Drilling thick fabric woven CFRP laminates with double point angle drills

Carbon fiber reinforced plastics (CFRPs) have many desirable properties, including high strength-to-weight ratio, high stiffness-to-weight ratio, high corrosion resistance, and low thermal expansion. These properties make CFRP suitable for use in structural components for aerospace applications. Drilling is the most common machining process applied to CFRP laminates, and it is difficult due to the extremely abrasive nature of the carbon fibers and low thermal conductivity of CFRP. It is a challenge for manufacturers to drill CFRP materials without causing any delamination on the work part while also considering the economics of the process. The subject of this study is the drilling of fabric woven type CFRP laminates which are known to be more resistant to delamination than unidirectional type CFRP laminates. The objective of this study is to investigate the influence of double point angle drill geometry on drilling performance through an experimental approach. An uncoated carbide and two diamond coated carbide drills with different drill tip angles are employed in drilling experiments of aerospace quality thick fabric woven CFRP laminates. Force and torque measurements are used to investigate appropriate drilling conditions based on drill geometry and ideal drilling parameters are determined. Tool life tests of the drills were conducted and the condition of the diamond coating is examined as a function of drilling operational parameters. High feed rate drilling experiments are observed to be favorable in terms of drill wear. Feed is observed to be more important than speed, and the upper limit of feed is dictated by the drill design and the rigidity of the machine drill. Hole diameter variation due to drill wear is monitored to determine drill life. At high feeds, hole diameter tolerance is observed to be more critical than hole exit delamination during drilling of fabric woven CFRP laminates.     

CFRP drilling: Fundamental study of local feed force and consequences on hole exit damage.

Carbon Fiber-Reinforced by Plastic (CFRP) is now commonly used in the aircraft industry. The main challenge is to manufacture this difficult-to-cut work material, considering its quality criteria and economical aspects. Drilling is the main machining operation required for the assembly of the aircraft structure. In this paper, results are presented and discussed regarding exit delamination studied at a local scale. Because of the anisotropic properties of CFRP, the fiber cutting modes change with the composite sequence combined with the drill revolution parameters. The local feed forces generated by the cutting edge on the hole bottom may be correlated with delaminating aspects. A posttreatment method is proposed to analyze precisely these feed force and cutting torque distributions. Appropriate ply sequences are identified in order to limit the mechanical load concentration and the risk of delamination or uncut fibers     

碳纤维复合材料在航空领域中的应用现状及改进

碳纤维复合材料(CFRP)具有质轻高强、高模量和低膨胀优势,在“陆海空天”等领域应用潜力和需求极大. 其中热塑性碳纤维复合材料(CFRTP)近年来快速发展,CFRP和CFRTP与金属结构件的连接技术受到国内外学者和工业界的广泛关注. 文中综述了金属材料与CFRP连接的主要技术及界面的结合机理;对比分析了不同连接工艺的成形原理,总结了金属与CFRP连接界面的不同作用效果;结合发展现状及未来工程应用需求,对金属/CFRP高质量连接技术瓶颈的突破方向进行了展望.     

电镀金刚石钻头钻削碳纤维复合材料研究

碳纤维复合材料钻孔加工时极易产生分层、毛刺、撕裂等缺陷,是典型的难加工材料。针对碳纤维复合材料特点,以电镀金刚石钻头为研究对象,从钻削轴向力、钻孔出口质量等方面分析电镀金刚石钻头钻孔特点,并与硬质合金麻花钻进行对比,得出结论：电镀金刚石钻头钻削碳纤维复合材料时钻削轴向力较小,钻削质量较好,更适合于碳纤维复合材料的加工;钻头转速提高有利于减小钻孔缺陷的产生,钻削轴向力随钻头转速的升高而降低,随钻头直径的增大而增大;最后,通过多元线形回归方法得出电镀金刚石钻头钻削力经验公式。     

Mechanistic force modeling for milling of carbon fiber reinforced polymers with double helix tools

Carbon fiber reinforced polymers (CFRP) have emerged as the material of choice to satisfy increasing demand for lighter aircrafts. Machinability characteristics of CFRPs are quite different than those of metals; therefore, special tool designs have been developed for CFRP machining. The double helix end mill design compresses the upper and lower sides of the laminate using opposite helix angles that eliminate delamination. A mechanistic force model for double helix tools is developed based on milling force data obtained on flat end mills. The proposed model can be used to improve double helix tool designs and to optimize milling process parameters.     

Evaluation of drilling parameters on thrust force in drilling carbon fiber reinforced plastic (CFRP) composite laminates using compound core-special drills

Drilling is the mostly used secondary machining of the fiber reinforced composite laminates, while the delamination occurs frequently at the drill exit in the workpiece. In the industrial experiences, core drill shows better drilling quality than twist drill. However, chip removal is a troublesome problem when using the core drill. Conventional compound core-special drills (core-special drills and step-core-special drills) are designed to avoid the chip removal clog in drilling. But the cutting velocity ratio (relative motion) between outer drill and inner drill is null for conventional compound core-special drills. The current study develops a new device and to solve the problems of relative motion and chip removal between the outer and inner drills in drilling CFRP composite laminates. In addition, this study investigates the influence of drilling parameters (cutting velocity ratio, feed rate, stretch, inner drill type and inner drill diameter) on thrust force of compound core-special drills. An innovative device can be consulted in application of compound core-special drill in different industries in the future.     

Feasibility study of the rotary ultrasonic elliptical machining of carbon fiber reinforced plastics (CFRP)

Carbon fiber reinforced plastics (CFRP) are used for various aircraft structural components because of their superior mechanical and physical properties such as high specific strength, high specific stiffness, etc. However, when CFRP are machined, rapid tool wear and delamination are troublesome. Therefore, cost effective and excellent quality machining of CFRP remains a challenge. In this paper, the rotary ultrasonic elliptical machining (RUEM) using core drill is proposed for drilling of holes on CFRP panels. This method combines advantages of core-drill and elliptical tool vibration towards achieving better quality, delamination free holes. The cutting force model and chip-removal phenomenon in ultrasonic elliptical vibration cutting are introduced and analyzed. The feasibility to machine CFRP for RUEM is verified experimentally. The results demonstrate that compared to conventional drilling (CD), the chip-removal rate has been improved, tool wear is reduced, precision and surface quality around holes is enhanced, delamination at hole exits has been prevented and significant reduction in cutting force has been achieved.     

Cutting edge rounding: An innovative tool wear criterion in drilling CFRP composite laminates

An evenly and smoothly distributed abrasion wear, observed along the entire cutting edge of an uncoated carbide drill bit in drilling CFRPs, is due to the highly abrasive nature of the carbon fibres. A very few researchers have only quoted this wear mode as being responsible for giving rise to the rounding of the cutting edge, or its bluntness. However, this wear feature has seldom been investigated, unlike the conventional flank wear in practice. This paper offers a new approach in unveiling and introducing the cutting edge rounding (CER) – a latent wear characteristic as a measure of sharpness/bluntness – of uncoated cemented carbide tools during drilling CFRP composite laminates. Four different types of drills (conventional and specialised) were tested to assess the applicability and relevance of this new wear feature. Mechanical loads (drilling thrust and torque) were recorded, and the hole entry and exit delamination were quantified. For the utilised tools, the accruing magnitude of CER was also recorded, in parallel with studying their conventional flank wear. Very appreciable correlations between the CER and the drilling loads, and also the quantitative delamination results are observed. Results reveal that this new wear type develops almost similarly for the selected tools and is practically independent of their respective conventional flank wear patterns. Moreover, a distinct, non-zero magnitude of the CER for a very fresh tool state may provide researchers with some lucid information in further studying the results during wear tests, more emphatically. The CER correlations with quantitative delamination results are noticed quite comparable to those of the conventional flank wear via statistical linear regression analyses.     

Performance of cryogenically treated WC drill using tool wear measurements on the cutting edge and hole surface topography when drilling CFRP

Drilling CFRP poses major challenges from the perspective of rapid tool wear and poor hole quality, with the development of higher strength fibers further accentuating these problems. Cryogenic treatment is one way of increasing tool hardness thereby improving tool life and hole quality through longer retention of cutting edge sharpness. In this study, tool wear is monitored by measuring flank wear, cutting edge flatting (CEF), peak flatting (PF) and cutting edge surface roughness (CESR). While flank wear is unable to distinguish the better performance of the cryo-treated drill from the untreated drill, the other wear parameters are able to account for the better hole quality (exit delamination) produced by this drill. CEF and PF are direct measures of the extent of cutting edge blunting unlike flank wear which measures wear along the flank due to rubbing. Fiber pullout is the primary reason for deterioration in surface finish and Rz and Rv are better measures than Ra in estimating surface quality.     

Acoustic emission for controlling drill position in fiber-reinforced plastic and metal stacks

Drilling a stack made of fiber-reinforced plastic and metal layers is investigated and presented in this paper. Improvement of performance can be achieved if the process parameters will be adapted to the various drilled materials and drill position. Therefore, the true position of the drill should be precisely known. An algorithm for real time monitoring of drill position is suggested. Drill position is defined by analyzing acoustic emission signals from a sensor located near the drilling point. During drilling of CFRP and Al-stacks, it could be proved that material changeover can be identified ahead of time, especially when using stepped drills.     

Detecting debonding flaws at the epoxy–concrete interfaces in near-surface mounted CFRP strengthening beams using the impact–echo method

This paper discusses the feasibility of using the impact–echo method (IE method) to assess the debonding flaws at the epoxy–concrete interfaces of near-surface mounted carbon fiber reinforced polymers (NSM CFRPs). The effectiveness of NSM CFRP strengthening is influenced by whether epoxy bonds CFRP bars and existing concrete structures effectively and thoroughly, thereby mutually transmitting the stress in the reinforced zones with CFRP bars. Because of poor construction quality, aging, and deterioration, debonding occurs between epoxy and concrete surfaces, severely inhibiting the reinforcement effectiveness of NSM CFRPs. The IE method is based on the principle of stress waves and was adopted in this study as the nondestructive debonding flaw assessment technique. To clarify the characteristics of the stress wave propagation caused by impact forces on NSM CFRP structures, numerical analysis and experimental test were performed on the bar-like epoxy specimens with a cross-sectional size of 13×13 mm. The types of specimens used in this study included the bar-like pure epoxy specimen, bar-like epoxy specimens containing CFRP bars, and the concrete beams containing bar-like epoxy specimens with and without CFRP bars. For the concrete beams embedded with bar-like epoxy specimens, various debonding statuses at the epoxy–concrete surfaces were considered. The numerical analysis revealed that the impact responses in the bar-like pure epoxy specimen and bar-like epoxy specimens containing CFRP bars were dominated by the first few cross-sectional modes of vibration, and the IE test verified this result. The numerical and experimental results indicated that for the concrete beams containing bar-like epoxy specimens with and without CFRP bars and without debonding flaws, the impact response spectra each featured one high-amplitude peak at the fundamental mode frequency, referred to as the dominant frequency. When debonding occurred at the epoxy–concrete interfaces of the concrete beams containing bar-like epoxy specimens with and without CFRP bars, the impact responses showed that the dominant frequencies decreased significantly. When the dominant frequencies instead increased to levels nearly equal to those of the fundamental frequencies of the bar-like epoxy specimens, the epoxy thoroughly debonded from the concrete interfaces. According to the characteristics of the aforementioned impact responses, the IE method is capable of detecting the debonding flaws at epoxy–concrete interfaces in NSM CFRP strengthening.     

Orthogonal cutting mechanics of multi-directional carbon fiber reinforced polymer with interlaminar bonding effect

This paper determines the chip formation mechanism, fiber-matrix failure modes, and cutting forces in orthogonal cutting of multi-directional carbon fiber reinforced polymer (MD CFRP) with interlaminar bonding effect. The cutting experiments show that the varying chip formation angles with different fiber orientations in cutting unidirectional plies converge for MD CFRP. A new analytical mechanics model for cutting MD CFRP is developed to predict the chip formation angle and failure modes based on the minimum energy principle for all plies. The model with experimental validation reveals the different cutting mechanisms between UD and MD CFRPs.     

New type drill with three major cutting edges

Drilling performance of a new type twist is described. The new type drill has three major cutting edges, three chisel edges and three flutes. This drill is an efficient means of making a hole with high accuracy and no reaming. Recently, some research on drill materials including surface coating, drill point geometry, cutting forces and tool life have been carried out in order to increase the cutting performance of drills. All the drills used in these experiments were ordinary drills with two major cutting edges. The authors calculated the point geometry of the new type drill, and also examined the cutting characteristics of the drill with respect to cutting forces, hole accuracy and tool vibration. The experimental results showed that the whirling vibration which frequently occurs in an ordinary drill with two major cutting edges disappears when the new type drill is used, and thereby rifling marks do not results on the hole surface. Consequently, a hole with high roundness and straightness is obtained.     

CFRP/金属异种材料激光连接工艺的研究现状

树脂基碳纤维增强复合材料（CFRP）作为新型轻质结构复合材料,广泛应用于航空航天器件的零部件中。CFRP与航空常用金属传统连接主要有胶接和机械连接,但具有一定的局限性。激光连接技术具有能量密度高,可控性好等特点,可用于复合材料和金属的连接。本文针对当前树脂基CFRP与金属（铝合金、钛合金、钢等）激光连接接头成形机理以及接头缺陷进行了综述,同时分析了焊接工艺、组织结构优化以及焊前金属表面处理对接头成形的调控,并对CFRP/金属激光连接的发展趋势提出了展望。     

Hole quality assessment following drilling of metallic-composite stacks

The use of material stacks comprising titanium, carbon fibre reinforced plastics (CFRPs) and aluminium is expanding for structural aerospace applications, especially where high mechanical loads exist such as for aircraft wing and tail-plane components. Here, the production of bolt/fixation holes is essential to the manufacturing process in order to facilitate part assembly. The paper outlines an analysis of hole quality/integrity following drilling of titanium/CFRP/aluminium stacks under flood cutting fluid and spray mist environments. Uncoated and coated (CVD diamond and hardmetal) tungsten carbide drill performance is evaluated against key response measures including hole size, out of roundness, cylindricity, burr height, hole edge quality, average surface roughness (R_a), microhardness (of the metallic elements) and swarf morphology. Burr height (up to 0.5 mm) was observed to be greater at the hole exit (aluminium) compared to hole entry (titanium) while delamination was significantly reduced when machining CFRP in the stack configuration as opposed to a standalone arrangement. Spiral shaped continuous aluminium swarf was prevalent while both short and long helical chips were found with the titanium material when cutting wet. In contrast, the CFRP layer typically produced dusty black composite particles suspended in the soluble oil of the coolant emulsion.     

Taguchi analysis of delamination associated with various drill bits in drilling of composite material

This paper presents a prediction and evaluation of delamination factor in use of twist drill, candle stick drill and saw drill. The approach is based on Taguchi’s method and the analysis of variance (ANOVA). An ultrasonic C-Scan to examine the delamination of carbon fiber-reinforced plastic (CFRP) laminate is used in this paper. The experiments were conducted to study the delamination factor under various cutting conditions. The experimental results indicate that the feed rate and the drill diameter are recognized to make the most significant contribution to the overall performance. The objective was to establish a correlation between feed rate, spindle speed and drill diameter with the induced delamination in a CFRP laminate. The correlation was obtained by multi-variable linear regression and compared with the experimental results.     

Investigation of wear resistance of drill bits with WC,Diamond-DLC,and TiAlSi coatings with respect to mechanical properties of rock

Drilling is an important engineering operation with extensive application in many fields of industry including mining engineering, oil and gas exploration and exploitation, civil engineering, groundwater management, etc. Drill bits must be able to endure enormous stresses that gradually wear them down during the drilling operation. In rock drilling, wear resistance is a key determinant of the drill bit lifetime and hence the drilling cost, thus basically affecting the choice of drilling method for any given rock type. With the advent of new wear-resistant materials, they can be used to improve the resistance of drill bits against wear and erosion. This study investigated the wear resistance of drill bits with tungsten carbide (WC) coating, DLC-Diamond coating, and titanium-silica‑aluminum (TiAlSi) coating when drilling in three types of hard rock, namely Khoshtinat Granite (A₁), White Natanz Granite (A₂) and Nehbandan Granite (A₃). The drilling tests were performed on cuboid specimens using a drilling machine at rotation speeds of 850, 900 and 950 rpm and penetration rates of 12, 18 and 24 mm/min. The results showed that for any fixed drilling conditions, the wear rates of the TiAlSi drill bit in A₁, A₂, and A₃ were respectively 48%, 52%, and 60% lower than those of the WC drill bit. In the same rocks, the Diamond-DLC drill bit also showed 42%, 44.25%, and 55% lower wear rates than the WC drill bit. in addition to the observed changes in wear rate of the drill bits, the surface roughness created by these drills represents the optimum performance of the TiAlSi drill bit. It was observed that, as the mechanical properties of the rock (uniaxial compressive strength, Mohs hardness, Schimazek's abrasivity index and Young's Modulus) increased, the tested drill bits showed wider differences in terms of wear resistance. As the TiAlSi drill bit had the lowest wear rate (27%) and after that, the Diamond-DLC drill bit showed a better wear (30%) performance than the WC drill bit (60%).     

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.     

Anisotropy in carbon fiber reinforced polymer (CFRP) and its effect on induction thermography

Lock-in induction thermography was carried out on uniaxial, biaxial, and multi-ply woven carbon fiber reinforced polymers (CFRPs) at an induction frequency of approximately 300 kHz and at modulation frequencies of 0.4–36 Hz to detect defects in them. The measurements were performed in reflection and transmission modes. In the uniaxial material, heating occurred far away from the excitation coil. Lines of minimal heating were observed. The modulation frequency dependence of the amplitude showed an inverse proportionality, indicating volume heating. The multi-ply woven CFRP plates with impacts of 3 and 5 J showed distinct contrast because of cracked fibers. The results were compared with those obtained using optically excited thermography and X-ray tomography.     

Numerical and experimental investigation of manufacturing and performance of metal inserts embedded in CFRP

Due to their outstanding specific mechanical properties, carbon fibre reinforced plastics (CFRP) exhibit a high application potential for lightweight structures. With respect to multi-material design and to avoid drilling of structural CFRP parts to join them to other components, embedded metal elements, so called inserts, can be used. The inserts consist of a shaft and a baseplate which is embedded between the fibre layers. So far, only punctiform inserts have been subject to research. One feasible geometry are linear inserts which have not been studied yet. In this work, the performance of two different types of linear inserts will be investigated. The shapes are based on a punctiform insert which is made out of a threaded shaft welded onto a baseplate whose performance under different types of loading has been investigated before. The first type of linear inserts has the same cross-section as the reference punctiform insert but is of a linear form. The second type is a quasi-linear insert which consists of a baseplate with the same dimensions as the first linear inserts and three threaded shafts welded onto it. All samples are manufactured by resin transfer moulding (RTM). Depending on the geometry of the insert and the preforming concept it is potentially possible to maintain the fibre continuity. For the inserts with a continuous shaft and in the proximity of the insert, it is necessary to cut fibres of the top layers which are aligned perpendicular to the shaft. For the quasi-linear insert, it is possible to maintain the fibre continuity as the fibres are guided around the circular shafts. Additional to mechanical tests that are carried out, mould-filling and curing simulations are performed for different inserts to analyse the influence of the process parameters onto the part quality. In the main series of tests, the specimens are characterized regarding their failure behaviour and load bearing capacity under quasi-static loads. The results of the experiments show that, compared to the punctiform reference insert, the linear load introduction elements exhibit higher load bearing capacity. However, the linear load introduction elements are inferior regarding specific load bearing capacity and furthermore increase process complexity during preforming and production.