Fatigue Weaknesses Of Surfaces

Author:  Almen, J.O.
Source:  Product Engineering, Nov. 1950, p. 118-140
Doc ID:  1950001
Year of Publication:  1950
Abstract:  
The commonly accepted theories by which we try to explain fractures that occur in structural materials are based upon data obtained from static tests. These theories work well enough when aplied to the kind of loads from which they were derived, but they fail when the conditions of loading are changed. We are now slowly awakening to the fact that these theories are incomplete, inaccurate, and often misleading. The remarkable progress that has been made toward more effective use of materials in modern machine structures has been accomplished in spite of our faulty understanding of the behavior of metals under the various kinds of loads that are encountered in practice. Because of inadequate understanding of the fundamentals that are involved, designers of high duty machines cannot proceed by orderly application of engineering principles. Instead, machine parts are dimensioned by a curious mixture of engineering formulas, empirical correction factors, and plain guessing based on unorganized experience. Under the present circumstances, experience is far more important in solving machine design problems than training in engineering "fundamentals." No doubt progress will be more rapid and superior designs will be produced as we more clearly comprehend the fundamental nature of failure under each of the various methods of loading. It is hoped that the following discussion of surface weakness will help in some small measure toward a more complete understanding of problems associated with strength of materials including the role, for good or for evil, that is played by manufacturing processes.