Paul J. Callus

Tensile properties and failure mechanisms of 3D woven GRP composites

Abstract Tensile tests were performed on glass reinforced polymer (GRP) composites with three-dimensional (3D) orthogonal, normal layered interlock, and offset layered interlock woven fibre architectures. The mechanical properties and failure mechanisms under tensile loading were similar for the three composites. Cracks formed at low strains within the resin-rich channels between the fibre tows and around the through-thickness binder yarns in the composites, although this damage did not alter the tensile properties. At higher applied tensile stresses the elastic modulus was reduced by 20–30% due to inelastic tow straightening and cracking around the most heavily crimped in-plane tows. Further softening occurred at higher strains by inelastic straightening of all the tows. Composite failure occurred within a localised region and the discrete tow rupture events that have caused tow lock-up and pullout mechanisms in other 3D woven composites were not observed.

The Effect of Ply Orientation on the Performance of Antennas in or on Carbon Fiber Composites

In this paper, the anisotropic conductivity efiect of quasi- isotropic carbon flber laminates on conformal load-bearing antenna structures (CLAS) is presented. The conductivity of a quasi-isotropic IM7/977-3 CFRP laminate is measured using waveguide techniques. The results show that orientation of the surface ply relative to the polarization of the incident E-fleld has a major in∞uence on the re∞ectivity. This difierence is attributed to the fact that carbon flbres oriented parallel to the E-fleld plies behave as good conductors, while ofi-axis plies present as lossy dielectric layers with a flnite conductivity. This anisotropic behavior of the ply layers is shown to have a distinctive in∞uence on the operation of both microstrip patch and slot antennas.

An affordable methodology for replacing metallic aircraft panels with advanced composites

This paper describes an affordable approach to substitute metallic honeycomb panels with more durable stiffened graphite/epoxy composite panels. Although the technology is generic, it is being demonstrated on a honeycomb panel for the Royal Australian Air Force (RAAF) F-111 aircraft. This panel was selected with the aim of validating the design, manufacture and certification approaches. Future work will focus on airworthiness certification, operational testing, addressing service related issues, and composite substitution of other types of metallic components.

The Role of Fibre Orientation on the Electromagnetic Performance of Waveguides Manufactured from Carbon Fibre Reinforced Plastic

Aircraft skins manufactured from carbon fibre reinforced plastic (CFRP) can simultaneously support structural load and act as antennas. This offers the potential for disproportionately large antenna elements and arrays, and thus enhanced aircraft capability. The efficient design of such structures requires that the link between CFRP microstructure and electromagnetic performance be established. This paper presents a method of predicting the electromagnetic attenuation of waveguides manufactured from CFRP. The method considers both the orthotropic, complex conductivity of CFRP, high in the fibre direction and low transverse to it, and the local electric fields in waveguides, which vary with location and frequency. The method was validated experimentally using waveguides manufactured from aerospace grade IM7/977-3 prepreg tape with [0 90]s, [90 0]s and [±45]s ply stacking sequences.

Split-Ring Resonator Loading for the Slotted Waveguide Antenna Stiffened Structure

Slotted waveguide antenna stiffened structure (SWASS) utilizes hat-stiffeners on thin skins or blade stiffeners in sandwich structures as microwave waveguides. By machining slots through the outer skin and into the waveguide, large slotted waveguide antenna arrays may be integrated into a load-bearing structure. However, the slot length is typically resonant with half-wavelength spacing, thereby degrading the load-bearing capacity of the structure. This letter demonstrates a simple method to achieve comparable gain from a slot radiator with subresonant length by means of a single split ring. The adverse structural impact of the individual slot and the slot array may therefore be reduced.

A Novel Method of Conductivity Measurements for Carbon-Fiber Monopole Antenna

The relation between loss resistance and conductivity for monopole antennas over a ground plane is derived. Its accuracy was confirmed by good agreement between it and finite element and method of moments simulations. This relation may be used to calculate conductivity of any material that radiates. A Wheeler Cap system was setup. System losses were determined by equating the loss resistance of a 70 mm long × 1.17 mm diameter copper monopole at resonance with that predicted using the published conductivity for this material. The loss resistance of a 70 mm long × 2 mm wide × 2 mm thick monopole antenna manufactured from an aerospace grade carbon fibre reinforced plastic (CFRP) laminate, IM7/977-3 prepreg tape with a [0 45 90-45]2s stacking sequence, was measured then corrected for system loss. The conductivity of the CFRP laminate was calculated to be 51,000 S/m at 980 MHz.

The strong diamagnetic behaviour of unidirectional carbon fiber reinforced polymer laminates

Carbon fibers are finite conductors with a weak diamagnetic response in a static magnetic field. When illuminated with a high-frequency alternating electromagnetic wave such that the skin depth is greater than the fiber diameter, carbon-fiber composites are shown to exhibit a strong dynamic diamagnetic response. The magnetic susceptibility (χm) is controlled by the polarization angle (θ), which is the angle between the incident electric field and conductor direction. A closed form solution for this behaviour was derived using Maxwells equations and an understanding of the induced conductor currents. The equation was verified using simulation and free space “wall” and waveguide measurements on unidirectional IM7/977-3 carbon fiber reinforced polymer laminates. The measured responses ranged from non-magnetic at θ = 90°, χm = 0, up to strongly diamagnetic at θ = 30°, χm = −0.75, over the 8-18 GHz bandwidth. The experimental results are in good agreement with theoretical predictions and computational simulat...

Capacitively Fed Cavity-Backed Slot Antenna in Carbon-Fiber Composite Panels

A new form of a conformal load-bearing antenna structure that consists of a cavity-backed slot in a carbon-fiber reinforced polymer (CFRP) panel is presented. The antenna is fed via a coaxial cable and patches that capacitively couple energy into the highly conductive carbon fibers without the requirement to abrade off the nonconductive epoxy resin layer on the surface. Computational simulations, which are validated by experiments, show that this feed configuration is as effective as soldering in a brass slot antenna. Backing the slot with a CFRP cavity enhances gain and front-to-back ratio by 2 and 13 dB, respectively. Gain is increased further by orienting the inner surface ply of the CFRP cavity to the same direction as the local E-field. Finally, the dimensions of the slot and cavity are optimized to minimize the antenna size. The resultant low-profile cavity is 14% of the volume of the original antenna design, but maintains similar gain and resonant frequency.

A Frequency Selective Polarizer Using Carbon Fibre Reinforced Polymer Composites

Unidirectional carbon/epoxy composite laminates are highly orthotropic, with their conductivity and permittivity being strongly dependent on the incident angle relative to the flbre orientation. This paper presents a novel frequency selective polarizing subre∞ector manufactured from unidirectional carbon flbre reinforced polymer (CFRP), placed a certain distance from a conducting ground also made from CFRP laminate. Theoretical analysis, computational simulation, and experimental measurements are conducted to investigate the efiects of separation ofiset, laminate thickness and incident angle on the performance of a re∞ector manufactured from a unidirectional IM7/977-3 CFRP. The results show that this new re∞ector reduces the cross polarization at S-band by 13dB while remaining a good re∞ector at X-band and the incident angle has minimal efiect on the frequency response of the polarizer. The single re∞ector can support two orthogonal polarized frequencies, unlike traditional wire grid polarizer screens.

FEA Evaluation of the Mechanical and Electromagnetic Performance of Slot Log-Spiral Antennas in Conformal Load-Bearing Antenna Structure (CLAS)

Conformal load-bearing antenna structures (CLAS) have been attracting the attention of aerospace industries in recent years. This type of multifunctional structures combines the features of conventional antennas with load-bearing capacity and has important applications in military and commercial airplanes especially for Unmanned Aerial Vehicles (UAVs). Equiangular slot spiral antennas are an alternative to traditional rectangular slots because of its wideband radiation characteristics. However, the mechanical characteristics of such a spiral antenna integrated into a structure are so far largely unexplored. In this paper, the electromagnetic (scattering parameter, radiation pattern and gain) and mechanical properties (stress concentration factor (SCF)) of spiral antennas is investigated using finite element analysis (FEA). The results lead to a recommendation for using this type of antenna for future CLAS concepts.Copyright