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Home  >  Journal list  >  MATERIALS TRANSACTIONS  >  Vol.51  No.12 (2010)  >  pp.2304-2310

MATERIALS TRANSACTIONS
<<Previous article Vol.51  No.12 (2010)   pp.2304 - 2310 Next article>>

Tensile Strength Enhancement by Shortening Glass Fibers with Sub-Millimeter Length in Bulk Molding Polymer Compound

Michael C. Faudree1) and Yoshitake Nishi2)
1) Center of Foreign Language, Tokai University
2) Graduate School of Science and Technology, Tokai University


Effects of short fiber with sub-millimeter length on tensile strain, εf and tensile strength, σf were investigated for a bulk molding compound (BMC) glass fiber reinforced polymer (GFRP) composite with 20 mass% E-short glass fibers. The εf and σf values of BMC-GFRP samples with 0.44 mm short fibers were almost 40 and 60% higher than that of BMC-GFRP samples with long fibers (3.2 and 6.4 mm in length), as well as more than 65 and 110% higher than that of the filled fiber free resin, respectively. The reduced fracture strain (εf⁄εf,6.4) and reduced tensile strength (σf⁄σf,6.4) as a function of the fiber end density, ρE (cm−3) were expressed by the following equations with linear regression as (εf⁄εf,6.4=1.21×10−7ρE+0.965) and (σf⁄σf,6.4=1.51×10−7ρE+0.998). Acoustic emission (AE) analysis detected microcracking was increased threefold by shortening the mean fiber length from 6.4 to 0.44 mm. Scanning electron microscopy (SEM) results showed increased fiber/matrix debonding at fiber ends and along fiber lengths in the 0.44 mm samples compared with that reported for 6.4 mm samples. The fiber debonding is thought to impart an internal strain field in the matrix surrounding each fiber resulting in volume expansion sites, hence compressive stress sites which absorb energy from an approaching crack tip front in the nearby vicinity halting the crack’s advance. Therefore, more microcracks can be tolerated increasing fracture strain. Moreover, the critical crack length range for thermoset polymers was calculated to be approximately 0.50<2ac<5.0 mm from reported KIC results in the literature, and is greater than the 0.44 mm mean fiber length. The probability of a microcrack propagating above 0.44 mm before it encounters a matrix compressive site, therefore is reduced. Furthermore, increased microcracking in the vicinity of the main crack tip acts to reduce the main crack tip stress concentration. All of these serve to prevent crack propagation resulting in enhancement of fracture strain in the BMC-GFRP.


Keyword:
glass fiber reinforced polymer (GFRP), bulk molding compound (BMC), short fiber composite, fiber length, micro-crack toughening, fracture strain, fracture stress, acoustic emission

Received: April 08, 2010
Accepted: October 04, 2010 , Published online: November 25, 2010
Copyright (c) 2010 The Japan Institute of Metals

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