Process Modeling of Microtextured Region Breakdown in Ti-6242 to Improve Dwell Fatigue Properties
The titanium alloy Ti-6242 (Ti-6Al-2Sn-4Zr-2Mo) has been the structural material of choice for use in high-pressure compressors for gas turbine engines of aircraft due to its high strength-to-weight ratio and excellent high temperature mechanical properties. However, Ti-6242 is susceptible to dwell fatigue at low temperature due to crack growth on low-angle boundaries along the primary α grains, in particular small faceted cracks that link up to form a “quasi-cleavage” surface. These low-angle boundaries occur within so-called microtextured regions (MTR) that consist of many neighboring primary α grains with similarly oriented c-axes of the hexagonal close-packed atomic lattice. Post-mortem observations of specimens have revealed facet clusters along the fracture surface within the MTR. In this work, the crystal plasticity finite element method in WARP3D is used to model the processing of MTR within Ti-6242. Several configurations of MTR within representative microstructures have been simulated under single and multiple loading axes to determine the critical strain to break down the MTR and introduce more disorientation into the microstructure. These and ongoing computational studies provide a complement to experimental data from AFRL and mill processors that demonstrate a load direction dependence on the texture evolution of this material.
Support: Air Force Research Laboratory
3D Experimental and Computation Studies of Crystallographic Effects on Creep and Fracture in Salt Rock
Rock salt, a sedimentary rock classified as an evaporate, forms as a result of evaporation of inland seas or any enclosed bodies of water and can be found in nature as bedded or domal formations. Salt domes often trap oil, gas, and other minerals around their edges. Drilling through rock salt to reach oil reservoirs poses many challenges, including long-term wellbore stability/integrity, casing collapse due to lateral pressure, and drilling fluid-salt interaction. The accuracy with which the fracture behavior of any material can be simulated, including geological materials like rock, hinges upon the fidelity of both the engineering model and the geometrical representation of the cracked body. A key limitation of existing phenomenological creep models is that rock salt’s anisotropic response is not represented at the microstructural level. Presently, there is an apparent lack of crystal-orientation-sensitive models in the literature for creep and fracture in 3D rock specimens coupled with direct measurements of 3D crystal structure. Thus, this unprecedented research aims to combine nondestructive 3D x-ray diffraction (3DXRD), 3D synchrotron micro-computed tomography (SMT) in-situ experimental measurements, and 3D crystal-plasticity modeling to enhance current understanding of creep and crack formation and growth mechanisms in polycrystalline rock.
Support: National Science Foundation Collaborators: Khalid Alshibli
Creep-Life Prediction for Ferritic-Martensitic Steels Using Crystal Plasticity Modeling
Creep rupture strength is one of the high temperature material properties that are employed in establishing the allowable stresses for materials employed in high temperature reactors. Gaining mechanistic understanding of long term deformation and degradation mechanisms such as creep, grain boundary cavitation, and thermal aging will provide guidance for model-based extrapolation of accelerated creep-fatigue experimental data. Therefore, microstructural finite element models are being pursued that combine grain boundary mechanics with dislocation density-based crystal plasticity models. Grain boundary mechanisms under consideration include cavitation, embrittlement, and sliding. The creep deformations within the grains are captured via the evolution of statistically stored dislocations and geometrically necessary dislocations. Additionally, the effect of grain size distribution and misorientation are taken into account when developing the microstructural models. Combining these crystal plasticity and grain boundary mechanisms enables the modeling of creep response across the phases of primary, secondary, and tertiary creep at high temperatures and service load conditions.
Interface Integrity and Debonding in Polymer-Matrix Composites
Composite materials are prevalent across many civil and defense applications due to their excellent mechanical properties such as high strength-to-weight ratio and directionally-dependent stiffness. However, defects in these materials tend to occur at the interfacial bonds either between the fiber and matrix at the micro-scale or between the lamina at the meso-scale. The ability to computationally predict the onset of cracking or debonding in these materials is a critical component both for the initial design of structural components as well as the inspection and life-prediction of in-service materials. The aim of this project has been to develop a Discontinuous Galerkin approach to numerically predict the onset of cracking in these composite materials. By employing an internal damage variable defined along the interface, the method captures the transition between bonded and debonded configurations in a seamless fashion with fewer numerical tuning parameters. In particular, simulations of both slowly-varying as well as impact loading demonstrate that the pre-cracked response is entirely unaffected by the presence of the interface elements. Thus, high-fidelity modeling of composite materials can be performed across a range of in-tact and damaged conditions.
Support: Air Force Research Laboratory
Frictional Response of Bolted Metallic Surfaces
If you have ever heard or felt a rattle in your car while driving on the highway, then you recognize that controlling the behavior of joints is important for maintaining comfortable and safe performance. Simulation of bolted structures remains a significant challenge due to the interaction of surface roughness and lubrication effects at the micro-scale with observed sliding and friction at the macro-scale. The goal of this project has been to develop a multiscale modeling framework for characterization and prediction of vibration and dissipation in bolted joints. Rather than directly resolving the surface roughness profile, the interaction between asperities is incorporated into a physics-based friction model through a statistical summation process. This multiscale constitutive model is embedded within a Discontinuous Galerkin interface formulation that provides unbiased treatment of the contacting surfaces. The resulting framework has been applied to model quasi-static and dynamic hysteresis of bolted lap-joints, yielding response comparable to those observed in experiments.
Support: National Science Foundation
Modeling Heterogeneous Materials Using Low-Order Tetrahedral Elements
Adding reinforcing particles to materials such as elastomers and plastics enhances mechanical properties, including toughness, conductivity, and stiffness, and may also produce emergent functionality if properly designed. Also, deformation and plasticity within grains of polycrystaline materials accommodates load shedding and re-directioning to sustain larger far-field applied forces. Computational modeling of such heterogeneous materials often requires the use of tetrahedral finite elements due to the existence of robust mesh generators that can resolve the complex geometric features. However, traditionally linear tetrahedral elements produce poor-quality solutions except on extremely-refined meshes due to locking and other pathologies. Therefore, we developed a stabilized low-order tetrahedral element for modeling incompressible materials undergoing large deformations that is free from numerical tunable parameters. The element’s enhanced approximation capabilities facilitates the computational proto-typing of such multi-functional materials.
Support: National Science Foundation
© 2018 UTK, Timothy Truster