Critical Thrust Force Research Articles

Evaluation of a novel approach to a delamination factor after drilling composite laminates using a core–saw drill

Drilling is the most commonly applied method for hole making of fiber-reinforced materials owing to the need for structure joining. Delamination is the most common defect during drilling because of the heterogeneity of both the fibers and the matrix. The delamination, in general, is an irregular shape and size, containing long and fine breaks and cracks at the exit of the drilled hole, especially in the drilling of carbon-fiber-reinforced plastic (CFRP). On the other hand, a core–saw drill is designed to reduce the threat of chip removal in drilling composite materials. Since the thrust force of core–saw drill is distributed toward the periphery, the core–saw drill allows a larger critical thrust force than the twist drill at the onset of delamination when drilling composite materials. The aim of this paper is to present a novel approach of the equivalent delamination factor (Fed) to characterize drilling-induced delamination using a core–saw drill and compare it with the adjusted delamination factor (Fda) and the conventional delamination factor (Fa). The experimental results indicated that the Fed obtained is considered suitable for characterizing delamination at the exit of a hole after drilling CFRP.

Effect of Peripheral Drilling Moment on Critical Thrust Force Using Twist Drill in Drilling Composite Materials

Composites have become valuable construction materials in the aerospace, defense, automobile, and civil industries due to their superior mechanical properties. However, there are significant differences between the machining of metals and alloys and that of composites, because composites are anisotropic and inhomogeneous. Drilling with a conventional twist drill is characterized by a relatively large thrust force due to a negative rake and negligibly small cutting speed at the chisel edge. Such a large thrust force causes defects and damages the composites, leading to poor hole quality, and reduced in-service life under fatigue loads. Although significant efforts have been made to realize the thrust force of twist drill, there are few papers reporting the effect of peripheral drilling moment (torque) on delamination in drilling composite materials. In this paper, an attempt is made to develop the critical thrust of the twist drill with peripheral drilling moment using linear elastic fracture mechanics (LEFM) and energy conservation. The theoretical results agree well with the practical experience in industries.

Drilling thick composite materials using large diameter drills

Composite materials are widely used in aeronautical structures. Assembling the various parts involved requires machining operations, especially drilling. When drilling, a number of defects that diminish the structure breaking strength are propagated. Delamination on hole exit is considered to be the main such defect, this being directly related to the drilling axial force. Determining the drilling critical thrust force at delamination is thus crucial. A number of studies have been conducted into this question but are only applicable to small diameter drilling operations cases. This paper proposes an orthotropic analytical model with the aim of calculating the drilling critical thrust force with a large diameter drill. New assumptions are then proposed. The plate element located under the cutter is broken down into a number of zones in relation to the cutter different active parts. A digital model is developed to validate analytical modelling. Punching tests were also conducted to validate the choices of boundary conditions.

Analytical models of composite material drilling

Drilling of composite material structures is widely used for aeronautical assemblies. When drilling, damage to the composite laminate is directly related to the cutter geometry and the cutting conditions. Delamination of the composite materials at the hole exit as directly related to the axial force (F Z) of the cutter is considered to be the major such defect. To address this issue, an orthotropic analytical model is developed in order to calculate the critical force of delamination during drilling and a number of hypotheses for loading are proposed. This critical axial load is related to the delamination conditions (propagation of cracks in the last layers) and the mechanical characteristics of the composite material machined. A numerical model is also drawn up to allow for numerical validation of the analytical approach. A comparison between these analytical and numerical modellings and experimental results from quasi-static punch tests led to the choice of the loading hypothesis closest to the experimental conditions. The selection of corresponding load permits to model the drilling critical thrust force on delamination and then to optimise the cutting conditions. The dimensions and geometrical shape of the cutter are of considerable importance when it comes to choosing this load. The present article focuses on the case of the twist drill, which is commonly used to drill thick plates. However, this work can be adapted to different cutter geometries.

Analysis of delamination in drilling composite materials by core-saw drill

Machining of fibre reinforced plastic is a process that is substantially different from metals. Drilling is one of the long practiced methods for generating holes for riveting and fastening structural assemblies. The size of the delamination zone has been shown to be related to the thrust force developed during drilling and there is a critical thrust force below which no damage occurs. The critical thrust force at the onset of delamination is predicted and compared with the twist drill, core drill and saw drill. Based on the proposed model, an innovative drill can be developed for industry.

Effects of peripheral drilling moment on delamination using special drill bits

The delamination damage caused by the drilling thrust has been recognized as one of the major problems during drilling. Based on the earlier concentrated-load model proposed, the thrust force is the main cause to delaminate at the drill exit in workpiece. However, the central concentrated loading (twist drill) is considered a special case of the general condition also with circular loading (saw drill, candle stick drill and core drill). A comprehensive critical thrust force model considering the effects of the peripheral moment for saw drill, candle stick drill and core drill compared with twist drill is constructed in the present study. In this analysis, the critical thrust force associated with the effect of distributed peripheral moment causing the onset of delamination in use of special drills is predicted and discussed.

Effect of pilot hole on thrust force by saw drill

The applications of composite materials are numerous, especially in the structural parts of aerospace, automotive and marine industries. Owing to the marked anisotropy and macroscopic heterogeneity of composite materials, the mechanics of machining used is different when compared to metals. Delamination is one of the most concerns of applying the fiber-reinforced composite materials in various industries. A hole is pre-drilled to eliminate the thrust caused by the chisel edge of twist drill; thus, the threat for delamination is significantly reduced. Saw drills eliminate the chisel and utilize the peripheral distribution of the thrust in drilling. An analytical approach to identifying the role of the pilot hole was proposed to reduce the thrust force-induced delamination during saw drilling. The predicted critical thrust force is in good agreement with the experimental values.

Drilling of uni-directional glass fiber reinforced plastics: Experimental and finite element study

The use of Fiber Reinforced Plastics (FRP) has increased manifold over the last few years. Generally developed for aerospace and other high-end applications, composites are now making inroads into the automotive and general engineering market. Thus good quality and cost-effective manufacturing of FRP composites becomes imperative. Drilling of holes is an important machining operation to ascertain the assembly operations in intricate composite parts. The research endeavors in the field of drilling of composite materials have focused on optimization of the operating variables, tool geometry and theoretical modeling of the critical thrust force. The work done in the Finite Element (FE) modeling of the drilling of FRP composite materials is limited. The present initiative is an attempt to investigate statistically the relative significance of the drilling parameters on the thrust force and torque. The results of analysis of variance (ANOVA) are then used to make assumptions for developing a Finite Element model for predicting drilling induced damage. The FE results are found in good agreement with the experimental results.

Effect of tool wear on delamination in drilling composite materials

Among all machining operations, drilling using twist drill is the most frequently applied for secondary machining of composite materials owing to the need for structure joining. Delamination is mostly considered as the principal failure model in drilling of composite materials. Drill wear is a serious concern in hole-making industry, as it is necessary to prevent damage of cutting tools, machine tools and workpieces. The industrial experience shows the worn drill causes more delamination. This paper presents a comprehensive analysis of delamination caused by the drill wear for twist drill in drilling carbon fiber-reinforced composite materials. The critical thrust force at the onset of delamination for worn drill is predicted and compared with that of ideal drill. The experimental results demonstrate that though the critical thrust force is higher with increasing wear ratio, the delamination becomes more liable to occur because the actual thrust force increases to larger extent, as the thrust factor ( Z) illustrates. Compared to sharp drill, the worn twist drill allows for lower feed rate below which the delamination damage can be avoided.

Effect of deviation on delamination by saw drill

Various cutting techniques are available to drill holes, but drilling is the most common way in secondary machining of composite materials owing to the need for structure joining. Twist drills are widely used in the industry to produce holes rapidly and economically. Since the twist drill has a chisel edge, increasing the length of a chisel edge will result in an increase in the thrust force generated. Whereas, a saw drill has no chisel edge; it utilizes the peripheral distribution of the thrust force for drilling. As a result, the saw drill can achieve better a machining quality in drilling composite laminates than twist drill. The deviation of cutting edge that occurs in saw drill would result in an increase of thrust force during drilling, causing delamination damage when drilling composite materials in particular. A comprehensive model concerning delamination induced by the thrust force of a deviation saw drill during drilling composite materials has been established in the present study. For a deviation saw drill, the critical thrust force that triggers delamination increases with increasing β. A lower feed rate has to be used with an increasing deviation saw drill in order to prevent delamination damage. The results agree with real industrial experience. A guide for avoiding the drill deviation during drill regrinding or drill wear has been proved analytically by the proposed model, especially when the deviation ratio ( β) affects the critical thrust force. This approach can be extended to examine similar deviation effects of various drills.

Modeling and Optimization of Process Parameters for Defect Toleranced Drilling of GFRP Composites

Drilling is a frequently practiced machining process for fiber-reinforced plastics (FRP) in industry owing to the need for component assembly in mechanical structures. Drilling experiments are performed on a (0/±45/90)2s 3-mm-thick glass fiber-reinforced laminate using 4-, 6- and 8-mm-diameter HSS drills. The machining response of the quasi-isotropic laminate was studied by monitoring the thrust and torque. The performance of the HSS drills for different cutting conditions was studied by measuring the tool wear. Delamination due to drilling is a major concern in machining a composite laminate and is analyzed by using linear elastic fracture mechanics, classical plate bending theory, and the mechanics of composites. A mechanical model for evaluating the critical thrust at which delamination is initiated at different ply locations has been used, and the critical thrust force at the onset of delamination has been found to be 70 N. The present work analyzes data on the thrust force, torque, and tool life by using a group method data handling (GMDH) algorithm. An optimization algorithm using simulated annealing with a performance index is then applied to search for the optimal process parameters for delaminatien constrained drilling.

Effects of special drill bits on drilling-induced delamination of composite materials

Drilling is the most frequently employed operation of secondary machining for fiber-reinforced materials owing to the need for joining structures. Delamination is among the serious concerns during drilling. Practical experience proves the advantage of using such special drills as saw drill, candle stick drill, core drill and step drill. The experimental investigation described in this paper examines the theoretical predictions of critical thrust force at the onset of delamination, and compares the effects of these different drill bits. The results confirm the analytical findings and are consistent with the industrial experience. Ultrasonic scanning is used to evaluate the extent of drilling-induced delamination. The advantage of these special drills is illustrated mathematically as well as experimentally, that their thrust force is distributed toward the drill periphery instead of being concentrated at the center. The allowable feed rate without causing delamination is also increased. The analysis can be extended to examine the effects of other future innovative drill bits.

Effect of eccentricity of twist drill and candle stick drill on delamination in drilling composite materials

Drilling is the most frequently employed operation of secondary machining for fiber-reinforced materials owing to the need for structure joining. Delamination is one of the serious concerns during drilling. Practical experience shows that an eccentric twist drill or an eccentric candle stick drill can degrade the quality of the fiber reinforced material. Comprehensive delamination models for the delamination induced by an eccentric twist drill and an eccentric candle stick drill in the drilling of composite materials have been constructed in the present study. For an eccentric twist drill and an eccentric candle stick drill, the critical thrust force that will produce delamination decreases with increasing point eccentricity ξ. The results agree with industrial experience. The need for control of drill eccentricity during drill regrinding has been proved analytically by the proposed models.

Critical thrust force at propagation of delamination zone due to drilling of FRP/metallic strips

A critical thrust force at propagation of delamination zone due to drilling of an infinite FRP/metallic strip is obtained by the definition of energy release rate. The critical thrust force is derived in terms of the size of the delamination zone. The differences of the results by the previous and present methods are discussed.

Critical thrust force at delamination propagation during drilling of angle-ply laminates

A new formulation for the critical thrust force necessary to propagate the delamination generated during the drilling operation of an antisymmetric angle-ply laminate is proposed by modeling the delamination zone as an elliptical shape. The critical thrust force is analytically derived with the consideration of bending, twisting and mid-plane extension of the delamination zone. And then to maximize the critical thrust force, the optimal design of an angle-ply laminate is performed to find the optimal number of fiber per millimeter, optimum diameter of fiber and optimum lamination angle using ADS (Automated Design Synthesis).

Critical thrust force at delamination propagation during drilling of angle-ply laminates

A new formulation for the critical thrust force necessary to propagate the delamination generated during the drilling operation of an antisymmetric angle-ply laminate is proposed by modeling the delamination zone as an elliptical shape. The critical thrust force is analytically derived with the consideration of bending, twisting and mid-plane extension of the delamination zone. And then to maximize the critical thrust force, the optimal design of an angle-ply laminate is performed to find the optimal number of fiber per millimeter, optimum diameter of fiber and optimum lamination angle using ADS (Automated Design Synthesis).

Comprehensive analysis of delamination in drilling of composite materials with various drill bits

Beside the twist drill, the effects of various drill geometries were rarely discussed in analytical fashion. This study presents a comprehensive analysis of delamination in use of various drill types, such as saw drill, candle stick drill, core drill and step drill. In this analysis, the critical thrust force at the onset of delamination is predicted and compared with the twist drill.

Study on vibration drilling of fiber reinforced plastics with hybrid variation parameters method

Delamination is recognized as one of the major causes of damage during drilling of fiber reinforced plastics (FRPs), which not only reduces the structural integrity, but also has the potential for long-term performance deterioration. It is difficult to produce precise quality holes with high efficiency by conventional drilling method. In this paper, a new vibration drilling technology is presented using a hybrid variation parameters method, based on the model for predicting critical thrust forces and the model for predicting thrust and torque during vibration drilling of FRPs. From the drilling experiments, it was found that both precise quality holes and high efficiency could be obtained by this new drilling method.

Analysis of delamination in drilling composite materials using core drill

Delamination is one of the serious concerns during drilling. This paper presents a comprehensive analysis of delamination when using core drill. The critical thrust force at the onset of delamination is predicted and compared with a twist drill. The well known advantage of industrial use of a core drill is fundamentally illustrated. A guide for drill design can be developed based on the proposed models, and this approach can be extended to examine the effects of other special drill designs.

Drilling of Aramid and Carbon Fiber Polymer Composites

Drilling tests were conducted on aramid and carbon fiber-reinforced composite laminates using an instrumented machining center. Machining parameters for the damage-free drilling of these materials were established together with semi-empirical relationships between drilling forces and cutting parameters. The drilling force responses as a function of various feed rates and drill sizes were characterized to define the key process stages taking place during a drilling cycle. Using a previously established delamination-based criterion for initiating damage during drilling, the critical feed rate for damage-free drilling was established for the two composite material types: aramid and carbon fiber-reinforced polymer laminates. Independent measurements of the opening-mode delamination crack energy release rates were conducted on both materials to determine the critical thrust force for damage initiation. This study establishes the key process stages exhibited by carbon and aramid fiber composites during drilling, the critical threshold feed rates to avoid damage, and machining relations that can be utilized for the design of intelligent controllers for efficient drilling of composite laminates.