Assessment of power law creep constants of Gr91 steel using small punch creep tests

A method for determining the power law creep constants using the small punch (SP) creep test is studied. We performed elastic-plastic-secondary creep finite-element (FE) analysis of Gr91 (ASTM A387 GR91 CL2) steel using the properties at 565 °C to investigate the evolution of stress and strain rate at the weakest location of the SP creep specimen, i.e. at the annular region located at about 0.7 mm from the centre of the specimen. Empirical relations that correlate the applied load to the equivalent stress and the punch displacement rate to the equivalent creep strain rate are suggested on the basis of the finite-element stress analysis results. These simple relations enable us to achieve the constitutive relation of equivalent stress and equivalent creep strain rate under small punch creep test condition. To validate this approach, SP creep tests were conducted and creep constants were evaluated by using the proposed relations. These evaluated creep constants were then compared with those measured from standard uniaxial creep test. It is shown that creep constants evaluated from the SP creep test and the proposed method are in a good agreement with those from the uniaxial creep test.     

Finite element analysis of notched bar skeletal point stresses and dimension changes due to creep

In this investigation, finite element calculations have been performed to obtain the creep stress distributions generated in circumferentially notched bar test‐pieces. They have also been made to determine the relation between axial extension and notch throat diameter changes. It has been found that an approximate skeletal point can be identified where the stress state is insensitive to the power law stress dependence of creep. Consistent trends in skeletal point stress ratios to those given in an existing Code of Practice for notch bar creep testing have been obtained. Nevertheless updated values, particularly for sharp notches, are proposed and these have now been inserted into a new version of the Code of Practice. In contrast, the link between extension and notch throat diameter changes has been found to depend on the creep stress index as well as the notch geometry. It is anticipated that the analysis can be used to establish the multi‐axial creep stress deformation and rupture behaviour of materials.     

A small cantilever beam test for determination of creep properties of materials

The application of small specimen creep test techniques in the evaluation of creep properties of materials in‐service has been increasing. To obtain the creep data accurately and conveniently, a new creep test method with small cantilever beam specimens is proposed. Analytical equations are derived that can convert the load to equivalent uniaxial stress and the displacement rate to equivalent uniaxial strain rate. Three types of the cantilever beam specimens are designed. The optimal configuration of the cantilever beam specimens is recommended with the aid of finite element method, which is further validated by the cantilever beam and uniaxial specimen tests. The results show that parameters obtained from the cantilever beam tests correspond reasonably well with those from uniaxial tests at low stress levels. With a relatively large equivalent gauge length, the cantilever beam specimen allows the small creep strain rate data obtained with a high accuracy.     

Determination of creep behaviour of Ti60 alloy by small punch creep test

The small punch (SP) creep test has distinct advantages in the creep property assessment of materials at elevated temperatures. However, there are few creep properties of Ti alloys obtained by the SP creep test in the current literature. In this paper, the SP creep behaviour of Ti60 alloy has been evaluated under various loads in the range 550–800 N over a temperature range 873–973 K. The SP creep curves obviously indicated the primary, secondary and tertiary stages of creep. The test results have been compared with those of conventional creep tests. The European Code of Practice (CoP) for Small Punch Testing, Dorn equation and Monkman–Grant relationship have also been used to analyse the results of the SP creep tests. The ratio of load of the SP creep tests to equivalent stress of conventional creep tests, the load exponent value of steady deflection rate and activation energy for creep deformation were estimated from the SP creep tests. In conclusion, it was found that dislocation creep may be the main mechanism that dominates the SP creep deformation of Ti60 alloy in the range of load and temperature.     

Comparative study of the estimation of creep crack growth behaviour of TiAI by using a precrack and a notch CT specimens

The fatigue crack growth behaviour of Ti-48.2%A1 intermetallics (full lamellar structure) strongly depends on the microstructure, that is lamellar alignment, and it grows along the lamellar direction. Therefore, it is difficult to introduce a straight fatigue precrack in a CT specimen to provide a standardized specimen for the estimation of the crack growth rate. Concerning high temperature creep brittle materials, since notch opening displacement at the time of creep crack initiation is small, a sharp notched specimen can keep similar stress singularity as a fatigue precracked specimen. In this paper, comparative research on the characteristics of creep deformation and crack extension was performed by using a fatigue precracked and a sharp notched CT specimens. Based on this result, a mechanical model was constructed and finite element stress analyses on the creep deformation for both specimens were performed. By these analytical results, the different behaviour of the characteristics of creep deformation and crack extension between a precrack and a sharp notch specimens were verified mechanically. Furthermore, the validity of using a sharped notched CT specimen to estimate creep crack growth rate was assured for creep brittle materials.     

Nanoindentation creep deformation behaviour of high nitrogen nickel-free austenitic stainless steel

Nanoindentation tests of the high nitrogen nickel-free austenitic stainless steel (HNS) were performed with peak load in a wide range of 100–600?mN to investigate the nanoindentation creep deformation behaviours. The results of the nanoindentation creep tests have demonstrated that the load plateaus, creep strain rate and creep stress of the cold-rolled HNS are larger and its creep stress exponent is smaller than the solution-treated HNS. The analysis reveals that the obvious creep deformation behaviour in the cold-rolled HNS arises from the rapidly relaxed dislocation structures in the initial transition regime, while the small creep deformation behaviour of the solution-treatedHNS is mainly attributed to that the stable dislocation structures for the intensive interactions between dislocations.     

Proposal of a new concept on creep fracture remnant life for a precracked specimen

Abstract

The creep life time of a smooth specimen can be predicted using existing laws for creep deformation and steady state creep rate. When crack growth behaviour is involved, it is necessary to construct a law of creep crack growth rate to predict creep fracture life. Creep fracture life can be measured by integrating the law of creep crack growth rate. One example is the creep crack growth rate, represented by the parameter Q*. In this study, we investigated the applicability of this prediction method to creep fracture remnant life for a cracked specimen. The Ω criterion is proposed to predict creep fracture remnant life for a smooth specimen for creep ductile materials. In this study, the correlation between Q*_L derived from the paremeters Q* and Ω is investigated. The correlation between Q_L* and Ω provided a unified theoretical prediction law of creep fracture remnant life for high-temperature creep-ductile materials in the range from smooth to precracked specimens.     

Creep behaviour of AISI 316H stainless steel under stress-varying creep loading conditions: primary creep regeneration

Primary creep regeneration (PCR) is an important reported observation from creep under stress-varying conditions for several alloys. For a specimen deforming in the secondary creep regime, a stress reversal leads to an enhanced creep rate upon reloading due to reactivation of the primary creep regime (i.e. PCR). This paper focuses on an investigation of the PCR phenomenon during stress-varying creep loading for AISI 316H stainless steel at 650°C. The experimental observations clarify the influence of different parameters (e.g. forward creep stress level, reverse stress magnitude and forward and reverse accumulated inelastic strain) on the extent of PCR activation. In addition, a correlation between the extent of PCR activation and inelastic strain accumulation during the reverse loading period was found, which was employed to develop an empirical–phenomenological model for prediction of the creep behaviour of the alloy after stress transients (e.g. stress reversals).     

Use of small specimen creep data in component life management: a review

Small specimen creep testing techniques are novel mechanical test techniques that have been developed over the past 25 years. They mainly include the sub-size uniaxial test, the small punch creep test, the impression creep test, the small ring creep test and the two-bar creep test. This paper outlines the current methods in practice for data interpretation as well as the state-of-the-art procedures for conducting the tests. Case studies for the use of impression creep testing and material strength ranking of creep resistant steels are reviewed along with the requirement for the standardisation of the impression creep test method. A database of small specimen creep testing is required to prove the validity of the tests.     

Experimental Study and Numerical Modelling of Creep and Stress Relaxation of Dielectric Elastomers

Dielectric elastomers (DEs) are gaining acceptance as potential actuator materials because of their exhibition of a large amount of deformation when stimulated by electrostatic forces. However, time‐dependent behaviour such as creep and stress relaxation still pose a great challenge for the design, modelling and control of the DE‐based actuators. In this work, attempts are made for experimental estimation and modelling of creep and relaxation properties of one of the most widely used dielectric acrylic elastomers, VHB 4910. Experimental investigation shows that the material possesses strong time‐dependent creep and stress relaxation. It has been shown that creep and stress relaxation characteristics vary with the holding stress and holding strain respectively. Creep and stress relaxation properties are also shown to depend on the number of cycles in the case of cyclic loading. Results also show that Findley's power law can successfully model the creep and stress relaxation behaviour of the VHB 4910 elastomer.     

Extension of fibre bundle models for creep rupture and interface failure

We present two extensions of the classical fibre bundle model to study the creep rupture of heterogeneous materials and the shear failure of glued interfaces of solid blocks. To model creep rupture, we assume that the fibres of a parallel bundle present time dependent behaviour under an external load and fail when the deformation exceeds their local breaking threshold. Assuming global load sharing among fibres, analytical and numerical calculations showed that there exists a critical load below which only partial failure occurs while above which the system fails globally after a finite time. Approaching the critical point from both sides the system exhibits scaling behaviour which implies that creep rupture is analogous to continuous phase transitions. To describe interfacial failure, we model the interface as an array of elastic beams which experience stretching and bending under shear load and break if the two deformation modes exceed randomly distributed breaking thresholds. The two breaking modes can be independent or combined in the form of a von Mises type breaking criterion. In the framework of global load sharing, we obtain analytically the macroscopic constitutive behaviour of the system and describe the microscopic process of the progressive failure of the interface.     

Investigation the effect of crystal orientation of nickel‐based single crystal superalloys on bending creep tests

The relationship between the three points bending creep test and the uni‐axial creep test on the single crystal superalloy was investigated by using crystal plasticity slip theory with a three‐dimensional (3D) finite element model. The purpose of the present work is to build the relationship between bending creep and conventional uni‐axial tensile creep in determining crystallographic creep parameters for face centered cubic (FCC) nickel‐based single crystal superalloys. To this aim, the bending creep performed on [001]‐, [011]‐, and [111]‐oriented nickel‐based single crystal superalloys were respectively investigated, and the data was compared with those obtained with uni‐axial tensile creep counterparts. It shows that the determination of crystallographic creep stress exponent is independent of crystallographic orientations, and the results agree reasonably well between bending creep test and uni‐axial tensile creep test. The findings may shed some light on understanding of the crystal structures and its time‐dependent deformation mechanisms with the bending creep method.     

Creep fracture behaviour of SUS304H steel with vanadium addition based on small punch creep testing

The high-temperature creep fracture behaviour and creep strength of SUS 304H containing a minor addition of V were investigated in this study. A series of small punch (SP) creep and uniaxial creep tests were performed at 700 °C. The load and punch displacement rates obtained from the SP creep tests were used to derive conversion equations to determine the equivalent stress and creep strain rates. A converting equation is suggested in this study so that Norton’s secondary power law creep constants obtained from the SP creep testing can be in agreement with those obtained from the uniaxial tensile creep tests. The creep strength of the modified SUS 304H steel containing V was shown to be superior to that without V based on the current results and other available results for type 304H steel.     

Numerical modelling of small disc creep test

Abstract

The small disc creep test is described. The disc creep curves observed need to be related to the creep properties of the material obtained by conventional testing. This relationship should include a means of assessing the creep damage which is initially present in the material and therefore to an estimate of remanent life. There are difficulties with such correlations and these are shown to be related to an imperfect understanding of the complex nature of the small punch test. This understanding can be improved by suitable physical modelling of the deformation occurring. The present paper describes a suitable finite element model and verifies the model against experimental observations of disc creep tests. The model uses a realistic creep deformation law which includes strain hardening, thermal softening and damage accumulation for 0·5Cr0·5Mo0·25V steel. It is shown that the results obtained from the test are sensitive not only to the initial condition of the specimen but also to the conditions of the test (e.g. the values of frictional forces at the punch/specimen interface). The paper makes suggestions of how the model may be used to optimise the disc creep test.     

Room temperature creep and its effect on flow stress in a X70 pipeline steel

This paper studied the phenomenon of room creep deformation and its effect on tensile property of a X70 pipeline steel under stress-control loading pattern using round tensile test specimen. Significant time-dependent deformation under constant load was observed in the steel at room temperature, and the deformation is not only dependent on loading stress rate but also dependent on the loading process. It is also found that the loading-unloading-reloading process reduces the subsequent creep strain, while the occurrence of room temperature creep obviously enhances the subsequent yielding strength and the flow stresses.     

微结构对泡沫材料蠕变性能的影响

针对较低密度开孔泡沫的正四面体模型,通过引入支柱结点处的体积修正使新模型能够用于预测较大密度范围内的泡沫材料的蠕变性能,并且基于该修正模型,分析了斜支柱的弯曲变形机制以及剪切变形机制对蠕变应变率的影响。结果表明:当泡沫材料的相对密度较低时,支柱的弯曲变形机制决定了其蠕变速率;而当相对密度较高时,支柱的剪切变形作用机制开始主导其蠕变速率。通过与实验结果的比较验证了本文预测的有效性。      

Numerical investigation of creep crack growth in cross‐weld CT specimens. Part II: influence of specimen size

A numerical investigation of the influence of specimen size on creep crack growth in cross‐weld CT specimens with material properties of 2.25Cr1Mo at 550 °C is performed. A three‐dimensional large strain and large displacement finite element study is carried out, where the material properties and specimen size are varied under constant load for a total of eight different configurations. The load level is chosen such that the stress intensity factor becomes 20 MPa √m regardless of specimen size. The creep crack growth rate is calculated using a creep ductility‐based damage model, in which the creep strain rate ahead of the crack tip perpendicular to the crack plane is integrated taking the degree of constraint into account. Although the constraint ahead of the crack tip is higher for the larger specimens, the results show that the creep crack growth (CCG) rate is higher for the smaller specimens than for the larger ones. This is due to much higher creep strain rates ahead of the crack tip for the smaller specimens. If, on the other hand, the CCG rate is evaluated under a constant C * condition, the creep crack growth rate is found to be higher for the larger specimens, except when the crack is located in a HAZ embedded in a material with a lower minimum creep strain rate; then, the creep crack growth rate is predicted to be higher for the smaller specimen. In view of these results, it is obvious that the size effect needs to be considered in assessments of defected welded components using results from CCG testing of cross‐weld CT specimens.     

Experimental and numerical investigation of the creep behaviour of Ni‐based superalloy GH4169 under varying loading

The high‐temperature creep experiment of Ni‐based superalloy GH4169 under the constant loading and varying loading conditions was conducted by using the round bar specimens. The creep time‐strain curves under different loading conditions were obtained to study the high‐temperature creep behaviour of GH4169 superalloy. At the same time, the longitudinal and lateral sections near the fracture of creep specimens were observed by the optical microscope, and the specimens with smaller grain corresponded to the larger creep strain rate. In view of the dispersion of the creep curves, the corresponding data processing method was put forward, and on this basis, a model that can describe the 3 stages of creep with certain physical meaning was established. The simulation results are in good agreement with the experimental results, especially the creep deformation under the varying loading condition. The predicted results of the relative time hardening model are closer to the experiment compared with time hardening and strain hardening model. The creep model is realized by the user's material subroutine code in a commercial FEM software package, which can be used as the basis of creep analysis for engineering structures.     

Estimation of anisotropy of mechanical properties in Mg alloys by means of compressive creep tests

A detailed knowledge of dependence of mechanical properties on orientation in materials prepared by directional processes may present an important factor influencing the design of construction parts. Toward this end, the compressive creep testing of short specimens may be useful. Three different magnesium-based materials were subjected to this testing: (i) pure magnesium, (ii) magnesium matrix composite reinforced with 10 vol.% of titanium, and (iii) magnesium alloy WE54. All three materials were prepared through a powder metallurgical route with final hot extrusion. The specimens for creep tests were cut in such a way that their longitudinal axis (i.e., the direction of compressive creep stress) and the axis of extruded bar contained a predestined angle. Two extreme cases can be observed: In pure Mg and in Mg-Ti composites, the dependence of the creep rate is very sensitive to the orientation especially at small inclinations from extrusion axis. The greatest creep resistance is observed in specimens with stress axis parallel to the extrusion axis, the lowest at declinations from 45 to 90°. On the other hand, in WE54 no orientation dependence was observed. Possible explanations of the behavior based on microstructural observations are discussed. __________ Translated from Problemy Prochnosti, No. 1, pp. 125–128, January–February, 2008.