The long time evolution of structural hierarchy in cross-linked natural rubber, i.e., cis-polyisoprene, was investigated in uniaxially stretched and partially retracted state using real time
birefringence, optical microscopy, and wide angle X-ray scattering techniques. During the deformation,
the needlelike domains that contain highly oriented crystals develop in the stretching direction. The
constrained holding of these samples up to 14 days results in the development of additional crystals
adjacent to original needlelike domains exhibiting lower orientation. Further holding up to 47 days results
in lateral overgrowths of lamellar crystals emanating from these oriented crystals. Growth branching
was found to be increasingly suppressed with the long-term crystallization that lead to the formation of
two-dimensional crystals that grew the longest. Slight retraction after stretching results in a higher
crystalline orientation with lower crystallinity due to retraction-induced melting of crystalline regions
with lower orientation. The population of lateral lamellar overgrowths on oriented crystals was found to
be larger in these stretched and retracted samples than the samples subjected to stretching only.
Temperature dependent WAXS analysis of these crystallites in stretched and stretched−retracted samples
indicates that the crystals with differing perfection and tilt angle with respect to the stretching direction
melt earlier than their more perfectly aligned counterparts.
This article collects the development of a frontal composite structure for electric light vehicles (concretely within L7e European category), which has been required to fulfil energy absorption capabilities for pedestrian protection. An initial design made of a composite sandwich structure (glass fibre skins with polyvinyl chloride foam core) was proposed and a prototype was manufactured and tested against impact. Then, a numerical model was created and the impact test was simulated by the finite element method. After adjusting the numerical model to the real performance of the component, the initial material configuration of the sandwich composite was optimized according to design objectives involving safety, current regulations and repairability.
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