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Fracture fatigue entropy (FFE) is considered to be independent of the loading amplitude in fatigue experiments, and this conclusion has been applied to fatigue life prediction of composites and metals with satisfactory prediction accuracy. However, the existing research work mainly focuses on high-cycle fatigue, and it is not clear whether the FFE is independent of the loading amplitude in low-cycle fatigue, which requires more in-depth research. Fatigue experiments were conducted on three CFRP laminates with different ply orientations, and the FFE values corresponding to low-cycle fatigue were obtained by thermographic analysis. The results show that the FFE value is no longer independent of the loading amplitude under low-cycle fatigue conditions. On the contrary, it decreases with the increase of fatigue loading amplitude and also varies with different ply orientations. The relationship between low-cycle fracture fatigue entropy and loading amplitude of composites with different ply orientations was analyzed and modeled, and a novel low-cycle fatigue life prediction method based on FFE was developed. • FFE of CFRP under the LCF loading is no longer independent of loading amplitude. • The value of FFE for LCF decreases with the increase of ply angle. • The relationship between FFE, loading amplitude and ply angle is modeled. • A novel method is developed to predict LCF life based on the concept of FFE. [ABSTRACT FROM AUTHOR]

• Six different preloaded CFRP laminates under high velocity impact are investigated. • Preloading conditions reduce the ballistic limit and energy absorption rate. • Failure modes including delamination are identified and discussed. • An analytical model has been proposed to predict ballistic limit velocity. In this research, we explore the effects of complex in-plane quasi-static loads on the high-velocity impact response of Carbon Fiber Reinforced Plastic (CFRP) composite laminates, grounded in the practical context of various working loads combined with potential high-velocity impacts experienced by aircraft. The specific complex in-plane loads studied include uniaxial tension and compression, biaxial tension and compression, and a combination of tension and compression preloading. The results show that the complexity of in-plane preloads, along with pre-compression, reduces the ballistic limit velocity. Biaxial preloading, especially in compression, greatly compromises the specimen's ballistic resistance. The study also proposes a theoretical model based on energy conservation principles to predict the ballistic limit velocity of CFRP laminates under various preloading conditions. The research provides valuable insights for designing composite structures and highlights the importance of considering in-plane preloads in assessing thin CFRP ability to withstand ballistic impacts. [Display omitted] [ABSTRACT FROM AUTHOR]

Delamination in composite laminates is one of the most common and critical defects, it would significantly reduce the stiffness and strength of components. This paper aims at reducing the drilling-induced delamination to improve hole quality. First, drilling tests are carried out to examine the distribution and amplitude of thrust force on drill under different cutting conditions. Empirical thrust force models are developed using experimental data. Then, analytical models of critical thrust force (CTF) are separately established considering two drilling states, considering the actual interactions between the drill and laminate. Anisotropy of composite, tool geometry and load distribution on drill are considered. Comparisons with experimental data and existing models show that the proposed models are capable of predicting CTFs under different loading conditions. In addition, the critical feed rates at different ply locations are determined. This study can provide guidance for the selection of cutting parameters to reduce drilling-induced delamination. • Drilling tests are carried out on CFRP laminates to examine the effects of chisel edge and cutting parameters on the thrust force and delamination. • Analytical mechanical models are proposed to predict the critical thrust force of delamination onset under different cutting conditions. • Critical feed rates at different uncut plies are calculated to avoid drilling-induced delamination. [ABSTRACT FROM AUTHOR]

The heat dissipation caused by internal friction plays an important role during fatigue property prediction based on self-heating data but it is still unclear for Carbon Fiber Reinforced Polymer (CFRP). In this paper, fatigue tests and thermal data analysis are carried out in order to investigate internal friction and Fracture Fatigue Entropy (FFE) of CFRP laminates with various stacking sequences. The fatigue limit is firstly estimated based on an improved graphic method developed by our previous study and then used to mark the beginning of fatigue damage occurs. It is found that the relationship between internal friction and applied stress amplitude is non-linear which is different from metals. Quadratic function is employed to characterize the internal friction region below fatigue limit and the trend line is used to separate heat dissipation related to damage and internal friction beyond fatigue limit. Results show that the internal friction effect of cross-ply laminates is significantly greater than that of unidirectional laminates. FFEs estimated by the heat dissipation related to damage are found to be independent of applied stress amplitude, which can be used to predict S–N curve of CFRP laminates efficiently and precisely. • The relationship of internal friction and stress amplitude is found to be non-linear for CFRP laminates. • Internal friction effect of cross-ply laminates is significantly greater than that of unidirectional laminates. • FFEs estimated by the heat dissipation related to damage are found to be independent of applied stress amplitude. • A new method based on FFE is developed to predict S–N curve of CFRP laminates efficiently and precisely. [ABSTRACT FROM AUTHOR]