Process Monitoring for Vibration-assisted Drilling

Ultrasonic vibration-assisted drilling, which can obtain the minimization of geometrical defects, provides a solution for the drilling of carbon fiber-reinforced plastic (CFRP) by adding a displacement of a micro-scale amplitude with a frequency to the tool tip. However, single vibration mode in drilling of CFRP cannot satisfy the different requirements of various drilling stages in CFRP and limits the applications of current ultrasonic vibration-assisted drilling of CFRP. To overcome the limitation and improve the performance of vibration-assisted drilling process in CFRP components, a novel variant-dimension vibration-assisted drilling system (VD-VADS) is proposed. The VD-VADS is specially designed to be capable of delivering variant-dimension vibrations at the tool tip for drilling holes in CFRP by producing one-dimension longitude vibration, two-dimension elliptic vibration, and three-dimension composite vibration, which can be transferred to adapt to various drilling stages in CFRP. An analytical model of the VD-VADS’s locus will be formulated to facilitate the prediction of the loci of the tooltip and design and realization of the prototype device. The control strategy of variant-dimension vibration mode of the proposed VD-VADS is established and realized for various drilling stages in CFRP. To validate the machining performance of the VD-VADS, five groups of drilling experiments under the conventional drilling and various vibration-assisted drilling processes were performed. The performance tests and comparison results of the obtained holes between the conventional drilling and various vibration-assisted drilling processes indicate that the proposed VD-VADS is capable of generating holes with better overall quality than the current single vibration-assisted drilling processes.

Carbon Fiber Reinforced Plastics (CFRP) is widely used in the structural components of aircraft. Upon retirement, aircraft generate a large amount of CFRP waste, typically disposed of in landfills as industrial waste. However, for CFRP to become a sustainable industry staple in the future, methods for recycling and reusing it must be established. Domestic and overseas venture companies are actively working on exploring and establishing technologies to extract carbon fibers. However, despite ongoing research, there remains a scarcity of initiatives aimed at recycled carbon fiber (rCF) utilization. To aim for a complete circular economy in the future, it is desirable to ensure sufficient mechanical properties when turning rCF into CFRP. This paper introduces efforts aimed at improving the mechanical properties of rCFRP.

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

Carbon fiber reinforced plastic (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. In order for CFRP materials to be used in aircraft structures or machine elements, high quality holes must be created on them efficiently. However, it is difficult for conventional methods such as drilling, helical milling and so on to meet the requirements. Therefore, this study proposes a novel method for creating holes on CFRP products. This method is just performed by tilting the workpiece at a certain angle on the basis of conventional helical milling (CHM) and hence called tilt helical milling (THM). In this paper, the processing principle of the THM is described in detail at first. An experimental apparatus is then constructed by installing a work holding unit produced in house onto a CNC milling machine. Finally, experiments are carried out on the apparatus. The obtained experimental results demonstrated that the THM is capable of creating holes on CFRP with lower burrs and chippings formation compared with those by CHM. By this new method, high quality holes could be obtained with high efficiency compared with conventional methods.

In many applications of automotive and aircraft industries the use of material combinations such as compound materials made from carbon fiber reinforced plastics (CFRP) and metal materials like steel, titanium or aluminum alloys is significantly increasing. For these industries, the lightweight and mechanical properties of the reinforced plastic materials gain more and more importance. When machining material combinations, a number of distinctive material related effects occur and hamper the straight-forward implementation of machining processes. These effects mainly derive from the distinctive chipping behavior of the material combination caused by the different material characteristics. Thus, the drilling process of CFRP/metal stacks is to be regarded as a challenging task due to the requirements of machining efficiency and quality aspects. In this regard, vibration assisted drilling at low frequencies but high amplitudes opens up significant opportunities for improvements of the machining processes. The feed rate is superimposed by a controlled harmonic motion in order to create an intermittent cutting state. The potential of vibration assisted drilling lies in the reduction of cutting forces and tool wear.

Carbon fiber reinforced plastics (CFRP) has been widely used in various aircraft structural components. However, it is difficult for conventional methods such as drilling and helical milling to meet the requirements on high quality and efficient holes creation. Hence a so-called tilt helical milling (THM) method has been proposed. This new method is performed by replacing the revolving motion of the tool in conventional helical milling (CHM) with a conical pendulum motion, in which the tool axis is tilted towards the hole axis at a certain angle. As a step toward the establishment of the new method, in this work, the fundamental drilling characteristics of CFRP by the THM is elucidated by experimentally investigating the effects of tilt angle on thrust force and delamination factor. The obtained experimental results demonstrated that thrust force and delamination factor can be reduced with THM technique. In addition, THM can achieve better hole surface finish than CHM.

The Al foams made by space-holder method were physically inserted into carbon fiber reinforced plastic (CFRP) composite thin-wall tubes to obtain the composite structure of Al foam-filled CFRP composite thin-wall tubes. Quasi-static compression tests of CFRP composite tubes, Al foams and Al foam-filled CFRP composite thin-wall tubes were carried out to study their compression properties. Meanwhile, digital image correlation (DIC) was applied to analyze their deformation modes. Furthermore, the compressive properties, energy absorption properties and failure modes of Al foam-filled CFRP composite thin-wall tubes at different temperatures (25-150℃) were studied. The results show that Al foams as fillers change the compression deformation behavior of CFRP composite thin-walled tubes from the scattering flowering failure of a single CFRP composite tube to the fiber layer fracture failure of a foam-filled tube. Comparing to CFRP composite thin-walled tubes, the stress fluctuations of Al foam-filled CFRP composite thin-wall tubes decrease obviously. With environmental temperature increasing, both the compressive properties and energy absorption properties of CFRP composite thin-walled tubes, Al foams and Al foam-filled CFRP composite thin-wall tubes decrease. But the interaction between Al foams and CFRP composite thin-walled tubes is enhanced, the enhancement effect of Al foams on CFRP composite thin-walled tubes is more obvious at high temperature.

Multi-objective optimisation in vibration-assisted drilling of CFRP/Al stacks

The study presents results of experiments and numerical analyses concerning thin-walled shells used as components of aircraft structures. The solution, which is proposed here, consists in stiffening such elements by means of integral ribs. A comparative analysis has been carried out between the suggested design solution and the reference structure. In the experimental part of the study, an optical scanner with digital image correlation has been used. Nonlinear numerical analyses have been carried out with the use of software based on the finite element method.

Carbon fibre reinforced plastics (CFRPs) are increasingly used in the aerospace industry because they offer lightweight construction and design flexibility. As sustainability becomes more important in business and to consumers, these materials help vehicles use less fuel and become more durable. The aerospace industry started the development of CFRP materials in the 1970’s, after which were also introduced in other industries, such as marine and automotive, leading to higher volumes of CFRP materials in subsequent years. This thesis is focussed on prepreg CFRP manufacturing and the detection of specific production flaws found in an industry survey. During the CFRP prepreg manufacturing process, raw CFRP layers are stacked during the layup phase and then cured to reach the component’s design mechanical and geometric specifications. To assure the right quality of the end product, the product is tested for flaws in the non-destructive evaluation (NDE) phase. The criticality of any detected flaw needs to be evaluated and if required reworked or rejected. This research presents an industry survey of typical CFRP manufacturing flaws and evaluates possible NDE technologies to detect these flaws in-situ in the manufacturing process. Analysis is performed by literature review and experimental tests. NDE process monitoring system should decrease the number of rejected and repaired products and increase the efficiency of the manufacturing process. Three NDE techniques were investigated in depth in this thesis to make a detailed design of a monitoring system: Lamb waves, laser displacement sensing and fibre optic sensors. This thesis concludes that the two latter technologies are promising for industrialization to improve the CFRP manufacturing process. The costs of production flaws in the CFRP manufacturing process are normally hidden in the cost structure of the end product. To address this the research also investigated the financial impact of rework and rejection of products in a CFRP manufacturing process and the estimated financial benefits of implementing an NDE process monitoring system. Overall, this research shows that the potential to detect flaws in-situ, the impact of rework and rejection and the financial feasibility of implementing a novel NDE process monitoring system will increase the efficiency and effectiveness of the CFRP manufacturing process.

Carbon-fiber-reinforced plastic (CFRP) composites are intensively used in aircraft and aerospace industry thanks to their superior properties. Comparing to the conventional drilling (CD), vibration-assisted drilling (VAD) is a novel machining technique suitable for drilling CFRP. Still, multi-mode excitations with elliptical locus and low vibration performance limit the applications of current VAD schemes for CFRP. To overcome these limitations and improve the overall performance, an innovative longitudinal-torsional complex-mode ultrasonic vibration-assisted actuator with single excitation and an elliptical locus is presented employing a piezoelectric transducer and a stepped horn with spiral grooves. The proposed actuator is specially designed to deliver elliptical vibration and assure high vibration performance of a tool tip. Analysis of the actuation mechanism for the longitudinal-torsional composite vibration mode is discussed, and its simplified model is developed. A detailed design process of this actuator is given. Its vibration characteristics are verified with both finite-element simulation and experimental modal analysis using a swept sine test. It is demonstrated the developed prototype achieved longitudinal-torsional elliptical vibration. To validate the machining performance of the actuator, two groups of drilling experiments were performed. These indicate that the proposed actuator is capable of drilling CFRP with improved machining performance.

Recyclable carbon fiber-reinforced plastics (CFRPs) with controlled degradability and stability were developed by adding bisphenol A (BA)-type epoxy resins containing acid-degradable acetal linkages (BA-CHDMVG) to the matrix resins of CFRPs. BA-CHDMVG was synthesized by the reaction of the hydroxyl groups of BA with vinyl ether containing a glycidyl group (cyclohexane dimethanol vinyl glycidyl ether (CHDMVG)) and added to conventional BA-type epoxy resins (jER828 and jER1001). The acetal linkage-containing CFRPs (BA-CHDMVG-based CFRPs) were prepared using a mixture of these epoxy resins as the matrix. The obtained CFRPs exhibited almost the same tensile and thermal properties as those of the conventional BA-based CFRP (BA-based CFRP). One of the BA-CHDMVG-based CFRPs, (BA-CHDMVG)40-(jER1001)60-based CFRP (content in the matrix resin: BA-CHDMVG, 40 wt%; jER1001, 60 wt%), underwent smooth degradation through hydrolysis of the acetal linkages incorporated into the matrix resin upon treatment with 0.1 mol l−1 hydrochloric acid in a tetrahydrofuran (THF)/water (9/1 v/v) mixed solvent for 24 h at room temperature to produce strands of carbon fibers. On the other hand, there was no change in the appearance of the same BA-CHDMVG-based CFRP board upon acid treatment in water with no organic solvent. To determine their stability against aqueous acid, the BA-CHDMVG-based CFRP boards were immersed in 0.1 mol l−1 aqueous acetic acid solution at room temperature for 30 days and then subjected to tensile testing. The BA-CHDMVG-based CFRPs after the acid treatment exhibited almost the same tensile properties as those of the BA-CHDMVG-based CFRPs before the acid treatment. These results indicate that the BA-CHDMVG-based CFRPs are stable toward acid in the normal living environment, such as acid rain. Recyclable carbon fiber-reinforced plastics (CFRPs) with controlled degradability and stability were developed by using acid-degradable acetal linkage-containing epoxy resin (BA-CHDMVG). The obtained BA-CHDMVG-based CFRPs exhibited almost the same tensile and thermal properties as those of the conventional BA-based CFRPs and underwent smooth degradation through hydrolysis with the treatment of hydrochloric acid in a tetrahydrofuran (THF)/water (9/1) mixed solvent. On the other hand, in the absence of organic solvents, the BA-CHDMVG-based CFRPs have sufficient stability toward acid in the normal living environment.

In recent years, the demand for carbon fiber reinforced plastics (CFRP), which have excellent mechanical properties, is increasing in various fields. In particular, the amount of CFRP used accounts for more than 50% of the body structure weight of the state-of-the-art airplanes. Moreover, in such airplanes, stack material, which is a combination of CFRP and titanium alloy (Ti-alloy), is frequently used. Therefore, a novel high-efficiency end-milling technology for cutting CFRP and Ti-alloy simultaneously is required. It is known that for restraining the occurrence of tool wear, diamond coating, which has high hardness, is useful. On the other hand, in the case of machining of Ti-alloy, several problems arise due to the machining heat. Consequently, in this study, we focus on cBN (cubic boron nitride). In order to compare diamond coating and cBN, end-mills, which were electroplated diamond grains, were also fabricated. In this study, as a cutting experiment, side milling of stack material, which is a combination of CFRP and Ti-6Al-4V, was carried out using the fabricated tools. Then, we discuss their cutting performance by measuring the CFRP temperature and chip temperature, tool wear, surface roughness, and surface integrity. As a result, it could be expected to precisely cut the stack material with the fabricated electroplated cBN end-mill. However, it is seen that improvement of the tool shape or the electrodeposition process is required.

Preparation and characterization of carbon fiber reinforced plastics (CFRPs) incorporating through-plane-stitched carbon fibers

"Since bumper reinforcements are positioned at front/rear ends of vehicles, weight reduction of the bumper reinforcements improves vehicle dynamic performance by reducing a yaw moment of inertia. CFRP (Carbon Fibre Reinforced Plastic) composites are attractive lightweight materials due to their excellent specific strength and rigidity. However, because of their relatively high cost, applications of CFRP materials to vehicle structural parts are limited. In this study, we developed a lightweight structural part which consists of a thin-walled Aluminum bumper reinforcement with a unidirectional CFRP sheet, in order not to increase part cost by reducing amount of Aluminum and by using only a little amount of CFRP. Compared to Aluminum, unidirectional CFRP sheets have even higher tensile strength and modulus. When vehicles crush, bumper reinforcements may be subjected to bending force. If a unidirectional CFRP sheet adhered on a tensile side of an Aluminum bumper reinforcement, not only Aluminum thickness on the tensile side but also thickness on the compression side can be reduced due to movement of a bending neutral axis. Bending strength of the developed parts can’t be predicted by a full plastic moment which could be used to predict metal parts’ bending strength because CFRP don’t deform plastically. In this study, based on Bernoulli-Euler theory, the bending neutral axis was decided considering elastic/plastic areas of the Aluminum bumper reinforcement, and bending strength of the part was predicted. To valid the calculation method, three-point bending tests on the parts were carried out. Experimental data of bending strength were in the range of predicted bounds. In addition, after the peak load, the load decreased gradually, like conventional all metal bumper reinforcements, without delamination of the unidirectional CFRP sheet. In order to launch the developed part, robustness of part’s performance was also evaluated. Finally, the part was adapted to a rear bumper reinforcement of LEXUS RC-F. The part weight is 11 % lighter compared to a conventional all Aluminum bumper reinforcement."