PSI - Issue 17

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

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critical structures. There are more and more researches aimed at studying behavior of composite materials under cyclic effects M.Haggui et al (2019), Movahedi-Rad et al (2019), Chen G. et al (2019), Fouchier, N. et al (2019), Růžek, R. et al (2018), Strizhius V. (2016). For composite materials based on a polymer matrix, the stages of changes in residual strength and stiffness properties during fatigue damage accumulation have been revealed Philippidis T.P., Passipoularidis V.A. (2007), Van Paepegem W. et al (2005), Maragoni L. et al (2016). Researchers outline direct and indirect methods for assessing residual properties of specimens of composite materials Philippidis T.P., Assimakopoulou T.T. (2008), Matvienko Y.G. et al (2016), Plekhov O. et al (2005), M.Haggui et al (2018), A. Maleki et al (2018), Dattoma, V., Giancane S. (2013), 21. Lobanov D.S. et al (2015). The direct methods are based on the results of quasi-static tests after preliminary cyclic exposure without fatigue failure. The indirect methods are focused on the use of non-destructive control systems, such as infrared thermoscanning and recording of acoustic emission signals in the process of cyclic loading. Increased temperatures can significantly affect deformation and fracture, fatigue life and the survivability of polymer composites. The work aims at an experimental study of the influence of increased temperatures on deformation and fracture of polymer composite materials (PCM) under cyclic conditions. To achieve the aim, a series of experiments were carried out using the research facilities of the Center for Experimental Mechanics (http://www.ckp-rf.ru/ckp/353547/). The study focused on the composite material specimens made by serial production technology based on VPS-48 fiberglass prepreg and VSE 1212 binder with a reinforcement scheme [0º/90º] 8 using the autoclave molding method. Mechanical properties of the composite were determined under preliminary quasistatic uniaxial tension tests taking into consideration ASTM D 3039 recommendations. The tests were carried out using Instron 5882 electromechanical system equipped with a temperature chamber with a working temperature range from - 100 to +350ºС. The traverse speed was 2 mm/min. The groups of specimens were tested at temperatures of 22, 120 and 200ºС. To measure the longitudinal deformation of the specimens, Advanced Video Extensometer (AVE) Instron 2663-821 non-contact video extensometer was used. Its operation is based on determining the coordinates of the contrasting (white or black) coordinates of the measuring base marks printed on the working part of the specimen using the high resolution digital video camera. The use of the video extensometer is justified by the fact that it does not exert an additional mechanical effect on the specimen surface in the working area, and can also be used together with a heat chamber throughout the entire temperature range without restrictions. The field of view of the video extensometer is 200 mm, the video signal digitizing rate is one frame of information in 20 microseconds described by Lobanov D.S. et al (2018). The specimens were preliminary thermostatically- controlled under increased temperatures of 120 ºС and 200ºС. The temperature control mode included linear heating of the specimens to the selected temperature at a rate of 10°C/min and holding for 2 hours for the entire group of specimens and 0.5 hours after each subsequent fitting of the specimen. To select the cyclic loading parameters, the values of the elastic modulus, tensile strength, and relative elongation were determined. The methods of cyclic tests under increased temperatures complied with recommendations of the corresponding ASTM D3479 standards. The fatigue tests were carried out using Instron Electropuls E10000, the electromechanical system equipped with a temperature chamber with a working temperature range from - 100 to +250ºС . At increased temperatures of 120 and 200ºС the thermostatically -controlled mode was similar. The specimens had thermomechanical fatigue life tests (the maximum number of fatigue fracture cycles (N max ) at the parameters of cyclic loading: σ max = 0,5∙σ B , w here σ B – strength limit , R = 0,1, ν = 20 Hz and a sinusoidal form of cycles. The study of changes in the residual strength and stiffness properties of the polymer composite specimens at a temperature of 120°C was carried out with a combination of the above techniques. The technique included quasistatic tensile tests with the determination of the nominal values of strength limit σ B and Young's modulus E at increased temperatures. Later the value of fatigue life of the specimens N max was determined during cyclic tension wirth a frequency of ν = 15 Hz, coefficient of skewness R = 0, 1 and the value of maximum stresses in cycle σ max = 0, 5 σ B . After that the specimens were subjected to cyclic pre-exposure with cycle life in the range (0,1 – 0,8) N max . The values of residual strength and stiffness characteristics of the studied PCM specimens were determined during quasistatic tensile tests at increased temperatures. 2. Material, Experimental Facilities and Testing Techniques