Instrumented Puncture Impact Test
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Instrumented puncture impact test
General
The instrumented puncture impact test (also known as the instrumented free-falling dart test) extends the conventional methods described in ISO 6603-1 and ISO 7765-1 (see also: puncture impact test) to include the recording of the load (force)–time and load–deformation behaviour, which allows the material behaviour to be better characterised. In addition, fewer test specimens are required for the instrumented methods.
The tests are described in standards ISO 6603-2 and ISO 7765-2. The former describes the test on plates (thickness between 1 mm and 4 mm) and the latter on films (thickness up to 1 mm). The test setup itself is similar for both standards, so that an identical testing machine can be used in terms of basic design and measurement technology (see also: impact loading free-falling dart test). There are greater differences in the characteristic values obtained from the load–deformation diagrams.
The following sections explain both methods in terms of testing and evaluation aspects.
Instrumented puncture test for solid plastics (sheets)
The instrumented puncture impact test with electronic measurement data acquisition for determining the puncture behaviour of solid plastics is carried out in accordance with ISO 6603-2 [1] and represents an extension of the conventional puncture impact test in accordance with ISO 6603-1 [2].
The permissible test specimen thickness of 1 mm to 4 mm distinguishes this method from ISO 7765-2 [3], which is to be used for films (with thicknesses up to 1 mm).
This test characterises the multiaxial impact behaviour of plastics perpendicular to the plate plane. The advantage here is that the load–time or, directly, the load–deformation behaviour is recorded at a practically constant test speed, and measured values or characteristic values for characterising the impact behaviour can be determined from these diagrams. If the temperature and/or the test speed is varied, this test method can also be used to determine brittle-tough transitions as a function of temperature or frequency.
Equipment technology/test procedure
For experimental purposes, free-fall machines with instrumented falling bolts are used (Fig. 1), e.g. Fractovis devices from Instron/Ceast (see: impact loading free-falling dart test).
| Fig. 1: | FRACTOVIS instrumented drop bolt testing system from Instron/Ceast and illustration of the preferred test specimens for plastics |
The geometry (diameter) of the falling bolt is defined in standard [1] and is usually 20 mm, sometimes 10 mm.
The force is measured by a piezoelectric force transducer or strain gauges (DMS) applied near the tip of the falling bolt.
The measurement is typically triggered by a trigger (force increase or light barrier). The light barrier can also be used to determine the actual impact speed by means of a flag mounted on the drop bolt that passes through the light barrier shortly before the point of impact.
If there is no measuring system, the deformation is calculated from the load-time data using double integration, knowing the impact velocity.
To avoid friction (see also: friction force) with the test specimen surface and falsification of the measured values, the drop bolt can be lubricated with oil or grease.
During the test, a test specimen, which can also be taken from a component and usually has a diameter or edge length of 60 mm, is pierced perpendicularly to its surface by the impact body at a practically constant velocity of 4.4 m/s (corresponding to a fall height of approx. 1 m) (see Fig. 2)
| Fig. 2: | Instrumented drop hammer test on clamped plate test |
If an acceleration device is applied, higher test speeds can also be achieved (20 m/s).
The test specimens can be placed freely on the table or secured with a clamping device (see: specimen clamping) to prevent slipping and sliding.
Optionally, the system can be equipped with a catch device to prevent multiple impacts in the case of non-breaking test specimens.
Evaluation of the load–deformation behaviour of brittle, ductile and tough plastics
The resulting diagram can then be evaluated using measurement technology in order to assess the material behaviour under impact stress. Based on the shape of the diagram and the fracture pattern of the test specimens, it is possible to distinguish between brittle, tough or very tough fractures (see: microscopic fracture features for plastics) (Fig. 3). ISO 6603-2 provides for classification into the following four groups:
- Damage due to flow followed by deep drawing (YD)
- Damage due to flow with subsequent stable crack formation (YS)
- Damage due to flow with subsequent unstable crack formation (YU)
- Damage without flow with subsequent unstable crack formation (NY)
| Fig. 3: | Schematic load–deformation behaviour (a) ductile or tough, (b) brittle and (c) highly tough plastics in an instrumented puncture test [3] |
Comparisons between individual plastics can only be made using this method if the test specimen production, test specimen dimensions, surface properties and test conditions are comparable. The thickness of the test specimen plays a particularly important role here. The following measured variables and material parameters can be specified as a result of the puncture test:
- maximum load FM,
- deformation at maximum load lM,
- energy at maximum load EM,
- puncture deformation lp and
- puncture energy Ep.
The puncture point is defined as the point at which the load has fallen to half the maximum load.
Annex A of ISO 6603-2 also allows the determination of a damage point D (FD, lD). This is characterised by a significant drop in load before reaching the maximum load. As there is considerable scope for interpretation here, the criteria for determining this point must be agreed separately. The damage point determined in this way differs from the damage parameters that can be determined in film testing according to ISO 7765-2.
Instrumented puncture impact test for films
The instrumented puncture impact test with electronic measurement data acquisition for determining the impact strength of films using the falling hammer method is carried out in accordance with ISO 7765-2 (see also: film testing) [3] and extends the test variant described in ISO 7765-1 [4] to include the recording and evaluation of load–time and load–deformation diagrams, thereby enabling better material characterisation. In addition, fewer test specimens are required than with the conventional method.
This test characterises the multiaxial impact behaviour of plastic films perpendicular to the film plane for films with a maximum thickness of 1 mm. The advantage of this test standard is that the load-time or directly the load-deformation behaviour is recorded at a practically constant test speed and measured or material values for characterising the impact behaviour can be derived from these diagrams. If the temperature and/or test speed is varied, this test method can also be used to determine brittle-tough transitions as a function of temperature or frequency.
Equipment technology/test procedure
For experimental procedures, free-fall test machines with instrumented impact bolts are used (Fig. 4), for example devices of the Fractovis type from Ceast (now Instron GmbH). The geometry (diameter) of the falling bolt is specified in standard [3] and is usually 20 mm, sometimes 10 mm, with a piezoelectric force measuring cell or strain gauge (DMS) applied near the tip to measure the force. To avoid friction (see also: friction force), the falling bolt can be lubricated with oil, grease or talcum powder.
| Fig. 4: | Instrumented free-falling dart test systems (a) HIT230F from ZwickRoell GmbH & Co. KG, Ulm, and (b) PRIMUS from Coesfeld, Dortmund |
During the test, a film or test specimen, which must not be taken from near the edge, with a diameter of 80 mm perpendicular to its surface is pierced by the impact body at a practically constant velocity of 4.4 m/s (corresponding to a fall height of approx. 1 m) and the load–deformation diagram is recorded at the same time (see Fig. 5).
The force measurement is carried out by a piezoelectric force measuring cell or strain gauges applied near the tip of the falling bolt.
The recording of the measured values is typically triggered by a trigger (force increase or light barrier). The light barrier can also be used to determine the actual impact speed by means of a flag mounted on the falling bolt that passes through the light barrier shortly before the point of impact.
| Fig. 5: | Instrumented free-falling dart test on clamped film test specimens |
As the film is flexible, it should be secured using the clamping device to prevent slipping and slippage.
In the case of very tough films, instead of fracture, pronounced deep-drawing behaviour may occur. In this case, it must be ensured that the drop bolt is not slowed down or caught during deep drawing.
Load–deformation behaviour, measured variables and fracture patterns
The recorded load–deformation diagrams can then be evaluated using measurement technology in order to assess the material behaviour under impact stress. Based on the shape of the diagram and the fracture pattern of the film, brittle, ductile or very ductile fractures can be distinguished from one another (see: microscopic fracture features for plastics) (Fig. 6).
| Fig. 6: | Schematic load–deformation behaviour (a) brittle, (b) tough and (c) highly tough plastic films in the instrumented falling hammer test [5, 6] |
Comparisons between individual plastics can only be made using this method if the test specimen production, test specimen dimensions, surface properties (see also: surface) and test conditions are comparable. The thickness of the film or test specimen plays a particularly important role here.
As a result of the puncture test, the puncture work WT, the energy at maximum load WM and the energy at the point of damage WF can be given as average values, which are calculated using the following measured variables:
- maximum load FM,
- deformation at maximum load sM,
- damage deformation sF and
- damage load FF.
The damage point at which FF and sF are determined is defined as the point at which a sharp drop in force occurs, which is considered the initiation point according to ISO 7765-2. Since, in contrast to the other parameters, there is greater scope for interpretation here, the criteria for determining this point must be agreed separately.
When testing sheets in accordance with ISO 6603-2 [1], a damage point (FD) can also be defined in accordance with Annex A. However, according to the standard, this is located before the maximum load.
A puncture point, as defined in ISO 6603-2, is not specified in the film test in accordance with ISO 7765-2.
See also
- Puncture impact test
- Film testing
- Impact loading free-falling dart test
- Brittle-tough transition
- Tear test
References
| [1] | ISO 6603-2 (2023-06): Plastics – Determination of Puncture Impact Behaviour of Rigid Plastics – Part 2: Instrumented Impact Testing |
| [2] | ISO 6603-1 (2000-03): Plastics – Determination of Puncture Behaviour of Rigid Plastics – Part 1: Non-instrumented iIpact Test |
| [3] | ISO 7765-2 (2025-05): Plastics Film and Sheeting – Determination of Impact Resistance by the Free-falling Dart Method – Part 2: Instrumented Puncture Test |
| [4] | ISO 7765-1 (1988-12): Plastic Film and Sheeting – Determination of Impact Resistance by the Fee-falling Dart Method – Part 1: Staircase Methods |
| [4] | Reincke, K., Grellmann, W.: Approaches to Characterise the Mechanical Properties of Films and Elastomers. In: Grellmann, W., Langer, B.: Deformation and Fracture Behaviour of Polymer Materials. Springer Series in Materials Science 247, Springer, Berlin Heidelberg (2017) 225–236 (ISBN 978-3-319-41877-3; e-Book: ISBN 978-3-319-41879-7; see AMK-Library under A 19) |
| [6] | Reincke, K.: Testing of Polymeric Films. In: Grellmann, W., Seidler, S. (Eds.): Polymer Testing. Carl Hanser Munich (2022). 3rd Edition, pp. 643–678 (ISBN 978-1-56990-806-8; e-Book ISBN 978-1-56990-807-5; ePub ISBN 978-1-56990-802-2; see AMK-Library under A 22) |