Impact Loading High-Speed Testing
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Impact loading, plastics, high-speed testing
General
When using plastics or composite materials for lightweight construction in machines, vehicles or aircraft, in the event of explosions in containers or pipelines, in crash situations or even when processing these materials, sudden impact stresses can occur which superimpose themselves on the static/dynamic load collective. These usually cause locally greatly increased deformation rates and are further intensified by the presence of stress peaks at sharp notches or edges, multiaxial stress states and low temperatures [1].
Effects of high deformation rates
Since the molecular relaxation and retardation mechanisms of plastics require sufficient reaction time, high impact speeds or very low temperatures significantly influence damage, strength, deformation and fracture behaviour, as the time required for the material to react to the impact loading is no longer available. The decisive factor for the effect of the sudden stress on the plastic is the generated strain rate dε/dt or the characteristic exposure time, as shown by different impact loadings (Table 1).
| impact-process | maximum speed | typical load | maximum strain rate |
|---|---|---|---|
| building construction: jackhammer | 5 m/s | 5 · 10-3 s | 1 s-1 |
| Automobilbau: crash | 20 m/s | 5 · 10-2 s | 500 s-1 |
| ballistics: projectile penetration | 2.000 m/s | 1 · 10-4 s | 1,000,000 s-1 |
| manufacturing: machining | - | - | 1,000,000 s-1 |
| astronomy: meteorite impact | 10,000 m/s | 1 · 10-6 s | 10,000,000 s-1 |
With the increase in the deformation rate or the decrease in the test or examination time, a transition from isothermal to adiabatic testing occurs, as the heat generated can no longer be dissipated to the environment within the short test duration. As a result of this, the material values of the dynamic yield strength, the tensile strength and the modulus of elasticity increase, whereas the deformation characteristics show a decrease. Of particular significance here is the reduction in impact strength, which promotes deformation-free critical brittle fracture. Of interest here is not only the bend loading, as in the impact bending test, but often also the tensile loading at high test speeds or strain rates.
Schematic structure of a high-speed testing machine
For such tests, drop bolt testing systems with a so-called reverse suspension or specially equipped high-speed testing machines (Fig. 1) can be used, which allow a maximum test speed of 80 m/s (see: high-speed tensile test and servo-hydraulic testing machine).
However, most of these testing systems can only be operated at test speeds of up to 20 m/s, as evaluation problems arise in this test range due to the superposition of vibrations and the inertial load.
| Fig. 1: | Schematic diagram of a high-speed testing machine for rapid tear tests |
In these high-speed testing systems, the test specimen, including the holder, is pre-accelerated over a specified run-up distance and then loaded by the moving cylinder piston until fracture occurs.
In some cases, even in this speed range, it is possible to control the strain rate in order to maintain constant test conditions.
As with the pendulum impact testers or drop bolt test systems, optional equipment with a temperature control or media chamber is also available here.
Device systems
Technical implementation variants of these test systems are shown in Fig. 2.
| Fig. 2: | High-speed tensile testing machines from (a) Instron GmbH, Darmstadt, and (b) Fa. ZwickRoell GmbH & Co. KG, Ulm] |
During the test, the test software records the force-strain diagram in high resolution, which can then be evaluated offline using the available evaluation routines or special software for graphical and mathematical analysis.
Experimental results for determining the strength of PP-GF composites are presented in the article High-speed tensile test.
See also
- Impact Loading Free-falling Dart Test
- High-Speed Tensile Test
- Impact Loading Plastics
- Instrumented Puncture Impact Test
- Compression After Impact Test
References
| [1] | Grellmann, W.: Impact Loading. In: Grellmann, W., Seidler, S. (Eds.): Polymer Testing. Carl Hanser, Munich (2022) 3rd Edition, pp. 143–156 (ISBN 978-1-56990-806-8; E-Book: ISBN 978-1-56990-807-5; see AMK-Library under A 22) |