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|Title: ||顆粒物質受自由落體衝擊之力學分析;Mechanical Response of Granular Bed Subjected to Impact of a Free-drop Projectile|
|Keywords: ||顆粒;衝擊;顆粒床;拋射體;particle;impact;granular bed;projectile|
|Issue Date: ||2014-10-15 17:19:22 (UTC+8)|
|Abstract: ||本研究主旨在探討不同剛性及形狀的顆粒物質在透明壓克力罐體容器中受一自由落體衝擊作用之力學行為。利用加速規黏貼於自由落體上，記錄衝擊過程中的加速度值進而得知速度、灌入深度及顆粒對落體之阻力，並利用高速攝影機拍攝衝擊過程。透過廣義虎克定律將罐體上三個不同高度位置所量測的應變值計算應力，藉以分析罐壁在顆粒受衝擊過程中應力隨高度變化之情形。本研究選擇兩種不同材質探討不同剛性顆粒受衝擊下，顆粒與罐體間力學行為之差異 ; 此外，選擇球形、二款橢圓形、藥丸形、雙球形共五種不同形狀之ABS顆粒進行衝擊實驗，並將實驗結果分別依不同顆粒形狀、顆粒長寬比、顆粒角數進行比較分析以探討其受自由落體衝擊之力學響應。對於高剛性之顆粒則選擇球形及藥丸形兩種形狀之不銹鋼顆粒來探討高剛性顆粒的形狀效應。|
;The purpose of this study is to investigate the effects of particle stiffness and shape on the interactions between a granular bed and an acrylic container and the relevant mechanical responses at various positions during impact by a free-drop projectile. The acceleration data recorded by an accelerometer are used to calculate the penetration depth and velocity of the projectile and vertical drag force. Variations of strains in the container wall are measured through strain gages attached at three heights and used to calculate the relevant stresses through a generalized Hooke’s law. Two kinds of particle material, namely steel and ABS, are selected to characterize the effect of particle stiffness. In addition, the experimental results of five selected particle shapes, namely spherical, ellipsoidal I, ellipsoidal II, cylindrical, and paired ABS particles, are compared to characterize the effects of particle shape, aspect ratio, and particle angularity. For the high-stiffness particle, the experimental results for steel particles of spherical and cylindrical shapes are also compared to characterize the shape effect.
Experimental results of the spherical ABS particles and steel particles show the peak hoop stress in the container wall is of tensile stress and decreases gradually from the top of the container to the bottom. The peak axial stress is of compressive stress except for the top position and decreases gradually from middle to bottom due to various types of particle movement. For the high-stiffness particles, a greater vertical force leads to a shallow penetration and the container wall is subjected to a greater force during impact. It indicates that particles of a larger stiffness are easy to transmit force and dissipate less energy on deformation. The projectile does not completely penetrate into the steel granular bed leading to a greater extent of confined compression for the particles located at the bottom of the container.
For spherical, ellipsoidal, and cylindrical particles, cylindrical particles have a greater interlocking effect, resulting in a greater stiffness of the granular bed. Therefore, a greater vertical drag force is generated by the cylindrical particles and leads to a smaller penetration depth. A less extent of particle movement in the granular bed of cylindrical particles results in a lower hoop and axial ratio. For the spherical particles, interactions between particles are intensified for their smaller size and exert more drag force on the projectile. The hoop and axial stress ratio is greater due to a greater extent of particle movement and a deeper penetration depth. An increase in the aspect ratio of ellipsoidal particles causes a greater stiffness of the granular bed, but the interlocking effect of the ellipsoidal particle is insignificant. For a smaller aspect ratio, as the size is smaller than that of a larger aspect ratio, the interactions between particles are intensified and exert more drag force on the projectile. On the other hand, a higher particle angularity results in a greater interlocking effect and a less extent of particle movement in the granular bed of paired particles. The vertical drag force, hoop stress ratio, and axial stress ratio of the paired particles are smaller than those of the spherical particles.
For spherical and cylindrical steel particles, the variation of peak hoop stress in the container wall shows a similar trend to that of ABS particles. For cylindrical steel particles, the axial stress shows an opposite trend to that of ABS particles at the bottom position. It indicates a less extent of particle movement in the granular bed of cylindrical steel particles due to a greater interlocking effect, resulting in a decrease in the axial stress at the bottom position. Apparently, the shape effect is more pronounced for hard particles.
|Appears in Collections:||[機械工程研究所] 博碩士論文|
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