Fatigue Damage Models for Laminated Composite Structures

Many cyclic loaded structures show damage after a certain number of cycles even though the maximum stress in a cycle is far below static strength. This phenomenon is called fatigue. It is a critical criterion and has to be considered for appropriate dimensioning of engineering structures, which are in many cases subjected to repeated loadings. Especially in the field of laminated composite materials, fatigue is still content of extensive research due to their complex damage mechanisms. The present work focuses onto the investigation of the current state of finite elemente analysis (FEA) software packages in the field of fatigue of laminated composites. Due to the motivation of a possible application to a composite rim, where problems with fatigue delaminations occur, the focus of the assessment lies on interlaminar fatigue damage. To achieve these objectives, a certain theoretical basis is needed. Therefore, the first part of the thesis contains a summary of fundamentals in fracture mechanics, laminated composites, fatigue modeling in general and state of the art fatigue methods for laminated composites. In the second part, extensive reviews of theories of selected FEA software manufacturers are given, namely Siemens Samtech Samcef and 3DS Abaqus. Former manufacturer, which meanwhile integrated the software package into their product lifecycle management (PLM) environment NX, the theory, which is based on continuum damage mechanics (CDM), focuses on intralaminar fatigue damage. For interlaminar damage, a cohesive zone model is suggested, but no specific fatigue theory is developed until now. Since the fatigue model was still not implemented into the software, no assessment could be done. Abaqus implemented a low cycle fatigue tool for interlaminar crack growth, which is based on linear elastic fracture mechanics (LEFM) and Paris law for fatigue crack growth. Furthermore, the onset of a crack is considered in an additional criterion. However, after an extensive practical assessment, it was concluded that the method is still very limited in its capabilities and shows some unreasonable behavior. Accordingly, its applicability onto complex structural components such as a composite rim is not recommended and hence was not done. In conclusion, the tools for calculating the fatigue behavior of composite materials mentioned in this work are not yet fully applicable for evaluating practical problems. Based on the knowledge obtained in the reviews and the assessments, a proposal for the treatment of fatigue in laminated composite materials is given for future work.submitted by Andreas Peter PröllUniversität Linz, Masterarbeit, 2017(VLID)219544