梯度多胞金属的实验研究、多尺度分析和耐撞性设计

发布时间：2018-01-11 07:31

本文关键词：梯度多胞金属的实验研究、多尺度分析和耐撞性设计　出处：《中国科学技术大学》2017年博士论文　论文类型：学位论文

【摘要】：梯度多胞金属继承了均匀多胞金属优异的力学性能,具有良好的抗冲击能力和吸能效果,且由于包含密度的变化,具有很强的可设计性,受到了越来越多的关注。梯度多胞金属通常可以分为连续密度梯度多胞金属和层级密度梯度多胞金属。由于制造技术的限制,目前大部分实验研究选用的试样为层级密度梯度多胞金属,即用具有不同密度的均匀多胞金属热压或粘接而成,无法准确描述梯度多胞金属的力学行为。研究人员转而采用理论分析或有限元分析,如基于冲击波模型的理论分析和基于Voronoi技术的有限元分析。此外,研究人员还对梯度多胞金属进行了耐撞性设计研究,但大都是在一系列的密度分布组合或不同的密度梯度中筛选耐撞性能较好的密度分布,并没有充分利用梯度多胞金属的可设计性。本文主要基于Voronoi技术发展了梯度多胞金属的三维细观模型,用以更加准确地开展梯度多胞金属的有限元分析,并且率先提出了基于冲击波模型反向设计梯度分布的耐撞性设计方法,以期实现吸能材料从传统的被动选择过渡到主动设计。用实验方法考察了梯度泡沫铝的基本力学性能。由于难以按照特定密度分布制造闭孔连续梯度多胞金属,先用间接测量的方法估算出选用的梯度泡沫铝试样的密度分布,采用线性拟合得到密度分布的近似表达式,并开展梯度泡沫铝的准静态单轴压缩实验。在实验中发现了试样由胞元较大的一端开始渐近压溃的现象。根据这一现象通过建立理论模型推导了压缩载荷-位移关系,并用刚性-理想塑性-锁定(R-PP-L)模型近似材料的应力-应变关系,对实验数据进行了拟合,得到了 R-PP-L模型的力学参数。然后,用改造过的霍普金森压杆开展了梯度泡沫铝的动态实验,结合文献中提出的"双应变片"测量法,通过计算得到了试样支撑端的反力;利用高速摄影机和数字图像相关技术求得子弹的速度历史曲线,发现4组实验的结果吻合得较好。基于应力波理论,建立了质量块撞击梯度多胞杆的冲击波模型,考察了密度梯度对梯度多胞金属抗冲击特性的影响。考虑一系列具有相同平均密度、不同密度梯度且密度分布线性增加的梯度多胞金属杆,推导出了质量块冲击下质量块的速度和波阵面位置对时间的微分方程组,并采用R-PP-L模型的材料参数,利用经典龙格-库塔法对微分方程组进行数值积分求解。结果表明,密度梯度对梯度多胞金属试样的冲击响应有着很大的影响,并发现存在质量块的减速历史振荡非常小的情形,且此时最大减速度的值最小。对比理论分析的结果与实验结果,发现在冲击过程的前期,理论和实验的结果吻合得非常好;随着速度的下降,实验得到的支撑端反力明显大于理论分析结果,而质量块的速度下降比理论分析结果更快。推测其主要原因是R-PP-L模型对于材料特性的模拟较为简化,并未考虑材料的硬化,且压实应变是固定的,影响了理论分析的结果。基于三维Voronoi技术发展了梯度多胞材料的细观模型并开展力学行为的研究。基于变胞元尺寸法和三维Voronoi技术,构造了一系列具有相同平均密度且密度分布为线性的梯度多胞试样,并通过验证胞元的相对密度和统一修正壁厚的方法得到具有准确密度分布的三维Voronoi模型。然后利用ABAQUS/Explicit软件进行有限元分析,考察梯度多胞结构在准静态压缩下的力学行为和动态冲击下的抗冲击特性和吸能效果。发现梯度多胞试样在准静态压缩下存在应变持续增加应力逐渐增加的阶段,这不同于均匀多胞试样的平台段,从变形图上可以观察到由一端开始逐渐压溃的现象。在动态冲击下,观察到密度梯度对梯度多胞金属的动态力学性能同样有着很大的影响,当梯度多胞金属密度大的一端靠近支撑端时可以更好地保护撞击物,而密度小的一端靠近支撑端时对于支撑端处的结构有很好的保护作用。从能量吸收的角度看,密度梯度对梯度多胞金属的能量吸收同样存在较大的影响。提出了根据耐撞性要求反向设计梯度多胞材料的密度梯度的方法。基于冲击载荷的历史曲线,应用非线性塑性冲击波模型,反向推导出了梯度多胞杆的密度分布,并考察冲击力历史为线性变化的耐撞性情形,得到了当冲击力历史分别为恒定、线性增加和线性减少时的密度分布表达式。根据理论分析结果设计出的三种密度分布,构建了相应的三维细观有限元模型,通过数值模拟验证反向设计得到的冲击力响应的结果。通过对比发现,当冲击力历史为恒定或线性增加时,在有限元分析中梯度多胞杆的变形模式与假设的相同,得到的冲击力历史曲线与设定的值吻合得较好;当冲击力历史为线性减少时,冲击过程中试样两端均发生了变形,这和理论分析中的假设冲突,两者的结果也存在一定的差别。从变形图样中还发现,初始的设计中梯度多胞杆靠近支撑端的一部分材料几乎未发生压溃,即设计是偏保守的。针对这样的情况,提出了优化设计的方法,即通过限制波阵面第一次到达支撑端时靠近支撑端处材料的波后应变来缩短梯度多胞杆的长度。所以,根据耐撞性反向设计梯度多胞材料的密度分布的设计方法能够实现。
[Abstract]:The gradient of cellular metal inherits the uniform mechanical properties of cellular metal excellent, good shock resistance and energy absorption effect, and because of the changes of density, can be well designed, has attracted more and more attention. The gradient of cellular metal can usually be divided into continuous density gradient and the level of cellular metal the density gradient of cellular metal manufacturing technology. Due to restrictions, most of the current sample experimental study was used in hierarchical cellular metal density gradient, i.e. with different density uniformity of cellular metal or hot pressing bonding, cannot accurately describe the mechanical behavior of cellular metal gradient. The researchers turn by theoretical analysis and finite element analysis based on the model, such as shock wave theory analysis and finite element analysis based on Voronoi technology. In addition, the researchers also conducted crashworthiness studies to design gradient of cellular metal, but are Screening of Crashworthiness density distribution of good performance in density distribution or a combination of a series of different density gradient, and can not make full use of the design gradient of cellular metal. This paper is mainly based on the Voronoi technology development model of three-dimensional meso gradient of cellular metal, used to accurately carry out gradient of cellular metal the finite element analysis, and proposed based on shock wave model of reverse design gradient distribution crashworthiness design method, in order to achieve the energy absorbing material from passive to active. The transition from the traditional design of the basic mechanical properties of aluminum foams were investigated by experimental method. According to the specific density distribution due to the continuous gradient of cellular manufacturing cell the metal, first with the indirect measurement method to estimate the density gradient distribution of aluminum foam specimen of the linear fitting is used to get approximate expressions for the density distribution, and carry out the global gradient Foam aluminum the quasi-static uniaxial compression experiment. The sample cell by the end of larger asymptotic collapse phenomenon began in the experiment. According to this phenomenon by establishing the theoretical model of compression load displacement relationship, and rigid - perfectly plastic locking (R-PP-L) model to approximate the material stress strain relationship, fit the experimental data, the mechanical parameters of the R-PP-L model. Then, Hopkinson pressure bar with a modified to carry out the dynamic experimental gradient of aluminum foam, combined with the proposed "double gauge" method, obtained by calculating the force supporting sample end; the speed of history curve image correlation technique to obtain the bullet speed camera and digital, found 4 sets of experimental results agree well. Based on the stress wave theory, established the impact mass impact gradient cellular rod wave model, effects of density gradient on the ladder Effect of cellular metal impact properties. Consider a series with the same average density, different density gradient and density gradient distribution of linear increase of cellular metal rod, deduced the mass under the impact of mass velocity and wave front position of time differential equations, and the material parameters of the R-PP-L model numerical solution of differential equations by using Runge Kutta method. The results show that has great influence on the shock response of density gradient gradient of cellular metal specimen, and found that there are very small oscillation situation deceleration history of mass, and the maximum velocity is the lowest. Compared with the results of theoretical analysis with the experimental results, found early in the impact process, the theoretical and experimental results agree well with the speed of decline; the support end force is significantly greater than the result of theoretical analysis, and quality Liangkuai decreased the rate of results than the theoretical analysis. The main reason is that faster simulation for material properties of the R-PP-L model is simplified, not considering the material strain hardening, and the compaction effect is fixed, the results of the theoretical analysis. Research on 3D Voronoi technology development of the mesoscopic model gradient of cellular materials and carry out mechanical behavior based on the variable cell size method and 3D Voronoi based technology, to construct a series with the same average density and density distribution of linear gradient cell samples, and got the 3D Voronoi model accurate density distribution through the validation of cellular relative density and uniform wall thickness correction method. Then using finite element analysis ABAQUS/Explicit software, anti shock characteristics of gradient cellular structures in the quasi-static compression and dynamic mechanical behavior under the impact and effect of energy absorption. Found cellular gradient The sample in the presence of increasing strain stress increasing stage under quasi static compression, which is different from the uniform cellular sample platform, observed by the end began to collapse from the phenomenon of deformation. In the dynamic map can be observed under the impact of density gradient also has great influence on the dynamic mechanical properties of gradient cellular metal, when the end of the gradient of cellular metal density near the end of the support can better protect the impactor and the end of low density near the end of the support structure supporting end has a good protective effect. From the perspective of energy absorption, density gradient on the gradient of cellular metal energy there are also greater absorption effect. Based on the crashworthiness requirement of density gradient reverse gradient design of cellular materials. The history curves of impact load based on the application of nonlinear plastic shock wave model, reverse thrust Density gradient of cellular rod is derived, and the effects of impact resistant situation impact history as a linear change, is obtained when the impact force history were constant, linear increase and linear decrease in density distribution expressions. According to the theoretical analysis of three kinds of density distribution results to design, construct the three-dimensional finite element model the corresponding micro, through numerical simulation verified the impact of reverse design response results. By comparison, when the impact force history to increase the constant or linear time, deformation model and hypothesis in the finite element analysis of cellular rod of the same gradient, the impact force history curve obtained with set values agree well when the impact force history; linear decrease, in the process of impact specimen are deformed, and this assumption conflict in theoretical analysis, the results also have certain difference. From the deformation pattern in the hair Now, the initial gradient in the design of cellular rod near the support end of a portion of the material had hardly been crushing, namely the design is conservative. In view of this situation, put forward the method of optimization design, which is close to the end of support material by limiting the wave front first arrived at the support end after strain wave shorten the gradient cellular rod length. Therefore, according to the design method of the density distribution of the crashworthiness design of reverse gradient cellular materials can be achieved.

【学位授予单位】：中国科学技术大学
【学位级别】：博士
【学位授予年份】：2017
【分类号】：TG146.21;TB383.4

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