A magnetic anisotropy sensor is used for nondestructive measurement of stress on surfaces of a ferromagnetic material, such as steel. The sensor is built on the principle of the magneto-strictive effect in which changes in magnetic permeability due to deformation of the material is measured in a nondestructive manner, which then can be translated into the absolute values of stresses existing on the surface. This technique was applied to measure stresses of H-beams used in tunnels. The results show that this method could be one of the promising technologies for non-destructive stress measurement for safe construction of underground structures.

Une sonde magnetique anisotropique est utilisee pour des mesures non-destructives de stress sur la surface d'un metal ferromagnetique, comme l'acier. La sonde est construite sur le principe de l'effet magneto-strictif dans lequel les changements dans la permeabilite magnetique due à la deformation du metal est mesuree avec une manière non-destructive, qui peut être alors traduite en valeurs absolues des stress existants à la surface. Cette technique a ete appliquee pour la mesure de stress des poutres en H utilisees dans les tunnels. Les resultats montrent que cette methode peut être l'une des technologies promettantes concernant les mesures nondestructives de stress pour des constructions sûres de strcutures souterraines.

Ein magnetischer Anisotropie-Sensor wird fuer die zerstörungsfreie Messung von Spannungen an Oberflachen von ferromagnetischen Materialien wie z.B. Stahl verwendet. Der Sensor kann Änderungen in der magnetischen Durchlassigkeit auf Grund von Verformungen des Materials erkennen, die dann in Absolutwerte der Spannungen an der Oberflache umgerechnet werden können. Die Methode wurde auf Messung von Spannungen von H-Balken in Tunnels angewendet. Ergebnisse zeigen, dass dies eine vielversprechende Methode fuer die zerstörungefreie Messung im Tiefbau ist.

Introduction

A magnetic anisotropy sensor is used for nondestructive measurement of stress on surfaces of H-beams used as structural elements in tunnel support systems. The sensor is built on the principle of the magneto-strictive effect in which changes in magnetic permeability due to deformation of a ferromagnetic material, such as steel, are measured and converted to absolute values of stresses existing on the surface of the material. Proper treatment of boundary conditions allows determination of stress tensor completely on surface of H-beam flanges, for example. The level of estimation error, as investigated so far, is around 10 to 20MPa, which is within an acceptable range considering that yielding and ultimate load capacity are discussed in the range greater than 250MPa for normal steel. The sensor was used first to measure stresses on surfaces of four straight H-beams having the length of 2m. Three of them were then bent in a factory to make curved H-beams for tunnel support with approximate radius of 6m, for which the second measurement was conducted. Interpretation of the results from a series of stress measurements shows that (1) significant level of stresses exist already in straight H-beam, (2) alteration of stress level during a bending procedure is significant and part of H-beam after bending experience some plastic deformation.

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