Fatigue Loading and Laboratory Testing

Important for designers involved in fatigue-sensitive product or structure, is the fatigue response of materials to various loadings that might occur throughout the design life of the part being manufactured. That is, the designer is interested in the effects of various loading and associated stress that will in general be a function of the design configuration and the operational use of the part. The simplest fatigue stress spectrum to which an element may be subjected is a zero-mean sinusoidal stress-time pattern of constant amplitude and fixed frequency, applied for a specified number of cycles. Such a stress-time pattern is often referred to as a completely reversed cyclic stress.

It is reported that the more complicated stress-time patterns are produced when the mean stress, stress amplitude, or both mean and stress amplitude change during the operational cycle. This type of quasi-random stress-time pattern might be encountered in an airframe structural member during takeoff, maneuvers, and landing. Instrumentation of existing machines, such as operational aircraft, provide some useful information to the designer if their mission is similar to the one performed by the instrumented machine. Recorded data from accelerometers, strain gages, and other transducers may in any event provide a basis from which a statistical representation can be developed and extrapolated to future needs if the fatigue processes are understood.

Basic data for evaluating the response of materials, parts, or structures are obtained from carefully controlled laboratory tests. There are fatigue-testing machines that range from being very simple to very complex. Various types of testing machines and systems are used. As an example computer-controlled fatigue testing machines are widely used in modern fatigue testing laboratories. Usually such machines take the form of precisely controlled hydraulic systems with feed-back to electronic controlling devices capable of producing and controlling virtually any strain-time, load-time, or displacement-time pattern desired.

Special testing machines for component testing and full-scale prototype testing systems are not found in the general fatigue-testing laboratory. These systems are built up especially to suit a particular need, for example, to perform a full-scale fatigue test of a bridge structure. For example, the very complex testing systems used to test a full-scale prototype, produce very specialized data applicable only to the particular prototype and test conditions used. For the particular prototype and test conditions the results are very accurate, but extrapolation to other test conditions and other pieces of hardware is usually difficult, if not impossible.

Simple smooth-specimen laboratory fatigue data are very general and can be utilized in designing virtually any part of the specimen material. To use such data in practice requires a quantitative knowledge of many pertinent differences between the laboratory and the application, including the effects of non-zero mean stress, varying stress amplitude, environment, size, temperature, surface finish, residual stress pattern, and others. Fatigue tesdng is performed at the extremely simple level of smooth specimen testing, the extremely complex level of full-scale prototype testing, and everywhere in the spectrum between.

0 0

Post a comment