Postgraduate Research Conference 2009
The Prize Winners
PRESENTATION: Indentation into Polymeric Foams
student: E.A Flores-Johnson
supervisor: Dr Qingming Li
Abstract
Polymeric foams are widely used as core materials for sandwich constructions in the
automotive and aerospace industries because of their light weight and high stiffness. During the
service life of foams or foam core sandwich structures, low velocity impacts by foreign objects
are likely to occur [1]. Consequently, a good understanding of penetration response of both
sandwich structures and foam cores is necessary in order to predict and assess their impact
damages. This research is motivated by lack of knowledge in the study of indentation behaviour
of polymeric foams. The penetration performance of PMI (polymethacrylimide) Rohacell 51WF
under quasi-static indentation with conical, truncated-conical, flat and spherical indenter nose
shapes was reported. Numerical simulations were carried out using the explicit finite element
code LS-DYNA and results were compared with experimental measurements.
Quasi-static indentation test
Rohacell 51WF foam was tested with series of quasi-static indentation tests using a range of
steel indenters shown in Table 1. The indenters were mounted in a standard 200 kN INSTRON
servo-hydraulic testing machine. The load was applied at a nominal strain rate of 8.3x10-4s-1.
The indenters were pushed to 50 mm maximum depth into the foam specimens. The geometry
of the 100 x 100 x 100 mm cube was used.
Typical force-indentation curves all indenters are depicted in Fig. 1. Based on experimental
results observed, the indentation responses by different indenters are summarised below:
(a) Conical indenters: Force increases gradually with the depth of indentation as the contact area between the indenter and the specimen increases. As indentation increase, the crush zone in the surrounding area of the indenter increases.
(b) Truncated indenter: Force-indentation curves exhibit an initial small elastic regime due to stress singularity and strain localization at the perimeter of the indenter tip. After the elastic regime a continuous and gradual increase of load is observed mainly due to crushing beneath the indenter, tearing of the cell walls.
(c) Flat indenter: Force-indentation curves exhibit an initial elastic regime at very low strains until reaching a peak load which indicates the onset of the plastic collapse and crushing of the cells. Plastic regime is characterized for a steady increase of the force as a consequence of the additional force required to tear the cell walls at indenter’s perimeter.
(d) Spherical indenter: Force-indentation curves present an initial elastic regime and steadily
rises with the increase of the contact area with further indentation. An increase of the force is
observed during plastic regime and it is attributed to the same tearing, crushing mentioned
before.
Numerical simulation of quasi-static indentation
Numerical simulations were carried out with LS-DYNA explicit finite element code. The
problem was considered to be axisymmetric. Implicit time integration was used with hourglass
control option. The material model selected in this investigation was
(MAT_CRUSHABLE_FOAM) [2].
Figure. 1 Experimental and simulation results of the indentation force.
Fig. 1 shows a comparison between experimental results and numerical simulations and a good agreement is observed. Fig. 2 shows that a good reproduction of the indentation mechanisms of Rohacell 51 WF foam is observed. Crushed material (denser mesh in black colour) is observed beneath the indenter nose and around the indenter body as observed in the cross-sectioned specimens.
Conclusions
Quasi-static indentations shown that penetration load depends on the geometry of the
indenter. Crushing and tearing were found to be the main indentation mechanisms to contribute
to the indentation force. Numerical simulations were found to agree well with quasi-static
indentations experimental data.
References
[1] Abrate S, (1998), Impact on Composite Structures. New York. Cambridge University Press.
[2] Hallquist JO, ed. LS-DYNA Keyword User’s Manual, Version 971. May 2007, Livermore
Software and Technology Corporation: Livermore CA.