Estimating parameters of permissible defects in structural fiberglass based on theory of critical distances

In the process of manufacturing products from composite materials, many defects can occur: cracks, chips, scratches, dents, impact defects, air macro inclusions, and others. Such defects can significantly reduce both the static and fatigue resistance of structures. The purpose of this work is to determine the size of defects that do not affect the strength characteristics of products made of STEF composite material using the point and linear approaches of the theory of critical distances. In the course of the work, a series of tensile tests were carried out on flat specimens of STEF structural fiberglass for electrical purposes. In addition to the experiment, numerical simulation of the tensile processes of these specimens was also carried out. The studied specimens were strips without stress concentrators and with a concentrator in the form of V-shaped notches with different rounding radii at the concentrator top and notch depth. The results obtained were used to determine the material constants according to the theory of critical distances. In this case, two approaches of the theory of critical distances were used, i.e. linear and point ones. To analyze the experimental results, finite element models were built using the ANSYS software package; and numerical simulation was carried out, which resulted in the obtained linearized maximum principal stresses on the central line passing through the top of the stress concentrator. Based on the results of the work, the values of the critical distances of the composite were determined, obtained by using the point and linear methods. On the basis of the data obtained, the sizes of permissible defects in the studied fiberglass were established, which do not affect the strength characteristics of the material. The results obtained can be used to predict the strength characteristics of real products with a complex geometry, as well as to diagnose damaged structural elements.