Methodological improvement of stable crack growth analysis in filled TPS/PBS blends using optical light microscopy, confocal laser scanning microscopy and X-ray micro computed tomography

Understanding stable crack propagation in natural fiber-reinforced biopolymers remains a challenging task, particularly about the measurement and interpretation of stable crack propagation (Δa). A major cause of this is the highly complex fracture surfaces of fiber-filled materials. This study investigates stable crack propagation ( Δa ) of a thermoplastic starch and polybutylene succinate (TPS/PBS) blend filled with ground hemp fibers (HFm) at fiber contents of 0, 10 and 15 wt.%. The test specimens were tested using an instrumented notched-bar impact test (I-NBIT) equipped with a stop block configuration (StB) and compared with the results of the conventional notched-bar impact test (NBIT). To evaluate fracture surfaces, complementary characterization methods were used including optical light microscopy, confocal laser scanning microscopy (CLSM), and X-ray micro computed tomography (μCT). The results show good agreement between optical light microscopy and CLSM in the projected crack propagation Δa , while CLSM enabled three-dimensional (3D) surface topography reconstruction of the crack, thereby improving the reproducibility of the crack length measurement. Furthermore, μCT provides additional insights into fracture-related features that are only partially accessible by surface-based optical methods, especially for test specimens with higher filler contents and increased fracture surface complexity. This enables analysis of the 3D crack path and the true fracture surface geometric, leading to a more precise assessment of Δa . Overall, the complementary use of optical light microscopy, CLSM, and μCT enables a more detailed characterization of Δa and shows that method selection significantly influences the understanding of fracture behavior in natural fiber-reinforced bioplastics.