The Effect of Hardness on Eddy Current Residual Stress Profiling in Shot-Peened Nickel Alloys

Recent research results indicate that eddy current conductivity measurements can be exploited for nondestructive evaluation of subsurface residual stresses in surface-treated nickel-base superalloy components. According to this approach, first the depth-dependent electric conductivity profile is calculated from the measured frequency-dependent apparent eddy current conductivity spectrum. Then, the residual stress depth profile is calculated from the conductivity profile based on the piezoresistivity coefficient of the material, which is determined separately from calibration measurements using known external applied stresses. This paper presents new results that indicate that in some popular nickel-base superalloys the relationship between the electric conductivity profile and the sought residual stress profile is more tenuous than previously thought. It is shown that in delta-processed IN718 the relationship is very sensitive to the state of precipitation hardening and, if left uncorrected, could render the eddy current technique unsuitable for residual stress profiling in components of 36 HRC or harder, i.e., in most critical engine applications. The presented experimental results show that the observed dramatic change in the eddy current response of hardened IN718 to surface treatment is caused by very fine nanometer-scale features of the microstructure, such as γ′ and γ″ precipitates, rather than micrometer-scale features, such as changing grain size or δ phase and carbide precipitates.     

Eddy Current Assessment of Near-Surface Residual Stress in Shot-Peened Inhomogeneous Nickel-Base Superalloys

Recently, it has been shown that shot-peened nickel-base superalloys exhibit an approximately 1% increase in apparent eddy current conductivity at high inspection frequencies, which can be exploited for nondestructive subsurface residual stress assessment. Unfortunately, microstructural inhomogeneity in certain as-forged and precipitation hardened nickel-base superalloys, like Waspaloy, can lead to significantly larger electrical conductivity variations of as much as 4–6%. This intrinsic conductivity variation adversely affects the accuracy of residual stress evaluation in shot-peened and subsequently thermal-relaxed specimens, but does not completely prevent it. Experimental results are presented to demonstrate that the conductivity variation resulting from volumetric inhomogeneities in as-forged engine alloys do not display significant frequency dependence. This characteristic independence of frequency can be exploited to distinguish these inhomogeneities from near-surface residual stress and cold work effects caused by surface treatment, which, in contrast, are strongly frequency-dependent.     

On the Influence of Cold Work on Eddy Current Characterization of Near-Surface Residual Stress in Shot-Peened Nickel-Base Superalloys

Shot-peened nickel-base superalloys exhibit 1–2% increase in apparent eddy current conductivity (AECC), which can be exploited for nondestructive residual stress assessment. Experimental evidence indicates that the excess AECC is due in part to elastic strains, i.e., residual stress, and in part to plastic strains, i.e., cold work. The very fact that the conductivity increases rather than decreases was originally thought to indicate that there was no significant cold work contribution to the observed AECC increase. This assumption was also supported by X-ray diffraction (XRD) results on fully relaxed specimens showing that the cold work induced widening of the diffraction beam only partially vanishes when both the residual stress and the AECC completely disappear due to thermal relaxation. However, we show in this paper that assuming that the conductivity change is entirely due to residual stress via the piezoresistivity of the material could result in an unacceptable overestimation of the magnitude of the compressive residual stress. Therefore, we investigated the different mechanisms through which cold work could influence the AECC in surface-treated nickel-base superalloys. It was found that neither the magnetic susceptibility nor the piezoresistivity of the material is affected significantly by cold work up to 50% plastic strain level, but the electrical conductivity does substantially increase due to microstructural changes. Based on these observations, we suggest that in future research the complex variations caused by cold work should be modeled by at least two main types of cold work parameters rather than by a single one in order to properly account for the otherwise contradictory effects of plastic deformation on eddy current conductivity and XRD measurements.     

Eddy Current Assessment of Near-Surface Residual Stress in Shot-Peened Nickel-Base Superalloys

It is shown in this paper that, in contrast with most other materials, shot-peened nickel-base superalloys exhibit an apparent increase in eddy current conductivity at increasing inspection frequencies, which can be exploited for nondestructive residual stress assessment of subsurface residual stresses. It has been found that the primary reason why nickel-base superalloys, which are often used in the most critical gas-turbine engine components, lend themselves easily for eddy current residual stress assessment lies in their favorable electro-elastic behavior, namely that the parallel stress coefficient of the eddy current conductivity has a large negative value while the normal coefficient is smaller but also negative. As a result, the average stress coefficient is also large and negative, therefore the essentially isotropic compressive plane state of stress produced by most surface treatments causes a significant increase in conductivity parallel to the surface. The exact reason for this unusual behavior is presently unknown, but the role of paramagnetic contributions cannot be excluded, therefore the measured quantity will be referred to as apparent eddy current conductivity. Experimental results are presented to demonstrate that the magnitude of the increase in apparent eddy current conductivity correlates well with the initial peening intensity as well as with the remnant residual stress after thermal relaxation.     

Residual Stress Profile Assessment by Eddy Current for Shot Peened Nickel Superalloy

Eddy current (EC) measurements have shown promise toward becoming a nondestructive method of residual stress characterization, particularly for nickel-base superalloys. However, previous studies on shot-peened materials have shown apparent discrepancies between directly measured residual stress profiles and those determined from EC data. Here, we report a study of the inter-relationship among electrical conductivity deviation, residual stress and texture of shot peened materials, in order to improve understanding of the piezoresistivity effect that is essential to the on-going efforts to make EC measurements a viable technique for residual stress assessment. Specifically, we develop a macroscopic piezoresistivity theory for polycrystalline materials influenced by texture. The theory was applied to analyze the swept high frequency eddy current data obtained from a shot peened Inconel 718 sample, which was found to exhibit shot-induced texture in the near surface region using X-ray diffraction (XRD) and orientation imaging microscopy (OIM). The residual stress profile of the peened sample was inverted from EC data using a physics model-based approach, and was found to agree with the residual stress profiles measured independently using the standard layer removal XRD technique.     

Thermoelectric Nondestructive Evaluation of Residual Stress in Shot-Peened Metals

Abstract

This paper presents a noncontacting thermoelectric method that can be used to characterize the prevailing residual stress in shot-peened specimens. This novel method is based on magnetic detection of local thermoelectric currents in the compressed near-surface layer of metals when a temperature gradient is established throughout the specimen. Besides the primary residual stress effect, the thermoelectric method is also sensitive to the secondary “material” effects of shot peening (local texture, increased dislocation density, hardening), but it is entirely insensitive to its “geometrical” by-product, i.e., the rough surface topography. Our experimental results in copper indicate that the developed method is more sensitive to residual stress effects than to the secondary material effects, but unequivocal separation of residual stress relaxation from the parallel decay of secondary cold-work effects is generally not feasible. However, since the ratio of residual stress to cold work is primarily determined by the material and the specific surface treatment used, the thermoelectric method still offers the unique capability of nondestructively monitoring thermomechanical relaxation below the treated surface. Preliminary results on IN100 nickel-base superalloy are also presented to demonstrate that the proposed method might be applicable to a wide range of alloys including high-strength, high-temperature engine materials.     

Investigation of the influence of residual stresses on the thermophysical and thermoelastic properties of silicon nitride ceramic by photothermal and photoacoustic methods

Photodeflection and photoreflection microscopy have been used to show that residual stresses do not influence the thermophysical parameters of silicon nitride ceramic. It was demonstrated that photoreflection microscopy can provide information on the thermophysical properties of ceramics at the level of individual grains. It was established that the theory of photoacoustic signal formation with a piezoelectric recording method based on the Murnaghan model with allowance for the stress dependence of the thermoelastic coupling coefficient can qualitatively explain these experimental data and can be used to estimate the thermoelastic parameters of the ceramic. Pis’ma Zh. Tekh. Fiz. 24, 40–48 (November 12, 1998)     

Calibration of residual stress measurements obtained from EDM hole drilling method using physical material properties

Abstract

When using the electrodischarge machining (EDM) hole drilling method to measure residual stress, the stress induced by the metallurgical transformation layer formed during the drilling process can lead to significant measurement errors. In this study, it is shown that provided the dielectric fluid retains a high level of purity, the stress induced during the drilling process is determined primarily by the thermal conductivity and carbon equivalent of the specimen. Accordingly, these two material properties are used as the basis of two calibration equations designed to compensate the residual stress measurement obtained using the EDM hole drilling method. It is shown that the calibration equations reduce the discrepancy between the actual simulated residual stress values produced by the prestress loadings and the measured residual stress values to less than 8 MPa. The calibration equations provide a significant improvement in the accuracy of the measurement results obtained using the EDM hole drilling technique.     

Mechanical property requirements for aero gas turbine materials

Abstract

The outstanding performance of current military and civil aero gas turbine engines is linked closely to the way in which modern design and manufacturing techniques have become totally integrated with materials designed specifically for operation within the hostile environment of a gas turbine. Advanced titanium alloys are used extensively throughout the compressor and nickel-base superalloys dominate materials application in the turbine. In spite of current achievements, the engine designer is still under severe competitive pressure to improve engine performance still further and this will inevitably lead to even more demanding material requirements. The present paper outlines the continuing trends in engine development and describes the impact these are having on materials technology in general and the mechanical property requirements of nickel-base superalloys in particular.

MST/512     

Pressure dependence of the thermoelastic quotient for three glasses

The interrelationship between the mechanical work done on a material in the elastic range and changes in its thermodynamic properties, that is, between stress and strain, on the one hand, and temperature and entropy, on the other, is known as the Thermoelastic effect. The phenomenon is exactly adiabatic and is characterized by the thermoelastic quotient commonly referred to as thermoelastic constant. The thermoelastic effect can be used for stress analysis by monitoring the stress fluctuations by means of infrared radiometry, Also, it can be applied to study the anharmonicity in materials by measuring the temperature changes associated with adiabatic pressure changes, In this paper thermodynamic expressions are derived for the pressure derivative of the thermoelastic quotient under adiabatic as well as isothermal conditions, The derived expressions are applied to investigate the thermoelastic effect for the three glasses, namely, silica glass, soda-lime silica glass, and lead-silica glass, The isothermal pressure derivative of the thermoelastic quotient is evaluated for the three glasses. The isothermal volume derivative of the Gruneisen function is calculated.     

Effect of Tensile Stress on Hall Coefficients of Nickel-Base Superalloys

This paper reports on Hall Effect measurements on nickel-base superalloys and their stress dependency. The work is motivated by the desire to develop a nondestructive method of characterizing the near-surface protective residual stress in metals. Our approach is based on the assumption that the Hall coefficient deviates under the stress. It is anticipated that stress measurements based on the Hall Effect are less contaminated by cold work and other effects than conductivity-based measurements such as eddy current. The paper focuses on the two superalloys, Inconel 718 and Inconel 600. The challenge is that Hall coefficients are small in metals, and the stress-induced changes are even smaller. To measure the small effect, the lock-in technique was used, with AC injected current and AC magnetic field. It was found that the Hall coefficients indeed vary proportionally to the stress. The proportionality coefficients are significantly larger than what are estimated from the volumetric effect in a free carrier gas model. The temperature and injected current dependences of the Hall coefficients were also measured, while no dependence on the magnetic flux density was observed.     

The influence of dynamic strain aging on fatigue and creep-fatigue characterization of nickel-base solid solution strengthened alloys

Abstract

The nickel-base solid solution alloys, Alloy 617 and Alloy 230, have been observed to exhibit serrated yielding or dynamic strain aging (DSA) in a temperature/strain rate regime of interest for intermediate heat exchangers (IHX) of high temperature nuclear reactors. At 800°C, these nickel-base alloys are prone to large serrated yielding events at relatively low strains. The presence of DSA introduces challenges in characterizing the creep-fatigue and low cycle fatigue behaviour. These challenges include inability to control the target strains as a result of DSA induced strain excursions and distorted hysteresis loops. Methods to eliminate or reduce the influence of DSA on creep-fatigue testing have been investigated, including varying the strain rate, stepping to the target strain, and adjusting servo-hydraulic tuning parameters. It has not been possible to eliminate the impact of serrated flow in the temperature range of interest for these alloys without compromising the desired test protocols.     

Direct Measurements of Magneto-caloric Effect of Gd5Si2Ge2 Alloys in Low Magnetic Field

Direct measurements of the magneto-caloric effect of Gd₅Si₂Ge₂ alloys under heat treatment conditions are investigated by measuring adiabatic temperature change (ΔT _ad) in a magnetic field of 1.5 T using a homemade magnetocaloric effect measuring apparatus. The crystal structure, microstructure as well as the chemical composition of the alloys are measured using X-ray diffraction (XRD), scanning electron microscope (SEM) with energy dispersive X-ray Detector (EDX). It is found that the microstructure of the alloys could be fully homogenized and the impurities in the alloys could be remarkably removed via appropriate heat treatment. As a result, the maximum adiabatic temperature change (ΔT _ad) of the alloy annealed at 1573 K increases by 105% from 1.9 to 3.9 K for a magnetic field change from 0 to 15 kOe when compared to the as arc-melted material, while the magnetic ordering temperature slightly reduced.     

Mechanical properties of conventionally cast,directionally solidified,and single-crystal superalloys

Abstract

In this paper the authors compare the creep and low-cycle fatigue properties of conventional, directionally solidified, and single-crystal castings produced from nickel-base superalloys. A brief historical review describes the reasons for the evolution from wrought to cast product through directionally solidified to modern single-crystal (‘monocrystal’) castings. The influence of microstructural variations produced by the casting conditions, such as porosity and grain size, on creep and low-cycle fatigue properties are illustrated. The important aspects of postsolidification heat treatment, hot isostatic pressing, and the damaging effects of impurities are described for conventional castings. The results of controlling the microstructures produced by directional solidification especially by high temperature gradient solidification are demonstrated by comparing the creep properties of directionally solidified materials with those of the conventionally cast alloys in long-term tests. The creep and low-cycle fatigue properties depend on the stress direction relative to the crystallographic directions of the material for both directionally solidified and single-crystal castings. For single crystals, individual alloys show variable dependences of properties on the crystallographic directions. Directionally solidified materials show advantages in thin sections and are less sensitive to the effects of impurities compared to conventional castings.

MST/329     

Numerical and experimental investigation of thickness effect on residual stress measurement

This article studies the effect of thickness on residual stress measurements both numerically and experimentally. First, the released strain of aluminium specimens with different thicknesses subjected to incremental hole drilling is analysed using an efficient finite element model. From these results, the cumulative calibration coefficients and their variations as a function of plate thickness are determined. Then, the residual stress profile of a 1-mm-thick aluminium plate subjected to incremental hole drilling is investigated by in-plane three-directional moiré interferometry. A comparative study between the principle stress data calculated using the special calibration coefficients that match the thickness and the general ASTM calibration coefficients that are designed for thick plates is carried out. The analysis results demonstrate that the thickness significantly influenced the release strain. Moreover, the mismatch between the calibration coefficients and the thicknesses results in a deviation in the stress data of more than 10%, and the deviation increases with decreasing thickness.

This paper is part of a Themed Issue on Measurement, modelling and mitigation of residual stress.     

Time-dependent creep stress redistribution analysis of thick-walled functionally graded spheres

Time-dependent creep stress redistribution analysis of thick-walled spheres made of functionally graded material (FGM) subjected to an internal pressure and a uniform temperature field is performed using the method of successive elastic solution. The material creep and mechanical properties through the radial graded direction are assumed to obey a simple power-law variation. Total strains are assumed to be the sum of elastic, thermal and creep strains. Creep strains are time, temperature and stress dependent. Using the equations of equilibrium, compatibility and stress–strain relations a differential equation, containing creep strains, for radial stress are obtained. Ignoring creep strains, a closed-form solution for initial thermoelastic stresses at zero time is presented. It has been found that the material in-homogeneity parameterβ has a substantial effect on thermoelastic stresses. From thermoelastic analysis the material identified by β=2 in which a more uniform shear stress distribution occurs throughout the thickness of the FGM sphere is selected for time-dependent stress redistribution analysis. Using the Prandtl–Reuss relations and Norton’s creep constitutive model, history of stresses and strains are obtained. It has been found that radial stress redistributions are not significant, however, major redistributions occur for circumferential and effective stresses. It has also been concluded that stresses and strains are changing with time at a decreasing rate so that there is a saturation condition beyond which not much change occurs. Indeed after 50 years the solution approaches the steady-state condition.     

Thermoelastic analysis for an opening crack in an orthotropic material

The thermoelastic analysis of an opening crack embedded in an orthotropic material is made under applied uniform heat flux and mechanical loadings. To simulate the case of an opening crack filled with a medium, a thermal-medium crack model is proposed. The thermally permeable and impermeable cracks are the limiting ones of the proposed thermal-medium one. The crack-tip thermoelastic fields induced by a crack in an orthotropic material are determined in closed forms. The elastic T-stress can be also obtained explicitly. The effects of applied mechanical loadings and the thermal conductivity of crack interior on the heat flux at the crack surfaces and the mode-II stress intensity factor are investigated through numerical computations. The obtained results reveal that an increase of the thermal conductivity of crack interior decreases the mode-II stress intensity factor. And when an applied mechanical loading is increasing, the mode-II stress intensity factor is rising.     

Thermoelectric Nondestructive Evaluation of Residual Stress in Shot-Peened Metals

This paper presents a noncontacting thermoelectric method that can be used to characterize the prevailing residual stress in shot-peened specimens. This novel method is based on magnetic detection of local thermoelectric currents in the compressed near-surface layer of metals when a temperature gradient is established throughout the specimen. Besides the primary residual stress effect, the thermoelectric method is also sensitive to the secondary ``material' effects of shot peening (local texture, increased dislocation density, hardening), but it is entirely insensitive to its ``geometrical' by-product, i.e., the rough surface topography. Our experimental results in copper indicate that the developed method is more sensitive to residual stress effects than to the secondary material effects, but unequivocal separation of residual stress relaxation from the parallel decay of secondary cold-work effects is generally not feasible. However, since the ratio of residual stress to cold work is primarily determined by the material and the specific surface treatment used, the thermoelectric method still offers the unique capability of nondestructively monitoring thermomechanical relaxation below the treated surface. Preliminary results on IN100 nickel-base superalloy are also presented to demonstrate that the proposed method might be applicable to a wide range of alloys including high-strength, high-temperature engine materials.     

铜及铜合金带材导电率的涡流测试方法

采用涡流法选择不同的测试频率,分别对不同厚度不同叠加层的几种铜带材进行了导电率测试试验,并与双电桥法进行了对比。结果表明,只要测试频率和叠加层数合适,用涡流法完全适合对铜及铜合金带材进行导电率测量,非常适用于规模化生产铜及铜合金带材导电率快速检验的需要。     

Finite-Element Calculations of Remote Field Eddy Current Interactions with Slit Defects

Abstract

Remote field eddy current (RFEC) excitation is a promising approach for detection of the very fine axial cracks typical of stress corrosion cracking (SCC) in pipelines. Interactions between adjacent cracks or slits can enhance responses in some cases. Detailed finite-element modeling was undertaken to establish the behavior and interactions of multiple slits such as those occurring in SCC. Three different field/slit configurations are considered, with anomalous source models used to aid interpretation of the results. The study noted that magnetic perturbations generated by ferromagnetic material tend to be vanishingly small, and that the interactions between multiple cracks give minimal enhancement, indicating that eddy current rather than magnetic field excitation is best for the detection of SCC. With eddy current excitation, field perturbations are generated by even very fine slits, and are larger in non-ferromagnetic material. For non-ferromagnetic pipes, the perturbations tend to merge as a circumferential separation between parallel axial cracks decreases, resulting in significant interaction and signal enhancement.