Recent Developments in Measuring Creep Strain in High Temperature Plant Components

Accurate measurements of creep strain are necessary to evaluate the condition and predict the remaining life of power plant constituent materials. Optical techniques are appropriate for this purpose as they are a non‐contact method and can therefore be used to measure strain without requiring direct access to the surface. Within this class of techniques, the Auto‐Reference Creep Management And Control (ARCMAC) camera system can be used to calculate the strain between two points using a series of silicon nitride (SiN) target spheres (the ARCMAC gauge). There are two iterations in system design, the Conventional ARCMAC and Digital Single‐Lens Reflex (DSLR) ARCMAC. Experiments are conducted to determine the absolute limit of accuracy of the systems in comparison to a strain gauge, and the relative accuracy across several orders of magnitude until specimen failure. In addition, tests have been performed using the ARCMAC gauge at elevated temperatures to evaluate the effect of temperature on the gauges and to investigate whether its accuracy diminishes in creep conditions. It was found that both conventional and DSLR ARCMAC systems can be accurate to 60 με or less. In accelerated creep tests, the ARCMAC gauge produced similar agreement to a linear variable displacement transducer when used to measure creep strain. Strain variations (under 500 με) were noted on a steel plate subjected only to operational temperature and no stress. This error is very reasonable compared to a critical strain value of 93 000 με in a given high temperature‐service material. Digital image correlation (DIC) results using the DSLR ARCMAC system show approximately 4% error in measurement for plastic strains in the specimen. The two measures of strain measurement (using ARCMAC and DIC) can serve to complement each other.     

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.     

UTILIZATION OF STRAIN GAUGES OVER LONG PERIODS OF TIME AT ELEVATED TEMPERATURES

Tests are described for the evaluation of a particular strain gauge/cement combination under steady loading at elevated temperatures over long periods of time. Although the strain gauges showed a relaxation of their reading with time, due to creep of the gauge backing and cement, it proved possible to correct for this effect to within the scatter associated with the specimens and the instrumentation. It is concluded that application of such corrections to other strain gauge/cement combinations should be possible.     

Influence of Boundary Conditions on the Finite Element Creep Stress and Strain Analysis of a Double Shear Specimen for Isotropic Creep Conditions

The present study investigates the influence of boundary conditions on creep finite element stress and strain results for a double shear creep specimen which was developed by Mayr et al. [1]. The results of the study show that a preliminary analysis [2], which was performed using the finite element code ABAQUS using RIGID SURFACE as a boundary condition for loading is conservative, because it predicts that a homogeneous state of shear stress is only maintained up to shear strains of the order of 10%. Using a more realistic boundary condition for loading (ABAQUS option: CONTACT PAIR) it can be shown that the homogeneous state of shear stress in the specimen is maintained throughout creep up to much higher shear strains than originally estimated. Further anisotropic finite element creep stress analysis of our double shear creep specimen will therefore be based on this more realistic loading condition.     

Characterization of the elevated temperature static load crack extension behavior of type 304 stainless steel

Creep crack extension rates in Type 304 stainless steel, obtained as a function of temperature over the range 650–800°C and as a function of specimen geometry at 750°C, are empirically correlated with both the net section stress and the apparent stress intensity factor. The results indicate that the stress intensity correlation is strongly dependent on specimen geometry, whereas the net section stress correlation appears to be generally valid. A direct correspondence between crack extension and local (crack tip) displacement is noted when creep crack extension rates at 750°C are compared with COD obtained from actual castings of the crack tip. By introducing the concept of a miniature creep specimen at the crack tip, a physical model for creep crack growth is developed, based on local stress relaxation and strain accumulation, that is consistent with both experimental observation and existing theories of steady state creep.     

Self-compensated high plastic resistance strain gauges for use up to 300°C

The development of a high plastic strain gauge for use up to 300C is presented. This kind of gauge can measure strain up to 8% and possesses good self compensation on carbon steel. Basic analysis, gauge wire, adhesive material, gauge structure and examples of application are also described.     

Development of a Vibrating Wire Strain Gauge for Measuring Small Strains in Concrete Beams

Abstract:  A vibrating wire strain gauge capable of measuring strains in concrete elements to an accuracy of better than 0.5  μ ɛ is presented. This offers some advantages over conventional electrical resistance gauges, the quoted accuracy of which is typically 3  μ ɛ , and which are often considered unsuitable for concrete because of their inability to span cracks. While vibrating wire gauges are potentially more accurate, they are prone to significant errors because of temperature changes. In the purpose-built gauge described here, temperature correction is achieved using an unstrained reference gauge. The vibration data are analysed using a moving-window Fourier transformation in order to identify and remove the geometrically nonlinear portion of the response. The resulting system is accurate, economical and easy to use. The gauges have been used to study the behaviour of cracked concrete specimens. Typical results are presented and discussed.     

Variation of monotonic strain in copper thin films during fatigue testing

The plastic deformation behavior is important to evaluate the fatigue damage of copper thin films. In spite of the importance of the deformation behavior of thin films with load cycling, there is no trial to measure the strain directly on the surface during fatigue testing because of difficulties in measuring the strain.In this study, the monotonic strain of copper films of 12 μm thickness during fatigue testing was measured by using the DIC method. The DIC method provides full field deformation of the specimen with high precision, and can directly measure the strain of the gauge section without any assumption. With increasing number of cycles, the monotonic strain increases similarly to the conventional creep curve, and the fluctuations of the strain on the specimen surface seriously increases and the increase in the fluctuation range is very large compared to that in the mean strain due to the roughness of the specimen.     

Correcting for the effects of temperature on strain gauges —-use of computer–logger on Milford Haven Bridge project

The effects of temperature changes on strain gauge readings depend partly on the characteristics of the gauge and measuring equipment, and partly on the thermal properties of the structure being tested. The effect on the gauge can be numerically corrected, or compensated for by experimental techniques, and if required, the effect of temperature changes on the structure can be reduced by careful selection of the time of reading. Correction of Demec, vibrating wire and electrical resistance strain gauge outputs is discussed.
Experience with a computer controlled data logger used to correct for thermal effects on a 1/4-bridge electrical resistance strain gauge installation is described. The use of this system on the Milford Haven Bridge is estimated to have reduced potential temperature errors from between ±100 to 200 μstrain to about ±20 to 30 μstrain.     

Viscoplastic–plastic modelling of IN792

At high temperatures metallic materials behave in a viscous manner exemplified by strain rate dependence, stress relaxation and creep deformation. At low temperatures however, these effects are extremely small, and the behaviour is strain rate independent and shows no or very small relaxation effects. Finally there exists an intermediate region, in which the material behaviour is close to strain rate independent for high strain rates but at the same time shows time dependent inelastic effects, such as stress relaxation and creep. For IN792 this occurs at temperatures around 650 °C. The article describes the extension of a power-law viscoplastic model describing the behaviour of IN792 at 850 °C, also to describe the behaviour at 650 °C, by bounding the elastic–viscoplastic stress-space by a plastic yield surface. The model parameters have been estimated using data from creep test and tailored step relaxation tests, and the model fits well to both the step relaxation data aimed at resembling relevant component conditions and long term creep data.     

Protection of vibrating wire surface strain gauges

To protect vibrating wire gauges against damage from construction work and weather, a cover has been designed which permits access to the gauge for manual and automatic readings and which does not interfere with gauge performance. The protected gauges have been used to measure strains on steel plates and stiffeners forming the diaphragm, webs and flanges of the box girder of the Milford Haven Bridge. 94 of the 100 gauges installed were still operating satisfactorily after 16 months use in very adverse conditions.     

Creep properties of a U‐type notch plate in Ni‐based superalloy

In the present investigation, the effect of notch on creep rupture behavior and creep rupture life of a Ni‐based superalloy has been assessed by performing creep tests on smooth and U‐notched plate specimen under 0°C. The finite element analysis coupled with continuum damage mechanics are carried out to understand the stress distribution across the notch throat and the creep damage evolution under multi‐axial stress state. The creep rupture life of U‐notched specimen is much larger than that of plane plate specimen under the same stress condition, indicating that there is a strengthening effect on notch specimen. Creep rupture life increases with increasing the notch radius, the smaller notch radius can induce the creep rupture easier. The effect of notch on the creep damage is also studied. It is found that the location of the maximum creep damage and the maximum equivalent creep strain initiates first at the notch root and gradually moves to the inside as the notch radius increases.     

Ratcheting‐fatigue behaviour of bainite 2.25Cr1MoV steel with tensile and compressed hold loading at 455°C

The ratcheting behaviour of a bainite 2.25Cr1MoV steel was studied with various hold periods at 455°C. Particular attention was paid to the effect of stress hold on whole‐life ratcheting deformation, fatigue life, and failure mechanism. Results indicate that longer peak hold periods stimulate a faster accumulation of ratcheting strain by contribution of creep strain, while double hold at peak and valley stress has an even stronger influence. Creep strains produced in peak and valley hold periods are noticeable and result in higher cyclic strain amplitudes. Dimples and acquired defects are found in failed specimen by microstructure observation, and their number and size increase under creep‐fatigue loading. Enlarged cyclic strain amplitude and material deterioration caused by creep lead to fatigue life reduction under creep‐fatigue loading. A life prediction model suitable for asymmetric cycling is proposed based on the linear damage summation rule.     

Effects of anelastic deformation on high-temperature stress relaxation of polycrystalline MgO and Al2O3

High-temperature (1160 to 1450 C) deformation of dense polycrystalline (10 to 90 m) Al₂O₃ and MgO doped with Fe (up to 2.65 cation %) was studied by stress relaxation, dead-load creep and creep recovery. In some cases, all three deformation tests were conducted on a single specimen. A comparison of strain rate-stress data calculated from both stress relaxation and dead-load creep experiments revealed discrepancies in both the magnitude of the strain rates and the dependence between the strain rate and stress. These differences were attributed to the existence of anelastic deformation effects. The correction of stress relaxation data in the low stress regime for linear anelasticity led to strain rate-stress data in reasonably close agreement with results obtained from dead-load creep tests conducted in the viscous creep regime. Creep recovery experiments indicated that anelastic deformation in these ceramic materials was relatively insensitive to changes in temperature and grain size over the range of variables studied.     

Creep in polycrystalline aluminium

Abstract

The present paper reports a study of creep in large grained polycrystalline aluminium. Two techniques were employed to understand this process in more detail. The first was stereoimaging to measure the local strain in the specimen. This measurement was made by photographing the specimen at various times during the creep test, which was carried out in a scanning electron microscope, and using these images to calculate the strain. The second technique was analysis of electron backscattering diffraction patterns, as generated in a scanning electron microscope. This technique allowed examination of changes in crystallography that accompanied the creep process. The results of the experiments showed that strain built up in different grains at different rates. There was never a discontinuity in strain across the grain boundary, and strain relaxation was observed in different grains at different times during the test. Recrystallisation was also observed to occur. In some cases, an existing grain migrated into another grain, presumably through strain induced grain boundary migration. In other cases, there appeared to be nucleation of a new grain with a different orientation.     

Creep rupture and viscoelastoplasticity of polypropylene

Observations are reported on isotactic polypropylene in tensile tests with various strain rates, relaxation tests at various strains, and creep tests with various stresses at room temperature. Constitutive equations are derived for the viscoelastic and viscoplastic responses of semicrystalline polymers at three-dimensional deformations with small strains. The stress-strain relations involve eight material constants that are found by fitting the experimental data. The model is applied to the numerical analysis of creep failure of polypropylene under various deformation modes (uniaxial tension, equi-biaxial tension, shear, multiple-step creep tests).     

Piezo-resistive semi-conductor gauges

Though the piezo–resistive semi–conductor strain gauge has a very high gauge factor compared with wire or foil resistance strain gauges, some precautions ate necessary in its use if a reasonable degree of stability and accuracy are to be attained. The introduction of thermistors in the bridge circuit can compensate for errors caused by temperature changes.     

Calibration of a Speckle Interferometry Full-Field Strain Measurement System

Abstract:  This study describes the calibration of a full-field speckle interferometry strain measurement system using the calibration specimen and protocol defined in the Standardisation Project for Optical Techniques of Strain measurement (SPOTS) standard. The specimen was based on the monolithic embodiment of a four-point bending test and was manufactured from aluminium following the SPOTS design. Strain-gauge rosettes attached to the upper and lower faces of the beam were used to derive two correction factors of an analytical expression that predicted the strains generated in the gauge section of the beam. Following the SPOTS protocol, the comparison of measured and predicted strains yielded two fit parameters and their associated uncertainties for each of three displacement-load steps which indicated the closeness of the data sets. An acceptable calibration was achieved for the single normal component of in-plane strain considered in this study, for each load step employed. For the highest load range, which generated a maximum strain of approximately 810  μ strain in the gauge section, the overall calibration uncertainty was found to be 35.3  μ strain, which in relative terms can be expressed as 2.2% of the strain measurement range for which the instrument was calibrated.     

Finite Element Stress and strain analysis of a double shear creep specimen

This paper reports on creep stress and strain results obtained by the finite element method (FEM) for a double shear specimen which was recently developed [1]. It considers stress redistribution during creep, small changes in specimen geometry during testing and high local stress states at the positions where the specimen is loaded. The results of the present study show that the unconstrained double shear test technique proposed in [1] yields viable results for shear strains of at least up to 0.10. ([1]: C. Mayr et al., Materials Science and Engineering, A 199 (1995) p. 121)