PSI - Issue 13

Markus Berchtold et al. / Procedia Structural Integrity 13 (2018) 676–679 Markus Berchtold/ / Structural Integrity Procedia 00 (2018) 000 – 000

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2.2. Environment There are time related mechanisms such as oxidation, corrosion or creep that may a play role for the formation of a new surface during crack initiation and propagation. Depending on the relevance of such mechanisms a significant frequency effect can be expected. For example, it is reported that some investigations show a significant frequency dependency of Aluminium alloys on fatigue life. The Aluminium alloy AW-5083 shows almost now frequency effect at 20kHz on fatigue life in inert atmosphere but in air [3]. 2.3. Strain rate The strain rate is proportional to the testing frequency. During loading in resonant condition the strain rate is not constant. It follows a sinus function. It is thought that the strain rate of irreversible deformation could affect fatigue life significantly. An influence of the testing frequency at 20kHz on fatigue life could be found on quenched and tempered steel 50CrMo4 depending on the strength of the material. It was concluded that the found correlation of fatigue life and testing frequency is related to the strain rate and is typical for cubic body centred metals. The frequency effect is mainly seen on the left side, of finite life of the S – N curve. [3]. For metastable austenitic steel (1.4301, AISI 304) a frequency effect related to the transformation of crystallographic structures was found during testing at 1000Hz with the GIGAFORTE. The analyses showed that higher amounts of strain-induced Martensite and lower plastic strain amplitudes are observed when the cyclic experiments are carried out at lower frequency, promoting higher fatigue strengths [2, 5].

3. Temperature records

RUMUL could look into heating up behaviour of material samples in the last year. The specimens have been provided by interested laboratories. For Temperature re-cording a type K thermocouple was attached on the specimen. Compressed air was used to mitigate heating up if required. Load ratio was selected -1 for all tests.

Material

Specimen

Gauge diam. 7 mm

Testing condition

Freq.

Load amplitude 8.5 kN (220 MPa) 18.6 kN (373 MPa) 5.52 kN (280 MPa) 17 kN (337 MPa)

Temp.

Nodular Iron round, cyl. and hour glass, w. thread

load increasing 0.2*10 6 cycles / step compr. air cooling load increasing, 2*10 6 cycles / step compr. air cooling load increasing 0.5*10 6 cycles / step no cooling load increasing, 10 6 cycles / step no cooling, (Fig.2)

1111 Hz

38°C

9% Cr-steel

round, hour glass, w. thread round, hour glass, w. thread round, cyl. w/o thread

8 mm

1024 Hz

26 ° C 54°C 1) 35°C

Ti alloy

5 mm

996 Hz

Ferritic steel HV 30 ~ 220

8 mm

1023 Hz

62°C 2)

1) 2)

cooling temporarily off

temperature is not stabilizing, probably softening effect

4. Summary and Outlook

The RUMUL GIGAFORTE is an efficient tool for testing very high number of load cycles in a reasonable time. Common specimen types and sizes can be used. Depending on material and load the specimen may heat up. The heating is usually low or moderate and can be mitigated with compressed air cooling, continuous testing is possible. In Fig. 1 the 1000Hz Fatigue Testing Machine RUMUL GIGAFORTE is shown with small sound enclosure, whereas in Fig. 2 RUMUL GIGAFORTE is shown with round specimen without thread, thermocouple attached with tape.