Instrumented Falling Weight Impact Testing: Greater understanding in a fraction of the time!
Greater understanding of impact resistance of polymers and composites leads to materials and application innovation across a wide variety of industries. Along with quality assurance and standard reference data, impact testing also provides a fundamental understanding of a materials structure and allows for assessment in relation to end-use function.
Advanced Impact Analysis
Advanced impact analysis is now easier than ever thanks to systems like the Instrumented Falling Weight Impact Tester from Ray-Ran (RR-IFWT), with its powerful tupDAQ ® software. Utilising fundamental force and velocity sensor technology, an impact event can be automatically identified and defined, and an extremely wide range of data points and measurable values calculated within an instant. In the case of the RR-IFWT, there are 47 measurable values available to users, ranging from typical parameters such as Peak Energy, Total Energy and Maximum Force, to custom cursor points selected from scalable graphs that allow users to evaluate specific curve characteristics.
For any application that uses service critical parts, impact testing is an imperative activity as the simulated high strain rate conditions the test provides cannot be replicated through any other means of mechanical testing. A better understanding of the impact properties leads to improvement of design tolerances and a greater understanding of the materials suitability for a given application.
The Ray-Ran Instrumented Falling Weight Impact Tester
For example, graphite fibre epoxy laminate materials with extremely high tensile and flexural strengths have excellent suitability for aerospace structures such as outer surface aircraft panels as well as automotive body panels and wind turbines, due to their high ‘specific’ properties. However, even low velocity impacts, such as someone stepping on the component, can introduce hidden interlaminar defects that can compromise the integrity of the end-use structure. This could mean that the material cannot be used to its full tensile or flexural potential without the impact properties first being improved upon. Not only is this important for safety, but also allows development in areas such as light-weighting and product design, all applicable for the automotive and aerospace industries, as well as energy, construction, and manufacturing.
Innovation from impact testing
This potential for innovation in component manufacture is derived from impact testing often providing the best representative conditions for a structure’s end-use or the ‘worst-case scenario’ conditions that a structure is expected to endure. It therefore has the potential to be the most important test for development of a company’s given application.
Through Impact testing, the materials used in automotive body components can be optimized to provide the best combination of energy absorbance and impact strength.
With increasingly wide usage of polymers and composites for advanced applications, it is only through impact instrumentation that appropriate analysis can be achieved to keep pace with the current rate of component and materials innovation.
So, from where has this technology developed? The most prevalent method for evaluating a material’s impact toughness in industry and academia has always been by means of a falling projectile striking a flat plate, supported only by its edges. The impact toughness value achieved, also referred to as the impact strength, relates to the amount of energy absorbed by the material through deformation or damage during the impact event.
The impacting head, referred to as the ‘TUP’ features a leading nose, specific to the testing standard being adhered to, that is connected to strain-gauged load cells of a specific rating suitable for the impact energy.
Historically, this means that the crucial figure is therefore the lowest energy of impact that will cause catastrophic failure in a test specimen (or the average energy required to break the specimen). This being the case, impact strength is typically determined through a ‘staircase’ method. The ‘staircase’ method involves repeatedly performing drops onto the specimen with incremental increases or decreases made to either the dop mass or drop height, thus increasing or decreasing the impact force and energy. With a new specimen being used for each drop, an impact that does not result in failure is followed by an impact of greater energy. Similarly, impacts that do result in failure of the specimen are followed by impacts of incrementally lower energy. Following a high number of iterations, the average impact energy that results in failure of the specimen can be calculated, giving you the impact strength of the material.
The Staircase method is a valid option for determining an overall result but does little to describe the intricate relationship of the integral parameters that define an impact: these are time and displacement, which give rise to velocity and force and displacement which give rise to energy. Different types of material behave entirely differently under impact, and it’s only through measurement of velocity and force (and thus the other parameters mentioned) that characteristic behaviours of certain materials can be modelled. As a result, Instrumented Impact testing has increased in popularity over the course of the last decade and is recognized as an advanced research technique by several international testing standards, including ASTM D7136, ASTM D3763-02 and ISO 6603.
Here the tupDAQ ® software test screen shows hysteresis in the red force reading following a test. This means that there is a non-linear relationship between the displacement and force indicating that energy is absorbed by the material through plastic deformation during the impact event. Below the graphical display is a customisable table providing various numerical results from the test.
To give an example of how an impact would be further interrogated with instrumentation, we can compare a stiff metallic material to a polymer. The maximum displacement of the metal specimen would occur coincidently with the maximum force of impact. The typical behaviour of a polymeric material on the other hand, would be to absorb energy over the course of the displacement, owing to the characteristic viscoelasticity of the material. This is evidenced by hysteresis in the force vs. displacement curve for the material, meaning that maximum displacement does not occur coincidently with maximum force, but does occur when the maximum amount of energy has been absorbed. For something like an automotive bumper component, researchers would be looking to separate the points of maximum force and maximum displacement as much as possible, whilst not reducing the overall impact strength required of the material. This would mean that the component can absorb the energy of an impact more effectively without reaching the point of catastrophic failure.
For each drop, there are known conditions. These include the mass of the projectile, the height from which it is dropped and the acceleration under gravity. The only instrumentation required to complete the system and provide data for the five impact defining parameters (time, force, velocity, displacement, and energy) are therefore sensors for force and velocity at the point of impact. To measure force, strain-gauged loadcells in the impacting head are used. For velocity, a detector block that reads the velocity of a ‘flag’ connected to the falling assembly is used. With just these sensors, an extensive array of data is recorded. The information is then subject to data reduction algorithms that provide a precise and usable resultant dataset automatically, with minimal user input.
The mechanical system, consisting of the sample holder, guiding rails and falling assembly, connects to the tupDAQ computer-based software through a data logger and signal conditioning box. The box takes the instantaneous readings from the force sensor and the velocity sensor and processes the information to identify the impact event and reduce the data to a manageable size for output.
A system that can provide instantaneous access to such a wide variety of calculations, with resolution into 1000ths of a millisecond range, saves countless hours of effort by removing the time cost of physically resetting the system for the repeated drops that would otherwise be needed, and by automating the lengthy computations and calculations from the test data that would be required to achieve the same result. Where time is saved, the process of development and innovation is expedited, and in the case of Instrumented Falling Weight Impact testing, this couldn’t be more apparent.
Impact testing with Industrial Physics
If you would like to learn more about the Ray-Ran Instrumented Falling Weight Impact Testing, check out the product page here! For other sample testing and impact testing requirements, contact us today!