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The composite materials with fibrous reinforcement are widely present in ballistic protection and offer both high protection level and lightweight to armored structures. However, these materials could potentially still be improved with a better understanding of the influence of the composite material properties on ballistic performance. During an impact, the mechanical behavior of the composite material is mainly controlled by the dynamic mechanical behavior of its components (matrix and reinforcement). For ballistic protection materials, the fibrous reinforcement can be a woven, a unidirectional or a nonwoven structure, itself composed of a multitude of yarns intertwined, or layered in the case of a unidirectional structure. Among several inner parameters, the mechanical behavior of these complex structures depends on the mechanical behavior of these single yarns. Thus, characterizing the mechanical behavior of single ballistic yarns can widely help to predict the impact behavior of these fibrous structures.
To respond to this yarn characterization, we have developed a new device, the Split Flying Mass (SFM), in order to test yarns under dynamic uniaxial tension and determine their mechanical behaviors in the longitudinal direction. The SFM device is composed of three main parts: the support, the flying mass and the yarn sample which is maintained by its two ends to the support and the flying mass. A gas gun propelled the SFM device at a velocity from 20 m/s to 40 m/s. Aramid yarns are tested and two different sample lengths are used (5 mm and 20 mm) to reach strain rates within the range from 1000 s-1 to 4000 s-1. During the test, we measure the displacement of the flying mass when it applies a longitudinal tension on the yarn which undergoes an elongation up to its rupture.
After data treatment, we obtain the evolution of the velocity of the flying mass versus time which depends on the mechanical behavior of the yarn (evolution of the stress vs. strain, ultimate stress and strain). The comparison of our results with ones of a Photo Doppler Velocimetry device allows validating this new dynamic uniaxial tension device. Four phases are defined on the velocity versius time curve. Amond them, we identify two important phases which are the yarn elongation and the progressive yarn rupture. Thanks to an analytical approach proposed to model these two phases, we could estimate the longitudinal Young modulus of the yarn under dynamic loading. These results also provide us information about the specific energy absorbed by the yarn and what could be failure mechanisms of yarns under a dynamic tension. Thus, this new test of yarns under dynamic loading gives promising results and, with further work, could lead to a better knowledge of ballistic yarns. This knowledge about dynamic behavior of yarns would then be considered for improving future numerical models.
