The largest stress for which Hooke’s law applies（适用）or the highest point on the straight-line portion of the stress-strain diagram is the proportional limit（比例极限）. The largest stress that a material can withstand without being permanently（永久地）deformed is called the elastic limit（弹性极限）. This value is seldom actually measured and for most engineering materials including structural steel is synonymous with（与..意思相同）the proportional limit. For this reason the term proportional elastic limit is sometimes used.

虎克定理适用的最大应力或者应力-应变图的直线部分的最高点是比例极限。一种材料能承受的且不发生永久变形的最大应力称为弹性极限。这个值实际上很少被测出，而且对包括结构钢在内的大多数工程材料来说，该值与比例极限同义。因此，术语弹性比例极限有时被采用。
The stress at which there is a decided（明显的）increase in the elongation or strain without a corresponding increase in stress is said to be the yield stress. It is the first point on the stress-strain diagram where a tangent to（..的切线）the curve is horizontal. The yield stress is probably the most important property of steel to the designer, as so many（很多的）design procedures are based on this value. Beyond the yield stress there is a range in which a considerable increase in strain occurs without increase in stress. The strain that occurs before the yield stress is referred to as the elastic strain; the strain that occurs after the yield stress, with no increase in stress, is referred to as the plastic strain. Plastic strains are usually from 10 to 15 times the elastic strains.
在某个应力上伸长或应变有明显的增加而应力没有相应的增加，该应力称为屈服应力。它是应力-应变图上其所在曲线的切线是水平的第一个点。对设计者来说，屈服应力可能是钢材的最重要的特性，因为很多的设计方法是基于该值的。超过屈服应力后，有一段范围中应变发生相当大的增加而应力没有增加。屈服应力发生之前的应变称为弹性应变；屈服应力发生之后的没有应力增加的应变称为塑性应变。塑性应变通常是弹性应变的10到15倍。
Yielding of steel without stress increase may be thought to be a severe disadvantage when（而）in actuality（实际上）it is a very useful characteristic. It has often performed the wonderful（出色的）service of preventing failure due to omissions（忽略）or mistakes on the designer’s part（在..方面）. Should（倒装，表示如果..） the stress at one point in a ductile steel structure reach the yield point, that part of the structure will yield locally without stress increase, thus preventing premature failure. This ductility allows the stresses in a steel structure to be readjusted（重新调整）. Another way of describing this phenomenon is to say that very high stresses caused by fabrication, erection, or loading will tend to equalize（使..平衡） themselves. It might also be said that a steel structure has a reserve（储备）of plastic strain that enables it to resist overloads and sudden shocks. If it did not have this ability, it might suddenly fracture, like glass or other vitreous（玻璃质的）substances.
没有应力增加的钢材屈服可能被认为是一个严重的缺点，而实际上它是一个非常有用的特性。它已常常有效地防止了由于设计者方面的忽略或错误而导致的破坏。如果延性钢结构中的某一点应力达到屈服点，则该结构部分将局部屈服而没有应力的增加，这样便阻止了过早破坏。延性允许钢结构中的应力进行再调整。另一种描述这个现象的方法是说由制造、安装或加荷引起的非常高的应力将往往使它们自身得到平衡。也可以说钢结构具有塑性应变的储备以使它能抵抗超载和突然的冲击。如果它不具有该能力，它可能会象玻璃或其它玻璃质的物质那样突然断裂。
Following（在..之后）the plastic strain there is a range in which additional（额外的）stress is necessary to produce additional strain. This is called strain-hardening. This portion of the diagram is not too important to today’s designer because the strains are so large. A familiar stress-strain diagram for mild or low-carbon structural steel is shown in Fig. 10-1. Only the initial part of the curve is shown here because of the great（很多的）deformation which occurs before failure. At failure in the mild steels the total strains are from 150 to 200 times the elastic strains. The curve will actually continue up to its maximum stress value and then “tail off”（减小）before failure. A sharp（急剧的）reduction in the cross section of the members takes place (called “necking”颈缩) followed by failure.
塑性应变后有一段范围，在该段范围内要产生额外的应变必须有额外的应力。这段称为应变硬化。这部分图对今天的设计者来说不太重要，因为应变太大了。一张熟悉的低碳结构钢的应力-应变图如图10-1所示（图10-1是典型的低碳结构钢在室温下的应力-应变图）。这里只显示了曲线的初始部分，因为很多的变形发生在失效之前。在低碳钢失效时，总的应变是弹性应变的150到200倍。曲线实际上将继续至它的最大应力值，然后在破坏之前应力变小。紧接着构件的破坏，其横截面会发生急剧的减小（称为颈缩）。
The stress-stain curve of Fig.10-1 is typical of（对..是典型的）the usual ductile structural steel and is assumed to be the same for members in tension or compression. (The compression members must be stocky（粗短的）because slender compression members subjected to compression loads tend to bend laterally（侧向地）, and their properties are greatly affected by the bending moments so produced.) The shape of the diagram varies with the speed of loading, the type of steel, and the temperature. One such variation is shown in the figure by the dotted line（虚线）marked upper yield（标明屈服上限）. This shape stress-strain curve is the result when a mild steel has the load applied rapidly, while the lower yield is the case for slow loading.