Methods for determining the interlaminar fracture toughness according to Mode I (GIC) of layered composites are considered. Tests were conducted on two types of DCB specimens of sheet fiberglass, STEF-1/3 and STEF-1/4, with thicknesses of 3 and 4 mm, respectively, when delaminated with different initial lengths. Additional parameters, adjustment factors, and values of GIC of fiberglass during delamination according to ASTM D5528 using the conventional beam theory with a perfectly integrated DCB specimen – \({G}_{IC}^{I}\) , the compliance calibration (CC) method – \({G}_{IC}^{CC}\) , and the modified beam theory (MBT) method – \({G}_{IC}^{MBT}\) . A significant difference in the fracture of two fiberglass materials during their delamination was demonstrated. For STEF-1/3 with a relatively large glass-fabric structure, significantly larger jumps were observed on the force-displacement diagrams, and the GIC values were lower than those for STEF-1/4 across all methods. For each set of experimental data, linear regression equations were calculated, allowing comparison of the results of determining GIC for each method. It is shown that the CC and MBT methods for fiberglass data provide a more reliable estimate of GIC compared to the conventional beam theory and the assumption of a perfectly embedded specimen.
