冲击载荷下梯度多孔金属力学行为研究

发布时间：2018-07-13 15:08

【摘要】：相对连续介质材料而言,多孔材料一般具有相对密度低、比强度高、比表面积高、重量轻、隔音、隔热、渗透性好等优点。同时,多功能梯度设计可以利用材料的某一方面或者是几方面的设计特点来达到均匀材料所不具备的性能。而多孔金属材料作为一种集物理功能与结构一体化的新型工程材料,其优越的特性和高孔隙的开放式结构,使其可设计成为密度按照一定规律分布的多功能梯度结构,满足服役于各种极端环境时多功能集成的重大战略需求。因此,梯度多孔材料引起了学术界和工程界的莫大兴趣。近来的研究发现多孔材料引入梯度后会有着比均匀材料更加优越的吸能特性,但不同研究得到的结论存在一些争议。这说明梯度泡沫的吸能特性并不是一成不变的优越于均匀泡沫,因此,并不能笼统地认为梯度材料的力学特性一定优于均匀材料,要视实际情况而定,比如压缩程度,梯度分布,载荷情况等等,这也正是多功能梯度设计的目的之一。目前对密度梯度多孔金属的抗冲击特性研究还并不系统,强动载荷下层梯度多孔材料的能量分布与能量耗散机理并不是十分清楚;应力波在梯度多孔金属中的传播规律等问题还有待进一步完善。因此,本文开展了以下工作。首先,工作讨论了层梯度圆弧蜂窝和Voronoi随机蜂窝在恒定速度载荷下的变形机理、应力响应和吸能特性。考虑密度梯度以后,除了惯性效应,不同层的准静态屈服强度也是影响变形模态的一个重要因素,尤其是在中低速撞击载荷下的变形;当冲击载荷足够高时,惯性效应掩盖了屈服强度的影响成为主导梯度蜂窝变形的唯一因素。冲击端压缩应力受冲击速度和密度梯度的影响,而材料的准静态屈服应力以及梯度蜂窝的变形模态成为影响固定端应力水平的决定性因素。其次,研究发现基底材料应变强化会影响闭孔泡沫动态力学性能,为了完善该方面的研究,本文采用了三维Voronoi模型对基底材料应变强化效应进行了较为系统的研究,发现存在应变强化效应收敛现象。再次,先前研究中用来预测梯度泡沫塑性响应的不同分析模型,主要是从R-P-P-L模型拓展而来,即忽略了泡沫压缩过程中的应变强化效应,而只考虑平台应力和自锁应变(即密实应变)两个关键参数,这与泡沫实际的变形情况有一定的差别。因此,在本文中,为了考虑了应变强化效应,借用非率敏感的刚性-塑性强化模型(R-PH模型)并拓展到连续梯度泡沫中,得到考虑应变强化的应力波传播模型。同时,通过控制局部胞元参数的方法有效地控制密度分布,得到连续梯度Voronoi随机泡沫细观模型。理论分析和有限元结果对比发现分析模型能够很好的预测梯度泡沫在恒定速度载荷下的响应,这也为进一步理解梯度泡沫动态响应机理提供了一种行之有效的方法。
[Abstract]:Compared with continuum materials, porous materials generally have the advantages of low relative density, high specific strength, high specific surface area, light weight, sound insulation, heat insulation, good permeability and so on. At the same time, the multifunctional gradient design can make use of the design characteristics of one aspect or several aspects of the material to achieve the performance that the uniform material does not have. As a new type of engineering material which integrates physical function and structure, porous metal material can be designed as a multi-functional gradient structure with density distribution according to certain law because of its superior characteristics and open structure with high porosity. To meet the major strategic needs of multifunctional integration in various extreme environments. As a result, gradient porous materials have attracted great interest in academia and engineering. Recent studies have found that porous materials with gradient will have better energy absorption characteristics than homogeneous materials, but the conclusions of different studies are controversial. This shows that the energy absorption characteristics of gradient foams are not always superior to those of uniform foams. Therefore, the mechanical properties of gradient materials cannot be generally considered to be superior to those of homogeneous materials, depending on the actual situation, such as the degree of compression. Gradient distribution, load conditions and so on, this is one of the objectives of multifunctional gradient design. At present, the research on the impact resistance of density gradient porous metals is not systematic, and the energy distribution and energy dissipation mechanism of the gradient porous materials under the strong dynamic load are not very clear. The propagation law of stress waves in gradient porous metals needs to be further improved. Therefore, this paper carries out the following work. Firstly, the deformation mechanism, stress response and energy absorption characteristics of layer gradient circular arc honeycomb and Voronoi random honeycomb under constant velocity load are discussed. After considering the density gradient, in addition to the inertia effect, the quasi-static yield strength of different layers is also an important factor affecting the deformation modes, especially in the case of moderate and low velocity impact load, when the impact load is high enough, The inertia effect masked that the effect of yield strength was the only factor leading to the gradient honeycomb deformation. The compressive stress at the impact end is affected by the impact velocity and density gradient, while the quasi-static yield stress of the material and the deformation mode of the gradient honeycomb are the decisive factors affecting the stress level at the fixed end. Secondly, it is found that the dynamic mechanical properties of closed cell foam will be affected by strain strengthening of substrate material. In order to perfect the research, a three dimensional Voronoi model is used to study the strain strengthening effect of substrate material systematically. It is found that the strain strengthening effect converges. Thirdly, the different analytical models used in previous studies to predict the plastic response of gradient foam are mainly extended from the R-P-P-L model, that is, the strain strengthening effect in the process of foam compression is neglected. Only two key parameters of platform stress and self-locking strain (i.e. compaction strain) are considered, which is different from the actual deformation of foam. Therefore, in this paper, in order to consider the strain hardening effect, the non-rate-sensitive rigid-plastic hardening model (R-PH model) is used and extended to the continuous gradient foam, and a stress wave propagation model considering strain strengthening is obtained. At the same time, the continuous gradient Voronoi stochastic foam mesoscopic model is obtained by controlling the density distribution effectively by controlling the local cell parameters. The comparison between theoretical analysis and finite element analysis shows that the analytical model can well predict the response of gradient foam under constant velocity load, which provides an effective method for further understanding the dynamic response mechanism of gradient foam.
【学位授予单位】：太原理工大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：TG113.25;TB383.4

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