Abstract: Wood-plastic composite(WPC)belonging to bio-composites is a kind of non-toxic, recyclable and eco-friendly material. It has developed rapidly for two decades since the end of the 20th century. But now the development of WPC comes to a bottleneck. Low mechanical properties, especially poor toughness, make the application of WPC narrow, which is one of the main reasons what restrict the development of the WPC. Many researches show that adding fibers into WPC to produce multiple structural composite, is an effective way to improve the strength of WPC. In this paper, the current research status of fiber reinforced WPC is summarized. Fibers used as reinforcement for composites are divided into natural cellulose fibers, synthetic fibers, inorganic non-metallic fibers and metal fibers. The production methods and reinforcing effects of the WPCs reinforced with fiber, such as glass fiber, mineral fiber, carbon fiber, aramid fiber, polyethylene terephthalate fiber and hemp fiber et al., are reviewed. The results show that different fibers have different effects on strength or toughness of WPCs. There is a "critical value" for the addition quantity of short fibers. When the fiber content is less than the "critical value", the addition quantity and enhancement effects are positively correlated, however, contrarily negative correlated is observed when the fiber content above the "cirtical value". It is obviously that the effect of continuous glass fiber on the impact strength which increases 20 times. The natural cellulose fiber (NCF) reinforced WPCs have been used in auto parts field with high added value in Europe, though the application is not very popular. The enhanced mechanisms of fiber reinforced WPCs such as crazing mechanism of shearing band, rigid particles strengthening theory, multiple slit theory and the theory of composite mechanics, et al. are also summarized. These theories can be used to explain the mechanism of fiber reinforced WPC. And the latter two theories are most used. This article also points out that there is no one theory which can fully disclose mechanical behavior of WPC because of its complex structure tend after adding fiber. The influences of fiber addition on the mechanical properties, moisture and thermal properties of the composites are summarized. It is find that adding fiber can not only improve the mechanical strength of WPC, but also have a positive effect on reducing the moisture absorption and improving the thermal stability. Besides, some fibers can also improve the crystallinity of the matrix. The article concludes with the industry prospects and the problems of fiber-reinforced plastic composites. These issues include that improving production efficiency, developing special equipment for fiber-reinforced plastic composites, developing technology for continuous fiber-reinforced plastic composites and pioneering markets for high performance and high value-added WPC.

Key words: wood-plastic composites, fiber, mechanical properties, impact strength, enhanced mechanism