ABSTRACT

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 magnetostrictive 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 20 MPa, which is within an acceptable range considering that yielding and ultimate load capacity are discussed in the range greater than 250 MPa for normal steel.