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Siegmund, Thomas

Publications and source records attributed to Siegmund, Thomas.

11 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

Transient creep‐fatigue crack growth in creep‐brittle materials: Application to Alloy 718

Abstract This study presents incremental finite element computations of creep‐fatigue crack growth in Alloy 718 at in air. Alloy 718 is representative of creep‐brittle materials, in which viscoplastic deformation is restricted near the tip of a growing crack. The computations predict crack growth using a unique combination of an irreversible cohesive zone formulation and a strain gradient viscoplastic material model based on the Kocks–Mecking formulation. Cohesive zone damage parameters are estimated using sustained loading and constant‐amplitude cyclic loading experiments. Computations of crack extension under three different waveforms containing overloads all predict post‐overload retardation. The amount of retardation depends strongly on the overload ratio, consistent with experiments in the literature using similar waveforms. Analysis of the crack‐tip fields demonstrates retardation is associated with unloading in the highly deformed material near the advancing crack tip. Dynamic recovery and geometrically necessary dislocations are shown to significantly influence post‐overload crack extension.

36 MATERIALS SCIENCE↗
DOE OSTI2020

The roles of yield strength mismatch, interface strength, and plastic strain gradients in fatigue crack growth across interfaces

An irreversible cohesive zone model was used to understand how plastic strain gradients and yield strength mismatches affect fatigue crack growth across bi-material and multilayer interfaces for a range of interface strengths. Parametric studies identified regimes of penetration through the interface, deflection into the interface, and a transition where cracks extend on both paths. Plastic strain gradients strongly influence the transition regime, and a yield strength mismatch is necessary to cause deflection. For crack growth across an interlayer, interactions between the interfaces occur when the crack-tip plastic zone spans the interlayer. Plastic strain gradients promote penetration through the interlayer.

42 ENGINEERING↗
DOE OSTI2018

New Mechanistic Models of Creep-Fatigue Interactions for Gas Turbine Components (Final Report)

This report documents activities for DE-FE0011796 New Mechanistic Models of Creep-Fatigue Interactions for Gas Turbine Components. As part of the computational mechanics work two rate dependent strain gradient plasticity models are developed. One model is an extension of the Norton model and the other model an extension of the Kocks-Mecking model of unified viscoplasticity. Both models are used to compute results for time dependent deformation problems (gradient creep, void growth, a rotating turbine disk). Detailed numerical verification studies are reported. A plasticity model appointing for the strength differential effect in IN 718 is implemented and employed to analyze indentation experiments. Crack growth simulations are conducted with a cohesive zone model approach combining a rate independent fatigue damage component with a time dependent damage model. Crack growth simulations reported for several combinations of the cohesive zone model with the strain gradient plasticity formulations. Plastic strain gradients lead to local suppression of plastic strain which alters the fatigue crack growth response and crack tip fields locally. It also contributes to the condition of crack deflection and bifurcation at interfaces and interlayers as relevant for the understanding of the crack growth response in certain conditions of IN 718 manufactured additively. IN 718 from a conventional and additive manufacturing route (including a HIP process) are considered. Detailed microstructure characterization is reported. Fatigue crack growth at room temperature and at 650°C are reported for all three material variants, and microscale mechanisms are described.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Final Report for Project 13-4791: New Mechanistic Models of Creep-Fatigue Crack Growth Interactions for Advanced High Temperature Reactor Components

This project developed and validated a novel, multi-scale, mechanism-based model to quantitatively predict creep-fatigue crack growth and failure for Ni-based Alloy 617 at 800°C. Alloy 617 is a target material for intermediate heat exchangers in Generation IV very high temperature reactor designs, and it is envisioned that this model will aid in the design of safe, long lasting nuclear power plants. The technical effectiveness of the model was shown by demonstrating that experimentally observed crack growth rates can be predicted under both steady state and overload crack growth conditions. Feasibility was considered by incorporating our model into a commercially available finite element method code, ABAQUS, that is commonly used by design engineers. While the focus of the project was specifically on an alloy targeted for Generation IV nuclear reactors, the benefits to the public are expected to be wide reaching. Indeed, creep-fatigue failure is a design consideration for a wide range of high temperature mechanical systems that rely on Ni-based alloys, including industrial gas power turbines, advanced ultra-super critical steam turbines, and aerospace turbine engines. It is envisioned that this new model can be adapted to a wide range of engineering applications.

36 MATERIALS SCIENCE↗