High Strain Rate Testing of Materials – Part 1

Polymers, composites and some metallic materials are viscoelastic and strain-rate sensitive. Under high strain rates the micro mechanisms by which these materials deform is different than that experienced at low strain rates. Consequently, use of quasi-static stress-strain data may not produce accurate and reliable predictions of material and product performance at highstrain rates. The use of such data in simulation and FEA leads of improper design of engineering components. An understanding of the mechanical properties of polymers over a range of strain rates, temperatures, and frequencies is thus an imperative requirement. As well as being governed by the composition and microstructure of the materials, these properties are highly dependent on a number of external factors.  Common applications where the high strain rate properties are critical are composite and steel material properties in high speed crash analysis of automotive and aerospace structures, high speed ballistic impacts and drop impacts of consumer durables and electronic items.

Most polymers and composite materials exhibit time and temperature dependent mechanical behaviour. This can be inferred by their rate dependent Young’s modulus, yield strength, and postyielding behaviour. Over a range of strain rates from low to high the mechanical properties of these materials may change from gel-like to rubbery to ductile plastic to brittle like ceramics. Along with these strain rate effects, polymers also exhibit large reversible deformations in addition to incompressibility.

Viscoelastic properties of materials play a very critical part in defining the short and long-term behaviour of metals, polymers and composites. To fully characterize this time, frequency and temperature dependent properties of the materials it is important to characterize them in the defamation modes and the rates at which this materials and their products will perform underfield service conditions.

Quasi static characterization test methods assess the properties of the material under static conditions. This serves as a good starting point in product design but when the goal is of full field 360 degree characterization of properties to serve the full range from implicit to explicit FEA simulations for drops impacts, to high speed deformation cases thenthe use of such data will lead to wrong simulation and interpretation of results. 

Different types of testing techniques are used to generate data under high speed and dynamic conditions.Each test method satisfies a specific range of strain rates and deformation characteristics. Electro-mechanical test systems,Servo-hydraulic test systems and Split Hopkinson bar testing apparatus are typically used to characterize the properties of these materials at progressively high strain rates. Complexities in applying this testing techniques come from multiple factors such as sample gripping, calculation of strain and strain rates, test data acquisition and analysis of the test data to generate the right response curve.

Figure 1: Electromechanical and Servo-hydraulic Test Setup at AdvanSES

AtAdvanSES,We have capabilities to test these materials characteristics using all the three testing apparatus mentioned above.

Figure 2: Split Hopkinson Pressure Test SHPB Test Setup at AdvanSES

Strain rate is the change in strain of a material with respect to time. Longer testing time is related to low strain rate,and shorter testing time iscorrelated to higher strain rates.

When a sample in a tensile test is gradually stretched by pulling the ends apart, the strain can be defined as the ratio {\displaystyle \epsilon }ε between the amount of stretchon the specimen and the original length of the band:

ε(t) = L(t) – L0/L0 

Where, L0 is the original length of the specimen and L(t) is the length at time t. Then the strain rate is defined by,

where v(t)is the speed at which the ends are moving away from each other. The unit is expressed as time-1.