A nondestructive test method determines fatigue of a test ferromagnetic construction material by quantifying a change in effective stress due to aging of the test material, in which the magnetic susceptibility (.chi..sub.c) of the test material is measured in its aged state under a magnetic field having a specified intensity (H) according to a relation as expressed by a first equation: c=.chi..sub.c H.sup.3. The magnetic susceptibility (.chi..sub.c) so measured and the magnetic field intensity (H) are put into the first equation, to obtain a susceptibility coefficient (c) of the test material. The susceptibility coefficient (c) so obtained is put into a second equation: .sigma.={log (c)-a}/b, where a and b are known constants determined by an internal structure of the test material, to obtain a current tensile stress (.sigma.) of the test material. The current tensile stress (.sigma.) of the test material so obtained is compared with a known, initial tensile stress (.sigma..sub.0) of the same test material, to determine a change in effective tensile stress of the test material.

A strain sensor is formed by integrally stacking the following elements an amorphous magnetic metal layer (amorphous layer) having a reverse magnetostriction effect, two print sheets which are bonded to the upper and lower surfaces of the amorphous magnetic metal layer through insulating layers to form one coil, and an insulating layer arranged on a surface of the coil. This strain sensor is produced in the following steps the step of forming a first conducting pattern by arranging a plurality of first conductors on a first insulating layer to be substantially parallel to each other, the step of forming a second insulating layer on the first conducting pattern, the step of forming an amorphous magnetic metal layer having a reverse magnetostriction effect on the second insulating layer to cross the first conductors constituting the first conducting pattern, the step of forming a third insulating layer on the amorphous magnetic metal layer, the step of forming a second conducting pattern constituted by a plurality of second conductors on the third insulating layer to cross the amorphous magnetic metal layer and to connect the first conductors constituting the first conducting pattern in series with each other, thereby forming a coil spirally wound around the amorphous magnetic metal layer together with the first conductors, and the step of forming a fourth insulating layer on the second conducting pattern.

Torque in a shaft is measured by a torque sensor comprising at least one strain gage. The strain gage has a magnetic circuit comprising a magnetostrictive, soft ferromagnetic element. A drive mechanism applies a first and a second magnetomotive force to the magnetic circuit. The second magnetomotive force has a sense opposite that of the first magnetomotive force. A sensing mechanism senses the state of magnetization of the ferromagnetic element and a detecting mechanism determines the magnitude of that magnetic field applied to the ferromagnetic element by the second magnetomotive force which reduces the magnetization of the element to zero. Torque is derived from the strain indicated by a change in the coercive field of the ferromagnetic element.

Strain within a substrate is measured by attaching thereto a magnetic circuit comprising a magnetostrictive, soft ferromagnetic element, and sensing a change in the coercive field of the element caused by strain therewith.

A system for, and method of, empirically determining stress in a molded package and a power module embodying the system or the method. In one embodiment, the system includes: (1) a sensor, having a magnetic core exhibiting a known complex permeability in a control environment, that is embedded within the molded package and therefore subject to the stress and (2) a measurement circuit, coupled to the sensor, that applies a drive signal to the sensor, measures a response signal received from the sensor and uses the drive signal and the response signal to determine a complex permeability under stress of the core. The magnitude of the stress can then be determined from the core's complex permeability under stress.