Quantitative evaluation of inclusion homogeneity in composites and the applications

Microstructures are critical for designing composite materials with admirable properties required by various applications. However, desired inclusion arrangements are pre-required and have to be reached by appropriate processing as well as quantitative evaluation. Up to date, visual characterization like transmission electron microscope is still the "golden rule" for measuring inclusion arrangement, together with qualitative descriptions in most studies. In order to establish structure-property relationship, optimize processing parameters and even predict composite performances, quantitative evaluation of inclusion arrangement becomes a must. This review focuses on the numerous developed mainstream quantitative approaches and their recent advances. Methods, including ASTM index (American Society of Testing Materials), Morishita's index, quadrat method, linear intercept distance, free-path spacing, nearest neighbor distance, inter-particle distance, free-space length, functional analysis, Delaunay triangulation and Dirichlet tessellation, are classified into three categories: methods based on inclusion features, methods based on inclusion position and methods based on inclusion position and inter-distance, which are introduced in the manuscript. Definition, advantages, updated progress and recent application examples of specific method are included. The differences between each methods and their limitations are summarized.