Issue 53

E. M. Strungar et alii, Frattura ed Integrità Strutturale, 53 (2020) 406-416; DOI: 10.3221/IGF-ESIS.53.31 413 Special attention is paid to analysis of deformation fields pattern evolution for assessment of development of the healed reach-through breakdown defects and formation of macro-damage conditions in three-layer composite fiberglass laminate panels. The authors suggested a method of study of the defect effect on residual strength of the elements of composite structures based on the joint use of the systems of deformation fields and temperatures registration. They assessed the effect of cyclic tests on behavior of defect-containing composites with the following parameters: asymmetry coefficient R= -0.25; cycle amplitude Na = 25kN; sinus wave shape; frequency – 0.25Hz; the number of cycles n=200. The minimal values at loading constituted -10kN, the maximal ones – 40kN (Fig. 10). It is shown that the bearing capacity of a sample with a defect applied is 30% lower than that of a defectless sample. Cyclic loadings under the above-discussed modes resulted in another strength reduction by 10% [12]. T HE USE OF DIGITAL IMAGE CORRELATION METHOD FOR SOLUTION OF FIBER OPTIC TECHNOLOGY TASKS mong the present-day and promising diagnostic instruments, the monitoring systems based on the introduction of fiber optic deformation sensors (FODS) which are effective both under production conditions and during operation of various purposes products and objects, are of special interest. Introducing FODS into the composite structure, it is necessary to solve a number of technical and scientific problems. One of the trends of the use of DIC method may be the use of this instrument to solve the problems of fiber optic technologies [14,27]. The final part of the paper shows that the use of video systems for three-dimensional analysis of the displacement and deformation fields makes it possible to develop recommendations for location and calibration of built-in fiber optic deformation sensors FODS (taking into account the calibration coefficient provides the increase of precision of deformations recorded by FODS by 25%), and assess the sensitivity to loading modes [27]. Besides, the paper studied the organization of the equilibrium fissure growth mode. This study provides the results of experiments on registration of deformations under cyclic loading, and the development of recommendations on registration the damages based on the results of measuring the deformations during cyclic loading. In this case, we provided not an instant development of the damage area, but its development with a rate that allows us to speak about significant effect of the number of cycles on destruction. To analyze the deformation fields and determine the locations of the optical sensors to record the origin and development of damage in the polymer composite material, we implemented a multi-stage cyclic loading according to Fig. 11 diagram. The transition from one stage to another was carried out with the decrease in the value of the load amplitude Pa by more than 5%. A sample with a lateral cutout was used as the study object, it served as a stress concentrator, in the vicinity of which the damages first occur under loading. The longitudinal deformation fields and the corresponding images obtained using a microscope are shown in Fig. 12[14]. Figure 11: Staged strain mode. Similarly, this technique was applied to carbon fiber composite samples; a loading-time diagram with the marked points corresponding to longitudinal strain fields is shown in Fig. 13. The criterion for transition from one stage to another was the change in the value of the loading amplitude Pa by 3, 10, 50%. In the course of a staged cyclic loading necessary for recording the origin and development of damages in the polymer composite material, a more equilibrium fissure growth is observed for fiberglass samples, than for carbon fiber composite samples. The presented technique is necessary for determining the locations of optical sensors during registration of origin and development of damages in a polymer composite material A