AM-In
this paper the existing biaxial fatigue theories are reviewed. The effect of isotropy, mean stress, phase angle, and notches on biaxial fatigue is discussed. An approach based on equivalent stress is proposed. The exactness and consistency of this approach is verified with experimental results of full scale test artickr. The analysis indicates that this simple approach can be used with conhdence in predicting the linear cumulative damage in full scale structural components, which are experkncing multiaxial stress bading.
INTBODUCTION
ANY EFQCENT structural member must have three primary attributes, namely: (1) the ability to perform its intended function. (2) adequate service life. (3) capabk of being produced at reasonable cost. The first and third attributes are, in most cases, capable of accurate analysis and are given primary importance in the design stage. The second attriie, adequate service life, is usually estimated, approximately, by the available material data. The aircraft structure is the most obvious exampk where functional requirements demand light weight and, therefore, high operating stresses. The fatigue analysis then becomes of primary importance.
To ensure the safety of aircraft structures, the Civil Aeronautical Board requires aircraft structures: (a) to be demonstrated to have satisfactory fatigue strength by comparative experience; or (b) be analyzed and tested so that the major elements are shown to have an adequate fatigue strength; or (c) the structure be designed so that, if a failure does occur, it will not become catastrophic. Similar requirements are used by the Air Force for military aircrafts.
It is particularly important at this time, therefore, to realize how the current state of engineering knowledge does permit the design with structural safety. The degree of safety of any specific structure is merely a question of the degree to which the proper existing eng&&ng state-of-the-art and testing techniques can be applied economically to any design. The majority of data available to engineers is generated under uniaxial stress cycling conditions. However, most of the structural members must be able to withstand combinations of alternating and static (mean) stresses. Therefore, a precise knowledge of the manner in which combined stresses cause failure is of primary importance. The amount of available information on the fatigue characteristics of structural components experiencing combined loading is rather limited. Because of its importance in engineering application, it is highly desirable to make a study of this phenomenon, and, in particular, to be able to predict biaxial fatigue damage from basic uniaxial data.
A general state of stress in a structural component can be described by three principal stress components and their direction as an arbitrary function of time. However, in order to correlate the theoretical investigation with the experiments, it is necessary to simplify the fluctuating stress in such a way that parameters can be identified and their influence observed. Therefore, any criteria for fracture phenomenon must be based on these three principal stresses, and the component is considered to be experiencing a triaxial state of stress. It is well established that most fatigue cracks start at the surface of structural components where a biaxial state of stress exists. The three most inthrencing reasons for fatigue cracks starting at the surface under biaxial loading conditions are: (I) many parts are stressed by bending or torsion and the highest stresses occur at the surface. (2) surface stresses are increased by the unavoidable stress raisers, such as notches, grooves, holes and scratches. (3) metallurgical evidence that the crystalline grams at the surface are inherently weaker under stress because there are no restricting grains adjacent to them.
The effect of a combined state of stress on the fatigue strength of metals has been investigated sporadically since 1916. The problem has been difficult and time consuming because of the large number of tests involved, the difliculty of designing suitable testing machines, the difficulty of 731