Abstract
This research presents a comprehensive experimental investigation on the machinability effects of variable drilling parameters (feed rate, cutting speed and thrust force), drill diameters and chips formation mainly on delamination and surface roughness, in addition to other drilling-induced damage on both natural and synthetic fibre reinforced polymer (FRP) composites: hemp fibre reinforced polymer (HFRP) and carbon fibre reinforced polymer (CFRP) composite laminates respectively, using double-fluted coated high speed steel (HSS) drills under dry machining and compressed air cooling conditions. It also describes a thermo-mechanical models for predicting and analysing onset push-out delamination during FRP composite machining.After a broad and critical literature survey on FRP composites and their drilling has been carried out, three principal stages of experimental, and an analytical works were conducted to investigate and analyse the influence of both conventional drilling (CD) and ultrasonically-assisted drilling (UAD) techniques on different specimens of HFRP and CFRP composites. Stage 1 involved the CD of 5 specimens of 197 x 197 mm, 7.5 mm thickness HFRP composite laminates of aspect ratios (AR) of 00 (neat), 19, 23, 30 and 38, using diameter holes of 5.0 and 10.0 mm for delamination and surface roughness respectively, among other drilling-induced damage. Taguchi’s technique was used in the design of experiment. The results obtained show that increase in cutting speed reduced delamination factor and surface roughness of drilled holes. However, increase in feed rate caused an increase in both delamination factor and surface roughness. Feed rate and cutting speed had the greater influence on delamination and surface roughness respectively, when compared with aspect ratio, while an increase in fibre AR caused a significant increase in both delamination factor and surface roughness. The optimum results occurred at cutting speed and feed rate (drilling parameters) of 20 mm/min and 0.10 mm/rev, respectively, when drilling specimen of AR 19.
The stage 2 experiment described a comprehensive investigation on the machinability effects of CD parameters, drill diameters and chips formation on the same drilling-induced damage on an optimal specimen of 19-HFRP and MTM 44-1/CFRP composite laminates, using the same specimen dimensions, drills, drilling parameters and condition. The results obtained depict that an increase in feed rate and thrust force caused an increase in delamination and surface roughness of both specimens, different from cutting speed. But HFRP and CFRP specimens have greater surface roughness and delamination-drilling damage respectively. Also, increased drill diameter and types of chips formation caused an increase in both delamination and surface roughness of both specimens as the material removal rate (MRR) increased. Evidently, the minimum surface roughness and delamination factor of the two specimens for an optimal drilling are associated with feed rates of 0.05-0.10mm/rev and cutting speed of 30m/min. Stage 3 of the research focused on the benefits of UAD technique compared with the CD, initially on the first 5 hemp fibre/thermoplastic polycaprolactone (HF/PCL) composite specimens under similar drills, drilling parameters and condition. The results obtained show that UAD technique further confirmed and validated the optimal performance of specimen with AR of 19 (19-HF/PCL) composites, because of the minimum value of thrust force and machining time recorded, when compared with other aspect ratios and CD technique. The 19-HF/PCL laminate has maximum thrust force of 90N and 75N during UAD and CD respectively, which were the lowest force reduction at minimum drilling-induced damage, with the lowest machining time of 30 seconds for both. But comparatively, an improved drilled holes, optimal drilling and nearly 40 % of an average drilling forces (thrust and toque) reduction were recorded with UAD of hemp fibre/thermoset vinyl ester (HF/VE) composite specimens, when compared with both CD and HF/PCL specimens, respectively.
Conclusively, the stage 4 addressed the theoretical aspect of this research through application of analytical method. Hence, in this last stage, an analytical thermo-mechanical model is proposed to predict critical feed rate and critical thrust force at the onset of delamination crack on CFRP composite cross-ply laminates, using the principle of linear elastic fracture mechanics (LEFM), laminated classical plate theory (LCPT), cutting mechanics and energy conservation theory. The delamination zone (crack opening Mode I)is modelled as an elliptical plate. The advantages of this proposed model over the existing models in literature are that the influence of drill geometry (chisel edge and point angle) on push-out delamination are incorporated, and mixed loads condition are considered. The forces on chisel edges and cutting lips are modelled as a concentrated(point) and uniformly distributed loads, resulting into a better prediction. The model is validated with models in the literature and the results obtained show the flexibility of the proposed model to imitate the results of existing models. Evidently, it can be summarily concluded that the quality of the drilled holes and total machinability of the FRP composites depend on the nature and properties of the composite specimens, drill designed geometry, drilling parameters, conditions and techniques.
| Date of Award | Aug 2017 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Hom Dhakal (Supervisor), Ivan Popov (Supervisor) & Asa Barber (Supervisor) |