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Optical measurement of crack propagation experiments
 

Seyed Iman Saberi Fathi

 

As part of this thesis an optical method for crack growth detection of plastic specimens under simultaneous mechanical strain (variable load) and media exposure (for example, liquid hydrocarbons) was developed. To implement this approach, both a customized software and a suitable image processing system (including hardware) was developed. The realized system allows automatic data acquisition (load cycle, crack length, crack opening, etc.) in real time. Sinusoidal force signal was applied to the specimen under test and the pictures were taken at maximum displacement in a user-defined time interval.

The software-based technical analysis (measurement including distortion correction) was carried out with the calibration of the recorded (crack growth) images using a reference image (calibration grid) with a known geometry. A graphical representation based on the detected crack growth rate was utilized (da/dN) for a defined media temperature environment on fracture mechanics parameters (stress intensity factor K). This representation helps to estimate kinetic crack propagation values, which are used for lifetime calculation of components under similar stresses (strain or ambient medium).

 

Figure 1: Configuration of cameras and lighting. Figure 2: Entire setup to realize the experiments.

In addition, the optically determined crack lengths were compared (direct method) with the measured values from an indirect method for the detection of the crack length (crack compliance via extensometer). At the end, air was the only possible ambient medium because the liquid ambient media (chemically aggressive) were not realistic with the av

Keywords: optical measurement, image processing, crack propagation, polymer material, fracture mechanics

September 4, 2014