以有限元素法模擬探討子彈衝擊編織布之行為

Abstract

本研究主要利用有限元素分析(Finite element analysis)探討纖維編織布受子彈衝擊後之行為與吸能機制。首先藉由代表性單元體來觀察纖維間摩擦力對於等效纖維紗束之剪力模數的影響，隨著紗線厚度方向壓 應力逐漸增加，其等效剪力模數會隨之增加，而當壓縮應力相同時，增加纖維間摩擦係數亦會提升等效剪力模數。根據單層編織布的衝擊模擬，觀察圓球速度歷程變化以及編織布能量吸收情形，比較編織布不同剪力模數、纖維間摩擦係數、幾何排列形狀以及邊界條件之影響。結果顯示纖維編織布材料的剪力模數愈低則較不易發生破壞，並能延遲阻擋衝擊的時間，使得抗彈能力較佳；當編織布纖維間無摩擦力亦或編織布幾何排列為雙跨距編織法時，纖維間都會容易產生滑移之現象，導致抵抗衝擊能力會較差；觀察纖維編織布在不同邊界條件之下受衝擊後之行為發現，若邊界有受到拘束則變形波傳遞較快，不受拘束方向的變形波傳遞較慢，因此根據衝擊物之餘速發現四邊固定之編織布對於抗彈能力最佳。 為了有效率地分析多層編織布受鋼球衝擊之行為，本研究將單層以及三層全編織模型進行簡化為單尺度混合元素模型(single-scale hybrid element model)，建立矩形等效模型(central-patch model)以及十字型等效模型(center-cross model)做比較，鋼球分別以40m/s、100m/s及300m/s之速度衝擊，觀察編織布受衝擊後鋼球之速度歷程、編織布動能及應變能變化與摩擦耗能歷程，並比較全編織模型與簡化模型受衝擊後之行為及運算時間。研究結果顯示簡化模型不但可減少運算時間，其中矩形等效模型在鋼球速度300m/s衝擊下較能與全編織模型受衝擊結果達到一致，而十字型等效模型不論受到三種速度衝擊下皆能達到與全編織模型同樣之結果。

This research aims to investigate the ballistic impact behaviors of woven fabric and its energy absorption using finite element analysis. The representative volume element (RVE) model was employed to observe the inter-fiber friction effect on the effective shear modulus. With the increment of the compressive stress through thickness direction, the effective shear modulus gradually enhanced. Increasing the coefficient of inter-fiber friction enhanced the equivalent shear modulus with the same compressive stress. According to the simulation of the ballistic impact of a single-ply woven fabric, the effects of the equivalent shear modulus, friction between yarns, geometry and boundary conditions of woven fabric are studied by observing the projectile velocity and energy transformation histories. Modeling results show that the lower equivalent shear modulus of the woven fabric contributed to delaying fabric failure, and caused the anti-bullet capability enhanced. In the conditions such as no inter-fiber friction or double span woven fabrics, yarns are sliding easily and causing poor resistance against bullets. In different boundary conditions of woven fabrics during ballistic impact, the strain wave propagated faster at the constrained boundaries than at the free boundaries. Thus the fabric can effectively reduce the projectile residual velocity when four edges were clamped. In order to analyze multilayer woven fabric during ballistic impact effectively, this research presents a single-scale hybrid element model to simulate the 1-ply and 3-ply woven fabrics. This technique involves modeling the central-patch model and center-cross model. The projectile velocity histories, internal energy, kinetic energy and sliding energy of the woven fabrics with impact velocities of 40m/s, 100m/s and 300m/s were investigated. It was found that there is a good agreement between the central-patch model and the full woven model in the impact velocity of 300m/s. The center-cross model shows very good agreement with the full woven case in three kinds of impact velocity. Moreover, the effective models can reduce much computing time than the full woven model.