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Pitch-based carbon fiber reinforced epoxy composites are used in specialized applications for their high-modulus and thermal conductivity; however, little data on their fatigue performance are available in the open literature. In this study, fatigue behaviors of ultra-high-modulus pitch-based carbon fiber and standard-modulus polyacrylonitrile (PAN)-based carbon fiber were compared in woven quasi-isotropic epoxy matrix composites subject to uniaxial tension. It was found that the pitch fiber composite possessed higher normalized tensile fatigue strength and that its stress-life (S-N) curve is less steep. Extrapolation suggests the pitch fiber composite is more fatigue-resistant in higher cycle regimes (specifically, N > 107). Cyclic loading of the pitch fiber composite resulted in minimal matrix damage, and the eventual fractures were localized and fiber-dominated for all stress levels. Cyclic loading of the PAN fiber composites resulted in widespread matrix cracking and delamination. The difference in fatigue behavior is attributed to the different strain levels attained at similar stress levels and the consequent difference in matrix damage development. [ABSTRACT FROM AUTHOR]

Tensile quasi-static and fatigue behaviors of out-of-autoclave, quasi-isotropic, E-glass/epoxy composites with woven and stitched fiber architectures are compared. Some specimens were cyclically loaded to failure, while others were tested for residual modulus and ultimate strength. Both laminates survived 107 cycles at a maximum stress of 94.8 MPa but failed prior to 107 cycles at a maximum stress greater than or equal to 124.1 MPa. For the stress-lifetime diagram and residual strength, the woven laminate had superior properties below 104 cycles and the stitched laminate had superior properties above 105 cycles. While intralaminar crack density was higher in the stitched laminate, the woven laminate demonstrated a higher percentage of delaminated area. [ABSTRACT FROM AUTHOR]

Fiber volume fraction is a driving factor in mechanical properties of composites. Micromechanical models are typically used to predict the effective properties of composites with different fiber volume fractions. Since the microstructure of 3D-printed composites is intrinsically different than conventional composites, such predictions need to be evaluated for 3D-printed composites. This investigation evaluates the ability of the Voigt, Reuss, and Halpin–Tsai models to capture the dependence of modulus and strength of 3D-printed composites on varying fiber volume fraction. Tensile coupons were printed with continuous carbon fiber-reinforced Onyx matrix using a Markforged Mark Two printer. Specimens were printed at five different volume fractions with unidirectional fibers oriented at either 0 ° , 45 ° , or 90 ° to obtain longitudinal, shear, and transverse properties, respectively. It is shown that the Voigt model provides an excellent fit for the longitudinal tensile strength and a reasonable fit for the longitudinal modulus with varied fiber content. For the transverse direction, while the Reuss model fails to capture the transverse modulus trend, the Halpin–Tsai model provides a reasonable fit as it incorporates more experimental parameters. Like conventional composites, addition of fibers degrades the transverse strength, and the transverse strength decreases with increasing fiber volume fraction. The shear modulus variation with fiber content could not be fitted reasonably with either Halpin–Tsai model or Reuss model. [ABSTRACT FROM AUTHOR]

Hybrid fiber composites offer designers a means of tailoring the stress–strain behavior of lightweight materials used in high-performance structures. While the longitudinal stress–strain behavior of unidirectional hybrid fiber composites has been thoroughly evaluated experimentally and analytically, relatively little information is available on the transverse behavior. The objective of the current investigation is to present data on the transverse modulus of elasticity of unidirectional composites with five different ratios of carbon and glass fiber and to compare the data with predictive and fitted models. The transverse modulus increases monotonically with the proportion of glass fiber in the composite. Finite element analysis was used to evaluate different ways to model voids in the matrix and allowed the unknown transverse properties of the carbon fibers to be backed out using experimental data from the all-carbon composite. The finite element results show that the transverse modulus can be accurately modeled if voids are modeled explicitly in the matrix region and if modulus is calculated based on stress applied along the minimum interfiber distance path between adjacent fibers arranged in a rectangular array. The transverse modulus was under-predicted by the iso-stress model and was well predicted by a modified iso-stress model and a modified Halpin–Tsai model. [ABSTRACT FROM AUTHOR]

Laminated fiber reinforced polymer composites are known for high specific strength and stiffness in the plane of lamination, yet relatively low out-of-plane impact damage tolerance due to matrix dominated interlaminar mechanical properties. A number of factors including the toughness of the matrix can influence the response of composites to impact. The objective of the current investigation is to evaluate the ballistic impact response of carbon/epoxy tubes with variable amounts of nanosilica particles added to the matrix as a toughening agent. Mass density, elastic modulus, glass transition temperature and Mode I fracture toughness of the matrix materials were measured. Tubes manufactured with these matrix materials were ballistically impacted using a round steel projectile aimed at normal incidence across the major diameter. After impact, the tubes were nondestructively inspected and subjected to mechanical tests to determine the residual shear strength in torsion. Increasing concentrations of nanosilica monotonically increased the modulus and fracture toughness of the matrix materials. Tubes with nanosilica had smaller impact damage area, higher residual shear strength, and higher energy absorbed per unit damage area versus control materials with no nanosilica. Overall, the addition of nanosilica improved the impact damage resistance and tolerance of carbon/epoxy tubes loaded in torsion, with minimal adverse effects on mass density and glass transition temperature. [ABSTRACT FROM AUTHOR]

ABSTRACT: Adhesives that are stable in Li‐ion battery electrolytes are required to realize the potential of new battery designs that integrate structural elements with energy storage. Here, several polymers, commercial adhesives, and sealants were investigated to bond and seal a Li‐ion battery sandwich panel. Gravimetric electrolyte uptake measurements were compared with Hansen solubility parameters to predict long‐term durability of the materials exposed to battery electrolyte. The durability of adhesively bonded joints with an epoxy adhesive, which was selected as the lowest electrolyte uptake material, was examined using single lap shear strength tests and three‐point bending tests in a fabricated sandwich panel. The strength of the epoxy decreased after exposure to battery electrolyte due to solvent uptake in the bond. The addition of lithium hexafluorophosphate to the ethylene carbonate/dimethyl carbonate mixture severely decreased the strength with respect to the pure solvents. In device testing, the sandwich panel did not show any visible damage or leakage when loaded to above 1000 N during three‐point bending tests. Using sol extraction measurements and differential scanning calorimetry analyses, the optimized curing temperature for the epoxy adhesive ranged from 80 to 100 °C. At these temperatures, the cured adhesive had a highly crosslinked structure with low sol extraction. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 46059. [ABSTRACT FROM AUTHOR]