PSI - Issue 17

M.P. Tretyakov et al. / Procedia Structural Integrity 17 (2019) 865–871 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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1. Introduction

The development of the models of mechanics of deformable solids, which describe the postcritical stage and the corresponding fracture conditions, is important for improving the methods of refined strength analysis of critical structures, predicting their behavior and estimating survivability, including the description of the processes at the crack tip. The wide applicability of polymer composite materials in the manufacturing of critical structural elements that operate under conditions of complex thermomechanical effects, determines the actuality of studying the occurrence of limiting states and the destruction of composites under various loading conditions. From the analysis of the literature it follows that little attention is paid to the postcritical stage of deformation of polymer composites of various classes. The falling branch of the strain curves is not considered as a source of information about the material behavior and is often discarded when presenting experimental data and, as a result, it is not considered when designing and evaluating the safety of structures. Nevertheless, information on the availability and conditions for the implementation of the postcritical deformation stage of polymer composites seems to be necessary for a refined assessment of the bearing capacity and prediction of the survivability of the critical composite parts and can be used in the design and safety assessment of structures (Lobanov et al. (2015, 2017, 2018), Vallons et al. (2014). Analysis of the deformation diagrams of typical polymer composites with different reinforcement directions, obtained with the common types of static tests (tensile, bending, interlayer shear) revealed that the types of loading correspond to the types of drop-down sections of the deformation diagrams. Moreover, the most pronounced postcritical behavior is manifested in the testing of composites under three-point bending and bending according to the short beam method. Among the common types of implementation of drop-down areas of the deformation diagrams of polymer composites, four main ones can be distinguished: • the drop -down branch of the diagram is implemented as an insignificant softening area, followed by a dynamic breakdown; • full step -down branch with equilibrium load redistribution areas; • the drop -down branch of the diagram begins with a dynamic load breaking by a value from 10 % to 50 % of the maximum load, followed by an equilibrium destruction section; • combined realization including the main types. The degree of implementation and the nature of the postcritical stage of deformation depends on the implementation of various mechanisms of destruction of polymer composites. In the works of Dattoma and Giancane (2013), Pohoryles et al. (2017), Lobanov et al. (2015) to identify the mechanisms of destruction in studies are widely used additional systems for recording acoustic emission signals, recording and analysis of deformation fields and infrared thermography. Despite the fact that modern testing systems have high rigidity characteristics, in the process of loading on any testing machine, the specimen deformation is accompanied by elastic deformation of the machine parts, including the frame, thrust, couplings, gripping devices, load-measuring device. The greater this deformation, the more pliable the loading system, by which is understood the aggregate of all bodies, which are deformed because of the transfer of the load on the sample. The importance of the concept of “loading system” from the point of view of the study of postcritical deformation processes was noted earlier in the work of Vildeman et al. (1997, 2008), Bazant et al. (2004). The dependence of the postcritical behavior of metallic materials on the rigidity parameters of the loading system and the type of stress-strain state is considered in the works of Tretyakov et al. (2016, 2018), Vildeman et al. (2016). The aim of the work is an experimental study of the postcritical behavior patterns of layered fiber composite materials in bending experiments using the short beam method for various parameters of the rigidity of the loading system, loading rate and elevated test temperatures.

2. Materials and methods

The tests were carried out on samples of structural fiberglass (GFRP) on the basis of UPPS-48 prepreg and epoxy binder VSE 1212 with laying layers [0º / 90º] 12 manufactured by autoclave molding and samples of structural carbon-