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Shyha I.,University of Birmingham | Soo S.L.,University of Birmingham | Aspinwall D.K.,University of Birmingham | Bradley S.,Composite Research Center | And 2 more authors.
Key Engineering Materials | Year: 2010

Following a review on the machinability of CFRP composites and multilayer stacks typically comprising metallic and composite material elements, the paper details experimental results when drilling 30 mm thick titanium/CFRP/aluminium workpiece stacks. Testing utilised a modified fractional factorial design based on an L18 Taguchi orthogonal array. This comprised four factors, three of which were at three levels and one at two levels and involved tool coating, cutting speed, feed rate and machining environment. Tools evaluated involved hardmetal and diamond coated carbide in addition to uncoated tungsten carbide drills. Response variables were principally tool wear and cutting force/torque with an end of test criteria of 300μm flank wear. Peeling of the CVD diamond coating occurred within the first several holes drilled however this was not a limiting factor in terms of tool life. Principal damage occurred when drilling through the titanium (Ti-6Al-4V) rather than the aluminium (Al 7050) or CFRP (unidirectional "UD" laminates) sections. Best tool life/performance (310 drilled holes) was obtained with the more conventional uncoated carbide drills at lower cutting speed and feed rate. Typically thrust forces increased from 300 N for the first hole to ̃2200 N for last hole drilled while torque values were generally below 600 N.cm for worn tools. © (2010) Trans Tech Publications.

Shyha I.,University of Birmingham | Soo S.L.,University of Birmingham | Aspinwall D.,University of Birmingham | Bradley S.,Composite Research Center
Journal of Materials Processing Technology | Year: 2010

Composites use in the aerospace industry is expanding, in particular carbon fibre reinforced plastics (CFRP) for structural components. Machinability can however be problematic especially when drilling, due to CFRP's inherent anisotropy/in-homogeneity, limited plastic deformation and abrasive characteristics. Following a brief review on composites development and associated machining, the paper outlines experimental results when twist drilling 1.5 mm diameter holes in 3 mm thick CFRP laminate using tungsten carbide (WC) stepped drills. The control variables considered were prepreg type (3 types) and form (unidirectional (UD) and woven), together with drill feed rate (0.2 and 0.4 mm/rev). A full factorial experimental design was used involving 12 tests. Response variables included the number of drilled holes (wear criterion VBBmax ≤ 100 μm), thrust force and torque, together with entry and exit delamination (conventional and adjusted delamination factor values calculated) and hole diameter. Best results were obtained with woven MTM44-1/HTS oven cured material (3750 holes) while the effect of prepreg form on tool life was evident only when operating at the higher level of feed rate. Thrust forces were typically under 125 N with torque values generally below 65 Nmm over the range of operating parameters employed. Finally, the delamination factor (Fd) measured at hole entry and exit ranged between ∼1.2-1.8 and 1.0-2.1 respectively. © 2010 Elsevier B.V. All rights reserved.

Shyha I.S.,University of Birmingham | Soo S.L.,University of Birmingham | Aspinwall D.K.,University of Birmingham | Bradley S.,Composite Research Center | And 3 more authors.
International Journal of Machine Tools and Manufacture | Year: 2011

The use of material stacks comprising titanium, carbon fibre reinforced plastics (CFRPs) and aluminium is expanding for structural aerospace applications, especially where high mechanical loads exist such as for aircraft wing and tail-plane components. Here, the production of bolt/fixation holes is essential to the manufacturing process in order to facilitate part assembly. The paper outlines an analysis of hole quality/integrity following drilling of titanium/CFRP/aluminium stacks under flood cutting fluid and spray mist environments. Uncoated and coated (CVD diamond and hardmetal) tungsten carbide drill performance is evaluated against key response measures including hole size, out of roundness, cylindricity, burr height, hole edge quality, average surface roughness (Ra), microhardness (of the metallic elements) and swarf morphology. Burr height (up to 0.5 mm) was observed to be greater at the hole exit (aluminium) compared to hole entry (titanium) while delamination was significantly reduced when machining CFRP in the stack configuration as opposed to a standalone arrangement. Spiral shaped continuous aluminium swarf was prevalent while both short and long helical chips were found with the titanium material when cutting wet. In contrast, the CFRP layer typically produced dusty black composite particles suspended in the soluble oil of the coolant emulsion. © 2011 Elsevier Ltd. All rights reserved.

Ramadas C.,Composite Research Center | Balasubramaniam K.,Indian Institute of Technology Madras | Joshi M.,Composite Research Center | Krishnamurthy C.V.,Indian Institute of Technology Madras
NDT and E International | Year: 2011

The quantitative assessment of both, the length and width, of a delamination in quasi-isotropic composite laminates, using experimentally generated B- and D-scan imaging techniques employing ultrasonic Lamb waves was presented. Distinct zones were identified in B- and D-scans and the sizes of zones were expressed in terms of the width and length of delamination, the probe separation distance, and the probe diameter. B-scans generated through numerical simulations gave an insight and significance of each zone. B- and D-scans were used to estimate the width and length of delaminations, respectively. The estimated width and length of delaminations were found to be in concurrence with the actual values. © 2011 Elsevier Ltd.

Ramadas C.,Composite Research Center | Padiyar J.,Indian Institute of Technology Madras | Balasubramaniam K.,Indian Institute of Technology Madras | Joshi M.,Composite Research Center | Krishnamurthy C.V.,Indian Institute of Technology Madras
NDT and E International | Year: 2011

In this article, the complete characterization of the interface delamination in a composite T-joint using ultrasonic guided Lamb waves is discussed. Correlation between the numerically simulated and experimentally obtained B-scans was established. In the B-scan images, different Zones were identified and could be utilized for differentiating the healthy and delaminated T-joints. The length and the width of delamination were computed from traversing the transducers in two orthogonal directions (i.e. called the B-scans, and the D-scans). For the first time, backward propagating mode (called Turning Lamb Modes) that are generated at corner of the T-joint was observed and verfied. Using a single linear line scan of the Turning Lamb Modes, both the width and length of interface delamination could be computed. In the present study, the experiments were carried out employing air-coupled ultrasonic transducers. © 2011 Elsevier Ltd. All rights reserved.

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