52 V. Ilari et al. Beyond its energy absorption capability, cork also possesses a remarkable recovery capacity. This means that its deformation is mostly elastic, allowing it to return to its original shape once the load is removed. This characteristic is particularly advantageous, because it allows the material to maintain its energy absorbing capacity almost unchanged, even after repeated impacts [6] [7]. To characterize the visco-hyperelastic behavior of cork, quasi-static and dynamic compression tests were carry out with a strain rate between 10−3 and 103 s−1. The resulting stress-strain curves were used to calibrate the constitutive model parameters that describe cork’s mechanical response. In particular, the hyperelastic behavior was modeled by the Ogden hyperfoam formulation; while viscoelasticity was described through a Prony series model (generalized Maxwell model). Additionally, the unloading phase was analyzed to evaluate damage evolution of the material and the potential release of stored elastic energy that can occurre after the impact. The Mullis effect was incorporated to describe the unloading phase [8]. To validate the proposed constitutive model, numerical simulations were performed using the finite element Abaqus/ Explicit software. The experimental compression tests were reproduced in the simulation environment, integrating the material properties through user-defined subroutine (VUMAT). Finally, the energy absorption performance of a cork based shock absorber was evaluated through puncture tests, conducted using a drop tower. These tests were numerically reproduced in Abaqus/Explicit software. The comparison between numerical and experimental results serves as a crucial validation step, ensuring the model’s accuracy in predicting the mechanical response of cork under different loading conditions. Materials and Methods Compression tests were performed on agglomerate cork samples (Fig.1a) with a base size of 12 x 12 mm and a height of 15 mm. They have a density of 140 kg/m3. Tests with a strain rate of 10−3, 10−1 and 101 s−1 were conducted through a servo-pneumatic machine model Siplan® (Fig.1b), equipped with a 3 kN load cell and capable of reaching a piston speed of 100 mm/s, while dynamic tests with a 103 s−1 strain rate were performed with a Split Hopkinson Pressure bar (SHPB). (a) (b) Fig. 1 a) Agglomerated cork sample; b) Servo-pneumatic testing machine For the tests conducted at 10−3 and10−1 s−1, the samples were placed on a flat plate connected to the load cell, located on the fixed crosshead of the machine. The piston was then moved with precision, until it touches the upper surface of the sample, marking the start of the test. For the tests conducted ad 101 s−1, however, the piston was initially raised to a sufficient distance from the sample, allowing it to complete the acceleration phase from zero to a 100 mm/s velocity. This strategy allowed to keep the deformation of the samples as uniform as possible, compatible with the capabilities of the machine. To analyze the relaxation phase of the material in tests with10−3 and10−1 s−1 strain rate, the deformation was manteined unchanged at the end of the loading ramp for a period equal to the duration of the ramp itself. Subsequently, the load was progressively reduced by moving the piston backwards until the applied load is completely removed. Dynamic test at 103 s−1 strain rate were performed using a Split Hopkinson Pressure Bar (SHPB), shown in Fig.2. The calibration and the fitting of this system for the study of soft materials are described in [9] [10]. Furthermore, the SHPB
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