Tham khảo tài liệu 'engineered interfaces in fiber reinforced composites part 11', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 284 Engineered interfaces in fiber reinforced composites giving rise to long fiber pull-out lengths whereas this mechanism was apparently absent with the SVF coating. The effectiveness of the intermittent bonding concept has been confirmed under adverse environmental conditions such as hygrothermal aging Atkins and Mai 1976 . In follow-up studies with Kevlar fiber-epoxy matrix systems Mai 1983 1988 Mai and Castino 1984 1985 the coatings based on SVF and a blend of polyester-polyether resins Estapol were explored. The effects of hygrothermal aging percentage coating over a repeated fiber length fatigue damage strain rate and temperature on tensile strength modulus impact fracture toughness and pullout toughness of the composite were investigated. The fracture toughness of composites with Estapol coated fibers was increased by some 200-300 particularly at high temperatures and low strain rates as shown in Fig. without sacrificing other strength properties. Fig. . Fracture toughness R of Kevlar 49-epoxy matrix composites a under varying strain rates in three-point bending and b at different temperatures under impact loading O uncoated fibers 41 63 and 100 Estapol coated fibers A silicone vacuum fluid SVF coated fibers. After Mai and Castino 1984 . Chapter 7. Improvement of transverse fracture toughness with interface control 285 The tensile debonding model associated with the intermittently bonded interface schematically shown Fig. appears to be rather unrealistic in unidirectional fiber composites as the stress state near the crack tip should be three-dimensional in nature Kim and Mai 1991a . The model certainly needs further verification as it requires complicated stress conditions to be satisfied. Nevertheless there is no doubt that the longitudinal splitting promoted by the weakened interface increases the interfaced debonding and subsequent fiber pull-out with large contributions to the composite fracture toughness. The beneficial effect of the tensile
