Silk fibre composites: Towards mechanical improvements by viscoelastic prestressing

Being a 100 % natural, biodegradable and biocompatible material, high strength silk fibre has attracted increasing interest as a reinforcement for composite materials. This is the first paper to investigate the mechanical improvements of silk fibre composites by using viscoelastic prestressing. The long-term viscoelastic behaviours for both raw and degummed silk fibres were firstly investigated and confirmed; and the recovery stress measurement showed that raw silk could build up a notably greater compressive stress (∼57 MPa) than degummed silk (∼26 MPa) eventually. Then, the mechanical properties of raw silk fibre composites (RSFCs) and degummed silk fibre composites (DSFCs) were studied by tensile, three-point flexural and Charpy impact testing. Prestressed RSFCs and DSFCs demonstrated increases in strength and stiffness of up to 10 % compared with corresponding control (unstressed) counterparts, which could be mainly attributed to the imparted compressive stresses within the matrix by prestressed fibres. Prestressing also increased the impact strength of DSFCs by ∼16 %; however, this was observed to reduce by almost 20 % for RSFCs as the prestress facilitated fibre fracture and impeded matrix cracking. Since these modest mechanical property improvements were achieved with low fibre volume fractions (∼10 % or less), greater benefits would be expected at higher fibre concentrations. • A “green” fibre pre-treatment method：viscoelastic prestressing (VP) was applied. • Long-term viscoelastic behaviours for raw and degummed silk fibres were revealed. • Effects of VP on silk composite mechanical properties were studied. • VP offers mechanical improvement without the increase of structural size or weight.