PAMM · Proc. Appl. Math. Mech. 17, 317 – 318 (2017) / DOI 10.1002/pamm.201710128
Investigation of strain-rate effects in Al foams and Ni/Al hybrid foams
on different scales
, Jutta Luksch
, Markus Felten
, Martin Reis
, David R. Sory
, Andy D. Pullen
, William G.
, Martin Larcher
, Georgios Valsamos
, and George Solomos
Saarland University, Applied Mechanics, Campus A4.2, D-66123 Saarbrücken
Imperial College London, Institute of Shock Physics, Blackett Laboratory, UK SW7 2AZ, London
Imperial College London, Department of Civil & Environmental Engineering, UK SW7 2AZ, London
European Commission, Joint Research Centre, Directorate for Space, Security and Migration Safety and Security of
Buildings, I-21027 Ispra
Open-cell metal foams are a new class of cellular materials with structural features resembling those of lightweight load-
bearing materials such as cancellous bones and wood. Their high stiffness-to-weight ratio coupled with their typical long,
ﬂat stress-strain response make them ideal candidates as cost-effective shock energy absorbers in crashworthiness, impact
loading and blast mitigation strategies. The macroscopic mechanical properties of foams are strongly inﬂuenced by both the
mechanical behaviour of single pores at the mesoscopic level and the struts and their structure at the microscopic length-scale,
based on a strong structure-property relationship. This is shown in the present contribution where an experimental-numerical
investigation has been conducted demonstrating the existence of strain-rate effects at different hierarchical scales. Micro
inertia effects arising due to the pore geometry as well as further strain-rate effects stemming from the rate-sensitivity of the
Ni coating in Ni/Al hybrid foams are also outlined.
2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1 Ni/Al hybrid foams and strain-rate effects
Ni/Al hybrid foams are a type of open-cell materials consisting of an Al substrate foam coated with nanocrystalline nickel (Ni),
which strengthens the struts leading to signiﬁcantly improved global properties such as a tenfold energy absorption capacity
compared to pure aluminium foams . The mechanical properties of the Ni/Al hybrid foams can be tailored by the coating
thickness and coating properties, which make Ni/Al hybrid foams a multifunctional material.
Cellular materials such as foams are mainly used under dynamic loading conditions. Consequently, strain-rate effects are of
importance. Strain-rate effects in cellular materials may arise e.g. in closed-cell foams by pressure increase due to entrapped
air. In open-cell foams, strain-rate effects may be caused by rate-dependent strut materials. A further mechanism in both types
of foams, which might be most important, are micro inertia effects arising through the speciﬁc pore structure.
The aim of the present study is to report an experimental-numerical procedure to investigate the strain-rate-dependent effects
of Ni/Al hybrid foams arising in individual struts, individual pores as well as in macroscopic material behaviour. This is of
importance in order to understand the inﬂuence of the pore structure on strain-rate-dependent mechanical properties (such as
energy absorption capacity) in this kind of foams, and it can lead to improved design of foam-based components.
2 Numerical and experimental investigation of strain-rate effects
Since Al is not a rate-dependent material, strain-rate sensitivity in open-cell Al foams can only arise from micro inertia effects.
However, Ni is a rate dependent material, thus the Ni coating is expected to affect the rate-dependent behaviour of Ni/Al
hybrid foams. Quasi-static, drop-weight and split-Hopkinson pressure bar tests performed on macroscopic foam specimens
have shown a rate-dependent behaviour when comparing the plastic collapse stress (PCS) σ
at different strain-rates for Al
foams and Ni/Al hybrid foams [2, 3]. The slope of the PCS as well as of the plateau stress σ
as function of the strain-rate and
density is given in Table 1. Clearly, there is no rate effect for the plateau stress of the Al foams. This indicates that rate effects
must be due to micro inertia resulting from the speciﬁc pore geometry. Ni/Al hybrid foams manifest rate effects for both the
PCS and the plateau stress as a result of the rate dependency of the Ni coating. Further, in order to get a deeper understanding
of the micro inertia effects, drop-weight test were performed on individual pores of the two foam types. These tests proved the
existence of micro inertia effects, arising from the pore structure, and outlined increased rate effects for Ni/Al hybrid foams,
which become stronger with increasing coating thickness.
Table 1: Strain rate effects expressed as slope of the property vs. strain-rate curves.
property Al foams Ni/Al hybrid foams
- MPa cm
s MPa cm
Corresponding author: e-mail firstname.lastname@example.org, phone +49 681 302 2169, fax +49 681 302 3992
2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim