THE old methods of structural and machine design, based on large doses of empirical formulas reinforced by factors of safety, are rapidly becoming outmoded in the fields of airplane and automobile engineering, due to the increasing importance of weight saving requirements, and new methods involving accurate stress measurements are being demanded, Mr. Lipson emphasizes.
The author describes four methods of experimental stress analysis that are used in the Chrysler laboratories, namely: photoelasticity, Stresscoat, extensometers, and electric strain gages. He also shows how they are applied in given engine problems involving evolution of a new design, determination of the cause of failure, and a comparison between various designs.
In the utilization of the four methods of experimental stress analysis described, there are two principal means of procedure: static tests, using simulated service loading, and dynamic tests, under actual service conditions.
Static load testing, Mr. Lipson says, is usually the most convenient and generally favored because the problem of instrumentation is greatly simplified. The penalty for this simplification lies in the fact that the investigation is often conducted on the basis of loads that may or may not correspond to operating conditions. Dynamic load testing demands more complex instrumentation and so far it is less thoroughly developed. A comprehensive stress analysis investigation requires both phases of testing: dynamic tests should establish the mode and magnitude of operating loads, while static measurements will determine the corresponding stresses. Refinements on instrumentation and technique are necessary to promote greater accuracy and speed but this particular phase of experimental stress analysis is developing in a satisfactory manner.