Effect of porosity and microstructure defects in out-of-plane properties of 3D printed composite materials made by continuous fiber reinforcement

Abstract

This research presents a comprehensive experimental and predictive analysis of 3D printed composite materials (3DPCM) made with Onyx reinforced Kevlar fibers. The mechanical behavior was characterized through three in-plane (tensile, compression and flexural) and four out-of-plane (Mode I, Mode II, Mixed-Mode I/II fracture, and short-beam strength) tests to evaluate both intralaminar and interlaminar responses. In-plane results revealed a strong dependence on fiber orientation, with the 0° fiber orientation achieving higher tensile and flexural, while the 90° fiber orientation exhibited slightly greater compressive modulus. Out-of-plane results demonstrated higher Mode I fracture toughness in 90° fiber orientation, whereas Mode II and mixed mode responses were dominated by shear effects, following the relationship 𝐺𝐼𝐼𝑐 >𝐺𝐼𝑐 > 𝐺𝐼/𝐼𝐼𝑐. Microstructural analysis identified voids, matrix peeling, fiber exposure, and poor impregnation as the key defects influencing crack initiation and delamination. Also, void content of the samples demonstrated an impact in mechanical properties as in traditionally made composites, the higher the void content the higher the mechanical variation. Finally, a machine-learning predictive model was developed to predict load-displacement curves in Short-Beam Strength tests, enabling accurate prediction of mechanical responses (𝑅2 > 0.94) based on number of fiber layers and fiber orientation configuration. These findings highlight the strong coupling between printing parameters and mechanical performance, providing valuable insights for the design and optimization of 3DPCM.