Crack arrest unstable

KEYWORDS Z-Fiber , Acoustic Emission (AE), delamination, unstable crack propagation, crack arrest, Z-pin blocks, IM7/977-2... 

Figure 9.21a shows rapid force oscillations during crack growth in a 5 mm thick Charpy specimen impacted at lms , which contained a sharp pre-crack. The HDPE had a density of 955 kg m . A number of crack arrest locations are visible on the fracture surface (Fig. 9.21b), so the crack appears to advance in an unstable manner. However, as the crack advances fastest at the specimen mid-thickness, it is not possible to monitor its position by photography, which would only record the surface crack growth through the shear lips (S). 

A special case is shown in figure 5.16(c). On reaching a load Fq, the crack propagates unstably for a certain distance and then becomes arrested. This is called pop-in. 

At a constant temperature, the increase of e involves an increase of crack propagation is generally of the stick-slip type (see Fig. 12.4), at low e, and becomes brittle-unstable when e increases (Galy et al., 1994). KIca (for arrest) remains at an almost constant value (Kinloch, 1985). 

Evans (1975), Evans and Charles (1977), and Emery (1980) performed more refined fracture mechanics studies regarding the onset and arrest conditions Bahr et al. (1988) and Pompe (1993) extended this work and considered the propagation of multiple cracks while Swain (1990) found that materials showing non-linear deformation and A-curve behaviour have a better resistance to thermal shock. More specifically, the behaviour of a crack in the thermal shock-induced stress field was deduced from the dependence of the crack length on the stress intensity factor. Unstable propagation of a flaw in a brittle material under conditions of thermal shock was assumed to occur when the following criteria were satisfied ... 

At higher concentrations of fibres or at intermediate concentrations when a few fibres around the crack tip are orientated perpendicular to the notch plane, the loading curve increases linearly up to a maximum load Pi as the load is transferred onto the fibres at the crack front and a process zone develops. Fracture of the fibres lying normal to the notch plane results in unstable crack propagation until it is arrested by a packet of fibres favourably orientated then the applied load must be increased to create a new frontal process zone. Tlierefore the successive unstable crack extensions result in a saw-tooth like loading curve behaviour (types 3 and 3 loading curves in Table II, associated with Figures I OB and lOE, I OF respectively). 

Standard Mode I Double Cantilever Beam specimens for delamination testing of a unidirectional (UD) IM7/977-2 composite were Z-pinned with two separate blocks of Z-Fiber reinforcement. The reinforced beam configuration was such as to provoke an unstable delamination, propagating between the two Z-pin blocks. Crack resistance curves for these specific geometry specimens of IM7/977-2 indicate that the unstable delamination cracks are arrested by the second Z-pin block, with the crack propagation resistance being dictated primarily by the Z-pinning density within a block. Acoustic emission analysis is used to interpret visual observations and other test data. 

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.

---