Critical States of Laminated Polymer Composite under Quasi-Static Deformation after Preliminary Low-Velocity Impact Loads

The study results of the influence of preliminary localized impacts of different intensity on the mechanical behavior of laminated polymer composite with the use of modern testing and diagnostic equipment is presented in this paper. Drop-weight three-point bending tests followed by quasi-static tension were carried out. The data of change in residual strength and deformation characteristics of the studied composite were obtained. The nonlinear dependence of residual strength and stiffness on the energy of preliminary impacts was noted. On the basis of the previously developed approach that is based on the analysis of fatigue sensitivity diagrams, impact sensitivity diagrams were plotted and threshold values of dynamic impact energy, corresponding to the transition to critical states, were determined. Deformation and fracture processes during quasi-static tension of damaged fiber-reinforced plastic (FRP) specimens were analyzed on the basis of data, obtained by the VIC-3D video system. Using the method of digital images correlation (DIC), stress-strain curves of damaged specimens were plotted. The evolution of inhomogeneous longitudinal strain fields on the FRP’s surface is presented. The processes of damage accumulation and fracture of FRP specimens during quasi-static tension were analyzed, based on the data of acoustic emission signals that were registered with the AMSY-6 system. The relationship between the change in impact energy and the registered parameters of acoustic response signals was established. Three characteristic frequency ranges, correlated with matrix cracking, delamination and fiber breakage, were identified. The conclusion was made about the significant influence of preliminary dynamic impacts on the deformation and fracture patterns of laminated-fiber polymer composite materials.