Andrea Incardona, Application Engineer at materials testing instrumentation manufacturer Instron, explains how computer aided engineering (CAE) simulations require high-quality, well-defined and extensive data to yield reliable results. A significant amount of early characterisation work involves establishing basic properties such as density, stiffness and tensile strength.
Instron
Quasi-static testing remains a fundamental technique for characterising material properties. However, although this method gives reliable outputs at ultra-low velocities, it’s less effective for characterising materials under impulsive loads, forcing scientists to extrapolate data gathered at low velocity.
The risk of financial loss from pursuing a material formula that ultimately doesn’t give the required properties is a great concern for materials developers. In many industries that use rigid plastics, films, other polymers and composite materials, the need for 100% reliability in material properties is non-negotiable.
Safety is a key factor too. In safety-critical industries such as aerospace and automotive, dynamic condition testing is often mandated to comply with standards such as the Underwriters Laboratories family.
Take electric vehicles (EVs). Within EV batteries, the cell has polyethylene-dividing films with selective porosity separating the electrodes. UL2580 demands that this separating polymer is characterised under dynamic conditions to guarantee performance safety.
However, meeting regulatory standards is not the only role of dynamic testing. Advanced instrumentation enables flexible experimental conditions outside standardised frameworks, for example, original equipment manufacturers’ (OEMs) internal specifications and allows assessment of different materials.
Challenges with dynamic testing
The high-speed nature of dynamic testing requires great precision to capture material behaviour reliably. Deformation and failure happen within milliseconds, too fast for traditional sensors and data loggers to record. This cheapens the quality of output data, making the overall characterisation profile less accurate.
Several dynamic testing methods exist, each suited to different applications. Split Hopkinson Pressure Bar (SHPB) provides high strain-rate mechanical properties, while Pendulum Impact Testing (PIT) is commonly used for fracture toughness assessment.
When coupled with high-speed imaging or Digital Image Correlation (DIC), these methods can also provide valuable elongation and displacement data. These metrics are important to predict failure modes and identify elastic versus plastic behaviour.
Drop tower tensile impact testing
Drop tower tensile impact testing is a developing measurement field that allows material characterisation scientists to find tensile strength, elongation, displacement and the stress-strain curve.
It involves suspending a material coupon in a vice and then dropping a striker at a predetermined velocity to apply longitudinal strain. Similar to other impact systems such as PIT, the dependent variable is the energy absorbed to failure, i.e. until the sample breaks.
Drop tower tensile impact testing provides valuable high strain-rate data, reducing the need for extrapolation from quasi-static tests. Comprehensive profiles do require a combination of drop tower results with other test methods and numerical modelling to predict behaviour across a broad strain-rate spectrum.
Simulations that can be trusted to represent real-world behaviour not only bring peace of mind to developers over their investments, they also accelerate the pace of advances. In this way, cutting-edge experimentation techniques drive forward technical industries at increasing speeds.
What does drop tower testing add to materials characterisation?
There are multiple technological features that can be applied to drop tower testing that make it appealing for materials characterisation. The first is integration of high-speed imaging. Drop tower instrument manufacturers can pair their machines with high-speed cameras (HSC) to show the failure mode in successive stages, despite the breakage happening in the order of milliseconds.
When combined with a force sensor, failure propagation analysis can show the material sample’s progression from elastic to plastic deformation, it reaching the elongation at break and then the full fracture. This kind of insight is invaluable for predicting real-world behaviour but was not available prior to recent instrumentation advances.
Digital Image Correlation (DIC) is another powerful tool in combination with drop tower tensile impact testing. Although force displacement may be the output value from this test, end users need a stress-strain curve.
While general calculations using impact velocity and percentage deformation give an average strain across the length of the material sample, DIC allows the investigator to characterise localised stress and strain. This is done by applying a visual pattern to a test specimen, such as black dots on a light coloured sample.
By exporting the HSC images into dedicated software, users can analyse the movement of the pattern and map stress and strain onto the material’s surface. This granular level of analysis represents a leap forward for materials characterisation and the potential for technological advances.
Finally, variable sampling frequency is essential when considering high-velocity tensile impact testing. Brittle materials, like some composites, break very quickly under high stress. If sampling a brittle material at the higher velocities that dynamic conditions require, for example above ten metres per second, only a high sampling frequency will allow the tester to detect the force profile at high resolution.
Conversely, a ductile material will have a longer breakage window, perhaps 50 ms compared against 10 ms with the brittle counterpart. A high frequency will not deliver a full failure curve, underlining the importance of variability. For this reason, advanced drop tower instruments have broad sampling frequency ranges and sophisticated data acquisition systems with more than 60,000 acquisition points.
Moving beyond unreliable extrapolations
Although quasi-static impact testing is still a useful tool in the arsenal of materials characterisation scientists, the time to move beyond feeding complex models with approximations has come.
Advanced methods like drop tower tensile impact testing provide surety through experimental data collection. Manufacturing industries can create safer, more reliable products and eliminate the risk involved with investing resources in materials for which they don’t have complete characterisation profiles.