PSI - Issue 18

Markus Berchtold et al. / Procedia Structural Integrity 18 (2019) 532–537 Markus Berchtold/ Structural Integrity Procedia 00 (2019) 000–000

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A material specific basic damping is always present when deforming a solid material. Microscopic plastic deformation during cyclic loading leads to additional damping and it is almost completely transferred to heat. The damping energy and corresponding heat that is produced per load cycle and volume is constant for an even axially loaded specimen. The produced heat per time is proportional to the frequency. The resulting material temperature depends as well on the present heat losses, for example the heat flow to the fixture and to the ambient atmosphere. Convectional cooling can be used to control the temperature during testing. Some material do not show the above described linear relation between temperature and testing frequency, with higher frequency the temperature does not increase as expected [4]. This finding may point to hardening (resp. softening) mechanism which can be strain rate related (see section 2.3) 2.2. Environment There are time related mechanisms such as oxidation, corrosion or creep that may play a 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 aluminum alloys on fatigue life. The aluminum alloy AW-5083 shows almost now frequency effect at 20 kHz on fatigue life in inert atmosphere but in air [3]. 2.3. Strain Rate The strain rate is proportional to the testing frequency. However during loading in resonant condition the strain rate is not constant, it follows a sinusoidal function. It is thought that the strain rate of irreversible deformation could affect fatigue life significantly. A relevant effect can be expected when the highest strain is present during the load cycle. Therefor the relation between frequency and a possible effect on fatigue strength is non-linear. An influence of the testing frequency at 20 kHz 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. Fatigue Testing with the GIGAFORTE 3.1. Fatigue life of steel 50CrMo4, 37HRC Tests were carried out in cooperation with Hochschule Osnabrück - University of Applied Sciences on the RUMUL TESTRONIC (95 Hz), GIGAFORTE (1000 Hz) and an Ultra Sonic testing system (20 kHz). On 1000Hz round specimen with diameter 6mm and hour glass shape were used and tested at RUMUL. Compressed air cooling was used. On 1000Hz and on stress amplitude 500MPa and higher the heating up was significant and difficult to control. The tests show the fatigue life results that are gained on 1000Hz are within the population of the results of the tests carried on 95Hz or below, the latter is most likely related to the specimen heating up. The fatigue life results on 20 kHz are significantly higher, it is related to frequency and size effects [6] (as found in other investigations, see 2.3).