PSI - Issue 17

654 Dmitrii S. Lobanov et al. / Procedia Structural Integrity 17 (2019) 651–657 Lobanov Dmitrii S., Staroverov Oleg A./ Structural Integrity Procedia 00 (2019) 000 – 000 = ⁄ (2) where u a is the value of the amplitude of sample’s displacements during tests; P – is the load value under cyclic tension; n t – is a relative number of cycles up to fracture at different temperatures. 4

Fig. 2. Diagrams of changes in the relative values of the stiffness of the sample R during fatigue testing at various temperatures

A decrease in the relative value of the specimen stiffness R by 0.1 corresponds to a decrease in the specimen stiffness by 10%. Under the test temperatures of 120°C and 200°C, the dependences of R stiffness alterations had a similar view; the curves appeared on the straight section of the diagram after a smaller number of cycles compared to the curves obtained during the tests at room temperature. For the specimens tested at 200°C, fracture occurred with numerous delaminations throughout the working area of the specimen (Fig. 3); in the diagram of R specimen relative stiffness, this type of fracture corresponds to the interval (0.9 - 1) n’.

c

a

b

Fig. 3. Fracture surfaces of specimens after fatigue life tests at 22 (a), 120 (b) and 200 ° C (c)

Under the testing temperatures between 22 and 120°С with respect to ASTM classification, the specimens fractured according to the LGM mechanism; under higher temperature of 200°С there comes the shift o f fracture mechanisms into DGM and DMM (Fig.3). A significant destruction of the binder leads to a disruption of interlayer adhesion and development of multiple delaminations in the working area of the fiberglass specimens. 4. Constructing Diagrams of Fatigue Sensitivity of Fiberglass Composites at Increased Temperatures The relations of the residual properties alterations on preliminary cyclic loads and increased temperature (120°C) are presented in the form of the fatigue sensitivity diagram by Wil'deman V. E. et al (2018). The diagram of fatigue sensitivity in relative coordinates K Bn ’ – n’ , where ′ = ⁄ (3) is the factor of retention of the static strength in cyclic loading, with σ Bn and σ B — the strengths after and before