Review of long Fiber thermoplastic composites: Processing and mechanical performance

Long-fiber thermoplastic (LFT) composites are widely used in lightweight structural applications due to their specific stiffness, damage tolerance, compatibility with volume manufacturing, and ability to retain longer fibers, thereby improving load transfer. During molding, heterogeneous microstructures develop, including aligned skin layers, randomly oriented cores, and transitions between charge and flow dominated regions, resulting in spatially varying properties. This review addresses current research gaps by examining the relationships between manufacturing conditions, microstructural evolution, and the resulting mechanical behavior of LFTs, with particular emphasis on recent developments in direct long-fiber thermoplastic (D-LFT) processing and their effects on fiber length and orientation distributions. It critically evaluates how these microstructural characteristics, together with strain rate, environmental exposure, and the differences between charge- and flow-dominated regions, influence tensile, flexural, shear, fatigue, and impact performance, highlighting the origins of variations in mechanical properties and the associated failure mechanisms, which have received limited attention in previous studies. The review summarizes recent advances in numerical modeling, highlighting the remaining challenges in accurately representing microstructural complexity and predicting structure–property relationships. Finally, it broadens the perspective beyond the automotive focus of previous studies by reviewing emerging applications of LFTs across diverse structural and engineering sectors.