Compression impact behaviour of agglomerated cork at intermediate strain rates

Abstract

This work analyses the influence of intermediate strain rates, between 25 s -1 and 75 s -1 , on compressive behaviour of agglomerated cork. Quasi-static and dynamic compressive tests of an agglomerated cork with two different densities were carried out. This work complements previous research focused on higher strain rates. The influence of strain rate on less studied properties as densification strain or Poisson ratio is also studied. Digital Image Correlation (DIC) is applied in the estimation of the strain distribution in the longitudinal and transversal directions for both materials quantifying their heterogeneity through the specimen. Under the studied dynamic conditions, no significant influence of strain rate has been found in the mechanical properties. Young modulus, plateau stress, and Poisson ratio are higher in the densest agglomerate, whereas the densification strain is similar in both material densities.Differences in properties such as Young's modulus, Poisson ratio, plateau stress, and densification strain have been found between quasi-static and dynamic conditions for both agglomerates. In the agglomerate with the lowest density, all properties are higher in dynamic conditions than in quasi-static conditions. In the material with the highest density, Young modulus and plateau stress increase significantly, whereas the Poisson ratio decrease.

The Compressive Behavior and Crashworthiness of Cork: A Review

“…Agglomerated cork can be assimilated to a multiscale foam and the random orientation of the granules intended to obtain an almost isotropic material at the macro level is actually responsible for material anisotropy at the micro level. Through DIC analysis, Gomez et al [ 52 ] disclosed a severe heterogeneity in the specimens responsible for a strong difference in the strain of different points in the sample and similar results were reported by Le Barbenchon et al [ 47 ] and by Sasso et al [ 53 ], who identified several localization bands with different regions experiencing different strains, as reported in Figure 4 .…”

“…cork can be assimilated to a multiscale foam and the random orientation of the granules intended to obtain an almost isotropic material at the macro level is actually responsible for material anisotropy at the micro level. Through DIC analysis, Gomez et al [52] disclosed a severe heterogeneity in the specimens responsible for a strong difference in the strain of different points in the sample and similar results were reported by Le Barbenchon et al [47] and by Sasso et al [53], who identified several localization bands with different regions experiencing different strains, as reported in Figure 4. Moving to strain rate sensitivity, many studies investigated the effect of test speed on agglomerated cork exploring the quasi-static regime [47,49,50], i.e., 10 −5 -10 −1 s −1 , the intermediate range [52], i.e., 25-75 s −1 , the low-high range [51,53] i.e., 10 −3 and 100-600 s −1 , and the whole strain rate range [48], i.e., 10 −3 -10 −1 and 60-200 s −1 .…”

“…If design parameters are fundamental to tailoring agglomerated cork’s performance, other test conditions can modify agglomerated cork’s response, as already pointed out for natural cork, including sample orientation [ 47 , 48 ], strain rate [ 47 , 48 , 49 , 50 , 51 , 52 , 53 ], moisture [ 42 ], temperature [ 49 , 50 ] and number of loading cycles [ 47 , 54 ].…”

“…Strain rate proved to play a strong effect even on the two main peculiar features of agglomerated cork, i.e., the Poisson’s ratio and the dimensional recovery. Gomez et al [ 52 ] showed an increase of 33% in a 140 kg/m 3 agglomerated cork’s Poisson’s ratio moving from the quasi-static to the high velocity regime. This study also pointed out a progressive decrease in this parameter up to 12% strain advising to include Poisson’s ratio dependency on both the strain and strain rate in a numerical model to obtain a reliable analysis of this natural material.…”

“…This means that the introduction of fillers in the polymeric binder is another suitable way to tailor agglomerated cork's compressive properties. If design parameters are fundamental to tailoring agglomerated cork's performance, other test conditions can modify agglomerated cork's response, as already pointed out for natural cork, including sample orientation [47,48], strain rate [47][48][49][50][51][52][53], moisture [42], temperature [49,50] and number of loading cycles [47,54].…”