Investigation of the correlations between the microstructure and the tensile properties multi-scale composites with a polylactic acid matrix, reinforced with carbon nanotubes and carbon fibers, with the use of the fiber bundle cell theory

In this paper, we established a relationship between the microstructure and the tensile properties of nano- and hybrid composites with a thermoplastic, semi-crystalline poly(lactic acid) (PLA) matrix, and reinforced with carbon fibers and carbon nanotubes, using the fiber bundle cell model. The microstructure of the matrix and the volume ratio of the mobile amorphous, rigid amorphous and crystalline fractions were determined by differential scanning calorimetry and the elastic modulus of these fractions was measured by atomic force microscope. These data served as input parameters for the fiber bundle cell model, in which each structural unit corresponded to a single bundle of fibers. For the parameters for which no experimental method was available, we determined structure-independent constants. With these model parameters, we fitted a fiber bundle cell model curve to the averaged tensile curves of each material. By analyzing the fitted model, we concluded that the mobile and the rigid amorphous fraction of the matrix has a key role in the properties of the initial, load-carrying section of the tensile curve. The crystalline fraction and the carbon nanotubes elongated the failure section. By examining the damage maps, we found that overlapping interphases in the vicinity of carbon fibers and carbon nanotubes improved the load transfer between the carbon fiber and the matrix, thus allowing better utilization of the reinforcing effect of the carbon fibers.