Investigating Strain Rate Effects on the Anisotropic Mechanical Properties of Lignin‐Filled Glass/Polypropylene Long‐Fiber Thermoplastics Manufactured via Direct Compounded Compression Molding ( LFT‐D )

ABSTRACT Fiber‐reinforced composites are employed in various sectors, such as the transportation sector, where they are subjected to various loading conditions, including high loading rates. In this study, the tensile properties of lignin‐filled glass/polypropylene (PP) long‐fiber thermoplastics (LFT) were investigated. These materials were manufactured using a direct compounded compression molding process, and tests were conducted at loading rates ranging from quasi‐static (0.0001 ) to elevated strain rates of up to 200 . The samples were examined in four material orientations, namely, 0°, +45°, −45°, and 90°, to understand their anisotropic behavior with three different percentages of lignin filler (0%, 14%, and 21%). Tensile strength was significantly enhanced at elevated strain rates, stabilizing beyond 100 . For 0% lignin, strength increased from 100 MPa (quasi‐static) to 221 MPa (100 /200 ), with 132% and 98% improvements for 14% and 21% lignin, respectively. While 14% lignin increased stiffness, 21% lignin weakened interfacial bonding, reducing mechanical strength. Strong anisotropy was observed, with the highest tensile strength (221 MPa) in the 0° direction and the lowest (74 MPa) in the 90° direction. SEM analysis revealed increased fiber pullout and matrix‐fiber debonding at higher lignin content, further reducing mechanical performance.