Crack initiation and arrest

The typical effect of rate on the measured value of K,c and the associated type of crack growth can clearly be seen in Fig. 4 for an epoxy polymer based upon a DGEBA resin and cured with various amounts of TETA. The values of K ci and K ca are those for crack initiation and arrest (Fig. 2b) respectively and the difference between them characterises the amount of crack jumping which has taken place. When the difference... 

M. G. Jenkins, A. S. Kobayashi, K. W. White, and R. C. Bradt, Crack Initiation and Arrest in SiC Whisker/Al2C>3 Matrix Ceramic/Ceramic Composites, Journal of the American Ceramic Society, 70[6], 393-395 (1987). 

Figure 2 Stress-Temperature Diagram for Crack Initiation and Arrest. 3...

Figure 16 Crack initiation and arrest loads for phenolic FRP-wood interface.

Associated with the development of the Master Curve concept, which is discussed in Section 10.3.4, studies have concluded that it is possible to develop correlations describing a relationship between crack initiation and arrest toughness (Wallin, 2001). These studies have focused on clarifying which elements of the Master Curve approach should be modified for assessing crack arrest and finding possible correlations between initiation and arrest parameters. Due to different mechanisms and differences in factors controlling fracture initiation and arrest events (e.g. the local properties are crucial for crack initiation, but not so critical for crack arrest), the weakest link theory applied in the Master Curve approach is not directly suitable for crack arrest. 

Ravi-Chandar, K. and Knauss, W.G., An investigation into dynamic fracture. I - Crack initiation and arrest. Int. J. Fract., 25, 247-262 (1984). 

The higher the crack propagation rate in a sample gets (a > 107 m/s) the more the relation between d and Kj - and even the measured value of Kic - depend on the parameters of the experiment. As shown in Fig. 9.7 Kg increases monotonically up to a Kic of MN/m / if oq is kept constant [12, 13]. In other experiments (Charpy impact tests, crack initiation and arrest in DCB specimens) K-values at higher crack speeds (2 to 300 m/s) are obtained which lie below the Kic [14, 18]. 

Figure 4. Typical schematic load-displacement traces for double torsion test specimen (a) continuous (stable) crack growth (b) discontinuoxis (unstable) crack growth showing initiation and arrest loads.

Polymer blends, in which the constituent polymers are incompatible, tend to improve fatigue resistance. For example, natural rubber and polybutadiene show good resistance to fatigue, crack initiation, and growth because of the formation of heterogeneous polymer phases a crack growth in one polymer phase is arrested at the boundary with the adjacent polymer phase. 

The possibility of multiple initiation and arrest of the crack in the DCB specimen validates the application of the method of mechanical compliance for determining the characteristic energy Gjc. Values of Gic for the adhesive-bonded joints under test were calculated from... 

Figure A7-7 shows the material properties jc and A)a (intensity factors for crack initiation and for crack arrest of a propagating crack), with reference to a typical pressure vessel steel.

The cracks initiate and propagate in the form of decohesions in the aggregate-matrix interface and these regions deserve more particular attention. The physical and chemical composition, as well as the structure of the interface, is essentially different from these of the bulk matrix. Cracks are often arrested just at the interface layer a few micrometers before reaching an obstacle in the form of an aggregate grain or a fibre (cf. Chapter 7). 

This simple model explains why the oscillations can be sinusoidal or sawtooth shaped, depending on the shape of the limit cycle, and why the amplitude of oscillations changes with the imposed velocity V. It shows that Gj for crack initiation and G for crack arrest [the upper and lower points of the G(v) curve] differ from G and G j (except for the relaxation cycle) and are not a material property. 

Upon unloading the joint, the force-displacement trace not returning to the origin may indicate that plastic deformation of the substrates has occurred see ISO (2009) for full procedures. Such an occurrence would invalidate the values of Qc determined. The presence of stick-slip crack propagation in the continuously loaded BS or ISO methods requires some special considerations to be taken into account. Again, fiill details are provided in ISO (2009). Under these conditions, steady-state propagation values of Qc are not determined, instead values at crack initiation and crack arrest are obtained. 

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