Uniaxial Creep Deformation Research Articles

Branching of axial compression histories of solid cylinders undergoing thermo-chemo-mechanical changes

This work considers the uniaxial compressive creep of a solid elastomeric circular cylinder under a fixed dead load as it undergoes time dependent and temperature dependent changes in its macromolecular structure. The changes occur at temperatures exceeding 100 °C and consist of chemical scission of macromolecular network junctions and their re-crosslinking to form new networks in new reference configurations. The scission process causes softening due to modulus reduction and the re-crosslinking causes stiffening due to deformation of newly formed networks. A condition is developed for the time when the homogeneous uniaxial creep deformation history develops an non-homogenous barrel shaped branch. Branching times and the corresponding compression ratio are calculated from this condition using mechanical and chemical scission properties from the experimental literature.

Finite-Element Analysis of Waspaloy Using Sinh Creep-Damage Constitutive Model Under Triaxial Stress State

The creep deformation and damage evolution of nickel base superalloy (Waspaloy) at 700 °C are studied using the classic Kachanov–Rabotnov (KR) and a recently developed Sin-hyperbolic (Sinh) model. Uniaxial creep deformation and Bridgman rupture data collected from literature are used to determine the model constants and to compare the KR and the Sinh solutions. Finite-element (FE) simulations on a single eight-node element are conducted to validate the accuracy of the FE code. It is observed that KR cannot predict the creep deformation, damage, and rupture life of nickel base superalloys accurately using one set of constants for all the stress levels. The Sinh model exhibits a superior ability to predict the creep behavior using one set of constants for all the stress levels. Finite-element analysis (FEA) on 3D Bridgman notched Waspaloy specimen using the Sinh model is conducted. The results show that the Sinh model when combined with a representative stress equation and calibrated with experimental data can accurately predict the “notch effect” observed in the rupture life of notched specimen. Contour plots of damage evolution and stress redistribution are presented. It is demonstrated that the Sinh model is less stress sensitive, produces unconditional critical damage equal to unity at rupture, exhibits a more realistic damage distribution around the crack tip, and offers better crack growth analysis than KR.

The influence of pre-compression on the creep deformation and failure behaviour of Type 316H stainless steel

The influence of pre-compression to 8% plastic strain at room temperature has been examined on the tensile properties, uniaxial creep deformation, creep crack initiation and growth behaviour of Type 316H stainless steel at 550°C. Uniaxial creep and crack growth tests have been performed on the pre-compressed (PC) material and the results compared to existing long term (>15,000h) and short term test data on as-received (AR) (i.e. uncompressed) material. Pre-compression has been found to increase the materials subsequent yield stress in tension. Therefore the extent of non-linearity observed on the load–displacement curves of uniaxial creep rupture and crack growth tests on PC material is limited compared to AR material. In addition pre-compression causes a significant reduction in creep ductility and rupture time, although similar average and minimum creep strain rates are found in PC and AR materials. The creep crack growth (CCG) data on PC and AR materials have been characterised using the steady state creep C* parameter employing appropriate validity criteria and geometry dependent fracture mechanics parameter solutions. The CCG results are compared to the creep crack growth prediction models. Based on the creep properties, creep ductility and metallurgical observations of the fracture behaviour of the AR and PC materials, it has been shown that short term creep crack growth tests on PC material may be used to predict long term creep crack initiation (CCI) and CCG behaviour of AR material at 550°C.

Finite element analysis of uniaxial and multiaxial state of stress on creep rupture behaviour of 2.25Cr–1Mo steel

Effect of multiaxial state of stress on creep rupture behaviour of 2.25Cr–1Mo steel has been investigated. Multiaxial state of stress during creep test has been introduced by incorporating two different circumferential U-notches in the uniaxial creep specimen. Creep tests on both plain and notched specimens were carried out at 873K over a stress range of 90–210MPa. The creep rupture life of the material was found to increase in presence of the notch with the consequent decrease in creep rupture ductility. The increase in rupture life was found to be more for relatively sharper notch. Finite element analysis coupled with continuum damage mechanics was carried out to understand the creep damage evolution during uniaxial and multiaxial creep exposures. The predicted uniaxial creep deformation and rupture life of the material were in good agreement with the experimental results. The creep rupture life of the notched specimen has been predicted and was found to be controlled predominantly by von-Mises stress. The creep damage in this steel under multiaxial state of stress was predominantly in the form of microstructural degradation with some evidences of intergranular creep cavitation especially at lower stresses.

Constitutive equations that describe creep stress relaxation for 316H stainless steel at 550°C

The prediction of the stress relaxation behaviour of welding induced residual stresses in thick section 316H austenitic stainless steel welded component provides an input for quantifying reheat crack initiation observed in the heat affected zone. The cracks occur after service at a temperature range from 490 to 520°C. The present work reviews some of the widely applied stress relaxation models. The relative strengths and weaknesses of these existing models are discussed. An improved constitutive equation derived from a forward uniaxial creep deformation law is proposed. The relative importance of the parameters selected in the new constitutive model, when compared with experimental data, is discussed. The importance of a better understanding of the role of the internal stress and its measurement is highlighted.

Constitutive equations that describe creep stress relaxation for 316H stainless steel at 550°C

The prediction of the stress relaxation behaviour of welding induced residual stresses in thick section 316H austenitic stainless steel welded component provides an input for quantifying reheat crack initiation observed in the heat affected zone. The cracks occur after service at a temperature range from 490 to 520°C. The present work reviews some of the widely applied stress relaxation models. The relative strengths and weaknesses of these existing models are discussed. An improved constitutive equation derived from a forward uniaxial creep deformation law is proposed. The relative importance of the parameters selected in the new constitutive model, when compared with experimental data, is discussed. The importance of a better understanding of the role of the internal stress and its measurement is highlighted.

Molecular mobility of poly(methyl methacrylate) glass during uniaxial tensile creep deformation

AbstractAn optical photobleaching method has been used to measure the segmental dynamics of a poly(methyl methacrylate) (PMMA) glass during uniaxial creep deformation at temperatures between Tg − 9 K and Tg − 20 K. Up to 1000‐fold increases in mobility are observed during deformation, supporting the view that enhanced segmental mobility allows flow in polymer glasses. Although the Eyring model describes this mobility enhancement well at low stress, it fails to capture the dramatic mobility enhancement after flow onset, where in addition the shape of the relaxation time distribution narrows significantly. Regions of lower mobility accelerate their dynamics more in response to an external stress than do regions of high mobility. Thus, local environments in the sample become more dynamically homogeneous during flow. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 1713–1727, 2009

High‐temperature dislocation plasticity in the single‐crystal superalloy LEK94

The evolution of the dislocation structure that forms during uniaxial creep deformation in the single-crystal superalloy LEK94 of low density and with Re additions was analysed using transmission electron microscopy. The material has a gamma/gamma'-microstructure consisting of gamma'-cubes (L12 phase, 80 vol.%) separated by thin gamma-channels (face-centred cubic). <100> tensile creep tests were performed at 980 and 1020 degrees C at stresses of 200 and 240 MPa. The microstructure was investigated at three characteristic stages of creep (directly after loading, at 5% strain and after rupture) to show the evolution of the dislocation structure during high-temperature creep. It was found that in the early stages of creep, a(0)/2<011> dislocations form within the gamma-channels. Later on, dislocation networks form and gamma' cutting processes with a(0)/<001> superdislocations are observed. The results are in line with observations made for other superalloy single crystals in the high-temperature low-stress creep regime.

Creep Deformation Behavior and Failure Diagnostic Diagrams of a Columnar-Grained Nickel-Based Superalloy

Uniaxial creep deformation of a columnar-grained cast Ni-based superalloy has been analyzed in the temperature range of 1035-1315 K. Two-stage heat treatment of the material resulted in an average γ′ particle size of 0.4 µm and inter-particle distance of 0.04 µm. At 1035 K, the alloy showed predominantly primary creep with very low tertiary creep strains. At temperatures above 1035 K, a pseudo-tertiary creep regimen is predominant. Faceted fracture, coarsening of γ′ particles, decohesion of the matrix around the particles, and rafting of the coarsened γ′ particles have been observed as failure modes over the temperature range studied. An analysis is presented to understand creep deformation behavior of the material in the secondary and tertiary creep stages, by expressing them as power functions of time. Failure diagnostic diagrams have also been constructed for the material over the temperature range investigated.

Multiaxial creep and cyclic plasticity in nickel-base superalloy C263

Physically-based constitutive equations for uniaxial creep deformation in nickel alloy C263 [Acta Mater. 50 (2002) 2917] have been generalised for multiaxial stress states using conventional von Mises type assumptions. A range of biaxial creep tests have been carried out on nickel alloy C263 in order to investigate the stress state sensitivity of creep damage evolution. The sensitivity has been quantified in C263 and embodied within the creep constitutive equations for this material. The equations have been implemented into finite element code. The resulting computed creep behaviour for a range of stress state compares well with experimental results. Creep tests have been carried out on double notched bar specimens over a range of nominal stress. The effect of the notches is to introduce multiaxial stress states local to the notches which influences creep damage evolution. Finite element models of the double notch bar specimens have been developed and used to test the ability of the model to predict correctly, or otherwise, the creep rupture lifetimes of components in which multiaxial stress states exist. Reasonable comparisons with experimental results are achieved. The γ ′ solvus temperature of C263 is about 925 °C, so that thermo-mechanical fatigue (TMF) loading in which the temperature exceeds the solvus leads to the dissolution of the γ ′ precipitate, and a resulting solution treated material. The cyclic plasticity and creep behaviour of the solution treated material is quite different to that of the material with standard heat treatment. A time-independent cyclic plasticity model with kinematic and isotropic hardening has been developed for solution treated and standard heat treated nickel-base superalloy C263. It has been combined with the physically-based creep model to provide constitutive equations for TMF in C263 over the temperature range 20–950 °C, capable of predicting deformation and life in creep cavitation-dominated TMF failure.

Cavitational Strain Contribution to Tensile Creep in Vitreous Bonded Ceramics

Analysis of the role of cavitation during uniaxial creep deformation in vitreous bonded ceramics reveals that the cavity volume contributes only to the strain in the direction parallel to the tensile stress regardless of the shape and orientation of cavities. Creep asymmetry results from the fact that cavitation preferentially contributes to axial tensile strain while the strain observed under the same conditions in compression is produced only by volume‐conserving mechanisms. The contribution of cavitational strain in the axial tensile strain is equal to the volume fraction of cavities and proportional to the difference between tensile and compressive strains in the axial direction. The density change method and a newly proposed method based on the difference in the axial strains were used for separating the cavitational from the true tensile strain in self‐reinforced silicon nitride. Both methods consistently revealed more than 90% contribution of cavitation to the total tensile strain. Cavitation is concluded to be the dominant mechanism of tensile creep deformation in vitreous bonded ceramics because the reported volume fractions of cavities during their deformation are usually in the range of 70–90% of tensile strain.

Creep deformation and crack growth behavior of a single-crystal nickel-base superalloy

Uniaxial creep deformation and crack growth data are presented on the single-crystal nickel-base superalloy SC16, which is a candidate material for industrial gas turbine applications. All testing was performed at 900 °C. The uniaxial experiments were conducted with the loading direction aligned approximately along the [001] crystallographic axis of the material. Under these conditions, a small primary region followed by mainly tertiary creep was obtained, and failure initiated from cracks at interdendritic pores. The crack growth experiments were performed on single-edge notch tension specimens and compact tension test pieces containing deep side grooves to examine state-of-stress effects. A selection of crystallographic orientations was also examined. Little effect of stress state and orientation was obtained. It has been found that the creep crack growth characteristics of the alloy can be predicted satisfactorily from a model of the accumulation of damage at a crack tip using the creep fracture mechanics parameter C* and assuming plane stress conditions.

Uniaxial Creep Deformation of Shape Memory Polymer of Polyurethane Series.

The uniaxial creep deformation of shape memory polymer of polyurethane series at temperatures higher than the glass transition temperature was investigated experimentally. The main results are summarized as follows. ( 1 ) The time-independent strain is recoverable and does not vary under cyclic loading. The time-independent strain is represented by the sum of linear and nonlinear terms of stress. ( 2 ) The time-dependent strain is resolved to be a recoverable component and an irrecoverable component. The recoverable strain does not vary under cyclic loading. The irrecoverable strain appears only during the first loading process. ( 3 ) A creep limit exists for the time-dependent strain. The time-dependent strain is represented by a power function of time.

Deformation and subcritical crack growth under static loading

Abstract To examine the effect of creep deformation rate on crack growth, an analytical relationship between the kinetics of crack growth in an inert environment and the deformation kinetics is developed by combining a tensile ligament instability model for crack growth with Hart's model for uniaxial creep deformation. The modified model was examined in terms of the steady state creep and crack growth data of Landes and Wei on AISI 4340 steel at 297, 353 and 413 K (i.e. below the homologous temperature of about 450 K). The model was found to be effective in describing the crack growth response at high stress intensity factor K levels and showed a threshold crack growth behavior at lower K levels. Changes in crack growth response with deformation mechanism and the need for additional experimental confirmation are discussed.

A numerical analysis of void growth in tension creep

The uniaxial creep deformation of a material containing a doubly periodic square array of circular cylindrical voids is studied under plane strain conditions. A variational principle for incompressible nonlinear viscous behavior is used as the basis for implementing a finite element solution. The effects of large, time dependent deformation are included in the formulation. A power law creep relationship is used and results are obtained for the influence of different stress exponents on the overall ductility. The void shape evolution and zones of concentrated deformation are described under increasing overall strain.