Estimation of residual fatigue life of polymer composites after preliminary low-velocity impact

Abstract

Composites are replacing traditional metals and alloys in many industries, including aviation, space, building, automotive, petrochemical, due to their lower weight and physical and mechanical characteristics, which allows them to be introduced into critical high-loaded structures and parts. At the same time, in the process of operation, composite structures can be in conditions of complex thermomechanical action. Data on changes in the residual mechanical characteristics, available damages allow increasing the prediction life, survivability and safety of destruction of composite structures.The paper presents the results of studying the processes of accumulation of fatigue damage and destruction of CFRP specimens with layouts [0º/90º]n and concentrators of an operational nature. The study was carried out with the implementation of a preliminary transverse impact by a falling load according to the three-point bending and subsequent cyclic stretching. In accordance with the developed technique, the value of the ultimate strength (σB) of the carbon fiber under study was determined from quasi-static tensile tests. The nominal fatigue life (Nmax) is obtained from tests with the parameters with the maximum stress in the cycle σmax=0.75∙σB, the asymmetry coefficient R=0.1 and the frequency ν=20 Hz. During the cyclic tests, the frequency and amplitude of the loading were selected in such a way that the heating of the sample did not exceed 20°C relative to the temperature of the beginning of the test. From experiments on impact bending, the potential energy (Emax) was determined, at which the samples were destroyed. Further tests for preliminary impact according to the three-point bending were carried out in the range from 0.3 to 0.9 of the destruction energy (e’). Single blows were made across the entire width of the samples. After each impact, a visual inspection of the samples was carried out. Microstructural studies and analysis of fracture patterns of prototypes under complex shock impacts and cyclic loading were performed using a Carl Zeiss SteREO Discovery V12 stereomicroscope.As a result, experimental dependences of the influence of different intensity of impact bending on the residual fatigue life of carbon fiber specimens were obtained. Analysis of the diagram of the change in the residual fatigue life versus the impact bending intensity made it possible to reveal the presence of a shock sensitivity threshold. Impact actions with energies exceeding the threshold value led to a significant decrease in fatigue life, by more than 10% of the nominal value of Nmax, for the studied CFRP composites.