Journal of Composite Materials

Results from cyclic multiaxial tests on unidirectional [0₄] and angle-ply [±45], silicon carbide/titanium (SiC/Ti) tubular specimens are compared with theoretical predictions from a micromechanical model that includes imperfect bonding between the fiber and matrix. Comparison of theory and experiment indicates that damage to the fiber/matrix interface often is a major factor in the stress-strain response of SiC/Ti composites. The axial shear (torsional) response of a [0₄] tube displayed a distinct knee associated with damage at the shear stress of 276 MPa (40 ksi). Transverse tensile (internal pressure) loading of a [0₄] tube and axial tensile loading of a [±45], tube resulted in inelastic deformation due to damage well before initial yielding was predicted to occur in perfectly bonded composites. Substantial differences were observed in the effects of damage on the response of [0₄] and [±45], tubes subjected to combined loading.

The fracture mechanism of impact-induced delamination is studied in carbon fiber/PEEK (polyetheretherketone) cross-ply laminates under drop weight impact. The study is based on the energy theory of fracture mechanics and the concept of crack arrest toughness. The damaged laminate is modeled by a finite element method which simulates delaminations and transverse cracks. The numerical results are combined with test data to study the delamination behavior. It is found that the delamination occurs in a deflection-controlled condition and is a process of Mode II dominated unstable crack growth and subsequent arrest. The fracture behavior can be described by strain energy release rate and the delamination size is governed by the delamination arrest toughness of the composite.

Potential of using pitch-based high modulus carbon fiber was investigated as a reinforcement in cementitious composites for structural reinforced concrete (RC) members. For this purpose, effects of carbon fiber mechanical properties on the mechanical properties of CFRC reinforced with CFRP rods were studied through the three-point flexural test by using several pitch-based high modulus carbon fiber rods of varying fiber moduli and strengths. For the specimens with a fiber volume fraction larger than the critical volume fraction, the flexural strength is found to exceed the CFRC matrix strength and is linearly proportional to the sum of all rod strengths, and the flexural modulus after matrix cracking is found to also be linearly proportional to the sum of all rod stiffnesses.

This paper deals with a new bending test method for advanced composites. Although the conventional three and four-point bending tests are convenient to obtain bending modulus and strength, composites may fail at the loading nose due to the stress concentration, because composite materials have high anisotropy and they fail in a brittle manner.

To compensate for the above shortcoming, we developed a new bending test method which is based on a buckling of a column. Both strength and modulus could be measured. In the present paper, another refined methodology to evaluate the bending modulus is discussed in addition to the bending strength. 1300 and T800 carbon/epoxy unidirectional and quasi-isotropic laminates with various specimen lengths were tested. The superiority of the present method was demonstrated especially for evaluating the strength.

In this paper, an analysis is developed to predict the stress redistribution in the presence of single and multiple fractured fibers in a unidirectional composite material. This analysis includes the effects of constituent properties, fiber volume fraction, and crack size on the strain concentrations experienced by the adjacent fibers. These effects are not included in other predictions such as shear lag or those of Hedgepeth and Van Dyke. In addition, the predictions are compared with direct experimental measurements obtained from model composite tests.

An analytical model of free vibration of a delaminated composite laminate in prebuckled states has been developed. The formulation is based on a new constrained model which includes both effects of the compressive force and bending-extension coupling. These two effects on the natural frequency of delaminated plates have not been studied by such a model before. It is found that the compressive force, laminate lay-up, delamination length, and delamination locations in the thickness-wise and spanwise directions are significant factors to determine the vibration characteristics. Experiments have been conducted to validate this analytical model. Good agreements between the analytical results and test data have been obtained.

Non-viscous flow effect in liquid molding processes has been analyzed and evaluated. Ergun equation is used to describe the non-viscous flow through fibrous media. Overall flow resistance of the fibrous media is considered as the summation of the viscous inertial components. The generalized permeability of the fibrous media is found to be not only a function of the structural parameters such as fiber diameter and porosity, but also related to the fluid properties and flow velocity. Two-dimensional mold filling flow equation has been established. In order to evaluate the magnitude of the non-viscous flow effect in various process conditions, solutions of simplified one-dimensional filling flow cases are discussed. Parametric study with numerical simulations has been carried out to assess the non-viscous flow effect on the filling time and the inlet fluid pressure.