Impact Loading Free-falling Dart Test
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Impact loading free-falling dart test
Higher deformation rates
Modern high-performance plastics for lightweight construction applications in mechanical engineering, the automotive and aerospace industries, and for containers and pipelines in the chemical industry require precise knowledge of how these materials behave under impact loads at high stress rates, especially in crash situations (see also: high-speed tensile test). Such impact loads overlap with the existing static/dynamic loadings and can thus cause critical conditions that can lead to component failure of plastic components. This is caused by locally greatly increased deformation rates, especially in the presence of stress peaks at sharp notches or edges, multiaxial stress states and/or reduced operating temperatures [1].
In plastics, the molecular relaxation and retardation mechanisms require a sufficient reaction time, which cannot be guaranteed at high impact speeds and/or very low temperatures. As a result, the damage, strength, deformation and fracture behaviour of these high-performance plastics is significantly influenced by the velocity dε/dt or frequency f or the exposure time tB of the impact loading, as illustrated by the different impact loads in Table 1.
| impact process | maximum velocity | typical load duration | maximum strain rate |
|---|---|---|---|
| building construction: jackhammer | 5 m/s | 5 · 10-3 s | 1 s-1 |
| automotive engineering: crash | 20 m/s | 5 · 10-2 s | 500 s-1 |
| ballistics: bullet penetration | 2,000 m/s | 1 · 10-4 s | 1.000.000 s-1 |
| manufacturing technology: machining | - | - | 1,000,000 s-1 |
| astronomy: meteorite impact | 10,000 m/s | 1 · 10-6 s | 10,000,000 s-1 |
Table 1 shows that conventional or instrumented tensile impact test or instrumented Charpy impact tests cannot be used to investigate or simulate the behaviour of materials at such high stress rates, as the maximum achievable test speed of 3.9 m/s is insufficient, even with locally excessive deformation rates. In addition, these tests, which are used industrially for the toughness characterisation of plastics, as well as high-speed tensile tests, are dominated by a uniaxial stress and strain state that cannot reflect the real situation of components in use.
High-speed testing equipment
To investigate the impact behaviour of components under high test speeds and multiaxial stress, drop bolt test systems are therefore usually used, which are optionally equipped with temperature control chambers. When equipped with an additional acceleration system, test speeds of up to 20 m/s can be achieved instead of 4.4 m/s (Fig. 1).
| Fig. 1: | Schematic diagram of the FRACTOVIS drop bolt testing system from Instron/Ceast |
In these high-speed test systems, a drop bolt with different diameters (10, 12.5 or 20 mm) is raised to the specified height and then released to fall freely. The required impact energy is determined by the drop height and the additional masses attached. When using the test facility's acceleration system, the linkage is preloaded via springs, whose energy provides additional acceleration. Shortly before the impact of the test specimen, the impact velocity of the drop bolt is determined by an optical measuring section. The load–time diagram is recorded using strain gauges or a piezoelectric force transducer, which is usually installed close to the tip of the drop bolt. If the measuring system is optionally equipped with a three-point bending arrangement (Fig. 2) (see: bend loading), conventional or instrumented impact or instrumented bend tests according to CHARPY can be performed at test speeds of up to 20 m/s.
| Fig. 2: | Optional CHARPY testing device for drop bolt testing systems |
The load–deflection diagram is determined from the recorded load–time curve, knowing the impact velocity v0, and the characteristic values are then calculated in accordance with ISO 179-1 [2]. Problems can arise when evaluating instrumented Charpy impact tests (ICIT), as the experimental conditions change with increasing test speeds as a result of the initial load and strong vibrations can interfere with the useful signal. A meaningful fracture mechanical toughness assessment or evaluation according to ISO 179-2 [3] may then no longer be possible (see: fracture mechanical testing).
See also
- Impact loading plastics
- High-speed tensile test
- Impact loading high-speed testing
- Instrumented puncture impact test
- Puncture impact test
- Compression After Impact Test (CAI)
References
| [1] | Grellmann, W.: Impact Loading. In:Grellmann, W., Seidler, S. (Eds.): Polymer Testing. Carl Hanser, Munich (2022) 3rd Edition, pp. 143–156 (ISBN 978-13-56990-806-8; E-Book: ISBN 978-1-56990-807-5; see AMK-Library under A 22) |
| [2] | ISO 179-1 (2026-03): Plastics – Determination of Charpy Impact Properties – Part 1: Non Instrumented Impact Test |
| [3] | ISO 179-2 (2020-05): Plastics – Determination of Charpy Impact Properties – Part 2: Instrumented Impact Test |