Hole Exit Delamination Research Articles

Bi-directional drilling of CFRPs: From principle to delamination suppression

Carbon Fiber Reinforced Plastics (CFRPs) are widely used in aerospace, automotive and other transportation industries because of their excellent material and mechanical properties. Nevertheless, up to now, drilling is still regarded as the most important process in the machining of CFRPs, and the delamination is one of the most challenging damage in drilling process. In this concern, we propose and implement a bi-directional drilling method for CFRPs hole-making. In this paper, the exit delamination formation mechanism was firstly analyzed to clarify the effect of bi-directional drilling on delamination suppression. The critical thrust force was calculated by considering both conventional and bi-directional drilling conditions. Based on the model, it can find that bi-directional drilling process itself can provide support without employing any extra facility. Finally, experimental validation indicates that the calculated critical thrust force showed a reasonable agreement with the experimental results based on conventional and bi-directional drilling. The hole exit delamination damage under the bi-directional drilling reaches the desired suppression effect with a reduction of 11.3%–27.9% compared to conventional drilling based on experiment results. Through an in-depth understanding of critical thrust force, the findings of this paper not only provide a new processing technology for drilling CFRPs, but also open a new avenue to improve the quality of drilling CFRPs by reducing delamination.

Effect of workpiece preheating on tool wear and delamination at the hole exit in high feed drilling of carbon fiber reinforced plastics with diamond-coated tools

High feed drilling of carbon fiber reinforced plastics (CFRP) poses a risk of serious tool wear and delamination caused by high drilling thrust force despite improving productivity. However, by taking advantage of the proper thermal softening of the epoxy matrix in CFRP, a decrease in forces related to tool wear and delamination is achievable under high feed. Through this study, thermal softening of epoxy matrix by preheating was effective in reducing thrust force and tool wear in the drilling of CFRP with diamond-coated tools. Hole exit delamination factors decreased by adjusting workpiece temperature to the proper value using a developed preheating system under high feed (0.2 mm/rev).

BPNN-ACO application on minimization of hole delamination during GFRP drilling process

Delamination defects during the drilling process on the glass fiber reinforced polymer (GFRP) have a great contribution to the component failure. Hence, it is necessary to properly choose the combination of machining variables to minimize hole entry delamination (EnDel) and hole exit delamination (ExDel) during drilling process simultaneously. This study underlines the modelling and minimizing the EnDel and hole ExDel during GFRP drilling process by combining a backpropagation neural network (BPNN) method and ant colony optimization (ACO). The varied drilling parameters were type of drill point angle, feeding speed, and cutting speed. The optimum BPNN architecture could be obtained by using 3-4-2-5-8-2 network architecture with tansig activation function. The optimum GFRP drilling parameters that can minimize EnDel and ExDel simultaneously were 116° of drill point angle, 51.3 mm min−1 of feeding speed and 4975 rpm of spindle speed.

An analytical delamination model of drilling aramid fiber–reinforced plastics by brad drill

Aramid fiber–reinforced plastic (AFRP) composites have been widely used in aerospace, defense, and automotive industries. Proven by practical experience, a brad drill can effectively reduce drilling-induced damages in drilling of AFRP due to its unique tool geometry. While the delamination mechanism and the associated analytical model of the brad drill are crucial for improving hole quality and machining accuracy, these aspects were rarely discussed in the literature. This study reports the first work on the delamination mechanism in the context of drilling AFRP by brad drill through detailed analysis of tool/material interactions. A novel analytical model of the critical thrust force (CTF) is proposed for the prediction of both hole wall and hole exit delamination. Model analysis shows that the hole wall delamination is more likely to occur prior to hole exit delamination when the remaining uncut material is thinner than 0.38 mm. When the thickness of the uncut material is greater than 0.4 mm, no hole wall delamination will occur. Through analysis of thrust force components generated by different cutting edges on the total CTF, the contribution of cutting spurs on the thrust force is found to be more effective at suppressing hole wall delamination. The model and the associated experimental validation of this study have laid down a foundation for the design and optimization of drill bits in the future.

Delamination formation and suppression during rotary ultrasonic elliptical machining of CFRP

As a superior hole manufacturing process, rotary ultrasonic elliptical machining (RUEM) has been successfully employed for drilling carbon fiber reinforced plastics (CFRPs) recently. The delamination of CFRP during drilling is generally considered as the most undesirable form of damage and the most challenging failure mode. In order to reduce the delamination in this novel process, it is important to understand the formation and suppression mechanisms during RUEM of CFRP. In this study, the delamination formation in both core drilling (CD) and RUEM was observed and analyzed. The variation trend of delamination factor with feed speed as well as cutting speed was obtained. The experimental results show that, compared with CD, RUEM method can effectively reduce hole exit delamination by 5.4%–19.3% between 1/2 plies and 0.7%–8.4% between 2/3 plies at the feed rates from 50 to 100 μm/rev. Moreover, the delamination suppression mechanism in RUEM was fully analyzed and verified. In a word, in the industry practice, RUEM can be considered as a competitive and promising technique to drill CFRP compared to other delamination suppression techniques.

Multi-response optimization of carbon fiber reinforced polymer (CFRP) drilling using back propagation neural network-particle swarm optimization (BPNN-PSO)

An integrated approach has been applied to predict and optimize multi-performance-characteristics, being optimum thrust force (FTh), torque (M), hole entry delamination (FDen) and hole exit delamination (FDex), in the drilling process of carbon fiber reinforced polymer (CFRP). The drilling operation was performed by using a full factorial design of experiments with two different drill geometry (DG), three diverse levels of spindle speed (n), and feeding speed (Vf). The quality characteristics of FTh, M, FDen, and FDex were smaller the better. Back propagation neural network (BPNN) was first performed to model the drilling process and to predict the optimum drilling responses. Particle swarm optimization (PSO) was executed to attain the best combination of drilling parameters levels that would give optimum performance. The influences of drill geometry, speeds of spindle, and feeding speed on the responses were examined by using the response graphs. In addition, the scanning electron microscope (SEM) photos of the drilled hole are also provided to show the difference of the hole quality before and after optimization. The outcome of the confirmation experiment disclosed that the integration of BPNN and PSO managed to substantially predicted and enhanced the multi-performance characteristics accurately.

Study on chisel edge drilling behavior and step drill structure on delamination in drilling CFRP

In this paper, a twist drill was used to drill carbon fiber reinforced polymer composite (CFRP) materials to study the influence of the chisel edge drilling behavior on delamination at different parameters. The test results show that a high spindle speed is beneficial to the chisel edge to remove the material when the chisel edge is about to drill the bottom of the plate, and the chisel edge drilling behavior has important effect on delamination. In order to reduce the impact of chisel edge on delamination, three kinds of step drills (different ratio of primary drill bit diameter to secondary drill bit diameter (k)) were designed to drill CFRP at different cutting parameters. The method of combining theory with practice is used to analyze delamination of step drill, and to optimize the step drill structure. Experimental results demonstrates that the hole entry quality of the step drills are better than that of the twist drill. k has great impacts on hole exit delamination. The step drill, k is about 0.5, can achieve the best hole exit quality under the large processing parameters, therefore it's suitable for drilling of CFRP.

In Situ Qualitative Inspection of Hole Exit Delamination at Bottom‐Ply during Drilling of Woven CFRP Epoxy Composite Laminates

AbstractExploiting very high speed digital videography, an in situ examination of the hole exit delamination at the bottom‐most ply during drilling holes in the selected woven CFRP epoxy laminates is presented. At the beginning, a rotating elastic bulge of the carbon fibers at the bottom‐ply, which is just the impression of the protruding drill chisel edge, was always observed. Following the elastic bulging, a few, initial cracks along the weak fiber/matrix interfaces appeared. Thereafter, tensile failures in the carbon fibers were seen. The exact location of the initiation of these fiber failures specifically depends on the actual drill‐hole position with respect to the woven configuration of the bottom‐ply. A visual model for the weak interstitial or undulated regions at the bottom‐ply is also proposed in this paper, showing the undulating fibers, which are susceptible to mostly tensile failures under the drilling loads. During a sub‐completion drilling‐phase at the bottom‐ply, various cracks were seen to be propagating mostly via linear paths. Also, the exit delamination at the bottom‐ply during a sub‐completion drilling‐phase was always observed as to be divided into various small, independent localized contours—each of which propagated almost independently through within several individual warps/wefts during drill‐feed. Also, the shape of each such tiny contour within a single warp/weft was identified as elliptic, which is observed around an entire drill‐hole in unidirectional (UD) composites as reported in literature. It was also observed that the overhanging cantilever‐like fibers at the bottom‐ply are really difficult to cut, once their base‐location or their exit delamination contour reaches outside the hole nominal diameter. Moreover, by referring to some very basic cutting angle configurations for the orthogonal trimming of UD‐composites as found rarely in archival literature, an illustrative model diagram is also proposed for the drilling of the selected laminate material. This idea is also approximately validated via a few visual observations. The proposed visual model is generally an attempt in correlating the observed peripheral hole quality (delamination) with various instantaneous tool/fiber engagement configurations occurring across the entire drill‐hole periphery at the bottom‐ply, during the very last drilling‐phase.

Subsequent drilling on pilot holes in woven carbon fibre reinforced plastic epoxy laminates: the effect of drill chisel edge on delamination

Experimental results on thrust force and quantitative delamination are compared after drilling selected woven epoxy carbon fibre reinforced plastic (CFRP) laminates using two different drill bits. The results were acquired from two separate drilling modes: the normal drilling mode and the subsequent drilling conducted on some predrilled pilot holes. The use of pilot holes in drilling fibre reinforced plastics (FRPs) has been recommended by various researchers, following the basic concepts of drilling of metals and theoretical modelling. According to researchers, the drilling thrust force is directly correlated to the hole exit delamination; therefore, a pilot hole should minimize the hole exit delamination by nullifying the so-called chisel edge effect of a drill bit. A chisel edge, which is always regarded as the most inefficient part of a drill bit, is the biggest contributor to the thrust force. In this paper, despite the observed tremendous drops (as high as 92 per cent) recorded in the experimental thrust force magnitudes while conducting subsequent drilling on predrilled pilot holes, no improvement could be witnessed in the quantitative results on the induced hole entry and exit delamination. The main inference of this research is that the main cutting edges of a drill (along with their corners) do play a decisive role with regard to the hole exit delamination, instead of the role of the chisel edge alone.

Experimental Investigations on Drilling GFRP Epoxy Composite Laminates Using Specialized and Conventional Uncoated Cemented Carbide Drills

The growing demand of the fiber reinforced plastics (FRPs) in lightweight engineering has led to a need for assessing and quantifying the workpiece material defects, like hole exit delamination, during their drilling process. This article presents the experimental investigations on in-process drilling load profiles and a quantitative analysis of the induced hole exit delamination in drilling glass fiber reinforced plastic (GFRP) epoxy composite laminates, using various uncoated carbide drill bits (two specialized and a conventional one). The aim of studying and analyzing the in-process load profiles, in this work, is necessary in better comprehending the interaction between the tool and the workpiece material, with respect to the induced peak loads. It may further be helpful for determining and assessing the root cause of the induced delamination damage. Moreover, it is shown that the design and geometry of a particular drill bit also plays a vital role regarding its cutting loads and delamination results. The specialized tools were found to be much better than their conventional counterpart. Furthermore, subsequent drilling was conducted onto some predrilled pilot-holes using the same specialized drills, to test the chisel edge effect. The research emphasizes a need to pay attention to the quality and design of a drill bit (cutting edge corner).