GRAPHITE FLAKES EXFOLIATION

The extra-ordinary capability of graphene for reinforcement can be exploited if its properties are maintained in the composite material & and sufficient stress-transfer from the matrix to reinforcement is accomplished. This presupposes that the graphene in the composite is defect-free, monolayer (or a few layers), high-aspect-ratio and biaxially stretched.

Graphene oxide is produced by exfoliation of graphite flakes in a prototype machine, which develops and controls the required fluid flows on graphite flakes, suspended in a mixture with solvent(s). Two disks (Fig. 1, EFLI & PMGH, OD=160 mm) are placed in a face-to-face arrangement at a fix distance, they are counter rotating and the suspension (denoted by yellow color in the drawing) is pressurized and circulated by a piston pump. Laminar flow develops in the comparatively large horizontal flow path between the disks, which flow orients the graphite flakes (due to the velocity gradient) parallel to the flow direction, before entering the peripheral narrow flow zone. The horizontal extensional flow is axis symmetric (along ZZ’ axis) and also plane symmetric with reference to the horizontal plane of symmetry, separating the two disks. As the disk periphery is approached the flow path becomes thinner and for the last 4mm before the periphery (indicated by arrows) the extensional & shear strain rates exceed the required for graphene exfoliation. The gap between the disks at the distance of 4mm before the periphery is 532μm but it is rapidly reduced to a 25μm gap at the periphery of the disks, IP & LM (exaggerated in the drawing). The geometry of this part satisfies specific conditions, the extensional radial strain rate remains almost constant along the stream lines and its maximum does not exceed 5x10^4 /s. The shear strain rate is independently controlled by the disks counter rotation, with an average value exceeding 10^(4 )/s but less than 10^5 /s. In this narrow region the graphite flakes are initially subjected to extensional, radial and tangential strain rates, aligning further the graphene orientation and also enhancing biaxial stretching. At the same time sufficient shear strain forces exfoliate graphene (combined flows of Hagen-Poiseuille and Couette). The present design, which will be optimized, is based on the more efficient extensional flow, requires a low flow rate (0,5 L/s), low circulation pressure (< 1 MPa), low circular velocity of the disks (5 rps) and generates a maximum speed of 40m/s. The exfoliation process can be repeated many times along the flow route by modifying the local geometry. Also, the exfoliation process can continue by linking the exit flow with the entrance (circulation). Exfoliation and functionalisation are attempted in one step.

HIGH STRENGTH, HIGH DAMPING CAPACITY NANOCOMPOSITE

A PEEK-CNT nanocomposite with stretched and aligned CNTs has been developed at the IMMG Laboratory, exhibiting improved strength (more than 200% increase) and damping characteristics (more than 400% increase) than the matrix material with the addition of only 3% vol. CNTs.

A novel (patent pending No. 20080100037) manufacturing process has been developed in-house which allows the production of the nanocomposite in a continuous production line with increased cost effectiveness without compromising the good and repeatable quality of the product. The process uses purified and PEEK-specific functionalized multi-walled carbon nanotubes, which are sandwiched between two thin PEEK membranes and subsequently fused with the polymer matrix through a process involving hot rolling and stretching-annealing cycles.

The predominant mechanism responsible for the observed unsurpassed behaviour of CNT nanocomposites in damping compared to other engineering materials of equal stiffness is a shearing stick-slip mechanism observed when the shear stress maximum at the ends of a stretched CNT, aligned to the direction of loading, exceeds the characteristic interfacial shear strength causing slippage of the CNT in the matrix. This phenomenon, which, according to recent works, is observed in compression as well, is responsible for the formation of a hysteretic, energy-absorbing loop during cyclic loading-unloading that can be repeated many times without any degradation of the material properties.

The resulting nanocomposite has also demonstrated unsurpassed strength and damping capacity at elevated temperatures, remarkable creep resistance and resistance to chemical attack and ultraviolet radiation due to the properties of the PEEK matrix.

HIGH-STIFFNESS AND HIGH-STRENGTH DIRIS CORE AND PANEL.

The DIRIS (DIrectionally Reinforced Integrated Single-yarn) multilayered honeycomb panel is composed of a honeycomb core and two skins from composite material with glass or carbon fibers in a thermoplastic polymer matrix. The DIRIS core is made from a single tape from the composite material wound in successive layers thus forming a characteristic grid shape which is composed of equilateral triangles. In the triangular gaps formed by the winding of the longitudinal reinforcement, triangular prismatic cells from the same material are ultrasonically bonded. The fibre reinforcement of the cells runs at ±45° to the vertical thus reinforcing essentially the resulting honeycomb core in shear. On the flat surfaces of the core two skins made from the same U-D reinforced material are placed on each side at the 0° and 90° directions and bonded through hot pressing. Due to the materials, the geometry and the manufacturing method the proposed panel exhibits higher shear, flexural and compressive strength to weight ratio than the existing panels with similar reinforcement.

Apart from its advanced shear and flexural behaviour, the DIRIS design offers a number of advantages compared to other commercially available sandwich panels with similar reinforcement such as:

  • High operational temperature (up to 260° C continuously for PEEK) and elevated resistance to fatigue and solar radiation.
  • High fiber content per thermoplastic matrix volume (up to 60% for S2 glass - fibers) with the use of thin tapes, thus achieving higher strength.
  • Easy, fast and repeatable local repairing due to the higher strength and to the ability of the used thermoplastic material to bond autogenously either ultrasonically with the application of local heat and pressure.
  • Exceptionally high skin-core delamination resistance and better damping properties due to the continuous crossing reinforcement in various directions and the high available contact surface between the core and the skins.
  • Smaller difference in the mechanical strength of the matrix and the fibers by using high strength thermoplastic polymers reinforced with carbon nanotubes.

Applications of the DIRIS core and panel include membrane connectors for tension fabrics and satellite floors and structural elements for space applications (ESA project 21102/07/NL/PA, 2007).

ULTRA-FAST PIEZO-ACTUATED HYDRAULIC SERVOVALVE.

IMMG staff has replaced the conventional electromagnetic actuation mechanism of a commercially available servovalve with a piezoelectrically driven actuation system which acts directly on the main servovalve piston. As it was demonstrated by detailed fluid mechanical studies and experimental observations, the new configuration has managed to produce pressure fluctuations in the vessel characterized by frequencies as high as 600 Hz and amplitudes that go beyond 40% of the initial steady pressure value.

This technological innovation stresses the fact that piezoelectric actuation can extend considerably the current limits of high speed testing (around 200 Hz in present machines), leading to a host of important implications for the industry (aeronautics, structures, etc.). In an attempt to advance further, IMMG staff have formulated specific concepts that aim at improving the piezostack structure (piezomaterial and electrodes configuration) from a mechanical point of view in order to withstand and operate efficiently under higher loadings than the ones presently employed. The relevant investigation has produced promising results which in conjunction with the use of the most advanced piezoceramics available worldwide, lead to reliable operation of the stack for large number of cycles. In short, the new piezoelectrically actuated servohydraulic valve is capable to perform under high loading at very high frequencies for long operation times, features that are unique in present day high speed testing. An essential characteristic of the device is its capability to interrupt a fast developing phenomenon at a predetermined (stress or strain) stage and therefore to allow specimen examination.

IMMG has demonstrated these capabilities by coordinating a four year Brite - EuRam European project (BRE2 - CT94 - 0962) and investigating buckling phenomena developing on laminated GRP and CFRP shells. Buckling was initiated and interrupted at predetermined stages using the piezoelectric servo-valve.