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On the optimization of laser shock peening induced residual stresses
Citation Link: https://doi.org/10.15480/882.1523
Publikationstyp
Doctoral Thesis
Date Issued
2018
Sprache
English
Author(s)
Advisor
Referee
Title Granting Institution
Technische Universität Hamburg-Harburg
Place of Title Granting Institution
Hamburg
Examination Date
2017-12-06
TORE-DOI
There is a strong economic motivation of the aircraft industry to explore novel residual stress-based approaches for the fatigue life extension, repair, and maintenance of the growing fleet of ageing aircrafts, although the effect of residual stresses is not taken into account by the established damage tolerance evaluation methods. Laser shock peening — the most promising life enhancement technique — has already demonstrated great success in regard to the mitigation of fatigue crack growth via deep compressive residual stresses. However, no comprehensive model exists which allows the prediction of generated residual stress fields depending on the laser peening parameters.
Furthermore, the hole drilling method — a well-established technique for determining non-uniform residual stresses in metallic structures — is based on measuring strain relaxations at the material surface caused by the stress redistribution while drilling the hole. However, the hole drilling method assumes linear elastic material behavior and therefore, when measuring high residual stresses approaching the material yield strength, plastic deformation occurs, which in turn leads to errors in stress determination.
In the light of these two points, the present work aims to optimize the laser shock peening process in regard to high residual stress profiles, their correct measurement by the hole drilling method and demonstration of the fatigue crack growth retardation through the laser peening treatment on the laboratory scale.
First, the methodology for the correction of the residual stresses approaching the material yield strength when measuring by the hole drilling is established and experimentally validated. The correction methodology utilizes FE modelling and artificial neural networks. In contrast to the recent studies, the novelty of this methodology lies in the practical and elegant way to correct any non-uniform stress profile for a wide range of stress levels and material behaviors typically used in industrial applications. Therefore, this correction methodology can be applied in industry without changing the procedure of hole drilling measurement.
Second, the laser shock peening process is optimized in regard to the generated residual stress profiles using design of experiments techniques. The strategy involves laser peening treatment with different parameters and subsequent measurement of induced residual stress profiles through hole drilling. The measured stress profiles are subjected to correction using the neural network methodology. After that the regression model is fitted into the experimental data in order to find the relationship between the laser peening parameters and the stress profiles’ shapes. In the final stage, it is experimentally demonstrated that the established regression model provides an accurate prediction of the residual stress profile when using defined laser peening parameters and vice versa.
Third, the regression model obtained in the design of experiments study is used for generating the desired residual stresses in the C(T)50 AA2024-T3 specimens for the fatigue crack propagation test. Significant retardation of the fatigue crack propagation of specimens due to the presence of deep compressive residual stresses is experimentally demonstrated on the laboratory scale.
Furthermore, the hole drilling method — a well-established technique for determining non-uniform residual stresses in metallic structures — is based on measuring strain relaxations at the material surface caused by the stress redistribution while drilling the hole. However, the hole drilling method assumes linear elastic material behavior and therefore, when measuring high residual stresses approaching the material yield strength, plastic deformation occurs, which in turn leads to errors in stress determination.
In the light of these two points, the present work aims to optimize the laser shock peening process in regard to high residual stress profiles, their correct measurement by the hole drilling method and demonstration of the fatigue crack growth retardation through the laser peening treatment on the laboratory scale.
First, the methodology for the correction of the residual stresses approaching the material yield strength when measuring by the hole drilling is established and experimentally validated. The correction methodology utilizes FE modelling and artificial neural networks. In contrast to the recent studies, the novelty of this methodology lies in the practical and elegant way to correct any non-uniform stress profile for a wide range of stress levels and material behaviors typically used in industrial applications. Therefore, this correction methodology can be applied in industry without changing the procedure of hole drilling measurement.
Second, the laser shock peening process is optimized in regard to the generated residual stress profiles using design of experiments techniques. The strategy involves laser peening treatment with different parameters and subsequent measurement of induced residual stress profiles through hole drilling. The measured stress profiles are subjected to correction using the neural network methodology. After that the regression model is fitted into the experimental data in order to find the relationship between the laser peening parameters and the stress profiles’ shapes. In the final stage, it is experimentally demonstrated that the established regression model provides an accurate prediction of the residual stress profile when using defined laser peening parameters and vice versa.
Third, the regression model obtained in the design of experiments study is used for generating the desired residual stresses in the C(T)50 AA2024-T3 specimens for the fatigue crack propagation test. Significant retardation of the fatigue crack propagation of specimens due to the presence of deep compressive residual stresses is experimentally demonstrated on the laboratory scale.
DDC Class
620: Ingenieurwissenschaften
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