Creep Tension Tests on Hastelloy Alloy X Bar, Predicting its Behavior and Applying the Data in a Design Problem

Creep is a continuing deformation at elevated temperatures under constant stress. Creep data are important in the design of many machine members subjected to high temperatures. Examples of these would be steam and gas turbine components, thermal cracking equipment in oil refineries, jet air-craft, missiles, rockets and mobile reactors. The second law of thermodynamics shows us that the efficiency of an ideal heat engine can be increased by increasing the temperature of the working fluid. The optimum strength-to-weight ratio is an essential requirement in aircraft structural design. But all these facts bring us the problem of higher creep rates for the same stresses. In some applications, the permissible creep deformations are critical, e.g. Allowance for creep is important in turbine blades for proper operation to maintain small clearances between the moving and stationary parts and in others no significance. The creep phenomenon has been known for the last 80 years, but knowledge of the phenomenon has advanced rapidly since World War II, due to more demand for high temperature materials, resulting in today’s successful new alloys. Creep deformation can be determined in the laboratory with a testing machine but conducting such tests for the entire life of the machine element is impracticable. As we know, the operating characteristics of an original model can be obtained from a small scale model by means of dimensional analysis. The idea was to have lead creep curves similar to those of the original material in order to obtain direct extrapolation from low temperature lead tests to operating conditions. Cases occur where parts are subjected to combine bending and direct load. For example, turbine blades are subjected to a b3ending action due to the transverse loading applied by the steam, and to a direct load due to centrifugal force. Under creep conditions, parts subjected to this combined straining action when the predominant action is bending are much more favorably stressed than would be the case if the material were elastic and when the predominant action is direct load the stress conditions will approach those for elastic condition. So analyzing the forces acting on a machine element is an important step in a design problem