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Physics and Chemistry of Solids

Experimental study of dynamic behaviour at short timescales
 

The group has a wide range of experimental facilities at its disposal. These range from simple sample preparation equipment such as soldering irons, to custom built facilities that occupy whole rooms. Many of the facilities have been designed and constructed in house and therefore it is possible for them to be adapted to suit current experimental needs.

In addition to working in the laboratory, a number of the pieces of equipment can be taken off site for use either in demonstrations or to instrument large scale experiments.

While the equipment detailed in this section represent our current and available capabilities the group continually looks to expand and improve our experimental facilities, and are always happy to discuss this with interested parties.

 

Plate impact and ballistics

As well as a 50mm bore gun (normally used for shock and terminal ballistics studies), we have a separate light gas-gun facility with a set of interchangeable barrels. These have a range of bore sizes from 5mm to 25mm and a maximum firing velocity of 1500 m/s for a 15g (or lighter) projectile. Normally the barrel is matched to the diameter of the projectile it is desired to fire, thus avoiding the complexity of having to strip away a carrier (or sabot). But if it is desired to fire very small particles such as sand grains, for example, sabots can be used. The specimen is normally placed within an environmental and expansion chamber which can be evacuated or alternatively filled with an atmosphere of choice. The chamber has viewing ports for high-speed optical and X-ray flash photography. Electrical and optical signals can also be fed in and out through cables.

As with the large plate impact facility, the advantages of firing into a (rough) vacuum are:

  • The gun is silenced.
  • The target is not dislodged by an airblast ahead of the projectile.

The kind of projects that have been performed with this facility include:

  • Ballistic impact on, for example, laminated windows
  • 'Symmetrical' Taylor impact (for validating constitutive models of metals and other materials)
  • Hailstone impact.
  • Single solid-particle erosion damage studies
 

Mechanical testing

The ability to investigate material properties across a range of strain rates and loading configurations is very important, as properties can vary significantly. In order to access these regimes it is necessary to utilise a number of different  pieces of apparatus. Over a number of years we have built and developed the equipment described below. This development is ongoing as the challenges of new projects and technological advancements require different and innovative approaches.

Instrons

The two Instron machines are screw driven low strain rate apparatuses that are capable of loading samples in compression and tension, in that the machines go up and down. With specific sample geometries this can be extended to more complex loading such as shear, Brazilian disc or three point bend. The machines have a maximum loading of 10 kN (smaller more modern machine) and 100 kN (older bigger machine) and allow for the force and extension to be measured by the machine (for fine measurements of extension other gauges are available). Monitoring of samples with a variety of other diagnostics is also carried out on a routine basis, including optical, x-ray and electronic based measurements. Control of temperature is also well established using either liquid nitrogen to cool or a variety of heating apparatus to achieve above room temperature conditions. This has proved to be of great interest particularly in relation to polymer properties where time-temperature superposition has been studied extensively. 

The Dropweight

While the dropweight is a simple piece of equipment (the name itself is a fairly good summary of the operation) we have developed a facility that allows for a number of sophisticated diagnostics to be employed during loading events. The dropweight provides strain rates of around 1000 per second, which is similar tot he Hopkinson Bar. The difference is that the size of the dropweight gives an effectively (on the timescale of the sample deformation) infinite loading pulse allowing for different physics to be probed. Additionally the size of the sample that can be effectively loaded is larger, and granular samples can be tested (to an extent) without the need for confinement required on the Hopkinson Bar system (which is horizontal).

In a typical experiment, the weight (mass ca. 5.5 kg) drops from a height of approximately 1.3 m. The drop weight consists of an aluminium alloy plate which slides between two external guiding rods. The material to be tested is compressed between toughened glass anvils at an impact velocity of typically 4.5 m/s producing a maximum impact pressure in the specimen of approximately 1 GPa. The dropweight can be fitted with a force transducer to look at mechanical properties, a periscopic arrangement to allow for high speed photography (for example to look at hot-spot ignition of energetic materials) through the glass anvils and more recently we have developed a mass spectrometer to allow for the sampling of reaction gasses.

Hopkinson Bars

The group has a suite of bars that cover three of the main loading configurations, compression, tension and torsion. The compression bar has interchangeable bars of differing impedances to allow for better coupling of stress waves into the sample. These range in impedance from magnesium through aluminium and steel to tungsten. Owing to the mechanical complexities of the systems and the loading methods, both the tension and torsion bars only have a single set of bars.

A wide variety of specimen types have been used in the bars, including metals, polymers, ceramics and geological materials. The main use of the bars has been to determine stress-strain relationships which are often subsequently used to define constitutive relations for use in computational modelling (for example through collaboration with QinetiQ). A variety of diagnostics have been utilised in addition to the strain gauges mounted on the bars, including high speed photography. Additional confinement can sometimes be provided through the use of collars around specimens and temperature control over a range from approximately -100 to 600 degrees Celsius is possible.

 

Erosion

The two types of erosion facilities that have been developed in the laboratory over the 40 years or so that the group has been researching in this area can be separated by the erodent used, either solid or liquid erodents.

The solid particle erosion facility consists of a long tube connected to the laboratory high pressure air system. A flow valve controls the amount of air allowed into the system and can be calibrated to give a range of velocities. Sand or other solid particles are introduced via a venture section in the tube and the flow of sand can be controlled via a hopper system so that the erodent flux is well known. Different barrels are available for the system to alter the area of target being eroded, and also the velocity range (which is around 200 m/s maximum). A wide range of carefully particle sized sand fractions are available and the target chamber is fitted with an industrial vacuum cleaner so that silica dust cannot escape into the laboratory. As well as standard erosion experiments, the facility has also been used to examine phenomena such as light emission from high velocity sand impact on helicopter rotor blades.

The liquid erosion facility consists of two separate pieces of equipment, the Single Impact Jet Apparatus (SIJA) and the Multiple Impact Jet Apparatus. The SIJA uses a small gas powered gun to fire a single pellet into a reservoir of water which has a small hole on the opposite side to the impact face. The pellet compresses the water and forces it out of the hole as a jet. Owing to the air resistance this jet becomes hemispherical in flight and therefore approximates a drop when impacting the target surface. Velocities of up to 1200 m/s can be reached without much difficulty. The MIJA utilises the same principle, but incorporates a multiple shot capability and computer control. This has allowed similar machines to be produced for industrial use. The main use for both machines has been investigating materials for the aviation industry, where high speed droplet impact is of a high importance.

 

Laser flyer

We have an experimental laser-launched flyer system.  This consists of a Q-switched Nd:YAG laser with a half-joule pulse energy, and an array of optics to condition, analyse and focus the beam.  The focussed pulse falls on a film, a few microns thick and supported by a transparent substrate.  The area irradiated is of order a square millimetre.  The high energy density in this region converts a thin layer of the film to plasma, driving the remaining thickness forward at speeds of a few kilometres per second. 

There are two main strands of research using the laser-launched flyer system.  The first is improvement of the system itself, studying beam conditioning and flyer design to optimize flyer speed and shape.  The main recent innovations in this field have been a spatial filter system, and the development of layered flyers.  The latter are based on the observation that the mechanical properties required for a good flyer, and the optical and thermal properties required for efficient plasma generation, may not occur in the same material.  By forming an "ablation layer" between the incident light and the flyer itself, both requirements can be met. 

The second strand consists of characterizing the response of energetic materials to shocks of nanosecond duration and tens of gigapascal magnitude.  Using a hundred-micron standoff the flyer is given space to form before striking the material's surface, but the impact takes place early enough that oxidization does not destroy the flyer.  Given efficient coupling between the flyer and the explosive, this system holds out the promise of providing excellent timing control for detonation, and eliminating potentially hazardous primary explosives from explosive systems.

 

High rate diagnostics

The ability of obtain relevant information at with high temporal resolution is critical for a large amount of the research conducted in the group. Much of this equipment is relatively specialised, but is also transportable (as opposed to being a build in part of a larger experimental apparatus) from experiment to experiment and can also be used off site if required.

Interferometers

The Doppler shift of light from a moving target can be used to determine the velocity of that surface. We have two different types of interferometry set up, VISAR and Het-V (or PDV), which differ in how they set up the interference fringes. VISAR splits the reflected light and delays one portion by slowing it down through a known length of glass, whereas Het-V compared the Doppler shifted light with un-shifted light, setting up a beat frequency (in our system the unshifted light is obtained from back reflectance at the end of the optical fibre going to the target). We have a single beam and a 3 beam VISAR system as well as a single beam and a dual beam Het-V system.

Temperature

The group has investigated a number of method for measuring temperature changes on short timescales, and has a number of pieces of equipment either available or in development to make such measurements. A simple 3 channel pyrometer has been used in a couple of projects, as have fast acting thermocouples. A current on-going project is examining the possibility of fabricating temperature gauges from thinly deposited gold films.

Chemistry

Probing the evolution of chemical reaction on short timescales is a difficult task. In order to try and get some information on these events the group has two optical spectrometer setups. The first is a Princeton Instruments gated spectrometer that allows for high resolution visible spectra to be obtained. Owing to the high temporal resolution, the amount of light needed to obtain a suitable signal to noise ratio is quite high. If less light is available then we also have access to a highly image intensified spectrometer which can detect down to the level of single photon emission events. In addition to optical spectrometry we have recently developed a mass spectrometer system that is primarily used to sample gasses evolved during reactions on the dropweight apparatus.

Gauges

The group has the capacity to operate a number of gauge systems including semiconductor strain gauges (that are normally used on the Hopkinson Bar), Manganin stress gauges and PVDF gauges. We have a variety of amplifiers, power systems and recording oscilloscopes and these allow us to field other types of gauges that might be specific to certain experiments.

High Speed Cameras

We have the capability of taking pictures from a few frames per second to 100 million per second. The cameras complement each other, with different cameras being suitable for different applications.  Many of the high-speed cameras currently available commercially are derived from cameras developed within the group.  In addition, we have two cameras which can take streak photography records, and a single shot X-ray facility.

Our suite of high-speed cameras includes a Phantom 6410 (1,500,000 fps), a Phantom 1610 (1,000,000 fps) and an Ultra UHSi (200,000,000 fps).

 

Dynamic Mechanical Analysis

The group has acquired a TA Instruments Discovery DMA 850 Machine. The machine is capable of measuring the complex modulus and loss tangent of material samples, by subjecting them to mechanical oscillations, as well as other important mechanical properties such as damping, creep, and stress relaxation. The frequencies that can be achieved are 0.001 to 200 Hz and temperatures from -150 to 600°C. Tests can be also carried out in controlled humidity conditions at a temperature range of 5 to 120˚C. We are able to carry out measurements under tension, compression, single and double cantilever bending, 3-point bending, shear, and penetration, depending on the sample shape and material.

 

Microscopy

Within the group we have an Olympus BH2-UMA optical microscope for conducting optical microscopy, with a Nikon D7000 camera using Camera Control Pro 2 to capture images of the object being viewed.  The Microscope has a selection of magnifications available ranging between 1x and 50x, though when viewed through the eyepieces there is an additional 10x magnification not present for the camera.  The microscope also has various filters and polarisers for inserting into the line of sight to allow the contrast of the images to be enhanced.  Optical microscopy has been used for investigating the microstructure of geological materials, looking at damage in energetic materials and for observing the retention of water within granular materials.

Another type of microscopy available within the group is atomic force microscopy using a Veeco Enviroscope, which allows for imaging of features on the µm to nm scales in a variety of modes such as contact and tapping mode.  In addition to the aforementioned modes, the AFM is capable of using more uncommon imaging techniques such as magnetic force microscopy, electronic force microscopy, lateral force imaging and surface potential detection along with a force mode for measuring adhesive forces.  The Enviroscope has a built in vacuum chamber, temperature control for samples and a set-up for imaging under fluids.

The group also has access to the Cavendish electron microscopy suite which houses a variety of electron microscopes including SEMs, TEMs, STEMs and ESEMs as well as facilities such as ion beam milling machines.  More information about these microscopy tools can be found at http://www.emsuite.phy.cam.ac.uk/

 

Sample preparation

The group has a number of dedicated areas and facilities for sample preparation:

  • Spot welder - this is mostly used for joining thermocouples together to ensure good bonding between the wires.
  • Soldering irons
  • Fume hoods are available for both sample preparation, experiments and post experimental analysis where chemicals that can cause respiratory issues are used.
  • 3 roll mill. This apparatus can be used for example to reduce the size of particles for use as pigments in inks.
  • Ovens. We have a number of ovens which can fulfil a variety of purposes. A large capacity oven is available for big items, there is a smaller general purpose oven and an oven with programmable temperature gradients for annealing plastics and other more complex operations.
  • Furnace - this allows for sample processing up to 1200 degrees Celsius.
  • 30 ton press - a simple hand operated hydraulic press that is useful for pressing pellets of materials. We have a variety of pistons and dyes that can be used to make samples of different sizes.
  • Edwards evaporator - this allows for thin films of metal to be evaporated onto surfaces. Patterns such as gauges can be produced using a suitable mass.
  • Spin coating - this is a technique for thin film coating surfaces.
  • Lapping/polishing machines - we have access to a variety of surface finishing machines which can be used to prepare samples for electron microscopy work for example.
  • In the laboratory as a whole there are extensive workshop facilities, including a student workshop. This allows for a very extensive level of component manufacturing to be done on site.