Surface Rolling and Other Methods for Mechanical Prestressing of Metals - SAE J811 AUG81

Author:  SAE
Source:  SAE: J 811 Aug81
Doc ID:  1981093
Year of Publication:  1981
Abstract:  
1.1 INTRODUCTION: The word "prestressing" implies that a stress is applied prior to service. for the pruposes of this discussion this is a true but insufficient, definition. It must be extended to say, by virtue of a localized pressure on the surface of a part, that the surface of the part in the vicinity is stressed in tension beyond its elastic limit. When the pressure is removed, the surface elements tend to retain part of the total deformation experienced under pressure. Since this is resisted be subsruface layers which did not exceed the elastic limit, the surface and adjacent layers are left in a state of compressive residual stress. the two most widely used methods of mechanical prestressing probably are surface rolling and shot peening. Since the process of shot peening has been rather widely discussed in previously published leterature, the greater part of this manual is concerned with surface rolling and its theory, load specification, tooling, control and effects. Methods briefly considered include hammer peening, cold pressing, and treatment of small holes in balls or tapered pins. The general aims of this manual are: 1) To give the reader a general understanding as to what mechanical prestressing is and whether it may be expected to help him with his product. 2) To help him choose a process. 3) To help him get started in tool design and preparation of test samples. At this stage of development of the art, the optimum prestressing conditions and degree of performance improvement should be established by objective and destructive tests, unless one has ample previous experience on similar materials and products. Mechanical prestressing methods affect the surface layers of a part in at least three ways, the relative amounts being affected by the process and the material: 1) Compressive residual stresses. 2) Cold work or strain hardening. 3) surface geometry or finish. It is usually quite difficult to assess the individual contributions of these effects on the improvement in performance attained. The consensus, however, is that the compressive residual stress is the most potent of the three. These effects are discussed in later sections of this reprot. While mechanical prestressing methods are now used in many various industries, most of their development and application has occurred in the transportation industry. This might be attributable to the intense competition which forster development work, continually driving toward the attainment of maximum strength in minimum space and weight with low cost alloys and processing. In meeting these goals, prestressing methods have made some of their most impressive accomplishments. The following list of parts is not intended to be comprehensive, but rather gives an idea as to the variety of parts where worthwhile gains in performance have been obtained: 1) Aircraft- Propellers, engine parts, wheels. 2) Maarine- Propeller shafts, engine crankshafts. 3) Automotive- Coil and leaf springs, torsion bars, front axle spindles, crankshafts, wheels. 4) Railroad- Car axles. Significant increases in performance have been obtained by prestressing techniques. Under certain conditions, the fatigue strength of specimens has been more than doubled (3). Improvements of this order are not possible in all fatigue situations, nor in all materials. Furthermore, where they are possible, the optimum prestressing conditions must be worked out by objective performance tests. Once these are established and production specificaitons are set up, the processing engineer must insist that these specifications be met on every piece processed. Mechanical prestressing is unique because there are not simple, nondestructive, or even destructive, tests which can be routinely used to check whether a particular part has been properly prestressed, although certain laboratory methods can be used to measure residual stress. One must inspect the process rather than the part. Even then, subsequent processing steps can reduce or cancel the benefits obtained. these steps could include honing or other finishing, straightening, overheating and so forth.