Studying the dynamics of microdefect growth in carbon fiber reinforced plastics under mechanical loading by means of ultrasonic microscopy

Abstract

Strength characteristics of carbon fiber reinforced plastics (CFRPs) are investigated by nondestructive means as microstructural changes in a material’s bulk under external mechanical loads. CFRP microstructure is studied experimentally via pulsed ultrasonic microscopy at the level of mechanical deformation resulting in degradation of a material’s properties. The process of composite deformation is studied by means of stepped stretching. Acoustic emissions are used to identify the stage preceding final destruction (the accumulation of microcracks, fibers breaking, and delamination) as an indicator of a material’s degradation. Pulse acoustic microscopy is used to observe the accumulation of microcracks in individual layers of a material. To study the behavior of a CFRP microstructure upon mechanical loading, tensile stress was applied to samples with cross-ply packing of fibers (0°, 90°) and (45°, −45°). It is shown that the brittle fracturing of reinforcing fibers is typical of CFRPs with fiber orientation (0°, 90°), and is accompanied by growing areas of stress concentration and a rise in of acoustic emission activity, with a subsequent increase in the signal energy and the formation of extensive interlaminar delamination. Acoustic emission shows a low level of activity for CFRP samples with fiber orientation (45°, −45°), which is accompanied by the formation of structural microdefects that are clearly visible in acoustic images.