Automated thermographic monitoring of crack growth in short fiber reinforced polymers under cyclic loading

Accurately measuring crack growth during fracture mechanics testing is often time-consuming and can be highly dependent on user experience, as manual tracking requires individual judgment to identify the crack tip, which may vary between operators due to effects such as matrix crazing, fiber bridging, or small-scale surface irregularities. To address this challenge, a novel thermographic approach was developed by integrating Digital Image Correlation (DIC) techniques for the automatic detection of crack growth in short fiber reinforced polymers. The method was experimentally validated on two representative materials: polypropylene reinforced with 40 wt% short glass fibers (PPGF40) and polyetheretherketone reinforced with 30 wt% short glass fibers (PEEKGF30) and was able to measure crack growth rates greater than ∼1 × 10−6 mm/cycle. Tests were conducted at room temperature under cyclic loading conditions with a load ratio of R = 0.1. For PPGF40, manual and automated crack length measurements were performed in parallel, showing R2 values consistently above 0.8. For PEEKGF30, comparison of crack kinetic curves obtained by both methods for different specimens using logarithmic data also yielded R2 values above 0.8, except for one test where R2 was 0.72. Furthermore, a detailed investigation into the fracture mechanisms and the influence of stress intensity on crack propagation was carried out, providing deeper insight into material behavior under fatigue loading. The observations indicate that the fracture mechanisms along the crack growth path vary with the stress intensity, with higher stress intensities leading to a more ductile failure in the matrix and more fiber pullout.