Thomas Bouchenot

Controlled fragmentation of multimaterial fibres and films via polymer cold-drawing

Polymer cold-drawing is a process in which tensile stress reduces the diameter of a drawn fibre (or thickness of a drawn film) and orients the polymeric chains. Cold-drawing has long been used in industrial applications, including the production of flexible fibres with high tensile strength such as polyester and nylon. However, cold-drawing of a composite structure has been less studied. Here we show that in a multimaterial fibre composed of a brittle core embedded in a ductile polymer cladding, cold-drawing results in a surprising phenomenon: controllable and sequential fragmentation of the core to produce uniformly sized rods along metres of fibre, rather than the expected random or chaotic fragmentation. These embedded structures arise from mechanical–geometric instabilities associated with ‘neck’ propagation. Embedded, structured multimaterial threads with complex transverse geometry are thus fragmented into a periodic train of rods held stationary in the polymer cladding. These rods can then be easily extracted via selective dissolution of the cladding, or can self-heal by thermal restoration to re-form the brittle thread. Our method is also applicable to composites with flat rather than cylindrical geometries, in which case cold-drawing leads to the break-up of an embedded or coated brittle film into narrow parallel strips that are aligned normally to the drawing axis. A range of materials was explored to establish the universality of this effect, including silicon, germanium, gold, glasses, silk, polystyrene, biodegradable polymers and ice. We observe, and verify through nonlinear finite-element simulations, a linear relationship between the smallest transverse scale and the longitudinal break-up period. These results may lead to the development of dynamical and thermoreversible camouflaging via a nanoscale Venetian-blind effect, and the fabrication of large-area structured surfaces that facilitate high-sensitivity bio-detection.

An Analytical Stress-Strain Hysteresis Model for a Directionally-Solidified Superalloy Under Thermomechanical Fatigue

Cyclic plasticity and creep are the primary design considerations of 1st and 2nd stage gas turbine blades. Directionally-solidified (DS) Ni-base materials have been developed to provide (1) greater creep ductility and (2) lower minimum creep rate in solidification direction compared to other directions. Tracking the evolution of deformation in DS structures necessitates a constitutive model having the functionality to capture rate-, temperature-, history-, and orientation-dependence. Historically, models rooted in microstructurally-based viscoplasticity simulate the response of long-crystal, dual-phase Ni-base superalloys with extraordinary fidelity; however, a macroscopic approach having reduced order is leveraged to simulate LCF, creep, and creep-fatigue responses with equally high accuracy. This study applies uncoupled creep and plasticity models to predict the TMF of a generic DS Ni-base, and an anisotropic yield theory accounts for transversely-isotropic strength. Due to the fully analytic determination of material constants from mechanical test data, the model can be readily tuned for materials in either peak- or base-loaded units. Application of the model via a parametric study reveals trends in the stabilized hysteresis response of under isothermal fatigue, creep-fatigue, thermomechanical fatigue, and conditions representative of in-service components. Though frequently considered in design and maintenance of turbine materials, non-isothermal fatigue has yet to be accurately predicted for a generalized set of loading conditions. The formulations presented in this study address this knowledge gap using extensions of traditional power law constitutive models.Copyright

A Microstructurally-Informed, Continuum-Level Life Prediction Model for Thermo-Acousto-Mechanically Fatigued Ti-6242S and IN617

Cumulative damage theory has been extended to develop a deterministic fatigue life prediction model for materials subjected to superimposed thermo-acousto-mechanical cycling. This loading profile is exhibited by particular external panels of reusable hypersonic cruise vehicles. Both creep and fatigue (LCF) test data have been acquired for two candidate fuselage materials: a Ni-base alloy (IN617) and a near-alpha titanium alloy (Ti-6242S). Although both materials exhibit considerable strength against isothermal fatigue and/or creep damage, their oxidative responses gives rise to surface-form cracks under cyclic temperature conditions. Damage modules for fatigue, creep, and coupled environmentalfatigue life are developed to predict fatigue life under service conditions. Although a module is also included for acoustic cycling, the combined model is demonstrated for available data.

Approach for Stabilized Peak/Valley Stress Modeling of Non-Isothermal Fatigue of a DS Ni-base Superalloy

Turbine blades derived from directionally solidified (DS) Ni-base superalloys are increasingly employed in the first and second stages of gas turbine engines, where thermal and mechanical cycling facilitate cyclic plasticity and creep. The elongated grains, which are aligned with the primary stress axis of the component, provide (1) greater creep ductility, and (2) lower minimum creep rate in solidification direction compared to other directions. Tracking the evolution of deformation in these structures necessitates a constitutive model having the functionality to capture rate, temperature, history, and orientation dependence. Historically, models rooted in microstructurally based viscoplasticity simulate the response of long-crystal, dual-phase, Ni-base superalloys with extraordinary fidelity; however, a macroscopic approach having reduced order is leveraged to simulate low-cycle fatigue (LCF), creep, and creep-fatigue responses with equally high accuracy. This study applies uncoupled creep and plasticity models to predict the thermomechanical fatigue (TMF) of a generic DS Ni-base, and an anisotropic yield theory accounts for transversely isotropic strength. The microstructure of the subject material contains γ-matrix (FCC Ni) and γ′-particles (cuboidal Ni3Al). Because of the fully analytic determination of material constants from mechanical test data, the model can be readily tuned for materials in either peak- or base-loaded units. Application of the model via a parametric study reveals trends in the stabilized hysteresis response of under isothermal fatigue, creep fatigue, idealized thermomechanical fatigue, and conditions representative of in-service components. Though frequently considered in design and maintenance of turbine materials, non-isothermal fatigue has yet to be accurately predicted for a generalized set of loading conditions. The formulations presented in this study address this knowledge gap using extensions of traditional Ramberg-Osgood and Masing models.