Research On Method For Stress Intensity Factor Calculation Of MSD Structures

Multiple site damage (MSD) is one of the important characters of aging aircraft subjected to long-term fatigue, whose existence would obviously reduce the residual strength, shorten the crack growth life and debase the safety capacity. MSD would be fatal threat to the integrity of the aircraft structure, so finding out the development law of MSD is very important and full of application value in engieering. When a structure with MSD is evaluated through damage tolerance technique, the first important step is to calculate the stress intensity factor (SIF) at its crack tip. Then the characters of crack propagation, crack linkup law and the residual strength of the structure can be assessed. Finite element method (FEM), is computationally expensive, to perform SIF calculations when a little more complex MSD configuration is considered. Traditional cross-term method has very complicated steps to figure out the complex stress functions. Compouding method has very simple calculating process, but the lack of information on cracks emanating from multiples makes the calculating process difficult to be programmed.The proposed complex function method is used to calculate the SIF of cracks emanating from multiple holes. Combining the method with compounding method, the SIF calculation of finite MSD structures was achieved. The calculation process is easy to operate and the results are reliable which can be calculated by programmes. In addition, fatigue life prediction of MSD structures was also operated in this paper. By studying the SIF and crack growth rules of MSD structures, a series of conclusions with practical value in engineering can be obtained.The research details are as follows:(1) Discussed various methods for SIF calcaulating including the complex function method. Analysed the merits and demerits of every method concerned in its calculating process and results, also the applicability of calculating SIF of MSD structures.(2) Proposed an analytical method for SIF calculating of infinite MSD structure. The properties of complex functions are used to evaluate the stress function. The approximate superposition method is applied to solve SIF problems on multiple holes. The calculating process of the method is simple and the calculating results are almost accurate especially for longer cracks.(3) Based on the complex function method proposed earlier in this paper, a modified complex function method was brought forward to solve SIF problem on infinite MSD structures. This modified method aims to remove the errors arising from the calculating process in the complex method. In this way, the SIF of every crack with all lengths would be calculated. The resultant hole-egde crack is substituted by an isolated crack in an infinite plane, use the earlier proposed complex method to calculate the SIF of the equivalent crack, can obtain the accurate SIF. This modified method not only has very simple calcaulating process but also has reliable calculating results.(4) Combining with the complex function method proposed in this paper with other known mehods, every effect factor of SIF in MSD structures can be assessed. Use the basic theory of compounding method, the SIF of finite MSD structures can be obtained. Applying to method to some typical MSD structures, important conclusions on SIF of MSD structures can be achieved.(5) Introduced the definition and classification of fatigue and the predition methods of fatigue life. Several important problems in crack growth life prodiction and three typical formulas in describing the crack growth rate were emphasized. Cimbining with the SIF of the MSD structure obtained from the method proposed and applying Paris Law, the crack growth life can be obtained. Crack growth life of some typical MSD structures is predicted, regularity conclusions can be achieved.