نظارت آنلاین از انتشار صوتی برای کنترل کیفیت در حفاری کامپوزیت های پلیمری

Automation and optimization of the manufacturing processes play an important role in enhancing productivity. For this, monitoring and diagnostic systems are becoming increasingly necessary to asses the response of materials in manufacturing. In this paper, acoustic emission (AE) sensing was employed for on-line detection of workpiece status and to improve the process stability and workpiece quality by minimising associated defects. Drilling trials were conducted on woven glass fabric/epoxy with high speed steel (HSS) drills to determine the relationship between AE rms and cutting parameters. The variation of AE rms and power are in close correlation to the flank wear and hole shrinkage. The experimental results show that AE is very sensitive to the response of the drilling environment.

The use of composite materials has grown rapidly in recent years, especially in the aerospace and automotive industries due to their useful properties such as, high specific strength, high specific stiffness, good corrosion resistance and good fatigue performance. These properties can be tailored to suit many engineering applications. Even though near-net manufacturing of composite materials is possible, drilling will remain an unavoidable operation, particularly in assembly practices. There is a considerable difference between the machining of metals, their alloys and that of composite materials due to their anisotropy and inhomogeneity. Fiber reinforced polymer composites pose considerable problems in drilling such as delamination, fiber pull-out, hole shrinkage, spalling, fuzzing and thermal degradation [1], [2] and [3]. Many researchers proposed that the quality of drilled surfaces is strongly dependent on drilling parameters. Among the defects caused by drilling, delamination is the most critical. Delamination can result in a lowering of bearing strength and can be detrimental to the material durability by reducing the structural integrity of the material, resulting in long-term performance deterioration. Drilling induced delamination occurs both at the entrance and the exit planes of the workpiece. Investigators have studied analytically and experimentally cases in which delamination in drilling have been correlated to the thrust force during exit of the drill and there is a ‘critical thrust force’ below which no damage occurs [2], [3] and [4]. A rapid increase in feed rate at the end of drilling will cause the cracking around the exit edge of the hole. It was also stated that the larger the feeding load, the more serious is the cracking [5]. The ultimate goal of on-line monitoring of composite drilling is to produce defects-toleranced high quality holes. Numerous types of sensors are available for monitoring the drilling environment. Inasaki et al. [6] mentioned that among those, acoustic emission (AE) sensor is considered to be one of the practical and potential candidates. AE can be used as an active, intelligent indicator for monitoring process status. AE generated during drilling of composite laminates mostly emanate from sources like matrix deformation, fiber cutting, friction mechanisms, delamination and tool wear. Everson and Cheraghi [7] identified the characteristics of these individual sources in terms of AE signal. Jiaa and Dornfled [8] examined the use of acoustic emission for detecting delamination of composite laminates during drilling. Their results showed a linear energy level increase in AE based on the size of entry and exit hole in the composite. Lee et al. [9] investigated the application of AE for process monitoring of dull or broken twist drills. Mizutani et al. [10] used AE during drilling of composites. AE generated during drilling of composite laminates carries valuable information on the state of material being cut. Ravishankar and Murthy [11] stated that the AE signature is highly dependent on the type, nature and surface characteristics of the tool and the laminate. Sreejith and Krishnamurthy [12] used AE to study the performance of carbon/phenolic composite machining with PCD and PCBN tools. They explained the tool behavior while machining at different cutting speeds. In this work, an experimental study on the drilling of GFRP has been carried out to study the influence of drilling parameters (cutting speed and feed rate) on thrust force and flank wear which influences the hole shrinkage and also to monitor the material response through AE to assess the relationship between monitored parameters.

The results of drilling studies on GFRP composite using HSS drills were presented. The effect of cutting parameters, e.g. cutting speed and feed rate were analysed. Thrust force, tool wear, hole shrinkage, AE power and AE rms were measured. Some of the important observations are: • The thrust is seen to increase distinctly with feed rate and it is ascertained that for the drilling of woven glass fabric/epoxy laminate of fiber volume fraction 0.4 with HSS tools, the best cutting parameters are 18.85 m/min cutting speed and 0.02 mm/rev feed rate for minimum thrust. • Monitored parametric influence on flank wear, showed minimum wear with best cutting parameters. Also, relatively closer variation in flank wear was seen with minimum feed rate. • Minimum hole shrinkage was seen with 18.85 m/min drill speed, indicating stable (minimum stressing) drilling performance. •Though of mixed mode, monitored AE signal shows dominant peak around 70 kHz. Increase of AE power around 30 holes, indicated that 30 is the limiting number of holes to be drilled for defect tolerancing. • Feature plane representation of data clearly demarcates the significance of drilling speed on drilling performance.