PSI - Issue 28

Jan Seyda et al. / Procedia Structural Integrity 28 (2020) 1458–1466 Author name / Structural Integrity Procedia 00 (2019) 000–000

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Fig. 1. Dimensions of PA38-T6 specimens, given in millimeters.

The materials used for specimen surface replication are 125 µm thick cellulose acetate foil and a set of auxiliary tools. The micro photographs were taken on a metallographic microscope Delta Optical MET-1000-TRF equipped with DLT-Cam PRO 12MP digital camera with dedicated image processing software DLTCamViewer. In order to capture features of the whole surface of specimen’s working part, the first test was conducted in strain-control mode. Next, based on the recorded displacement history, the second test was performed in displacement-control mode. The tests were paused every 10% of expected fatigue life to make the replica on both sides of the specimen. The cellulose acetate sheet was attached by spraying a drop of acetone between tape and specimen. The acetone melts the tape, which sticks to the specimen. Next it evaporates for 7 to 10 minutes. When the cellulose acetate was dry again, it was gently taken off with a plastic pincer and put between microscope slide glass. Very small details of specimen surface topography imprint on the replica. Moreover, the replica can be observed using transmitted light. Thanks to that, there is no interference caused by light reflection or curved surface effect. The greatest advantage of this technique is an ability to track the crack development history. To show the capabilities of this technique, the image of replica and direct image of specimen surface are compared in Fig. 2. Shots were captured in such a way that the vertical direction is in line with the specimen axis.

Fig. 2. Comparison of crack images: (a) taken directly on specimen surface and (b) from replica.