Research On Mechanisms Of Delamination Suppression During Drilling Glass Fiber/Epoxy Resin Composite Plates

Composites are materials which are fabricated from combined components of different properties.Due to their good mechanical properties and flexibility of tailoring,they have become the cutting-edge materials in industries such as aerospace,construction,transportation and the demand has been rising every year.Among these composite materials,the most popular one is the composite reinforced by fiber with thermoset resin as matrix.During the manufacture of parts made by fiber reinforced composites,drilling accounts for nearly 60% of the whole process.Drilling induced delamination tremendously deteriorates the structural stiffness.Therefore,drilling-induced delamination fo fiber reinforced polymers has become the common issue existed in their manufacturing process.There are two major types of drilling-induced delamination of laminated composites: one locates at the vicinity of drilling entrance,the other one is often observed around the drilling exit.This article studied the drilling process of woven glass fiber reinforced epoxy composite using high speed steel twist drills,explained the dynamic behavior of delamination and analysed the suppression mechanisms of delamination as well as the safe-range of the drilling parameters.The main contents are as follows:(1)Research on the suppression mechanism of entrance delamination during drilling glass fiber/epoxy resin composite plates.Based on the image module in the Image processing library,PIL,of Python platform,the image processing program was designed and compiled.Determining the center of the drilled holes,searching the outer pixels of the delamination area,calculating the area of delamination can be realized through the program.The program can also automatically output the delamination-angle figure with pixel precision.Through the analysis of different drilling stages,entrance delamination was found to occur when the drill tip is fully penetrated into the material.The experimental data of entrance delamination showed that the delamination area increases with the increment of feed rate.Based on plate theory and linear elastic fracture mechanism,the analytical model of entrance delamination was established.After defining the entraence delamination coefficient M based on the machenical properties of the drilled material,the positive relationship between the feed rate and delamination area was explained,determining that the key of suppressing entrance delamination is to decrease the level of feed rate.(2)The mathematical model establishment of the relationship between drilling thrust force and feed rate and experimental research for drilling glass fiber/epoxy resin composite plates.The relationship between the thrust force and the horizontal and vertical cutting forces from the orthogonal cutting model of fiber reinforced materials was obtained with space geometry of twist drill.The general analytical model containing unknown parameters of drilling thrust force was established with the relationship between the cutting depth of each tooth of the drill bit,the infinitesimal length of the cutting edge,rake angle and the drilling thrust force.The Num Py module of Python platform was used to calculate the key variables,therefore simplifying the thrust model,and to denoise the thrust-time curves obtained through drilling experiments.With the experimental data,the curve?fit module of Python language was applied to determine the detailed thrust model for the drilling conditions of this article.Via experimental validation,the thrust model in this article well matched the experimental values.As a result,it can be used to calculate critical drilling feed rate for exit delamination suppression.(3)Research on the suppression mechanism of exit delamination during drilling glass fiber/epoxy resin composite plates.There are two parts of edges in twist drills: the chisel edge and the main cutting lips,and these two parts both showed the potential to induce exit delamination.The analytical models of these two types of exit delamination were established based on the plate theory and linear elastic fracture mechanism.It was found that the delamination induced by chisel edge was sensitive to the remaining thickness under the drill bit,which significantly influences related critical thrust force.However,model results showed that despite the change of drilling feed rates,the high level of the critical thrust force of the delamination induced by the cutting lips was very hard to achieve in practical drilling processes.Experimental results showed that the application of the critical thrust model for chisel edge were able to effectively suppress the delamination and the delamination suppression results via the critical thrust model for main cutting lips were consistent with experimental results.This article proposed a step-feed delamination-suppression method for the entire drilling process of fiber reinforced polymers with the studies of the three main contents above,achieving entrance and exit delamination suppression in one single drilling procedure.Experimental results showed that,compared with fixed feed drilling,the area of entrance delamination reduced by 41% and the area of exit delamination reduced by 45%.