On the post-mortem fracture surface morphology of short fiber reinforced thermoplastics

The fracture surface morphology of short fiber reinforced thermoplastics (SFRTs) has often been used to assess qualitatively the degree of fiber–matrix interfacial adhesion. Mechanical properties such as tensile strength, fracture toughness and failure strain, etc. are then correlated with the morphology. Fracture surfaces showing fibers surrounded by a large amount of matrix material is commonly regarded as indication of strong fiber–matrix interfacial adhesion while smooth fibers are characteristic of weak interfacial adhesion. Many experimental results of SFRTs have been so interpreted. However, it is shown in this paper that strictly speaking, such interpretations are generally incorrect. Moreover, the amount of matrix material does not provide a quantitative measure of the adhesion. Correct implication of the morphology of fracture surfaces is clarified. Short glass fiber reinforced polyamide 6,6/polypropylene (PA 6,6/PP) blends toughened by rubber are employed as examples for SFRTs since the PA 6,6/PP blend system by changing PA 6,6 concentration in the matrix blend represents a wide range of matrix materials. It is demonstrated that the fracture surface morphology of such composites is dependent on both fiber–matrix interfacial adhesion strength and matrix shear yield strength. Consequently, tensile failure strain is well correlated with the post-mortem fracture surface morphology of these SFRTs.