Study on the Machinability of Natural Fiber Reinforced Composite Materials

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Li, Huaizhong
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Dao, Dzung V
Prusty, Gangadhara
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Interest in natural fiber reinforced composites (NFRCs) is increasing rapidly thanks to their numerous advantages such as low cost, biodegradability, eco-friendly nature, relatively good mechanical properties, and a growing emphasis on the environmental and sustainability aspects of engineering materials. However, large scale use of NFRCs is still considered as challenging due to the difficulties in manufacturing, limited knowledge of its machinability and appropriate parameter settings, and being prone to machining-induced defects. These materials are known as hard-to-machine materials due to their heterogeneous structure, mechanical anisotropy and tendency to damage while exposed to mechanical stresses. High rejection rate of composite parts at the assembly stage because of poor quality hole due to several vital drilling induced damages such as matrix cracking, fiber pull-out, delamination, fiber and matrix separation and thermal degradation is a serious concern for manufacturing industries. Among all these defects, delamination was found to be the most vital life-limiting factor which affects the mechanical strength and structural integrity of the component significantly in terms of dimensional tolerances and load carrying capability. Therefore, the main objective of this research is investigating the influence of drilling process parameters on the machinability of flax/poly(lactic acid) bio-composites along with characterization, modelling, and condition monitoring of drilling operation through extensive experimental and analytical investigations. The effect of key drilling parameters and tool geometry such as cutting speed, feed rate, drill diameter, drill material and point angle at different levels were studied experimentally to analyse the relations between resultant quality of the produced holes, cutting forces and size of delamination. Damages and defects associated with the drilling process such as delamination, fiber breakage, fiber pull-out, and matrix cracking were studied through qualitative measurements, optical microscopy and scanning electron microscopy examination. Experimental results revealed that the choice of drill bit in terms of diameter, material and point angle has a considerable effect on the machinability and hole performance. Drilling with HSS drills resulted in nearly 60% lower thrust force and better hole quality compared to that with carbide drills. In addition, the analysis of variance (ANOVA) was applied to identify the significance of each individual cutting parameter. Analytical model was developed to predict the critical thrust force related to the onset of delamination propagation during drilling FF/PLA laminates. The delamination zone was modelled as an elliptical plate, with clamped edge and the analytical model developed based on theory of virtual work, LEFM methodology and theory of plate bending. An experimental investigation was carried out, in addition to the analytical model, through a punching test on different configurations of blind hole to characterize the critical thrust force at the onset of delamination. The developed model has been verified by experimental data and compared with the results of existing models and the presented model considering the effect chisel edge and cutting edges. Based on the results, the predicted values by the proposed model present better correlation with the experimental values than those predicted by other models. A relationship exists between cutting variables (thrust and cutting forces), tool wear and the final quality of the drilled hole. Accordingly, the quality of drilled holes can be improved by in-process monitoring in order to record the whole process status through measuring the thrust force and other indicators. An experimental investigation on online monitoring and non-destructive evaluation of drilling operation using vibration, acoustic emission and thrust force signals was conducted and the correlation between the cutting parameters, delamination, cutting thrust force and the pattern of the signals was detected. The response of material through acceleration, force and AE signals were analysed using different signal analysis tools and statistical parameters to derive the features of signals that can express the key characteristics of machining condition. It is observed that the AE rms values are affected by variation in the cutting parameters and it follows a similar trend as observed in the case of drilling thrust force by varying cutting conditions. The variation of vibration and acoustic emission signals were in correlation with delamination factor and damage severity. Four major damage mechanisms have been identified generally as the main sources of AE energy wave in drilling of FF/PLA composites namely fiber breakage, delamination, matrix cracking and friction. A process for detection and discrimination of various damage mechanisms can be correlated to the frequency of damages. Furthermore, among several statistical parameters applied on the effective segment of the time signals, Kurtosis was found the most competent statistical parameter for condition monitoring of the drilling process to to differentiate between poor and good quality of the drilled holes and enhance the quality of composite component. The findings from this research concluded that damage severity can be assessed through AE parameter analysis and it has a considerable potential for the application of in-process monitoring.

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Thesis (PhD Doctorate)
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Doctor of Philosophy (PhD)
School of Eng & Built Env
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natural fiber reinforced composites
drilling process parameters
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