In Situ Diagnostics of Damage Accumulation in Ni-Based Superalloys Using High-Temperature Computed Tomography

The design, operation, and performance of a laboratory-scale X-ray computed tomography arrangement that is capable of elevated-temperature deformation studies of superalloys to 800 °C and possibly beyond are reported. The system is optimized for acquisition of three-dimensional (3D) backprojection images recorded sequentially during tensile deformation at strain rates between 10−4 and 10−2 s−1, captured in situ. It is used to characterize the evolution of damage—for example, void formation and microcracking—in Nimonic 80A and Inconel 718 superalloys, which are studied as exemplar polycrystalline alloys with lesser and greater ductility, respectively. the results indicate that such damage can be resolved to within 30 to 50 μm. Collection of temporally and spatially resolved data for the damage evolution during deformation is proven. Hence, the processes leading to creep fracture initiation and final rupture can be quantified in a novel way

Failure in notched tension bars due to high-temperature creep: Interaction between nucleation controlled cavity growth and continuum cavity growth

AbstractFor axi-symmetrically notched tension bars [Dyson, B.F., Loveday, M.S., 1981, Creep Fracture in Nimonic 80A under Tri-axial Tensile Stressing, In: Ponter A.R.S., Hayhurst, D.R. (Eds.), Creep in Structures, Springer-Verlag, Berlin, pp. 406–421] show two types of damage propagation are shown: for low stress, failure propagates from the outside notch surface to the centre-line; and for high stress, failure propagates from the centre-line to the outside notch surface. The objectives of the paper are to: identify the physics of the processes controlling global failure modes; and, describe the global behaviour using physics-based constitutive equations.Two sets of constitutive equations are used to model the softening which takes place in tertiary creep of Nimonic 80A at 750°C. Softening by multiplication of mobile dislocations is firstly combined, for low stress, with softening due to nucleation controlled creep constrained cavity growth; and secondly combined, for high stress, with softening due to continuum void growth. The Continuum Damage Mechanics, CDM, Finite Element Solver DAMAGE XX has been used to study notch creep fracture. Low stress notch behaviour is accurately predicted provided that the constitutive equations take account of the effect of stress level on creep ductility. High stress notch behaviour is accurately predicted from a normalized inverse cavity spacing d/2ℓ=6, and an initial normalized cavity radius rhi/ℓ=3.16×10−3, where 2ℓ is the cavity spacing, and d is the grain size; however, the constants in the strain rate equation required recalibration against high stress notch data. A void nucleation mechanism is postulated for high stress behaviour which involves decohesion where slip bands intersect second phase grain boundary particles. Both equation sets accurately predict experimentally observed global failure modes

The evaluation of high-stress creep ductility for 316 stainless steel at 550 °C by extrapolation of constitutive equations derived for lower stress levels

The CDM-based constitutive equations for the creep of 316 Stainless Steel at 550 °C are reviewed. During creep tests carried out under these conditions, it has been observed that as time elapses inelastic straining takes place due to time independent plasticity and to creep. It has been recognised that at high stress levels the time dependent plastic strain accumulated during constant load creep tests forms a major part of the inelastic strain and dominates over the creep strain. Hence, due to the plastic strain the true stress level is not constant during the test. The time independent plastic strain has been evaluated using a stress–strain curve obtained at a high strain rate, and the creep strains have been evaluated for the relevant stress history by integration of the constitutive equations. Minimum creep rates and lifetimes have been extrapolated from low stresses to higher stresses using linear stress versus logarithmic plots. In this way, the creep strain–time history, the minimum creep rates, lifetimes and ductilities have been evaluated. In the stress range 325–450 MPa a lower shelf ductility of 1.1% has been found. The model is also shown to predict the isochronous rupture locus determined from multi-axial test data obtained from a range of different sources

Predictions of thermo-mechanical behavior of a Nicalon-CAS 0°-90° ceramic matrix composite from constituent materials properties

A physics-based model, developed previously by the authors, for the thermo-mechanical behavior of ceramic matrix composite tows is used to study the performance of a Nicalon-CAS 0°/90° laminate. For this material, the room temperature uni-axial stress—strain curve and the transverse thermal conductivity—strain curves are available from a previous experimental investigation; these curves have been used as benchmarks to assess the fidelity of the model. For the stress—strain behavior, a dynamic fiber failure model is utilized which assumes that the local transverse stressing arising from the 0°/90° composite lay-up instantaneously deactivates fiber pullout and initiates dynamic fiber failure; and hence, triggers catastrophic failure of the axially stressed regions of the tow. The model is shown to accurately predict the experimentally measured stress—strain-failure behavior of the 0°/90° laminate. For the transverse thermal conductivity behavior, the predictions show good agreement with experimental results. Furthermore, it has been confirmed that the effect of the degradation of transverse thermal conductivity is due to strain driven growth of wakedebonded cracks. </jats:p