Indentation creep

Abstract

From the hot hardness test, information can be obtained concerning the time dependent flow, or creep, of the material beneath the indenter. Analysis of these data for selfsimilar indentation (i.e. indentation using a pyramid or cone) leads to equations from which the power law creep exponent and activation energy for creep can be derived, within limits imposed by the approximations of the method. The technique is used to analyse hot hardness data for metals and ceramics, drawn from a number of sources.

MST/1484     

Determination of the creep exponent of a power-law creep solid using indentation tests

Based on dimensional analysis, we analysed the indentation of a rigid indenter into a power-law creep solid for which the relationship between the stress and the strain rate is given by . It is shown that under a described condition the creep exponent n can be determined without invoking the detail knowledge of the indenter profile and the shape of the indented solid. The result reported herein should be useful for interpreting the data of nanoindentation into a power-law creep solid in the case that the indented solid is not a flat half-space and/or the indenter has tip defects. The performance of the simple method to evaluate the creep exponent is examined by using numerical experiments and its limitations also discussed. An erratum to this article can be found at     

数字散斑相关技术在高分子材料蠕变测试中的应用

将数字散斑相关计算与压痕硬度测试相结合,探讨在洛氏硬度计上测试高分子材料的蠕变性。首先使用CCD连续采集压头移动图像,然后进行散斑计算,根据计算结果绘制出压头位移-时间曲线,据此建立压入深度与时间、材料的蠕变与时间以及应力与时间之间的关系式,最后通过线性拟合得到被测材料的蠕变指数。通过对高密度尼龙、有机玻璃及环氧树脂的试验证实,该方法是可行的,试验得到三种材料的蠕变指数分别为0.12、0.012、0.07。试验中还发现,根据深度计算的硬度值与通过表盘读出的硬度值是不同的,在比较不同高分子材料的硬度时,前者比后者更合理。     

Finer-Scale Extraction of Viscoelastic Properties from Nanoindentation Characterised by Viscoelastic–Plastic Response

Abstract:  Motivated by recent progress in viscoelastic indentation analysis, the identification of viscoelastic properties from materials exhibiting elastic, viscous and plastic material behaviour by means of nanoindentation is dealt with in this paper. Based on existing solutions for pure viscoelastic material behaviour, two methods allowing us to consider the effect of plastic deformation are presented: (i) the so-called double-indentation technique, with the second indentation characterised by pure viscoelastic material response and (ii) the use of spherical indenter geometries instead of commonly used pyramidal indenters avoiding plastic deformation at all. Both methods are applied to three different polymers, giving access to the model parameters of the fractional dash-pot which is used to describe the viscoelastic behaviour. The results obtained are compared with results from standard (single) indentation tests using a Berkovich indenter. Moreover, the influence of the maximum load, determining the amount of plastic material response, on the identified model parameters is investigated. Finally, the creep-compliance functions identified by nanoindentation are compared with the respective macroscopic creep-compliance functions obtained from bending-beam rheometer tests.     

Effects of Loading Conditions on Deformation Process in Indentation

Static indentation experiments are typically performed to characterize the mechanical properties of a material of interest by a rigid indenter of known geometry to various depths. In contrast, dynamic indentation of materials has not been fully studied. Evaluating material performance under dynamic loading conditions is a challenge and we demonstrate that various modelling schemes may be appropriate for different flavours of dynamic indentation. In order to compare underlying thermo-mechanics and deformation processes in a static and dynamic indentation process, indentation of a rigid indenter into a workpiece to a fixed chosen penetration is extensively studied. A nonlinear strain rate and temperature-sensitive material model is used to characterize the macroscopic response of a titanium-based beta-alloy to indentation.     

Impression creep deformation behaviour of 316LN stainless steel

The impression creep deformation behaviour of 316LN SS was investigated from microstructure, substructure, microhardness and profilometry studies of the creep deformed region. Impression creep tests were conducted on 316LN SS in the temperature range of 923–973?K, at different punching stresses in the range of 472–760?MPa. The impression creep deformation was characterised by a hemispherically shaped plastic zone which developed around the indentation. The study revealed the distinct regions under the punch undergoing deformation to different extents. The deformation was found to occur predominently on (111) planes. The dislocations in the highly deformed region were well dispersed in the matrix. The size of the plastic zone was estimated to be ～1·5 times the diameter of the indenter based on the microhardness and profilometry studies. The critical spacing to be maintained between the adjacent indentations was estimated to be >5 times the diameter of indenter.     

Determination of creep properties of P91 by small punch testing

Abstract

Small punch creep testing was conducted to determine creep properties of P91 at 873 K. The procedure followed the European Code of Practice for Small Punch Tests. Results of small punch creep test and finite element analysis were compared and discussed. Based on the results, a practical method was proposed to determine the parameters of Norton creep law more conveniently for material which obey this law. The validity of the proposed method was verified and it is shown that the parameters of the Norton creep law can be determined from only one experimental curve directly.     

Numerical Study of Creep Damage Behavior of Thin Film/Substrate Systems under Indentation

A numerical study has been performed on the creep damage development of the thin film/substrate systems by the Kachanov‐Rabothov damage law. The emphasis was to study the influence of the modulus ratio of the substrate to the thin film, the size of the indenter and the indentation stress. Results show that two obvious damage zones are found ahead of the indenter. One is at the edge of the indenter, the other is at the interface ahead of the indenter edge. The influence of the modulus ratio of the substrate to the thin film on the indentation damage is not obvious before a certain creep time, and later, the greater modulus ratio of the substrate to the thin film has the smaller damage rate. And the indentation depth rate and the damage rate are also affected by the size of the indenter and indentation stress.     

Two different small size testing technique for the creep residual life evaluation of an equiaxed Ni-based superalloy for GT blade and vane application

Two miniaturised creep testing techniques have been applied to a polycrystalline Ni-based superalloy employed for gas turbine blades and vanes. The test campaign was conducted on as-delivered material to assess both testing techniques and evaluate their effectiveness in determining creep residual life and creep residual properties of operated material. The first technique involved small size uniaxial creep testing, conducted on samples with a diameter lower than 3 mm. The diameter effect was deeply studied and a calibration was done to correlate results from standard creep test pieces with those from small diameter samples. The second technique involved the small punch creep test, and the assessment on virgin material enabled determination of the correlation factor, k_sp, to compare small punch test results with uniaxial creep tests. All the collected results were related to the coarse grain macrostructure that characterises this class of alloys and that increases the scatter of the collected experimental results.     

Micro mechanics based on vacancy diffusion coupled with damage mechanics related to creep deformation and prediction of creep fracture life

Abstract

In situ observations of crack growth and damage progression were conducted under creep conditions for P92 and titanium aluminides inter-metallic compound. A proposed analysis of stress induced particle diffusion was applied to stress induced vacancy diffusion. Results obtained from this analysis were successfully correlated with the experimental behaviour of macroscopic damage progression and a theoretical characteristic of creep deformation was derived. It was found to be in good agreement with experimental characteristics of creep deformation. Furthermore, the experimental characteristics of creep damage progression which concern voids and micro crack formations at grain boundary were found to be well correlated with those of deformation. From these results, correlation between vacancy diffusion in nano-scale, creep damage in mezzo-scale and creep deformation in macro-scale were successfully realized.     

Impression creep; a new creep test

A new indentation creep test is introduced in which the indenter is a circular cylinder with a flat end. Unlike conventional indentation tests, a steady-state velocity is observed in this new test shortly after a transient period during which the indenter makes a shallow impression on the surface of the specimen; hence the name impression creep. This steady-state velocity is found to have the same stress and temperature dependences as the conventional undirectional creep tests using bulk specimens. Three possible mechanisms are analysed in detail, bulk diffusion, surface diffusion, and dislocation creep. They have different stress and indenter-size (radius) dependences. Experimental results on succinonitrile crystals are reported.     

Incubation periods and indentation creep in lead

The experimental variation of the microhardness of lead with dwell time and temperature is reported. At homologous temperatures close to 0.5T _m, and using low loads (i.e. 0.1 N), a period of zero indentation creep of the order of 300 sec is observed. This phenomenon, called the incubation period, and the subsequent indentation creep behaviour, are explained in terms of restructuring and recovery of the dislocation network beneath the indenter.     

Computer Simulation of the Indentation Creep Tests on Particle-Reinforced Composites

A systematical simulation has been carried out on the indentation creep test on particle-reinforced composites.The deformation ,failure mechanisms and life are analyzed by three reasonable models.The following five factors have been considered simultaneously:creep property of the particle,creep property of the matrix,the shape of the particle, the volume faction of the particle and the size(relative size to the particle )of the indentation indenter.For all the cases,the power law respecting to the applied stress can be used to model the steady indentation creep depth rate of the indenter,and the detail expressions have been presented.The computer simulation is analyzed by the two-phase model and the three-phase model.Two places of the stress concentration are found in the composites.One is ahead of the indentation indenter, where the high stress state is deduced by the edge of theindenter and will decrease rapidly near to a steady value with the creep time The other one is at the interface,where the high stress state is deduced by the misfit of material properties between the particles and matrix.It has been found that the creep dissipation energy density other than a stress parameter can be used to be the criterion to model the debonding of the interfaces.With the criterion of the critical creep dissipation energy density, a power law to the applied stress with negative exponent can be used to model the failure life deduced by the debonding of interfaces.The influences of the shape of the particles and the matching of creep properties of particle and matrix can be discussed for the failure.With a crack model,the further growthe of interface crack is analyzed, and some important experimental phenomena can be predicted.The failure mechanism which the particle will be punched into matrix has been also discussed.The critical differences between the creep properties of the particles and matrix have been calculated, after a parameter has been defined.In the view of competition of failure mechanisms, the best matching of the creep properties of the two phases and the best shape of the particles are discussed for the composite design.     

A three dimensional view of high temperature creep damage

Abstract

A three dimensional view of creep voids in a hydrogen reformer tube is presented. By separating and reconstructing various microstructural features present, the proper representation of creep voids in 3D can be fully studied. Useful measurements of parameters such as void volume, void-to-void distance, and grain boundary angles were obtained. The data presented here represent the initial collection of creep void information for use in various creep void nucleation and growth models. Additional data are currently being collected for material subjected to different service conditions.     

闭孔铝泡沫在静压痕下的力学响应

采用球形压头对闭孔铝泡沫材料进行了准静态压痕实验,研究了不同直径、铝泡沫相对密度及边界条件对铝泡沫的压痕硬度、吸能能力及能量吸收率的影响。研究表明,铝泡沫在球形压头作用下的响应曲线可采用幂函数形式进行描述,幂函数指数随相对密度的增大而线性增加。铝泡沫压痕处的断面显示铝泡沫变形被严格限制在压头之下,铝泡沫的压痕变形是局部的不均匀变形。铝泡沫的压痕硬度及吸能能力均随压头直径的增大而线性减小,但它们却均随铝泡沫相对密度的增大而线性增大;能量吸收率不随压头直径和铝泡沫相对密度而变化。在一定压痕深度范围内,刚性基础和简支条件对铝泡沫的压痕响应影响可以忽略不计。最后基于实验数据分别建立了压痕硬度和吸能能力与压头直径及铝泡沫相对密度的关系。      

Micro–macro creep damage simulation for welded joints

Abstract

Assessment of Type IV creep damage in pipe welds is important for residual life prediction of fossil power plants. Actual creep damage is firstly microscopic, such as the initiation and coalescence of small defects, and lastly macroscopic, such as the propagation of crack-like defects. In this paper, an outline of the micro – macro combined creep damage simulation on the basis of the grain-boundary-fracture-resistance model is shown, and is applied to the creep damage simulation of both low-alloy steel welds and high-chromium steel welds. Firstly modelling of the simulation for low-alloy steel welds such as 2.25Cr –1Mo steel (ASME P-22) is discussed, and application examples are shown such as creep rupture specimens in the laboratory and welds in actual power piping. Secondly a trial is carried out to reproduce the number density of small defects in the heat-affected zone of welded joints of modified 9Cr –1Mo steel (ASME P-91). The possibility of predicting the microscopic damage is demonstrated.     

Dynamic nanoindentation testing for studying thermally activated processes from single to nanocrystalline metals

Nanoindentation experiments are widely used for assessing the local mechanical properties of materials. In recent years some new exciting developments have been performed for also analyzing thermally activated processes using indentation based techniques. This paper focuses on how thermally activated dislocation mechanisms can be assessed by indentation strain rate jump as well as creep testing. Therefore, a small overview is given on thermally activated dislocation mechanism and how indentation data from pointed indenters can be interpreted in terms of uniaxial macroscopic testing. This requires the use of the indentation strain rate as introduced by Lucas and Oliver as well as the concepts of Taylor hardening together with Johnson expanding cavity model.These concepts are then translated to nanoindentation strain rate jump tests as well as nanoindentation long term creep test, where the control of the indenter tip movement as well as the determination of the contact are quite important for reliable data. It is furthermore discussed, that for a steady state hardness test, the interpretation of the hardness data is straightforward and comparable to macroscopic testing. For other conditions where size effects play a major role, hardness data need to be interpreted with consideration for the microstructural length scale with respect to the contact radius.Finally strain rate jump testing and long term creep testings are used to assess different thermally activated mechanisms in single to nanocrystalline metals such as: Motion of dislocation kink pairs in bcc sx-W, Grain boundary processes in nc-Ni and ufg-Al, and the Portevin-le Chatelier effect in ufg-AA6014.     

A new creep damage assessment method for metallic material under variable load conditions at elevated temperature

It is significant for structural design and maintenance to assess behaviour and life under varying load conditions. For structures operating in high‐temperature environments, creep is one mechanism responsible for material failures. In this paper, different damage accumulation rules were reviewed, and a new creep damage assessment method was proposed based on the creep damage tolerance parameter λ and load factor Φ. By introducing the creep damage tolerance parameter λ and the minimum creep rate, the loading process and creep behaviour of the material are taken into account in a damage assessment. The parameters in the model can be obtained by the simple variable load creep test, and the remaining life and strain can be predicted using uniaxial creep test data. To analyse the applicability and accuracy of this new model, the strain histories, the life of step load, and the constant load creep from experiments on a titanium alloy at 500°C were obtained, and the prediction results of the novel and previous methods were carefully investigated.     

Calibration and traceability of notch creep strain measurements

Abstract

Blunt circumferential (Bridgman) notched tensile creep testpieces are used for determining material property data under a multi-axial stress state at high temperatures. Aspects of calibration and traceability for diametral strain measurement are considered, with particular reference to creep strain measurement on Bridgman notched testpieces. Details are given of a new laser interferometer calibrator rig used to provide traceability to the National Measurement system for diametral and axial extensometers used for notch creep strain measurement.     

A comparison of three weld consumables for P92 under creep

Abstract

To encourage realistic power plant weld heat-affected zone failures within reasonable test times in laboratory creep testing, a welding consumable for P92 steel with the best creep performance was sought, to avoid failures in the weld metal itself. Therefore, a short term creep test programme was undertaken to evaluate weld pads manufactured using three commercially available consumables for P92, the results of which are presented in this paper. The effects of weld consumable composition are discussed both in general and with specific reference to the behaviour observed. The best overall and the poorest performing weld consumables were identified by composition. Microstructural investigations revealed the presence of precipitate free zones, that more readily formed using the poorest performing consumable as the result of retained δ–ferrite, since its deposited chemical composition led to the lowest value of δ–ferrite formation temperature. A consumable not based on P92 but on the stronger steel alloy FB2 did not perform as well as expected, confirming previous suggestions that the compositional factors which make a parent material creep-strong do not apply on a one-to-one basis to weld metals.