Crack growth, unstable

It follows from equation (7.8) that unstable crack growth occurs if the slope of the straight line corresponding to the stress intensity factor at constant applied stress is greater than or equal to the slope of the tangent line to the fracture resistance curve at the same point (Fig. 7.6). Also the applied stress intensity factor becomes higher than the fracture resistance of the material. 

General criterion of stable/unstable crack growth in a brittle material. 

Conditions for stable/unstable crack growth in a layered structure (a) a range of crack length parameters for stable crack growth in a laminate with a residual compressive stress in a top layer (b) stable/unstable crack growth in a laminate with a residual tensile stress in a top layer. 

Fig. 12 Force versus time response from IWP specimens showing stable and unstable crack growth (a) Standard IWP specimen (b) IWP specimen with a pre-crack.

It was already mentioned that for the beam tested, transverse cracking was mostly associated with unstable crack growth. It is worth adding that a micrographic study of the transverse cracks showed that the release of energy induced by transverse cracking was not enough to lead to the initiation of a crack for delamination between the 90° and the 0° layer. Such an observation was made on carbon-epoxy systems [8]. 

Figure 7. SEM fractographic observation of an area of unstable crack growth, in a 2 L45 sample tested at 2 m/s.

The Griffith formalism, therefore, requires that the quantity acr /a be a constant. The left-hand side of Eqn. (2.22) represents a crack-driving force, in terms of stress, and the right-hand side represents a material property that governs its resistance to unstable crack growth, or its fracture toughness. From previous consideration of stress concentration, Eqn. (2.12), it may be seen that, as /o 0,... 

KEYWORDS fincture, impact mode I. mode II, mixed-mode I/II, adhesive joint composite, falling wedge test, fr-acture failure envelope, stick-slip, unstable crack growth, epoxy... 

The most notable aspect of these arrest results is the fact that the arrest values are only approximately 17 % of the initiation values as a result of the unstable crack growth. These low values are most likely due to the extreme time dependence of the chosen adhesive, along with artifacts associated with kinetic energy effects due to the rapid crack growth. Crack. jump distances as great as 150 mm were observed in static DCB testing, although 40-60 mm jumps were more common. These are comparable to stick-slip results collected by Blackman et al.[7]. who show Jumps of up to 100 mm. 

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.

Equation 48 ean be rearranged and integrated between an initial crack length and a final crack length af, where unstable crack growth initiates. 

---