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.     

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。      

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.     

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.     

Drilling in carbon/epoxy composites: Experimental investigations and finite element implementation

Drilling carbon fibre reinforced plastics (CFRPs) is typically cumbersome due to high structural stiffness of the composite and low thermal conductivity of plastics. Resin-rich areas between neighbouring plies in a laminate are prone to drilling-induced delamination that compromises structural integrity. Appropriate selection of drilling parameters is believed to mitigate damage in CFRPs. In this context, we study the effect of cutting parameters on drilling thrust force and torque during the machining process both experimentally and numerically. A unique three-dimensional (3D) finite element model of drilling in a composite laminate, accounting for complex kinematics at the drill-workpiece interface is developed. Cohesive zone elements are used to simulate interply delamination in a composite. Experimental quantification of drilling-induced damage is performed by means of X-ray micro computed tomography. The developed numerical model is shown to agree reasonably well with the experiments. The model is used to predict optimal drilling parameters in carbon/epoxy composites.     

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.     

Prediction of Delamination Onset and Critical Force in Carbon/Epoxy Panels Impacted by Ice Spheres

Polymer matrix composite structures are exposed to a variety of impact threats including hail ice. Internal delamination damage created by these impacts can exist in a form that is visually undetectable. This paper establishes an analysis methodology for predicting the onset of delamination damage in toughened carbon/epoxy composite laminates when impacted by high velocity ice spheres (hailstones). Experiments and analytical work focused on ice sphere impact onto composite panels have determined the failure threshold energy as a function of varying ice diameter and panel thickness, and have established the ability to predict the onset of delamination using cohesive elements in explicit dynamic finite element analysis. A critical force associated with damage onset was found to be independent of the ice diameter and thus can be expressed as a function of basic panel-describing parameters, namely bending rigidity and interlaminar fracture energy. Critical force can be used as a failure criterion in simpler models (e.g., shell elements) when predicting the onset of delamination by high speed spherical ice impact.     

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.     

Delamination and Surface Roughness Analyses in Drilling Hybrid Carbon/Glass Composite

Previous attempts at drilling of fibrous composites have been challenging due to the limited mechanical properties and presence of severe delamination damage at the top and bottom surfaces of the drilled hole. With the recent introduction of hybrid carbon/glass fiber reinforced polymer (HFRP) composites in both research literature and industrial applications, the need for evaluating their drillability is inevitable prior to their final usage. This is mainly because of the unique properties of HFRP composites as compared to the single-type FRP composites. Therefore, this paper aims to present a research initiative that will elucidate the high-range parametric effects of drilling control factors on delamination damage and surface quality. Taguchi methodology and statistical analysis of variance were applied to determine the performance of the drilling process. Experimental results revealed that delamination damage and surface quality values were strongly influenced by the feed and special tool geometries rather than the spindle rotational speed. Changes in the feed are likely to contribute to increase in the thrust force and strain rate on the workpiece. Confirmation tests have shown the closeness of the calculated values via a regression model and additive rule with the experimental values. This indicates that the regression model from the response surface can be employed to estimate delamination damage and surface roughness during drilling of HFRP composite.     

Drilling tool geometry evaluation for reinforced composite laminates

In this work, a comparative study on different drill point geometries and feed rate for composite laminates drilling is presented. For this goal, thrust force monitoring during drilling, hole wall roughness measurement and delamination extension assessment after drilling is accomplished. Delamination is evaluated using enhanced radiography combined with a dedicated computational platform that integrates algorithms of image processing and analysis. An experimental procedure was planned and consequences were evaluated. Results show that a cautious combination of the factors involved, like drill tip geometry or feed rate, can promote the reduction of delamination damage.     

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).     

碳纤维增强环氧树脂复合材料层合板干涉连接插钉轴向力建模与分析

碳纤维增强环氧树脂复合材料（CFRP）构件干涉配合连接的插钉轴向力过大会引起层合板弯曲和分层,严重影响产品的安全性。针对CFRP层合板的高锁螺栓干涉连接过程,分析了其制孔、插钉及拧紧等装配连接工艺,将其干涉插钉过程划分为4个阶段,并对各个阶段进行了详细的力学行为分析;对螺栓杆处和倒角处的挤压力和摩擦力分别进行力学建模,并结合各作用力的边界条件与阶段划分,构建了干涉插钉全过程的轴向力模型;通过ABAQUS有限元模拟了CFRP层合板干涉插钉工艺过程,并开展了干涉螺栓安装实验,对比分析了层合板孔周径向挤压应力分布和插钉轴向力变化规律,解析结果与模拟和实验结果吻合较好,为后续CFRP层合板的插钉分层损伤和工艺优化研究奠定基础。      

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.     

Delamination analysis of carbon fibre reinforced laminates: Evaluation of a special step drill

Drilling of carbon fibre/epoxy laminates is usually carried out using standard drills. However, it is necessary to adapt the processes and/or tooling as the risk of delamination, or other damages, is high. These problems can affect mechanical properties of produced parts, therefore, lower reliability. In this paper, four different drills – three commercial and a special step (prototype) – are compared in terms of thrust force during drilling and delamination. In order to evaluate damage, enhanced radiography is applied. The resulting images were then computational processed using a previously developed image processing and analysis platform. Results show that the prototype drill had encouraging results in terms of maximum thrust force and delamination reduction. Furthermore, it is possible to state that a correct choice of drill geometry, particularly the use of a pilot hole, a conservative cutting speed – 53 m/min – and a low feed rate – 0.025 mm/rev – can help to prevent delamination.     

Optimization of process variables in drilling of carbon fiber reinforced plastics using various multi criteria decision making approaches

Carbon fiber-reinforced polymers are one of the lightweight materials used in structural design due to their exceptional mechanical performances. The drilling operation is indispensable as it facilitates the assembling of various manufactured components. However, drilling of fibrous laminates is deemed difficult in comparison to the traditional metals because of the anisotropic and non-homogeneous nature. The present work addresses the parametric effect on the drilled hole delamination and further reduces it with an optimal combination of parameters for multi-objectives using different multi-criterion decision-making techniques. Initially, the response surface-based regression model of delamination as a function of three static inputs has been developed, further revised with induced thrust as well as mean torque for the improvisation of the prediction capability. Finally, for the overall improvement, a decision-making model has been used that includes grey relation analysis, technique for order performance by similarity to ideal solution, and VIšekriterijumsko Kompromisno Rangiranje method. The delamination was found to be minimum at a low drill point angle (100°), high spindle rotation (2150 min⁻¹ ), and low feed rate (0.025 mm/rev) due to reduced thrust force. The mean absolute prediction error was significantly improved considering root mean square torque rather than axial thrust with process variables.     

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.     

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.     

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.