本文選題：多胞材料 + 均勻蜂窩��； 參考：《中國科學技術大學》2017年博士論文

【摘要】：多胞材料是一種內部含有大量空隙并由一定的胞結構組成的具有明顯多尺度特征的材料,具有高比剛度和高比強度,作為輕質結構被廣泛應用于沖擊防護和吸能領域。多胞材料在動態(tài)沖擊情形下呈現出應力增強和變形局部化的典型特征,基于連續(xù)體定義的宏觀名義應力-應變曲線失去物理意義,不能描述多胞材料在動態(tài)沖擊情形下的本構行為。由于沖擊端的應力增強,傳統的基于端面的名義應力不能反映材料中所處的真實應力狀態(tài)。在高速的情形下,變形集中于撞擊端并像沖擊波一樣傳播。為了描述這一特征,已有文獻中已經提出了一系列的沖擊波模型,例如經典的R-PP-L(率無關、剛性-理想塑性-鎖定)模型和R-PH(率無關、剛性-塑性硬化)模型。但這些模型都是在準靜態(tài)的應力-應變曲線上近似得到的,因此很有必要對動態(tài)沖擊下的多胞材料的應力-應變的關系進行研究。為了獲得多胞材料在動態(tài)壓潰下的局部的應力信息,本文發(fā)展了一種拉格朗日截面上工程應力的計算方法。利用該方法研究了多胞材料中的應力分布信息,結合局部應變場計算方法研究了多胞材料的動態(tài)應力-應變行為。本文基于細觀有限元模型提出了截面工程應力的計算方法,得到了多胞材料中的局部應力信息。截面工程應力定義在拉格朗日截面位置上,由兩部分應力組成,即節(jié)點傳遞應力和接觸引發(fā)應力。由節(jié)點傳遞的應力是通過基體材料單元所傳遞的節(jié)點力所引起的,而由接觸引發(fā)的應力是由胞壁間的接觸所引起的。在沖擊初始階段,接觸引發(fā)應力幾乎為零,節(jié)點傳遞應力與截面工程應力相等。在發(fā)生接觸之后,節(jié)點傳遞應力變化不大,接觸引發(fā)應力急劇增加幾乎等于截面應力。接觸引發(fā)應力對應力增強起決定作用。通過截面應力計算方法研究了均勻蜂窩在恒速壓縮下的應力歷史和應力分布。結合應力變化歷史和局部應變歷史得到了不同沖擊速度下的應力-應變歷史曲線。在低速壓縮下,應力-應變關系與準靜態(tài)應力-應變曲線幾乎重合。但是在中速和高速壓縮下,應力和應變從波前初始壓潰狀態(tài)經歷塑性壓潰階段變化到波后動態(tài)壓實狀態(tài)。應力-應變的歷史曲線包含Rayleigh線的發(fā)展過程,隨著沖擊速度的提高,Rayleigh線的斜率變大,即沖擊波速度變大。沖擊波波后的應力-應變狀態(tài)點全部位于準靜態(tài)應力應變曲線的右側,即同等應力下,動態(tài)壓實應變大于準靜態(tài)壓實應變。由應力分布證實了塑性沖擊波在試件中的傳播,得到了沖擊波速度與沖擊速度的關系并與沖擊波模型做了比較�；跊_擊波速度與沖擊速度的關系以及一維沖擊波理論,提出了一種分段模型,基于分段模型推導得到了均勻蜂窩在動態(tài)情形下的率無關本構模型。梯度的引入使得多胞材料呈現不同的力學性能,研究了梯度蜂窩在恒速壓縮情形下的應力分布情形。直接通過應力分布觀察到梯度蜂窩中存在單波和雙波傳播模式�；赗-PP-L和R-PH假設推導了梯度蜂窩沖擊波傳播的單波和雙波理論模型并由截面應力方法進行驗證。R-PP-L模型不適合表征多胞材料在動態(tài)壓潰下的力學行為。R-PH模型得到的應力分布和沖擊波速度與有限元結果相近。關于多胞材料在動態(tài)加載下的初始壓潰應力在已有的文獻中存在看似矛盾的認識。通過局部應力和應變信息對多胞材料在動態(tài)加載下的初始壓潰行為進行了全面的分析。采用了截面應力計算方法和局部應變場計算方法來決定基于細觀有限元模型的均勻蜂窩在不同加載情形下的初始壓潰應力和初始壓潰狀態(tài)的應變率。在恒速壓縮下,初始壓潰應力與準靜態(tài)初始壓潰應力相等但是小于直接撞擊情形下的初始壓潰應力。也就是說,初始壓潰應力在不同的沖擊情形下是不同的,即使基體材料是沒有應變率效應的。當局部應變率大于一個臨界應變率時,發(fā)展了初始壓潰應力與應變率之間的冪次關系來描述蜂窩中初始壓潰應力的應變率效應。小于臨界應變率的時候,初始壓潰應力沒有應變率效應。研究了均勻蜂窩在不同的加載情形下的初始壓潰行為的變形機理,發(fā)現塑性壓縮波前方的初始壓潰區(qū)域的變形模式在不同的沖擊情形下是不同的。
[Abstract]:Multi cell material is a kind of material with a large number of gaps and a certain number of cellular structures, with high specific stiffness and high specific strength. As a lightweight structure, it is widely used in the field of impact protection and energy absorption. Multi cell materials exhibit stress enhancement and deformation localization under dynamic impact. Characteristics, the macroscopic nominal stress strain curve based on the continuum definition loses physical meaning and can not describe the constitutive behavior of multi cell materials under dynamic impact. Due to the increase of the stress at the impact end, the traditional nominal stress based on the end face can not reflect the real stress state in the material. In high speed case, the deformation is concentrated in the case. In order to describe this feature, a series of shock wave models have been proposed in the literature, such as the classical R-PP-L (rate independent, rigid ideal plastic locking) model and R-PH (rate independent, rigid plastic hardening) model, but these models are all approximated on a quasi-static stress-strain curve. Therefore, it is necessary to study the stress-strain relationship of multi cell materials under dynamic impact. In order to obtain the local stress information under dynamic crushing of multi cell materials, a method of calculating the stress on the Lagrange section is developed. The dynamic stress strain behavior of multi cell materials is studied by the local strain field calculation method. Based on the mesoscopic finite element model, the calculation method of the stress of the section engineering is proposed and the local stress information in the multi cell material is obtained. The section engineering stress is defined on the Lagrange section position, which is composed of two parts of stress, that is, node transfer. Stress and contact cause stress. The stress transmitted by the node is caused by the nodal force transmitted by the matrix material unit, and the stress caused by contact is caused by contact between the cell walls. In the initial stage of the impact, the contact stress is almost zero, the transfer stress of the node is equal to the cross section stress. After the contact, the stress is equal to the cross section stress. The stress change of the node is little, and the sharp increase of contact stress is almost equal to the cross section stress. The contact initiation stress corresponds to the force enhancement. The stress history and stress distribution of the uniform honeycomb under constant velocity compression are studied by the cross section stress calculation method. Stress strain history curves under the velocity of shock. Under low velocity compression, the stress-strain relationship is almost coincided with the quasi-static stress-strain curve. However, under the medium speed and high velocity compression, the stress and strain change from the initial stage of the pre wave crushing to the dynamic compaction state. The historical curve of stress strain includes Rayleigh. With the increase of the impact speed, the slope of the Rayleigh line becomes larger, that is, the velocity of the shock wave becomes larger. The stress strain state after the shock wave is all located on the right of the quasi-static stress-strain curve, that is, under the same stress, the dynamic compaction strain is larger than the quasi static compressive strain. The stress distribution confirms the plastic shock wave in the test. The relation between shock wave velocity and impact velocity is obtained and compared with shock wave model. Based on the relationship between shock wave velocity and impact velocity and one dimensional shock wave theory, a piecewise model is proposed. Based on the piecewise model, the rate independent constitutive model of uniform honeycomb is derived. The stress distribution in the gradient honeycomb under constant velocity compression is studied. The single wave and double wave propagation mode in the gradient honeycomb are observed directly through the stress distribution. Based on the hypothesis of R-PP-L and R-PH, the theoretical model of the single wave and double wave propagation of the gradient honeycomb wave is derived. The surface stress method is proved that the.R-PP-L model is not suitable for the characterization of the mechanical behavior of multi cell materials under dynamic crushing. The stress distribution and the shock wave velocity are similar to the finite element results. The initial pressure stress in the dynamic loading of multi cell materials appears to be contradictory in the existing literature. The local stress is through the local stress. The initial crushing behavior of multi cell materials under dynamic loading is comprehensively analyzed and the strain information is used to determine the strain rate of the initial crushing stress and the initial crushing state of the homogeneous honeycomb based on the meso finite element model. The initial crushing stress is equal to the quasi static initial crushing stress but less than the initial crushing stress in the case of direct impact. In other words, the initial crushing stress is different under different impact conditions, even if the matrix material has no strain rate effect. When the local strain rate is larger than a critical strain rate, the initial pressure is developed. The power relation between the crushing stress and the strain rate is used to describe the strain rate effect of the initial crushing stress in the honeycomb. When the critical strain rate is less than the critical strain rate, the initial crushing stress has no strain rate effect. The deformation mechanism of the initial crushing behavior of the uniform honeycomb under different loading conditions is studied, and the initial crushing pressure in front of the plastic compression wave is found. The deformation modes of the region are different under different impact situations.
【學位授予單位】：中國科學技術大學
【學位級別】：博士
【學位授予年份】：2017
【分類號】：TB30

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