Investigation of Strain‐Rate Effects in Ni/PU Hybrid Foams under Low‐Impact Velocities

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.

High Strain‐Rate Compression Experiments on Ni/Polyurethane Hybrid Metal Foams Using the Split‐Hopkinson Pressure Bar Technique

“…The obtained results for the normalized PCS are in good agreement with the study of potential strain-rate effects on open-cell Ni/PU hybrid composite metal foams with an initial length of l ¼ 20 mm by Felten et al [10] conducted at low strain rates via drop tower experiments. However, the increase in the density normalized PCS for specimens with an initial length of l 02 ¼ 20 mm resulting from the strain-rate sensitivity under dynamic conditions has been found to be 57%, instead of the 66% observed by Felten et al [10] . This slightly reduced values of the dynamic PCS in the given investigation may result from the thermal softening of the material.…”

“…DIC has been applied in several studies to analyze the micromechanical deformation mechanisms of open-cell metal foams. [4,10,36] This 2D analysis technique visualizes the local strain distribution on the surface of a specimen under loading conditions and therefore provides a detailed analysis of the micromechanical deformation of open-cell metal foams. Figure 10 shows the local strain distribution of a specimen with an initial length of l 01 ¼ 10 mm at certain macroscopic strain states after the PCS.…”

“…In recent years, a novel class of open‐cell Ni/polyurethane (PU) hybrid foams has been increasingly investigated, as the production process via electrochemical deposition enables the precise adjustment of the mechanical properties. [ 9 ] Felten et al [ 10 ] studied potential strain‐rate effects of open‐cell Ni/PU hybrid composite metal foams under low impact velocities up to 550 s −1 using a drop tower setup equipped with a high‐speed camera and an infrared camera. They observed a constant enhancement of 66% for the density normalized PCS under dynamic impact conditions and associated this strain‐rate sensitivity with the microinertia effects described by Calladine and English.…”

“…[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.…”

Characterization of Energy Absorption and Strain Rate Sensitivity of a Novel Elastomeric Polyurea Foam

“…Recent studies suggest using natural surface features as an appropriate pattern for DIC‐based macroscopic strain measurements in low‐density foams. [ 28–30 ] Image contrast originated from the natural surface features of the foam samples, i.e., cells and pores, was used for image correlation purposes in this work. Figure 2b shows the histogram of the foam's natural contrast.…”