Analyzing objects in images for estimating the delamination influence on load carrying capacity of composite laminates

The use of fiber reinforced plastics has increased in the last decades due to their unique properties. Advantages of their use are related with low weight, high strength and stiffness. Drilling of composite plates can be carried out in conventional machinery with some adaptations. However, the presence of typical defects like delamination can affect mechanical properties of produced parts. In this paper delamination influence in bearing stress of drilled hybrid carbon+glass/epoxy quasi-isotropic plates is studied by analyzing objects in images. Conclusions show that damage minimization is an important mean to improve mechanical properties of the joint area of the plate and the appropriateness of the image processing techniques used. decreases. At some point, the loading exceeds the interlaminar bond strength and delamination occurs – figure 1. Push-out delamination caused by this piercing action can be reduced if thrust force during drilling is minimized, Hocheng & Dharan (1990). The reduction of force can be accomplished by different ways and so several studies had been published with the aim to suggest alternative drilling methods to avoid delamination onset. Piquet et al (2000) listed some basic rules to be observed in drill geometry design. Tungsten carbide should be used as material, a rake angle of 6o will reduce peel-up delamination, a great number of cutting edges, up to six, will facilitate heat removal by increasing tool/part contact length. Park et al. (1995) applied the helical-feed method to avoid fuzzing and delamination. Dharan & Won (2000) proposed an intelligent machining scheme to avoid delamination. Stone & Krishnamurthy (1996) studied the implementation of a neural thrust force controller that updates feed rate every three spindle revolutions. Tsao & Hocheng (2004) compared three different drills with varying parameters and concluded for the major importance of feed rate and drill diameter on delamination. The importance of feed rate was also established by Davim et al. (2003). A comprehensive summary of the steps towards free-delamination holes can be found in Hocheng & Tsao (2005). Another possible approach is the execution of a pilot hole in order to reduce thrust force during drilling. The pilot hole reduces the chisel edge effect on the drill thrust force and, consequently, delamination hazard. Reduction of thrust force can reach 50%, according to the work of Won & Dharan (2002). Recently, Tsao & Hocheng (2003) also studied the effect of chisel length and pilot hole on delamination. According to these authors the pilot hole diameter should be around 15 to 20% of final drill diameter to minimize delamination risk. Finally, the use of a sacrificial backup plate, if possible, can reduce delamination by providing a support for the uncut plies of a laminate.