Delamination in drilling GFR-thermoset composites
Introduction
Drilling is often a final operation during assembly; any defects leads to rejection the part represent an expensive loss. In the aircraft industry, for example, drilling-associated delamination accounts for 60% of all part rejections during final assembly of an aircraft [1]. The economic impact of this is significant considering the value associated with the part when it reaches the assembly stage. The quality of the drilled holes such as waviness/roughness of its wall surface, axial straightness and roundness of the hole cross-section can cause high stresses on the rivet, leading to its failure. Stress concentration, delamination and microcracking associated with machined holes significantly reduce the composites performance [2], [3], [4].
In machining composite parts, a finish comparable to metals cannot be achieved because of inhomogeneity and anisotropy of materials [4], [5]. FRP composites consist of a load-carrying fiber component, whose geometrical orientation depends on force directions is enveloped in the matrix, which mainly provides the fixation of the fibers and the distribution of forces. The physical properties of fiber and matrix are quite different and together with the fiber orientation they determine, by their combination, the performance as well as the machinability of the composite [6]. The resulted chip from machining the metals were classified into continuous, sectional (discontinuous) and broken chip. Thermoset composites are brittle and possess less capability for plastic deformation than metals and thus their chips tend to fracture earlier to give discontinuous chips in powder and scrap form [7].
Drilling in fiber reinforced thermoset composites results in a series of mini-fiber fractures, fiber pullouts, and matrix cracking. Cutting variables in drilling process (feed and speed) have great influence on the thrust force and torque and hence on the quality of the machined holes. Low feeds in some cases improve the surface roughness due to the reduction of thrust force. In other cases drilling at lower feeds and high speed leads to increased temperature generated, assisted by a low coefficient of thermal conduction and a low transition temperature of plastics. The accumulated heat around the tool edge destroys the matrix stability and produces fuzzy and rough cuts [7], [8].
Delamination can often be the limiting factor in the use of composite materials for structural applications. In drilling polymeric composite materials the thrust force has been cited as the cause of delamination by several investigators [1], [2], [8], [9], [10], [11], [12], [13], [14], [15], [16] and some [1], [8], [15], [16], [17] have been predicted the critical thrust at the onset of delamination using linear elastic fracture mechanics and classic plate bending theory. One application of this theory is to control feed respect to the depth of drilling [18].
Delamination has been measured using different techniques [1], [4], [5], [14], [19], [20]. Chen [14] measured the damaged zone using X-ray as a non-destructive technique. This method required coating the hole edge with tetrabromoethane. He used the delamination factor (the ratio of the maximum diameter in the damage zone to the hole diameter) in order to analyze and compare the delamination degree in the drilling of carbon FRP composite laminates. Enemuoh et al. [5] and Davim and Reis [20] measured the delamination in AS4/PEEK and CFRP composite materials using a tool marker’s microscopes at 5× and 30× magnification respectively. Caprino and Tagliaferri [19] measure the extent of the damaged area of drilled GFRP composites using visual inspection at 10× magnification, with the help of a strong light source located on the back. Visual measurement of hole delamination diameter for graphite–epoxy composites was achieved by Stone and Krishnamurthy [1] using CCD camera with the aid of visual digitizer that used to capture a frame from the camera to produce the images. Tagliaferri et al. [4] used the diffusion phenomena of the liquid into the material through the cut surface to measure the damaged area around the hole with optical microscope at 10× magnification. It was noted that the actual value was strongly affected by the immersed time. Therefore they carry out all the measurements after 24 h after immersion the specimens in the liquid, in order to reliably compare the results from different tests. The latter technique is not applicable when comparing the damaged zone for different composite materials where each composite material has different diffusion coefficient [21]. Also the specimen cannot be used for farther mechanical measurements such as notched strength and pin bearing strength where the absorption of the liquids significantly reduces the mechanical properties of FRP composites [22].
The main objective of the present work is to study the influence of drilling and material variables on thrust force, torque and delamination of GFRP composites. Drilling variables are cutting speed and feed. Material variable include matrix type, filler and fiber shape. Drilling process will carried out on cross-winding/polyester, continuous-winding with filler/polyester, chopped/polyester, woven/polyester and woven/epoxy composites. Based on strain-gage sensor, the thrust force and torque will be measured using two-component dynamometer. While the delamination size will be measured using a simple and inexpensive technique.
Section snippets
Materials
Drilling processes were conducted on five different types of E-glass fiber reinforced thermosetting composites. The constituent materials and fiber volume fractions (Vf) for each composite type were illustrated in Table 1. Two thermoset resins were used in this study: polyester and epoxy. The different types of thermoset composites are: continuous-winding with filler/polyester, cross-winding/polyester, chopped/polyester, woven/polyester and woven/epoxy. The former two types were supplied from
Behavior of thrust force and torque during drilling of different composite materials
Fig. 3, Fig. 4, Fig. 5, Fig. 6, Fig. 7 illustrate a complete drilling cycles for continuous-winding, cross-winding, chopped, woven/polyester and woven/epoxy composites respectively. The thrust force and torque in these figures were measured at the lowest cutting conditions, n=455 rpm and f=0.03 mm/rev. The wave diagram of continuous-winding composites (Fig. 3) shows a gradual increase of the thrust force, up to 25 N (point 1–2), was due to the cutting in polyester resin at the outer surface of
Delamination mechanisms
Two mechanisms of delamination associated with drilling FRP composites were observed in the present study. They are known as peel-up at entrance and push-out at exit [7], [8], [10], [20], [31], [32]. Peel-up occurs as the drill enters the laminate and is shown schematically in Fig. 25(a). After the cutting edge of the drill makes contact with the laminate, the cutting force acting in the peripheral direction is the driving force for delamination. It generates a peeling force in the axial
Conclusions
- 1.
The constituent materials of the composite specimens, specimen thickness and machining time have a significant effect on the behavior of thrust force and torque over the machining time. At minimum cutting variables the thrust force of continuous-winding, woven/epoxy and chopped composites were suddenly dropped from the maximum value to zero at the drill exit with significant push-out delamination. On the other hand gradual decreases in thrust force was observed for cross-winding composites
References (33)
- et al.
A neural network thrust force controller to minimize delamination during drilling of graphite–epoxy laminates
J. Mach. Tools Manufact.
(1996) - et al.
Effect of drilling parameters on the finish and mechanical properties of GFRP composites
J. Mach. Tools Manufact.
(1990) - et al.
Quality definition and assessment in drilling of fiber reinforced thermosets
CIRP
(1989) - et al.
Mechanical model for predicting thrust and torque in vibration drilling fiber-reinforced composite materials
J. Mach. Tools Manufact.
(2001) - et al.
Experimental analysis of drilling damage in thin carbon/epoxy plate using special drills
Composites, Part A
(2000) Some experimental investigations in the drilling of carbon fiber-reinforced plastic (CFRP) composite laminates
J. Mach. Tools Manufact.
(1997)- et al.
Damage development in drilling glass fiber reinforced plastics
J. Mach. Tools Manufact.
(1995) - et al.
Study of delamination in drilling carbon fiber reinforced plastics (CFRP) using design experiments
J. Compos. Struct.
(2003) - et al.
Investigation into drilling laminated printed circuit board using a torque–thrust–temperature sensor
CIRP
(1988) - et al.
Delamination-free and high efficiency drilling of carbon fiber reinforced plastics
J. Compos. Mater.
(1995)
The effect of fastener hole defects
J. Compos. Mater.
Neural network based sensor fusion for on-line prediction of delamination and surface roughness in drilling AS4/PEEK composites
Transactions of NAMRI of SME
Machinability of some fiber-reinforced thermoset and thermoplastics in drilling
Mater. Manufact. Process.
Delamination during drilling in composite laminates
J. Eng. Ind., ASME
Study on cause of internal damage of drilled GFRP
Key Eng. Mater.
Drilling damage of GFRP and residual mechanical behavior––Part II: Static and cyclic bearing loads
J. Compos. Technol. Res.
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