PSI - Issue 54

Johannes Kaiser et al. / Procedia Structural Integrity 54 (2024) 26–33 Johannes Kaiser et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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It should be noted that these specimens do not comply with the specified dimensions of ASTM standard E399 due to their smaller thickness. However, these specimens allow more accurate imaging of a plane stress ratio (ESZ) and neglect of the transverse contraction coefficient in the thickness direction, allowing cameras to be focused throughout the test. 3.2. 3D digital image correlation system in combination with a thermography camera for determination of crack tip heating In order to resolve the temperature increase in the fracture process zone at the crack tip as accurately as possible, a combined setup consisting of a thermographic camera and a 3D image correlation system is used. For this purpose, the setup already presented in [13] was extended accordingly. For the image correlation setup consisting of two CMOS cameras with a resolution of 5 MPx and a maximum acquisition rate of 75 Hz were used. VICSnap, version 8, was used as the acquisition software and VIC-3D, version 8, from Correlated Solutions, Irmo, USA, as the evaluation software. The recording rate of the cameras was adjusted according to the test speed between 20 Hz for the lower test speeds and 75 Hz for the higher test speeds. The thermographic camera used was a Flir SC6811 from FLIR Systems, Wilsonville, USA. This has a thermal resolution of approx. 18 mK and can record images with 640 x 512 pixels in full screen mode at a recording rate of 565 Hz. When the image lines are reduced, a recording rate of 16,404 Hz is possible, but this also reduces the integration time of the detector. A temporal synchronization of the images and assignment of the force-displacement information of the tensile testing machine was realized via a trigger box, which triggers a start trigger when a pre-force of 10 N is reached and starts the recordings. To determine the glass transition temperature of the amorphous plastics, dynamic differential scanning calorimetry (DSC) measurements are carried out according to DIN EN ISO 11357-1. The test programm is carried out in a temperature range between -20 °C and 280 °C at a heating or cooling rate of 10 K/min. In addition, the thermal diffusivity was determined using laser flash analysis (LFA) on an LFA 447 from Netzsch GmbH & Co. KG, Selb, Germany. For this purpose, cylindrical test specimens were pressed with the aid of a plate press and tested at 25 °C, 50 °C and 100 °C. The PC test specimens were additionally measured at 150 °C. 3.4. Determination of the mechanical properties The mechanical properties were determined in accordance with DIN EN ISO 527 on a UPM 1476 universal testing machine from Zwick-Roell, Ulm. The test temperatures were 25 °C, 50 °C, 75 °C and 100 °C. In addition, the tests were carried out at the following test speeds at each temperature: 0.5 mm/min, 1 mm/min, 5 mm/min, 50 mm/min and 250 mm/min. To determine the transverse contraction number, a dot pattern was applied to each test specimen and the displacement in the x- and y-directions was recorded and evaluated using a camera system. The fracture mechanical properties and the crack tip heating are determined in standard climatic conditions at 23 °C and 50 % relative humidity on a UPM 1455 universal testing machine from Zwick-Roell, Ulm. The test speed is varied between 5 mm/min and 250 mm/min. For the combined measurement with the digital image correlation, a fine, randomly distributed speckle pattern is applied with the aid of an airbrush gun or a high-temperature coating. The crack depth is determined afterwards with the aid of the digital image correlation evaluation software. 4. Results und discussion 4.1. Thermal properties The temperature influences both the mechanical behavior and the fracture behavior. For amorphous polymers, the glass transition temperature is of particular interest. It should be noted that, strictly speaking, this is not a temperature 3.3. Determination of the thermal properties