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Yield and Plastic Flow

Trong tổng quan của chúng ta về đường cong căng thẳng căng thẳng căng trong Module 4, chúng tôi mô tả sản lượng như là một phân tử sắp xếp lại vĩnh viễn bắt đầu tại một căng thẳng đủ cao, biểu hiện σY hình. 1. Các quá trình năng suất là rất phụ thuộc vào vật liệu, được liên quan trực tiếp đến phân tử di động. | Yield and Plastic Flow David Roylance Department of Materials Science and Engineering Massachusetts Institute of Technology Cambridge MA 02139 October 15 2001 Introduction In our overview of the tensile stress-strain curve in Module 4 we described yield as a permanent molecular rearrangement that begins at a sufficiently high stress denoted ƠY in Fig. 1. The yielding process is very material-dependent being related directly to molecular mobility. It is often possible to control the yielding process by optimizing the materials processing in a way that influences mobility. General purpose polystyrene for instance is a weak and brittle plastic often credited with giving plastics a reputation for shoddiness that plagued the industry for years. This occurs because polystyrene at room temperature has so little molecular mobility that it experiences brittle fracture at stresses less than those needed to induce yield with its associated ductile flow. But when that same material is blended with rubber particles of suitable size and composition it becomes so tough that it is used for batting helmets and ultra-durable children s toys. This magic is done by control of the yielding process. Yield control to balance strength against toughness is one of the most important aspects of materials engineering for structural applications and all engineers should be aware of the possibilities. Figure 1 Yield stress ƠY as determined by the 0.2 offset method. Another important reason for understanding yield is more prosaic if the material is not allowed to yield it is not likely to fail. This is not true of brittle materials such as ceramics that fracture before they yield but in most of the tougher structural materials no damage occurs before yield. It is common design practice to size the structure so as to keep the stresses in the elastic range short of yield by a suitable safety factor. We therefore need to be able to predict 1 when yielding will occur in general multidimensional .