Fatigue Fracture Mechanism of Advanced Composite Materials (Micro-, Meso- and Macroscopic Approach)

混合模式delamination crack growth tests at elevated temperature(473K)were carried out using unidirectionally carbon fiber reinforced thermoplastics(CFRTP)under static creep condition.The crack propagation rate，da/dt，became higher in terms of mode I component of total energy release rate，GⅡD2，as mixed mode ratio，GⅡD2/GⅡD2 D2，decreased.This implied that crack propagation rate was accelerated by mode II component of total energy release rate，G I D2 IIⅡD2。It was found that creep deformation area around the crack tip by mode II component was larger than that by mode I.Fractographic observation showed that matrices were elongated along the direction of main stress and fracture occurred at the interface between fiber and matrix.Reversed plane bending tests were conducted on three kinds of SiC reinforced aluminum composites，namely whisker reinforced casting 6061aluminum composite，and particle reced cuminum composites……More surface-notched specimens.Initiation and growth behavior of small fatigue cracks were continuously monitored by a plastic replication technique and investigated in detail.Crack initiation mechanism on a whisker reinforced casting composite was found characterized by the interface cracking between matrix and SiC whiskers，where the lump of SiC whiskers was composed in a manufacturing process，and that on particle reinforced P/M composite was cycle slip deformation in matrix in the vicinity of SiC reinforcement particle.Fatigue strength of both materials was higher than that of unreinforced alloy due to the fact that SiC whiskers or tightly bounded matrix/particle interface restricted cyclic deformation of matrix，which resulted in the delay or fatigue crack initiation。While in a particle reinforced casting composite fatigue crack easily initiated at a casting defect at matrix/particle interface，and this premature crack initiation resulted in the lowest crack initiation strength.Fatigue crack growth test under constant amplitude loading was conduced on a grain-oriented3%silicon iron using servo-hydraulic fatigue loading system。The configuration of specimen surface around fatigue crack tip was transferred to a thin replica film and examined by means of an atomic force microscope(AFM)。Discrete slip lines which emanated from crack edges were observed clearly along the two preferred slip directions(±55°)。It implies that two preferential slips operated to almost identical extent simultaneously and fatigue crack grew in near mode I.The slip deformation during loading and unloading was observed successively and it was found that the operation of slips behind the fatigue crack tip resulted in crack closure and that at the crack tip brought about crack growth.The surface displacement during one cycle was measured quantitatively by using image processing technique.The pattern of the relationship between load and slip deformation was correlated well with that between load and crack tip opening displacement，CTOD，and the slipping distance near the crack tip corresponded to the ideal CTOD。Less：Less

Mixed mode delamination crack growth tests at elevated temperature (473K) were carried out using unidirectionally carbon fiber reinforced thermoplastics (CFRTP) under static creep condition. The crack propagation rate, da/dt, became higher in terms of mode I component of total energy release rate, GィイD2IィエD2, as mixed mode ratio, GィイD2IィエD2/GィイD2IIィエD2, decreased. This implied that crack propagation rate was accelerated by mode II component of total energy release rate, GィイD2IIィエD2. It was found that creep deformation area around the crack tip by mode II component was larger than that by mode I. Fractographic observation showed that matrices were elongated along the direction of main stress and fracture occurred at the interface between fiber and matrix.Reversed plane bending tests were conducted on three kinds of SiC reinforced aluminum composites, namely whisker reinforced casting 6061 aluminum composite, and particle reinforced P/M and casting 2024 aluminum composites, with shallow … More surface-notched specimens. Initiation and growth behavior of small fatigue cracks were continuously monitored by a plastic replication technique and investigated in detail. Crack initiation mechanism on a whisker reinforced casting composite was found characterized by the interface cracking between matrix and SiC whiskers, where the lump of SiC whiskers was composed in a manufacturing process, and that on particle reinforced P/M composite was cycle slip deformation in matrix in the vicinity of SiC reinforcement particle. Fatigue strength of both materials was higher than that of unreinforced alloy due to the fact that SiC whiskers or tightly bounded matrix/particle interface restricted cyclic deformation of matrix, which resulted in the delay or fatigue crack initiation. While in a particle reinforced casting composite fatigue crack easily initiated at a casting defect at matrix/particle interface, and this premature crack initiation resulted in the lowest crack initiation strength.Fatigue crack growth test under constant amplitude loading was conduced on a grain-oriented 3% silicon iron using servo-hydraulic fatigue loading system. The configuration of specimen surface around fatigue crack tip was transferred to a thin replica film and examined by means of an atomic force microscope (AFM). Discrete slip lines which emanated from crack edges were observed clearly along the two preferred slip directions (±55°). It implies that two preferential slips operated to almost identical extent simultaneously and fatigue crack grew in near mode I. The slip deformation during loading and unloading was observed successively and it was found that the operation of slips behind the fatigue crack tip resulted in crack closure and that at the crack tip brought about crack growth. The surface displacement during one cycle was measured quantitatively by using image processing technique. The pattern of the relationship between load and slip deformation was correlated well with that between load and crack tip opening displacement, CTOD, and the slipping distance near the crack tip corresponded to the ideal CTOD. Less