Drilling analysis of chopped composites

This work investigates the effects of the drilling parameters, speed, and feed, on the required cutting forces and torques in drilling chopped composites with different fiber volume fractions. Three speeds, five feeds, and five fiber volume fractures are used in this study. The results show that feeds and fiber volumes have direct effects on thrust forces and torques. On the other hand, increasing the cutting speed reduces the associated thrust force and torque, especially at high feed values. Using multivariable linear regression analysis, empirical formulas that correlate favorably with the obtained results have been developed. These formulas would be useful in drilling chopped composites. The influence of cutting parameters on peel-up and push-out delaminations that occurs at drill entrance and drill exit respectively the specimen surfaces have been investigated. No clear effect of the cutting speed on the delamination size is observed, while the delamination size decreases with decreasing the feed. Delamination-free in drilling chopped composites with high fiber volume fraction remains as a problem to be further investigated.     

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.     

Analysis of fracture mechanisms and surface quality in drilling of composite materials

The aim of this work is to clarify the interaction mechanisms between the drilling tool and material. Drilling tests were carried out on glass/polyester and carbon/epoxy composites using different twist drills. The cutting tools and machined surfaces were examined by optical microscopy, scanning microscopy and surface profilometry to study composite damage and tool wear. Among the defects caused by drilling, delamination appears to be the most critical and may occurs at both the entrance and exit planes. A prediction model of thrust force for drilling without delamination is proposed. __________ Translated from Problemy Prochnosti, No. 1, pp. 48–51, January–February, 2008.     

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.     

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.     

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。     

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.     

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.     

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.     

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。      

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.     

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.     

A review of modeling and control during drilling of fiber reinforced plastic composites

Drilling induced damage in polymer–matrix composites (PMCs) is a research area of immense engineering importance. Various approaches have been tried worldwide to minimize drilling induced damage. In this study, a review of automated drilling operation has been done. Various mathematical modeling methods used for dynamic phenomenon of drilling in PMCs and conventional materials have been discussed. Drilling of fiber reinforced plastic composites can be modeled using empirical techniques, neural network/fuzzy-logic and transfer function modeling methods. This paper brings state-of-the-art in the control of drilling process. The drilling of fiber reinforced plastic composites can be controlled using neural network, fuzzy logic, supervisory, PI, PID, pole placement and adaptive controllers. Results indicate that thrust force and torque have not been controlled simultaneously for delamination free drilling in PMCs. Critical thrust force has also not been precisely tracked. There is a need to create a combined mathematical model consisting of thrust force, torque and feed rate coupled with a suitable control law for simultaneous control of thrust force as well as torque for delamination free drilling of composites.     

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.     

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。     

Assessment of drilling-induced damage in CFRP under chilled air environment

Drilling holes in carbon fiber-reinforced polymer (CFRP) laminates are more prone to incur damage during machining. Surface damage could be considerably minimized through the adoption of cryogenic assisted machining. The economic and safety implications associated with cryogenic technology necessitate the exploration of alternate technologies. In this research work, the effects of cutting velocity (100, 125, and 150?m/min) and feed rate (0.03, 0.06, and 0.09?mm/rev) on thrust force, surface roughness, delamination, and acoustic emissions are studied during the drilling of CFRP laminates under chilled air environment and compared with dry drilling. The output parameters are found to be much influenced by feed rate than cutting velocity. Under high feed rate and cutting velocity, the delamination factor, surface roughness, and acoustic emissions are, respectively, reduced by 13.2, 10.5, and 7.4% for the drilling performed under chilled air environment over dry condition. About 9.9% increased thrust force is observed for chilled air-assisted drilling under the identical machining condition.     

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.     

The effect of thermoplastic coating on the mechanical properties of woven fabric carbon/epoxy composites

The effect of a polyetherimide (PEI) coating on the mechanical properties of woven fabric carbon/epoxy composites was investigated by thermal mechanical analysis, fractographical analysis and mechanical properties measurements. PEI coating enhanced the mechanical properties of carbon/epoxy composites mainly through the improvement of matrix properties. This was because most of the PEI coated on the carbon fiber diffused into the bulk of epoxy matrix due to its good miscibility with epoxy resin. As for mechanical properties of woven fabric carbon/epoxy composites, the extent of improvement by PEI coating highly depended on the applied stress state. Among the mechanical properties, mode II delamination resistance of carbon/epoxy composites showed the highest increment because matrix shear property played an important role in delamination resistance of woven fabric carbon/epoxy composite. Because of the woven geometry of carbon fiber, the improvement in impact property of carbon/epoxy composite was trivial except the large amount of PEI coated case.     

Fractographical analysis on the mode II delamination in woven carbon fiber reinforced epoxy composites

Mode II delamination phenomena of woven fabric carbon/epoxy composites were investigated by scanning electron microscopy. End notch flexural (ENF) test was used to examine the mode II delamination. Woven fabric composites showed two peculiar crack propagation patterns due to the complexity of woven geometry. In warp yarn region, crack propagated with forming a shear band and breaking the fiber/matrix interface. In fill yarn region, however, no shear band was observed. Considering these crack patterns, matrix shear property and fiber/matrix interfacial strength played an important role in enhancing the delamination properties of woven fabric carbon/epoxy composites. Due to the woven geometry, matrix rich positions, which are interstitial and undulated region, were formed in woven carbon/epoxy composite. In these regions, matrix fracture and complex crack path were mainly observed.