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本科毕业论文翻译范文

作者:第一论文网 更新时间:2015年10月26日 21:04:51

外文文献翻译
英文:
Interaction between Concrete and Reinforcement
Several aspects of interaction between concrete and reinforcing steel in reinforced concrete materials are briefly discussed in this section.
  It has been customary to consider the two constituents, concrete and steel reinforcement, as separate contributors to the overall stiffness and strength using the principle of superposition. In most practical applications, full kinematic continuity between concrete and steel is usually assumed in order to simplify the solutions. However, the two materials are highly unequal in their behavior: Young’s modulus for steel is one order higher than that of concrete, and as opposed to concrete, the stress-strain relation for steel is symmetrical in tension and compression. This lack of material compatibility results in bond failure and sliding of reinforcement bars, and cracking (RILEM, 1957).
   Several important mechanisms of reinforcement-concrete interaction are show in Fig. 1.37.  In case a, the bar is pulled while the concrete is kept in place so that sliding of the bar occurs. Such a situation can appear in a structure subjected to large shear forces (e.g. , for beams at the supports ), when high deformation gradients are present or at anchoring zones of single reinforcement bars. In a finite element analysis, the pullout effect can be modeled relatively easily by means of discrete or discrete or distributed springs which simulate the contact forces along the face of the bar (Ngo and Scordelis, 1967). Such springs must be given nonlinear characteristics in accordance with experimental findings for pullout.
  In case b, the concrete and the reinforcement are both subjected to tension so that larger cracks form. The sketch depicted shows the concrete between two such primary cracks. The opening of cracks occurs at the same time as bond failure and relative movement between the bar and the concrete takes place. The shear forces at the contact surface feed tension stresses into the concrete between the cracks. The concrete hangs on to the bar and contributes to the overall stiffness of the system. This stiffness effect, which is often called tension stiffening, may be quite significant for concrete beams under normal working loads.
 
FIGURE 1.37 interactive effects between concrete and reinforcement.
(a) Pullout effect.   (b) Tension-stiffening effect.   (c) Dowel effect.
 
FIGUE 1.38 Gradual softening to account for tension stiffening in concrete after cracking.
  Since case b corresponds to uniform tension (or bending), there is no resulting pullout effect and the model suggested for case a would not be effective here. However, this stiffening effect may be accounted for in an indirect way by assuming that the loss of tension strength in concrete appears gradually. This form of the tension stiffening effect, first introduced by Scanlon (1971), has been represented by a descending branch of the concrete stress-strain curve as shown in Fig.1.38. A more involved alternative model for the interaction in case b is discussed by Bergan and Holand (1979).
  An alternative approach of representing the stiffening effect is to increase the steel stiffness and stress. This additional steel stress corresponding to the same strain in steel represents the total tensile force carried by both the steel and the concrete between the cracks. This added stress is lumped at the level of the steel and oriented in the same direction for reasons of convenience.
  In case c of Fig.1.37, there is a major shear deformation after tension cracking first has occurred. The bars act as dowels under such conditions, taking concentrated shear forces. This dowel effect can be incorporated in a continuum model by using an equivalent shear stiffness and shear strength for the cracked concrete, just as proposed previously for the aggregate interlocking effect.

SUMMARY
  The nonlinear response of reinforced concrete material is caused by two major effects, cracking of the concrete and plasticity of the reinforcement and of the compression concrete. Only the nonlinearity due to cracking is considered in this chapter; the nonlinearity due to the plasticity of the compression concrete is studied in the following chapter. Obviously, cracking and plasticity can occur simultaneously, a situation that must be accommodated in a general constitutive model of reinforced concrete. This is discussed in Chapter 2.
  This chapter discusses the mathematical modeling of uncracked and cracked concrete based on the linear theory of elasticity. A brief summary of some typical experimental data for concrete under uniaxial, biaxial, and triaxial states of stress in first examined (Sec.1.2). those experimental results already available are found adequate as a basis for forming accurate mathematical models for the failure surface of concrete ranging from one-to five-parameter models (Sec.1.3). The usual three-parameter strength models such as Mohr-Coulomb and Drucker-Prager with tension cutoffs can be used to define failure surfaces for practical use (Sec.1.4). However, more advanced functions with five parameters that fit the experimental evidence better have also been suggested and discussed in detail in Sec.1.5. All the experimental results refer to proportional loading paths; very little is known about the failure criteria. Examples of using these failure surfaces as yield or loading functions of plasticity theory for concrete are given in this book.
  In spite of its obvious shortcomings, the linear theory of elasticity combined with criteria defining failure of the material is the most commonly used material law for uncracked and cracked concrete. The elastic-fracture formulation as described in SEC.1.6 can be quite accurate for some reinforced concrete structures such as panels and shells where the stress state is predominantly of the biaxial tension-compression type and where the tensile cracking is the major cause of the nonlinear behavior of the structure. However, these formulations fail to identify inelastic deformations, a shortcoming that becomes apparent when the material experiences unloading. This can be improved to some extent by introducing the nonlinear theory of hyper-or hypoelasticity. This is described in Chapter2.
  Another major problem is to assess what happens with the concrete after the condition of maximum carrying capacity has been reached. In one limit, for perfectly plastic response, the initial failure surface does not change its position and shape. This is known as a perfectly plastic material. In the other limit, for perfectly brittle behavior, local instabilities suddenly occur during which the initial failure surface partially collapses. In reality, there are no brittle or ductile materials; instead, media are subjected to environments under which they exhibit brittle or ductile postfailure response. A simple approach to the material degradation problem for a fractured concrete is given in Sec.1.7, where a crushing coefficient is suggested to identify the mode of failure being either a pure cracking, a pure crushing, or a mixture of the above. Another approach as suggested by Argyris et al.(1976) is that a collapse of the Mohr-Coulomb failure surface with tension cutoffs occurs after the ultimate strength constraint has been reached. The collapsed failure surface can be described as (1) a zero tension material, (2) a frictional material without cohesion, and (3) a cohesion material without friction. 
  It has been customary to consider the two constituents, concrete and steel reinforcement, as separate contributors to the overall stiffness and strength using the principle of superposition. It is normal to assume full kinematic continuity between concrete and steel, at least at nodal points or element boundaries in a finite element analysis. In reality, the two materials are highly unequal in their behavior in stiffness and strength. This lack of material compatibility results in bond failure and sliding of reinforcing bars, local deformations, and cracking. A discussion of the interaction between concrete and reinforcement is given in Sec.1.8 where the interactive effects such as dowel action, tension stiffening, and aggregate interactive effects such as dowel action, tension stiffening, and aggregate interlocking are explained physically and modeled mathematically in the form of a continuum model by using an equivalent shear stiffness and shear strength for the cracked concrete or a spring-type model by assuming that the loss of tension strength in concrete appears gradually.
  In finite element analysis of concrete structures, two principally different approaches have been employed for crack modeling. The most customary procedure is to assume that cracked concrete remains a continuum; that is, the cracks are “smeared out” in a continuous fashion. This continuous model for cracking is used in Sec.1.9.2 to study the crack propagation of a thick-walled concrete cylinder under increasing internal pressure. In this sample problem, the engineering fracture theory is applied using brittle, ductile, and intermediate collapse models, together with stress transfer rules for fractured elements.
  An alternative to the continuous models is the introduction of discrete cracks. This is normally done by disconnecting displacement parameters for adjoining elements. This approach is applied in Sec.1.9.1 to study the nonlinear behavior of reinforced concrete beams. The difficulty of such an approach is that the location and orientation and orientation of the cracks are not known in advance. Thus geometric restrictions imposed by the preselected finite element mesh can hardly be avoided.


中文:
混凝土和钢筋之间的相互作用
    本节间要论述钢筋混凝土材料中混凝土和钢筋之间相互作用的几个方面。
习惯上通常采用叠加原理考虑把混凝土和钢筋看作分别对总体刚度和强度做出贡献的两个组成部分。在大多数实际应用中,为了简化求解,通常假定混凝土和钢筋之间是运动连续的。然而,这两种材料的性质大不相同:钢的杨氏模量比混凝土的高一个数量级,并且钢在拉伸和压缩下的应力—应变关系是一样的,这与混凝土不同。材料的不相容导致发生钢筋粘着破坏和滑移、局部变形以及开裂(RILEM,1957)。
图1.37显示钢筋—混凝土相互作用的几个重要机理。在图(a)中,当混凝土固定在适当位置,拉拔钢筋时钢筋就会发生滑动。在承受巨大剪力(例如梁的支承处)的结构中,当高变形梯度出现时或在单根钢筋的锚固区,就会出现这种情形。在有限元分析中,可采用离散或分散的弹簧模拟沿钢筋面的接触的方法比较容易的模拟拔出效应(Ngo和Scordelis,1967)。依据拔出试验结果,这样的弹簧必须具有非线性特征。
在图(b)中,混凝土和钢筋两者都受拉以致形成大裂纹。简图中示意画出了两条主裂纹之间的混凝土。裂纹张开同钢筋与混凝土之间的粘着破坏和相对运动同时发生。接触面上的剪力把拉应力传入裂纹之间的混凝土。混凝土紧紧握裹钢筋提升系统的总刚度。通常称这个刚度效应为拉伸强度化,它对于正常工作荷载下的混凝土梁是十分重要的。
 

图1.37  钢筋与混凝土之间相互作用效应
(a)拔出效应;(b)拉伸强化效应;(c)榫合效应

 
图1.38   混凝土开裂后拉伸强化的逐步软化现象

由于图(b)相当于均匀拉伸(或弯曲),没有拔出效果,因而为图(a)提出的模型在此不适用。不过,通过假设混凝土逐步出现抗拉强度的损失可以间接考虑强化效应。首先由Scanlon (1971) 引入的这一拉伸强化效应形式已于图1.38中以混凝土应力—应变曲线的下降部分表示。针对图(b)中的相互作用,Bergan 和 Holand (1979) 论述了更多的可供选择的模型。
另一个表示强化效应的方法是增加钢的刚度和应力。对应于钢中同样应变的附加应力,代表由钢和裂纹间的混凝土两者传递的总拉力,为方便起见,这一附加应力集中在钢的同样位置上并朝向同一方向。
在图1.37的图(c)中,在拉伸开裂最初发生以后有一个较大的剪切变形。在此情况下,钢筋作为接榫承受集中剪力。正如先前为骨料连锁效应提出的那样,对于开裂混凝土可以使用等效剪切刚度和剪切强度将这个榫合效应结合起来成为一个连续体模型。

总结
钢筋混凝土材料的非线性响应是由混凝土开裂、钢筋及受压混凝土塑性这两种主要原因引起的。本章只考虑由开裂引起的非线性,下一章将研究受压混凝土塑性引起的非线性。显而易见,开裂和塑性可能同时产生,在一般钢筋混凝土的本构关系中必须包含这一情况。这将在第2章中论述。
本章根据线弹性理论论述了未开裂和开裂混凝土的数学模拟。在单轴、双轴和三轴应力状态混凝土一些典型实验数据的简略总结首次得到检验(1.2节)。那些已经是可靠的实验结果,适合作为建立从一参数到五参数模型(1.3节)的混凝土破坏面精确数学模型的依据 。在实际应用中,可以用拉伸断裂的Mohr-Coulomb 和Drucker-Prager 那样常用的三参数强度模型来确定破坏面(1.4节)。不过,在1.5节中还提到并详述了与实验验证符合较好的五参数更高级的函数。所有实验结果皆归属于比例加载方式。在破坏准则方面,与高应力循环有关联的一般加载历史的影响知之甚少。本书中将给出使用这样破坏面作为混凝土屈服或塑性理论家在函数的例子。
尽管存在明显的缺点,但线弹性理论与确定材料破坏的准则联合仍然是未开裂和开裂混凝土最普通使用的材料准则。对于一些应力状态主要是双轴拉-压型,拉伸开裂为结构非线性现象首要原因的钢筋混凝土结构,诸如板和壳一类,1.6节中论及的弹性断裂公式可能是很精确的,不过,这些公式不能鉴别非弹性变形,当材料经历卸载时,这个缺点就更明显。这可以通过引入超弹性或低弹性非线性理论,有一定程度的改进,将在第2章论述。
另一重要问题是判断混凝土在达到最大承载力状况以后会发生什么。一个极端是理想的塑性响应,初始破坏面并没有改变其位置和形态,这种材料视为理想的塑性材料;另一个极端是理想的脆性响应,在初始破坏面部分崩溃期间突然发生局部失稳。实际上,没有脆性或塑性材料,只存在一定外界条件下表现脆性或塑性破坏响应的材料。1.7节给出开裂混凝土材料简化的一个简单方法,其中提出了压碎系数来鉴别破坏模式是纯开裂、纯压碎还是两者混合。另一个由Argyris 等(1976)提出的方法是拉伸断裂的Mohr-Coulomb破坏面的崩溃在极限强度限制已经达到之后发生。崩溃的破坏面可描述为(1)零伸长材料;(2)无内聚力的摩阻材料;(3)无摩阻的黏性材料。
习惯上常用叠加原理,把混凝土和钢筋这两种组成材料看作分别对总刚度和强度起作用。在有限元分析中通常假定在混凝土和钢筋之间,至少在节点或单元边界上运动时连续的。实际上,这两种材料在其刚度和强度方面的性质上是很不相同的,这种材料的不相容导致钢筋的粘结破坏和滑移、局部变形和开裂。在1.8中曾对混凝土和钢筋间的相互作用作了讨论并对榫合、拉伸强化及骨料连锁等效应做了物理解释和数学模拟。在数学模拟中,对开裂的混凝土模拟是采用等效剪切强度与等效剪切刚度的连续体模型或是采用弹簧模型。而弹簧模型的建立是基于混凝土拉伸强度是逐渐丧失的假定的。
在混凝土结构的有限元分析中,两种原理上不同的方法已应用于开裂模拟。最惯用的方法是假设开裂混凝土保持为连续统一体;既裂纹是以连续模式“模糊”地表现出来的。1.9.2节使用开裂连续模型研究厚壁混凝土筒体在不断增大内压下的裂纹扩展。在这一例题中,工程断裂理论和断裂单元应力转移规则一起用于脆性、延伸和中间破坏模型。
另一个可供选择的连续模型是离散开裂模式的导入,通常这是通过连接单元间有不相连的位移来实现的。1.9.1节应用这一方法来研究钢筋混凝土梁的非线性特性,难点在于不知道下一步裂纹位置和方向,因此就不可避免地用预先给定的有限元网格来强加几何限制。