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

摘要

Effects of short fiber with sub-millimeter length on tensile strain, ε_f and tensile strength, σ_f were investigated for a bulk molding compound (BMC) glass fiber reinforced polymer (GFRP) composite with 20 mass% E-short glass fibers. The ε_f and σ_f 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 (ε_f/ε_f,6.4) and reduced tensile strength (σ_f/σ_f,6.4) as a function of the fiber end density, ρ_E (cm~(-3)) were expressed by the following equations with linear regression as (ε_f/ε_f,6.4 = 1.21 x 10~(-7)σ_E + 0.965) and (σ_f/σ_f,6.4 = 1.51 x 10~(-7) ρ_E + 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 < 2a_c < 5.0 mm from reported K_(IC) results in the literature, and is greater than the 0.44 mm mean fiber length. The probability of a microcrack propagating above 0.44mm 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.