Residual Stress Assessment in Surface-treated Nickel-base Superalloys

Experimental results are presented to illustrate that there exists a unique window of opportunity for eddy current NDE of residual stress in surface-treated nickel-base superalloys. In light of its frequency-dependent penetration depth, the measurement of eddy current conductivity has been suggested as a possible means to allow the nondestructive evaluation of subsurface residual stresses in surface-treated components. This technique is based on the so-called electro-elastic effect, i.e., the stress-dependence of the electrical conductivity. In contrast with most other materials, surface-treated nickel-base superalloys exhibit an apparent increase in electrical conductivity at increasing inspection frequencies. This observation by itself indicates that in these materials the measured conductivity change is dominated by residual stress effects, since both surface roughness and increased dislocation density are known to decrease rather than increase the conductivity and the presence of crystallographic texture does not affect the electrical conductivity of these materials, which crystallize in cubic symmetry. Experimental results indicate that the frequency-dependent apparent eddy current conductivity of shot-peened nickel-base superalloys can be used to estimate both the absolute level of the sub-surface stress and the penetration depth of the compressive layer. The eddy current results correlate well with residual stress profiles obtained by destructive X-ray diffraction measurements both before and after partial thermal relaxation. Introduction: Shot peening is known to improve the resistance to fatigue and foreign-object damage in metallic components due to its damage arresting qualities. This surface enhancement process, which introduces beneficial residual stresses and hardens the surface, is widely used in a number of industrial applications, including gasturbine engines. Modern aircraft turbine engine components are designed using a damage-tolerance philosophy that allows the prediction of a given component’s useful service life based on fracture mechanics and structural analysis. However, the fatigue life improvement gained via surface enhancement is not explicitly accounted for in current engine component life management processes and there would be a significant potential for increasing the predicted damage tolerance capabilities of components if beneficial residual stress considerations could be incorporated into the life prediction methodology. A major barrier to introducing subsurface residual stress information into life prediction models is the necessity to make accurate and reliable nondestructive measurements on shot-peened hardware. Due to its frequency-dependent penetration depth, eddy current inspection is an obvious candidate for use in characterizing the residual stresses resulting from shot peening [1-9]. Unfortunately, the eddy current conductivity is affected by a great number of variables beside residual stress, such as chemical composition, microstructure, plastic deformation, hardness, surface roughness, temperature, etc. Scientific evidence indicates that, even when the eddy current measurements are conducted with sufficient precision, the obtained parameters are affected by not only the existing residual stress profile, but also by the accompanying cold work [8] and surface roughness effects [7,10,11]. The penetration depth of the cold worked region is typically one third of that of the compressive residual stress, therefore, just like the surface roughness effect, cold work effects cannot be eliminated simply by an appropriate selection of the inspection frequency. Generally, cold work exhibits itself through lattice imperfections, e.g., increased dislocation density, and localized anisotropy caused by crystallographic and morphological texture. Separation of the residual stress and cold work effects requires careful optimization of the inspection method on a case-to-case basis. For example, crystallographic anisotropy strongly affects ultrasonic surface acoustic wave (SAW) measurements in all metals except those of very low elastic anisotropy like tungsten or aluminum, but has no effect on eddy current and thermoelectric measurements in metals that crystallize in cubic symmetry, a broad category that includes essentially all engine materials with the notable exception of titanium alloys [12,13]. In most metals the stress-dependence of the electrical conductivity is rather weak and the primary residual stress effect is very difficult to separate from the secondary cold work effect and, especially in shot-peened specimens, from the apparent loss of conductivity caused by the spurious surface roughness effect. In paramagnetic materials the electrical conductivity typically increases by approximately 1% under a maximum biaxial compressive stress equal to the yield strength of the material. However, the electrical conductivity measured on shot-peened specimens typically decreases with increasing peening intensity, often as much as 1-2%, which indicates that surface roughness and cold work effects dominate the observed phenomenon. We recently found that, in sharp contrast with most other materials, shotpeened nickel-base superalloy specimens exhibit an apparent increase in electrical conductivity at increasing inspection frequencies and demonstrated that the main reason for this behavior lies in the favorable piezoresistive properties of these alloys [14]. In the presence of elastic stress [τ] the electrical conductivity [σ] tensor of an otherwise isotropic conductor becomes slightly anisotropic. In general, the stress-dependence of the electrical resistivity is described by the fourth-order piezoresistivity [π] tensor [15,16]. Isotropic materials can be fully characterized by two independent parameters, namely the parallel (π11 / ) and normal (π12) piezoresistivity coefficients. For easier comparison to our eddy current measurements we are going to consider the stress-induced change in the electrical conductivity rather than in the electrical resistivity. In direct analogy to the well-known acoustoelastic effect, a widely used NDE terminology for the dependence of the acoustic velocity on elastic stress, we are going to refer to the stressdependence of the electrical conductivity as the electroelastic effect. In the three principal directions, the relative conductivity change can be expressed as follows