Unit 3 Structure-Property Relationships of Materials
Today’s materials can be classified as metals and alloys, as polymers or plastics, as ceramics, or as composites; composites, most of which are man-made, actually are combinations of different materials.
譯文：當今的材料可以分為金屬和合金，聚合物或者塑料，陶瓷或復合材料；復合材料，它們大多數是人造的，實際上是不同材料組合而成。
Application of these materials depend on their properties; therefore, we need to know what properties are required by the application and to be able to relate those specification to the material.
譯文：這些材料的應用取決於它們的性質；因此，根據應用的場合，我們需要知道什麽樣的性質是必需的，我們需要能夠把這些詳細說明同材料聯系起來。
For example, a ladder must withstand a design load, the weight of a person using the ladder. However, the material property that can be measured is strength, which is affected by the load and design dimension. Strength values must therefore be applied to determined the ladder dimensions to ensure safe use. Therefore, in general, the structures of metallic materials have effects on their properties.
譯文：比如，壹個梯子必須能經受住設計的載荷，也就是使用這個梯子的人的重量。然而，能夠測量的材料的性質是強度，它為載荷和設計尺寸所影響。強度值因此被用來判定梯子的尺寸大小以保證安全使用。因此，通常說來，金屬材料的結構對它們的性質有影響。
In a “tensile test” a sample is gradually elongated to failure and the tensile force required to elongate the sample is measured using a load cell throughout the test. The result is a plot of tensile force versus elongation.
譯文：在壹個張力測試實驗中，樣品緩慢地拉長直到失敗，並且拉長樣品所必需的拉力在整個測試過程中，用測壓元件測量。結果是拉力同伸長度之間的壹個圖。
The problem is that the load required to elongated the sample by a certain extent depends upon the dimension of the sample. This would be a big problem if , for example, mechanical property data were to be used to design a bridge, since it is clearly impossible to test an entire bridge. Thus what is clearly needed is to make the data from the tensile test independent of the size of the sample.
譯文：問題是，拉伸樣品到壹定程度所必需的載荷取決於樣品的尺寸。比如，如果機械性質的數據被用於設計壹座橋梁，既然測試整個橋梁是明顯不可能的，這樣，明顯需要的就是從與樣品尺寸無關的拉伸實驗中獲得數據。
Thus, what is clearly needed is to make the data from the tensile test independent of the size of the sample. To achieve this end, we use “stress” and “strain”.The “true” stress( ) is defined as : ,where F=force applied to the sample at any given instant and A= current cross-sectional area of the sample.