【正文】 l mechanisms of vibrationassisted cutting of unidirectional fibrereinforced polymer posites abstract This paper aims to reveal the material removal mechanisms and the mechanics behind the vibrationassisted cutting (VAC) of unidirectional fi bre reinforced polymer (FRP) posites. Through a prehensive analysis by integrating the core factors of the VAC, including fibre orientation and deformation, fibre–matrix interface, toolfibre contact and toolworkpiece contact, a reliable mechanics model was successfully developed for predicting the cutting forces of the process. Relevant experiments conducted showed that the model has captured the mechanics and the major deformation mechanisms in cutting FRP posites, and that the application of ultrasonic vibration in either the cutting or normal direction can significantly decrease cutting forces, minimise fibre deformation, facilitate favourable fibre fracture at the cutting interface, and largely improve the quality of a machined surface. When the vibrations are applied to both the cutting and normal directions, the elliptic vibration trajectory of the tool tip can bring about an optimal cutting process. There exists a critical depth of cut, beyond which the fibrematrix debonding depth is no longer infl uenced by the vibration applied on the tool tip. Keywords: Fibrereinforced polymer posites。Fibrematrix debonding 1. Introduction Fibrereinforced polymer (FRP) posites have been widely used in advanced structural applications due to their high strength and stiffness to weight ratio. However, the machining of FRP posite products is difficult because of the significant difference in the mechanical properties of the fibres and matrix. As a result, a machined FRP posite usually contains various damages, such as fibre pullout, fibre fragmentation, matrix cracking, fibrematrix debonding and delamination . To 32 date, most experimental investigations on the machining of FRP posites are on the following issues: effect of fibre or matrix types , influence of fibre volume fraction and orientations , role of tool materials and geometries , contribution of the depth of cut , and selection of processing parameters . These studies, however, are limited to the traditional machining methods, such as turning,milling and drilling, and are still facing the poor surface integrity problems highlighted above. To acplish a high quality surface of FRP posites, the mon understanding is that grinding is more appropriate , because the instant depth of cut of a single cutting edge in grinding is much smaller than the diameter of a fibre . Nevertheless, grinding is inefficient in many cases. On the other hand, vibrationassisted cutting, which adds a displacement of microscale amplitude at an ultrasonic frequency to the tip motion of a cutting tool, has been experimentally evidenced to be an effective method to cut many single phase materials such as metals and ceramics . In order to machine FRP posites effectively, the authors have developed a vibrationassisted cutting technique . This technique applies ultrasonic vibrations to the tool tip and based on the directions of the vibrations, the tool tip trajectories can be controlled to follow, ., an elliptic path named as an elliptic vibrationassisted (EVA) cutting. Their investigation has shown that the EVA can significantly reduce cutting forces and subsurface damages in a workpiece even by using a simple cutting tool. The effects of vibrations in cutting and normal directions on machining characteristics have also been studied using the novel vibrator developed. They found that the vibration in the cutting direction is more effective in reducing the cutting force, but that normal to the cutting direction facilitates the chip removal. When the vibration is applied to both the directions in an EVA cutting, the cutting force can be greatly reduced, the surface integrity of an FRP workpiece can be much improved, and the tool life can be largely extended. However, the mechanics and mechanisms of the EVA in the material removal process are still unclear. This has significantly hindered the optimisation and practical application of the EVA technique. The objective of this paper is to remove the above barrier through a detailed mechanics analysis to understand the science behind the EVA cutting of unidirectional FRP posites and thus to establish he essential fundamentals . The most imp ortant factors, such as cutting forces, fibre deformation, fibre fragmentation, and fibrematrix debonding, will be prehensively integrated in experiments will be carried out to examine the model established. 33 2. Mechanics modelling . The principle Fig. 1 illustrates the principle of an EVA cutting process, in which the cutting tool feeds at a feed rate, v, while it vibrates elliptically at an ultrasonic frequency with a microscale amplitude in the xzplane. The feed rate is smaller than the maximum vibration speed in xdirection, such that an intermittent cutting is generated in each vibration cycle of the tool. In a cycle, cutting takes place only when the tip wedges into the workpiece at time instant tb, and finishes when the tangential cutting direction is parallel to the fibre orientation at time instant te. Thus, if a fibre/matrix breakage happens during this process, chips form and are pulled out by the tool. To facilitate the breakage of the fibres and matrix, the cutting distance within a cycle of the tool vibration, Δ, is set to be smaller than the fibre diameter, D, so as to improve the surface quality as explored in the previous work . Assume that a and b are the vibration amplitudes in x and zdirections, respectively, f is the vibration frequency, ψ is the phase difference,r e is the radius of the cutting edge, ap