Multi-fiber Composites Research Articles

Experimental measurement of fiber/fiber interaction using Micro Raman Spectroscopy

Understanding the stress concentrations that develop due to fiber failure in multi-fiber composites is crucial because it dictates the failure mode and toughness of a composite. Composites with strong fiber/fiber interaction (high stress concentrations) are often brittle while those with low fiber/fiber interaction are more ductile. In this study, the effect of fiber failure on neighboring fibers in graphite/epoxy composites was studied using Micro Raman Spectroscopy (MRS). The effect of interphase toughness and strength on the stress concentration factor (SCF) and the radius of the zone of influence, r∞ was studied. It was found that the toughness of the interphase strongly affects SCF and r∞ A brittle interphase was found to reduce r∞ from 30 fiber diameters to 20 and caused SCF to drop more rapidly with inter-fiber distance. The maximum value of SCF was not affected by the toughness of the interphase and was found to be 1.46 ± 0.05. On the other hand, a 50% decrease in the interfacial shear stress (from 40 MPa to 20 MPa) resulted in a further reduction in r∞ down to 10 fiber diameters and a reduction in the maximum SCF to 1.16 ± 0.05. The results are explained and discussed from both micromechanics and energy viewpoints.

Comparison of adhesion data from the single fibre composite fragmentation method to a new multi-fibre composite method

When unidirectional multi-fibre composites of E-glass/polyester and Kevlar/polyester, respectively, are subjected to transverse tension, the AE events associated with fibre matrix interfacial failure started and eventually became saturated at low strains in the materials containing Kevlar when compared to those containing E-glass. This clearly demonstrates that E-glass adheres far better to polyester than Kevlar, and from stress measurements, the E-glass/polyester bond is stronger. This was attributed to the silane-sizing of the E-glass fibres which is known to promote adhesion with polyester. Single fibre composite (SFC) fragmentation tests involving AE gave similar interfacial shear strength values for the E-glass/polyester and Kevlar/polyester interfaces, which may be interpreted as similar levels of adhesion. This paper compares and discusses the degree of accuracy of the adhesion data from both methods. However, further experimentation with composites whose fibres were pretreated with a silicon-oil prior to composite manufacture showed clearly that the multi-fibre method would give a clearer idea of the level of fibre-matrix adhesion in composite materials.

Theoretical aspects of the fragmentation test

This paper presents a review of the status of the fragmentation test. The processes occuring during the fragmentation test and especially the modelling of the test are analysed. The limits of one-dimensional models are defined and the quality of bidimensional models is outlined, one-dimensional numerical analysis of the fragmentation process being presented as a powerful tool for understanding the basic interfacial phenomena. Finally, misconceptions in the analysis of some mechanical features of the test are emphasized. The most notable of these are that no analytical relationship between the critical fragment length and the mean fragment length can be established at present and that the ineffective length, which may allow the in-situ Weibull parameters of the fibre to be determined, has to be defined by using a probabilistic criterion. In conclusion, fragmentation phenomena still appear to be poorly understood, but the development of understanding is crucial for the micromechanical analysis of composites because the fragmentation test exhibits the basic damage modes which are present in multifibre composites.

Mesomechanical modeling approach to interface-related tensile behavior of FRM

A mesomechanical modeling approach was applied to estimate the influences of interfacial chemical reaction and interfacial bonding strength on tensile behavior and strength of fiber-reinforced metal matrix composites. First, a fracture mechanical model of single fiber coated with thin brittle surface layer was applied to describe strength of fiber on which interfacial reaction layer or artificial coating layer exists. With this model, influences of interfacial bonding strength between fiber and surface layer, difference in mechanical properties between fiber and surface layer, thickness of surface layer and discontinuity of surface layer on fiber strength were clarified. Second, a fracture model of multi-fiber composites in which shear-lag-analysis, fracture mechanics and a Monte Carlo simulation method were incorporated was applied to describe the fracture process and strength of composites. With this method, the influences of interfacial bonding strength between fiber and matrix in association with scat...

Micromechanics analysis of fibre reinforced cement composites

A three-dimensional finite-element modelling of fibre-reinforced cement composites has been adopted in this study to determine the linear elastic and non-linear properties of the composites, given the properties of the constituent materials. The micromechanics adopted for the analysis involves the formulation of constraint equations to incorporate the necessary boundary conditions. The results obtained from the numerical analysis compare favourably with the experimental results. The importance of a fully three-dimensional analysis of multi-fibre composites has been demonstrated.

Redistribution of stresses due to fracture of brittle fibers in a multifiber composite

Redistribution of stresses due to fracture of brittle fibers in a multifiber composite

Analysis of Twisting Stiffness for a Multifiber Composite Layer

The twisting stiffness of a rectangular cross section consisting of a single row of solid circular cross-section fibers embedded in a matrix is analyzed. The problem is formulated as a Dirichlet torsion problem of a multielement region and solved by the boundary-point least-squares method. Numerical results for a single-fiber square cross section compare favorably with previous relaxation-method results. New numerical results for three and five-fiber composites suggest that the torsional rigidity of a multifiber composite can be approximated from the torsional rigidities of single and three-fiber models.