PSI - Issue 13

Jürgen Bär / Procedia Structural Integrity 13 (2018) 947–952 Author name / Structural Integrity Procedia 00 (2018) 000 – 000

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to failure. The decreasing difference in the strain to failure between the experiments in water and air at higher crosshead speeds can be ascribed to an insufficient cooling due to the short duration of the experiments. Under fatigue loading beside dislocation glide other effects can cause heating of the specimen. Blanche et al. (2015) found a temperature increase during VHCF experiments on copper below the elastic limit and contributed the heating to oscillation of pinned dislocations. Beside this mechanism, the Snoek effect was considered by Mareau et al. (2012) to explain the temperature increase found in VHCF experiments on a ferritic steel. These results clearly indicate that the temperature increase is caused by individual heat sources which are distributed in the material. That leads to the consequence that the heating is localized and the measured values, even for a single pixel of a thermographic camera, are mean values for an area or volume of the material. The local temperature at the heat sources may be a multiple of the measured mean temperature and, therefore, an influence on the dislocations especially in materials showing a localized deformation in shear bands or persistent slip bands are probable.

0,30

Copper f = 1 Hz

0,25

0,20

0,15

0,10

0,05 temperature change without thermoelastic effect [K]

0,00

-200

-100

0

100

200

stress [MPa]

Fig. 5. Temperature change within a cycle in copper loaded with 200 MPa and a frequency of 1 Hz.

The experiments performed in this work confirm the results of Boulanger et al. (2004) that the temperature increase depends on the loading level and the loading frequency. This fact can be explained in more detail using figure 5. In this diagram, the temperature change due to dissipative effects within a cycle is shown. The temperature changes caused by the thermoelastic effect were subtracted from the measured values using an experimental determined thermoelastic constant. Starting from the stress s = 0 MPa (red arrow) in tension the temperature is decreasing. From about 120 MPa an increase with continuously rising slope due to dissipative energies up to the load maximum is observed. With unloading the specimen the temperature decreases, indicating that the cooling of the specimen dominates and potentially existing heating effects are negligible. In compression loading the temperature increases starting at about130 MPa with a curve progression comparable to that in tension. The cooling effect after the stress minimum is comparable to that in tension, too. The temperature gap between starting and ending point of the hysteresis loop represents the effective temperature increase within this cycle. The effective temperature increase within a cycle depends on the maximum stress as well as on the stress ratio and the loading frequency. At positive stress ratios beside the loss of the heating effect in compression the time for cooling of the specimen is enhanced. With enhancing the frequency, the time between the two heating effects in tension and compression is reduced resulting in a reduced time for cooling and, consequently, an increased heating effect leading to higher specimen temperature. At high loading frequencies, due to the reduced time for cooling, even at low loading levels a heating of the specimen can be achieved. This clearly illustrates the high temperature increase observed in VHCF-experiments (Ranc 2008, Blanche 2015, Illgen 2018), but the confirmation of the influence of the deformation induced heating on the cyclic lifetime is problematic. At first, an effective cooling must be realized and, due to the scatter of the cyclic lifetime for a statistically confirmed conclusion many specimens have to be tested.