Yogesh Kesrinath Potdar

Shot peening simulation using discrete and finite element methods

Modeling shot peening process is very complex as it involves the interaction of metallic surfaces with a large number of shots of very small diameter. Conventionally such problems are solved using the finite element software (such as ABAQUS) to predict the stresses and strains. However, the number of shots involved and the number of elements required in a real-life components for a 100% coverage that lasts a considerable duration of peening make such an approach impracticable. Ideally, a method that is suitable for obtaining residual compressive stresses (RCS) and the amount of plastic deformations with the least computational effort seems a dire need. In this paper, an attempt has been made to address this issue by using the discrete element method (DEM) in combination with the finite element method (FEM) to obtain reasonably accurate predictions of the residual stresses and plastic strains. In the proposed approach, the spatial information of force versus time from the DEM simulation is utilized in the FE Model to solve the shot peening problem as a transient problem. The results show that the RCS distribution obtained closely matches with that of the computationally intensive direct FEM simulation. It has also been established, in this paper, that this method works well even in the situations where the robust unit cell approaches are found to be difficult to handle.

Analytical Model to Predict Thermomechanical Relaxation of Shot Peening Induced Residual Stresses

Shot peening is widely used to improve the fatigue life of engine blades and rotors by inducing compressive residual stress on component surfaces. However, the residual stresses can relax due to exposure at high service temperature and mechanical loading. A physics-motivated analytical solution is developed to predict the residual stress relaxation at high temperature and under mechanical loading. In this thermomechanical relaxation model, the plastic strains in the shot peening layer and the substrate are obtained analytically by using linear kinematic hardening material law, and the plastic strain evolution at high temperature is modeled by using a recovery strain term. The final residual stress as a function of time, temperature, and mechanical loading is obtained analytically by combining this recovery strain with equilibrium and compatibility conditions. The whole method can be implemented into Microsoft Excel, and is easy to use and validate. As a special case, an analytical closed-form solution to predict the pure thermal relaxation of a shot peening residual stress is developed. The model predictions agree satisfactorily with published experimental measurements.

Forced Response Analysis of Steam Turbine Inlet Flow Control Valves

This work presents a comparison of two forced response methodologies implemented in commercial finite element software Ansys for describing the dynamic structural response of a steam turbine combined stop and control valve assembly in realistic operation conditions. The first method employs one-way coupling where the pressure field of a selected acoustic mode from an acoustic modal analysis on the valve cavity is scaled based on a pressure probe measurement and mapped onto the structure followed by a harmonic forced response analysis at the structure natural frequency. This method is called the decoupled model — it is fast and conservative as it assumes the acoustic and the structure modes to coincide providing a worst-case forced response estimate. The second method employs two-way coupling between acoustics and structure vibration. It takes five to ten times longer to run than the decoupled model because of the presence of non-symmetric system matrices and must be run multiple times with inputs spanning the operating condition range. However, the coupled model provides the opportunity for a more optimal design as it does not assume the acoustic and structure modes to line up. For the valve geometry studied in this work the effect of two-way coupling seems significant in some conditions where it can cause changes of up to 50% in the forced response.Copyright

Modeling Thermo-Mechanical Relaxation of Shot Peening Induced Residual Stresses During Engine Operation

Shot peening is widely used to improve the fatigue life of engine blades and rotors by inducing compressive residual stress. However, the residual stresses can relax due to exposure at high service temperature and mechanical loading. A physics-motivated analytical solution was developed to predict the residual stress relaxation at high temperature and under mechanical loading. In this thermo-mechanical relaxation model, the plastic strains in shot peening layer and substrate are obtained analytically by using linear kinematic hardening materials law, and then the plastic strain evolution at high temperature is modeled by using a recovery strain term. The final stress as a function of time, temperature and mechanical loading is obtained analytically by combining this recovery strain with equilibrium and compatibility conditions. The whole method can be implemented into Microsoft (MS) Excel, and is easy to use and validate. As a special case, an analytical closed-form solution to predict pure thermal relaxation of shot peening residual stress is developed. The model predictions agree satisfactorily with published experimental data.Copyright

Evaluation of Finite Element Based Correction of Layer Removal During Xrd Measurements for Geometries and Stress Distributions of Interest in Aircraft Engines

X-Ray diffraction has evolved as one of the most practical methods to measure surface residual stresses. The method is regularly used to measure sub-surface stresses by removing thin surface layers by electro-polishing. When such layer removal is performed, the measured stress needs to be corrected for the stress relaxation and redistribution that occurs because of layer removal. The underlying principles of mechanics for this method are described by Francois et.al. [1]. Most standards on XRD measurements (e.g. SAE HS-784 [2]) cite analytical solutions derived and published in 1958 by Moore and Evans [3] as a recommended correction for simple geometries. We have extended these corrections by developing and implementing analytical solution for internal holes - a geometry of interest in aircraft engine applications.