Tham khảo tài liệu 'plastics engineered product design part 6', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 180 Plastics Engineered Product Design Fig. is cut at an arbitrary cross-section and one part removed. To keep the body at rest there must be a system of forces acting on the cut surface to balance the external forces. These same systems of forces exist within the uncut body and are called stresses. Stresses must be described with both a magnitude and a direction. Consider an arbitrary point p on the cut surface in the figure where the stress s is as indicated. For analysis it is more convenient to resolve the stress s into two stress components. One acts perpendicular to the surface and is called a normal or direct stress Ơ. The second stress acts parallel to the surface and is called a shear stress T. Plastic materials subjected to a constant stress can deform continuously with time and the behavior under different conditions such as temperature. This continuous deformation with time is called creep or cold flow. In some appfications the permissible creep deformations are critical in others of no significance. But the existence of creep necessitates information on the creep deformations that may occur during the expected fife of the product. Materials such as plastic RP zinc and tin creep at room temperature. Aluminum and magnesium alloys start to creep at around 300 F. Steels above 650 F must be checked for creep. There are three typical stages. The initial strain takes place almost immediately consisting of the elastic strain plus a plastic strain near its end if the deformation extends beyond the yield point. This initial action in the first stage shows a decreasing rate of elongation that can be called strain hardening as in metals . The action most important to the designer s working area concerns the second stage that is at a minimum strain rate and remains rather constant. In the third stage a rapid increase in the creep rate occurs with severe specimen necking thickness reduction and ultimately rupture. It is important for the designer to work in the
