复合圆柱壳冲击压缩数值模拟及稳定性研究

针对复合圆柱壳在炸药爆轰作用下的动力学响应及在此过程中伴随的失稳问题,研究了其制造工艺中可能出现的缺陷以及圆柱壳中铜线螺旋角和直径对复合圆柱壳稳定性产生的影响。采用SPH-FEM耦合算法,建立了复合圆柱壳二维细节模型,并提出了一种基于圆柱壳内壁粒子速度历史的失稳判据,计算了在不同参数条件下复合圆柱壳的失稳时间及对应的压缩率,对影响复合圆柱壳稳定性的因素进行了评估。分析结果表明,在复合圆柱壳制备过程中存在的折返层缺陷和铜线直径对复合圆柱壳的稳定性有较大影响,而螺旋角度对其稳定性影响不大。

     

新型负刚度吸能结构力学特性分析

负刚度结构作为一种具有广泛应用前景的力学超材料, 在吸能、减振及降噪等领域呈现出显著的优势, 但传统负刚度结构较低的比能吸收效率以及多稳态非自主回弹等特征, 严重限制了其工程应用. 为解决该问题, 通过单胞构型设计, 提出了一种新型可自主回弹的三维负刚度结构. 该结构利用串联的负刚度单胞在加载?卸载过程中, 曲梁胞元的自主反弹, 实现结构循环加载和多次重复利用; 通过凹槽深度设计抑制单胞多稳态的出现, 并且通过调整侧壁厚度, 控制曲梁屈曲模态的形式, 从而增大负刚度临界载荷差值, 实现吸能效率的显著提升. 随后为实现在复杂载荷环境下的高吸能, 对结构尺寸进行梯度设计, 提出了一种梯度负刚度结构, 利用有限元方法比较分析梯度负刚度结构与均匀负刚度结构在不同载荷作用下的吸能效果. 研究结果表明, 该梯度结构因微结构尺寸的不同, 具有不同的负刚度临界载荷最大值, 从而使其在不同的冲击载荷环境下, 在实现自主回弹的基础上, 均呈现出较好的吸能效率. 该新型负刚度结构为振动控制和结构重组等工程应用提供了技术支持.      

大开口复合材料加筋壁板稳定性分析

运用大型有限元软件Patran/Nastran分析了大开口复合材料加筋壁板的稳定性,并对不同加筋方式下壁板屈曲特征值和屈曲模态图进行了比较。结果表明:补强提高了大开口复合材料壁板的稳定性,但往往无法达到很好的效果,需要通过加筋改善其稳定性;加筋复合材料壁板稳定性较原有模型有较大提高;加筋大开口复合材料壁板屈曲特征值随筋条距开口中心距离的增加而减小,其屈曲分界线均位于筋条布置处;纵筋大开口复合材料加筋壁板一阶屈曲特征值为2.13,而横筋只达到1.08;纵筋布置对复合材料壁板稳定性影响明显高于横筋布置,可在实际工程应用中适当增加纵筋的布置。     

基于应变模态和稀疏贝叶斯学习的网架结构损伤识别

传统稀疏贝叶斯学习算法进行损伤识别时需要对每个单元进行刚度损伤系数的迭代更新,当结构单元众多时,存在计算效率低和对振型的完备性要求高等问题.本文提出了损伤识别两步法,首先利用应变模态差指标进行疑似损伤单元的判断;接着以单元刚度损伤系数为目标参数,建立结构损伤识别的多层次稀疏贝叶斯学习模型,利用稀疏贝叶斯学习算法进一步识...     

动点的牵连运动分析与运动合成定理推证

点的合成运动分析中,复杂运动合理地分解和合成的关键在于牵连点的运动分析,牵连点是动系上与动点瞬时相重合的点,由于动点在动系上不断变迁,故牵连点随时间变动。本文指出,任一瞬时,牵连点均是动系上的某一“固定”点,仅随动系一起运动,采用解析方法提出了牵连点速度和加速度的一种表示方法,并推导了点的速度和加速度合成定理。本方法无需引入相对导数,易于学生理解。     

The construction of boundary analogues of variational methods to approximate weak solutions of boundary-value problems in the theory of elasticity

Methods of approximating weak solutions of certain boundary-value problems in the theory of elasticity are proposed based on expanding the approximate solution in a finite series in basis functions which identically satisfy a homogeneous differential equation in the domain. The coefficients of the expansion are found by constructing a boundary analogue of the method of least squares (BAMLS). It is proved that the approximate solution thus obtained converges to a weak solution of the problem. Sufficient conditions for the stability of the BAMLS, easily verifiable by computational means, are derived. The construction of a boundary analogue of the collocation method (BACM) is proposed on the basis of the BAMLS, combined with discretization of the scalar product by quadrature formulae. The BACM obtained is convergent and stable and possesses better computational properties than the BAMLS.     

Investigation of the fragmentation and oxygen reactivity of endohedral metallofullerenes M@C60

The stability, fragmentation, and chemical reactivity of endohedral metallofullerenes M@C₆₀ (M = Ca, Sr, La, and Gd) were examined with laser desorption/ionization mass spectroscopy. A laser ablation/sublimation procedure was used to generate and purify M@C₆₀ samples, which were then characterized by both time-of-flight and Fourier transform ion cyclotron resonance mass spectrometries. Collisional dissociation of the ionized endohedral fullerenes listed above revealed multiple C₂ loss (either as sequential C₂ loss or larger C_2n eliminations) to generate similar terminal fragmentation products (i.e. MC_44,46), which might be predicted due to the similar ionic radii of the encapsulated metals. The reactivity of the endohedral compounds with oxygenated compounds was found to be dependent on the charge state of the metallofullerene. Singly charged cations of the metallofullerenes (M@C_n⁺) were completely unreactive in the gas phase with oxygenated compounds such as ethylene oxide; however, the corresponding neutral species appear to react readily with oxygenated species. This point was demonstrated in that exposure of solid phase M@C₆₀ to ambient conditions (laboratory air at room temperature) leads to the ultimate destruction of the metallofullerenes concomitant with the formation of metal oxides and carbonates.     

NORMAL CONTACT STIFFNESS OF THE ELLIPTIC AREA BETWEEN TWO ASPERITIES

A new expression for contact deformation is given, and the normal contact stiffness between single asperities is derived according to Hooke’s law. A contact model between two ellipsoidal asperities is simulated by the FE method, the result compared with the theoretical solution. It is found that the curves of the normal contact stiffness versus the included angle in the principal curvature direction show similar trends and evolve as a cosine feature. The effects of the parameters on normal contact stiffness are found to show that normal contact stiffness increases and reaches the upper limit gradually with an increase in these parameters.