WO1983001836A1

WO1983001836A1 - Method for measuring fatigue strength of ferromagnetic materials non-destructively

Info

Publication number: WO1983001836A1
Application number: PCT/FI1981/000086
Authority: WO
Inventors: Pekka Ruuskanen; Pentti Kettunen
Original assignee: Pekka Ruuskanen; Pentti Kettunen
Priority date: 1980-05-20
Filing date: 1981-11-18
Publication date: 1983-05-26

Abstract

Method for using the mechanical Barkhausen noise phenomenon, caused in ferromagnetic materials by the dislocation motion and the changing external mechanical loading, as a non-destructive measuring method, especially in determining the fatigue strength of the material. The method has two embodiments, the first of which has the following stages. The test piece (a) is loaded externally (stage 1), e.g., by a cyclic of other corresponding method, increases in its amplitude continuously (Fig. 2 a), and thus a dominating lattice defect arrangement from the point of view of the irreversible magnetization changes is created into the test piece. Simultaneously the changes in the effective value, the frequency, the size distribution or the mean value of the mechanical Barkhausen noise pulses induced during the fatigue of the ferromagnetic material are measured (b-f). Then (stage 2) the external loading is taken off. After this (stage 3), the test piece (a) is loaded externally, e.g., by a cyclic loading increasing in its amplitude continuously (Fig. 2 a), and simultaneously the maximum value appearing as the effective value, the frequency, the size distribution or the mean value of the mechanical Barkhausen noise pulses induced during the fatigue of the ferromagnetic material is measured. The loading amplitude corresponding to the measured maximum value also corresponds to the endurance limit (fatigue limit) of the test piece (Fig. 2 b). In the second embodiment, the test piece is magnetized with a stable magnetic field (g, h) in which case stages 1 and 2 of the first application mode are omitted and the measurement of the maximum point corresponding to the endurance limit (fatigue limit) of the test piece is done according to stage 3 of the first application mode.

Description

Method for measuring fatigue strength of ferromagnetic materials non-dest ructively

This invention relates to a measuring method used in the determination of the fatigue strength of ferromagnetic materials.

Fatigue strength is presented by means of the so-called S-N curve which is experimentally determined by straining the test samples at different strain or stress amplitudes till the final fatigue fracture. One of the most important detail of the S-N curve is the endurance or fatigue limit, i.e. the limiting strain or stress amplitude under which the applied amplitudes are no longer capable of causing fatigue fracture or destruction of the samples. From this point of view of practical applications, the endurance- or fatigue limit is also the most important value of the fatigue strength. Determination of this limit in the above mentioned destructive method requires a period of 5 to

10 days and nights depending on the testing machine used.

There exist also some non-destructive methods to measure fatigue tendency, mainly to follow the development of the structure deterioration associated with fatigue. A characteristic of these methods is that they require a comparison of the measured results with those of earlier measurements or those of reference materials. These methods, however, are proven to be rather unreliable, and, furthermore, with them one cannot measure the most important fatigue strength, the endurance of fatigue limit. Examples of these methods are, among others, the fatigue gauge method (fatigue wire methods) and measure¬ ments of the coersitive force.

O PI The present non-destructive method of measuring the endurance or fatigue limit applies the so-called mechanical Barkhausen noise phenomenon. As a phenomenon, the mechanical Barkhausen noise formation is related to the irreversible changes in the magnetic state of the material (with the discontinuous movements of the domain walls under the influence of mechanical loading of the sample in a stable magnetic field or in a demagnetised state).

Better known is the "ordinary" Barkhausen noise effect which resulted in a changing external magnetic field. For the application of this "ordinary" Barkhausen noise effect in the non-destructive measuring technique a number of devices are developed. Also the U.S. patent n:o 3,427,872, for the measurement of the static loading or strain state of a material by means of a changing external magnetic field is based on this "ordinary" noise effect. This method cannot be applied, however, to the analyzing of the mechanical Barkhausen noise, neither for the de- termination of the endurance or fatigue limit of the material.

The method according to the present invention is characterized, in the attached patent claims. The method of the present invention offers the notable advantage^* that the endurance or fatigue limit can, in practice, be determined very quickly, e.g., in a period of 15 minutes. Therefore, the method of the present invention can in practice be applied to determine immediately whether the dynamic loading of a machine part is too large or not.

WIPO The method of the present invention is clarified in de¬ tail in the attached description referring simultaneously to the attached drawings, of which Figure 1 shows the measuring arrangements required as the method of the present invention is applied, and Figure 2 shows one embodiment of measurements according to the method in different sequences.

Details of the measuring arrangements (Figure 1)

a. the sample under loading or straining b. sensor into which the mechanical Barkhausen phenomenon induces electromotoric force (voltage) pulses or sig¬ nals; the sensor can be located away from the surface of the sample c. amplifier d. discriminator by which the wanted pulses are selected from the amplified noise signals for analyzing e. the measurement of size distribution, effective value, average of mean value, of frequency f. output (print out or recording).

Fig. 1 represents by horizontal lines, and additionally by the letter g, the magnetization device by which the specimen or test piece can be magnetized with a stable magnetic field, the direction of which can be changed. The letter h shows additionally the direct current source associated with the magneti.zation device. The magnetizat¬ ion device g and the direct current source h are used in the second embodiment of the present invention.

Measurement occurs in stages as follows:

Stage 1: By means of an external loading a lattice defect arrangement is created in the test piece, which ment dominates the irreversible magnetization changes inside the test piece. For example, the mechanical load¬ ing amplitude F is increased, either continuously as shown in Fig. 2 a or in steps, in which case each step is of the order of several amplitudes. Lattice defects, in this connection, mean, e.g., dislocations, by means of which the plastic deformation of the material, and hence also the fatigue, occurs. As the loading amplitude is increased the effective value RMS of the mechanical Barkhausen noise pulses increases in correlation with the loading amplitude as shown in Fig. 2 b.

Stage 2i The loading amplitude F is decreased to zero.

Stage 3: The loading amplitude F is increased again as shown in Fig. 2 c. If the first increase of the loading amplitude F is carried to a sufficiently high value, the effective value RMS of the induced mechanical Bark¬ hausen noise pulses goes through a maximum at a certain value of the loading amplitude F, as shown in Fig. 2 d. This value of the loading amplitude is found to correspond very accurately to the endurance limit (or fatigue limit) of the original non-fatigued material.

The sufficiently high value of the loading amplitude, into which the preliminary fatiguing is carried, it is not necessary to know beforehand. It can be found ex— perimentally by repeating stages 1 and 2 and at each repetition increasing the value of the maximum loading amplitude until the maximum point in the-RMS-value according to stage 3 appears. Experiments have shown that the required maximum loading amplitude is about 60-70.% of the tensile yield strength. With the magnetization device g associated with the second embodiment of the method the test material can be magnetized with a stable magnetic field during measurement. Then the magnitude of the induced noise pulses increases and the changes at the endurance limit (fatigue limit) appear, more clearly. By changing the direction of the magnetic field the magnetic state of the structure can additionally be influenced so that it becomes most effective in correlation to the direct- ion of the acting load. E.g., in iron-based metals the direction of the magnetic field must be parallel with that of the loading. It is also observed that:the largest pulses of the machanical Barkhausen noise are most sensitive against changes due to dislocations. When the largest pulses are selected for analysis by the dis¬ criminator d, the accuracy of the measured results in¬ creases.

By applying the magnetizing device g to the method of the present invention stages 1 and 2 of the first application mode can be left out. That is to say that by increasing the loading amplitude according to Fig. 2 c the maximum point of Fig. 2 d can be revealed, which very accurately corresponds to the endurance limit (fatigue limit) of the original non-fatigued material.

The method can be applied by using a generally known - apparatus.

Above, only two most beneficial application. modes of the method of the present invention are presented. It is quite possible, e.g., to measure changes in frequency, size distribution or mean value of the Barkhausen noise signals caused by dislocation motion instead of the above mentioned effective value RMS. Also the external loading, by which the maximum value of the Barkhausen noise of the present method is caused, can be other than a cyclic loading increasing in its amplitude. For example, repeated tension of compression can be used for this r purpose.

The embodiments according to the method of the present invention can vary within the limits of the attached patent claims.

OMPI

Claims

Patent claims;

1. The method using the mechanical Barkhausen noise phenomenon, caused in ferromagnetic materials by the dislocation motion and the changing mechanical loading, as a non-destructive method to measure fatigue strength, c h a r a c t e r i s e d in that it consists on the following stages:

1) the test piece is loaded externally, e.g., by a cyclic loading, increases in its amplitude continuously (Fig. 2 a), and thus a dominating lattice defect arrangement, from the point of view of irreversible magnetization changes, is created in the test piece, and simultaneously the changes in the effective value, frequency, size distribution or mean value of the mechanical Barkhausen noise pulses induced during the fatigue of the ferromagnetic material are measured (Fig. 2 b),

2) the external loading is decreased to zero,

3) the test piece is loaded externally by a cyclic or other loading method (Fig. 2 c) and simultaneously the maximum point appearing as the effective value, the frequency, the size distribution or the mean value of the mechanical Barkhausen noise pulεes induced during the fatigue of the ferromagnetic material is measured. The loading amplitude at which the maximum point appears corresponds to -a fatigue strength equal with the endurance limit (fatigue limit) of the test piece (Fig. 2 d).

2. A method according to the patent claim 1,^" c h a¬ r a c t e r i s e d in that 1 and 2 being replaced by magnetizing the test piece by a stable magnetic field_*