PSI - Issue 5

Ralf Urbanek et al. / Procedia Structural Integrity 5 (2017) 785–792 Author name / Structural Integrity Procedia 00 (2017) 000 – 000

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published a series of papers in 1800 (Lovell 1968) and Lord Thompson described the thermo-elastic effect at compressed and decompressed gas under adiabatic conditions in 1853 (Thompson 1853). The next leaps in thermographic investigation of stressed specimen is directly connected to the development of thermographic cameras. The SPATE 8000-System used by Wong et al. (Wong 1987) has a photon emission detector with oscillation mirrors scanning the surface to get a thermographic stress pattern. Later Focal plane arrays came to market which Gyekenyesi used for his thermo-elastic stress analysis (Gyekenyesi 1999). Bigger FPAs with more pixel (up to 640x512) and higher recording frequencies (up to 100 Hz) at full frame allow lock-in thermographic analysis described by Brémond (Brémond 2007). The investigation of stress pattern with the thermographic lock-in method depends on several factors. Only the most significant are shortly mentioned like loading frequency, loading level, and the surface preparation. Therefore, a calibration on a specimen with known stress pattern is necessary. Subsequently these conclusions are applied to specimen with more complex stress patterns to evaluate the influence of specimen shape and cracks on the experimental data.

2. Experimental Details

2.1. Specimen and fatigue experiments

In this work experiments with three different materials, an aluminum alloy, oxygen free Copper and a high alloyed steel (X5CrNi 18-10, AISI 304) were performed. The main properties of the three materials are given in table 1. All specimen have a length of 80 mm, a width of 12 mm and a thickness of 3.96 ±0.02 mm. One surface of each specimen was grinded and coated with a thin layer of black paint (16-24 µm) to enhance the emissivity for infrared radiation according to the investigations of Robinsons (Robinson 2010).

Table 1. Material properties 1 (Matweb 2017), 2 (Kupferinstitut 2017), 3 own measurements Material AA 7075 T7351 1 CU-DHP 2

X5CrNi18-10 3

density ρ

[kg/m³] [J/kg/K] [10 -6 /K]

2810 960 23.4

8940 386 16.8

7903 465 16.0

heat capacity c p

Coefficient of thermal expansion

In case of the high alloyed steel beside the simple rectangular shape, SEN-specimens with a notch depth of 1 mm, 2 mm and 3 mm were analyzed. For crack length measurement in some specimen with a notch depth of 1 mm pins for potential drop measurement were introduced beside the notch. Fig. 1 shows the corresponding technical drawing and a photograph of a painted specimen with notch and pins.

Fig. 1. Technical drawing of a notched specimen with pins and picture of the painted area.

The experiments were carried out with a servo-hydraulic testing machine with a DOLI EDC 580 controller. Fixed grips with a parallel guidance were used to suppress bending forces for almost pure uniaxial loading in tension and