Failure Of Fiber Composites Research Articles

Predicting fiber composite damage and failure

Considerable effort has been put into the development of models describing damage and failure of fiber composites. This paper aims at providing an overview of models for intralaminate fracture of continuous fiber-reinforced polymer composites under quasi-static load. The models are grouped into homogeneous ones, those with some effect of inhomogeneity between fibers and the matrix, and fully inhomogeneous ones. Even a world-wide competition was not able to decide for the one and only homogeneous model. Accounting for the inhomogeneity between fibers and the matrix allows being more close to the real behavior on the account of higher computational effort. They suffer, however, from the difficulty to gather reliable material data. In summary, it must be concluded that a fully satisfying solution is not yet available. Consequently, for the time being, a real test remains the authentic way to secure structural strength.

Resin matrix shear band and its special effect on stress concentration of fiber composites

A constitutive model is constructed to consider the resin matrix post-yield softening and progressive hardening behaviors. A user-defined material mechanical behavior (UMAT) subroutine is created, then the non-linear three-dimensional finite element analysis on the tensile processes of multi-fiber composites is conducted. The approximate 45° shear bands emanating from the matrix crack tip are found, being coincided with the experimental observations. The shear stress on the adjacent intact fiber/matrix interface is strongly influenced by the shear band and thus the stress concentration factor (SCF) changes obviously in the adjacent fibers. The distinct stress redistribution in the adjacent intact fibers implies the significant effect of the shear bands on the progressive fiber fracture initiation. As the inter-fiber spacing increases, the peak value of the SCF in the adjacent intact fiber decreases, whereas the overload zone becomes wider. The research has provided a helpful tool to evaluate the failure of fiber composites and optimize the composite performance through the proper selection of resin matrix properties and fiber volume fraction.

Computational micro-mechanics for probabilistic failure of fiber composites in tension

The design of a composite from fiber and matrix properties requires micro-mechanics modeling of strength. The strength of a composite has an inherent variability which is a manifestation of the underlying stochastic failure processes of the constituent fiber and matrix. For fiber-dominated tensile composite failures, the failure process starts with the breaking of weak fibers. The initial failure sites are isolated by micro-redundancy provided by local load-sharing of the neighboring fibers. Higher applied load leads to progressive clustering of fiber breaks which ultimately cause macroscopic failure. Probability modeling of the failure sequences of the fiber breaks and the spatial geometry of the clustering of the sites resulted in the local load-sharing model. This model requires numerical evaluation because it contains an extremely large matrix of the transitional probability of the occurrences of the cluster sites. This paper provides an accessible computational implementation of this model for computer platforms ranging from personal computers to supercomputers. The computational algorithm was made general by a three-section piecewise-linear, tri-modal model for fiber statistics. This tri-modal local load-sharing software numerically computes the cumulative distribution function (CDF) of the composite strength by using a very wide range of fiber statistics as input. This CDF can be used to analyze composite strength data in testing, to predict size effects in design, to monitor processing quality, and to quantify the reliability of composite structures.

Dynamic compressive failure of fiber composites

A model is presented for the dynamic compressive response of polymer matrix fiber composites. The model includes the effects of fiber misalignment and material nonlinearity as well as material inertia. The role of fiber bending stiffness is included via a couple stress formulation. The response of fiber composites to suddenly applied, constant compressive axial load is examined. It is found that under constant load, inertial effects contribute to a reduction in the critical stress for composite failure. This reduction is greatest for composites with long initial fiber imperfection wavelengths. For a given load, there is a range of initial fiber imperfection wavelengths that will result in composite failure. Within this range, there is a preferred wavelength, which results in the shortest failure time.

The failure of fibre composites and adhesively bonded fibre composites under high rates of test

The failure of fibre composites under high rates of test was studied in detail, using a fracture mechanics approach to determine values of the fracture energies. The present paper, Part III of the series, considers the fracture of two different fibre composite materials under mixed-mode tensile and shear (i.e. mixed-mode I/II) and shear (i.e. mode II) loadings. Part I considered the experimental aspects of the mode I fracture of the fibre composite materials, and of adhesive joints. Part II analysed the dynamic effects which are invariably associated with high-rate tests, and showed how these effects influence the observed behaviour of the test specimens.

The failure of fibre composites and adhesively bonded fibre composites under high rates of test

The dynamic effects which are commonly encountered during high-rate DCB tests with fibre composite and adhesively bonded fibre composite arms have been studied in detail. This paper, Part II of the series, follows Part I, which described the experimental aspects of the high-rate testing. Part III will report the results from mode II and mixed-mode I/II tests on the fibre-composite materials.

The failure of fibre composites and adhesively bonded fibre composites under high rates of test

The dynamic effects which are commonly encountered during high-rate DCB tests with fibre composite and adhesively bonded fibre composite arms have been studied in detail. This paper, Part II of the series, follows Part I, which described the experimental aspects of the high-rate testing. Part III will report the results from mode II and mixed-mode I/II tests on the fibre-composite materials.

On the Role of Interphases in the Transverse Failure of Fiber Composites

A numerical investigation into the effect of interphase properties on the transverse strength of unidirectional fiber-reinforced composites is carried out in the con text of plane strain linear elasticity. The phase geometry of the composite is modeled by a regular hexagonal array of fibers in a matrix, and the interphase behavior is represented by a constrained spring layer model. A variationally coupled finite element-boundary ele ment method is used to obtain the mechanical response of a cell containing a center fiber and parts of surrounding fibers. We assume matrix cracking to be the predominant mode of failure. It is demonstrated that the interphase properties significantly affect the overall cracking patterns. Issues regarding the strength and integrity of the composite are dis cussed.

Instability and Failure of Fiber Composites in Compression

This paper addresses the uniaxial compressive failure of unidirectional, fiber-reinforced polymer matrix composites. The material that was used in this combined experimental/numerical effort was APC-2/AS4. Failure was found to result in kink bands of distinct widths oriented at an angle to the line of loading. Kink bands were preserved by conducting a series of experiments where failure was confined. Fiber misalignments in the prepreg and from the lamination process were identified as the imperfections that, in conjunction with the nonlinear shear response of the matrix material, essentially govern the formation of zones of localized bending. This, in turn, causes fibers to break and into distinct kink bands. The process was modeled by considering a micro-section of material with wavy fibers under compression. The formulation accounted for material and geometric nonlinearities and various distributions of waviness. It was illustrated that failure is associated with a limit load instability, followed by localized fiber bending which leads to kinking.