The effect of vibratory drilling on hole quality in polymeric composites

https://doi.org/10.1016/j.ijmachtools.2005.05.023Get rights and content

Abstract

The anisotropy of fiber-reinforced plastics (FRP) affects the chip formation and thrust force during drilling. Delamination is recognized as one of the major causes of damage during drilling of fiber reinforced plastics, which not only reduces the structural integrity, but also has the potential for long-term performance deterioration. It is difficult to produce good quality holes with high efficiency by conventional drilling method. This research concerning drilling of polymeric composites aims to establish a technology that would ensure minimum defects and longer tool life. Specifically, the authors conceived a new drilling method that imparts a low-frequency, high amplitude vibration to the workpiece in the feed direction during drilling. Using high-speed steel (HSS) drill, a series of vibratory drilling and conventional drilling experiments were conducted on glass fiber-reinforced plastics composites to assess thrust force, flank wear and delamination factor. In addition, the process-status during vibratory drilling was also assessed by monitoring acoustic emission from the workpiece. From the drilling experiments, it was found that vibratory drilling method is a promising machining technique that uses the regeneration effect to produce axial chatter, facilitating chip breaking and reduction in thrust force.

Introduction

Advanced composite materials such as fiber reinforced plastics (FRP) are recognized as excellent materials for some structural components and are being increasingly used in various applications such as aircraft, ships, automobiles, machine tool and sports equipment, due to their excellent properties such as high specific strength, high specific stiffness, high damping, low thermal expansion, good dimensional stability and an unusual combination of properties not obtainable with metal alloys.

As structural materials, joining of composite structures could not be avoided. The efficiency of mechanical joint is largely dependent on the quality of machined holes. Due to their anisotropy, and non-homogeneity, FRP cause some problems in drilling such as fiber breakage, matrix cracking, fiber/matrix debonding, fiber pull-out, fuzzing, thermal degradation, spalling and delamination. Among the defects caused by drilling, delamination is the most critical. Delamination can result in 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 [1], [2], [3].

Several non-traditional machining processes such as laser machining, water-jet cutting, ultrasonic drilling, electro-discharge machining, etc., have been adopted for machining of FRP. However, the drilling of these materials remains at the conventional mechanical drilling level from the viewpoint of differential mechanical/thermal properties of composite and also economy and quality of the product [4].

Drilling of polymeric composites is a highly stochastic process due to anisotropy and inhomogeneity of composites. Technology for conventional drilling technique struggles to meet the need to enhance the quality of holes drilled in composites. Thus, searching for new drilling methods is imperative.

Vibratory drilling is a branch of vibration assisted cutting, which is fundamentally different from conventional drilling. The conventional drilling process is a continuous cutting process, but the vibratory drilling process is a pulsed intermittent cutting process. The new vibratory drilling technique has attracted extensive interest in recent years [5]. Both theoretical investigations and experimental results have indicated that with the thrust force being reduced by means of vibratory drilling in metals, the machining quality of the drilled holes can be improved. Thrust force has been considered as the cause of delamination in polymeric composites by several researchers and it is believed that there is a ‘critical thrust force’ below which the damage can be constrained/eliminated.

In 1991, H. Takeyama [6] verified by experiments that burrless holes could be obtained by means of vibratory drilling in steel. Using the same method in drilling carbon fibre-reinforced polymers, Zhang Qixin [7] found that the thrust is reduced and delamination-free holes can be obtained.

The compromise between machining quality and machining efficiency cannot be avoided whether by conventional drilling method or vibratory drilling method mentioned above, i.e. much lower cutting quantities must be adopted if delamination-free hole is needed, whereas, much higher cutting quantities must be adopted if machining efficiency is taken into account. Therefore, searching for new drilling methods is imperative in order to drill precise holes without sacrificing machining efficiency [8]. The solution can be effective process monitoring.

The development of a reliable monitoring technique for manufacturing process control is an essential step towards manufacturing automation and computer-integration pursuing high productivity of the system and quality of its products. New demands are being placed on monitoring systems in the manufacturing environment because of recent developments and trends in machining technology. To meet such demands, the present monitoring system should become more reliable and flexible. Numerous types of sensors are available for monitoring different aspects of the machining environments. Among the sensors, AE sensor is considered to be one of the potential candidates [9].

Jiaa et al. [10] examined the use of acoustic emission for detecting delamination of composite laminates during drilling. Their results showed a linear increase in energy level of AE based on the size of entry and exit hole in the composite. Lee et al. [11] investigated the application of AE for process monitoring for dull or broken drills. Mizutani et al. [12]used AE during drilling of composites. AE generated during drilling of composite laminates carries valuable information on the state of material being cut. It provides real-time information on damage progression.

Ravishankar and Murthy [13] in their results stated that the AE signature is highly dependent on the type, nature and surface characteristics of the tool and the laminate. Sreejith and Krishnamurthy [14] used acoustic emission 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.

This paper presents an improved technique; low frequency, high amplitude vibratory drilling which induces an axial vibration in the feed direction. By reducing the thrust this new step leads to a remarkable improvement in quality of the hole.

Section snippets

Workpiece

Woven glass fabric reinforced plastic (GFRP) composites of 4 mm thickness were used for conducting the drilling studies. The reinforcing material used was woven glass fabric. The matrix used was commercial quality epoxy resin LY-556 and the hardener HT-972. Due to low cure shrinkage, GFRP laminates are stable and free from internal stress. The laminate was made by compression molding. Care was taken to ensure complete wetting of the fibres and removal of entrapped air and excess resin. The

Evaluation of critical thrust force

During drilling, the material ahead of the drill point undergoes bending and consequently can experience fiber breakage/pullout and crazing of matrix material. This is reflected in debonding, delamination and hole shrinkage. Due to high modulus of elasticity of composite laminates, the failure in the form of delamination can be modeled by classical plate bending theory and linear elastic fracture mechanics (LEFM). This model allows evaluating the thrust force applied by the drill in terms of

Concluding remarks

The results of vibratory drilling studies on woven glass fabric composite using high speed steel drills were presented. Some of the major observations are:

  • The thrust is observed to increase distinctly with feed rate and it is ascertained that for the vibratory drilling of woven glass fabric/epoxy laminate of fibre volume fraction 0.4 with HSS tools, the best cutting parameters are 18.85 m/min cutting speed, 0.02 mm/rev. feed rate, 200 Hz frequency and 15 μm amplitude of vibration for minimum thrust.

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