FI62732C - SAETT ATT DEFINIERA UTMATTNINGSHAOLLFASTHET HOS FERROMAGNETISKT MATERIAL UTAN ATT BRYTA MATERIALET - Google Patents

Description

62732

METHOD FOR DETERMINING THE FATIGUE STRENGTH OF A FERROMAGNETIC MATERIAL WITHOUT BREAKING A SUBSTANCE

Sät at definiera utmattningshällfasthet hos Ferro-magnetiskt material utan att bryta materialet

This invention relates to a method for measuring the fatigue strength of a ferromagnetic material.

Fatigue resistance is stated in the so-called Using an S-N curve determined experimentally by stressing the specimens with different 5 load amplitudes until fracture. One of the most important details of the S-N curve is the tolerance or fatigue limit, which is the value of the load amplitude below which the stress does not cause fatigue fracture or specimen destruction. The tolerance and fatigue limit is also the most important value of fatigue strength in practice. Ko. it takes 5-10 days to determine the limit in the above way, depending on the quality of the substance testing machine.

Some non-destructive fatigue measurement methods are known which can be used to monitor the development of fatigue in a structure. These methods are characterized in that they require a comparison of the measurement results with previous measurements or with values obtained from a reference sample. Ko. however, the methods have proved to be quite unreliable and cannot determine the above-mentioned major limit, tolerance or fatigue limit. Examples include: fatigue wire-20 methods and coercive force measurements.

The non-destructive fatigue limit measurement method which is the subject of the present invention utilizes the so-called mechanical Barkhausen noise. Mechanical Barkhausen noise is a phenomenon previously known per se and is associated with irreversible changes in the magnetic state of the material (so-called discontinuous movements of domain walls) due to mechanical 2,62732 loading when the body is in a stable magnetic field or in a demagnetized state.

The so-called "Ordinary" Barkhausen noise, based on the effects of a variable external magnetic field, for which a number of devices have been developed for non-destructive measurement. The latter phenomenon is based on U.S. Pat. No. 3,427,872, with which the static load or yield state of a material can be determined by means of an external variable magnetic field. Ko. however, method 10 cannot be used to analyze mechanical Barkhausen noise and cannot determine the fatigue limit of a material.

The method according to the invention is characterized by what is stated in the appended claims. By using the method according to the invention, the significant advantage is achieved that the fatigue or tolerance limit can be determined very quickly in practice, e.g. in about 15 minutes. Thus, with the method according to the invention, it can be determined practically immediately whether the dynamic stress of a machine part is too high.

The method according to the invention is described in more detail in the following description with reference to the accompanying drawings, in which Figure 1 shows the measurement arrangements required for applying the method according to the invention and Figure 2 shows the measurement method.

Components related to the measurement arrangements (Figure 1): a. A load-bearing body b. A sensor in which mechanical Barkhausen noise induces smv pulses; the sensor may be detached from the surface of the object to be measured 3 62732 c. amplifier d. a discriminator for selecting from the amplified noise pulses of the desired magnitude for analysis e. measurement of size distribution, rms value, average or frequency f. printing g. magnetization sensor, with which the test piece can be magnetized with a stable magnetic field, the direction of which can be changed for 10 h. DC source connected to the magnetization sensor

The measurement is performed as follows:

An external load causes a lattice fault sequence on the object to be measured, which dominates the irreversible magnetization changes inside the object. For example, the mechanical load amplitude F is raised either continuously, as shown in Fig. 2a, or in stages, each stage containing several amplitudes. In this context, lattice defects mean e.g. dislocations through which the plastic deformation of the material and thus also fatigue-20 takes place. As the load amplitude is increased, e.g., the rms value of the induced mechanical Barkhausen noise RMS increases in proportion to the load amplitude, as shown in Figure 2b.

During the measurement, the test material g is magnetized by a stable magnetic field with the magnetization sensor g authorized for the application of the method. When the load amplitude is further increased, a clear maximum point in the rms value of the induced mechanical Barkhausen noise RMS is obtained with a sufficiently large amplitude value, e.g. This load amplitude has been found to correspond very closely to the fatigue or tolerance limit of the original non-fatigue material.

5 When the test piece is magnetized with a stable magnetic field, the level of Barkhausen noise generated increases and especially the changes at the fatigue limit become very clear. In addition, by changing the direction of the field, the magnetic state of the structure can be influenced in such a way that it is most advantageous in relation to the direction of the acting load. For example, for ferrous metals, the magnetizing field and the load must be parallel. It has also been found that the largest pulses of mechanical Barkhausen noise are the most sensitive to changes caused by dislocations. When the largest pulses are separated from the generated 15 noise by the discriminator d for analysis, the measurement results are made even more accurate.

Only one preferred embodiment for applying the method according to the invention has been presented above. It is possible, for example, to measure changes in the frequency, size distribution or average of Barkhausen noise caused by dislocation faults and groupings instead of the rms value. Also, the external load which causes the maximum value of Barkhausen noise according to the method can be something other than an alternating load of increasing amplitude, e.g. a pulsating load or the like.

Embodiments of the method according to the invention may vary within the scope of the appended claim.

Claims (1)

62732 5 Patent application: Method for using mechanical Barkhausen noise caused by dislocation movement and variable mechanical load in ferromagnetic materials as a non-destructive fatigue measurement method, characterized in that the test piece is externally loaded or a lattice structure dominant in the test piece for irreversible magnetization changes, simultaneously measuring changes in the rms value, frequency, size distribution or average of the maximum, mean, frequency or mean, frequency, frequency or average the value of the amplitude corresponds to the fatigue strength of the fatigue limit of the test piece (Figure 2b) with the test piece at the time of measurement m ag-neted with a stable magnetic field.