FI60934B - SAETT ATT DEFINIERA UTMATTNINGSHAOLLFASTHET HOS FERROMAGNETISKT MATERIAL UTAN ATT BRYTA MATERIALET - Google Patents

Description

I55FH [B] (11) ADVERTISEMENT] PUBLICATION, n g, m J ^ STa l J '' utlAggningsskrift C / ^ r \ Patent granted on 13 04 1932 • · ύ \? (45) '7 Patent Meddelat ^ T ^ (51) Kv.lk?/lnt,CI.3 G 01 N 27/72 FINLAND — FINLAND d) Ptt * nttih * k «mut— Pattnun * 6knlng 80l6l5 (22) H «K« mltpUvl - Ansdknlngtdag 20.05.80 (23) AlkupUvi —Glltlgh «t * d» g 20.05.80 (41) Become Public - Bllvlt offemllg 21.11.8l

Patent] a register held .... ........ ........

_. . ,. . , (44) Date of issue and date of issue. -

Patent-och registerstyrelsen Antokin utitgd och utl. ^ 0 Tampere 5k,

Pentti Kettunen, Liutuntie 15 D 21, 362I + O Nattari,

The present invention relates to a method for measuring the fatigue strength of a ferromagnetic material without breaking the material. This invention relates to a method for measuring the fatigue strength of a ferromagnetic material.

Fatigue resistance is stated in the so-called Using the S-N curve, which is determined experimentally by stressing the test pieces with different loading 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 part 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.

There are some non-destructive fatigue measurement methods that can be used to monitor the development of fatigue in the 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 e.g. fatigue wire methods and coercive force measurements.

This non-destructive fatigue limit measurement method utilizes the so-called mechanical Barkhausen noise. Mechanical Barkhausen noise is previously known as a phenomenon and is related to irreversible changes in the magnetic state of a material (so-called discontinuous movements of domain walls) due to mechanical loading when the body is in a stable magnetic field or demagnetized state.

Better known is the so-called "ordinary" Barkhausen noise, based on the effects of a changing external magnetic field, for which a number of devices have been developed for non-destructive measurement. The latter phenomenon is based on e.g. US.pat.no. 3,427,872, by which the static load or yield state of a material can be determined by means of an external variable magnetic field. Ko. however, the method cannot be used for the analysis of 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 claim. By using the method according to the invention, the significant advantage is achieved that this fatigue or tolerance limit can be determined in about 15 minutes.

Thus, ko. the method can in practice immediately detect when the dynamic stress of a machine part becomes too great.

The method according to the invention is described in more detail in the accompanying 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 step by step.

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Components related to the measurement arrangements (Figure 1): a. Subject under load b. Sensor to which mechanical Barkhausen noise induces smv pulses; the sensor may be detached from the surface of the object to be measured c. amplifier d. a discriminator for selecting pulses of the desired size from the amplified noise for analysis e. measurement of size distribution, rms value, average or frequency f. printing

The measurement is performed step by step as follows:

Step 1: An external load is applied to the object to be measured in a lattice fault order that dominates the irreversible magnetization changes inside the object. For example, the mechanical load amplitude F is increased 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 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.

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Step 2: Drop the load amplitude F to zero and start raising it again / as shown in Figure 2c. In this case, if the first fatigue is performed with a sufficiently large amplitude, e.g. a clear maximum point in the rms value of the induced mechanical Barkhausen noise RMS is detected with a certain value of the load amplitude, as shown in Fig. 2d. This load amplitude has been found to correspond very closely to the fatigue or tolerance limit of the original non-fatigue material.

The aforementioned "sufficiently large load amplitude" need not be felt. It can be found experimentally by performing steps 1 and 2 a sufficient number of times in succession, increasing the maximum load amplitude each time until the maximum point of step 2 appears. Experiments have shown that the required maximum load amplitude is about 60-70% of the tensile yield strength of the structure.

Only one preferred embodiment for applying the method according to the invention has been presented above. For example, it is very possible to measure changes in the frequency, size distribution, or mean of Barkhausen noise caused by dislocation movements and groupings instead of the rms value. Also, the external load which causes the maximum value of the Barkhausen noise according to the method can be something other than an amplifying alternating load, 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)

Claim: A method for using mechanical Barkhausen noise caused by dislocation movement and variable mechanical loading in ferromagnetic materials as a non-destructive fatigue measurement method, characterized in that the method comprises the steps of: 2a thus causing a lattice defect structure (lattice defect sequence) to dominate the specimen in terms of irreversible magnetization changes, simultaneously measuring changes in rms, frequency, size distribution, or mean of the Barkhausen mechanical noise induced during fatigue of ferromagnetic material. with a stepwise or continuously increasing alternating load (Fig. 2c), simultaneously measuring the ferr the maximum point in the rms value, frequency, size distribution, or average of the mechanical Barkhausen noise induced during fatigue of the magnetic material, the corresponding value of the load amplitude corresponding to the fatigue strength of the fatigue limit of the test piece (Figure 2d). i