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Blast mitigation using cementitious foams: Experimental investigation and theoretical development.

机译:使用水泥泡沫的爆破缓解:实验研究和理论发展。

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摘要

Blast vulnerability of buildings in an urban environment has motivated the Structural Engineering community to develop and implement blast mitigation strategies for existing structures. The work presented in this dissertation is aimed at researching the concept of a protective system for structural elements consisting of compressible, low-strength, cementitious foam. A cladding made of cement foam is intended for use as a sacrificial protective material, which would reduce the stress amplitude transmitted to the structural element from an applied blast pressure loading. The attenuation of the applied pressure loading is produced by the irreversible compaction of the material. Brittle shattering or spalling of structural elements, primarily made of reinforced concrete or brick masonry, can be prevented or suppressed in cement foam cladded structures.Experiments were conducted to investigate the quasi-static and dynamic response of cement foam. Instrumentation for accurately measuring the applied blast pressure and the stress pulse transmitted through the foam to the structural element were developed. Specimens made of cement foams of different densities and lengths were evaluated for different blast pressure loadings. The cementitious foams exhibit a concave stress-strain relationship associated with crushing of cells, which leads to densification of the material. Under blast loading, a compaction front which exhibits shock-type characteristics, forms and propagates in the foam. The compaction front creates an interface which separates the densified material consisting of crushed cells from the uncrushed cells. Experimental results identified a critical length, Lcr, which depends on the stress-strain response of the foam, required for completely attenuating an applied blast pressure loading. When foam of length larger than Lcr is used, there is a clear beneficial effect in reducing the amplitude of the transmitted stress. The blast pressure loading applied at one end of foam specimen is transmitted to the substrate as a rectangular shaped pulse. The stress amplitude of the rectangular pulse is close, but slightly higher than the crushing strength of cement foam. When a length smaller than Lcr , is used, there is an increase in the stress following the initial rectangular stress pulse. The stress enhancement can exceed the applied blast pressure amplitude when the length of foam is significantly smaller than L cr.Two approaches based on different representations of the bulk deformation of the cement foam were developed to predict the dynamic compaction of the cement foam under transient loadings. The first approach considers a rigid-plastic-plastic-locking (RPPL) idealization of the quasi-static stress-strain curve and results in a simplified one-dimensional 'shock' model. Closed form solutions for calculating Lcr and the transmitted stress when the length of foam exceeds Lcr were also obtained analytically for typical blast loadings. The second approach considers the actual stress-strain behavior of the foam, which, when combined with the conservation laws, results in a set of non-linear hyperbolic equations. A finite volume implementation of the hyperbolic conservation equations in a Lagrangian framework was developed. A Second-Order TVD version of WAF scheme based on the exact solution of the local Riemann problem successfully predicted the wave structure consisting of the elastic wave and compaction fronts propagating in foam materials undergoing dynamic compression. The transmitted stress predicted by the exact Riemann based finite volume method compares favorably with the experimental measurement when the length of the foam is both larger and smaller than Lcr. The finite volume simulation provides an insight into the wave structure in the foam following blast wave incidence and leads to an understanding of the stress enhancement phenomenon. There is a steady attenuation of the compaction front, which results in diminishing its amplitude as it travels in the material. A completely diminished compaction front results in a reduce level of transmitted when the length of foam exceeds Lcr. The reflection of a partially attenuated compaction front leads to the stress enhancement when the length of the foam is less than Lcr.A design guideline for using foam cladding to protect the structural elements made of quasi-brittle material, such as reinforced, un-reinforced concrete or masonry, for a prescribed level of projected threat from a blast is presented. The proposed design approach includes a two step process and considers damage mechanisms in two time scales associated with stress wave propagation and structural motion.
机译:建筑物在城市环境中的爆炸易感性促使结构工程界开发并实施现有结构的爆炸缓解策略。本文的工作旨在研究一种由可压缩,低强度,胶结泡沫组成的结构元件保护系统的概念。由水泥泡沫制成的覆层拟用作牺牲保护性材料,可降低施加的爆炸压力载荷传递至结构元件的应力幅度。施加的压力负载的衰减是由于材料的不可逆压实而产生的。在泡沫水泥包层结构中,可以预防或抑制主要由钢筋混凝土或砖石结构制成的结构元件的脆性破碎或剥落。进行了研究以研究水泥泡沫的准静态和动态响应。开发了用于精确测量施加的爆炸压力和通过泡沫传递到结构元件的应力脉冲的仪器。对于不同的爆炸压力载荷,评估了由不同密度和长度的水泥泡沫制成的样品。水泥泡沫表现出与孔的破碎相关的凹应力-应变关系,这导致材料的致密化。在爆炸载荷下,具有冲击型特性的压实前沿在泡沫中形成并传播。压实的前部产生一个界面,该界面将由压碎的小室组成的致密材料与未压碎的小室分开。实验结果确定了临界长度Lcr,该长度取决于泡沫的应力-应变响应,这是完全减弱施加的爆炸压力负荷所需的。当使用长度大于Lcr的泡沫时,在减小传递应力的幅度方面有明显的有益效果。施加在泡沫样品一端的冲击压力载荷以矩形脉冲的形式传输到基底。矩形脉冲的应力幅值接近,但略高于水泥泡沫的抗碎强度。当使用小于Lcr的长度时,在初始矩形应力脉冲之后应力会增加。当泡沫的长度显着小于L cr时,应力增强可能会超过施加的爆炸压力振幅。基于水泥泡沫整体变形的不同表示,开发了两种方法来预测瞬态载荷下水泥泡沫的动态压实。第一种方法考虑了准静态应力-应变曲线的刚性-塑性-塑性锁定(RPPL)理想化,并简化了一维“冲击”模型。对于典型的爆炸载荷,还可以通过分析获得用于计算Lcr和泡沫长度超过Lcr时传递应力的闭合形式解。第二种方法考虑了泡沫的实际应力-应变行为,将其与守恒定律结合起来,得出了一组非线性双曲方程。开发了在Lagrangian框架中双曲守恒方程的有限体积实现。基于局部黎曼问题的精确解的WAF方案的二阶TVD版本成功地预测了由弹性波和压实前沿在动态压缩的泡沫材料中传播的波结构。当泡沫的长度大于和小于Lcr时,通过精确的基于Riemann的有限体积方法预测的传递应力与实验测量结果相比具有优势。有限体积模拟可让您深入了解爆炸波入射后泡沫中的波结构,并有助于了解应力增强现象。压实前沿有一个稳定的衰减,这会导致其在材料中传播时振幅减小。当泡沫的长度超过Lcr时,完全减小的压实前沿导致传递水平降低。当泡沫的长度小于Lcr时,部分衰减的压实前沿的反射会导致应力增强。使用泡沫覆层保护由准脆性材料制成的结构元件(如增强,非增强)的设计准则给出混凝土或砖石建筑,以针对爆炸造成的预计威胁达到规定水平。所提出的设计方法包括两步过程,并考虑了与应力波传播和结构运动相关的两个时间尺度的损伤机理。

著录项

  • 作者

    Nian, Weimin.;

  • 作者单位

    City University of New York.;

  • 授予单位 City University of New York.;
  • 学科 Engineering General.Engineering Civil.Engineering Mechanical.
  • 学位 Ph.D.
  • 年度 2010
  • 页码 216 p.
  • 总页数 216
  • 原文格式 PDF
  • 正文语种 eng
  • 中图分类
  • 关键词

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