PSI - Issue 2_A

Jürgen Bär / Procedia Structural Integrity 2 (2016) 2105–2112 Author name / Structural Integrity Procedia 00 (2016) 000–000

2111

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constant temperature on the backside of the measuring peltier element the heat flow depends on the temperature of the specimen.

0,00 0,05 0,10 0,15 0,20 0,25 0,30 0,35 0,40 0,45 0,50

26 27 28 29 30 31 32 33 34 35 36 37 38

K max = 12.5 MPa  m K max = 15 MPa  m K max = 17.5 MPa  m K max = 20 MPa  m K max = 22.5 MPa  m

b

K max = 12.5 MPa  m K max = 15 MPa  m K max = 17.5 MPa  m K max = 20 MPa  m K max = 22.5 MPa  m

a

Heat Flow [W]

Specimen temperature [°C]

1

2

3

4

5

6

7

8

1

2

3

4

5

6

7

8

crack length a [mm]

crack length a [mm]

Fig. 9. (a) Heat flow measured by the peltier sensor and (b) mean specimen temperature in the analysis area.

The thermoelastic effect should also reflect in the range of the heat flow measurements. In figure 10a the range, i.e. the difference between the maximum and the minimum value in each cycle is plotted against the applied force. Within the scatter band no influence of the force on the heat flow range can be observed. When experiments were performed with a frequency of 0.2 Hz a linear dependence as in the case of the E-Amplitude values can be observed (figure 10b). This clearly indicates the comparability of the heat flow measurements and the lock in thermography. Due to the response characteristic of the peltier sensor the measurement of elastic effects is limited to low frequencies.

2000 3000 4000 5000 6000 7000 8000 9000 0.000 0.002 0.004 0.006 0.008 0.010 0.012 0.014 0.016 0.018 0.020 0.022 0.024 b f =20 Hz f = 0.2 Hz

0.0015

a

K max =12.5 MPa  m K max =15 MPa  m K max =17.5 MPa  m K max =20 MPa  m K max =22.5 MPa  m

0.0014

0.0013

0.0012

Heat flow range [W]

Heat flow range [W]

0.0011

2000 3000 4000 5000 6000 7000 8000 9000 0.0010

Force [N]

Force [N]

Fig. 10. Heat flow range as an indicator for elastic stresses for experiments at 20 Hz (a) and the comparison to 0.2 Hz (b).

4. Conclusion The experiments have shown that a lot of interesting results can be achieved with both methods. The E-Amplitude values of the thermographic measurement represent the elastic stress field in front of the crack tip. With knowledge of the emissivity of the paint the stresses can be calculated using equation 1. In case of the heat flow measurements information about elastic stresses are only obtainable at low loading frequencies and averaged over the area covered by the peltier element.