过程装备与控制工程专业英语翻译 17.doc

Reading Material 17Stress CategoriesThe various possible modes of failure which confront the pressure vessel designer are:(1) Excessive elastic deformation including elastic instability.(2) Excessive plastic deformation.(3) Brittle fracture.(4) Stress rupture/creep deformation (inelastic).(5) Plastic instability-incremental collapse.(6) High strain-low cycle fatigue. (7) Stress corrosion.(8) Corrosion fatigue.In dealing with these various modes of failure, we assume that the designer has at his disposal a picture of the state of stress within the part in question. This would be obtained either through calculation or measurements of the both mechanical and thermal stresses which could occur throughout the entire vessel during transient and steady state operations. The question one must ask is what do these numbers mean in relation to the adequacy of the design? Will they insure safe and satisfactory performance of a component? It is against these various failure modes that the pressure vessel designer must compare and interpret stress values. For example, elastic deformation and elastic instability (buckling) cannot be controlled by imposing upper limits to the calculated stress alone. One must consider, in addition, the geometry and stiffness of a component as well as properties of the material.The plastic deformation mode of failure can, on the other hand, be controlled by imposing limits on calculated stresses, but unlike the fatigue and stress corrosion modes of failure, peak stress does not tell the whole story. Careful consideration must be given to the consequences of yielding, and therefore the type of loading and the distribution of stress resulting therefrom must be carefully studied. The designer must consider, in addition to setting limits for allowable stress, some adequate and proper failure theory in order to define how the various stresses in a component react and contribute to the strength of that part.As mentioned previously, different types of stress require different limits, and before establishing these limits it was necessary to choose the stress categories to which limits should be applied. The categories and sub-categories chosen were as follows:A. Primary Stress.(a) General primary membrane stress.(b) Local primary membrane stress.(c) Primary bending stress.B. Secondary Stress.C. Peak Stress.The major stress categories are primary, sec9ondary, and peak. Their chief characteristics may be described briefly as follows:(a) Primary stress is a stress developed by the imposed loading which is necessary to satisfy the laws of equilibrium between external and internal forces and moments. The basic characteristic of a primary stress is that it is not self-limiting. If a primary stress exceeds the yield strength of the material through the entire thickness, the prevention of failure is entirely dependent on the strain-hardening properties of the material.(b) Secondary stress is a stress developed by the self-constraint of a structure. It must satisfy an imposed strain pattern rather than being in equilibrium with an external load. The basic characteristic of a secondary stress is that it is self-limiting. Local yielding and minor distortion can satisfy the discontinuity conditions or thermal expansions which cause the stress to occur.(c) Peak stress is the highest stress in the region under consideration. The basic characteristic of a peak stress is that it causes no significant distortion and is objectionable mostly as a possible source of fatigue failure. The need for dividing primary stress into membrane and bending components is that, as will be discussed later, limit design theory shows that the calculated value of a primary bending stress may be allowed to go higher than the calculated value of a primary membrane stress. The placing in the primary category of local membrane stress produced by mechanical loads, however, requires some explanation because this type of stress really has the basic characteristics of a secondary stress. It is self-limiting and when it exceeds yield, the external load will be resisted by other parts of the structure, but this shift may involve intolerable distortion and it was felt that must be limited to a lower value than other secondary stresses, such as discontinuity bending stress and thermal stress.Secondary stress could be divided into membrane and bending components, just as was done for primary stress, but after the removal of local membrane stress to the primary category, kit appeared that all the remaining secondary stresses could be controlled by the same limit and this division was unnecessary.Thermal stress are never classed as primary stresses, but they appear in both of the other categories, secondary and peak. Thermal stresses which can produce distortion of the most complete suppression of the differential expansion, and thus cause no significant distortion, are classed as peak stresses.One of the commonest types of peak stress is that produced by a notch, which might be a small hole or a fillet. The phenomenon of stress concentration is well-known and requires no further explanation here. Many cases arise in which it is not obvious which category a stress should be placed in, and considerable judgment is required. In order to standardize this procedure and use the judgment of the writers of the Code rather than the judgment of individual designers, a table was prepared covering most of the situations which arise in pressure vessel design and specifying which category each stress must be placed in.The potential failure modes and various stress categories are related to the Code provisions as follows:(a) The primary stress limits are intended to prevent plastic deformation and to provide a nominal factor of safety of the ductile burst pressure.(b) The primary plus secondary stress limits are intended to prevent excessive plastic deformation leading to incremental collapse, and to validate the application of the elastic analysis when performing the fatigue evaluation.(c) The peak stress limit is intended to prevent fatigue failure as a result of cyclic loading.(d) Special stress limits are provided for elastic and inelastic instability.Protection against brittle fracture are provided by material selection, rather than by analysis. Protection against environmental conditions such as corrosion and radiation effects are the responsibility of the designer. The creep and stress rupture temperature range will be considered in later condition.阅读材料17应力种类压力容器设计者们面临的各种各样可能的破坏模式包括：(1) 过度的弹性变形，包括弹性失稳；(2) 过度的塑性变形；(3) 脆性弯折；(4) 压力破坏/逐渐变形（非弹性）；(5) 塑性失稳-递增破坏；(6) 高应变低周期疲劳；(7) 应力腐蚀；(8) 腐蚀疲劳；为了解决这种种失效问题，我们假设设计者们在心里已经形成了问题部件的受力图。这可以通过计算或测量瞬变或连续操作时在整个容器上产生的机械或热应力获得。有人肯定会问：这些数字对设计来说意味着什么？它们能保证部件安全有效地运转吗？面对这种种的失效模式，设计者们必须比较和解释这些应力值。例如,弹性变形和弹性失稳（褶皱）不能仅归结于施加其上的限制应力。除材料的属性外，必须还考虑到部件的几何性质和刚度。塑性变形，从另一方面来说，可以由施加于其上的应力控制，但不同于疲劳和应力腐蚀破坏，峰值应力并不能解释所有的问题。还必须仔细考虑到产量的问题，所以必须仔细研究负载的类型和由此产生的应力分布。除了在构件上施加允许的限制应力外，设计者们还必须考虑足够的可能的失效理论来解释构件内的复杂的应力是如何相互作用，如何对构件产生力的。正如之前提到的，不同的铰支会产生不同的应力，在建立这些铰支前，有必要选择应力的种类来决定需要用何种铰支。应力的种类和其分支的选择如下：A. 主应力(a) 普遍的主要薄膜应力。(b) 内部主要薄膜应力。(c) 主要的弯曲应力。B. 副应力.C. 最大应力.主要的应力种类即主应力、副应力、最大应力。它们的主要特征可以简要地描述如下：(a) 主应力是由附加的满足内外受力和力矩平衡方程的必要负载产生的。主应力的主要特征是自身不受限制。如果主应力在整个厚度上的值超过材料的承受能力的话，防止破坏就全部依靠材料的应变刚度属性了。(b) 副应力是构件的自身约束产生的。它必须满足附加的应力模式而不仅仅是外部的负载平衡。副应力的基本特征是自身受限制。局部屈服和小变形可以满足不连续条件或者因热产生的应力。(c) 最大应力是允许条件下的最大应力。其基本特征是它不产生显著地变形，但是引起疲劳破坏最有可能的原因。需将主应力划分入薄膜和弯曲应力部分的时候，就是稍后即将讨论的，限制设计理论表明：计算的薄膜弯曲主应力可以比主要的薄膜应力高时。由机械载荷产生的自身薄膜应力的分类需要一些解释，因为这种应力确实有副应力的特征。它受自身限制，并且当它超过屈服值时，外部载荷就会被构件的其他部分抵消，但这种变化也许会包括不可改变的变形且似乎其必须小于其他副应力，比如不连续的弯曲应力和热应力。副应力可以被划分到薄膜和弯曲应力中，正如主应力那样，但将自身薄膜应力划分移到主应力中后，似乎剩下的所有副应力都可以由同一种限制控制，因此这种划分没必要。热应力从未被划入主应力中，但它们出现在另外两种类型中，副应力和最大应力。能够通过大部分抑制小膨胀而产生变形，以及不会引起严重变形的热应力，被归为最大应力。最大应力中有一种最常见的是由槽口产生的，槽口可能是一个小孔或一个圆角。这种应力集中的现象很常见，不需要更多的解释。很多情况下要把应力划分入哪种类型并不是很明显，此时就需要更仔细地判断。为了规范这个步骤，运用规范作者的判断而不是每个单独设计者的判断，出现了一份涵盖在绝大多数情况下该怎样设计和将应力归结为何种类型的表格。潜在的破坏形式和各种应力类型均和一下提供的规范有关：(a) 主应力限制的目的是防止塑性变形，并为抵抗韧性爆裂压力提供提供一个名义安全因素；(b) 主应力和副应力组合限制是用以防止过度的塑性变形导致的递增破裂，并用以确认疲劳评价是弹性分析的应用；(c) 最大应力是用以防止周期载荷的疲劳失效；(d) 特殊应力限制是用以防止弹性和非弹性失稳。脆性弯折的保护是由材料的选择提供的，而不是分析。针对环境条件的保护措施诸和辐射等是设计者的责任。渐变和应力破裂温度范围是在其他条件下要考虑的。