Defence testing: materials development and characterisation
To ensure safety and reliability across the UK defence sector, the testing of materials is fundamental to the development of new equipment, as Eoin O’Keefe explains.
In 2011, the Ministry of Defence (MoD) recognised that the UK possessed certain underpinning technical capabilities key to its interests. However, despite a strong heritage, work in the field was in danger of falling into disrepair, being scattered, difficult to access, or even disappearing altogether from wider UK industry if it wasn’t directly supported.
The first of these ‘fragile capabilities’ to be scrutinised was materials, with a particular focus on those used to control transmission, absorption, emission and reflection in the electromagnetic (EM) and electro-optic (EO) domains. In response to an open competition, QinetiQ, UK, led a small consortium from UK industry to bid to the Defence Science and Technology Laboratory (Dstl) under the MAST-STC programme – the MoD’s research area relating to materials and structures – to establish what has become the STAAR Facility, based at QinetiQ and Malvern Optical.
Translating the acronym proposed by Professor Ian Young at Dstl gives the first indication – ‘STAAR’ equates to spectrum technologies assessment and research. The STAAR Facility focuses on electromagnetic materials in terms of experts, equipment, models and methods. It is designed to accommodate the MoD and its suppliers to understand the EM and EO properties of conventional, advanced, smart and functional materials when designing new platforms or updating to extend the life of legacy military vehicles. This capability has been and will continue to be key in informing and supporting MoD operational analysis and procurement exercises and provide the MoD with well-equipped technical experts.
The new EM development and characterisation laboratories have been established in Farnborough, UK, for four years and populated with newly designed equipment. The EO facilities, including laboratories for materials synthesis, formulation, prototyping and characterisation have been established jointly in QinetiQ’s Griffiths building in Farnborough and Malvern Optical, a UK SME based near Worcester.
Material for defence safety and security
Many types of military and high-performance civilian vehicles require EM control and management treatment for areas as diverse as EM shielding, thermal management, infrared imaging, anti-reflection coatings and solar and aerothermal engineering. The gamut of materials with specialised EO and EM properties developed is wide ranging, including paints and coatings, appliques such as films and tiles, large-area devices, fabrics and other textiles, structural polymers and reinforced polymer composites, structured metals and metamaterials, ceramics and optical ceramics as well as optical glasses and polymers. Increasingly, many of the useful effects are achieved by incorporating materials such as graphene as fillers in the formulations. Most of the materials developed, their properties and even the characterisation equipment in the laboratory have a national security grading that prevents full disclosure, so what follows is ‘for indication only’ of what goes on in these laboratories.
EM STAAR Facility
The EM and microwave laboratories house a variety of capabilities for the characterisation of material properties, ranging from 1Hz to 112GHz, and over a range of high to low temperatures. Various intrinsic material measurements can be made, such as electrical permittivity, magnetic permeability and related parameters, including surface admittance and impedance, along with transmission and reflectivity measurements at a range of angles of occurrence and polarisations.
The equipment and techniques precisely characterise the way that EM energy interacts with matter (like asking ‘how does my microwave oven work?’ or ‘why doesn’t my mobile phone work as well indoors?’). The measurement techniques vary from rectangular waveguide and circular coaxial line and parallel plate capacitive techniques at low frequencies to higher frequency free-field techniques for planar and non-planar samples.
All of these methods have full four-port capability including instantaneous measurement of fully calibrated co- and cross-polar data in both reflection and transmission.
Waveguide techniques can be applied for the characterisation of the intrinsic properties of materials, which are placed into a waveguide line. New materials formulations can be designed to meet MoD requirements through computational modelling combinations of materials and their spatial distributions. The optimised material is fabricated as a panel and its EM properties characterised, before components are fabricated in the materials design and its radar cross section is measured.
EO STAAR Facility
In order to test some of the materials for military applications, they undergo an extensive characterisation regime. The EO STAAR labs include more than 15 different dispersion and Fourier transform spectrometers and spectroradiometers, which when used together with conventional and bespoke accessories, can accurately measure angle-dependent and spatial distribution of transmission, reflection and absorption of samples monoliths, coatings, powders, liquids and gasses over the EO spectrum (200nm to >50 microns). Optical cryostats and optical thermostats enable measurements of transmission absorption and reflection between 80K (-193°C) and 1,050K (777°C). The spectrometers and spectroradiometers are then supplemented with high resolution imaging systems.
Instruments for testing come from further afield too. USA-based Surface Optics Corporation designed the bi-directional reflectometer (SOC 200) and hemispherical directional reflectometer (SOC 100). These instruments measure other parameters such as the spatial and spectral content of reflections. In the SOC 200, one robotic arm moves and directs an illuminating source, either a continuum source such as a high-temperature black body or a narrow band laser, to a point anywhere on the hemisphere above a sample. Samples are positioned on a rotating stage at the centre of the instrument. A second robotic arm moves the detector to any measurement point over the hemisphere above the sample. By moving the illumination point, rotating the sample and moving the detection point, the SOC 200 collects the bi-directional reflectivity function (BRDF) of the material at any wavelength between the UV (250nm) and thermal infrared (20 microns). This measures the reflection in one direction when you illuminate a material from any other direction. The BRDF data produced is used to describe the optical properties of surfaces in numerical models. This can then predict the appearance of future vehicles in EO wavebands at the concept stage.
Using this equipment, the STAAR facility can make most conventional materials measurements, from complex optical constants to the dynamic range of active devices. But, for defence, safety and security applications, the materials properties, equipment and techniques have to go beyond ‘conventional’. Measurements not catered for by orthodox spectroscopy techniques are performed on breadboard optical benches using discrete optical components to arrange specialist illumination sources, detectors and sample measurement geometries. Malvern Optical has a particular interest in measurements of near-grazing reflectance and surface scattering, using expertise of optical design tools to develop a bespoke instrument presented at the IOM3 Advances in Camouflage Materials conference (See Materials World, May 2016, page 12). Meanwhile, QinetiQ is developing equipment and techniques to measure the optical properties of materials in free space at high temperatures – up to 3,000K (2,727°C). The high-accuracy non-contact ‘remote’ surface measurement technique avoids sensitive optical equipment being too close to ~50mm diameter objects at temperatures of 3,000K.
Many of the ‘orthodox’ laboratory measurement techniques are replicated in handheld or portable equipment, so measurements can be made whenever or wherever the sample is. The former Throckmorton or Pershore airfield site alongside Malvern Optical and other locations across the UK provides a convenient location for ‘field trials’ where the materials are tested and characterised in a military environment. When the sample is too big to bring into the laboratory or difficult to bring to a convenient field trial location – for example a 70 tonne tank on a transporter, a fast jet in flight, a heavy lift helicopter, or a ship – the ‘lab on wheels’ is brought into play together with the trained, equipped and experienced trials teams. The field trials equipment includes a wide array of calibrated imaging equipment and procedures, ranging from consumer photographic cameras to high-performance UV and cooled thermal imagers mounted on spectroradiometers, which measure illumination, and handheld spectrometers, all of which cover the UV to thermal IR.
This specialised equipment brings together materials scientists, chemists, physicists, human vision scientists and other disciplines across the MoD and industry to work closely and efficiently together, to maintain the expertise of the UK defence sector in both materials development and materials characterisation.
Dr Eoin O’Keefe FIMMM is a QinetiQ Fellow researching advanced materials for Defence Safety and Security applications and is chairman of the IOM3 Defence Safety and Security Committee.