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.
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.
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.
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.