PSI - Issue 13

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

948

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

2

2. Experimental Details

The investigations were undertaken on flat specimens of oxygen-free Copper and cylindrical specimens of the aluminum alloy AA 6082-T651. Tensile tests with different testing speeds were performed on a hydraulic testing machine. To determine the strain to failure the gauge length was marked with lines on the specimen surface. The extension of the specimen was measured with a digital microscope. The experiments were undertaken in laboratory air and, to enhance the heat exchange from the specimen to the environment, under the same loading conditions in water. Therefore, a chamber with a closed circulating system was used. The water was channeled directly onto the specimen surface to obtain an optimal heat dissipation. The temperature of the water was about room temperature and due to the amount of water even without additional cooling only a negligible change of the temperature was registered. The temperature change during tensile tests in air and the fatigue tests were measured with an Infratec ImageIR 8300hp camera system. The fatigue tests were performed on specimens of AA 6082 on different loading levels with a stress-ratio of R = 0 and on copper specimen with stress ratios of R = 0 and R = -1. Figure 1 shows the results of a tensile test on AA 6082-T651 with a crosshead speed of 100 mm/min, the corresponding strain rate, calculated from the maximum strain and the duration of the experiment, is about 0,085 1/s. The red curve shows the changes of the measured maximum temperature on the specimen surface during the tensile test. After a decrease in the elastic regime, the temperature rises due to dissipated energy caused by plastic deformation. Up to the maximum of the stress-strain curve, the temperature increases nearly linear, with the start of necking a steeper slope of the temperature curve is visible. With increasing strain, the slope is decreasing. The Infrared pictures indicate, that prior to the maximum of the stress strain curve the temperature of the specimen surface is uniform, after the stress maximum the highest temperatures were found in the necking region. In this experiment, an increase of the maximum temperature of about 16 K was observed. 3. Results 3.1. Tensile tests

250

10 12 14 16 18 20

200

150

D T max [K]

0 2 4 6 8

100

stress [MPa]

50

AA 6082-T651 100 mm/min

0

0

5

10

15

20

25

30

35

40

strain [%]

Fig. 1. Stress-strain curve and measured temperature increase in a tensile test with a crosshead speed of 100 mm/min on an AA6082 specimen.

The temperature increase in the tensile tests depends on the crosshead speed or rather the strain rate. In figure 2 the increase of the maximum temperature as a function of the crosshead speed is shown. With increasing crosshead speed a distinct increase of the temperature in tensile tests up to more than 50 K at a crosshead speed of 1000 mm/min can be observed. For the AA6082 specimens a slightly smaller temperature increase compared to copper was measured.