Elsevier

Composite Structures

Volume 63, Issues 3–4, February–March 2004, Pages 313-327
Composite Structures

Delamination in drilling GFR-thermoset composites

https://doi.org/10.1016/S0263-8223(03)00180-6Get rights and content

Abstract

Delamination is a major problem associated with drilling fiber-reinforced composite materials that, in addition to reducing the structural integrity of the material, also results in poor assembly tolerance and has the potential for long-term performance deterioration. Delamination-free in drilling different fiber reinforced thermoset composites is the main objective of the present paper. Therefore the influence of drilling and material variables on thrust force, torque and delamination of GFRP composites was investigated experimentally. Drilling variables are cutting speed and feed. Material variable include matrix type, filler and fiber shape. Drilling process was carried out on cross-winding/polyester, continuous-winding with filler/polyester, chopped/polyester, woven/polyester and woven/epoxy composites. A simple inexpensive accurate technique was developed to measure delamination size.

The results show that the presence of sand filler in continuous-winding composites not only raised the values of cutting forces and push-out delamination but also increased their values with increasing cutting speed. In contrast, increasing the cutting speed in drilling cross-winding, woven and chopped composites reduces the push-out delamination as a result of decreasing the thrust force. The thrust forces in drilling continuous-winding composite are more than three orders of magnitude higher than those in the cross-winding composites. Chopped composites have lower push-out delamination than those made from woven fibers. For the same fiber shape, the peel-up and push-out delaminations of woven/epoxy composite are lower than that for woven/polyester composites. Delamination, chipping and spalling damage mechanisms were observed in drilling chopped and continuous-winding composites. In drilling woven composites the delamination was observed at different edge position angles due to the presence of the braids that made by the interlacing of two orthogonal directions of fibers tows (warp and fill). Delamination-free in drilling cross-winding composites was achieved using variable feed technique.

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

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