Delamination in drilling GFR-thermoset composites

Delamination is a major problem associated with drilling fiber-reinforced composite materials that, in addition to reducing the structural integrity of the material, also results in poor assembly tolerance and has the potential for long-term performance deterioration. Delamination-free in drilling different fiber reinforced thermoset composites is the main objective of the present paper. Therefore the influence of drilling and material variables on thrust force, torque and delamination of GFRP composites was investigated experimentally. Drilling variables are cutting speed and feed. Material variable include matrix type, filler and fiber shape. Drilling process was carried out on cross-winding/polyester, continuous-winding with filler/polyester, chopped/polyester, woven/polyester and woven/epoxy composites. A simple inexpensive accurate technique was developed to measure delamination size.

The results show that the presence of sand filler in continuous-winding composites not only raised the values of cutting forces and push-out delamination but also increased their values with increasing cutting speed. In contrast, increasing the cutting speed in drilling cross-winding, woven and chopped composites reduces the push-out delamination as a result of decreasing the thrust force. The thrust forces in drilling continuous-winding composite are more than three orders of magnitude higher than those in the cross-winding composites. Chopped composites have lower push-out delamination than those made from woven fibers. For the same fiber shape, the peel-up and push-out delaminations of woven/epoxy composite are lower than that for woven/polyester composites. Delamination, chipping and spalling damage mechanisms were observed in drilling chopped and continuous-winding composites. In drilling woven composites the delamination was observed at different edge position angles due to the presence of the braids that made by the interlacing of two orthogonal directions of fibers tows (warp and fill). Delamination-free in drilling cross-winding composites was achieved using variable feed technique.     

Experimental investigation on drilling of natural flax fiber-reinforced composites

Natural fiber composites (NFCs) have strong potential to replace glass fiber-reinforced plastics. An instrumental operation in machining composite structures is hole making which facilitates assembly of parts. Understanding the effects of drilling process parameters on feature properties of NFCs has great benefits. In this regard, to make a good quality and accurate hole in composite structures, appropriate selection of drill bit and cutting parameters is important. This paper investigates delamination behavior and hole quality of flax/epoxy composite laminates in response to feed, spindle speed, and three different types of drill bit. As indicated by analysis of variance results, drill bit type and feed have greater influences on the thrust force. It appeared that delamination factor and surface roughness were significantly influenced by drill bit, but not by feed and spindle speed. The choice of drill bit has great impact on the delamination factor (67.27%) and surface roughness (74.44%), respectively.     

Machinability analysis in drilling woven GFR/epoxy composites: Part I – Effect of machining parameters

The present paper deals with the effect of machining parameters (feed, speed and drill diameter) on the thrust force and machinability of woven glass fiber-reinforced epoxy (GFRE) composites. The selected machinability parameters were delamination size, surface roughness, and bearing strength. The results show that, delamination-free in drilling GFRE composites was not observed, in the range of the investigated cutting parameters. Surface roughness instrument can be used as an indication for the position of the internal delamination damage in drilling GFRE composites. The high values of correlation coefficients between thrust force and the machinability parameters confirm the importance of reducing the thrust force to improve the load carrying capacity of composite structure assembled by rivets or bolted joints.     

Investigations on Drilling of Bidirectional Cotton Polyester Composite

Cotton fiber composites are currently used in the interior parts of automobiles, insulation boards, fiber boards, and various parts of high-friction mechanical assemblies. These composites are subjected to machining operations. Drilling is an indispensable machining operation for assembly of different parts. The present work aimed to carry out experimental and analytical investigations on drilling of bidirectional cotton polyester composite. Influence of feed, spindle speed, and drill point angle on machining characteristics like thrust force, torque, and delamination factor was studied by conducting experiments derived from Taguchi's L27 orthogonal array. To determine the significance of each drilling process parameters and their interactions, analysis of variance (ANOVA) test was conducted. Modeling of drilling parameters was carried out through multiple regression analysis and their optimization for minimizing cutting forces and machining induced damage was carried out using signal to noise ratio analysis.     

Experimental thrust forces and delamination analysis of GFRP laminates using candlestick drills

Drilling is usually performed using twist drills while assembling the composite components. However, it is necessary to adopt appropriate tool as the risk of delamination damage is high. Many researchers have performed experimental and theoretical study concerning drilling-induced delamination damage in composites utilizing special drills. The article performed drilling experiments of glass fiber reinforced plastic laminates utilizing candlestick drills. Ten candlestick drills with different drill tip geometries were compared in thrust forces and push down delamination. The results revealed the drilling behavior of the outer cutting edges and the outer drill tips for candlestick drill, and also indicated that the push down delamination depended on exit thrust force instead of maximum thrust force. What is more, thrust forces and push down delamination were significantly reduced when holes were produced using reasonable candlestick drill tip geometry. Finally, the analysis was useful for selecting appropriate candlestick drill tip geometry and conducting candlestick drill tip geometry optimization.     

Dynamic response of multidirectional composites in hygrothermal environments

Drilling is an essential operation in the assembly of the structural frames of automobiles and aircrafts. The life of the joint can be critically affected by the quality of the drilled holes. The main objective of the present paper is to investigate the influence of some parameters on the thrust force, torque and surface roughness in drilling processes of fiber-reinforced composite materials. These parameters include cutting speed, feed, drill size and fiber volume fraction. The quasi-isotropic composite materials were manufactured from randomly oriented glass fiber-reinforced epoxy, with various values of fiber volume fractions (V_f), using hand-lay-up technique. Two components drill dynamometer has been designed and manufactured to measure the thrust and torque during the drilling process. The dynamometer was connected with a data acquisition, which installed in a PC computer. This set-up enable to monitor and record the thrust force and torque with the aid of a computer program that designed using Lab View utilities.

The results indicate that the start point of torque cycle is delayed by few seconds (depending on the value of feed) than the thrust force. This time is consumed to penetrate the specimen by chiseling edge. After the thrust force reached its maximum value it is gradually decreased during the full engagement of the drill and goes to zero when both the chisel edge and the cutting lips have exit of the laminate. In contrast the torque was gradually increased up to the end of the cycle and sudden jump to a value about 10 times the peak value. Cutting speed has insignificant effect on the thrust force and surface roughness of epoxy resin. For glass fiber-reinforced epoxy composites (GFREC) with V_f=9.8–23.7% the thrust force and torque were decreased with increasing cutting speed. On contrast increasing feed, drill size and fiber volume fractions lead to increase the thrust force and torque. The drilled holes of GFREC with lower V_f ratio at lower feed have greater roughness than that drilled at higher feed. Specimens with high V_f ratio have a contrary behavior. Drill diameter combined with feed has a significant effect on surface roughness.     

硬质合金台阶钻结构参数及钻削工艺参数对CFRP开孔性能的影响

基于VMC850B立式加工中心和UltraPAC超声C-扫描仪,搭建了碳纤维增强树脂基复合材料(CFRP)钻削试验平台,探讨了台阶钻结构参数及钻削工艺参数对CFRP钻削过程中的钻削轴向力和分层因子的影响。结果表明,钻削工艺参数对第一段钻削轴向力影响较大,台阶钻结构参数对第二段钻削轴向力的影响较大;分层因子的大小与第一段钻削轴向力和第二段钻削轴向力有关,当第一段和第二段直径比d/D0.5时,分层因子主要与第一段钻削轴向力有关;减小分层的优水平组合为第一段直径2.8mm,第二段锋角95°,主轴转速7000r/min,进给速度2.5mm/s。     

基于CFRP层合板钻削轴向力时变曲线的钻头几何形状分析

为探索能够实现碳纤维增强复合材料（CFRP）层合板低损制孔的钻头几何形状,采用4种不同几何形状的钻头,对T800级CFRP层合板进行钻孔实验研究,分析了钻头几何形状对钻削轴向力的影响,探讨了钻削轴向力与分层损伤之间的关系。结果表明:轴向力归零速度与出口分层因子有较好的正相关性,可采用钻削轴向力归零速度来表征钻头几何形状对CFRP层合板钻孔的适用性能。同时,实验发现切削区域具有多阶段几何特征的钻头,在钻出工件底部时轴向力是分阶段缓慢归零,出口分层因子较小。      

Residual tensile strength monitoring of drilled composite materials by acoustic emission

Drilling results in damage such as delamination and matrix cracking around the hole and might ultimately causes degradation in the residual tensile strength of the drilled components. The damage induced during the drilling of composites can be detrimental to the mechanical behavior of the composite products. In this work, the effects of machining parameters (feed rate and cutting speed) and drill point angle on thrust force, the adjusted delamination factor and residual tensile strength are investigated. The Taguchi technique for the design of experiments was employed to analyze the thrust force, adjusted delamination factor and residual tensile strength of woven glass/resin epoxy. The results show that feed rate and drill point angle are the most important parameters. During tensile testing of drilled laminates, acoustic emission (AE) events were recorded. By feature extraction of AE time domain parameters, the suitable parameter for detecting the characteristics of thrust force and tensile force were determined. The AE mean power (MP) and cumulative count correlated well with thrust force and tensile force, respectively.     

碳纤维增强复合材料螺旋铣孔切削力及加工质量研究

针对传统钻孔方法加工复合材料时易导致分层、撕裂等缺陷的问题,采用螺旋铣作为新的制孔技术,根据飞机装配现场的实际加工条件,构建以机器人为移动载体、螺旋铣孔终端执行器为加工单元、螺旋铣孔专用刀具为切削工具的加工系统,采用该加工系统对碳纤维增强复合材料(CFRP)螺旋铣孔关键工艺参数进行正交试验,并讨论了刀具主轴转速、每齿进给量和轴向切削深度等工艺参数对切削力的影响规律;通过对加工缺陷的监测,探讨了切削力与CFRP分层、撕裂等缺陷之间的关系;最后对工艺参数进行优化,经试验验证,优化后轴向切削力较优化前降低26%以上,孔入口及出口处均无撕裂、毛刺,加工质量最优。     

Influence of process parameters on cutting force and torque during drilling of glass–fiber polyester reinforced composites

This research outlines the Taguchi optimization methodology, which is applied to optimize cutting parameters in drilling of glass fiber reinforced composite (GFRC) material. Analysis of variance (ANOVA) is used to study the effect of process parameters on machining process. This procedure eliminates the need for repeated experiments, time and conserves the material by the conventional procedure. The drilling parameters and specimen parameters evaluated are speed, feed rate, drill size and specimen thickness. A series of experiments are conducted using TRIAC VMC CNC machining center to relate the cutting parameters and material parameters on the cutting thrust and torque. The measured results were collected and analyzed with the help of the commercial software package MINITAB14. An orthogonal array, signal-to-noise ratio are employed to analyze the influence of these parameters on cutting force and torque during drilling. The method could be useful in predicting thrust and torque parameters as a function of cutting parameters and specimen parameters. The main objective is to find the important factors and combination of factors influence the machining process to achieve low cutting low cutting thrust and torque. From the analysis of the Taguchi method indicates that among the all-significant parameters, speed and drill size are more significant influence on cutting thrust than the specimen thickness and the feed rate. Study of response table indicates that the specimen thickness, and drill size are the significant parameters of torque. From the interaction among process parameters, thickness and drill size together is more dominant factor than any other combination for the torque characteristic.     

Analysis of delamination in drilling glass fiber reinforced polyester composites

Glass fiber reinforced plastic (GFRP) composite materials are finding increased application in aeronautical, automobile and structural applications. Drilling is a complex process, owing to their tendency to delaminate is used to join composite structures. In the present work, an attempt has been made to develop empirical relationships between the drilling parameters such as fiber orientation angle, tool feed rate, rotational speed and tool diameter with respect to delamination in drilling of GFR–polyester composites. The empirical relationship has been developed by using response surface methodology. The developed model can be effectively used to predict the delamination in drilling of GFRP composites within the factors and their limits are studied. The result indicated that the increase in feed rate and drill diameter increases the delamination size whereas there is no clear effect is observed for fiber orientation angle. The spindle speed shows only little effect on delamination in drilling of GFR–Polyester composites.     

Machinability analysis in drilling woven GFR/epoxy composites: Part II – Effect of drill wear

The current paper is a continuation of the author’s work on machinability analysis in drilling woven glass fiber reinforced epoxy (GFRE) composites. The present paper deals the effect of drill pre-wear on the machinability parameters in drilling GFRE composites, at different cutting conditions. Machinability parameters were characterized by thrust force, torque, peel-up and push-out delaminations, and surface roughness of drilled holes. The results showed that, the behavior of thrust force during drilling process was greatly affected by the drill pre-wear. This effect becomes extreme at high cutting speed and feed, which in turn increases delaminations and surface roughness. The scanning electron photographs demonstrate that, peaks and valleys in surface roughness profile were due to burning the matrix. The multi-variable linear regression models were fair to fit the experimental data. Therefore, considerable attention should be paid to record different models that improve the correlation between the machinability parameters and machining conditions.     

Optimization of delamination factor in drilling GFR–polypropylene composites

Glass fiber-reinforced polypropylene composites often replace the conventional materials due to their special or unique mechanical properties. As the applications of these composites increase for a number of industries, drilling of these composites is inevitable for subsequent composite product manufacturing stage. In the drilling of composites, the thrust force is induced during the drilling operation; as a result, it causes damage. This damage is characterized by the delamination factor, which depends on the machining parameters such as speed of the spindle, feed rate, and drill diameter. The study on the delamination in the drilling of glass fiber-reinforced polypropylene is limited and has been carried out comprehensively. The effect of machining parameters on delamination in the drilling of glass fiber-strengthened polypropylene (GFR-PP) composites is studied through the Box–Bhenken design. Response surface method, along with the desirability analysis, is used for modeling and optimization of delamination factor in the drilling. The result proves that the models are effectively used to forecast the delamination in the drilling of GFR-PP composites. Also, the result indicates that the foremost issue that influences the delamination is the feed rate.     

Experimental investigation and optimization of machining parameters in drilling of fly ash-filled carbon fiber reinforced composites

Particle-filled polymer composites have become attractive because of their wide applications and low cost. Carbon fiber reinforced polymer (CFRP) is well known as a difficult-to-cut material, which has very strong physical and mechanical characteristics. Machining of carbon fiber reinforced composites is essential to have functional upshots, out of which drilling is the key operation needed for fabrication. In this paper Taguchi L₂₇ experimental design is coupled with grey relational analysis (GRA) to optimize the multiple performance characteristics in the drilling of fly ash-filled carbon fiber reinforced composites. Experiments were conducted on a vertical machining center, and Taguchi L₂₇ experimental design was chosen for the experiments. The drilling parameters, namely spindle speed, feed rate, drill diameter and wt% of fly ash, have been optimized based on the multiple performance characteristics including thrust force, surface roughness, and delamination. The GRA with multiple performance characteristics indicates that the wt% of fly ash and drill diameter are the most significant factors that affect the performance. Experimental results have shown that the performance in the drilling process can be improved effectively by using this approach.     

Numerical Modeling of Exit-Ply Delamination During Drilling of CFRPs Considering Thermal Effects

Abstract

A finite element (FE) model for exit-ply delamination during drilling carbon fiber reinforced polymers (CFRPs) laminates is presented. The current FE model is developed to predict critical thrust force at the onset of delamination for 1 and 2 plies under the twist drill for various cutting temperatures. The interface behavior for delamination onset is modeled using surface based cohesive zone model (CZM). The numerical predictions for critical thrust force are compared with experimental thrust forces for various number of plies under the twist drill over a range of cutting temperature. Thrust force predictions were found to match with experimental data.     

Cutting force and hole quality analysis in micro-drilling of CFRP

Micro-drilling in carbon fiber reinforced plastic (CFRP) composite material is challenging because this material machining is difficult due to anisotropic, abrasive and non-homogeneous properties and also downscaling of cutting process parameters affect the cutting forces and micro-drilled hole quality extensively. In this work, experimental results based statistical analysis is applied to investigate feed and cutting speed effect on cutting force components and hole quality. Analysis of variance based regression equation is used to predict cutting forces and hole quality and their trend are described by response surface methodology. Results show that roundness error and delamination factor have similar trends to those of radial forces and thrust force, respectively. Non-linear trends of cutting forces and hole quality errors are observed during downscaling of the micro-drill feed value. Optimization results show that cutting forces and hole quality errors are minimum at a feed value which is almost equal to the tool edge radius rather than at the lowest feed value. Therefore, the presented results clearly show the influences of size effects on cutting forces and hole quality parameters in micro-drilling of CFRP composite material.     

EXPERIMENTAL INVESTIGATIONS OF DRILLING ON CFRP COMPOSITES

The paper outlines the various problems associated with the drilling of CFRP composites. The technique of dimensional analysis is used to investigate the complex correlation between thrust force, cutting speed, feed, hole diameter, point geometry and material thickness during the drilling of holes in CFRP composites, A new non-dimensional number (t/D), thickness of layered composites to drill diameter, is found to influence the thrust force. Four drill point geometries specifically found effective in drilling of FRP were tried and among them the eight facet drill point geometry was found to give the best results.     

Optimization of machining parameters at high speed drilling of carbon fiber reinforced plastic (CFRP) laminates

High speed machining is now acknowledged to be one of the key manufacturing technologies to ensure high productivity and throughput. Drilling of CFRP, though a challenging task, is being performed successfully at low spindle speeds. However high speed drilling in CFRP thin laminates has not been explored much. This paper reports an experimental investigation of a full factorial design performed on thin CFRP laminates using K20 carbide drill by varying the drilling parameters such as spindle speed and feed rate to determine optimum cutting conditions. The hole quality parameters analyzed include hole diameter, circularity, peel-up delamination and push-out delamination. Analysis of variance (ANOVA) was carried out for hole quality parameters and their contribution rates were determined. Genetic Algorithm (GA) methodology was used in the multiple objective optimization (using MATLAB R2010a software) to find the optimum cutting conditions for defect free drilling. Tool life of the K20 carbide drill was predicted at optimized cutting speed and feed.     

Investigations on drilling of hemp/glass hybrid composites

The present study aimed to examine the influence of tool geometry, spindle speed and feed on thrust force (TF) and delamination in hybrid hemp-glass composites. The pure glass polyester, pure hemp polyester and hybrid hemp/glass polyester composite specimens were prepared using hand layup technique. The drilling experiments were performed according to the full factorial design. Analysis of variance (ANOVA) was used to determine the impact of layering arrangement of fibers, feed (0.06, 0.18, 0.3?mm/rev), speed (1000, 3000, 5000?rpm), tool geometry (Plexi Point, Brad, Parabolic) and their interactions on TF and delamination. It was observed that drill geometry is major determinant for TF and delamination. Empirical models were developed using regression analysis and grey relational analysis was performed for optimizing the input parameters for TF and delamination at entry and exit.