The fracture toughness of natural fibre- and glass fibre-reinforced SMC

Abstract

The fracture toughness properties, in terms of stress intensity factor K Ic and strain energy release rate G Ic , of hemp fibre mat-reinforced sheet moulding compound (H-SMC) are measured using the compact tension (CT) method and compared with those of glass fibre-reinforced SMC (G-SMC). Three material parameters were considered for composite optimisation: fibre volume fraction, CaCO 3 filler content and hemp fibre surface treatments using either alkaline, silane or a combination of these two treatments. The highest fracture toughness for H-SMC composites was obtained at a fibre loading of around 30 vol.-%, while it was also shown that the fracture toughness properties of H-SMC are sensitive to mineral filler content. Surface treatment of the hemp fibres using a combined alkaline-silane treatment resulted in a significant improvement in fracture toughness of H-SMC composites. Optimised H-SMC composites exhibited fracture toughness properties similar to those of G-SMC at fibre contents of 20 vol.-%, with K Ic values of around 6 MPa.m −1/2 .

Fracture toughness analysis of a woven sisal-glass fiber hybrid composite

“…After trimming, the leaves were divided into strips along their length. Hand labor was used to extract the fibers (Figure 2), which were subsequently dried in the sun after being cleaned of dust with pure water [9,10].…”

Post-impact damage tolerance of natural fibre-reinforced sheet moulding compound

“…Surface treatment conditions followed previous work. [28][29][30] Alkaline treatment. Pre-dried hemp fibre mats were immersed in a 2 wt% NaOH solution for 1 h and subsequently rinsed in distilled water several times.…”

“…Previous studies already showed that particularly a combined alkaline-silane treatment resulted in a significant increase in work of fracture, tensile and impact strength of H-SMC. [28][29][30] Treatment of hemp fibres with alkaline removed impurities from the waxy fibre surface, resulting in more sites for subsequent silane molecules to form bonds with the ligno-cellulose fibre surface, which in turn lead to a considerable increase in tensile strength 28 and work of fracture. 30 It should be noted that although composite strength typically requires a strong fibre-matrix interface, composite toughness usually requires an optimized rather than maximized interface.…”

“…[28][29][30] Treatment of hemp fibres with alkaline removed impurities from the waxy fibre surface, resulting in more sites for subsequent silane molecules to form bonds with the ligno-cellulose fibre surface, which in turn lead to a considerable increase in tensile strength 28 and work of fracture. 30 It should be noted that although composite strength typically requires a strong fibre-matrix interface, composite toughness usually requires an optimized rather than maximized interface. If the interface is too strong, a composite may fail in a more brittle mode, leading to a reduction in toughness.…”

“…Both types of SMCs were made using an industrial relevant manufacturing process and materials. Previous studies from our group reported already on the quasi-static, fracture and impact properties of these materials, [28][29][30] but to date there have been no post-impact damage tolerance studies reported for H-SMC. Parameters considered for material optimization included (i) hemp fibre volume fraction and (ii) hemp fibre surface treatment.…”