Finite element analysis and microscopy of natural fiber composites containing microcellular voids

Abstract

Mechanical behaviors of chopped, natural fiber composites with and without closed-cell microcellular foamed structures are investigated using finite element analysis (FEA), with references to previously conducted experimental measurements of the stress-strain characteristics of analogous test specimens and their microscopic images. Analytical and FEA models are compared for a simple, aligned fiber case; the FEA models are then extended to composites having more general fiber arrangements. These FEA results are compared to experimental stressstrain data. Scanning electron microscopy (SEM) images of fracture surfaces from test specimens are analyzed, and correlations between numerically predicted behaviors and fracture paths, and fracture surface images are discussed. The FEA results reflect experimental trends well including the effects of fibers and the presence of voids. These have also been found to induce specific regions of stress concentration. Inspection of fracture surfaces reveals transition from ductile to brittle behavior at low strain rates, generally brittle failure modes, and little evidence of fibers stressed to their tensile limits.

Voids in fiber-reinforced polymer composites: A review on their formation, characteristics, and effects on mechanical performance

“…In general, this study is more useful for analysis of pure polymer materials and lacks the interaction of fibers, voids, and damage, although it aims to predict the knock-down strength of composites. In a similar study for a (natural-fiber) wheat straw/PP composite, Kern et al 287 built an FE model including short non-aligned fibers dispersed in the matrix containing spherical voids (microcells). They noted that strain and stress localization happen in the vicinity of voids, which reduces the stress near the fiber ends, causing a lower stress increase rate in the stress-strain curve.…”

“…McMillan 286 conducted a 2D numerical investigation on knock-down strength of resin-rich areas, where idealized elliptical voids were modeled as holes in a homogenous matrix. In a similar approach, but including short non-aligned fibers, Kern et al 287 modeled spherical voids distributed in the matrix.…”

Finite element models of natural fibers and their composites: A review

“…19,74 Comparing with these micromechanics models, the FE RVE simulation is more effective and accurate for investigating the mechanical properties of the composite material, taking into account of the properties of NFs and matrix, the volume fraction, morphology and distribution of NFs, the interfacial properties between NFs and matrix, and defects in the composite. Short FRCs Two modeling methods, direct RVE method 15,75 and orientation averaging method, 76,77 are often used in the FE modeling of short NFRCs. The direct RVE consists of sufficiently short NFs surrounded by the matrix.…”

“…Kern et al. 15 developed 3D FE models to investigate the mechanical properties of polymeric composites reinforced with short, arbitrarily-distributed wheat straw fibers and took into account the microcellular voids. The fibers were assigned in-plane orientation angles of 0°, 15°, 30°, 45°, 60°, 75°, 90°, 105°, with equal numbers of fibers at each orientation, as shown in Figure 7.…”

“…Finite element method (FEM) is increasingly used in modeling of NFs and the design of NFRCs. 7,13–15 The microstructure of NFs is complex and is affected by plant variety and growth conditions, such as place of origin and weather. It is a challenging task to analyze how the complicated microstructures of NFs and NFRCs affect the macroscopic behaviors, and then predict the macroscopic properties of the targeted materials.…”