Monitoring of hole surface integrity in drilling of bi-directional woven carbon fiber reinforced plastic composites

Abstract

Fiber-reinforced plastic is one of the top priorities lightweight materials with excellent mechanical properties for the aerospace industries in recent years. However, it is difficult to machine despite having unique properties due to its non-homogeneous and abrasive nature in alternate fiber and matrix layers. Thus, it is found to be a challenging task to drill hole on such hard-to-machine materials, which is highly essential for the development of most of the engineering structural components. The present work addresses various drilling-induced defects such as delamination, circularity error, and roughness variations in the hole surface during drilling of quasi-isotropic cross-fiber oriented bi-directional woven-type carbon fiber reinforced plastic laminate using a full factorial design of experiments for different drill geometry. The response surface methodology was considered for the regression model development, which was found to be highly significant. The machining forces with associated torque have also been acquired during drilling, which was divided and further analyzed in time domain to correlate with drilling flaws. The drilling-induced delamination was found to be higher at a high feed rate using a higher drill point angle due to substantial thrust force generation at the initial stages in the drilling cycle. However, the internal surface finish with associated circularity error was reduced for higher spindle speed with less feed rate using a low drill point angle because of low torque fluctuation at the final drilling phases. The axial thrust force was found to be a prime indicator of drilled hole surface delamination, whereas drilling torque precisely indicated internal surface roughness as well as circularity error. The global root mean square, along with a local peak of thrust and torque, both were highly essential to completely characterize the drilled hole quality.

Prediction of drilled hole quality in bidirectional woven carbon fiber reinforced plastic using wavelet packets of force–torque signals

“…The tungsten carbide drill point angle of 110°(110°-125°) having 10 mm diameter (drill spindle speed of 1225-3200 r/min; feed rate of 0.025 mm/rev-0.075 mm/rev) was found to be the best twist drill for the improvement of drilled hole surface integrity in the earlier work. 8 Therefore, the present work addresses the further enhancement of drilled hole surface quality using two different coated drills such as TiAlN-and TiN-coated tungsten carbide drills as an extension. Thus, three different types of carbide drills (Figure 1(i)) with a uniform diameter (Ø) of 10 mm were considered in this work, namely, uncoated drill (Type A), TiAlN-coated drill (Type B), and TiN-coated drill (Type C).…”

“…However, the root means square value of the sampled data during the initial four consecutive phases was primarily considered as it was found to indicate both average force or torque with associated fluctuation. 8 The mean value of the sampled data has been determined as the summation of all the contributing data (force or torque) divided by the corresponding total number of data (time instants) in a particular phase. Similarly, the quadratic mean average, that is, the root mean square (rms), the square root of each observation, and standard deviation of the observations (i.e., the deviation of a particular set of data from the arithmetic mean of that data set) in a specified time phase in the drilling cycle have also been measured.…”

“…Therefore, it should be minimized as it may not be eliminated during the drilling of these fibrous composites. 8 The drilling spindle speed, feed rate, and tool geometric features like drill point angle influence the hole surface quality. The feed rate plays a critical role in the delamination as it is strongly correlated with the thrust force.…”

“…The real-time process monitoring during drilling is highly essential to completely characterize the process dynamics which may be further applied for the prediction of hole quality features . 8,19 The thrust force and torque signals in drilling as sensory outputs are normally processed in the time, frequency, and time-frequency domains. The timedomain analysis is generally characterized as the local peak, whereas statistical mean (μ), standard deviation (σ), and root mean square (rms) as global.…”

“…The timedomain analysis is generally characterized as the local peak, whereas statistical mean (μ), standard deviation (σ), and root mean square (rms) as global. 8,20 These time-domain features sometimes may not be reliable as they cannot be captured by the frequencies present in the acquired force (or torque) signals. Thus, for eliminating these shortcomings, the Fourier transform technique (FFT) is generally used.…”