<p>With the rising need for lightweight car bodies, short fiber reinforced polypropylene is increasingly being used for load-bearing components. In many thermoplastic matrix systems, slow crack growth-controlled failure is the main factor determining the overall lifetime. Therefore, consideration of the durable life of short fiber reinforced thermoplastic systems must account for their robustness against slow crack growth. However, despite its significance, due to the complex crack tip-process zone interaction and the unique damage dissemination behavior of polypropylene matrix, the slow crack growth of short glass fiber-reinforced composites remains understudied. In this study, in order to address the aforementioned gap, slow crack growth of short glass fiber reinforced composites with varying reinforcement compositions was observed under static and cyclic loading conditions. Due to the inadequacy of conventional real-time crack length measuring techniques for application to polymeric materials, a novel technique based on image processing was applied. Not only the crack length but also the process zone evolution was measured using the scattering of the incident light due to the damage development in the form of crazes. Similar damage development in the process zone, regardless of the short glass fiber reinforcement composition or loading mode, was observed, suggesting the dominant role of polypropylene matrix even in composite systems. It is expected that the observational results of this study will enable the development of theoretical frameworks, such as the crack layer theory, for the accurate assessment of the lifetime of short fiber-reinforced polypropylene systems.</p>