Crack Propagation Behavior

This paper revealed the existence of adequate MFC content for the BF composite (PLA/BF composite), to improve the mechanical properties. 

In this study, M FC was used as an enhancer for a PLA-based composite using BF to increase mechanical properties such as bending strength and fracture toughness. The effect of weight content of MFC on those mechanical properties was examined. 

1) If MFC is used as enhancer in a biodegradable composite reinforced by BF, the bending strength as well as fracture toughness are improved. 

This study was supported by the high technology research project on Research and Development Center for Advanced Composite Materials of Doshisha University and the Ministry of Education, Culture, Sports, Science, and Technology, Japan. This study was possible also because of the contribution from Mr Naoya Yamashita (former graduate student of Doshisha University). 

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Total interpenetration of chains (X approaches Rg) is not necessary to achieve complete strength when M > M and x < 7). However, a word of caution while complete strength may be obtained in terms of critical fracture measures such as G c and A lc, the durability, measured in sub-critical fracture terms, such as the fatigue crack propagation rate da/AN, may be very far from its fully healed state at r. We have shown that while the weld toughness A lc increases linearly with interdiffusion depth X as K f. X, the fatigue crack propagation behavior of partially healed welds behaves as [1]... 

In a first testing series, the fracture behavior of the neat, fully crosslinked epoxy network was studied. A fully unstable crack propagation behavior was observed and the critical stress intensity factor, Kj (0.82 MPaxm ), and the critical energy release rate, Gj (0.28 kj/m ), were determined [87]. These are typical values for highly crosslinked epoxy networks prepared with DGEBPA and aromatic or cycloaliphatic diamines. 

Although the constant AKpvalues applied had different amplitudes the fatigue crack propagation behavior is quite similar for the three polymers. They all show... 

The surface region of the FGM specimen after cracking will be analogous to a plate with a crack in which residual tensile stress is acting in the surface layer. Recently, it has been reported that cracks tend to branch and deflect into a trajectory with mode II stress intensity, K, being equal to zero[12]. This behavior is understood to be noting that the shear associated K, component of the crack tip field continually deflects cracks until the K,, =0 condition is achieved[12][13]. The vertical crack propagation behavior will be discussed on the basis of the K,, =0 criterion associated with the estimation of the stress intensity factor for the cracks perpendicular and parallel to the specimen surface[14][15]. 

Gallagher, J. P., and Wei, R. P., Corrosion Fatigue Crack Propagation Behavior in Steels, Corrosion Fatigue Chemistry, Mechanics and Microstructure, NACE-2 (1972), 409. 

The figure shows the stress CF process, type B, purely time-dependent CF propagation, which is essentially a simple superposition of mechanical fatigue and SCC. Stress CF occurs only when > Ki qq. In this case, the cyclic character of loading is not important. The combination of true CF and stress CF results in type C, the most general form of CF crack propagation behavior (79). 

The TDCB specimen is shown to be suitable for FRP-wood interface fatigue tests. The FRP-wood interface displayed the typical crack propagation behavior under constant cyclic loading as observed by others. 

L. A. James, Fatigue—Crack Propagation Behavior of Inconel 718, HEDL-TME 75-80, Hanford Engineering Development Laboratory, Richland, Washington (1975). 

Connelly GM, Rimnac CM, Wright TM, Hertzberg RW, Manson JA (1984), Fatigue crack propagation behavior of ultrahigh molecular weight polyethylene J Orthop Res, 2, 119-125. 

Figure 1. Fatigue crack propagation behavior of neat resins.

Fig. 3.1-177 Crack propagation behavior of recrystallized P/M TalOW specimens at room temperature at a stress ratio off = 0.4 [1.202]...

Several studies have determined the threshold stress intensity for engineering plastics, eg, polycarbonate (5). The dt/6N values are in excellent agreement for growth rates > 10 mm/cycle but differ significantly for lower values. Additional threshold data have been reported for PMMA as a function of stress ratio R, where R = l min/ max (70). Other studies have been conducted to characterize the near threshold crack propagation behavior (12,71). Figure 21 (12) shows a a-N curve for polyethersulfone, including data at the near threshold level. Similarly... 

Choe, H., Chen, D., Schneibel, J.H. et al., 2001, Ambient to high temperature fracture toughness and fatigue crack propagation behavior in a Mo 12Si 8.5B intermetallic, Intermetallics 9 319. 

Source H.H. Smith, RS. Kullen, and D.J. Michel, Fatigue Crack Propagation Behavior ofTitanium Alloys6242Sand5621SatBevatedTemperature, Metall. Trans. A, Vol 19A, April 1988, p 881 -885... 

Fig. 29 shows that due to the material mismatch, when the distance between the sub-interface crack and the interface 8t approaches zero, the corresponding component at the crack tip is veiy high and is acting in such a direction that the crack tends to deviate away from the interface toward the centerline of the bond. As the distance ht increases, the corresponding K value drops drastically, which suggests that the sub-interface crack will deviate from the interface in a rather gradual fashion. This prediction is consistent with crack trajectory shown in the SEM micrograph of the DCB specimens with directionally unstable cracks in Fig. 10. Since differences in the material mismatch will result in variations in the stress distribution. Fig. 29 also indicates that the crack propagation behavior will also be different for different materials systems.

The analysis of the interface mechanics provides useful insights into crack propagation behavior in adhesively bonded joints. A finite element model for the DCB specimen was constructed using Franc2D/L [64], a convenient code for this task because of its capability for automatic remeshing in the vicinity of a growing crack. An adhesive layer (material 2) with thickness of / = 0.5 mm is sandwiched between two adherends (material 1) with thickness of = 6 mm, and... 

Crack propagation behavior was observed for a special specimen of the PLA/BF and PLA/BF/MFC composite in which a single long fiber was longitudinally embedded in the matrix as shown in Figure 9.8. In this test, constraint strain was applied to the specimen, having initiated transverse crack in the matrix. 

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