Endurance Of Machines Under A Few Heavy Loads

Author:  Almen, J. O.
Source:  Metal Progress, September, 1943, p. 435
Doc ID:  1943001
Year of Publication:  1943
Abstract:  
In the article on page 254 of last month's issue, entitled "The Useful Data to Be Derived From Fatigue Tests", a working hypothesis was established that the slope of the fatigue curve of parts made of heat treated steel, as measured on a log-log plot, may be considered a measure of effective stress. That is, if the line is flat, the effective stress is close to the amount computed for ideal shape and surface; if the line is steep, the test pieces contain stress raisers, either due to irregular surface contour or metallurgical irregularities. Fatigue curves for varying degrees of resulting stress concentrations converge toward a point near the tensile strength of the material, and located out at some considerable number of stress cycles. It will now be shown how this hypothesis has been used. It should be remembered that the tensile strength in a test in which the load is slowly increased is lower than in a tensile test in which the load is maintained for a very short time, as in a fatigue test, and also that there is a considerable variation in the tensile strength of any material as measured by a number of tensile test specimens. (See "Deformation and Fracture of Mild Steel Under Cyclic Stresses in Relation to Crystalline Structure" by Gough and Wood, Institution of Mechanical Engineers, March-October 1939). Therefore, the tensile strength on a fatigue chart would actually plot as a band and not as a line and would lie above the normal tensile value. Likewise, the lines of a fatigue plot would converge to a region above the normal tensile strength and would probably not meet at a point. However, the inclusion of these variables would considerably complicate the above hypothesis and since they occur in a region of the fatigue plot that has little or no practical value, they may, for the present, be ignored. The application of this hypothesis to the fatigue strength of machine parts has some important implications. A large variety of machine elements are constantly being tested for relative durability in the laboratories of industries engaged in the manufacture of light weight, high output machines. In most cases these fatigue tests are intended to compare one design, material or process with another design, material or process.