PhD Thesis, Aitor Lasaosa
Title: Estudio y desarrollo de metodologías para caracterizar perfiles de tensiones residuales, capas endurecidas por inducción y quemas de rectificado mediante técnicas no destructivas de emisión magnética de Barkhausen en componentes sometidos a fatiga
Defense Date: 15/03/2017
Director: Dra. Dña. Ane Martínez de Guereñu Elorza
Co-Director: Dra. Dña. Kizkitza Gurruchaga Echeverría
The quality assurance of manufactured components is becoming increasingly important, especially on those components where their catastrophic failure can lead to human casualties or loss of great amount of money. Traditionally, the quality controls have been done by destructive characterization of a set of components from each batch, which leads to uncertainty on the rest of the batch. In the last few decades, the quality assurance of the whole production by nondestructive evaluation is becoming more common. However, nowadays many properties still cannot be characterized by conventional nondestructive techniques.
The main objective of the present thesis work is the study and development of nondestructive methodologies to characterize three properties of components that are important in their fatigue life for whom nowadays conventional nondestructive techniques do not exist: residual stress depth-profile, induction-hardened layer (depth and surface hardness of the layer) and the defects that can occur during grinding process (grinding burns). The inspection of these properties in 100% of the components could be important for manufacturers (security coefficients could be reduced) and for customers (the maintenance could be enhanced due to a better knowledge of the fatigue life of the components).
In the present PhD dissertation, the magnetic Barkhausen noise (MBN) nondestructive technique is used due to its sensitivity to residual stresses and mechanichal hardness of the components. The MBN is an electromagnetic signal produced due to magnetic domain wall movement during magnetization processes of ferromagnetic materials.
Microstructure of components, residual stresses and other properties of the material affect the properties of this.
The residual stress profile has been studied analyzing the MBN in the time and frequency domains. New methodologies have been developed with the MBN in the time domain, in order to estimate the average residual stress depth-profile (average residual stress from surface to each depth) up to 130 µm depth with an estimation error smaller than 100 MPa. Moreover, with a novel methodology developed in the present study based on the MBN in the frequency domain, the residual stress depth-profile (residual stress at each depth, not the average) has been estimated up to 130 µm with an estimation error smaller than 70 MPa.
The induction hardened layer and the defects that can occur during its grinding (grinding burns) have been studied analyzing the MBN and the magnetization signals. A new methodology has been developed in order to characterize the depth of the hardened layer (case depth) and the surface hardness of this layer and to detect the grinding burns independently of the case depth, with a unique measurement.
This methodology has been implemented in an aeronautic ball screw shaft production line. The case depth has been estimated with an error smaller than 200 µm; and thresholds have been selected to detect ball screw shafts with surface hardness softer than 56 HRC and with grinding burns, independently of the case depth of the ball screw shaft.