Slip stick growth

Crack propagation observed in epoxies has also been described as smooth crack growth and slip stick growth. Smooth crack growth is more often observed at high strain rates and lower temperatures. Slip stick crack propagation is indicative of... 

The following simplified explanation for the observed power-law behaviour during stick-slip crack growth is a limiting case of a more general approach to SCG [43] based on crack layer theory [40, 41]. The crack layer in... 

Fig. 9a and b. Stable and unstable fracture behavior as observed on Instron recorder traces when testing constant compliance fracture specimens, a) trace, stable crack growth b) trace, unstable, stick-slip crack growth... 

One of the most curious aspects of crack growth in most epoxies is the apparently unstable manner by which propagation occurs, even over wide ranges of temperature and test rate. This behavior is commonly referred to as stick-slip , and is characterized by the crack growing in a series of discrete, unstable jumps. Even some of the earliest works on epoxy fracture report this mode of crack growth. The suspected origins of stick-slip fracture behavior in epoxies is discussed in a subsequent section. Unhke epoxies, thermoplastic polymers, such as poly(methyl methacrylate) and polystyrene, are characterized by stable, continuous crack growth. This mode of fracture sometimes can be observed in epoxies, in particular, when they are tested at fast rates and/or low temperatures. 

The effects of moisture on epoxy fracture are not conclusive. Scott et al. reported that an amine cured epoxy, normally displaying stick-slip fracture at room temperature and low rates, exhibited stable behavior when immersed in distilled water. Also, they found that the rate necessary to promote the unstable to stable crack growth transition at room temperature was increased by two orders of magnitude in the presence of the water. Yamini and Young , on the other hand, found that testing in water tended to suppress stable behavior and promote stick-slip fracture in an amine cured epoxy over a wider range... 

Repeated unstable delamination growth (stick-slip) in most specimens with symmetric [0°/90°]6s cross-ply lay-up, occurring throughout the test. 

Unstable delamination growth (stick-slip) has been observed to coincide with jumps from one side of the specimen mid-plane to the other during saw-tooth delamination propagation in the cross-ply laminates. 

The factor that is least controllable in the deduction of static and kinetic friction values from stick-slip experiments is the condition of the contacting surfaces. This explains why Brockley and Davis [20] in their study of the influence of the time of contact on were unable to obtain repeatable results, as shown in Fig. 8-15a. For each series of determinations made with a single placement of the rider on the track, the plot of against the time of quiescent contact shows satisfactory self-consistency. But when the experiment is repeated with a fresh placement of the rider on the track, a different curve is obtained, self-consistent but not a duplicate of the previous experiment. This was traced to the variability of the surface with location on the rubbing track. Furthermore, it was demonstrated that a model of time-dependent junction growth could be made to yield the following expression ... 

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. 

D. Maugis, Subcritical crack growth, surface energy, fracture toughness, stick-slip and embrittlement, J. Mater. Sci. 20, 3041-3073 (1985). 

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