Workpiece damping and its effect on delamination damage in drilling thin composite laminates

Abstract

The machining of the glass fiber reinforced plastic (GFRP) laminates may generate several kinds of damage. The most serious is the delamination that may occur during drilling, and which severely compromises the mechanical characteristics of the material around the hole. The main cause of delamination is generally held to be the thrust force exerted by the drill point; the most commonly used method for reducing delamination is to place a support under the workpiece. Paradoxically, while this practice does limit delamination, the force exerted by the drill point is actually greater than in unsupported drilling. The presence of the support does influence the delamination mechanism, but exactly how that mechanism works does not seem to be fully understood yet. This study analyzes the differences in delamination mechanisms when drilling with and without a support placed under the workpiece. This investigation has led to hypothesize two main differences in the mechanism. On the basis of this analysis, a new device was designed that counters the hypothesized delamination mechanism. A simple prototype of this device was built, and its effectiveness verified. The results show that the proposed device can drastically reduce delamination.

Introduction

Composite materials are becoming increasingly popular due to their mechanical properties, and to their favorable strength to weight ratio in particular. But the machining of this materials implies coping with problems that are not encountered in machining other materials, and the development of new working techniques.

Drilling is a particularly critical operation for fiber reinforced plastics (FRP) laminates because the great concentrated forces generated can lead to widespread damage. This damage causes aesthetic problems but, more importantly, may compromise the mechanical properties of the finished part [1], [2].

The major damage is certainly the delamination that can occur both on the entrance and exit sides of the workpiece. The delamination on the exit surface, generally referred to as push-down delamination, is as a rule more extensive, and is consequently considered the most dangerous.

It has also been observed [1] that the level of delamination is related to the thrust force, and that there is a critical value of the thrust force (dependent on the type of material drilled) below which the delamination is negligible.

Several articles in the literature deal with the modeling of push-down delamination. In particular, Ho-Cheng and Dharan [3] describe the delamination as a circular crack propagating in the resin-rich interlaminar region under the load of the drill point, and they propose a model that considers the axial thrust force exerted by the drill point as the only cause of the delamination. They also propose an analytic model to evaluate the critical thrust force. Jain and Yang [4] extend this theory, taking into consideration the anisotropy of the material, and hypothesizing that the cracks are elliptical. They also find that the load of the drill point is mainly due to the chisel edge, while the contribution of the cutting lips in the axial thrust direction is smaller. Similar models, suitable for carbon-fiber composites, have been developed [5], [6].

Although, all these models are in good agreement with experimental observations, DiPaolo et al. [7] have observed that the delamination mechanism is fairly different from the one hypothesized. They desume that the load exerted by the chisel edge is not the main cause of delamination cracking. In fact, they observe that the crack opens after the chisel edge exits the workpiece. They claim therefore, that it is the cutting lips of the drill point, acting nearer to the border of the hole, that most influence the opening of the crack. These authors also find that the crack generally does not remain in a single interply resin-rich region, but propagates throughout the plies. As the literature shows, the delamination mechanism is extremely complicated and very difficult to describe with analytic methods.

The most popular way of reducing delamination damage is to support the bottom plies of the laminate, thereby increasing the critical thrust force. This can be accomplished by means of a sacrificial additional ply or a support, predrilled if necessary, and set under the workpiece in the zone where the hole will be drilled. Obviously, the predrilled hole is coaxial with the drill point. Both methods are effective, but quite complex and not always feasible.

Other methods have been proposed, such as a variable feed-rate strategy [4], the KTR method [8], and drill points with a special geometry [9], [10].

Further hypotheses about the mechanism of delamination are proposed in the present paper, based on the observation of the forces and displacements recorded during drilling. Then a different system for reducing delamination is presented. This system is based on the constrain of the workpiece dynamics during machining in order to limit the actual feed rate. This can be done with a very simple device, and does not require time-consuming preparations. Results show a drastic reduction in the extension of the delamination, indicating that the proposed system could be advantageously applied in industry.