discussed in the context of theoretical modeling and its application to alloy development. Emphasis is placed upon those engineering alloys specifically designed for high-temperature applications, such as precipitation and dispersionstrengthened (DS) alloys and metal-matrix composites (MMCs). Currently, these theoretical models use one of two different approaches, (a) a phenomenological approach, which is used in such models as those based on the internal stress concept, and those based on empirical creep equations; and (b) micromechanical models that are based on dislocation mechanisms and the interactions of dislocations with solute atoms, second-phase particles, and other reinforcements such as fibers. All these theoretical models have a common goal, namely, the understanding of high-temperature strengthening mechanisms and the relationship between high-temperature strength and the micromechanical mechanisms during high-temperature plastic deformation of the alloys. These theoretical studies can provide information that is useful in alloy design and processing, such as the selection of alloy chemistry, and the optimization of phase microstructural features (e.g., reinforcement amount, shape, size, and distribution; matrix grain size; and matrix and reinforcement interfaces) by optimization of processing methods.
Physical Background and Simulation of Creep in Steels
Creep Characteristics of Engineering Materials [Working Title], 2019
The simulative accelerated creep test (ACT) was developed as a response to an overall need of gaining in a short time useful physical data for determining longterm behavior of materials exposed to operation under stress at elevated temperatures in power generation and chemical processing industries. Additionally, the recently frequent power plant shutdowns due to adding solar/wind power to the net, call for creep-fatigue data, which standard creep tests cannot provide. In response to these needs, a thermal-mechanical fatigue procedure-ACT-was designed, taking into account physical phenomena causing microstructure transformation during creep, in particular generation of dislocation substructures, their role in nucleation of voids and cracks, intensification of carbide precipitation, and decay of mechanical properties during long-time exposure to elevated temperatures. The actual ACT procedure generates adequate data for calculating true lifetime of the tested creep resisting material for a nominal stress.
Advanced Methods for Creep in Engineering Design
2018
There are many applications where the combination of stress and elevated temperature require creep to be considered during the design process. For some applications, an evaluation of rupture life for given conditions is sufficient, however, for components such as those in gas turbine aeroengines, the accumulation of creep strain over time and the effect this has on other phenomena, such as high-temperature fatigue must be considered. In this chapter, modern creep curve modelling methods are applied to alloys used in gas turbine applications over a wide range of test conditions. Also, different creep hardening modelling methods are discussed along with their application to transient creep showing the deficiencies of simplistic models. Models are related to micromechanical properties where possible, and creep damage models are evaluated and applied to different applications using finite element analysis (FEA).
On the main directions in creep mechanics of metallic materials
Mechanics - Proceedings of National Academy of Sciences of Armenia, 2020
for better comparison to one-dimensional results, equivalent statements for the stresses and the strains were introduced. Up to now, there is no creep mechanics theory which is as strict as continuum mechanics. However, there are many engineering theories through which more and more solutions for practical cases can be obtained. The paper is a state of the art report of creep mechanics for metallic materials and structures composed from these materials.
New observations on high-temperature creep at very low stresses
Materials Science and Engineering: A, 2009
Creep tests were conducted in compression to evaluate the flow behavior of aluminum at very high temperatures and low stresses. The experiments used two types of specimens: single crystals of 99.999% purity and oligocrystalline samples of 99.97% purity. Results obtained for the single crystals lie consistently between the conventional region of Harper-Dorn creep and the anticipated behavior based on an extrapolation of conventional 5-power creep. Using etch pit studies with the single crystals, it is shown there is no evidence for subgrain formation and the measured dislocation densities are consistent with data extrapolated from the conventional 5-power region. The creep results on single crystals suggest the stress exponent is close to ∼3 at low stresses. It is demonstrated these results are consistent with earlier data including the results of Harper and Dorn when their data are plotted in terms of the true applied stress without incorporating a threshold stress.
Constitutive description of high temperature creep in mechanically alloyed AlCO alloys
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 1994
Creep curves of four mechanically alloyed aluminum alloys were studied within the range of stresses from 20 to 180 MPa at temperatures of 623 and 723 K. Creep curves of pure aluminum at temperatures of 473-623 K were taken as reference material data. The curves were described by the McVetty equation. The primary and steady-state stages of creep can be interpreted as a result of changes in the internal stress following from competition between work hardening and recovery processes. The analysis makes it possible to divide the internal stress into two components, one being due to stress fields of dislocations and the other to the existence of dispersed particles. The latter component equals the threshold stress in steady-state creep.