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Standard Handbook of Machine Design P10

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Tham khảo tài liệu 'standard handbook of machine design p10', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | SOLID MATERIALS 7.21 Mechanical properties are discussed individually in the sections that follow. Several new quantitative relationships for the properties are presented here which make it possible to understand the mechanical properties to a depth that is not possible by means of the conventional tabular listings where the properties of each material are listed separately. 7.8 HARDNESS Hardness is used more frequently than any other of the mechanical properties by the design engineer to specify the final condition of a structural part. This is due in part to the fact that hardness tests are the least expensive in time and money to conduct. The test can be performed on a finished part without the need to machine a special test specimen. In other words a hardness test may be a nondestructive test in that it can be performed on the actual part without affecting its service function. Hardness is frequently defined as a measure of the ability of a material to resist plastic deformation or penetration by an indenter having a spherical or conical end. At the present time hardness is more a technological property of a material than it is a scientific or engineering property. In a sense hardness tests are practical shop tests rather than basic scientific tests. All the hardness scales in use today give relative values rather than absolute ones. Even though some hardness scales such as the Brinell have units of stress kg mm2 associated with them they are not absolute scales because a given piece of material such as a 2-in cube of brass will have significantly different Brinell hardness numbers depending on whether a 500-kg or a 3000-kg load is applied to the indenter. 7.8.1 Rockwell Hardness The Rockwell hardnesses are hardness numbers obtained by an indentation type of test based on the depth of the indentation due to an increment of load. The Rockwell scales are by far the most frequently used hardness scales in industry even though they are completely relative. The .