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This paper presents a new approach to predict the high cycle fatigue limit of defective material submitted to multiaxial loading. Defects are simplified to a half spherical void at the surface of a specimen. Finite Element (FE) method is used to determine stress distribution around defect for different sizes and loading levels. Papadopoulos high cycle fatigue criterion is used to calculate equivalent stress around defect. Based on stress analysis, a definition of affected area is proposed, in which the Papadopoulos criterion is violated. The evolution of the affected area, versus the amplitude of loading and defect size leads to determine fatigue limit for defective material. Results are in good agreement with experimental investigations and show that the affected area is a good parameter to predict the influence of a defect on multiaxial fatigue behaviour.

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Fiber reinforced composites have wide structural applications and vast research has been going on to improve their mechanical performance when subjected to quasi-static loading but, study of their dynamic behavior is still underdeveloped. For this reason, scientists have been continuously working on developing methods to improve their dynamic characteristics and addition of nanofillers suchs as Carbon Nanotubes (CNTs) as reinforcement is considered a possible solution for developing future generation high-quality fiber reinforced nanocomposites. In this study, composite specimens are manufactured using Epon 862 Epoxy resin and T300 6k carbon fibers, and each specimen contained different weight percentages of multi-walled Carbon nanotubes (MWCNTs) i.e. 0% as a reference, 0.5%, and 2%. Specimens were tested experimentally using the Split Hopkinson pressure bar device (SHPB) under different impact pressures to examine their dynamic response and damage behavior at high strain rates. During the dynamic compression tests, a high-speed camera was used to monitor and record the damage kinetics. The experimental characterization showed that the integration of CNTs in matrix has greatly influenced the dynamic response and damage mechanism of the Carbon Fiber Reinforced Polymers composite (CFRP). Mechanical behavior of specimens with each percentage demonstrated the enhancement of the mechanical properties and showed the increase of the dynamic characteristics and fracture resistance because of the increase in stiffness of matrix material and interfacial bonding between matix and fiber reinforcement.