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Electrical Engineering Mechanical Systems Design Handbook Dorf CRC Press 2002819s_4

Tham khảo tài liệu 'electrical engineering mechanical systems design handbook dorf crc press 2002819s_4', kỹ thuật - công nghệ, điện - điện tử phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | FIGURE 7.8 Generic moment curvature diagram showing curvature springback AK after unloading from the loaded curvature KL. FIGURE 7.9 Simple matched tool forming over a cylinder. No edge constraint is used so the sheet sees only a bending moment if no interface friction is assumed. Because the tooling imposes a fixed input curvature the maximum moment output is determined by the constitutive relationship of the material most importantly the yield stress and the thickness. The modulus E is most nearly constant but the moment of inertia I varies with thickness to the 3rd power. Not surprisingly in practice it is found the most sensitive parameters with respect to springback are the thickness the yield stress and the post-yield strain-hardening properties of the sheet. 7.2.2.1 Material Variations The most common variations in sheet material are the thickness yield stress and plastic flow properties. The thickness can vary owing to rolling mill variations and while some stock such as aluminum beverage can stock can be rolled to very low variations 0.0002 in. larger material can vary considerably. In some thicker material and up into plates of thickness 0.5 in. material specifications often call for only maintaining a minimum thickness for minimum service strength but have a very broad tolerance on maximum thickness. Perhaps more insidious from a process control perspective is variation of the constitutive properties of the sheet. If we imagine a linearly strain-hardening material there are at least three parameters of concern the elastic modulus E the yield stress ƠỊ and the equivalent plastic modulus EP Because the modulus E depends primarily on the crystalline structure of the material it is nearly constant for a given material independent of the particular alloy or working history. However both ơy and EP are very sensitive to the chemistry heat-treating and cold working history of the piece. Variations in ơy of up to 20 from supplier to supplier for a given alloy .