Công nghệ cơ khí hay kỹ thuật cơ khí là ngành ứng dụng các nguyên lý vật lý để tạo ra các loại máy móc và thiết bị hoặc các vật dụng hữu ích. Cơ khí áp dụng các nguyên lý nhiệt động lực học, định luật bảo toàn khối lượng và năng lượng để phân tích các hệ vật lý tĩnh và động, phục vụ cho công tác thiết kế trong các lĩnh vực như ô tô, máy bay và các phương tiện giao thông khác, các hệ thống gia nhiệt và làm lạnh, đồ dùng gia đình, máy. | EM 1110-2-6054 1 Dec 01 Figure 7-11. Curves for fatigue life of a stiffening member with a surface crack 1 in. cm 1 ksi MPa 7-21 EM 1110-2-6054 1 Dec 01 a. Structural steel standards. 1 In the 1930 s when many hydraulic steel structures were designed and built several structural steels were commonly in use. In the mid-1930 s structural steel could have been either ASTM A7-33T or ASTM A9-33T steel Ferris 1953 . A7 steel was generally regarded at the time as a steel for bridges whereas A9 steel was a steel for buildings. The primary differences between the two were that A7 steel had a lower maximum allowable phosphorus content and had a limit on sulfur content compared with A9 steel. A7 steel also was restricted to open-hearth or electric-furnace production and excluded the older acid-bessemer production. These compositional and production restrictions suggest that A7 bridge steel was recognized as the premium steel of the two. For a brief period 1932-33 structural steel also could have been supplied as ASTM A140 steel which was a tentative replacement for both A7 and A9 steels Ferris 1953 . 2 Steel identified as silicon steel on design drawings is mostly likely ASTM A94-25T structural silicon steel. This was a high-strength steel with a specified minimum silicon content that attained its high strength minimum yield point of 310 MPa 45 ksi and tensile strength of 552 to 655 MPa 80 to 95 ksi through a high level of carbon percent maximum . It also had limits on its phosphorus and sulfur contents. 3 An important characteristic of the early steels regardless of whether they were A7 A9 A140 or A94 silicon steel is that they had either no specified level or a high level of carbon in their composition. Consequently the carbon level was either not rigorously controlled or was moderately high with the result that the steels probably had and have only poor to fair weldability. The specification for A94 structural silicon steel specifically limits welding and .
