PSI - Issue 13

Jürgen Bär / Procedia Structural Integrity 13 (2018) 947–952

950

Author name / Structural Integrity Procedia 00 (2018) 000 – 000

4

3.2. Fatigue Tests The deformation induced heating was investigated in fatigue tests on different loading levels and different loading frequencies. In case of the aluminum alloy all tests were performed with a stress-ratio of R = 0. In case of this alloy a heating effect could only be observed at loading levels near the yield strength of about R p0.2 = 207 MPa. The curves in figure 4 show a starting peak, caused by plastic deformation in the first cycle, followed by an increase of the mean temperature. The increase of the slope in the temperature curves in the second half of the cyclic lifetime can be explained by crack propagation. The influence of the loading frequency is clearly visible in the experiments undertaken with a maximum load of 215 MPa. At a loading frequency of 5 Hz only a small increase of the temperature can be observed, whereas the temperature increase at 40 Hz is significantly higher.

2.0

0 1 2 3 4 5 6 7 8 9 10

1 Hz

5 Hz

10 Hz

20 Hz

40 Hz

s max = 205 MPa - 40 Hz s max = 210 MPa - 40 Hz s max = 215 MPa - 40 Hz s max = 215 MPa - 5 Hz

s max = 130 MPa, R = -1 s max = 140 MPa, R = -1 s max = 140 MPa, R = 0

1.5

0.5 temperature increase D T [K] 1.0

temperature increase D T [K]

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 0.0

0

600

1200

1800

2400

3000

relative lifetime N/N f

time [s]

Fig. 4. Temperature increase during fatigue tests performed on AA6082 and copper specimens.

In the experiments undertaken on copper, the influence of the loading frequency is clearly visible (figure 4 right). In these experiments, the frequency was increased stepwise from 1 to 40 Hz. On each level, the frequency was kept constant for 10 minutes, respectively. The investigated loading levels are considerable below the yield strength of R p0.2 = 220 MPa. In case of loading with a stress ratio of R = 0 only a negligible temperature change can be observed. Under fully reversed loading conditions (R = -1) at a maximum stress of 130 MPa at frequencies of 20 and 40 Hz a temperature increase is visible. At a maximum stress of 140 MPa even at 5 Hz the temperature rises noticeable. At higher frequencies a distinct increase of the temperature is found, whereby no equilibrium in the temperature is reached. The experiments have shown that under monotonic loading plastic deformation leads to an enhanced specimen temperature especially in the necking region. In case of copper, obviously an enhanced heat dissipation in the experiments undertaken in water leads to an increase of the strain to failure. Cullen and Korkolis (2013) found an enhanced ductility in interrupted tensile tests on AISI 304. They contributed that fact to the plasticity induced heating in tensile tests supporting necking in the region of the highest temperature. In the interrupted tests, the heating was less pronounced, leading to a delayed necking and therefore a higher strain to failure. The increase of the strain to failure measured in the experiments (figure 2) can hardly be explained by the measured increase of the maximum temperature. However, it must be considered that even the measured maximum temperature is a mean value within an area given by the spatial resolution of the camera. The highest temperatures were achieved directly at the heat sources, the moving dislocations. Due to the supposed high temperature the mobility of the dislocation is enhanced leading to a self-energizing process associated with a localization of the deformation resulting in a necking of the specimen. By cooling, this process can be diluted leading to a less pronounced localization of deformation and an increased strain 4. Discussion