PSI - Issue 42

Jürgen Bär et al. / Procedia Structural Integrity 42 (2022) 1061–1068 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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2. Experimental details 2.1. Experimental details

The investigations were undertaken on a SEN-Specimen of a high alloyed steel (AISI 304L). A fatigue crack with a length of a = 4.6 mm was introduced with a constant stress intensity K max = 15 MPa  m under fully reversed loading conditions (R = -1) and a loading frequency f L = 20 Hz. A picture of the crack is shown in figure 1.

Fig. 1. Micrograph of the fatigue crack produced with a constant stress intensity of K max =15 MPa  m.

For the lock-in-evaluation the pre-cracked specimen was loaded with a constant force of F max = 2.695 kN which corresponds to the stress intensity of K max = 15 MPa  m, under fully reversed loading conditions and a loading frequency of 20 Hz. The thermographic investigation was undertaken with an Infratec ImageIR 8300hp camera. Frames with 640 x 512 pixel were recorded with a camera frequency f C = 293 Hz. The load signal was delivered from the control electronics of the testing machine in form of an analogue signal and recorded by the camera in form of a 14-bit value. To achieve a high and equal emissivity, the surface of the specimen was coated with black paint with a thickness of about 20 µm. Thereafter measurements for the alternative evaluation method were performed with the same loading level but at a loading frequency of f L = 5 Hz. In this experiment thermographic images with quarter size (320x256 Pixel) were recorded with a camera frequency f C = 1,000 Hz. During the two thermographic investigations less than 500 loading cycles were applied on the specimen, thus nearly no crack propagation took place during the investigations. 3. Results 3.1. Lock-In evaluation The resulting images of the lock-in-evaluation according to equation 2 are shown in figure 2. This dissection provides 4 different images with the amplitude and phase shift information for the sine waves for the E- and the D Mode. In the E-amplitude as well as in the D-amplitude Image a high amplitude due to the plastic zone in front of the crack tip is visible. Furthermore, in the phase images a maximum is visible near the crack tip. For the E-Mode, that represents the thermoelastic effect, this phase shift is not understandable because according to equation 1 the thermoelastic effect is directly connected to the magnitude of the stress and therefore a phase shift cannot occur.