Fractal Description of Interlaminar Contact Development during Thermoplastic Composites Processing

Fabrication of layered thermoplastics and thermoplastic-matrix composites using processes such as tow placement, tape laying, and resistance welding, is fundamentally based on the principle of fusion bonding, which involves applying heat and pressure to contacting thermoplastic surfaces. One of the important processing steps— intimate contact—is considered in this paper. Interlaminar intimate contact development is a strong function of thermoplastic surface geometry. Profilometric measurements of thermoplastic prepreg tows show that surface roughness features can be found at several length scales, which implies that the surfaces have a fractal structure. In this paper, principles of fractal geometry are used to describe prepreg surfaces. Based on this description, an axisymmetric squeeze flow model is developed to relate degree of intimate contact to the process parameters—pressure, temperature, and time—and the fractal parameters of the surface. The model development and comparisons with available experimental data are presented and discussed in this paper.