Modern Physical Metallurgy and Materials Engineering Part 8

Tham khảo tài liệu 'modern physical metallurgy and materials engineering part 8', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Mechanical behaviour of materials 201 Figure S N curve for carburized and decarburized iron. F F 2 2 -. J 2 2 3 F L - L MR ------- ---- 2 bd2 4-Point bend Figure Bend test configurations. MoR modulus of rupture F applied force L outer span Li inner span b breadth of specimen d depth of specimen. known as the modulus of rupture MoR and expresses the maximum tensile stress which develops on the convex face of the loaded beam. Strong ceramics such as silicon carbide and hot-pressed silicon nitride have very high MoR values. The four-point loading method is often preferred because it subjects a greater volume and area of the beam to stress and is therefore more searching. MoR values from four-point tests are often substantially lower than those from three-point tests on the same material. Similarly strength values tend to decrease as the specimen size is increased. To provide worthwhile data for quality control and design activities close attention must be paid to strain rate and environment and to the size edge finish and surface texture of the specimen. With oxide ceramics and silica glasses a high strain rate will give an appreciably higher flexural strength value than a low strain rate which leads to slow crack growth and delayed fracture Section . The bend test has also been adapted for use at high temperatures. In one industrial procedure specimens of magnesia basic refractory are fed individually from a magazine into a three-point loading zone at the centre of an electric furnace heated by SiC elements. A similar type of hot-bend test has been used for the routine testing of graphite electrode samples and gives a useful indication of their ability to withstand accidental lateral impact during service in steel melting furnaces. Proof-testing is a long-established method of testing certain engineering components and structures. In a typical proof test each component is held at a certain proof stress for a fixed period of time loading and unloading .

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