2 simple stress and strain.ppt

机械工程专业英语English In Mechanical Engineering Prepared by liuwen From College of Mechanical Engineering Chongqing University Lesson 2 Simple Stress and Strain In any engineering structure the individual components or members will be subjected to external forces arising from the service conditions or environment in which the component works. If the component or member is in equilibrium, the resultant of the external forces will be zero but, nevertheless, they together place a load on the member which tends to deform that member and which must be reacted by internal forces set up within the material. There are a number of different ways in which load can be applied to a member. Loads may be classified with respect to time: Lesson 2 Simple Stress and Strain A static load is a gradually applied load for which equilibrium is reached in a relatively short time. A sustained load is a load that is constant over a long period of time, such as the weight of a structure. This type of load is treated in the same manner as a static load; however, for some materials and conditions of temperature and stress, the resistance to failure may be different under short time loading and under sustained loading. An impact load is a rapidly applied load (an energy load). Vibration normally results from an impact load, and equilibrium is not established until the vibration is eliminated, usually by natural damping forces. A repeated load is a load that is applied and removed many thousands of times. A fatigue or alternating load is a load whose magnitude and sign are changed with time. Lesson 2 Simple Stress and Strain It has been noted above that external force applied to a body in equilibrium is reacted by internal forces set up within the material. If, therefore, a bar is subjected to a uniform tension or compression, i.e. a force, which is uniformly applied across the cross-section, then the internal forces set up are also distributed uniformly and the bar is said to be