This study explores the potential of engineered interphases to enhance the performance of fiber-reinforced composites through an integrated approach combining theory, computation, and experiments. A theoretical axisymmetric modeling framework is introduced to evaluate stress distribution within a three-phase composite system subjected to axial pull-out loads. The composite comprises a cylindrical fiber embedded in a matrix with intermediate interphase whose modulus varies radially, following linear and power-law gradation profiles. A variational principle is employed to derive formulation for stress estimation across the composite components. To validate these theoretical insights, an axisymmetric finite element model incorporating a radially graded interphase is developed using a user-defined subroutine (UMAT) in ABAQUS. Comparative analysis demonstrates good agreement between theoretical predictions and finite element results. Notably, graded interphases reduce peak radial and shear stresses at the interphase by approximately 37% and 60%, respectively, compared to ungraded interphases. These predictions are corroborated by single-fiber pull-out experiments using specimens with stepwise modulus-graded interphases fabricated via an Object Connex260 PolyJet 3D multi-material printer. Experimental results confirm that graded interphases significantly enhance the composite's load-bearing capacity and toughness by mitigating peak interfacial stresses. This study underscores the transformative potential of strategically engineered interphases in optimizing composite system performance, offering insights into advanced material design for high-performance applications.

We report the energy absorption and piezoresistive self-sensing performance of 3D printed discontinuous carbon fiber (CF)-reinforced polyetheretherketone (PEEK) cellular composites. Experiments conducted on three different 2D lattices with hexagonal, chiral and re-entrant topologies of the same relative density (33%) and CF loading (30 wt%) reveal that the CF/PEEK hexagonal lattice (HL), due its relatively brittle response, shows about 40% and 9% decrease in specific energy absorption (SEA) under in-plane and out-of-plane compression, respectively, compared with PEEK HL. While the collapse response of PEEK HL is nearly insensitive to the strain-rate over 43 ≤ ε̇ ≤ 106 s−1, we observe a twenty-fold increase in peak stress and a five-fold increase in SEA under in-plane impact loading over the same range of strain-rates for the CF/PEEK HL. The CF/PEEK lattices exhibit pronounced piezoresistive response under both in-plane and out-of-plane compression with maximum sensitivity of 3.1 and 5.2, respectively, for the re-entrant lattice, offering insight into the damage-state. Higher damage sensitivity indicates faster percolation of new contacts due to folds forming between the cell walls within the lattice under compression. The energy-absorbing and strain- and damage-sensing nature of 3D printed CF/PEEK lattices demonstrated here offers insight into the design of lightweight, high-performance multifunctional lattices.

The study is focused on multifunctional performance of carbon nanotubes (CNT) and Graphene nanoplatelets (GNP) reinforced PEEK composites enabled via fused filament fabrication (FFF) additive manufacturing (AM) utilizing in-house nanoengineered filaments. Thermo-physical, mechanical and wear characteristics of electro-conductive PEEK nanocomposites are reported. The coefficient of thermal expansion (CTE) is found to decrease by 26% and 18% with the incorporation of 5 wt% GNP and 3 wt% CNT into PEEK polymer, respectively. The decrease in CTE provides better dimensional stability to resulting nanocomposite structures. Due to uniform dispersion of CNT and GNP in the PEEK matrix, the crystallization temperature and degree of crystallinity are both increased. The 3D printed PEEK nanocomposites reveal interfacial voids between the beads and infra-bead pores and thus exhibit lower density compared to that of the 3D printed neat PEEK. Young's and storage moduli are found to increase by 20% and 66% for 3 wt% CNT loading and by 23% and 72% for 5 wt% GNP loading respectively. However, the PEEK nanocomposites exhibit similar tensile strength to that of neat PEEK. The coefficient of friction obtained from fretting wear tests is found to decrease by 67% and 56% for 1 wt% CNT and 3 wt% GNP loaded PEEK nanocomposites, respectively and the decrease is attributed to reduced hardness and increased porosity. Multifunctional performance of carbon nanostructures reinforced AM-enabled PEEK composites demonstrated here makes them suitable for a range of applications such as orthopedics, oil and gas, automotive, electronics and space.