No abstract

Metal foams are often used as energy absorbers and lightweight materials. Inspired by a natural blueprint, open‐cell metal foams can significantly reduce the mass of a structure. The innovative manufacturing process of electrodeposition provides the possibility to customize the coating layer thickness of nickel (Ni) on a polyurethane (PU) precursor foam. Consequently, the mechanical properties can be adjusted according to the requirements of the expected application. Herein, quasistatic compression tests and low‐velocity impact tests are conducted on open‐cell Ni/PU hybrid foams to investigate the strain‐rate effects for strain rates in the range of 10−3 to 550 s−1. Furthermore, digital image correlation is performed with the intention of comparing the micromechanical deformation mechanisms under quasistatic loading with those under dynamic loading. For the first time, the heat evolution at different impact velocities of metal foams has been investigated with an infrared camera.

Metal foams belong to the class of porous materials. This bio-inspired material class has gained an increasing interest over the last decades as a result of its versatile advantages in the field of lightweight constructions. [1][2][3] Metal foams exhibit a high energy absorption capacity under compression, based on a constant stress level over a large strain regime in the stress-strain curve. Hence, cellular materials are predominantly applied as energy absorbers in packaging, automotive, aerospace, and defence industry. [4] The stress-strain curve under compression loading is divided into three different regions. [5] The first region outlines an elastic deformation in the majority of the struts, nevertheless yielding occurs in isolated struts. Therefore, this region is referred to as pseudoelastic. [6] The end of the pseudoelastic region is achieved as soon as the first pore-layer collapses under the so-called plastic-collapse stress (PCS). Starting from this point, a nearly constant stress plateau emerges, where the remaining pore layers gradually collapse. In this damage stage, distinct flow plateaus emerge on the macroscale. The rising stress at the end of the stress plateau is subsequently generated by an increasing contact among the collapsed struts. This area of densification constitutes hereafter the third region of the stress-strain curve. [2] The particular shape of the stress-strain diagram is moreover based on the specific cellular microstructure of 3D interconnected pores. [7] Since the foams have to resist multiaxial static as well as dynamic loads during application, an analysis of potential strain-rate effects under compressive loading is essential. [3,[8][9][10][11] Strain-rate sensitivity in cellular materials is generally known to be based on four main criteria. [4] The first criterion affects exclusively closed-cell foams and open-cell fluid-filled foams, where the pore fluid moves slower in comparison to the surrounding framework. As a result, the pressure inside the pore rises and strain-rate effects occurs. The second criterion refers to the pore-framework itself. Calladine and English [12] observed microinertia effects in cellular materials depending on the particular deformation mode. While the first deformation mode is dominated by bending and shows no strain-rate sensitivity, the second deformation mode is subdivided into two distinct deformation steps, leading to an overall strain-rate sensitivity. The first step involves a plastic compression of the structure followed by a time-delayed rotation at the plastic flow hinges. [13] Thus, this effect does only depend on the structure of the specimen, however, the third criterion includes furthermore the material properties. According to Gibson and Ashby, [14] the strain-rate effects of metal foams are correlated with the properties of the strut material itself. The last criterion is based on the shock-wave propagation and enhancement, which occur solely for impact velocities above 50 m s À1 . [15] Previously, different results on strain-rat...

In this paper Split Hopkinson pressure bar (SHPB) was used for dynamic testing of nickel coated polyurethane hybrid foams. The foams were manufactured by electrodeposition of a nickel coating on the standard open-cell polyurethane foam. High strength aluminium alloy bars instrumented with foil strain-gauges were used for dynamic loading of the specimens. Experiments were observed using a high-speed camera with frame-rate set to approx. 100-150 kfps. Precise synchronisation of the high-speed camera and the strain-gauge record was achieved using a through-beam photoelectric sensor. Dynamic equilibrium in the specimen was achieved in all measurements. Digital image correlation technique (DIC) was used to evaluate in-plane displacements and deformations of the samples. Specimens of two different dimensions were tested to investigate the collapse of the foam structure under high-speed loading at the specific strain-rate and strain.