ICIT – Influence of Pendulum Hammer Velocity
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ICIT – Influence of Pendulum Hammer Velocity
ICIT: ratio of fracture load to the amplitude of the inertial load
For the fracture mechanics analysis of impact force (F)-deflection (f) diagrams recorded in instrumented Charpy impact tests, in accordance with the criteria of linear-elastic fracture mechanics (LEFM) and elastic‒plastic fracture mechanics (see: J-integral concept and crack tip opening displacement concept), the condition applies that the fracture load Fmax must always be significantly greater than the amplitude of the inertial load F1 during unstable crack propagation (see: ICIT ‒ Experimental conditions) [1].
Application of the low-blow method
A tried-and-tested technique is to reduce the pendulum hammer velocity vH (low-blow technique [2‒4]). To achieve this, it is simply necessary to reduce the fall height, which simultaneously reduces the work done by the pendulum (see also: impact loading pendulum impact tester).
The effects of a reduced pendulum hammer velocity are shown in Fig. 1 using polypropylene (abbreviation: PP) as an example, where a significant change in material behaviour can be observed.
| Fig. 1: | Effect of a reduced pendulum hammer velocity on the material behaviour of polypropylene (abbreviation: PP) for s/W = 7, RT and a/W = 0.45 (a) and a/W = 0.2 (b) |
Figure 1 shows that, at a pendulum hammer velocity vH = 2.9 m/s and a/W = 0.45, the maximum impact load is significantly smaller than the amplitude of the inertial load F1; consequently, for specimens with a notch positioned almost centrally, a fracture mechanics analysis based on fracture mechanics concepts is not possible. The increase in the amplitude of the inertial load shown in the sub-figures is plotted against the decrease in the fracture load Fmax for a/W = 0.2 and a/W = 0.45 in Fig. 2.
| Fig. 2: | Dependence of the maximum impact load Fmax and the inertial load F1 on the pendulum hammer velocity for polypropylene (abbreviation: PP) at a/W = 0.2 and a/W = 0.45 |
Whilst at a/W = 0.2 the inertial loads (see Fig. 1a) remain smaller than the fracture loads even at the maximum possible hammer velocity of 2.9 m/s, this condition is no longer met for a/W = 0.45. For this a/W ratio, F1 and Fmax are identical at 2.6 m/s, and at vH > 2.6 m/s, F1 > Fmax, meaning that the relationship Fmax > F1 is not satisfied and no exact specification of fracture mechanical parameters is possible. The inertial loads lie in the range 8 N ≤ F1 ≤ 36 N and are virtually independent of the a/W ratio.
The expected linear relationship between the amplitude of the inertial loads and the pendulum hammer velocity can be confirmed for both a/W ratios [1]. For vH > 1.5 ms-1, a material-specific increase of 15 Ns/m is determined for polypropylene (abbreviation: PP), and for vH < 1.5 ms-1, a different increase has been observed experimentally, which makes sense given the condition F1 → 0 as vH → 0. The difference in the fracture load Fmax for a/W = 0.2 and a/W = 0.45 results from the change in the residual cross-sectional area (ligament × specimen thickness).
Recording diagrams suitable for fracture mechanics analysis
It is evident from this that evaluable F-f diagrams ‒ i.e. those satisfying the control conditions Fmax > F1 and tB > 2.3…3τ (see: ICIT ‒ Experimental conditions) ‒ for determining fracture mechanical characteristic values at high hammer velocities can only be expected for small a/W ratios. As the velocity decreases, the relationship between load and deflection becomes increasingly non-linear. Due to the increasing material embrittlement associated with decreasing test temperature, which is linked to a reduction in fracture times tB, the control condition Fmax > F1 cannot be met at low temperatures and high a/W ratios.
A check of the energy balance – which stipulates that the impact energy absorbed by the specimen must be less than one third of the total energy of the pendulum impactor – showed that these conditions are adequately met even at a hammer velocity of 1 ms-1 (~40° hammer falling angle) [1].
Whilst the fracture forces do not change significantly for vH > 1.5 … 2 ms-1 (see Fig. 2), it is demonstrated in [1] that the specimen deflection decreases as expected with increasing pendulum hammer velocity and can therefore be regarded as an indicator of the increasing embrittlement that occurs with rising velocity [5].
See also
References
| [1] | Grellmann, W.: Beurteilung der Zähigkeitseigenschaften von Polymerwerkstoffen durch bruchmechanische Kennwerte. Habilitation (1986), Technische Hochschule Leuna-Merseburg, Wiss. Zeitschrift TH Merseburg 28 (1986), No. 6, pp. 787–788 (Content, Summary) |
| [2] | Retting, W.: Untersuchung des Verhaltens von Kunststoff-Folien bei biaxialer Stoßbeanspruchung. Materialprüfung 8 (1966) 2, pp. 55‒60; https://doi.org/10.1515/mt-1966-080202 |
| [3] | Ortmann, R.; Man, J.: Wiss. Zeitschrift TH Magdeburg 24 (1980) 1, p. 101 |
| [4] | Holzmann, M., Man, J.: Dynamika Lomova Houzevnatost (Dynamische Bruchzähigkeit), Zvaranie (1977) 5‒9 pp. 1‒43 |
| [5] | Grellmann, W., Seidler, S. (Eds.): Kunststoffprüfung. Carl Hanser, Munich (2025) 4th Edition (ISBN 978-3-446-44718-9; E-Book: ISBN 978-3-446-48105-3; see AMK-Library under A 23) |